Revision of the genus Acrochordiceras Hyatt, 1877 (Ammonoidea, Middle Triassic): morphology, biometry, biostratigraphy and intra-specific variability

Authors


Abstract

Abstract:  The family Acrochordiceratidae Arthaber, 1911 ranges in age from latest Spathian to the middle/late Anisian boundary, and it represents a major component of ammonoid faunas during that time. The middle Anisian genus Acrochordiceras Hyatt, 1877 is the most widespread taxon of the family and occurs abundantly worldwide within the low paleolatitude belt. However, there is a profusion of species names available for Acrochordiceras. This excessive diversity at the species level essentially results from the fact that sufficiently large samples were not available, thus leading to a typological approach to its taxonomy. Based on new extensive collections obtained from the Anisian (Middle Triassic) Fossil Hill Member (Star Peak Group, north-west Nevada) for which a high resolution biostratigraphic frame is available, the taxonomy and biostratigraphy of the genus Acrochordiceras Hyatt, 1877 is herein revised with respect to its intra-specific variation. Morphological and biometric studies (c. 550 bedrock-controlled specimens were measured) show that only one species occurs in each stratigraphic level. Continuous ranges of intra-specific variation of studied specimens enable us to synonymize Haydenites Diener, 1907, Silesiacrochordiceras Diener, 1916 and Epacrochordiceras Spath, 1934 with Acrochordiceras Hyatt, 1877. Three stratigraphically successive species are herein recognized in the low paleolatitude middle Anisian faunas from Nevada: A. hatschekii (Diener, 1907), A. hyatti Meek, 1877 and A. carolinae Mojsisovics, 1882. Moreover, an assessment of intra-specific variation of the adult size range does not support recognition of a dimorphic pair (Acrochordiceras and Epacrochordiceras) as previously suggested by other workers (Epacrochordiceras is the compressed and weakly ornamented end-member variant of Acrochordiceras). The successive middle Anisian species of Acrochordiceras form an anagenetic lineage characterized by increasing involution, adult size and intra-specific variation. This taxonomic revision based on new bedrock-controlled collections is thus an important prerequisite before studying the evolution of the group.

Based on new field investigations, a refined zonation of the classical Anisian ammonoid faunas of north-west Nevada (Smith 1914; Silberling and Wallace 1969; Silberling and Nichols 1982) has been established in the Fossil Hill Member by Bucher (1988, 1992a, b, 1994) and Monnet and Bucher (2005a, b). This detailed biochronological succession provides an adequate temporal framework for studying evolutionary trends of ammonoids (Monnet 2005). Among Anisian ammonoid faunas, the Acrochordiceratidae Arthaber, 1911 represents one of the most important and abundant families within the low paleolatitudes. Acrochordiceras Hyatt, 1877 is the most important genus of the family and a key component of both Tethyan and Nevadan faunas. At both the family level and within the genus Acrochordiceras, increasing adult size, increasing involution and increasing intra-specific variability are the most significant evolutionary trends. Therefore, the genus may potentially play an important role in the description and understanding of patterns and processes in macroevolution. However, there is a profusion of specific names in the literature for Acrochordiceras. This excessive diversity essentially results from the fact that sufficiently large samples were not available, thus leading to a typological taxonomy. Hence, the purpose of this study is to revise the taxonomy of Acrochordiceras not only by means of classical descriptive phenetics, but also by means of quantitative morphometry, thanks to extensive collections recently sampled from the Fossil Hill Member (Star Peak Group, north-west Nevada). These new collections also enable study of the extent of intra-specific variation and ontogenetic development of successive assemblages of Acrochordiceras Hyatt, 1877.

Material and methods

Palaeogeography and biostratigraphy

Systematic bed-by-bed ammonoid collections were obtained from multiple sections in the northern Humboldt Range, Augusta Mountains, New Pass Range, and southern Tobin Range of north-west Nevada (Text-fig. 1). Detailed descriptions of these sections are given in Bucher (1988, 1992a, b, 1994) and in Monnet and Bucher (2005a). All specimens of Acrochordiceras described in the present paper were collected from the Fossil Hill Member, which is common to both the Favret and Prida Formations of the Star Peak Group of north-west Nevada (see Text-fig. 2 for specific localities and sampling points). The Fossil Hill Member, described in detail by Silberling and Wallace (1969) and Nichols and Silberling (1977), consists essentially of a monotonous succession of dark micritic, thin-bedded limestones alternating with calcareous shales and siltstones. Its depositional environment was below wave base, with anoxic bottom waters as evidenced by the undisturbed finely laminated rocks, hydrocarbon residues and absence of benthonic organisms. Fossil Hill strata of middle Anisian age vary considerably in thickness, ranging from near 0 to 200 m, thus reflecting pronounced lateral changes of sedimentation rates within an outer platform setting. Synsedimentary block faulting is largely responsible for these lateral variations, and a detailed depositional sequence has been established for several localities within the basin (Nichols and Silberling 1977; Bucher 1992b). The highly refined succession of ammonoid zones also provides additional control on depositional rates, and the resulting picture is that of an extensional basin divided into numerous faulted blocks.

Figure TEXT‐FIG. 1..

 Palaeogeographical positions of sampled localities from north-west Nevada during the middle Anisian. A, Palaeogeographical position of north-west Nevada during the Anisian (after Golonka and Ford 2000; Stampfli and Borel 2002; modified). B, Simplified geological map of north-west Nevada (after Wyld 2000; modified). C, Location map of sampled areas (Humboldt Range, Augusta Mountains, Tobin Range, New Pass Range).

Figure TEXT‐FIG. 2..

 Biochronological age of studied samples of Acrochordiceras and biostratigraphic ranges of species of Acrochordiceras in north-west Nevada (after Monnet and Bucher 2005b; modified). Radiometric calibration after Galfetti et al. (2007) and Ovtcharova et al. (2006).

Smith (1914), Silberling and Wallace (1969) and Silberling and Nichols (1982) established the foundations for the classical Anisian ammonoid faunas of north-west Nevada from both a taxonomic and biochronological point of view. Bucher (1988, 1992a, b, 1994) and Monnet and Bucher (2005a) deeply emended these works and proposed a further refined ammonoid zonation for the Fossil Hill Member. Finally, Monnet and Bucher (2005b) synthesized and correlated the biochronological scales between north-west Nevada and British Columbia. The distribution of acrochordiceratid species is extracted from this synthesis and corrected to account for the taxonomic revision proposed in this study (Text-fig. 2).

Measurements and statistical tests

Systematic collections made during the last two decades provide several thousand Anisian ammonoid specimens, and of these, more than 700 sets of measurements are made from c. 550 bedrock-controlled specimens of Acrochordiceras. Although the genus Acrochordiceras spans the entire middle Anisian, and most exposures of middle Anisian strata have been sampled bed-by-bed, the studied specimens are unevenly distributed in time and space (see Text-fig. 2). Each set of measurements includes classical geometric parameters of the ammonoid conch such as the shell diameter (D) and corresponding whorl height (H), whorl width (W) and umbilical diameter (U). In addition to the four dimensional parameters, counts of ventral ribs per half whorl (R) are also made. To sample individual ontogenetic changes, some specimens are measured at different diameters.

To assess the variability of these parameters for each species, the parameters are plotted in absolute values (H, W, U and R) and as a ratio of the size-related parameter D (H/D, W/D, U/D and R/D) to remove the influence of growth. The normality of each parameter is statistically tested numerically by means of a Lilliefors test with a confidence level of 95 per cent (for details about classical statistical tests, see e.g. Davis 2002 or Hammer and Harper 2005). This test has two major goals: normal distribution is usually a prerequisite for descriptive statistical analyses, and population variability of a species is expected to usually follow a Gaussian distribution of its morphology in comparison with living species. If classical biases are discarded (e.g. data entry error, poor measurement or taxonomic misidentification), note that a departure from normality may result from a change in the system that generated the data, such as a change in the relative proportions of the ammonoid shell through ontogeny (e.g. egressive coiling of the shell at maturity).

The growth trajectory of H, W, U and R is investigated to determine allometry with respect to D. For this, an allometric curve is derived for each species. Because allometric growth conforms to an exponential-like equation, the values of each parameter are fitted by a power equation by means of the reduced major axis fitting method (see Monnet and Bucher 2005a). All studied species are also quantitatively compared by means of box and mean plots. These plots schematize the distribution and the mean of the data, as well as their confidence intervals. Hence, these plots allow determination of whether the different species have a common distribution, i.e. determines whether they differ by their biometric parameters as indicated by the amount of overlap between them.

Intra-specific variability and covariation

Most of the species dealt with in this study were initially described from the Tethys and were differentiated from each other utilizing a typological approach. This approach does not account for intra-specific variation, covariation and ontogenetic changes. Hence, a species was usually based on very few specimens and thus a small amount of morphological change. However, living species are well known to not have such a narrow morphological variation. Even if morphological variation does not equate with genetic variation and even if morphological sibling species exist, most living species exhibit an intra-specific morphological variation, which is continuous and spread across a typical normal (Gaussian) distribution between two extreme morphotypes. The application of this ‘population’ approach to ammonoid taxonomy will likely lead to the synonymization of numerous ‘typologically’ defined species, but it is more consistent with the morphological characteristics of living species.

It is worth noting that within an assemblage of ammonoids obtained from a single bed, a sufficiently large sample, when available, will allow recognition of the First Buckman’s Law of Covariation (Westermann 1966). Indeed, most ammonoid species possess a continuous intra-specific variation ranging from ‘slender’ forms, which are relatively involute, compressed and weakly ornamented, to ‘robust’ forms, which are more evolute, more depressed, and with coarser ornamentation. Within a single species, the frequency of these variants is represented by a typical normal (Gaussian) bell-shaped curve. This mode of intra-specific variation has been well documented by Reeside and Cobban (1960), Westermann (1966), Kennedy and Cobban (1976), Silberling and Nichols (1982), Dagys and Weitschat (1993), Monnet and Bucher (2005a) and Hammer and Bucher (2005), among others. Usually, most of the diagnostic ornamental characters of such variable species are better expressed by the robust variants. Slender variants are less well discernible and tend to converge across closely related species or even genera, thus making recognition of intra-specific variation a crucial characteristic for correct identification.

