Abstract: Based on new, bed-rock controlled material from Oman and Utah, USA, the Early Triassic genus Guodunites, which was recently erected on the basis of scarce specimens from northwestern Guangxi, South China, is now shown to be a representative of Proptychitidae. This solves the question of the previously unknown phylogenetic affinity of this genus. The genus is restricted to the late middle Smithian, and to date, its biogeographical distribution comprises Oman, South China and Utah, thus indicating an essentially low palaeolatitudinal distribution during the Early Triassic. Its palaeobiogeographical distribution further strengthens the existence of significant equatorial faunal exchanges between both sides of the Panthalassa at that time. It also suggests that, in addition to the potential stepping stones represented by Panthalassic terranes, vigorous equatorial oceanic currents must have contributed largely to the dispersal of ammonoids during such time intervals.
Few ammonoid genera survived the Permo/Triassic mass extinction (c. 252 Ma). The Early Triassic ammonoid recovery followed a global increasing trend in diversity with at least two major evolutionary radiations occurring during the Smithian and early Spathian respectively (Tozer 1981b; Dagys 1988; Brayard et al. 2006). Compared with other marine organisms, this diversification was extremely rapid (less than c. 1 myr; Ovtcharova et al. 2006; Galfetti et al. 2007b) and was accompanied by the formation of pronounced latitudinal diversity gradients during both the Smithian and Spathian evolutionary radiations (e.g. Dagys 1988; Brayard et al. 2006, 2007b). The emergence of such latitudinal diversity gradients parallels the steepening of the Sea surface temperature gradient (SST; Brayard et al. 2006). Brayard et al. (2006, 2007b, in press) and Jenks (2007) have also suggested that the occurrence of several nearly-identical Early Triassic ammonoid taxa on each side of the Panthalassa aptly demonstrates their capacity for very long-distance dispersal across this 20000 km wide ocean (Text-fig. 1). Along with SST control, oceanic surface circulation and/or the use of terranes as stepping stones may also have contributed to the patterns of Early Triassic ammonoid distribution.
Guodunites, a Smithian ammonoid genus, was first described from northwestern Guangxi, South China by Brayard and Bucher (2008). However, the rare and poorly preserved Chinese material did not permit a systematic assignment at the suprageneric level. The recent discovery of Guodunites in Oman and Utah, USA enables us to establish its phylogenetic affinities and to expand its palaeobiogeographical distribution, thus reflecting low-latitude trans-oceanic dispersal between both sides of the Panthalassa.
Biogeographical and Biostratigraphical Occurrences
Guodunites was first described from the Luolou Fm of northwestern Guangxi, South China (Text-fig. 1B) by Brayard and Bucher (2008). Ammonoids are very abundant in the Smithian part of this formation, which essentially represents the basinal deposit portion of the Nanpanjiang Basin (Lehrmann et al. 2007). The major component of the early Smithian usually consists of a conspicuous, ledge-forming, c. 3 m thick, grey, thin-bedded limestone unit containing the Flemingites rursiradiatus beds of early Smithian age (Brayard and Bucher 2008). In contrast to the carbonate-dominated lower part of the Smithian, the middle and late Smithian portions of the Luolou Fm. consist of thin-bedded limestone alternating with dark shales (Galfetti et al. 2007a, 2008). Within this succession, Brayard and Bucher (2008) reported the association of Guodunites with Aspenites, Inyoites, Juvenites, Lanceolites, Owenites, Pseudaspidites, Pseudoceltites? and Pseudoflemingites, thus defining its biostratigraphical position at the end of the middle Smithian (Owenites koeneni beds, Inyoites horizon; Text-fig. 2).
In Oman (Text-fig. 1A), Early Triassic ammonoids occur in exotic blocks of typical Hallstatt facies limestone at Jebel Safra, Wadi Musjah and the famous Baid exotic (Tozer and Calon 1990; Blendinger 1995; Brühwiler et al. 2007). At Wadi Musjah (75 km SSW of Musqat), Guodunites occurs in an exotic block consisting of micritic, red, ammonoid-rich limestone with Owenites, Inyoites, Paranannites, Lanceolites, Aspenites and Dieneroceras, thus indicating a late middle Smithian age. Preservation of ammonoids from this locality is sometimes excellent and, in rare cases, organic remains of so-called ‘false colour patterns’ have been found on specimens of Paranannites (e.g. Klug et al. 2007).
