The type specimen consists of multiple disconnected remains that were collected over the course of several field seasons. However, given that all fragments were collected from the same lithofacies and stratigraphic interval within less than 10
, are diagnostic of Testudinata (sensu Joyce et al. 2004
), exhibit the same type of preservation and do not replicate each other anatomically, all are confidently assigned to a single species and one individual. The alternative would be the occurrence of either at least two individuals or possibly at least two sympatric Triassic basal turtle species in a small localized area that, in comparison with other localities from that time period and given the extreme scarcity of turtle material therein, is not to be expected.
The material that most closely diagnoses C. tenertesta as a turtle is a portion of the carapace consisting of one and a half dorsal vertebrae, the proximal portions of the associated ribs and the overlying dermal bone (a–f and a–f). Although the exact location of this fragment within the dorsal column is uncertain, we speculate that it originates from the anterior portion of the column, because the neural arches increase in height posteriorly as they do in Proganochelys quenstedti. The central articulations of the dorsals are triangular, fully platycoelous and notably small, roughly the size of the neural canal. As in modern turtles, the main body of each dorsal centrum is hourglass-shaped in ventral view and exhibits a well-developed ventral ridge. Although the contacts are not fully clear, it appears that the dorsal rib heads only incipiently contact two dorsal centra, an unambiguous synapomorphy of Testudinata. The ribs are unique among turtles, in that they are near laminar in thickness and stand upright (i.e. are oriented dorsoventrally), the basal condition seen in most amniotes. The chamber that is formed between the ribs and the overlying carapacial bones are thus reduced in size. The neural canal is clearly discernable at the ends of the specimen and highly ovoid in shape. The narrow bridge that connects the vertebral centra with the carapace is formed by the neural arches and increases in height from anterior to posterior in the preserved portion of the specimen. The overlying carapacial bone exhibits a low sagittal keel that fades posteriorly. A single cross-running suture is present, presumably subdividing what is preserved into two wide neurals, but the lateral portions are too damaged to assess the actual shape of these elements.
The carapace is furthermore represented by a crushed portion of the bridge, various costal portions with vertical rib parts (not shown) and a fragment from the posterior region of the shell that consists of one or two peripherals, two partial costals, possible portions of the neurals and two dorsal ribs (g
). Interestingly, in all elements, the dorsal ribs run elevated across the visceral surface of the costals instead of being tightly integrated into them. Thus, the dorsal ribs and costals appear to be independent osteological elements. The costals exhibit no evidence of fontanelles, indicating that the carapace was fully ossified. Sulci are not apparent, but we do not conclude that scutes were absent, as sulci are often faintly preserved in fossil turtles. The most striking feature of the carapace is its laminar thickness. Although the carapace reaches approximately 3
mm in thickness where the median keel runs the highest on the putative neurals, it quickly thins to less than approximately 1
mm in the remaining carapace. We estimate C. tenertesta
to have had a carapace length of 35
cm by comparison of the available material with the larger but approximately coeval turtle P. quenstedti
), so C. tenertesta
is the thinnest shelled turtle taxon known with a fully ossified shell.
The plastron is represented by the well-preserved left hypoplastron (h
). Unfortunately, scute sulci cannot be traced, thus rendering this identification somewhat tenuous. However, the transverse orientation of the posterior suture with the xiphiplastron and the shortness of the xiphiplastral branch of the hypoplastron allow us to conclude that this must indeed be a hypoplastron. By contrast, the anterior hyoplastral suture with the epiplastra is oblique in all basal turtles and the epiplastron branch of the hyoplastra is relatively longer. Unlike all preserved portions of the carapace, the hypoplastron is notably thick along the inguinal notch (approx. 10
mm at its deepest point), exhibits a thickened ridge that runs anteriorly towards the axillary notch (approx. 7
mm), but thins rapidly in all other directions, down to 2
mm. The inguinal notch is unique in not forming a sharp rim, but rather a sloping plate. Another fragment (not shown) is available which we interpret as the central portion of the left hypoplastron due to the presence of what appears to be the rim of the axillary notch, but this identification is tentative.
The available material of C. tenertesta
includes various remains of non-shell-related dermal armour, the most prominent of which is a well-preserved partial neck spine of which two prongs are preserved (i,j
, and i
). The possibility that this element represents the serrated posterior rim of the shell is excluded, given that sufficient amounts of this region are preserved otherwise (see costal element above) and that the interpretation of this element as being the posterior rim of the carapace would imply the unparsimonious conclusion that this taxon not only possessed supernumerary bones within the tip of every marginal scute, but also additional bones along the ventral margin of the carapace. Finally, by comparison with P. quenstedti
, the element is too large to be considered a tail spike. Despite its massive appearance, the element is hollow (Lucas et al. 2000
), and the thickness of the preserved dermal bone is similar to that of the carapace. In comparison with the neck spines of the coeval turtle P. quenstedti
), the surface is smooth, and each prong forms an angular cone that is delimited from neighbouring cones by the development of a sulcus, indicating that the neck spines were covered in the living animal by keratinous scutes, much similar to the shell. Finally, unlike the previously available material of P. quenstedti
, the new element is clearly formed by multiple osteoderms.
Prior to the discovery of more diagnostic material, the fragmentary neck spine was the only known remain of C. tenertesta
. Lucas et al. (2000)
noted the great resemblance of this bone to the neck spines of the Triassic turtle P. quenstedti
) and tentatively assigned it to Testudinata. However, this identification always appeared questionable (Gaffney et al. 2006
), because the composite nature of this element contrasts with the apparently single and fused morphology seen in P. quenstedti
. The recovery of the only known bona fide turtle material from the Triassic of the entire North American main continent from the same locality that produced the turtle-like neck spine leads us to firmly conclude that this spine indeed belongs to a turtle. Conversely, considering that the sutures of the shell (Gaffney 1990
) or between the cleithrum and the plastron (Joyce et al. 2006
) are obscured in the available material of P. quenstedti
, we suggest that this taxon probably possessed multipart neck spines as well, and that the sutures were obscured during ontogeny as were those of the remaining dermal armour.
As with any species known from a single individual, assessing the relative ontogenetic age of NMMNH P-16697 is difficult. Furthermore, given that the ontogeny of basal turtles is only poorly known, comparisons with extant turtles may perhaps lead to unjustified inferences (de Queiroz & Gauthier 1992
). From observations made in P. quenstedti
, it is apparent that larger individuals of that taxon have fused shells and neck spines, whereas the smallest known specimen at least displays sutures in the shell (Gaffney 1990
). In that regard, NMMNH P-16697 should be interpreted as an individual that has at least not yet terminated growth. No known Triassic or Early Jurassic turtle displays costal fontanelles, making it unclear whether the lack of such fontanelles signifies adulthood, as in many extant turtles. The absolute size of NMMNH P-16697, herein estimated at 35
cm, however, clearly reveals that it is not a hatchling, but rather a subadult to young adult individual that is not much smaller than the smallest known individual of P. quenstedti
). Given that the orientation of the ribs, the placement of the ribs relative to the thoracic column and the thickness of the shell are not known to change during post-natal ontogeny in extant turtles and that these features, where applicable, do not change in the post-natal ontogeny of amniotes, in general, we are confident in concluding for the moment that the unique rib orientation and placement and thinness of the shell are not juvenile features of C. tenertesta
, but rather represent the adult morphology as well. Additional materials will eventually test this assertion.