Systematic palaeontology

Taxonomic descriptions follow the terminology of Arkell et al. (1957). The material described herein is reposited at the Palaeontological Institute and Museum of Zürich (PIMUZ; Switzerland). Conventional abbreviations used in front of the year in the synonymy lists have (briefly) the following meanings (see Matthews 1973 for details): *→ the work validating the species; . → the authors agree on the identification and endorse it; ? → the allocation of this reference is subject to some doubt; non→ the reference actually does not belong to the species under discussion; p → the reference applies only in part to the species under discussion; v → the authors have examined the original material of the reference; year in italics indicates a work without description or illustration.

Order AMMONOIDEA Zittel, 1884
Suborder CERATITINA Hyatt, 1884
Superfamily CERATITACEAE Mojsisovics, 1879
Family ACROCHORDICERATIDAE Arthaber, 1911

Genus ACROCHORDICERAS Hyatt, 1877

Type species. Acrochordiceras hyattiMeek, 1877, p. 124, pl. 11, fig. 5a (only).

Emended diagnosis.  Moderately evolute to involute, compressed to depressed shell with a rounded and highly arched venter, abrupt umbilical shoulders and convex to flattened flanks. Shell ornamented with thin to thick, slightly sinuous to straight, rectiradiate and rounded ribs crossing the venter where they enlarge (plicate pattern), and with or without coarse lateral or umbilical nodes from which may arise up to three ribs. Mature body chamber with a smooth venter, simple ribs bearing marginal swellings and tuberculation arising at the ventro-lateral margin. Suture line ceratitic to subammonitic.

Remarks.  The genus Acrochordiceras exhibits a very distinctive morphology with its plicate ribbing, its single row of nodes from which arises several rounded ribs that become thicker on the venter, its high oval whorl section and a rounded venter. However, species of Acrochordiceras have a wide morphological variation and a certain number of species and even genera fall within this variation. Morphological and biometric studies (this work) show that only one species occurs in each stratigraphic level in north-west Nevada. Continuous ranges of intra-specific variation also allow us to synonymize (see below) HaydenitesDiener, 1907, SilesiacrochordicerasDiener, 1916 and EpacrochordicerasSpath, 1934 with Acrochordiceras Hyatt, 1877.

The genus EpacrochordicerasSpath, 1934 (type Acrochordiceras portisiMartelli, 1906, p. 132) was defined by Spath (1934, p. 401) as ‘Acrochordiceras with nontuberculate ribbing, which is lost or weakened at some stage, generally on the body-chamber’. Hence, Spath (1934, p. 393) separated acrochordiceratids mostly into two groups: Acrochordiceras Hyatt, 1877 with tuberculate forms and EpacrochordicerasSpath, 1934, which he defined as Acrochordiceras with nontuberculate ribbing. As will be demonstrated by this study, forms belonging to the genus Epacrochordiceras are nothing more than the compressed and weakly ornamented end-member variants of Acrochordiceras. It is a well-established pattern of intra-specific variation that tuberculation fades on compressed, slender variants of ammonoid species (see e.g. Kennedy and Cobban 1976; Monnet and Bucher 2005a).

The genus SilesiacrochordicerasDiener, 1916 (type Acrochordiceras damesiiNoetling, 1880, p. 334) was described by Spath (1934, p. 405) as like Acrochordiceras gr. carolinae but with a different, more subdivided suture line and more numerous umbilical nodes. We herein regard the minor differences between these two species as insufficient justification for separating them into two distinct genera, and they instead deserve only a distinction at the species rank. Furthermore, as will be discussed later, without this difference in suture line, S. damesii would be considered a variant of A. carolinae.

The genus HaydenitesDiener, 1907 (type H. hatschekiiDiener, 1907, p. 72) was based on a unique large, evolute shell with strong ribs and tubercles. Diener (1907, p. 72) was apparently puzzled by the several homeomorphic characteristics shown by this species because he cited four genera in the diagnosis. Although Spath (1934, p. 465) followed this point of view, several authors (Silberling and Nichols 1982, p. 23; Bucher 1992b, p. 148; Shevyrev 1995, p. 78) synonymized Haydenites with Acrochordiceras based on their opinion that Haydenites corresponds well with the mature body chamber of Acrochordiceras and is thus characterized by peculiar modifications. Waterhouse (2002, p. 17), following Diener and Spath, kept Haydenites and Acrochordiceras as distinct genera, mainly because he considered H. hatschekii and A. hyatti to be distinct species. However, his arguments were misdirected; indeed, as will be demonstrated in this study, H. hatschekii and A. hyatti are effectively distinct species, but this does not mean that Haydenites and Acrochordiceras are different genera. Waterhouse was also led astray by the absence of published specimens showing the transition between the phragmocone of Acrochordiceras and the mature body chamber of Haydenites. Indeed, as noted by Smith (1914, p. 114), the phragmocone of Haydenites is clearly similar to Acrochordiceras (plicate ribbing, lateral tubercles and moderately high-whorled shell). The material studied herein clearly shows the transition from the plicate and rounded shell of Acrochordiceras with its lateral row of tubercles to the high-whorled shell of Haydenites with ribs interrupted on the venter and its tubercles in a marginal position (see Pls 1, 2).

Figure EXPLANATION OF PLATE 1.

Figure EXPLANATION OF PLATE 1.

 Figs 1–10. Acrochordiceras hatschekii (Diener, 1907). 1–2, PIMUZ 27716, sample HB 97, Favret Canyon (Augusta Mountains); mature specimen. 3–5, PIMUZ 27717, sample HB 82, Congress Canyon (Humboldt Range). 6–7, PIMUZ 27718, sample HB 82, Congress Canyon (Humboldt Range). 8–10, PIMUZ 27723, sample HB 81, Congress Canyon (Humboldt Range).
All specimens from the mctaggarti Subzone (hyatti Zone, middle Anisian), Fossil Hill Member, north-west Nevada; ×1.

Figure EXPLANATION OF PLATE 2.

Figure EXPLANATION OF PLATE 2.

 Figs 1–10. Acrochordiceras hatschekii (Diener, 1907). 1–2, PIMUZ 27719, sample HB 82; mature specimen; ×1. 3–4, PIMUZ 27721, sample HB 82; submature specimen; ×0.85. 5–7, PIMUZ 27722, sample HB 81; ×1. 8–10, PIMUZ 27720, sample HB 81; ×1.
All specimens from the mctaggarti Subzone (hyatti Zone, middle Anisian), Fossil Hill Member, Congress Canyon (Humboldt Range, north-west Nevada).

Composition of the genus. Acrochordiceras is herein considered to consist of only four valid species (Text-fig. 3): Ammonites (Acrochordiceras) damesiiNoetling, 1880 (p. 334); Acrochordiceras carolinaeMojsisovics, 1882 (p. 141); Acrochordiceras hyattiMeek, 1877 (p. 124); and Haydenites hatschekiiDiener, 1907 (p. 72).

Figure TEXT‐FIG. 3..

 Schematic whorl sections, tubercle positions, and suture lines of the four species of Acrochordiceras recognized in this study.

In North America, several species have been referred to Acrochordiceras, but some of them can be excluded from this genus. Acrochordiceras foltzenseSmith, 1914 (p. 39, pl. 32, figs 13–14) from the late Anisian is presumably a pathologic Gymnotoceras as suggested by Spath (1934, p. 393) and Silberling and Nichols (1982, p. 23). This interpretation is concordant with pathologic specimens of various species described by Monnet and Bucher (2005a, e.g. Billingsites cordeyi, pl. 6, fig. 12), which are characterized by an unusual continuous ribbing on the venter, thus mimicking the typical plicate ribbing of Acrochordiceras. Acrochordiceras anodosumWelter, 1915 (p. 111, pl. 89, fig. 3) was placed within the genus ParacrochordicerasSpath (1934, p. 400) because of its much more evolute coiling, rounded whorl section, lack of tuberculation and a simpler suture line. This genus is also restricted to the early Anisian. Acrochordiceras inyoenseSmith, 1914 (p. 40, pl. 34, figs 11–13) was assigned to PseudacrochordicerasTozer, 1994 (p. 163) of late Spathian age (Neopopanoceras haugi Zone) and is characterized by a lower and broader whorl section, more numerous and finer ribs, and a simpler suture line. Acrochordiceras coyotenseBucher, 1992b (p. 152, pl. 6, figs 17–21) now belongs to the genus BradyiaBucher, 2002 (p. 282), which is characterized by constricted whorls and an absence of tubercles.

Occurrence.  The genus Acrochordiceras ranges throughout the entire middle Anisian. It is recorded from North America (Nevada), the Western Pacific (Primorye), the Tethys (Alps, Bosnia, Turkey, Iran, Himalayas, South China), and the peri-Tethys (Germanic Province).

Acrochordiceras hatschekii (Diener, 1907)
Plates 1, 2; Text-figures 3–7

Figure TEXT‐FIG. 4..

 Box (left) and mean (right) plots of H/D, W/D, U/D, and R/D for the revised species of Acrochordiceras from north-west Nevada (middle Anisian). Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 5..

 Normality test of studied species of Acrochordiceras from north-west Nevada (middle Anisian). The histograms show the number of specimens for successive ranges of H/D, W/D, U/D and log(R/D). The curve represents the estimated normal distribution of the data. The results of a statistical Lilliefors test are summarized by the label and the two values below the species name: N, normal distribution; A, not normal distribution; 1st value, Lilliefors test; 2nd value, threshold value to reject the null hypothesis that the data has a normal distribution. Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 6..

 Allometric curves of H/D, W/D, U/D, and R/D for the revised species of Acrochordiceras from north-west Nevada (middle Anisian). Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 7..

 Scatter diagrams of H, W, U (top left), and R (bottom left), and of H/D, W/D, U/D (top right), and R/D (bottom right) for Acrochordiceras hatschekii (Diener, 1907) from north-west Nevada (Intornites mctaggarti subzone, Acrochordiceras hyatti Zone, middle Anisian). Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

  • * . 1907 Ceratites (Haydenites) hatschekii Diener, p. 72, pl. 6, fig. 1.

  • . 1914 Haydenites hatschekii Diener; Smith, p. 114, pl. 33, figs 1–2, 3?

  • * . 1934 Haydenites hatscheki Diener; Spath, p. 465, text-fig. 155a, b (after Diener 1907, pl. 6, fig. 1).

