Tongemys enigmatica gen. et, 2022

Joyce, Walter G., Bourque, Jason R., Fernandez, Vincent & Rollot, Yann, 2022, An alternative interpretation of small-bodied turtles from the " Middle Purbeck " of England as a new species of compsemydid turtle, Fossil Record 25 (2), pp. 263-274 : 263

Tongemys enigmatica gen. et sp. nov.

Figs 1 , 2

Holotype.

NHMUK PV OR 48262/48265, a near complete shell preserved on two separately catalogued slabs ( Lydekker 1889a, fig. 3; Guerrero and Pérez-García 2021b, fig. 1a-c; Fig. 1 ).

Type locality.

Beckles’ Pit, Durlston Bay, Dorset, United Kingdom; Marly Freshwater or Cherty Freshwater Members, Purbeck Group, early Berriasian, Early Cretaceous (see Geological Settings above).

Nomenclatural acts.

This publication and its nomenclatural acts were registered at ZooBank on 2 August 2022, prior to publication. The LSID of the publication is urn:lsid:zoobank.org:pub:347E8CB2-6D5C-46C1-8269-C60629424822, that of the new genus LSID urn:lsid:zoobank.org:act:92EBB8E9-9FDB-403E-A619-170CC9936568 and that of the new species LSID urn:lsid:zoobank.org:act:5E4C6B25-01B6-4237-8C59-6DBC8DFBB978.

Etymology.

The genus name, Tongemys , is formed in honor of Haiyan Tong, a paleontologist who has consistently contributed to the field of descriptive turtle paleontology and systematics over the course of the last three decades. The epithet, Tongemys enigmatica , alludes to 150 years of taxonomic uncertainty obscuring the validity of this new taxon. The epithet is formed as a noun in apposition, not an adjective.

Diagnosis.

Tongemys enigmatica gen. et sp. nov. can be diagnosed as a representative of Compsemydidae by its relatively small size (carapace length smaller than 30 cm), a finely textured shell, a sutured bridge, the reduction to absence of a nuchal contribution to the anterior carapace margin, the reduction to absence of a contact between peripheral I and costal I resulting in a contact between the nuchal and peripheral II, the absence of a cervical and a posterolaterally sloping gular-humeral sulcus. Tongemys enigmatica gen. et sp. nov. differs from other compsemydids by the presence of a distinct nuchal notch, which is formed by an anterior protrusion of peripheral I, a residual contribution of the nuchal to the anterior carapacial margin, a laterally expanded nuchal that is wider than costal I (also in Selenemys lusitanica ), the convergence of the nuchal, peripheral I, peripheral II and costal I on to a single point, neurals II-VII as broad as long (also in Compsemys russelli and Compsemys victa ), four-sided neural I (also in Compsemys russelli and Compsemys victa ), narrow anterior peripherals (also in Peltochelys duchastelii ) that are much wider than tall, V-shaped peripherals in cross section, anterior and posterior to the bridge, a straight medial margin of costal VIII resulting in a trapezoidal space for the suprapygals (also in Peltochelys duchastelii ), restriction of vertebral I to the nuchal and costals (also in Selenemys lusitanica ), the convergence of vertebral I, marginal II, marginal III and pleural I on to a single point, development of a shallow anal notch only (also in Selenemys lusitanica and Peltochelys duchastelii ), lack of a sinuous mid-line sulcus (also in Peltochelys duchastelii ), a posterolaterally sloping gular/humeral sulcus that nearly crosses the epi-hyoplastral suture and restriction of gulars to epiplastra. The available material is not sufficient to allow differentiating Tongemys enigmatica from Calissounemys matheroni , but the latter appears to be larger and have a finer and more striated surface texture.

Referred material.

The following specimens from the type locality are referred, based on their small size and the presence of a nuchal that is wider than costal I and that shows a reduced contribution to the anterior carapacial margin: NHMUK PV OR 48263, a carapacial disc lacking peripherals ( Guerrero and Pérez-García 2021b, fig. 2d); NHMUK PV OR 48263c, the anterior half of a carapacial disk lacking the peripherals ( Guerrero and Pérez-García 2021b, fig. 2b); NHMUK PV OR 48263e, a partial carapacial disk lacking peripherals ( Guerrero and Pérez-García 2021b, fig. 2c); NHMUK PV OR 48264, a carapacial disk lacking peripherals ( Guerrero and Pérez-García 2021b, fig. 3b, c). The following specimens are referred, based on their small size and the presence of a laterally contracting mesoplastron: NHMUK PV OR 48343, a near complete plastron (Fig. 2 ; Guerrero and Pérez-García 2021b, fig. 4a); NHMUK PV OR 48344, a near complete plastron lacking much of the lobes ( Guerrero and Pérez-García 2021b, fig. 4b); NHMUK PV OR 48347, a partial plastron lacking the anterior and posterior lobes ( Guerrero and Pérez-García 2021b, fig. 4c); NHMUK PV OR 48355, a partial plastron lacking the anterior and posterior lobes ( Guerrero and Pérez-García 2021b, fig. 3d); NHMUK PV OR 48354, a disarticulated shell lacking nuchal, peripherals and most of the anterior and posterior plastral lobes ( Guerrero and Pérez-García 2021b, fig. 3a). The following specimens, also from the type locality, but too incomplete to yield much taxonomic information, are referred, based on their small size: NHMUK PV OR 48263a, a carapacial disk lacking the nuchal and the peripherals ( Guerrero and Pérez-García 2021b, fig. 2a); NHMUK PV OR 48345, a heavily-eroded carapace ( Guerrero and Pérez-García 2021b, fig. 1f); NHMUK PV OR 48351, a partially disarticulated carapacial disk lacking the nuchal and the peripherals ( Guerrero and Pérez-García 2021b fig. 2e). Although all referred specimens are incomplete, all provide sufficient character evidence to assess their taxonomic referral.