New material from western Utah comes from the Early Triassic Thaynes Fm in the Confusion Range (Text-fig. 1C). The ammonoid bearing beds pertaining to this article occupy the uppermost portion of the Smithian and mainly consist of a limestone unit, overlying a reddish shale interval almost devoid of ammonoid faunas (Bacon 1948; Hose and Repenning 1959; Newell 1959). Ammonoids from beds immediately underlying the Anasibirites beds are relatively well-preserved. Many are complete and retain much of the outer shell.
Guodunites occurs within the Inyoites beds (informal name introduced here) in association with Inyoites, Owenites, Aspenites, Lanceolites, Juvenites, Churkites and Pseudaspidites. Thus, occurrences of Guodunites are synchronous on both sides of Panthalassa. Moreover, in South China and Utah, the immediately overlying Anasibirites fauna of late Smithian age confirms the late middle Smithian biostratigraphical position of Guodunites.
Guodunites was initially described by Hyatt and Smith (1905) and Smith (1932) as Aspidites and Clypeoceras respectively, from the Meekoceras beds (Owenites subzone) in Union Wash, Inyo Range, California.
Systematic descriptions follow the classification of Tozer (1981a, 1994) and Brayard and Bucher (2008). The quantitative morphological ranges for each species are described using the four classical geometrical parameters of the ammonoid shell: diameter (D), whorl height (H), whorl width (W) and umbilical diameter (U). H, W and U are plotted in absolute values and percentages (H/D, W/D and U/D). All measurements are given in Table 1. Repository of figured and measured specimens is abbreviated PIMUZ (Paläontologisches Institut und Museum, Universität Zürich).
Table 1. Measurements of the classic geometrical parameters of the ammonoid shell.
Description. Moderately involute, platycone shell with a narrowly (inner whorls) to broadly (outer whorls) arched venter and slightly convex flanks with maximum thickness near mid-flanks. Egressive coiling at submature stage. Umbilicus relatively shallow and crateriform for juvenile specimens. Umbilical wall convexly rounded and inclined without shoulders. Ornamentation consists of distinct, fine strigation on flanks disappearing on venter. A few biconcave plications are visible on smaller specimens. Growth lines biconcave, strongly projected on the external part of the flanks. Outer whorls with thick, projected lirae. Suture line subammonitic, exhibiting a high, second lateral saddle, and a large first lateral saddle, which is delicately crenulated on its ventral side on the Chinese specimens (Text-fig. 3A–B) and on both sides on the material from Oman (Text-fig. 3C). Saddles gently turned towards umbilicus. Lobes deep with strong indentations ascending saddle sides. Auxiliary series suggests a twofold subdivision of elements.
Discussion. Specimens from Oman are better preserved than those from South China, on which no strigation is visible. Minor differences in suture line may also be due to preservation effects. The largest specimen of Guodunites monneti exhibits slightly projected lirae (as defined by Bucher et al. 2003). This character, as well as the subammonitic saddles, is also well recognized for the Omani and American material.
The poorly preserved specimen from Afghanistan assigned to Owenites slavini by Kummel and Erben, (1968) may be conspecific with G. monneti, but its suture line is not known. However, G. monneti clearly differs from Owenites slavini (Popov, 1962) by the much more indented suture line, greater evolution and flatter whorls, but it is rather similar in ornamentation.
1905 Aspidites hooveri Hyatt and Smith, p. 153, pl. 17, figs 1–12.
1932 Clypeoceras hooveri (Hyatt and Smith); Smith, p. 63, pl. 17, figs 1–12.
Occurrence. Common in CR-A, CR-B, JJ4-05, Confusion Range, Utah, USA; Inyoites beds, Smithian. Meekoceras beds, Owenites horizon, Union Wash, Inyo Range, California, USA.
Description. Involute, laterally compressed shell with a narrowly rounded venter. Slightly convex flanks with maximum thickness near mid-flanks. Juvenile stages with thicker whorls and broader rounded venter. Egressive coiling at submature stage. Umbilical wall inclined with rounded shoulders, forming a deep, crateriform umbilicus on inner whorls. Outer shell with distinct, projected lirae. On inner whorls, bundled lirae may impart weak folds on flanks. One specimen (PIMUZ27243; Pl. 2, fig. 17) is ornamented with a very delicate strigation. Suture line subammonitic with an architecture similar to G. monneti (Text-fig. 3D–G). Lobes broad and deep with strong indentations ascending saddle sides. First and second lateral saddles high. Second lateral saddle bent towards umbilicus. Third lateral saddle broad, asymmetrically indented. Auxiliary series with twofold subdivided elements.