  • ?1981 Acrochordiceras (Acrochordiceras) orientaleZharnikova, p. 32, text-figs 1b, 2; pl. 4, fig. 4.

  • non . 1981 Haydenites hatscheki Diener; Wang and He, p. 293, text-fig. 5; pl. 2, figs 8–11 (=A. carolinae).

  • p . 1982 Acrochordiceras hyattiMeek, 1877; Silberling and Nichols, p. 21, text-fig. 13b (non 13a?); pl. 4, figs 11–19 (non figs 20–28); pl. 5, figs 1–3 (non figs 4–7).

  • . 1992b Epacrochordiceras cf. E. alternans (Smith); Bucher, tabl. 1.

  • . 1992b Acrochordiceras aff. A. hyatti Meek; Bucher, p. 144, tabl. 1.

  • p . 2002 ?Haydenites hatschekiDiener 1907; Waterhouse, p. 18, text-fig. 11 (holotype); (non pl. 1, fig. 12 (=A. carolinae ?)).

Holotype.  The holotype, which consists of a mature body chamber from Spiti (Himachal Pradesh, India), was figured by Diener (1907, p. 72, pl. 6, fig. 1). Its suture line was subsequently figured by Waterhouse (2002, text-fig. 11).

Material and measurements.  Thirty-four measured specimens from Congress Canyon, Coyote Canyon (Humboldt Range; samples: HB 80, 81, 82) and Favret Canyon (Augusta Mountains; sample: HB 97). For measurements, see Text-figures 4, 7. Whorl height (H/D) has a mean of c. 42 per cent, whorl width (W/D) c. 36 per cent, umbilical diameter (U/D) c. 33 per cent, and ribbing density (R/D) c. 75 per cent.

Description.  Evolute shell with egressive coiling at maturity. Whorl section relatively compressed, high-whorled, high oval to subrectangular with a large rounded venter, rounded indistinct ventral shoulders, narrowly rounded umbilical shoulders, shallow umbilical wall and rounded flanks with maximum whorl width at mid-flank. At maturity, whorl section acquires a low-arched venter, flanks become flatter, and maximum whorl width moves to a marginal position. Ornamentation consists of relatively thin, numerous, straight to slightly sinuous, rectiradiate ribs that cross the venter and typically increase in strength from the umbilicus to the venter. Ribs also frequently alternate on each side of the venter. At maturity, ribs become sparser and enlarge into swellings on upper flanks, but do not cross the venter. Robust variants bear a lateral row of laterally elongated and sparse tubercles. Secondary ribs are intercalated or arise from lateral tubercles. At maturity, these lateral tubercles move to a marginal position. Ceratitic suture line with complete saddles (denticulations restricted to the lower part of their marginal walls) of nearly equal height (Text-fig. 3).

Remarks. Silberling and Nichols (1982, p. 23) suggested that H. hatschekii is simply a mature A. hyatti, which exhibits the unusual modification of its body chamber, and thus they are conspecific. Although we agree that the genus Haydenites is nothing more than the mature body chamber of an Acrochordiceras (see discussion in the previous genus description), we do not consider H. hatschekii and A. hyatti to be conspecific. Indeed, the two species are closely related, but they still have significant differences in their respective shell shapes and are kept separate. In fact, our bedrock-controlled samples show that H. hatschekii differs from A. hyatti by its more compressed whorl section, more evolute coiling, less indented suture line and its weakly tuberculate inner whorls. Furthermore, these two species do not coexist, but instead characterize two distinct biochronological units. A. hatschekii also differs from A. carolinae and A. damesii by its more compressed whorl section, more evolute coiling, less indented suture line and its row of tubercles in a lateral instead of umbilical position.

Occurrence.  North-west Nevada (Augusta Mountains, Humboldt Range); Intornites mctaggarti subzone, Acrochordiceras hyatti Zone, early middle Anisian. The species was initially described from Spiti (India).

Acrochordiceras hyattiMeek, 1877
Plates 3–5; Text-figures 3–6, 8, 9

Figure EXPLANATION OF PLATE 3.

Figure EXPLANATION OF PLATE 3.

 Figs 1–4. Acrochordiceras hyattiMeek, 1877. 1–2, PIMUZ 27724, sample HB 25, Straight Canyon (Humboldt Range); submature specimen. 3–4, PIMUZ 27725, sample HB 182, Coyote Canyon (Humboldt Range); submature specimen.
All specimens from the hadleyi Subzone (hyatti Zone, middle Anisian), Fossil Hill Member, north-west Nevada; ×1.

Figure EXPLANATION OF PLATE 4.

Figure EXPLANATION OF PLATE 4.

 Figs 1–18. Acrochordiceras hyattiMeek, 1877. 1–3, PIMUZ 27727, sample HB 156, southern Tobin Range. 4–6, PIMUZ 27728, sample HB 66, Coyote Canyon (Humboldt Range). 7–9, PIMUZ 27729, sample HB 156, southern Tobin Range. 10–12, PIMUZ 27730, sample HB 103, Favret Canyon (Augusta Mountains). 13–15, PIMUZ 27731, sample HB 103, Favret Canyon (Augusta Mountains). 16–18, PIMUZ 27732, sample HB 152, southern Tobin Range.
All specimens from the hadleyi Subzone (hyatti Zone, middle Anisian), Fossil Hill Member, north-west Nevada; ×1.

Figure EXPLANATION OF PLATE 5.

Figure EXPLANATION OF PLATE 5.

 Figs 1–15. Acrochordiceras hyattiMeek, 1877. 1–2, PIMUZ 27736, sample HB 150, southern Tobin Range; submature specimen. 3–4, PIMUZ 27735, sample HB 66, Coyote Canyon (Humboldt Range); mature specimen (flattened mature body chamber has been removed). 5–6, PIMUZ 27738, sample HB 150, southern Tobin Range. 7–9, PIMUZ 27737, sample HB 103, Favret Canyon (Augusta Mountains). 10–12, PIMUZ 27734, sample HB 103, Favret Canyon (Augusta Mountains). 13–15, PIMUZ 27733, sample HB 103, Favret Canyon (Augusta Mountains).
All specimens from the hadleyi Subzone (hyatti Zone, middle Anisian), Fossil Hill Member, north-west Nevada; ×1.

Figure TEXT‐FIG. 8..

 Scatter diagrams of H, W, U (top left), and R (bottom left), and of H/D, W/D, U/D (top right), and R/D (bottom right) for Acrochordiceras hyattiMeek, 1877 from north-west Nevada (Intornites mctaggarti subzone, Acrochordiceras hyatti Zone, middle Anisian). Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 9..

 A–B, Acrochordiceras hyattiMeek, 1877, PIMUZ 27726, sample HB 25, Straight Canyon (Humboldt Range, north-west Nevada); Fossil Hill Member; hadleyi Subzone (hyatti Zone, middle Anisian); mature specimen; ×0.80.

  • *p . 1877 Acrochordiceras hyatti Meek, p. 124, pl. 11, fig. 5a (lectotype) (non fig. 5 (=A. carolinae)).

  • . 1895a Acrochordiceras BalaramaDiener, p. 35, pl. 7, fig. 3.

  • . 1895a Acrochordiceras JoharenseDiener, p. 36, pl. 7, fig. 4.

  • non . 1905 Acrochodiceras hyatti Meek; Hyatt and Smith, p. 178, pl. 23, figs 8–11 (=A. carolinae).

  • . 1905 Acrochordiceras Balarma Diener; Noetling, pl. 14, fig. 3 (after Diener 1895, pl. 7, fig. 3).

  • ?1907 Acrochordiceras Balarama Diener; Diener, p. 102.

  • ?1913 Acrochordiceras cf. Balarama Diener; Diener, p. 62.

  • ?1913 Sibirites cf. prahlada Diener; Diener, p. 59, pl. 8, fig. 6.

  • p . 1914 Acrochordiceras hyatti Meek; Smith, p. 39, pl. 15, fig. 5a (after Meek 1877, pl. 11, fig. 5a) (non fig. 5 (=A. carolinae); after Meek 1877, pl. 11, fig. 5); (non pl. 4, figs 8–11 (=A. carolinae); after Hyatt and Smith 1905, pl. 23, figs 8–11).

  • p . 1914 Acrochordiceras alternans Smith, p. 38, pl. 32, figs 15–17; (non pl. 33, figs 4–5 (=pathologic Gymnotoceras ?)).

  • ?1914 Acrochordiceras Balarama Dien.; Arthaber, p. 180, pl. 13, fig. 8.

  • ?1934 Acrochordiceras (Acrochordiceras) halili Toula; Spath, p. 398.

  • non . 1934 Acrochodiceras hyatti Meek; Spath, p. 394, text-fig. 137a–c (after Hyatt and Smith 1905, pl. 23, figs 8–10 (=A. carolinae)); text-fig. 139c (after Hyatt and Smith 1905, pl. 23, fig. 11 (=A. carolinae)).

  • v . 1948 Acrochordiceras balarama Diener; Renz and Renz, p. 5.

  • ?1961 A. (Acrochordiceras) aff. balarama; Kiparisova, p. 149, pl. 29, fig. 12.

  • ?1961 A. (Epacrochordiceras?) sp. indet. Diener, 1895; Kiparisova, p. 148, pl. 29, fig. 11.

  • ?1968 Acrochordiceras (Epacrochordiceras) aff. pustericumMojsisovics, 1882; Zakharov, p. 139, pl. 28, fig. 1.

  • ?1968 Durgaites aff. dieneri (Mojsisovics, 1903); Zakharov, p. 133, pl. 26, fig. 2; pl. 27, fig. 1.

  • . 1968 Acrochordiceras (Acrochordiceras) subrotundum Shevyrev, p. 126, pl. 7, fig. 4; text-fig. 31a.

  • p ?1968 Acrochordiceras (Paracrochordiceras) alternans Smith; Shevyrev, p. 127, pl. 8, fig. 3; text-fig. 31b.

  • ?1968 Acrochordiceras (Paracrochordiceras) simplex Shevyrev, p. 128, pl. 7, fig. 5; text-figs 31c, 32.

  • non . 1972 Acrochordiceras sp. aff. A. hyatti Meek; Tozer, p. 32, pl. 10, figs 11–13.

  • . 1981 Acrochordiceras (Acrochordiceras) kiparisovaeZharnikova, p. 30, text-fig. 1a; pl. 4, figs 1–3.