Description.

Carapacial bones. The exact number of elements that comprise the carapace of Tongemys enigmatica is not known. No complete carapace is preserved, but the available material preserves a nuchal, eight neurals, eight pairs of costals, the anterior eight pairs of peripherals and one to two suprapygals. Three pairs of posterior peripherals and the pygal were likely present as well. Though incomplete, the shell looks to have been rounded, with exception of a distinct nuchal notch, which is framed by peripherals I (Fig. 1A, B ). In the smallest specimens (e.g. NHMUK PV OR 48263a; Guerrero and Pérez-García 2021b, fig. 2a), elongate distal rib ends suggest that fontanelles were present, but in more skeletally mature specimens, including the holotype, the costals appear to contact the peripherals, with the exception of a minor gap between the nuchal, costal I and peripherals I and II (Fig. 1A, B ). The surface of the shell is decorated by a fine texture consisting of small, evenly-spaced pits (Figs 1 , 2 ). The holotype, one of the largest available specimens, has an estimate carapace length of 14 to 15 cm. Smaller specimens, such as NHMUK PV OR 48264, had an estimated carapace length of only 8 cm.

The nuchal is wide and hexagonal with long anterolateral and posterolateral contacts with peripheral I and costal I, a short anterior contribution to the margin of the shell and a short posterior contact with neural I (Fig. 1A, B ). A lateral corner contact with peripheral II is interrupted by what looks to be a minute fontanelle. Among compsemydids, the nuchal of Tongemys enigmatica resembles that of Selenemys lusitanica by being wider than costal I, but differs by symplesiomorphically contributing to the anterior margin of the carapace. A clear lateral contact with peripheral II is present in all other compsemydids.

The available material suggests that eight neurals are present, of which the last is typically fused with the suprapygals. The former presence of an asymmetric, abnormal element in the holotype is hinted at by a notch at its posterior end (Fig. 1A, B ). The variable fusion of neural VIII with suprapygal I is not only documented for other compsemydids, such as Compsemys russelli , Compsemys victa and Selenemys lusitanica , but also Pleurosternon bullockii . The neurals resemble those of Compsemys russelli and Compsemys victa by being nearly as wide as long. The holotype resembles Compsemys russelli and Compsemys victa by possessing a four-sided neural I, while the remaining parts of the sequence are hexagonal with symmetrically short anterior sides. The remaining specimens, though fragmentary, are consistent with this arrangement, with the exception of NHMUK PV OR 48263 ( Guerrero and Pérez-García 2021b, fig. 2d), which resembles Peltochelys duchastelii by exhibiting a preneural, a hexagonal neural I with symmetrically short posterior sides and a square neural II. A square neural II also seems to have been present in Selenemys lusitanica .

Eight pairs of costals are present that are fully separated from their counterparts by the contiguous neural series (Fig. 1A, B ). As in all turtles, the costals evenly fan out from the anterior to the posterior. All costals have similar anteroposterior dimensions to one another, with the exception of costal I, which is considerably longer anteroposteriorly than costal II and costal VIII, which is much smaller and almost rudimentary relative to costal VII. The costals typically contact two neurals medially (see above). Costal I contacts peripheral II and III anterolaterally. A point contact may have existed with peripheral I anteriorly and peripheral IV posterolaterally. Costal VIII otherwise contacted the suprapygal complex posteromedially. The remaining contacts of the costals with the peripherals are not preserved.

