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Keywords:

  • Conchostraca;
  • Branchiopoda;
  • Maastrichtian;
  • Madagascar;
  • palaeoecology;
  • biogeography;
  • Antronestheriidae

Abstract

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Abstract:  A new spinicaudatan genus and species, Ethmosestheria mahajangaensis gen. et sp. nov., is described from the Anembalemba Member (Upper Cretaceous, Maastrichtian) of the Maevarano Formation, Mahajanga Basin, Madagascar. This is the first spinicaudatan reported from the post-Triassic Mesozoic of Madagascar. The new species is assigned to the family Antronestheriidae based on the cavernous or sievelike ornamentation on the carapace. Of well-documented Mesozoic spinicaudatan genera, Ethmosestheria mahajangaensis is most closely related to Antronestheria Chen and Hudson from the Great Estuarine Group (Jurassic) of Scotland. However, relatively poor documentation of the ornamentation of most Gondwanan Mesozoic spinicaudatan species precludes detailed comparison among taxa. Ethmosestheria mahajangaensis exhibits ontogenetic trends in carapace growth: a change in carapace outline from subcircular/subelliptical to elliptical, and from very wide juvenile growth bands to narrow adult growth bands. Ornamentation style, however, does not vary with ontogeny. Ethmosestheria mahajangaensis individuals lived in temporary pools in a broad channel-belt system within a semiarid environment; preserved desiccation structures on carapaces indicate seasonal drying out of pools within the river system. Specimens of Ethmosestheria mahajangaensis are preserved with exquisite detail in debris flow deposits; these are the first spinicaudatans reported from debris flow deposits. These deposits also contain a varied vertebrate fauna, including dinosaurs, crocodyliforms, turtles, and frogs. Rapid entombment of the spinicaudatan carapaces likely promoted early fossil diagenesis leading to highly detailed preservation.

Spinicaudatans (‘conchostracans’) are a group of weakly biomineralizing branchiopod crustaceans that primarily occupy ephemeral pools of freshwater. These organisms are important as environmental indicators (e.g. Vannier et al. 2003) and are also biologically interesting. Unlike most crustaceans, spinicaudatans do not shed their carapace during ecdysis (Tasch 1987). Consequently, the entire ontogenetic history of an individual is preserved in its carapace. Where spinicaudatans occur in the fossil record, their remains tend to be numerous. Large sample sizes combined with the preserved ontogeny provide a framework to interpret community structure and pond dynamics.

In this paper, we report the first spinicaudatan specimens, assigned to a new genus and species, from the Cretaceous of Madagascar. Although spinicaudatans currently inhabit Madagascar, fossil spinicaudatans have only been recorded previously from the Early Triassic strata of Madagascar (Shen et al. 2002). The units from which the new specimens occur include a rich and increasingly well-documented vertebrate fauna (see Krause et al. 2006, and references therein), and biological and ecological information provided by the new spinicaudatan specimens help to further characterize the palaeoecology and palaeoenvironment of the latest Cretaceous of Madagascar. The new spinicaudatan specimens comprise exquisitely preserved carapace material for which a high level of ornamentation detail can be observed. We discuss the palaeobiological, palaeoecological, and palaeobiogeographic implications of the new specimens.

Spinicaudatan stratigraphic and sedimentologic occurrence

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

The spinicaudatans are known from the Anembalemba Member (Maastrichtian) of the Maevarano Formation, Mahajanga Basin, near the small village of Berivotra (Text-fig. 1). The fossils were discovered in 1993 during the first field season of the ongoing Mahajanga Basin Project, conducted jointly by State University of New York-Stony Brook and the University of Antananarivo, Madagascar. The Anembalemba Member consists of two distinct sandstone facies, which are interpreted to have accumulated in a shallow and very broad channel-belt system characterized by a strongly variable discharge regime (Rogers et al. 2000, 2007; Rogers 2005). Palaeoenvironmental reconstructions suggest the landscape was a low-relief alluvial plain with a continental environment rich with vertebrates (Rogers and Hartman 1999; Rogers and Krause 2007). Evidence from calcareous palaeosols and calcareous soil nodules within the paleosols suggests a semiarid climate (Rogers et al. 2000, 2007). Vertebrate fossils recovered from the Anembalemba Member include fish, frogs, turtles, snakes, lizards, crocodyliforms, nonavian dinosaurs, birds, and mammals. Some of these taxa inhabited the shallow river system, while others lived on the floodplains and scavenged carcasses near the channels (Rogers et al. 2007).