Discussion. Guodunites hooveri is distinguished from G. monneti by its deeper umbilicus and its more involute coiling. Strong projected lirae are visible for all ontogenic stages.
Due to the very rare and poorly preserved material from South China, Brayard and Bucher (2008) were unable to determine the family assignment for Guodunites, but the new material from Utah and Oman enables us to define precisely its phylogenetic affinity. On one hand, both Guodunites species exhibit a delicate subammonitic suture line (see systematic description). This phenomenon is quite unusual in the Smithian, as is the ornamentation consisting of projected marked lirae. Thus, this genus embodies a combination of different morphological characters, which so far is unknown for this sub-stage. On the other hand, its overall platycone shell shape with relatively involute coiling is similar to a few other Smithian genera such as Pseudaspidites, Wailiceras, Clypites or Clypeoceras (see Brayard and Bucher 2008), thus bringing Guodunites closer to Proptychitidae. However, Guodunites differs from typical proptychitids by its umbilicus which is unusual as well, as most Proptychitidae have a small, deep umbilicus with a vertical wall. The platycone shell shape is also characteristic of the Smithian Ussuriidae (e.g. Ussuria, Metussuria and Parussuria). Yet, Ussuriidae always display an occluded umbilicus and a very high whorl height, as well as a very distinctive suture line.
The architecture of the Guodunites suture line is conclusive for the family assignment. It is similar to the proptychitid type of suture, especially the saddles gently turned towards the umbilicus (like Pseudaspidites) and the typical subdivided auxiliary series. Guodunites differs from the classical proptychitid suture line only by its subammonitic saddles (Text-fig. 3). The ussuriid suture line is typically more complex.
Some rare Flemingitidae (e.g. Subflemingites) also display an overall involute and laterally compressed shell. However, they do not exhibit lirae and a subammonitic suture line. Guodunites monneti and G. hooveri display a delicate strigation. However, this character is not diagnostic of any particular Smithian family, as it is also found in Arctoceratidae, Flemingitidae, Inyoitidae, Paranannitidae and Ussuriidae (see Brayard and Bucher 2008).
Thus, based on its shell morphology and the overall architecture of its suture line, Guodunites is considered as belonging to the family Proptychitidae. However, as the genus displays certain unusual characteristics such as subammonitic saddles and lirae, it appears to be one of the most complex derivatives of this family. Leyeceras rothi, another proptychitid, described from Guangxi by Brayard and Bucher (2008) also possesses lirae. However, this species exhibits thinner lirae, more evolute coiling and a simpler suture line. Hyatt and Smith (1905) and Smith (1932) made a correct family clustering for their specimens of Guodunites hooveri linking them to proptychitid genera (Aspidites and Clypeoceras, respectively). However, Guodunites clearly differs from other proptychitids by its subammonitic suture line and its projected lirae.
The new records of Guodunites from Oman and Utah significantly expand its palaeogeographical distribution, not only along the northern Gondwanan margin but also towards the eastern equatorial side of the Panthalassa (Text-fig. 1D). Thus, the genus is now regarded as a low-palaeolatitude taxon that crossed the entire Panthalassa.
