Peltochelys duchastelii Dollo, 1884

Joyce, Walter G. & Correspondence, Yann Rollot, 2020, An alternative interpretation of Peltochelys duchastelii as a paracryptodire, Fossil Record 23 (1), pp. 83-93 : 84-87

publication ID	https://doi.org/ 10.5194/fr-23-83-2020
persistent identifier	https://treatment.plazi.org/id/03C5A47F-FFF5-DF62-FCAC-FA05FB0BFAEB
treatment provided by	Felipe
scientific name	Peltochelys duchastelii Dollo, 1884
status

Treatment

Type specimen

IRSNB Ct. R.16, lectotype, a near complete shell ( Dollo, 1884, pl. 2.1–2; Hummel, 1929, fig. 2; Meylan, 1988, figs. 3, 4; PØrez-García, 2011, fig. 1; Fig. 1 View Figure 1 ).

Type locality and stratum

Bernissart, Hainault Province, Belgium ( Dollo, 1884); Sainte-Barbe Formation, Early Cretaceous (middle Barremian–early Aptian) ( Yans et al., 2005).

Diagnosis

Peltochelys duchastelii can be diagnosed as a representative of Compsemydidae , a clade of paracryptodires, by the presence of a finely textured shell, a sutured bridge, and a sinuous plastral midline sulcus; absence of a contact between peripheral 1 and costal 1; and the withdrawal of the nuchal from the anterior margin of the shell resulting from a midline contact of peripherals 1. Within Compsemydidae , Peltochelys duchastelii is most readily diagnosed by a clear contact of costal 8 with peripheral 11 (likely absent in Riodevemys inumbragigas ), a point contact of vertebral 1 with marginal 3 (absent in all others), the absence of mesoplastra (present in all others), a shallow xiphiplastral notch (deep in Toremys cassiopeia and Compsemys spp. ), and the presence of a single gular (likely double in all others).

Referred material

Four juvenile specimens, IRSNB Ct. R.17, 18, 291, and 292 ( Dollo, 1884, pl. 2.3–6; PØrez-García, 2011, figs. 2, 3; PØrez-García, 2015, figs. 1, 2) from the type locality .

Description

Although the lectotype of Peltochelys duchastelii has already been photographed and/or illustrated independently four times ( Dollo, 1884; Hummel, 1929; Meylan, 1988; PØrez-Garcia, 2011) and succinctly described three times ( Hummel, 1929; Meylan, 1988; PØrez-Garcia, 2011), we provide yet another visual documentation of this specimen ( Fig. 1 View Figure 1 ) but focus on highlighting differences in interpretation.

Carapacial bones

The lectotype of Peltochelys duchastelii possesses nine neural elements ( Fig. 1 View Figure 1 ). Hummel (1929) noted that a crack runs through the anterior part of the shell and that the two anterior elements may represent a single damaged neural. The alternative interpretation is the presence of a preneural or the presence of a well-formed supernumerary neural. We are unable to weigh in favor of either hypothesis because all purported sutures are covered in paint, obscuring the true nature of the margins of these elements. To maintain the homology of the more posterior elements, we designate the two anterior ones as neural 1a and 1b ( Fig. 1 View Figure 1 ). Neural 7 displays an unusual shape, which is suggestive of two fused neural elements but also consistent with a partially obscured regular neural. We therefore follow previous authors by counting this as a single element. Our final count of eight neurals and a potential preneural matches that of Hummel (1929) and Meylan (1988) but differs from that of PØrez-García (2011), who counted nine neurals, but these differences are semantic and not based on a deeper sense of homology.

The preneural and neural 1 jointly form a hexagonal element with short posterior sides. The shape of neurals 2 and 3 is somewhat unclear, as the original contacts are obscured by movement, but we believe it is most likely that neural 2 is rectangular, while neural 3 is hexagonal with short anterior sides. Neurals 4–6 are clearly hexagonal with short anterior sides, neural 7 is octagonal with short anterior and posterior sides, and neural 8 is an irregular element with four contacts with the surrounding bones.

There is agreement that the lectotype of Peltochelys duchastelii includes eight pairs of costal elements that lack a midline contact ( Fig. 1 View Figure 1 ). As a trend, these elements decrease in anteroposterior length from costal 1 to costal 8. The anterior two elements expand distally and are oriented to the anterior. Costal 3 is rectangular and oriented laterally. The posterior five costal elements are slightly expanded distally and are increasingly oriented posteriorly. In our interpretation of the peripheral series (see below), costal 1 lacks a contact with peripheral 1 but contacts peripherals 2 and 3. The remaining costals each have two contacts with the remaining peripheral series, including a clear contact of costal 8 with peripheral 11, as documented on the right side of the specimen.