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Figure TEXT-FIG. 1..  A, Index map to spinicaudatan location and geologic framework (modified from Rogers 2005). B, Generalized section for the Anembalemba Member. Stratigraphic position of spinicaudatan specimens (MAD93-73) is marked with a carapace outline; marine gastropod position (MAD93-15) with circle. BF indicates Berivotra Formation and MM indicates Masorobe Member (modified from Rogers et al. 2007). C, Ethmosestheria mahajangaensis occurs at Locality MAD93-73, located on the Berivotra Escarpment in the Anembalemba (Kmva) Member of the Maevarano (Kmv) Formation (see text). The Maevarano Formation is underlain by the Masorobe Member (Kmvs) and overlain successively in this area by the marine Berivotra (Kb) Formation and Betsiboka (Pb) Limestone. Stratotypes for the various lithostratigraphic units are shown in this figure (in italics) by section range lines (see Rogers et al. 2000). The formational contacts were mapped by JHH in the Berivotra area in 1993 and 1996 and interpreted from field observations and air photos (FTM – Foiben-Taosarintanin’l Madagsaikara – 92 FTM/IGN 125/400 Mission of the Betsiboka drainage, 1:40,000). The only other well-documented macroinvertebrate body fossil locality in the Anembalemba (or Maevarano) is Locality MAD93-15, from which marine snails were collected (Rogers et al. 2000). Other labels include national highway 4 (A.P. 4) and mile markers (e.g. PK 521).

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The two sandstone facies of the Anembalemba Member, known as Facies 1 and 2, are distinguished on the basis of structure, colour, and fossil content. Facies 1 is cross-stratified, typically displaying small- to medium-scale, tabular and trough cross-stratification. The sandstone beds are fine- to coarse-grained and poorly sorted, with a sizeable clay component. This facies is interpreted as the ‘normal’ discharge channel deposit (Rogers et al., 2000, 2007; Rogers 2005). Facies 2 is massive in structure, displaying fine- to coarse-grained clay-rich massive sandstone beds characterized by very poor sorting (Rogers et al. 2000, 2007). This facies is interpreted to represent debris flow deposits; the debris flows were instigated by high-intensity precipitation events on the typically parched floodplain (Rogers 2005; Rogers et al. 2007). Fossils are concentrated in Facies 2, which also includes the spinicaudatans (Krause and Hartman 1996; Rogers et al. 2000). Vertebrate fossils excavated from Facies 2 are exquisitely preserved, including bird claws retaining keratin and almost fully articulated vertebrate skeletons (Krause et al. 1999; Rogers et al. 2007; and references therein). The overall stratigraphic sequence represents a transgressive system which is eventually overlain by marine Berivotra Formation (Text-fig. 1A–B). Marine incursions occurred in the uppermost Anembalemba Member, and rare brackish or marine invertebrate snails have also been reported from Facies 2 (Hartman et al. 1994; Krause and Hartman 1996; Rogers et al. 2000).

Spinicaudatan fossils have only been recovered to date from one locality (MAD93-73). The fossils are from an east-facing blowout-form depression in the Anembalemba Member (Text-fig. 2A). The locality occurs on a minor bench or platform on the escarpment edge that faces the valleys of the Vavanamadirobe and Maevarano–Manarenja drainages. The spinicaudatans occur in Facies 2 (Text-figs 1B, 2A). At this locality, Facies 2 also includes dinosaur, turtle, and fish material and particularly dramatic ball and pillow load structures (Text-fig. 2B). MAD93-73 is 9.6 m below the top of the Maevarano Formation and base of the marine Berivotra Formation. MAD93-73 is located at 15° 54′ 3.306456′′ S and 46° 36′ 10.151475′′ E at a relative elevation of 194.6 m (WGS 84 datum).

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Figure TEXT-FIG. 2..  A, View of the collection site (MAD93-73); spinicaudatans are preserved in the grey debris flow deposits of Facies 2, between the dashed lines on this image. B, Soft sedimentary deformation features (X) occurring at the top of unit from which specimens were collected.

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Systematic Palaeontology

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Institutional abbreviations.  UA, University of Antananarivo, Antananarivo, Madagascar; AMNH-FI, American Museum of Natural History, Fossil Invertebrates, New York, USA.

The ‘Order Conchostraca,’ as commonly used, is a paraphyletic group; the phylogenetically based, monophyletic classification scheme proposed by Olesen (2000) and Martin and Davis (2001) is followed herein. Systematic terminology follows that of Olesen (2000) and Martin and Davis (2001) for higher-level classification and follows Chen and Shen (1985) for superfamilial and lower levels. Measurements and terminology of carapace features follow Tasch (1987) (see Text-fig. 3 for locations of measurements). All specimens were imaged uncoated using an environmental scanning electron microscope (ESEM) under low-vacuum conditions with a 20-kV beam.

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Figure TEXT-FIG. 3..  Schematic outline of an Ethmosestheria mahajangaensis carapace indicating locations of morphological features measured. Anterior is to the left. Length: maximum carapace length; Height: maximum distance from the dorsal margin to the hinge line; Hinge: length of the hinge line; A: distance from the maximum anterior bulge to the dorsal margin; B: distance from the maximum posterior bulge to the dorsal margin; C: distance from the maximum ventral bulge to the most anterior part of the valve; D: distance from center of the umbo to the anterior margin.

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Subphylum CRUSTACEA Brünnich, 1772 Class BRANCHIOPODA Latreille, 1817 Subclass PHYLLOPODA Preuss, 1951 Order DIPLOSTRACA Gerstaecker, 1866 Suborder SPINICAUDATA Linder, 1945 Superfamily EOSESTHERIOIDEA Zhang and Chen, 1976 (inZhang et al. 1976) Family ANTRONESTHERIIDAE Chen and Hudson, 1991
Genus ETHMOSESTHERIA gen. nov.