  • ?1981 Acrochordiceras (Epacrochordiceras) korobkoviZharnikova, p. 35, text-figs 5–6; pl. 4, fig. 6.

  • p . 1982 Acrochordiceras hyattiMeek, 1877; Silberling and Nichols, p. 21, text-fig. 13a? (non 13b (=A. hatschekii)); pl. 4, figs 27–28 (lectotype; after Meek 1877, pl. 11, fig. 5a), figs 20–26 (non figs 11–19 (=A. hatschekii)); pl. 5, figs 4–7 (non figs 1–3 (=A. hatschekii)).

  • . 1988 Acrochordiceras asseretoiFantini Sestini, p. 52, text-figs 11d, 12d; pl. 9, fig. 5.

  • ?1988 Acrochordiceras sp. in Arthaber, 1914; Fantini Sestini, p. 55.

  • . 1992b Epacrochordiceras alternans (Smith); Bucher, tabl. 1.

  • . 1992b Acrochordiceras hyatti Meek; Bucher, p. 146, tabl. 1.

  • . 1995 Acrochordiceras subrotundumShevyrev, 1968; Shevyrev, p. 78, text-fig. 45, pl. 9, figs 6, 5?

  • ?1995 Epacrochordiceras inflatum Shevyrev, p. 80, text-fig. 46a, pl. 10, fig. 6.

  • . 1995 Epacrochordiceras compressum Shevyrev, p. 81, text-fig. 46b, pl. 13, figs 1–2.

  • v . 2006 Acrochordiceras cf. A. hyatti; Ovtcharova et al., p. 466.

  • . 2007 Acrochordiceras hyatti; Lucas et al., fig. 2.3a.

  • . 2007 Acrochordiceras hyatti; Jenks et al., pl. 14, fig. J.

Types. Acrochordiceras hyatti was originally defined by Meek (1877, p. 124) and was based on material from north-west Nevada (New Pass Range or Humboldt Range). Of the two syntypes illustrated by Meek (1877, pl. 11, figs 5, 5a), only one actually corresponds to A. hyatti. As already discussed by Silberling and Nichols (1982) the two syntypes have morphological characteristics clearly belonging to different species of different stratigraphic age in north-west Nevada. Silberling and Nichols (1982) designated one syntype (Meek, 1877, pl. 11, fig. 5a) as the lectotype because it corresponds more closely to the description given by Meek (1877, p. 124), e.g. he describes the lateral tubercles as being ‘near or within the middle of each side’. The other syntype (Meek, 1877, pl. 11, fig. 5; refigured by Hyatt and Smith 1905 and Smith 1914) is characterized by an umbilical row of tubercles and thus belongs to A. carolinae (see below).

Material and measurements.  Two hundred and twenty-nine measured specimens from Favret Canyon (Augusta Mountains; sample: HB 103), Coyote Canyon (Humboldt Range; samples: HB 25, 66, 83, 145, 153, 182), Wildhorse Mine and McCoy Mine (New Pass Range; samples: HB 87, 116, 124, 157, 205, 547, 581), and the southern Tobin Range (samples: HB 150, 152). For measurements, see Text-figures 4, 8. Whorl height (H/D) has a mean of c. 43 per cent, whorl width (W/D) c. 45 per cent, umbilical diameter (U/D) c. 32 per cent and ribbing density (R/D) c. 75 per cent.

Description.  Moderately evolute shell with a wide umbilicus. Moderately compressed, high oval to rounded whorl section with a large, rounded venter becoming low-arched at maturity, rounded indistinct ventral shoulders, narrowly rounded umbilical shoulders with moderate umbilical depth, and convex flanks with maximum whorl width at mid-flank. Ornamentation consists of rounded, thin to thick, rectiradiate, slightly sinuous to straight ribs that cross the venter. Ribs usually thicken on the venter and may briefly alternate on the venter (e.g. Pl. 4, fig. 8). Robustly ornamented variants bear a lateral row of distant, radially elongated nodes from which arise two to three ribs; number of intercalated ribs between nodes varies from two to five, depending on strength of ribbing. Presence of these nodes depends on the strength of overall ornamentation, and their persistence throughout ontogeny is highly variable. On inner whorls (D < 25 mm), tubercles in the lateral position are actually of a parabolic nature (e.g. Pl. 4, fig. 10). Mature body chamber characterized by striking change in ornamentation consisting of fading ventral ribs, flatter flanks and a more quadrate whorl section, as well as prominent ribs on the ventro-lateral margin, which are projected forward, forming swellings proportional in size to strength of the ornamentation of earlier whorls. Ceratitic, but complex suture line (Text-fig. 3); saddles rounded and complete, but the lobes are crenulated high upon the sides of the saddles; deep first lateral lobe.

Remarks.  Population samples of this species show a wide range of variation, especially with regard to its geometry. Nevertheless, Acrochordiceras hyatti clearly differs from A. carolinae and A. damesii by its more evolute coiling, a more rounded whorl section with maximum whorl width at mid-flank rather than near the umbilical margin, lateral rather than umbilical tubercles, and a less subdivided suture line pattern. The differences between A. hyatti and A. hatschekii are more tenuous, but A. hyatti differs from A. hatschekii by its greater involution, more depressed whorl section and more indented suture line. In addition, the two species characterize different stratigraphic levels in north-west Nevada.

Among species of Acrochordiceras described by Smith (1914), Acrochordiceras alternans is a very instructive case. Smith (1914, p. 39) rightly pointed out that this ‘species’ is characterized by undivided fine ribs that alternate on the venter and by a lack of umbilical or lateral nodes. He also pointed out that ‘Hauer (1892, p. 272) had described a species from Bosnia, Acrochordiceras enode, which resembles A. alternans, but differs by having ribs that cross the venter without alternation and also by its deeply digitate instead of ceratitic septa’ (Smith 1914, p. 39). These ‘two species’ were subsequently placed in a new genus, EpacrochordicerasSpath, 1934 (p. 401). This type of morphology, characterized by finer ribs and a weakening or disappearance of nodes, corresponds to end-member variants of typically variable species of ammonoids. A. alternans is nothing more than the ‘slender’ variant of A. hyatti, and A. enode is the ‘slender’ variant of A. carolinae. The differences observed between A. alternans and A. enode are almost the same as those observed between A. hyatti and A. carolinae. Furthermore, the stratigraphic occurrence of these species is in agreement with this interpretation: morphotypes of A. alternans co-occur with A. hyatti and morphotypes of A. enode occur with A. carolinae.

Also, the specimen figured by Smith (1914, pl. 33, figs 4–5) as A. alternans is in our opinion a pathologic beyrichitine. This specimen shows a dense sinuous ribbing delimited by megastriae as in Gymnotoceras. The presence of ribs crossing the venter is a frequent pathologic pattern observed among Anisian ammonoids (e.g. Billingsites cordeyiMonnet and Bucher 2005a, pl. 6, fig. 12; Rieppelites shevyreviMonnet and Bucher 2005a, pl. 21, fig. 11). Hence, this latter character is not sufficient to diagnose Acrochordiceras.

Occurrence.  North-west Nevada (Augusta Mountains, New Pass Range, northern Humboldt Range, and southern Tobin Range); Ginsburgites americanus and Unionvillites hadleyi subzones, Acrochordiceras hyatti Zone, early middle Anisian. The species has also been recorded from Guangxi (South China), the Himalayas (Himachal Pradesh), the Caucasus, Primorye (Russia) and the Gulf of Ismid (Turkey) with a Bithynian age (early middle Anisian).

Acrochordiceras carolinaeMojsisovics, 1882
Plates 6–11; Text-figures 3–6, 10–12

Figure EXPLANATION OF PLATE 6.

Figure EXPLANATION OF PLATE 6.

 Figs 1–15. Acrochordiceras carolinaeMojsisovics, 1882. 1–2, PIMUZ 27739, sample HB 179, Favret Canyon. 3–4, PIMUZ 27740, sample HB 179, Favret Canyon. 5–6, PIMUZ 27741, sample HB 179, Favret Canyon. 7–9, PIMUZ 27742, sample HB 179, Favret Canyon. 10–12, PIMUZ 27743, sample HB 100, Muller Canyon. 13–15, PIMUZ 27744, sample HB 179, Favret Canyon.
All specimens from the nicholsi Subzone (taylori Zone, middle Anisian), Fossil Hill Member, Augusta Mountains (north-west Nevada); ×1.

Figure EXPLANATION OF PLATE 7.

Figure EXPLANATION OF PLATE 7.

 Figs 1–14. Acrochordiceras carolinaeMojsisovics, 1882. 1–3, PIMUZ 27745, sample HB 175. 4–6, PIMUZ 27746, sample HB 175. 7–8, PIMUZ 27747, sample HB 176. 9–11, PIMUZ 27748, sample HB 175. 12–14, PIMUZ 27749, sample HB 173.
All specimens from the escheri Subzone (taylori Zone, middle Anisian), Fossil Hill Member, Favret Canyon (Augusta Mountains, north-west Nevada); ×1.

Figure EXPLANATION OF PLATE 8.

Figure EXPLANATION OF PLATE 8.

 Figs 1–11. Acrochordiceras carolinaeMojsisovics, 1882. 1–3, PIMUZ 27750, sample HB 175. 4–6, PIMUZ 27751, sample HB 175. 7–9, PIMUZ 27752, sample HB 173. 10–11, PIMUZ 27753, sample HB 175.
All specimens from the escheri Subzone (taylori Zone, middle Anisian), Fossil Hill Member, Favret Canyon (Augusta Mountains, north-west Nevada); ×1.

Figure EXPLANATION OF PLATE 9.

Figure EXPLANATION OF PLATE 9.

 Figs 1–13. Acrochordiceras carolinaeMojsisovics, 1882. 1–3, PIMUZ 27754, sample HB 173. 4–6, PIMUZ 27755, sample HB 173. 7–8, PIMUZ 27756, sample HB 173. 9–10, PIMUZ 27757, sample HB 175. 11–13, PIMUZ 27758, sample HB 173.
All specimens from the escheri Subzone (taylori Zone, middle Anisian), Fossil Hill Member, Favret Canyon (Augusta Mountains, north-west Nevada); ×1.

Figure EXPLANATION OF PLATE 10.

Figure EXPLANATION OF PLATE 10.