The Smithian Owenites assemblage is now clearly known to be restricted to the intertropical belt as illustrated by the common occurrence of such ammonoid genera as Aspenites, Lanceolites, Inyoites and Owenites between the Tethys (e.g. South China, Oman) and the eastern Panthalassic basins (California, Nevada, Utah, Idaho) on the east (Brayard et al. 2006, 2007b, in press; Brühwiler et al. 2007; Jenks 2007; Brayard and Bucher 2008). Within this intertropical belt closely straddling the equator, the South China Block is considered as a key palaeogeographical area, at the interface between Tethys and Panthalassa (Text-fig. 1D). It has recently been shown to contain ammonoid genera that are common to the Tethys and the eastern side of the Panthalassic Ocean (Brayard and Bucher 2008). Still within this palaeoequatorial band, the South Primorye terrane (presently located in eastern Russia) also contains ammonoids such as Churkites suggesting faunal similarities and relationships between each side of the Panthalassa (Jenks 2007). The new occurrence of Guodunites in Utah further suggests that equatorial faunal exchanges between both sides of Panthalassa were intense during the Smithian. This probably reflects: (1) the action of the surface oceanic circulation on the dispersal of ammonoids, especially within the intertropical belt; and (2) the action of thermal constraints on the geographical ranges of ammonoids. The exact mode of ammonoid dispersal remains unknown, but comparison with present-day coleoids (Boletzky 2004) strongly suggests that they also had a high dispersal rate during a juvenile (pseudoplanktonic?) phase (e.g. Tozer 1982; Tanabe et al. 1993; Cecca 2002), which was probably promoted by intense oceanic surface circulation (Brayard et al. 2006). A precise location of the Early Triassic centres of speciation and diversification is almost impossible to attest at that time. On one hand, an eastward dispersal by a potential deep Equatorial Countercurrent can be considered, as already suggested by Newton (1988) in her theory of pantropic distribution. On the other hand, a westward dispersal by possible North and South Equatorial currents (in comparison to the present-day Pacific Ocean configuration, see Stewart 2005) can also be considered. In addition to the biogeographical ‘patterning’ resulting from temperature and currents, the occurrence of terranes (e.g. South Primorye, Chulitna, see Nichols and Silberling 1979; Tozer 1982; Brayard and Brühwiler in press) within the inter-tropical belt no doubt aided dispersal providing stepping stones, if one endorses that the habitat of adults was restricted to platforms, as opposed to a pseudoplanktonic mode of life for the embryonic and neanic stages (Bucher et al. 1996). However, the exact role terranes played in ammonoid dispersal will remain somewhat nebulous until more is known of ammonoid habitat during their various ontogenetic stages and more definitive information is available regarding the position of terranes with respect to each other and within Panthalassa (see Brayard et al. in press).
From another point of view, all Smithian trans-Panthalassic genera such as Churkites or Guodunites, generally have a relatively short time-range. Thus, their value for biostratigraphical correlation within the low-palaeolatitudes on opposite sides of this wide ocean, are quite high. It also emphasizes that very detailed but long-distance correlations are possible within Early Triassic sub-stages, at least for the Smithian. In contrast to conodonts (Paull 1988), this was clearly unsuspected for ammonoids and reinforces conclusions of the independent works of Brayard et al. (2007a, b, in press); Brühwiler et al. (2007); Jenks (2007) and Brayard and Bucher (2008).
The following end-Smithian interval
Until the late middle Smithian, several genera such as Guodunites present a well-constrained biogeographical distribution within thin latitudinal bands. This is the consequence of the formation of a relatively marked SST gradient at that time. By the late Smithian (i.e. Anasibirites beds), this pattern completely shifts towards a less complex biogeographical configuration where most ammonoids are cosmopolitan (e.g. Anasibirites, Wasatchites, Xenoceltites; Brayard et al. 2006, 2007b; Brühwiler et al. 2007). This collapse of the intertropical faunal band and the global distribution of genera probably correspond to a very weak SST gradient. This interval also coincides with: (1) a major ammonoid and conodont extinction (Brayard et al. 2006, 2007b; Orchard 2007; Brayard and Bucher 2008); (2) global perturbations of the C-isotope record (e.g. Galfetti et al. 2007a, b); and (3) a major change in the Boreal palynological record shifting from hygrophyte to xerophyte conditions (Galfetti et al. 2007c). All of these concomitant events lead us to hypothesize that a short global warming episode just before the Smithian/Spathian boundary is associated with a reorganization of the biosphere and especially of ammonoids.
During the Early Triassic, ammonoids rapidly recovered following a global increasing trend in diversity. By the Smithian, ammonoids formed steep latitudinal diversity gradients with genera and species distributed within very thin latitudinal bands. Coupled with this palaeobiogeographical pattern, a trans-Panthalassic distribution like that of Guodunites is striking, but does not appear as exceptional. Smithian genera occurring in both eastern and western equatorial sides of the Panthalassa suggest that ammonoid dispersal by oceanic currents was frequent and essential in their distribution and mode of life.
Acknowledgements. Technical support for photography and preparation was provided by Rosemarie Roth and Markus Hebeisen (Zürich). This paper is a contribution to the Swiss National Science Foundation project 200020-113554 (HB). Two anonymous reviewers provided constructive critics, which helped us to improve the manuscript.