Derivation of name.  From the Greek ethmos, a sieve or strainer, in reference to the sievelike structure of the carapace ornamentation.

Type species. Ethmosestheria mahajangaensis sp. nov. from locality MAD93-73 near Berivotra, Mahajanga Basin, Madagascar.

Diagnosis.  Carapace moderate in size, elliptical in outline, valves moderately convex, length approximately 1.5× greater than height; dorsal growth bands broad and flat transitioning to narrow bands between the tenth and fifteen bands, with large, sievelike, cavernous ornamentation formed by aggregation of numerous punctae; growth lines smooth; cavernous ornamentation throughout growth band except distal margin; distal margin of growth bands ornamented by linear bands of punctae. Soft part and egg morphology unknown.

Remarks. Ethmosestheria is placed within the relatively species-meager family Antronestheriidae based on its elliptical shape, numerous growth bands, and characteristically cavernous, sievelike ornamentation. Members of the Estheriteidae can also exhibit cavernous ornamentation, but growth bands of these taxa include an undulose region at the top of the growth band followed by cavernous textures continuing to the ventral edge, which is opposite of the pattern observed in Ethmosestheria.

The new genus differs from other taxa included within the Antronestheriidae, which previously included only Paleoleptestheria? chinensis Chen, 1976 (in Zhang et al. 1976; Duan and Chen 2000) (Mid Jurassic of southwest China and Thailand), PaleoleptestheriaNovojilov, 1954 (Late Jurassic of Mongolia), Pseudestherites Chen, 1976 (in Zhang et al. 1976) (Early Cretaceous of northwest China and Argentina), and AntronestheriaChen and Hudson, 1991 (Mid Jurassic of Scotland). Ethmosestheria exhibits ornamentation differences and a more elliptical shape than the other genera referred to the family. Paleoleptestheria? chinensis exhibits cavernous ornament with comparably sized meshes that are randomly distributed in the dorsal part of the valve, as in the new genus; however, the random distribution transitions to a linear ornamentation in distal growth bands (see Zhang et al. 1976, pls 34 and 35 and Duan and Chen 2000, pl 1), which differs from that observed in Ethmosestheria. The growth bands of Pseudestherites Chen, 1976 (in Zhang et al. 1976) narrow gradually, with no dramatic break between small and large band width that occurs as in Ethmosestheria. In addition, the density of cavernous ornamentation is higher in Pseudestherites. Ethmosestheria most closely resembles AntronestheriaChen and Hudson, 1991 in style and density of ornamentation. Antronestheria, however, is characterized by a row of deeply impressed fossae crowded along the upper margin of each growth band, while Ethmosestheria lacks this characteristic and possesses a dense row of punctae instead.

Antronestheriidae have been previously documented from the Mid Jurassic of Great Britain, China, and Thailand and the Early Cretaceous of China and Argentina (Chen and Hudson 1991; Gallego and Shen 2007). This is the first record of a Late Cretaceous antronestheriid and is only the second record of this family from a Gondwanan landmass. Unfortunately, most spinicaudatan taxa described from Gondwana have only been illustrated using light microscopy (e.g. Tasch 1987). Without SEM images, we are unable to diagnose the characteristics of the family in these specimens and thus cannot accurately determine taxonomic relationships without restudy.

Occurrence.  Facies 2, Anembalemba Member (Upper Cretaceous, Maastrichtian) of the Maevarano Formation, 9.6 m below the top of the formation at the base of the Berivotra Formation, in the eastern area of Berivotra, Mahajanga Basin, Madagascar.

Ethmosestheria mahajangaensis sp. nov. Text-figs 4A–D, 5A–E, 6A–C, 7A–C, 8A–F
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Figure TEXT-FIG. 4..  Holotype specimen of Ethmosestheria mahajangaensis (UA 9643). A, View of entire left carapace valve. B, View of antero-central portion of wide growth bands (bands 7 though 12) exhibiting cavernous ornamentation. C, View of cavernous ornament illustrating sieve texture. D, View of postero-dorsal portion of narrow growth bands (bands 27 through 38).

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Figure TEXT-FIG. 5..  Paratype specimen of Ethmosestheria mahajangaensis (AMNH-FI 56228). A, View of left carapace valve. Note the transition from narrow to wide growth bands between growth bands 12–14. B, View of desiccation cracks wrinkles with cracks on centro-dorsal region of carapace. C, Close up of ornamentation pattern in antero-central region of growth bands six to eight. D, Close up of pore cluster in a single sieve set in growth band seven. E, Transition from wide to narrow growth bands (growth bands 10 through 22) in central region of carapace.