 Figs 1–16. Acrochordiceras carolinaeMojsisovics, 1882. 1–3, PIMUZ 27759, sample HB 232, praebalatonensis Subzone (taylori Zone), Muller Canyon. 4–6, PIMUZ 27760, sample HB 234, praebalatonensis Subzone (taylori Zone), Muller Canyon. 7–9, PIMUZ 27761, sample HB 234, praebalatonensis Subzone (taylori Zone), Muller Canyon. 10–11, PIMUZ 25128, sample HB 739, mojsvari Subzone (shoshonensis Zone), Ferguson Canyon. 12–14, PIMUZ 25129, sample HB 739, mojsvari Subzone (shoshonensis Zone), Ferguson Canyon. 15–16, PIMUZ 25127, sample HB 739, mojsvari Subzone (shoshonensis Zone), Ferguson Canyon.
All specimens from the middle Anisian, Fossil Hill Member, Augusta Mountains (north-west Nevada); ×1.

Figure EXPLANATION OF PLATE 11.

Figure EXPLANATION OF PLATE 11.

 Figs 1–12. Acrochordiceras carolinaeMojsisovics, 1882. 1–2, PIMUZ 27762, sample HB 190, rieberi Subzone, Favret Canyon; submature specimen; ×0.85. 3–4, PIMUZ 27763, sample HB 190, rieberi Subzone, Favret Canyon; submature specimen; ×0.85. 5–7, PIMUZ 25130, sample HB 739, mojsvari Subzone, Ferguson Canyon; ×1. 8–9, PIMUZ 25126, sample HB 741, mojsvari Subzone, Oliver Gulch; ×1. 10–12, PIMUZ 27764, sample HB 170, wallacei Subzone, Favret Canyon; ×1.
All specimens from the shoshonensis Zone (middle Anisian), Fossil Hill Member, Augusta Mountains (north-west Nevada).

Figure TEXT‐FIG. 10..

Figure TEXT-FIG. 10..

 Scatter diagrams of H, W, U (top left), and R (bottom left), and of H/D, W/D, U/D (top right), and R/D (bottom right) for Acrochordiceras carolinaeMojsisovics, 1882 from north-west Nevada (Nevadisculites taylori and Balatonites shoshonensis zones, middle Anisian). Abbreviations: D, diameter; H, whorl height; R, number of ribs per half whorl; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 11..

Figure TEXT-FIG. 11..

 Distribution of shell compression (H/W) and degree of coiling (U/D) for Acrochordiceras carolinaeMojsisovics, 1882 from north-west Nevada (Nevadisculites taylori and Balatonites shoshonensis zones, middle Anisian). This diagram shows the normal distribution of these parameters and the position of each species herein synonymized with A. carolinae, thus illustrating the continuous range of shell compression and degree of coiling for all of the old typological species. Abbreviations: D, shell diameter; H, whorl height; U, umbilical diameter; W, whorl width.

Figure TEXT‐FIG. 12..

Figure TEXT-FIG. 12..

 A–C, Acrochordiceras carolinaeMojsisovics, 1882, PIMUZ 27765, sample HB 220, wallacei Subzone (shoshonensis Zone, middle Anisian), Fossil Hill Member, Favret Canyon (Augusta Mountains, north-west Nevada); mature specimen; ×0.40.

  • p . 1877 Acrochordiceras hyatti Meek, p. 124, pl. 11, fig. 5 (non fig. 5a).

  • * . 1882 Acrochordiceras Carolinae Mojsisovics, p. 141, pl. 28, fig. 14 (lectotype); pl. 36, fig. 3.

  • . 1882 Acrochordiceras Fischeri Mojsisovics, p. 142, pl. 33, fig. 8.

  • . 1882 Acrochordiceras pustericum Mojsisovics, p. 143, pl. 6, fig. 4.

  • v . 1887 Acrochordiceras Damesi Noetling; Hauer, p. 22, pl. 5, fig. 2.

  • v . 1892 Acrochordiceras enode Hauer, p. 24, pl. 7, fig. 1.

  • . 1895b Acrochordiceras sp. ind. Diener, p. 22, pl. 4, fig. 2.

  • v . 1896a Acrochordiceras undatus Arthaber, p. 79, pl. 7, figs 7–8.

  • 1896a Acrochordiceras pustericum Mojs.; Arthaber, p. 80.

  • 1896a Acrochordiceras enode Hauer; Arthaber, p. 81.

  • . 1896a Acrochordiceras nov. spec. indet. Arthaber, p. 81, pl. 7, fig. 10.

  • . 1896a Acrochordiceras erucosum Arthaber, p. 82, pl. 7, fig. 9.

  • 1896b Acrochordiceras pustericum Mojs.; Arthaber, p. 226.

  • . 1896b Acrochordiceras undatum Arth.; Arthaber, p. 226, 235, pl. 27, fig. 2.

  • ?1896 Acrochordiceras halili Toula, p. 168, pl. 19, fig. 10.

  • . 1905 Acrochodiceras hyatti Meek; Hyatt and Smith, p. 178, pl. 23, figs 8–11.

  • . 1905 Acrochordiceras Carolinae Mojs.; Airaghi, p. 249, pl. 8, fig. 6.

  • . 1905 Acrochordiceras undatum Arth.; Airaghi, p. 251, pl. 8, fig. 4.

  • . 1905 Acrochordiceras enode Hauer; Airaghi, p. 252, pl. 8, fig. 2.

  • . 1905 Acrochordiceras Halali Toula; Noetling, pl. 10, fig. 1 (after Toula 1896, pl. 19, fig. 10).

  • 1906 Acrochordiceras portisi Martelli, p. 132, pl. 6, fig. 2.

  • . 1907 Acrochordiceras cf. Carolinae Mojsisovics; Diener, p. 99, pl. 12, fig. 4.

  • ?1907 Acrochordiceras cf. ind. aff. pusterico Mojs.; Diener, p. 102.

  • . 1911 Acrochordiceras Ippeni Arthaber, p. 271, pl. 24, fig. 11.

  • . 1911 Acrochordiceras Haueri Arthaber, p. 272 (nov. nom. for A. DamesiHauer, 1887 non Noetling, 1887, p. 22, pl. 5, fig. 2).

  • . 1913 Acrochordiceras cf. enode Hauer; Diener, p. 61, pl. 7, fig. 7.

  • . 1913 Acrochordiceras cf. Haueri Arth.; Diener, p. 63, pl. 7, fig. 8.

  • . 1914 Acrochordiceras bythinicum Arthaber, p. 179, pl. 14, fig. 2.

  • ?1914 (?)Acrochordiceras sp. Arthaber, p. 180, pl. 13, fig. 9; pl. 14, fig. 1.

  • ?1914 Acrochordiceras Halili Toula; Arthaber, p. 181, pl. 14, figs 3–4.

  • . 1914 Acrochordiceras Haueri Arth.; Arthaber, p. 182, text-fig. 12; pl. 14, figs 5–6.

  • . 1914 Acrochordiceras pustericum Mojs.; Arthaber, p. 183, text-fig. 13; pl. 14, fig. 7.

  • ?1914 Acrochordiceras Endrissi Arth.; Arthaber, p. 184, pl. 15, fig. 1.

  • p . 1914 Acrochordiceras hyatti Meek; Smith, p. 39, pl. 4, figs 8–11 (after Hyatt and Smith 1905, pl. 23, figs 8–11); pl. 15, fig. 5 (after Meek 1877, pl. 11, figs 5) (non fig. 5a; after Meek 1877, pl. 11, fig. 5a).

  • 1927 Acrochordiceras pustericum Mojs.; Gugenberger, p. 144.

  • 1927 Acrochordiceras undatum Arthaber; Gugenberger, p. 144.

  • . 1934 Acrochodiceras hyatti Meek; Spath, p. 394, text-fig. 137a–c (after Hyatt and Smith 1905, pl. 23, figs 8–10), text-fig. 139c (after Hyatt and Smith 1905, pl. 23, fig. 11).

  • . 1934 Acrochodiceras (Acrochordiceras) carolinae Mojsisovics; Spath, p. 395.

  • . 1934 Acrochodiceras (Acrochordiceras) haueri Arthaber; Spath, p. 396.

  • 1934 Acrochodiceras (Acrochordiceras) fischeri Mojsisovics; Spath, p. 397.

  • . 1934 Acrochodiceras (Acrochordiceras) undatum Arthaber; Spath, p. 399.

  • . 1934 Acrochordiceras (Epacrochordiceras) enode Hauer; Spath, p. 403, text-fig. 137d (after Hauer 1892, pl. 7, fig. 1c).

  • . 1934 Acrochordiceras (Epacrochordiceras) pustericum Mojsisovics; Spath, p. 402, text-fig. 139d.

  • . 1934 Acrochordiceras (Epacrochordiceras) portisi Martelli; Spath, p. 404, pl. 18, fig. 2.

  • 1958 Acrochordiceras carolinae Mojsisovics; Sacchi Vialli and Vai, p. 69, pl. 4, fig. 17.

  • ?1960 Acrochordiceras sp. indet. Kummel, p. 6, pl.1, fig. 1.

  • . 1972 Acrochodiceras sp. aff. A. hyatti Meek; Tozer, p. 32, pl. 10, figs 11–13.

  • . 1972 Epacrochodiceras sp. indet. Tozer, p. 31, text-fig. 3D, pl. 7, fig. 1.

  • non . 1973 Acrochordiceras cf. carolinae Mojsisovics; Pelosio, p. 153, pl. 17, fig. 3a–b (=indet. paraceratitinae).

  • . 1976 Acrochordiceras (Epacrochordiceras) cf. pustericum Mojsisovics; Okuneva, p. 50, pl. 1, fig. 7.

  • . 1980 Acrochordiceras carolinae Mojsisovics; Gu et al., p. 350, pl. 2, figs 12–13.

  • . 1980 Acrochordiceras undatum Arthaber; Gu et al., p. 350, text-fig. 6c; pl. 2, figs 4–5, 10–11.

  • . 1980 Acrochordiceras fischeri Mojsisovics; Gu et al., p. 351, text-fig. 6b; pl. 2, figs 8–9.

  • . 1980 Acrochordiceras balarama Diener; Gu et al., p. 351, text-fig. 6a; pl. 2, figs 14–16.

