Friday 10 June 2022

Chelydropsis aubasi: A new species of Snapping Turtle from the Middle Eocene of France.

Snapping Turtles, Chelydridae, form an important part of modern North American freshwater ecosystems, but have a relatively poor fossil record, largely because their shells are relatively thin compared to other Turtle groups, and tend to disarticulate rapidly after death. However, the fossil record we do have suggests that the group originated in North America in the Late Cretaceous, with subsequent dispersals into Europe and Asia (where they arrived in the Oligocene or earlier and became locally extinct in the Pliocene) and South America (where they arrived in the Pleistocene or earlier, and are still present). 

The dispersal patterns of Snapping Turtles appear to have been different from those of other Turtle groups. The European Turtle fauna of the Cretaceous and Palaeocene comprised indigenous lineages of Thremydids, Helochelydrids, and Pleurosternids, with occasional influxes from elsewhere. This indigenous Turtle population disappeared around the time of the Palaeocene-Eocene Thermal Maximum, and was subsequently replaced by a new fauna including Carettochelyids, Pan-geoemydids, Pan-testudinids, Podocnemidids, and Trionychids. All of these Turtles are present in Asia, which led palaeontologists the theorise that this new influx came entirely from that continent, but more recent studies have suggested that some European groups, most notably the Trionychids, appear to be more closely related to North American faunas than to those of Asia, possibly implying a dispersal into Europe via Greenland. Snapping Turtles appeared to have arrived in Europe much later, with the Earliest known examples from the Late Oligocene, suggesting an arrival into Europe quite separate from the Palaeocene-Eocene Thermal Maximum, and its immediate after-effects. There have been reports of older Snapping Turtle remains from the Eocene of France, although none of this material has ever been formally described.

In a paper published in the journal The Anatomical Record on 6 June 2022, Walter Joyce of the Department of Geosciences at the University of Fribourg, Jean-Luc Landréat of Soissons in France, and Yann Rollot, also of the Department of Geosciences at the University of Fribourg, describe a Snapping Turtle from the Middle Eocene deposits of the Chéry-Chartreuve Sandpit in the Department of Aisne, France.

The Chéry-Chartreuve Sandpit lies half way between the towns of Soissons and Reims, and contained sediments laid down in a transition from a marine to a continental setting. The pit was worked as a quarry in the 1990s and 2000s, exposing a fosiliferous layer at the base of the sequence, from which numerous fossils were extracted by Pierre Louis, Maurice Sabatier, and Jean-Luc Landréat, with the blessing of the quarries' owner, Charles Aubas. An area of about 10 000 m² of the fossil-bearing layer was exposed during this process, the whole of which was removed. This produced an extensive Vertebrate fossil fauna, including Birds, Crocodilians, Fish, Mammals, and Turtles, the majority of which are very well preserved, with little or no crushing or diagenetic darkening. Relatively little of this assemblage has been described to date. The precise age of this deposit has not yet been determined, but it is probably Bartonian (Middle Eocene or between 42.1 and 37.71 million years old.

A simplified map of France highlighting the location of the middle Eocene locality of Chéry-Chartreuve in the Departmentof Aisne. All departments previously reported to have yielded Pan-chelydrid remains, all of which are Oligocene, are highlighted in orange. Joyce et al. (2022).

The Snapping Turtle is placed in the genus Chelydropsis, and given the specific name aubasi, in honour of the late Charles Aubas, who owned the Chéry-Chartreuve Sandpit quarry and both permitted and supported the activities of palaeontologists there. This species is described from a collection of shell elements including, a neural III and IV withpartial right costa, a right peripheral I, a right peripheral II, a right peripheral III, a left peripheral IV and V, a left peripheral VI, a left peripheral VII, a right peripheral VIII, a left peripheral XI, a left epiplastron, an entoplastron, a left hyoplastron a partial right hyoplastron, a partial left hypoplastron, the lateral portion of a right hypoplastron, and , a partial left xiphiplastron. 

Chelydropsis aubasi, Middle Eocene (MP15, early Bartonian) of Chéry-Chartreuve, Aisne, France. 3D models of select costals and neurals. (a) NMB CHC.33, a neural II; (b) NMB CHC.465, a neural II; (c) NMB CHC.3, a neural III and IV with partialright costal IV; (d) NMB CHC.157, a neural IV; (e) NMB CHC.431, two fused posterior neurals; (f) NMB CHC.156, a posterior neural; (g) NMB CHC.34, a posterior neural; (h) NMB CHC.155, a posterior neural; (i) NMB CHC.154, a posterior neural; (j) NMB CHC.158, aposterior neural; (k) NMB CHC.466, medial portion of a left costal II; (l) NMB CHC.32, medial portion of a right costal IV; (m) NMBCHC.509, distal portion of a costal. Abbreviations: Co, costal; ne, neural; Ve, vertebral scute. Joyce et al. (2022).

Fossils of Snapping Turtles are often highly incomplete, and the Animals themselves can be morphologically variable within species, while different species are often very similar, making it difficult to differentiate species. This has led to a high level of confusion in the taxonomy of fossil Snapping Turtles. A total of fifteen extinct species have been proposed from European fossils to date, although Joyce et al. suggest that three might be a more reasonable number (some experts in the field go as low as two), these being Chelydropsis decheni, found in deposits dating from the Late Eocene to the Middle Miocene, Chelydropsis murchisoni, dating from the Middle Miocene and Chelydropsis ponitica, from the Late Miocene to Early Pliocene.

Chelydropsis aubasi, middle Eocene (MP15, early Bartonian) of Chéry-Chartreuve, Aisne, France. 3D models of allavailable peripherals in standard views. Left elements are flipped along the long-axes to ease comparison with right elements. (a) NMB CHC.15, a right peripheral I; (b) NMB CHC.13, a right peripheral II; (c) NMB CHC.27, a right peripheral III; (d) NMB CHC.23, a left peripheral IV;(e) NMB CHC.23, a partial left peripheral V; (f) NMB CHC.26, a left peripheral VI; (g) NMB CHC.4, a left peripheral VII; (h) NMB CHC.28, aright peripheral VIII; (i) NMB CHC.19, a left peripheral XI. Abbreviations: Ab-f, axillary buttress facet; cr-f, costal rib facet; hyo-f, hyoplastralfacet; hyp-f, hypoplastral facet; ib-f, inguinal buttress facet; ma, marginal scute; nu-f, nuchal facet; Pl, pleural scute; Ve, vertebral scute. Joyce et al. (2022)

The new species, Chelydropsis aubasi, does show some resemblance to Chelydropsis decheni, the earliest known species, but Joyce et al. feel that the differences that it does posses, combined with the chronological separation from all known specimens of that species, warrants its description as a new species.

Chelydropsis aubasi, middle Eocene (MP15, early Bartonian) of Chéry-Chartreuve, Aisne, France. Photograph and interpretive line drawings of plastral elements in (a) ventral and (b) dorsal view, including NMB CHC.2 (a left epiplastron), NMB CHC.1(an entoplastron), NMB CHC.198 (a left hyoplastron), NMB CHC.199 (a partial right hyoplastron), NMB CHC.66 (a partial left hypoplastron), NMB CHC.505 (the lateral portion of a right hypoplastron), NMB CHC.251 (a partial left xiphiplastron). Abbreviations: Ab, abdominal scute; an, anal scute; ent, entoplastron; epi, epiplastron; Fe, femoral scute; Gu, gular scute; Hu, humeral scute; hyo, hyoplastron; hyp, hypoplastron; IG, intergular scute; IM, inframarginal scute; xi, xiphiplastron. Joyce et al. (2022).

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