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Figure TEXT-FIG. 6..  Paratype specimen of Ethmosestheria mahajangaensis (AMNH-FI 56227). A, View of entire left carapace with substantial exfoliation at antero-central and dorsal posterior regions. B, Close up of posterior exfoliation indicating layered nature of the carapace structure as well as medium-course sand grains of matrix; parts of growth bands 12 through 24 shown. C, Close up of central dorsal margin illustrating layered nature of carapace. Ornamentation of growth bands is restricted to the outer edge; where growth bands overlap, a smooth texture occurs.

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Figure TEXT-FIG. 7..  Paratype specimen of Ethmosestheria mahajangaensis (AMNH-FI 56230). A, View of left carapace valve, margin is entire but exfoliation along dorsal and posterior margin is significant. B, Close-up of ornamentation at wide to transitional growth bands (growth bands 9 through 15). Note disorganized array of punctae at anterior margin of growth band 9. C, Close-up of transition from disorganized to clustered punctae of growth band 9.

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Figure TEXT-FIG. 8..  Additional specimens of Ethmosestheria mahajangaensis. A, Right carapace valve (ANMH-FI 56229) and B, left carapace valve (UA 56226) are both mildly distorted by incurving of dorsal margin due to desiccation; note transition from wide to narrow growth bands. C, View of umbonal region of right carapace valve indicating pronounced elevation (UA 9644). D, Tubercle on wide growth band (AMNH-FI 56232). E, Possible juvenile specimen (AMNH-FI 56239b). A complete left carapace valve consisting of only 12 growth bands; absence of additional growth bands is not a preservation artifact, suggesting this is an immature specimen. F, Complete interior of a left carapace valve (AMNH-FI 56234).

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Derivation of name.  Named for the Mahajanga Basin in northwest Madagascar from which the species was collected.

Types.  Specimen UA 9643, a left carapace valve complete except for a limited region of exfoliation near the dorsal posterior, is designated as the holotype specimen. Four additional specimens AMNH-FI 56227–56230 are designated as paratypes; AMNH-FI 56229 is a right carapace valve, the others are left valves. Morphological characteristics of the type material are presented in the Appendix. The type material was collected from Locality MAD93-73 (Text-figs 1, 2) on 4, 5, and 11 August 1993 by Robarison J. Augustin, Gregory A. Buckley, Joseph H. Hartman, Charles A. Lockwood, David W. Krause, Laurent Randriamiarimanana, Christine E. Wall, and Roshna E. Wunderlich.

Additional material.  Multiple specimens of isolated carapace valves (AMNH-FI 56226, 56232, 56234, 56239a, 56239b, 56241; UA 9644–9647, 9649, 9654–9656) and a single example of an intact hinge line (UA 9652) from the Anembalemba Member of the Maevarano Formation, Mahajanga Basin. Morphologic data on these specimens is presented in the Appendix. Figured specimens, measured specimens, and additional material (UA 9648, 9650–9651, 9653, 9657–9672 and AMNH-FI 56231, 5623, 26535–56238, 56240, 56242–56253) including fragmented material are reposited at Université d’Antananarivo (UA) and the American Museum of Natural History (AMNH).

Diagnosis.  As for the genus.

Description

General features.  Carapace of moderate size, mean length in adult individuals is 6.97 mm (range: 6.14–9.41 mm), height averages 4.45 mm (range: 2.85–6.34 mm); carapace outline highly elliptical, length to height ratio approximately 1.56 (range: 1.42–1.76) (Table 1, Appendix). Juvenile carapace subcircular to subelliptical (first 10–13 growth bands). Carapace elongation becomes more pronounced in transitional and adult growth bands (Text-figs 4A, 6A, 8A, 9). Dorsal margin straight and relatively short (approximately 0.52× carapace length; Table 1). Umbo pronounced, extends above hinge line; umbonal position anterior, located approximately one-fourth hinge length behind anterior margin (Table 1). Anterior margin rounded, approaching subcircular; maximum anterior bulge is approximately 0.38× the distance from hinge to dorsal margin. Posterior margin protracted into elongate oval; maximum posterior bulge only approximately one-quarter of distance from hinge line to dorsal margin. Maximum ventral bulge is anteriorly situated, only slightly greater than one-third of distance from anterior to posterior (Table 1). There is no evidence for sexual dimorphism in this species.

Table 1.   Summary of carapace morphological characteristics of Ethmosestheria mahajangaensis (measurements in mm).
CharacterMeanStandard deviation95% Confidence intervals
Length6.971.476.22–7.73
Height4.450.983.96–4.93
A1.680.441.45–1.92
B1.460.960.91–2.02
C2.640.92.24– 3.04
D1.690.561.39–1.99
Hinge3.620.813.18–4.08
Length/Height1.60.271.43–1.76
Hinge/Length0.520.0690.476– 0.564
A/Height0.3820.0550.351–0.412
B/Height0.2570.0540.222– 0.291
C/Length0.3770.0560.339–0.415
D/Length0.2370.0490.205– 0.268
# Wide growth bands11.472.6510.10–12.83
# Transitional growth bands43.162.09–5.91
# Narrow growth bands18.236.1414.52–21.94
Total # growth bands29.218.8624.10–34.33
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Figure TEXT-FIG. 9..  Ontogenetic stages of Ethmosestheria mahajangaensis. A, Juvenile carapace in the 12th molt stage; juvenile growth bands are wide and subelliptical. B, Young adult carapace in 18th molt stage; adult growth bands are narrow and become posteriorly elongated. C, Older adult carapace in 30th molt stage; continuation of early adult growth patterns through adulthood.