The holotype is the only specimen to preserve a meaningful sample of peripherals (Fig. 1A, B ). As in most species of compsemydids, peripheral I is located anterolaterally to the nuchal and the two bones broadly contact the entirety of one another. As a result, peripheral I lacks a posterior contact with costal I. This characteristic is present among all unambiguous compsemydids. In contrast to other compsemydids, however, peripheral I is not a wedge-shaped element that forms a rounded anterior carapace margin, but rather is a rectangular element that forms minor anterior protrusions that frame a narrow anterior nuchal notch. The anterior margin of peripheral II lines up with the anterior margin of costal I. A medial contact with the nuchal is, therefore, absent. A short contact between these two bones is present in Selenemys lusitanica and Peltochelys duchastelii , a more extensive one in Compsemys russelli and Compsemys victa . The remaining peripheral elements are disarticulated from the rest of the shell, likely because they were not tightly sutured to the costals. The peripheral III-IV and peripheral VIII-IX contacts, however, seem to have aligned with the costal I-II and costal VI/VII contacts. The anterior peripherals are notably narrow, like those of Peltochelys duchastelii . The CT data reveal that all available peripherals have a V-shaped cross section, not just the bridge peripherals. This is a previously under-reported characteristic not only apparent among other European compsemydids, such as Peltochelys duchastelii , but also the pleurosternid Pleurosternon bullockii (clearly visible in NHMUK PV R 1891) and the helochelydrid Aragochersis lignitesta ( Pérez-García et al. 2020).

The suprapygals are not preserved in the holotype (Fig. 1A, B ). In three specimens, a single suprapygal is apparent that is fused with neural VIII (NHMUK PV OR 48263a, 48263 and 48354, Guerrero and Pérez-García 2021b, fig. 2a, c and fig. 3a, respectively). In two other specimens, two suprapygal elements are present, of which the anterior is fused with neural VIII (NHMUK PV OR 48351, 48264, Guerrero and Pérez-García 2021b, fig. 2e and fig. 3b and c, respectively). The fusion of the suprapygal element to neural VIII is alluded to by the unusual polygonal form of the resulting compound element, including angular concavities. The suprapygal elements in concert fill the triangular space between costals VIII. The medial margin of costal VIII is, therefore, straight, not stepped to account for differently-sized anterior and posterior suprapygals. A similar arrangement is seen in Peltochelys duchastelii .

Carapacial scutes. The carapace was likely covered by five vertebrals, four pairs of pleurals and twelve pairs of marginals (Fig. 1A, B ). As in all other compsemydids, but also Pleurosternon bullockii , a cervical is clearly absent. In the holotype, the intervertebral contacts are located over the middle of neural I, the posterior thirds of neural III and V and neural VIII (Fig. 1A, B ). Other specimens generally agree, although minor deviations are apparent. As a more extreme abnormality, at least one specimen exhibits medially split vertebrals (NHMUK PV OR 48263c, Guerrero and Pérez-García 2021b, fig. 2b), also noted by Guerrero and Pérez-García 2021c (fig. 2c). Vertebral I is the broadest vertebral element. It has straight anterior contacts with marginal I, which jointly form a moderate convexity. Furthermore, it contacts the full length of marginal II anterolaterally, pleural I posterolaterally and vertebral II posteriorly. The near contact of vertebral I with marginal III hinders marginal II from broadly contacting pleural II, the condition seen in most other turtles. Vertebral I resembles that of Selenemys lusitanica by not lapping on to the peripherals. An overlap on to peripherals I and II is developed in Compsemys russelli , Compsemys victa and Peltochelys duchastelii . An overlap on to peripheral I only is exhibited in Pleurosternon bullockii . Vertebrals II to IV are hexagonal elements (Fig. 1A, B ). They each have two lateral contacts with the adjacent pleurals and relatively straight anterior and posterior contacts with the adjacent vertebrals. Of the three, vertebral III is the widest, vertebral IV the narrowest. In the holotype, these vertebrals are distinctly narrower than the pleurals (Fig. 1A, B ), but in the most juvenile specimens, they are wider (e.g. NHMUK PV OR 48263a, Guerrero and Pérez-García 2021b, fig. 2a). This differs significantly from the condition seen in Pleurosternon bullockii , where the vertebrals are much broader than the pleurals, even though all known individuals have a much greater size. The outlines of vertebral V are not well preserved in any specimen, but the holotype suggests that this element was about as wide as vertebral IV, but trapezoidal in outline, as its anterior sulcus with vertebral IV is narrow (Fig. 1A, B ). This element appears to be particularly narrow in the holotype, as the vertebral V-pleural IV sulcus barely overlaps the most posterodistal portion of costal VIII, but is clearly located on costal VIII in other specimens. This is not confirmed to be regular by the remaining material (e.g. NHMUK PV OR 48263, Guerrero and Pérez-García 2021b, fig. 2d).

In the holotype, the interpleural sulci are straight, but, while the anterior two evenly cross costals II and IV (as in most turtles), the posterior one laterally crosses from costal VI on to costal VII (Fig. 1A, B ). This unusual position is not seen in other specimens (e.g. NHMUK PV OR 48263, Guerrero and Pérez-García 2021b, fig. 2d).