  • . 1981 Acrochordiceras cf. A. carolinae Mojsisovics; Wang and He, p. 293, pl. 4, figs 5–6, 14–16?.

  • . 1981 Haydenites hatscheki Diener; Wang and He, p. 293, text-fig. 5; pl. 2, figs 8–11.

  • . 1982 Acrochordiceras cf. A. carolinae Mojsisovics; Silberling and Nichols, p. 22, text-fig. 14; pl. 5, figs 8–9.

  • . 1987 Acrochordiceras cf. carolinaeMojsisovics, 1882; Vörös, p. 56, pl. 2, fig. 3.

  • non . 1988 Acrochordiceras cf. halili Toula, 1986; Prlj and Mudrenović, p. 16, pl. 5, fig. 5 (=indet. paraceratitinae).

  • non . 1988 Acrochordiceras (Epiacrochordiceras) enode (Hauer), 1892; Prlj and Mudrenović, p. 16, pl. 5, fig. 7 (=indet. paraceratitinae).

  • . 1988 Acrochordiceras bythinicumArthaber, 1914; Fantini Sestini, p. 54, text-fig. 11a; pl. 9, fig. 4.

  • ?1988 Acrochordiceras haliliToula, 1896; Fantini Sestini, p. 54, text-fig. 11b; pl. 10, fig. 2.

  • . 1988 Acrochordiceras haueriArthaber, 1911; Fantini Sestini, p. 55, text-fig. 11e; pl. 10, figs 1, 3?.

  • . 1988 Epacrochordiceras pustericum (Mojsisovics, 1882); Fantini Sestini, p. 55, text-fig. 11c; pl. 11, fig. 3.

  • . 1988 Acrochordiceras carolinae (Mojsisovics); Bucher, p. 726, fig. 2.

  • . 1988 Epacrochordiceras cf. E. enode (Hauer); Bucher, p. 726, fig. 2.

  • . 1991 Acrochordiceras cf. carolinaeMojsisovics, 1882; Tatzreiter and Vörös, p. 252, pl. 2, fig. 2.

  • 1991 Acrochordiceras fischeriMojsisovics, 1882; Tatzreiter and Vörös, p. 252.

  • . 1991 Acrochordiceras undatum Arthaber, 1896; Tatzreiter and Vörös, p. 256, pl. 2, figs 5a–b (after Arthaber 1896, pl. 15, fig. 2).

  • . 1991 Acrochordiceras (Acrochordiceras) cf. fischeriMojsisovics, 1882; Vu Khuc, p. 125, pl. 55, fig. 6.

  • . 1991 Acrochordiceras (Paracrochordiceras) sp.; Vu Khuc, p. 126, pl. 55, figs 7–9.

  • . 1992a Acrochordiceras carolinae (Mojsisovics); Bucher, text-fig. 2.

  • . 1992a Epacrochordiceras cf. E. enode (Hauer); Bucher, text-fig. 2.

  • . 1994 Acrochordiceras carolinae; Bucher, text-fig. 1.

  • . 1994 Acrochordiceras erucosum; Bucher, p. 2, text-fig. 1.

  • p ?2002 ?Haydenites hatschekiDiener 1907; Waterhouse, p. 18, pl. 1, fig. 12; (non text-fig. 11).

  • . 2003 Acrochordiceras carolinaeMojsisovics, 1882; Vörös, p. 83, text-fig. A13; pl. A1, figs 11, 12, 13, 14a–b.

  • v . 2005a Acrochordiceras carolinaeMojsisovics, 1882; Monnet and Bucher, p. 16, pl. 2, figs 5–9.

  • . 2007 Acrochordiceras carolinae; Jenks et al., pl. 17, figs D–E; pl. 35, figs E–F (after Monnet and Bucher, 2005a, pl. 2, fig. 6).

  • . 2007 Acrochordiceras sp.; Jenks et al., pl. 18, figs G–H.

  • . 2008 Acrochordiceras cf. carolinaeMojsisovics 1882; Stiller and Bucher, p. 552, fig. 3a–b.

Types. Acrochordiceras carolinae was originally defined by Mojsisovics (1882, p. 141) based on material from the Anisian Schreyeralm Limestone (Northern Calcareous Alps, Austria). The lectotype (pl. 28, fig. 14) is a small specimen exhibiting the main diagnostic features of the species: relatively involute shell with a subtriangular whorl section (maximum whorl width near the umbilicus), an umbilical row of tubercles and plicate ribbing.

Material and measurements.  Four hundred and forty-nine measured specimens from Favret Canyon, Muller Canyon, Oliver Gulch and Ferguson Canyon (Augusta Mountains; samples: HB 100, 162, 163, 165, 171, 173, 174, 175, 176, 179, 185, 187, 189, 190, 192, 202, 210, 220, 228, 229, 230, 232, 234, 582, 599, 739, 741) and Wildhorse Mine (New Pass Range; sample: HB 541). For measurements, see Text-figures 4, 10, 11. Whorl height (H/D) has a mean of c. 48 per cent, whorl width (W/D) c. 45 per cent, umbilical diameter (U/D) c. 24 per cent and ribbing density (R/D) c. 47 per cent.

Description.  Moderately involute shell with narrow umbilicus and slightly egressive coiling at maturity. Moderately compressed to depressed, high oval or subtriangular to rounded whorl section with rounded venter becoming low-arched at maturity and rounded indistinct ventral shoulders. Umbilicus with narrowly rounded umbilical shoulders and moderate to deep umbilical depth. Flanks convex with maximum whorl width low on flanks. Ornamentation consists of rounded, thin to thick, rectiradiate, slightly sinuous to straight ribs. Ribs thicken while continuously crossing the venter without alternating. Robust variants bear an umbilical row of radially elongated nodes from which arise two to three ribs. On average, two intercalated ribs occur between nodes. These umbilical tubercles may completely fade on slender variants. On inner whorls (D < 25 mm), ornamentation is also characterized by a lateral row of parabolic tubercles (e.g. Pl. 7, fig. 9; Pl. 11, fig. 10), which rapidly disappear as the umbilical row of nodes appears (transition at a diameter of about 30 mm). On robust variants, these parabolic tubercles may briefly co-occur with the umbilical nodes, thus leading to a brief bituberculate pattern (morphotype A. erucosumArthaber, 1896a; see Pl. 11, fig. 10 and Arthaber 1896a, p. 88, pl. 7, fig. 9). Hence, the umbilical row of nodes is distinct and not a continuation of the lateral row of parabolic tubercles of the inner whorls. At maturity, ribs tend to straighten and become more distant, while they do not cross the venter, instead they project forward at the ventral shoulder forming node-like swellings. In addition, umbilical nodes also tend to fade and move to a more lateral position at maturity. Suture line is subammonitic with smoothly crenulated saddles and some second order subdivision of the first lateral lobe (Text-fig. 3).

Remarks.  The wide range of morphological variation exhibited by Acrochordiceras carolinae well illustrates the Buckman’s First Law of Covariation (Westermann 1966), namely, a covariation of shell compression, coiling and robustness of ornamentation (i.e. the more depressed the shell, the coarser the ribs and the more evolute the coiling). Nevertheless, Acrochordiceras carolinae clearly differs from A. hyatti and A. hatschekii by its more involute coiling, a more rounded whorl section, umbilical rather than lateral tubercles, and a more subdivided suture line pattern. The differences between A. carolinae and A. damesii are more tenuous, but A. damesii has a more compressed and subquadrate whorl section, as well as two to three ribs per umbilical node (whereas this ratio is five to six for A. carolinae).

Because of the extremely large variation of Acrochordiceras carolinae, numerous species described from Europe are included within its broad morphological spectrum. Most Acrochordiceras species were characterized typologically by slight changes such as a more compressed shell, a coarser ribbing, or an absence of tuberculation. A careful examination of these ‘old typological’ species reveals the presence of intermediate forms among them, thus suggesting the existence of only one highly variable species. This pattern is well illustrated in Text-figure 11, which compares the distribution of whorl shell compression (H/W) and the degree of coiling (U/D) for various sizes of Acrochordiceras carolinae from north-west Nevada. Also included in the comparison are the holotypes of other Acrochordiceras species now included in carolinae. The figure clearly shows that A. carolinae has a wide range of intra-specific variability with a normal distribution and that each ‘old typological’ species falls within this variation.

As already pointed out by Spath (1934, p. 395), Acrochordiceras haueriArthaber, 1911 is an inflated (H/W c. 1), coarsely ornamented variant of A. carolinae and transitional forms exist between them. Acrochordiceras fischeriMojsisovics, 1882 represents an extreme robust variant of A. carolinae with its very depressed whorl section (H/W > 1) and coarser, more distant ribbing. As noted by Vörös (2003, p. 84), A. carolinae and A. fischeri show the same basic features of shape and ornamentation. The small differences in the degree of compression and coarseness of ornamentation are typical of ammonoid intra-specific variation, and many transitional forms can be identified between them in our collections from Nevada; Vörös (2003) also identified transitional specimens between these two ‘species’ from the Balaton Highland (Hungary). Hence, A. haueri includes variants of A. carolinae intermediate between A. carolinae and A. fischeri (Spath 1934, p. 396) with coarse ornamentation and an inflated whorl section; A. carolinae is more compressed and has more closely spaced ribs than A. haueri. On the other hand, the species A. undatumArthaber, 1896a corresponds to compressed, weakly ornamented variant of A. carolinae with faint tubercles, thinner ribbing and a more compressed whorl section.

The species Epacrochordiceras enode is herein considered as an extremely ‘slender’ variant of A. carolinae with a very compressed whorl section and lacking tuberculation. Indeed, as noted by Spath (1934, p. 399), A. undatum is intermediate between Acrochordiceras carolinae and Epacrochordiceras enode in that it is more compressed and has smaller tubercles than A. carolinae. Furthermore, Spath (1934, p. 399, 403) noted that A. undatum is transitional to Epacrochordiceras and that fragmentary body chambers cannot be identified at the species level, thus providing additional evidence that Epacrochordiceras is an end-member variant of Acrochordiceras and that many transitional forms exist between them. For instance, Spath (1934, p. 399) mentioned that certain specimens were typical of E. enode‘until the umbilicus was cleared of matrix and revealed faint tuberculation’ which diagnoses Acrochordiceras and not Epacrochordiceras. In addition, Vörös (2003, p. 84) examined the holotype of A. undatum and agrees that this species falls within the intra-specific variation of A. carolinae. Finally, although Vörös (2003) considered A. pustericum to be a distinct species, our material shows that the more slender variant of A. carolinae may completely lose tuberculation (e.g. Pl. 8, fig. 7; Pl. 9, fig. 11; Pl. 10, fig. 7; Pl. 11, fig. 3), thus leading to finely ribbed shells typical of A. pustericumMojsisovics, 1882. Note that Diener (1913, p. 62) already suggested that A. enode and A. pustericum are conspecific.