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Ornamentation.  Approximately 30 growth bands are present on the adult carapace (average 29.21, range: 22–40). First 10 to 13 growth bands wide (0.26 mm or greater) followed by a transition over the next two to four growth bands to narrow bands (0.13 mm or less) (Text-figs 4A, 5A, E); this transition may indicate juvenile versus adult carapace growth. Ornamentation consists of an array of large depressions approximately 25 μm in diameter typically consisting of 7–22 pits arranged in a circular to ovate pattern (Text-figs 4C, 5D). Depressions arranged within loosely defined rows at a spacing of approximately 0.1 mm (Text-figs 4C, 5B). Wider growth bands include up to 10 weakly defined rows of depressions, while narrow bands contain only two rows (Text-fig. 7B). Near dorsal margins of growth bands, depressions are incompletely developed, resulting in a linear appearance (Text-figs 4B, 5C). A single tubercle was observed on a wide growth band (Text-fig. 8D). Distal edge of growth band and growth lines smooth with no indication of serration.

Interior.  Interior surface smooth. Neither muscle scars nor pore canals observed. Original structure of carapace layering preserved; layers approximately 4–5 microns thick (Text-fig. 8F).

Remarks

Comparisons.  The combination of elliptical shape, sievelike ornamentation, and dramatic shift from broad to narrow growth bands distinguishes this species from all other spinicaudatan species previously described. Ethmosestheria mahajangaensis is most similar to Antronestheria kilmaluagensisChen and Hudson, 1991 from the Jurassic of Scotland, but that species is more circular in outline and the distal margin of each growth band includes a deeply impressed, crowded row of depressions that is distinct from the ornamentation observed in E. mahajangaensis. No similar species have previously been described from Gondwana.

Preservation.  Compared to many spinicaudatan fossils, this material is exquisitely preserved. Spinicaudatan carapaces are only weakly mineralized and can undergo relatively rapid degradation (see discussion in Martin 1992 and Vannier et al. 2003). Acutalistic taphonomic analyses of Orr et al. (2008) indicate that carapace material may retain integrity for up to 3 weeks under aqueous conditions. In the case of these specimens, desiccation features (Text-fig. 5b) indicate that these specimens, or at least some of them, encountered drying conditions prior to burial in addition to any aqueous degradation. The detail of ornamentation preserved in these specimens suggests that early fossil diagenesis resulted in stabilization of the carapace material.

To further characterize the preservation of these specimens, the chemical composition of the Ethmosestheria mahajangaensis carapaces was assessed using an energy-dispersive x-ray spectrometry (EDS) system at the University of North Dakota. Uncoated specimens were imaged under low-vacuum conditions in a chamber at an acceleration voltage of 20 KeV. Six sites on specimen UA 9646 were analyzed, all of which exhibited the same spectrum. Carapaces are preserved largely as calcium phosphate. Ca, P, and O combined contribute 73–85% of carapace weight; C contributes 10–24% weight; all other elements contribute only 2–5% of carapace weight. The chemical composition of the preserved carapaces differs significantly from that of the argillaceous sandstone matrix (see Rogers 2005 for discussion of sedimentology). The preservation of E. mahajangaensis carapaces as calcium phosphate mimics closely the carapace composition of modern spinicaudatans; EDS analyses of modern spinicaudatan carapaces reveal a calcium phosphate composition with accessory amounts of aluminum, silica, sulfur, iron, and zinc (Stigall Rode et al. 2005b; Stigall et al. 2008). The mineralized component of modern spinicaudatan carapaces occurs as part of a chitin–phosphate complex (Martin 1992), with the proportion of mineralized and chitinous components varying between taxa (A. Stigall pers. obs.).

Specimens of Ethmosestheria mahajangaensis, therefore, may represent preservation of material of original carapace composition. The original carapaces were unlikely preserved unaltered, but rather the original elements of the carapace recrystallized very shortly (within weeks) after organismal death into a more stable version of apatite. This style of preservation has been previously reported from the Jurassic of Antarctica (Stigall Rode et al. 2005b; Stigall et al. 2008). Furthermore, the preservation of these fossils within debris flow deposits (see Rogers 2005 and Rogers et al. 2007 for more detail) indicates that these specimens were rapidly buried and removed from the influence of predators, scavengers, and bioturbators.

Ontogeny.  Since spinicaudatans do not fully shed their carapaces during ecdysis (see review in Vannier et al. 2003), it is possible to examine ontogenetic change within a single carapace valve. Ethmosestheria mahajangaensis exhibits two types of ontogenetic change: (1) a transition from a subcircular to highly elliptical outline and (2) a transition from wide to narrow growth bands (Fig. 9).

