Tavachelydra stevensoni: A new species of Snapping Turtle from the Palaeocene of Colorado.

Snapping Turtles, Chelydroidea, are found today from northern South America to Southern Canada, forming an important component of many North American freshwater ecosystems. Despite being widespread and numerous, there are only five species alive today. Stem group-Chelydroids (i.e. species which are more closely related to living Chelydroids than to any other group), known as pan-Chelydroids, first appeared in the Late Cretaceous, although many fossils are fragmentary, as the shells of Snapping Turtles are less heavily fused than other Turtle groups and tend to disarticulate soon after death, limiting our understanding of this group. Due to this no Cretaceous pan-Chelydroids have been described to date, although post-Cretaceous species have been described from across Laurasia.

In a paper published in the Swiss Journal of Palaeontology on 5 August 2025, Tyler Lyson, Holger Petermann,Salvador Bastien,Natalie Toth,Evan Tamez‑Galvan, and Sadie Sherman of the Department of Earth Sciences at the Denver Museum of Nature & Science, and Walter Joyce of the Department of Geosciences at the University of Fribourg, describe a new species of pan-Chelydroid Turtle from the Early Palaeocene Corral Bluffs of the Denver Basin in Colorado.

The Coral Bluffs are a series of outcrops of Latest Cretaceous to Eocene outcrops in El Paso County in the southern Denver Basin, to the east of Colorado Springs. The sequence is well-dated, with three documented magnetic reversals 30n/29r, 29r/29n, and 29n/28r), a pollen-defined Cretaceous/Palaeocene boundary, and a volcanic ash layer which has been dated using lead and uranium isotopes. These bluffs have produced abundant Vertebrate remains from the Puercan North American Land Mammal Age, including Denverus middletoni, one of two known Early Palaeocene pan-Chelydroid Turtles, which together form the earliest described members of the group.

Geography, chronostratigraphy, and biostratigraphy of the Corral Bluffs Study Area within the Denver Basin from which specimens of Tavachelydra stevensoni were collected. (A) Map of Late Cretaceous through Eocene sediments within the Denver Basin showing the location  of the Corral Bluffs Study Area within Colorado Springs (highlighted by box and enlarged in part (B)) in the southwestern portion of the basin. (B)  High-resolution photogrammetric model of the eastern portion of the Corral Bluffs Study Area overlain on Google Earth with geographic locations  of Tavachelydra stevensoni denoted by red stars: (1) DMNH EPV.141854/DMNH Loc.19258; (2) DMNH EPV.143100/DMNH Loc. 20,053; (3) DMNH. EPV.134087/DMNH Loc. 7082; (4) DMNH. EPV.136265/DMNH Loc. 18,852; 5, DMNH EPV.143200/DMNH Loc. 6284. (C) Age, magnetostratigraphic, lithostratigraphic,  and biostratigraphic logs showing the stratigraphic placement of Tavachelydra stevensoni localities (red stars; see numbers from (B)). Stratigraphy is tied to the Geomagnetic Polarity Time Scale using remnant magnetisation of the rocks at the Corral Bluffs Study Area, two chemical abrasion–isotope dilution–thermal ionisation mass spectrometry uranium/lead-dated volcanic ash beds (yellow star; two dated ash samples represent the same volcanic ash locality and thus only one yellow star), and the palynologically defined K/Pg boundary (italicised dates). The lithostratigraphic log is a composite and shows that the sequence is dominated by intercalated mudstone and sandstone, reflecting a loosely anastomosing fluvial  environment. Pollen biozones are defined by diversification of Momipites spp. (fossil Juglandaceous pollen). Abbreviations: Ma, million years  ago; K/Pg, Cretaceous-Paleogene boundary. Lyson et al. (2025).

The new species is named Tavachelydra stevensoni, where 'Tavachelydra' is a combination of 'Tava' from the Ute/Nuuchiu name for Pike's Peak Tavá-Kaavi, which can be directly translated as 'Sun Mountain'), and -chelydra, a common suffix for Turtles, which derives from the Greek 'khéludros', meaning 'water serpent', while 'stevensoni' honours the late Brandon Stevenson, a dear friend of Tyler Lyson andlong-time  supporter of the Corral Bluffs project.

The new species is described from five specimens, DMNH EPV.141854, the holotype, which consists of a disarticulated,  but associated, skeleton, comprising a nearly complete carapace and plastron and a complete pelvis, and three paratypes, DMNH EPV.143100, an articulated complete carapace  and partial right hypo- and hypoplastron, DMNH EPV.143200, a hypo- and xiphiplastra, and DMNH. EPV.134087, a poorly preserved, but complete cranium. 

Tavachelydra stevensoni, DMNH EPV.141854 (DMNH Loc.19258), holotype, external view of shell. (A) Photograph and (B) interpretive line drawing of the carapace. (C) Photograph and (D) interpretive line drawing of the plastron. Abbreviations: Ab abdominal scale, An anal scale, Ce cervical scale, co costal, ent entoplastron, epi epiplastron, Fe femoral scale, Gu gular scale, Hu humeral scale, hyo hyoplastron, hypo hypoplastron,  Ig intergular scale, Im inframarginal scale, Ma marginal scale, nu nuchal, per peripheral, Pl pleural scale, pn postneural, spy suprapygal, py pygal, Ve vertebral scale, xi xiphiplastron. Arabic numerals denote neurals. Lyson et al. (2025).

Specimens of Tavachelydra  stevensoni are large, with carapace reaching almost 50 cm in length. This is four times the size of Denverus middletoni, the other pan-Chelydroid Turtle from the Denver Basin. Within the Coral Bluffs fauna only one Turtle, Axestemys infernalis, is larger, while a second, Neurankylus sp., is about the same size. It also appears to be one of the rarer species in a Turtle-rich fauna, and since other thin-shelled species, such as Hoplochelys clark, are relatively abundant, this appears to be a reflection of actual rarity rather than a reflection of poor preservational potential. All the known specimens of Tavachelydra  stevensoni have been found in deposits associated with ponds, rather than river channels (the most abundant environment in the Coral Bluffs deposits), suggesting that they preferred such an environment in life. The skull of Tavachelydra  stevensoni is large and broad, with flat biting surfaces, which suggests a durophagous diet (eating hard food, such as shellfish). This is noteworthy, as there is evidence that durophagous species and groups may have preferentially survived the End Cretaceous Extinction.

Reconstruction of Tavachelydra stevensoni basking on a log in a ponded water environment. Andrey Atuchin in Lyson et al. (2025).

A phylogenetic tree constructed by Lyson et al. placed Tavachelydra  stevensoni as the sister species to the extant Snapping Turtles, with later European and Asian species less closely related. This is unsurprising, as while these species are more recent, they are less likely to be ancestral to modern species restricted to North America. Denverus middletoni was also recovered as only distantly related to extant Chelydrids, indicating it was a member of a lineage which did not survive till today.

Cladogram of Chelydroid Turtles mapped  against the stratigraphic ranges for each taxon (black, type strata,  grey, age of referred material). Strict consensus tree from six most  parsimonious trees. Lyson et al. (2025).

See also...