Bucher (1994, p. 2) separated A. erucosumArthaber, 1896a (p. 88, pl. 7, fig. 9) from A. carolinae on the basis that some depressed forms of Acrochordiceras have a bituberculate immature stage and that this stage is visible only on Acrochordiceras ranging from the escheri subzone (taylori Zone) up through the entire shoshonensis Zone. However, A. erucosum is herein considered a junior synonym of A. carolinae because this brief bituberculate stage is common on those depressed end-member variants of tuberculated ammonoid species that have parabolic nodes on their inner whorls (see e.g. Monnet and Bucher 2005a, Rieppelites shevyrevi, pl. 21, fig. 13). In our opinion, this bituberculate stage merely reflects a variation in the timing of the disappearance of the parabolic nodes on the inner whorls. The fact that the bituberculate stage is not observed in the oldest representatives of Acrochordiceras carolinae may simply indicate an increase in the intra-specific variation of the younger forms of this species.

Occurrence. Acrochordiceras carolinae was initially described from the Anisian Schreyeralm Limestone of the Northern Calcareous Alps (Austria). The species and its synonyms have since been recorded from strata of Pelsonian age (Balatonites balatonicus Zone, late middle Anisian) in the Southern Alps (Italy), the Balaton Highland (Hungary), Han Bulog (Bosnia), the Gulf of Ismid (Turkey), Anarak (Iran), and the Himalayas (India, Nepal, Tibet). In north-west Nevada (Augusta Mountains, New Pass Range), the species ranges throughout the entire Nevadisculites taylori and Balatonites shoshonensis zones (middle Anisian).

Acrochordiceras damesii (Noetling, 1880)
Text-figure 3

  • v * . 1880 Ammonites (Acrochordiceras) Damesii Noetling, p. 334, pl. 15, figs 1, 1a, 1b.

  • non . 1887 Acrochordiceras Damesi Noetling; Hauer, p. 22, pl. 5, figs 2 (=A. carolinae).

  • * . 1934 Silesiacrochordiceras damesi (Noetling); Spath, p. 405, text-fig. 139a–b (after Philippi in Frech 1903, pl. 1, fig. 7a–b).

  • . 1990 Acrochordiceras damesi (Noetling, 1880); Dzik, p. 51, text-fig. 3 (holotype); pl. 13, fig. 1 (holotype).

  • . 1990 Acrochordiceras cf. ippeniArthaber, 1911; Dzik, p. 61, text-fig. 2; pl. 16, fig. 1.

  • . 1999 Acrochordiceras aff. damesiNoetling, 1880; Kaim and Niedzwiedzki, p. 97, figs 3a–e, 4a (after Dzik 1990, fig. 2c), 4b, 4c (after Dzik 1990, fig. 3c; holotype), 5b.

  • 1999 Acrochordiceras damesii; Urlichs, p. 345, fig. 5.

  • ? 2003 Acrochordiceras cf. damesii (Noetling, 1880); Vörös, p. 82, text-fig. A12; pl. A2, figs 1a–b, pl. A3, fig. 1.

Types.  The type material of the species consists of only a single specimen (Noetling 1880, p. 334, pl. 15, fig. 1) and comes from the Gogolin Beds of Pelsonian age (Balatonites balatonicus Zone) in Silesia (Poland). The holotype has since been redescribed and figured by Dzik (1990).

Description.  Moderately involute shell with a narrow umbilicus. Moderately compressed, subquadrate to high oval whorl section with a rounded, low-arched venter. Ornamentation consists of thick, rectiradiate, slightly sinuous ribs that cross the venter without alternation. Two or three ribs arise from coarse umbilical nodes with an additional intercalated rib. Suture line is subammonitic with poorly crenulated saddles and an unusually large first lateral lobe (Text-fig. 3; Dzik 1990, text-fig. 3c).

Remarks.  The species Acrochordiceras damesii has been controversial from the time it was first described, and its variability is unknown because it is based on a single, partially eroded and deformed specimen. Kaim and Niedzwiedzki (1999, p. 97) tentatively assigned all Silesian acrochordiceratids to A. damesii, but this assignment should be corroborated by better preserved material. From a morphological point of view, A. damesii is closely related to A. carolinae because both species have the same type of ornamentation and whorl shape (Diener 1907, p. 99; Silberling and Nichols 1982, p. 22). A. damesii may differ from A. carolinae in having a more compressed and subquadrate whorl section, as well as only two to three ribs per umbilical node (vs. a ratio of five to six for A. carolinae). Then again, the inner whorls of A. damesii have up to five ribs per node (Dzik 1990; Kaim and Niedzwiedzki 1999). However, all of these differences in ornamentation and shell geometry fall completely within the intra-specific variation of A. carolinae as revised in this study. The remaining diagnostic character of A. damesii is its peculiar suture line. Indeed, A. damesii differs from A. carolinae (and other Acrochordiceras species) by having a suture line with a broad and shallow first lateral lobe and slender saddles whose sides converge toward their crests (Diener 1907, p. 100). Because of the holotype’s poor state of preservation, the peculiarity of its suture pattern may be the result of distortion, and in fact, it has been a matter of considerable confusion (see e.g. Spath 1934, p. 406). Nevertheless, Noetling’s type was refigured by Philippi (inFrech 1903, pl. 1, fig. 7a, b) and then by Dzik (1990, fig. 3, pl. 13, fig. 1), both of which show the details of the suture line with its very wide, almost subcircular and moderately indented lateral lobe. This peculiar suture pattern has apparently been corroborated by new material described by Kaim and Niedzwiedzki (1999) from Silesia, but its age is slightly younger than the holotype.

It is noteworthy that A. damesii has been documented (with certainty) only from an isolated epicontinental basin (Germanic Basin), and its suture line differs significantly from the suture of Tethyan Acrochordiceras species. This difference may have resulted from simplification of the suture line in acrochordiceratids upon entering an epicontinental basin (Kaim and Niedzwiedzki 1999), similar to some other Muschelkalk ammonoids (see Urlichs and Mundlos 1985).

The specimen figured by Vörös (2003, pl. A2, fig. 1, pl. A3, fig. 1) as A. cf. damesii clearly corresponds to a mature body chamber with its straight ribs and without umbilical nodes, but with enlarged ribs at mid-flank and the ventral shoulder. However, its specific status remains indeterminate because of its poor state of preservation. Indeed, its compressed whorl section and poorly indented suture line resemble a slender variant of A. carolinae (as herein revised). Furthermore, its supposedly large size is not a diagnostic character because our material demonstrates that A. carolinae reaches shell diameters in excess of 200 mm.

Occurrence.  This species is a typical ammonoid of the Germanic Lower Muschelkalk (middle Anisian), but it is not known from elsewhere with certainty.

Discussion

Biostratigraphy

In spite of the intensive sampling of middle Anisian strata over a period of several years in the northern Humboldt Range, southern Tobin Range, Augusta Mountains, and New Pass Range, the number of specimens obtained from each locality and sample point (i.e. single bed) is highly variable. For instance, a sufficiently large number of specimens could not be obtained from the Anagymnotoceras spivaki and Proteusites fergusoni subzones (Text-fig. 2). Owing to deformation of tectonic origin, a large portion of the collections from the Acrochordiceras hyatti, Nevadisculites taylori and Balatonites shoshonensis zones in the northern Humboldt Range was not suitable for biometric measurements. Our sampling thus represents ‘slices’ of the lineage, which are unevenly distributed through time. However, in excess of 550 adequately preserved, stratigraphically controlled specimens of Acrochordiceras were measured for this study, most of which were obtained from localities in the other mountain ranges mentioned above.

Both geometric and ornamental characters were selected for the identification of species. The profusion of names available in the literature makes it unnecessary to introduce new species names. Old names were preferred when sets of specific characters as used in the present work matched those of previously described species, provided their stratigraphic occurrence was consistent with that of their Nevada counterpart. Dzik (1990) suggested that Acrochordiceras represents a single lineage that could be subdivided into successive, arbitrarily defined chronospecies: A. anodosumA. hyattiA. haliliA. damesiiA. ippeni. This lineage supposedly follows a gradient of increasing involution and tuberculation. However, this hypothesis is not based on stratigraphically controlled data, and it reflects the difficulty in defining the taxonomy of highly variable species. As demonstrated by our study, which is based on large bed-by-bed collections, there exist significant natural morphological differences between different samples of Acrochordiceras, and thus several morphospecies can be distinguished. Only three stratigraphically successive species are recognized in the middle Anisian strata of the Fossil Hill Member: Acrochordiceras hatschekii (Diener, 1907), A. hyattiMeek, 1877 and A. carolinaeMojsisovics, 1882. As shown in Text-figure 2, the duration of these species varies widely, from only one subzone to more than one zone.

The middle Anisian substage, which corresponds to the stratigraphic range of Acrochordiceras, comprises three zones in north-west Nevada: the Acrochordiceras hyatti, Nevadisculites taylori and Balatonites shoshonensis zones, in ascending order (Text-fig. 2). Only one species of Acrochordiceras is recognized within each subzone; thus, the studied species of Acrochordiceras constitute an anagenetic lineage. Acrochordiceras hatschekii (Diener, 1907) occurs in the Intornites mctaggarti Subzone (hyatti Zone). Acrochordiceras hyattiMeek, 1877 typically occurs in the Unionvillites hadleyi Subzone (hyatti Zone), but it also occurs rarely in the older Ginsburgites americanus subzone (hyatti Zone). Acrochordiceras carolinaeMojsisovics, 1882 ranges throughout the taylori and shoshonensis zones. So far, acrochordiceratids have never been recorded from strata younger than middle Anisian in Nevada or in other Anisian basins, even though they are still abundant in the latest middle Anisian Bulogites mojsvari subzone.