As noted above, the first 10–12 growth bands are subcircular to subelliptical in shape (Text-figs 5A, 8E, 9). Successive growth bands, however, become elongated posteriorly compared to the anterior direction (Text-figs 4A, 5A, 7A, 9). This results in a highly elongate adult form. This type of elongation is observed in other eosestherioids, such as CarapacestheriaShen, 1994, another species with distinctive juvenile versus adult growth features; however, not all members of the group display such a significant change of carapace outline during ontogeny (cf. EustheriaDepéret and Mazeran, 1912). The underlying genetic reason for this morphological transition is not clear at present.

The transition from wide to narrow growth bands in Ethmosestheria mahajangaensis relates to a shift in the rate of carapace accretion. The percent of new carapace material accreted within a single wide band is much larger than the percent of new carapace material accreted in a narrow band. Similar patterns have been observed in modern Eulimnadia (Weeks et al. 1997) and Carapacestheria (Stigall Rode et al. 2005a). The shift in carapace accretion rate has been attributed to the onset of sexual maturity and the transition in biological priorities from rapid juvenile growth to reproductive activities (Weeks et al. 1997).

Occurrence.  Facies 2, Anembalemba Member (Upper Cretaceous, Maastrichtian) of the Maevarano Formation, 9.6 m below the top of the formation at the base of the Berivotra Formation, in the eastern area of Berivotra, Mahajanga Basin, Madagascar.

Palaeoecological implications

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Spinicaudatans typically inhabit shallow, ephemeral pools (see review of spinicaudatan ecology in Vannier et al. 2003). They are indicative of both freshwater and seasonal conditions. The life cycle of spinicaudatans is specialized to take advantage of variable water conditions. In fact, for many species, eggs will not hatch from their resting stage unless they have been fully dried out and then rehydrated (Thiéry 1996a, b). Since the hatching of resting eggs is related to initial influx of water into the pond, entire cohorts of spinicaudatans will hatch at once (Thiéry 1996a, b). This is commonly documented in the fossil record by bedding plane assemblages in which the spinicaudatans are all of similar size and maturity (e.g. Vannier et al. 2003).

A similar pattern is evident in the Ethmosestheria mahajangaensis specimens examined in this study. The range in morphological characters, such as carapace length and number of growth bands, is relatively narrow; very few specimens exhibit carapace measurement values substantially different from the mean value of the specimens examined. (Note low standard deviation values and narrow confidence intervals in Table 1, Appendix.) Only two specimens collected represent possible juveniles (AMNH-FI 56239a and 56239b, Text-fig. 8E), and these specimens occur on the same matrix block. All other specimens represent adult forms with 27–40 growth bands. Since individual spinicaudatans can produce growth bands at variable intervals (Weeks et al. 1997), this overall degree of similarity of ontogenetic stage (i.e., well-established adults) suggests that these organisms hatched from eggs as a single cohort or at least within days of each other.

The high degree of similarity of age in these specimens might be considered somewhat surprising since these fossils are preserved in a debris flow deposit. Spinicaudatan fossils are almost always preserved within deposits of the ephemeral ponds in which they lived (e.g. Great Estuarine Group, Chen and Hudson 1991; Newark Supergroup, Olsen 1984). In that depositional setting, carapaces can be buried in situ without transport during the 3-week window of carapace integrity (Orr et al. 2008). In the Anembalemba Member, however, spinicaudatans have only been collected from the debris flow deposits of Facies 2 (Rogers 2005). Debris flows are not internally turbid, so abrasion to the specimens was minimal to inconsequential. Furthermore, this type of flow is also unlikely to move material far, if at all, from its original location; in fact, many vertebrate fossils recovered from these deposits are fully articulated (Rogers et al. 2007). The high degree of ontogenetic similarity in the preserved specimens further suggests that the specimens examined likely came from a single pool or, at most, immediately adjacent pools (within several meters) that were hydrated by the same rainfall event.

The interpretation of the spinicaudatan population hatching as a cohort from a single flooding event is consistent with the palaeoecological reconstruction for the Anembalemba Member. The Late Cretaceous climate of the Mahajanga Basin has been reconstructed as semiarid including a dry season and a season punctuated by intense rainfall events (Rogers 2005, Rogers and Krause 2007, Rogers et al. 2007). Potentially, the ephemeral pool(s) occupied otherwise dry areas of the broad channel belt complex and were hydrated by an initial rainfall event. Desiccation features, including carapace wrinkles (Text-fig. 5A–B) and inward warping of the distal margins of the carapace (Text-fig. 8A–B), have been observed in multiple specimens of Ethmosestheria mahajangaensis. These features are consistent with observations by ALS of the drying out of modern spinicaudatan carapaces. The presence of desiccation features suggests the ephemeral ponds, or at least parts of them, had fully dried out before initiation of the debris flow. The debris flow which encased the spinicaudatan carapaces, therefore, resulted from a subsequent event of intense precipitation. This scenario of two rainfall events separated by drying of the channel belt complex matches the environmental reconstructions of Rogers (2005).