The holotype best preserves the marginals (Fig. 1A, B ). Marginal I is relatively broad and contacts its counterpart medially, marginal II laterally and vertebral I posteriorly. It evenly covers the medial two-thirds of peripheral I and the anterior portions of the nuchal. The median intermarginal sulcus is symplesiomorphically located on the nuchal as in Selenemys lusitanica , but also Pleurosternon bullockii . Only remnants of the remaining intermarginal sulci are otherwise apparent. This, in return, suggests that the marginal-pleural sulcus was located near the peripheral-costal suture. In this regard, Tongemys enigmatica resembles other compsemydids, but differs markedly from Pleurosternon bullockii , where the marginals broadly overlap the costals.

Plastral bones. The plastron consists of an entoplastron and paired epi-, hyo-, meso-, hypo- and xiphiplastra (Figs 1 , 2 ). The plastral fore-lobe is relatively straight along the hyoplastral margin and has a transverse anterior margin, but otherwise is broadly rounded. The plastral hind-lobe is shorter than the fore-lobe and more evenly rounded. Only a shallow anal notch is apparent, as in Selenemys lusitanica . On the visceral side of the plastron, the skin-scute sulcus is located just medial inside the margin of the plastron (see black arrows in Fig. 1C ) and, therefore, lacks broad overlap, in contrast to the plastral hind-lobe of Pleurosternon bullockii . The space between the inguinal and axillary notches is significantly shorter than either lobe. There is no evidence of plastral fontanelles.

The epiplastron broadly contacts its counterpart along the mid-line, the entoplastron posteromedially and the hyoplastron along a posteriorly convex contact posteriorly (Figs 1 , 2 ). The hyo-, meso- and hypoplastron jointly form the bridge region. Possible dorsal contacts of the wing-like axillary and inguinal buttresses with the carapace are unclear, even in the CT data, because the plastron is displaced relative to the carapace, but the lack of extensive sutural surfaces on the underside of the costals suggest that a contact would have been minor, if present at all. As in other compsemydids and Pleurosternon bullockii , the plastral bones do not align to meet exactly at the mid-line. The mesoplastron narrows laterally to a tip. This condition is otherwise hinted at in Selenemys lusitanica . The mesoplastron is lacking in Peltochelys duchastelii . The hypoplastron is only about two-thirds the anteroposterior length of the hyoplastron. The xiphiplastron is attached to the hypoplastron along a transversely straight suture, which is stabilized on the visceral side by a pronounced process of the xiphiplastron that overlies the hypoplastron.

Plastral scutes. In NHMUK PV OR 48343, the only specimen that preserves the plastral scutes well, the plastron is covered by paired gulars, extragulars, humerals, pectorals, abdominals, femorals and presumably anals (Fig. 2 ). There is no evidence of inframarginals, but we cannot be certain of this observation. The gulars are large, blocky elements that do not overlap the entoplastron ventrally, but exhibit an asymmetric mid-line contact. Similar asymmetries are polymorphically developed in Pleurosternon bullockii as well. As in other compsemydids, but not Pleurosternon bullockii , the extragular/humeral sulcus slopes posteriorly and may have even lapped on to the hyoplastron posterolaterally. There is no evidence of a deeply sinuous mid-line sulcus. The humeral-pectoral, pectoral-abdominal and abdominal-femoral sulci are arranged relatively straight transversely. The humeral-pectoral sulcus is located mid-length between the entoplastron and the axillary notch, the pectoral-abdominal crosses the mesoplastron, and the abdominal-femoral laterally aligns with the inguinal notch. The anal-femoral sulcus is not preserved, but a telling break in the holotype suggests that it is orientated diagonally and did not cross on to the hypoplastron.

Figures

Figure 1. NHMUK PV OR 48262 / 48265, Tongemys enigmatica, holotype, Early Cretaceous (Barremian) of England: A. Photograph in dorsal view; B. Interpretive line drawing in dorsal view; C. 3 D model of plastron in dorsal view; D. 3 D model of plastron in ventral view. Abbreviations: co, costal; ent, entoplastron; epi, epiplastron; hyo, hyoplastron; hyp, hypoplastron; Ma, marginal scute; mes, mesoplastron; per, peripheral; Pl, pleural scute; Ve, vertebral scute; xi, xiphiplastron.

Figure 2. NHMUK PV OR 48343, Tongemys enigmatica, referred specimen, Early Cretaceous (Barremian) of England: A. Photograph in ventral view; B. Interpretive line drawing in ventral view. Abbreviations: AB, abdominal scute; EG, extragular scute; ent, entoplastron; epi, epiplastron; FE, femoral scute; GU, gular scute; HU, humeral scute; hyo, hyoplastron; hyp, hypoplastron; mes, mesoplastron; PE, pectoral scute; xi, xiphiplastron.