Inter- and intra-specific variability

From a classical descriptive and qualitative morphological approach, three species of Acrochordiceras have been recognized among sampled specimens from Nevada, and a fourth valid species is restricted to the Germanic Muschelkalk (Text-fig. 3). The distributions of studied parameters (H, W, U and R) have been investigated to determine whether they are consistent with their taxonomic assignment. The distribution of each ratio (H/D, W/D, U/D and R/D) was tested for normality by plotting histograms for each species, each section within a species and each sample within a section (when sufficiently large enough datasets were available).

For example, Text-figure 5 shows the results of the test at the species level for the parameters H/D, W/D, U/D and R/D. Note that R/D has been previously log-transformed because its distribution is skewed to the ‘right’. Histogram plots and the Lilliefors test of normality show that H/D, W/D, U/D and R/D of all studied species have a normal or near normal distribution with a confidence level of 95 per cent. However, certain parameters of some species do not have a normal distribution, such as the number of ribs of A. hyatti, which seems to have a bimodal distribution. In most cases, this slight departure from normality may be explained by the fact that the studied species exhibit allometric growth with changes of their geometric parameters during ontogeny. Hence, all parameters of all studied species are considered as conforming to a normal distribution. Text-figure 11, which illustrates the distribution of shape compression (H/W) and degree of coiling (U/D) for A. carolinae, demonstrates the normal distribution of these two parameters, as ranging from depressed and evolute to compressed and involute forms.

The inter- and intra-specific variation of A. hatschekii, A. hyatti and A. carolinae from north-west Nevada is also illustrated by box and mean plots in Text-figure 4. For instance, this shows that A. carolinae is usually but not exclusively (overlapping boxes) characterized by high-whorled, involute shells compared to A. hatschekii and A. hyatti. On average, A. hatschekii is also characterized by a more compressed whorl section, but slender variants of A. hyatti and A. carolinae may still have similar whorl width.

Ornamentation

The three studied species of Acrochordiceras from north-west Nevada exhibit only moderate differences in ornamentation. Their innermost whorls are characterized by parabolic lateral tubercles, and this early ontogenetic character is present in the three species without modification. In all cases, these parabolic tubercles are associated with megastriae, which correspond to a growth break during which partial resorption of the aperture and adjacent prominent relief took place. The angular unconformity between the growth lines on each side of the megastriae indicates that the mantle edge had rotated when secretion resumed with construction of a new shell segment (for definition, significance, and classification of megastriae, see Bucher and Guex 1990; Bucher et al. 1996). In Acrochordiceras hatschekii and A. hyatti, a row of lateral tubercles is added after the initial parabolic stage. In A. carolinae, the early parabolic tubercles remain in a lateral position, but the succeeding row of tubercles is located in a lower position, near the umbilical edge. The transition to adult ornamentation is invariably enhanced either by the migration of tubercles to a marginal position on depressed variants or by their ultimate fading on compressed variants.

Although greatly affected by intra-specific variation, the generally plicate pattern of ribbing does not change drastically between species of Acrochordiceras. However, as already pointed out by Silberling and Nichols (1982), ribs tend to alternate and may even crisscross on the venter of A. hatschekii (e.g. Pl. 2, fig. 3) and A. hyatti (e.g. Pl. 4, fig. 9). This peculiar and specific trait nearly always disappears in A. carolinae, whose ribs cross the venter without alternation. However, this pattern is found to occur very rarely in the very early representatives of A. carolinae (see Pl. 6, fig. 15). Hence, changes in the position of tubercles and ribbing patterns are neither interdependent nor synchronous.

The adult shape of Acrochordiceras is highly conservative in that its slightly egressive mature body chamber invariably evolves into a rectangular to subquadrangular whorl section along with a smooth venter and simple ribs that end in marginal swellings. The strength of the final ornamentation is usually proportional to that of the immature stage, but even the extremely involute, compressed and smooth variants develop the characteristic pattern of mature ribbing on their final body chamber.

Suture line

The suture line of Acrochordiceras is ceratitic to subammonitic (Text-fig. 3) with two lateral saddles and one auxiliary (between the siphon and the umbilical seam). Its two lateral saddles are roughly of equal height, and the lateral lobe is moderately indented and varies from shallow to deep. The differences between studied species involve only the frilling of the saddles. In A. hatschekii, the saddles are complete, not denticulate, whereas in A. hyatti, they are still complete, but their sides are slightly crenulated. Finally, in A. carolinae the suture is subammonitic with slightly frilled saddles. In the Germanic A. damesii, the suture is also subammonitic but differentiated by a very large, relatively shallow, first lateral lobe.

Ontogenetic trajectories

The accretionary mode of growth of mollusc shells provides invaluable insight into the entire ontogenetic development of individuals. Thus, the average ontogeny of assemblages from different species can be compared. For this purpose, an allometric curve (y = b xa; for a review of allometry, see Gould 1966) of H/D, W/D, U/D and R/D has been derived for each species to compare the growth trajectory of these parameters with respect to shell diameter (Text-fig. 6). The first pattern indicates that the three species have a parallel growth trajectory for all studied parameters, except whorl width of A. carolinae, as evidenced by the allometric exponents (a; ‘curve’s slope’). Hence, the ontogeny of these parameters is similar for each species, and the three species have the same set of rules, which control how whorl shape is scaled with respect to shell diameter. The three species thus differ only by a different allometric coefficient (b; ‘curve’s intercept’), which correspond to a different offset at the beginning of their growth.

The second pattern highlighted by Text-figure 6 is that whorl shape compression (H vs. W) and the number of ribs (R) decrease throughout ontogeny within each species (negative allometry), while the degree of involution (H vs. U) increases throughout growth (positive allometry). Changes in whorl height, whorl width and umbilical diameter largely depend on shell diameter, because their allometric exponents are close to 1. Thus, increasing involution and decreasing whorl compression throughout ontogeny mainly result from their scaling to shell size. On the other hand, decreasing ribbing density (R/D) throughout ontogeny is poorly explained by shell diameter because its allometric exponent is closer to 0. Hence, the number of ribs is relatively independent of shell diameter. These results also suggest that ribbing density and conch geometry (whorl shape compression and degree of involution) are nearly independent of each other during the ontogeny of a specimen. This result contrasts with the usual concept of intra-specific variability of ammonites, because ribbing density has been shown to correlate with the degree of involution within a species at similar shell diameters (i.e. the Buckman’s First Law of Covariation; see e.g. Morard and Guex 2003). The pattern of intra-specific variation inversely linking the geometry of whorl shape with R/D cannot be transposed into the ontogenetic development of a single individual.

Finally, from a taxonomic point of view, the allometric exponent (a) of whorl height (H) and umbilical diameter (U) can be used to distinguish A. carolinae from A. hyatti and A. hatschekii, while the allometric exponent of whorl width (W) can be used to separate A. hatschekii from A. carolinae and A. hyatti (Text-fig. 6).

Dimorphism

Generally, a typological approach has been utilized for acrochordiceratids, thus leading to the arbitrary use of two generic names for the middle Anisian forms: depressed and compressed phenotypes were assigned, respectively, to Acrochordiceras Hyatt, 1877 and EpacrochordicerasSpath, 1934. Dzik (1990) advocated the viewpoint that Acrochordiceras and Epacrochordiceras constitute a dimorphic pair, which may represent sexual dimorphism expressed in the degree of involution of the shell. He also argued that the two genera evolved in parallel with a permanent gap in their range of morphologies, especially in the degree of involution (see Dzik 1990, fig. 4) and absence of intermediate forms between these two genera. In addition, he corroborated his hypothesis by plotting the degree of involution of various specimens of Acrochordiceras and Epacrochordiceras through time (Dzik 1990, fig. 4).

However, it is noteworthy that Dzik’s dataset is based on a small compilation, which contains relatively few specimens having a poor stratigraphic provenance from different worldwide localities. Our large, bed-by-bed collections of Acrochordiceras from north-west Nevada strongly contradict Dzik’s dimorphism hypothesis. Indeed, as clearly demonstrated in Text-figure 11, biometric analysis shows that morphotypes previously assigned either to Acrochordiceras or to Epacrochordiceras are nothing more than extreme end-member variants of a highly variable species. Furthermore, there are many intermediate forms between Acrochordiceras and Epacrochordiceras, and their studied parameters (e.g. whorl shape compression and degree of involution in Text-figure 11) clearly conform to a Gaussian distribution. Therefore, the hypothesis of dimorphism between Acrochordiceras and Epacrochordiceras is excluded.

Conclusions

This study of new, large collections from the Anisian Fossil Hill Member (Star Peak Group, north-west Nevada) has enabled us to define the extent of intra-specific variation and ontogenetic development, and thus simplify the taxonomy of the middle Anisian acrochordiceratids, especially the genus Acrochordiceras Hyatt, 1877. Continuous ranges of intra-specific variation led us to synonymize HaydenitesDiener, 1907, SilesiacrochordicerasDiener, 1916 and EpacrochordicerasSpath, 1934 with Acrochordiceras Hyatt, 1877. Consequently, the biochronological significance of the acrochordiceratids is largely improved. Three stratigraphically successive species are recognized in the low paleolatitude middle Anisian faunas: A. hatschekii (Diener, 1907), A. hyattiMeek, 1877 and A. carolinaeMojsisovics, 1882. A fourth valid species, A. damesii (Noetling, 1880) is restricted to the Germanic Muschelkalk. For each biostratigraphic subdivision established in the Nevada succession, biometric and morphological study indicates that phenotypes previously assigned either to Acrochordiceras Hyatt, 1877 or to EpacrochordicerasSpath, 1934 are nothing more than extreme end-member variants of a continuous, highly variable species. Moreover, an assessment of intra-specific variation of the adult size range does not support recognition of a dimorphic pair (Acrochordiceras and Epacrochordiceras). The three successive middle Anisian species of Acrochordiceras from north-west Nevada form a lineage characterized by an increase in the degree of involution, adult size and intra-specific variation. The analysis of this trend and its macroevolutionary implications will be dealt with in a separate study.

Acknowledgements.  This study was supported by the Swiss National Foundation (projects no 200021-113616 and no 200020-113554). Jim Jenks (Salt Lake City) is deeply thanked for editing the English version of this work. The reviewers S. Lucas and A. Vörös are thanked for their constructive comments.

Editor. John Jagt

Ancillary