Biogeographic implications

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

During the Mesozoic, the former components of Gondwana rifted apart from each other (Torsvik et al. 2000). The fundamental pattern of breakup involved initial separation of East Gondwana (Indo-Madagascar, Australia, Antarctica) from West Madagascar (South America, Africa) during the Late Triassic to Early Jurassic. Subsequent fragmentation of the West Gondwana block in the Early Cretaceous separated South America and Africa, while the East Gondwana block separated into Indo-Madagascar and Antarctica-Australia. By the Late Cretaceous, Madagascar had rifted from the Indian subcontinent and was an isolated island continent (Text-fig. 10). Continental connections may have existed at various times between South America and Antarctica or South America and Africa, but existence and timing of these is unclear at present (Krause et al. 2006). Faunal evidence has been one of the primary tools used to resolve the timing of possible continental connections (e.g. Krause et al. 2006), and the new spinicaudatans could potentially improve current biogeographic models.

image

Figure TEXT-FIG. 10..  Late Cretaceous paleogeography of Madagascar and surrounding paleocontinents, derived using Ross and Scotese (2000). Land bridges may have connected South America and the Antarctic Peninsula and/or Indo-Madgascar and Antarctica at various times during the Cretaceous. Collection localities of: Late Cretaceous Ethmosestheria mahajangaensis (star); Early Cretaceous Argentinian and South Chinese of Psedestherites (circles); Mid Jurassic Antronestheria of Great Britain (square).

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Ethmosestheria mahajangaensis is assigned to the family Antronestheriidae, which is otherwise only known from the Mid Jurassic of Great Britain and China and Early Cretaceous of China and Argentina (Text-fig. 10). It is thus the youngest member of the family. Furthermore, the only other occurrence of this family in Gondwana is from the Early Cretaceous of Argentina (Gallego and Shen 2007). Ethmosestheria mahajangaensis is more closely related to Antronestheria from Great Britian than any known Gondwana taxon; however, most of the Cretaceous spinicaudatan fauna previously described from Gondwana (e.g. Tasch 1987) has not been examined using scanning electron microscopy, which precludes assignment of taxa to the Antronestheriidae. Additional data on African, Indian, and South American taxa are required, therefore, before robust statements of biogeographic patterns can be put forth.

At a higher taxonomic level, the superfamily Eosestherioidea has been recorded from multiple localities in Gondwana. Specifically, the superfamily has been documented from the Triassic of Chile and Argentina and the Cretaceous of Argentina and Brazil (Guérin-Franiatte and Taquet 1993; Gallego and Shen 2007; Gallego et al. 2004, 2005). Eosestheroideids, however, are not particularly useful biogeographically. The superfamily has been recorded from most palaeocontinents, since this taxon includes the genus Euestheria, which was a cosmopolitan genus during the Mesozoic (Chen and Hudson 1991).

The only other fossil spinicaudatans described from Madagascar are Early Triassic in age (Shen et al. 2002). The Triassic spinicaudatans are considered to be conspecific with Eusestheria (Magniestehria) truempyi, a species only otherwise known from Germanic Basin (Shen et al. 2002). The pattern of close biogeographic affinities of Malagasy spinicaudatans with European taxa in both the Triassic and Cretaceous is an interesting pattern. The current absence of Gondwana relatives of the Malagasy spinicaudatans may be partially due to undersampling; however, even if additional Gondwanan taxa were recovered, it would not negate the close relationship of the Malagasy and European spinicaudatan taxa.

The connection observed between Europe and Madagascar for spinicaudatans is not replicated in the biogeographic affinities of the vertebrate fauna. Instead, vertebrate taxa recovered from the Maevarano Formation are most closely related to Indian and South American taxa with almost no taxonomic affinity to Laurentia (Krause et al. 1997, 1999, 2006; Sampson et al. 1998; Rogers et al. 2000). The difference in biogeographic affinities between subsets of the fauna may partially relate to different dispersal processes/capabilities of the vertebrate versus spinicaudatan lineages. Dispersal of spinicaudatans can occur through both flooding events and predatory prey interactions (Vannier, pers. comm. 2007). Modern spinicaudatans eggs are dispersed by amphibians and birds, as the eggs pass undamaged through their digestive tracts. Spinicaudatans are known for high dispersal capacity (Tasch 1987; Chen and Hudson 1991), and one modern species (Cyclestheria hislopi) even has a cosmopolitan distribution (Roessler 1995).

Conclusions

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Exquisitely preserved spinicaudatan specimens collected from the Anembalemba Member of the Maevarano Formation (Maastrichtian) in the Mahajanga Basin are assigned to the new genus and species Ethmosestheria mahajangaensis. The carapace of this species records ontogenetic change in two ways: a change in carapace outline from subelliptical to highly elliptical, and a reduction in carapace accretion rate after the termination of the juvenile stage.

Ethmosestheria mahajangaensis likely inhabited temporary, freshwater pools on a broad floodplain that was subjected to seasonal aridity and intense precipitation. Carapace deformation indicates that the spinicaudatans experienced at least partial drying out of the ephemeral pond prior to initiation of the debris flow by a severe precipitation event that buried the specimens (Rogers 2005). This is the first report of Cretaceous spinicaudatans from Madagascar and, to the best of our knowledge, the first report of spinicaudatans preserved in debris flow deposits. Ethmosestheria is more closely related to Antronestheria from the Jurassic of Scotland than to any Gondwanan genus. Most Cretaceous spinicaudatan genera from Africa, India, and South America, however, require revision incorporating SEM imaging to reveal critical characters with which to more accurately assess phylogenetic relationships. Consequently, at this time, the family Antronestheriidae may be actually present, but currently unrecognized, or truly absent in other regions of Gondwana.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Acknowledgements.  We would like to thank the members of the Mahajanga Basin Project, particularly Dave Krause for all his efforts toward the publication of these fossils. Specifically, we wish to thank Virginia Heisey for preparation from matrix of many of the specimens; and permissions and support during the 1993 field season by the late B. Andriamihaja (Université d’Antananarivo), E. Simons, P. Wright, B. Rakotosamimanana, the staff of the Madagascar Institute pour la Conservation des Environnements Tropicaux, and the kindness and help of the villagers of Berivotra. This manuscript was improved by comments and review by Chen Peiji, David Krause, Patrick Orr, Ray Rogers, Shen Yanbin, and Jean Vannier. Imaging was completed with the assistance of Lisa Park at the Environmental Scanning Electron Microscope Lab at the University of Akron and by Joseph Hartman at the University of North Dakota. ALS research was supported by Ohio University Department of Geological Sciences and Provost’s Undergraduate Research Fund. JHH research and field work were supported by the National Science Foundation (Co-PI and collaborative grants with D.W. Krause as lead PI–DEB-9224396, EAR-9418816, EAR-9706302).

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  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix
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Appendix

  1. Top of page
  2. Abstract
  3. Spinicaudatan stratigraphic and sedimentologic occurrence
  4. Systematic Palaeontology
  5. Palaeoecological implications
  6. Biogeographic implications
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Appendix

Appendix 1:

Morphological characteristics of specimens examined

SpecimenMaterialCarapace shape measurementsGrowth band dataCarapace ratio data
LengthHeightABCDHinge# Wide bands# Bands in transition# Narrow bandsTotal # bandsSite of transition Height/L Hinge/L A/H B/H C/L D/L
UA 9643Holotype. Exteterior of left valve; partial exfoliation in dorsal region9.375.042.101.873.992.183.891082038Between 11–171.860.420.420.370.430.23
UA 9644Right umbo only; shows pronounced elevation>7>7
UA 9645Left valve exterior, contorted into conical shape by deformation6.145.35∼3.431341330Between 11–151.15
UA 9646Left valve exterior9.416.342.191.652.374.191232237Between 13 and 151.480.450.350.260.25
UA 9647Exterior of right valve, margins complete but anterior half exfoliated8.135.302.311.342.631.684.2727>27<3.15 from umbo1.530.530.440.250.320.21
UA 9649Intact umbo113822Between 11 and 14
UA 9652Two valves joined across intact umbos>>4.474.181.52∼4.471.971031528Between 11 and 140.36
UA 9654Exterior of right valve, margins eroded, more in posterior than ant. Possible subadult.>4.30>2.81.15?1.780.81>2.40113?>14Around 12
UA 9655Interior of right valve6.462.851.000.592.100.893.49103>20>33Between 11 and 142.270.540.350.210.330.14
UA 9656Exterior of right carapace valve, complete outline, but large sections exfoliated6.804.802.340.972.211.203.13101∼18∼29Between 11 and 121.420.460.490.200.330.18
AMNH-FI 56226Exterior of a left valve; good ornament, crushed in central section and dorsal hinge8.774.991.94∼1.873.792.745.05871227Between 9 & 15;1.760.580.390.430.31
AMNH-FI 56227Left valve exterior, complete except distal dorsal margin∼7.354.881.390.912.891.313.639121132Between 10 and 210.280.19
AMNH-FI 56228Exterior of left valve7.395.121.541.432.862.023.931222236Between 13 and 141.440.530.300.280.390.27
AMNH-FI 56229Right exterior, dorsal posterior and and anterior eroded; complete outline present7.164.031.511.271.941.674.961011829At row 111.780.690.370.320.270.23
AMNH-FI 56230Left valve exterior, umbo and anterior complete, posterior exfoliated∼7.254.531.522.892.13>3.811422440Between 15 and 160.34
AMNH-FI 56232Exterior of a right valve, complete except for dorsal anterior margin6.514.131137
AMNH-FI 56234Complete interior of a left valve6.764.601.991.312.801.433.39∼20>201.470.500.430.280.410.21
AMNH-FI 56239aPossible juvenile. Complete left valve, significant exfoliation5.913.571.520.772.551.582.921212No transition1.660.490.430.220.430.27
AMNH-FI 56239bPossible juvenile. Complete left valve, lots of exfoliation4.062.671.030.571.560.942.151212No transition1.520.530.390.210.380.23
AMNH-FI 56241Complete interior of a right valve6.804.851.891.412.942.083.55??27>272.31 mm behind umbo1.400.520.390.290.430.31