The lectotype of Peltochelys duchastelii possesses two distinct suprapygal elements ( Fig. 1 View Figure 1 ). Suprapygal 1 is a large, trapezoidal element with a short anterior contact with neural 8, two elongate lateral contacts with costal 8 and a broad, convex posterior contact with suprapygal 2. Suprapygal 2 is a lenticular element that has a convex anterior contact with suprapygal 1, a short anterolateral contact with costal 8, and three similarly sized posterior contacts with peripherals 11 and the pygal. The pygal is rectangular.

Substantial differences exist in the interpretation of the nuchal and the peripheral series, which we justify more extensively in the Discussion below. We side with Dollo (1884) and Hummel (1929) by recognizing the presence of 11 pairs of peripherals, but we go further by concluding that the nuchal did not contribute to the margin of the shell. In our interpretation, the nuchal is a heptagonal element that has a broad anterolateral contact with peripheral 1, a short lateral contact with peripheral 2, an elongate posterolateral contact with costal I, and a short posterior contact with neural 1a. The point at the anterior margin of the slightly displaced nuchal suggests that this element did not contribute to the anterior margin of the shell. The alternative interpretation of Meylan (1988) and PØrez-García (2011) suggests that the nuchal contributes broadly to the anterior margin of the shell, the plesiomorphic condition for turtles. In contrast to Meylan (1988), we are unable to find paired nuchal processes in the lectotype of Peltochelys duchastelii , in part because the visceral side of the nuchal is covered from view. This agrees with the observations made by PØrez-García (2011), who also was unable to find any structures on the visceral side of the nuchal.

In our interpretation of the peripheral series, peripheral 1 is damaged, but the remaining triangular part of the bone highlights a broad posteromedial contact with the nuchal, the absence of a contact with costal 1, and a clear posterolateral contact with peripheral 2 ( Fig. 1 View Figure 1 ). The pointed tip of the nuchal suggests that peripheral 1 had a midline contact with its counterpart, but its depth would have depended on the presence of a nuchal notch. The remaining 10 pairs of peripherals form a continuous ring that surrounds the central carapacial disk.

Carapacial scutes

The carapace of Peltochelys duchastelii is covered by 5 vertebrals, 4 pairs of pleurals, and at least 11 pairs of marginals ( Fig. 1 View Figure 1 ). The anterior margin of the shell is damaged, obscuring the likely presence of another pair of marginals and perhaps also a cervical. To maintain homology with other turtles, we label the most anterior preserved marginal as marginal 2, which results in a count of 12 pairs of marginals. The interpretation is supported by external observations. In particular, the interpleural sulci of the pleurals laterally contact marginals 5, 7, and 9, as in all other turtles, including those with 11 pairs of marginals. The alternative interpretation, as suggested but not explicitly discussed by Meylan (1988) or PØrez-García (2011), in contrast, suggests highly apomorphic contacts of the interpleural sulci with marginals 4, 6, and 8.

Vertebral 1 is the broadest vertebral element and therefore clearly covers the median portions of peripheral 2 ( Fig. 1 View Figure 1 ). It has a broad lateral contact with marginal 2, a broad posterolateral contact with pleural 1, a broad posterior contact with vertebral 2, and an unusual lateral point contact with marginal 3. Possible contacts with marginal 1 and the cervical, if present, are obscured by damage. Vertebrals 2–4 have irregular hexagonal outlines created by their sinuous lateral margins with the pleurals. They are narrower than vertebral 1 and decrease in width from anterior to posterior. Vertebral 5 is a heptagonal element that is about as wide as vertebral 2. It has an anterolateral contact with pleural 4 and similarly sized posterior contacts with marginals 11 and 12. In our counting system, the intervertebral sulci are located at neurals 1, 3, and 6, and suprapygal 1 and the interpleural sulci are located as costals 2, 4, 6, and 8. As suggested above, the interpleural sulci run laterally into marginals 5, 7, and 9. The marginals are narrow elements that do not cover the costals or suprapygals.

Plastral bones

There is full agreement that Peltochelys duchastelii lacks mesoplastra ( Fig. 1 View Figure 1 ). The plastron therefore consists of an entoplastron and paired epiplastra, hyoplastra, hypoplastra, and xiphiplastra. The anterior plastral lobe has a straight anterior margin and covers much of the anterior carapacial opening. The posterior plastral lobe is narrower than the anterior plastral lobe and only partially covers the posterior carapacial opening. A rounded, shallow anal notch is present. Neither lobe shows signs of kinesis.

The buttresses are visible on the left side of the lectotype, but disarticulation and crushing obscure their contacts ( Fig. 1 View Figure 1 ). We therefore conclude that the axillary buttresses are well developed and likely contacted the costals but also that the contacts with the peripherals are unclear. A lack of a sutural articulation site at peripheral 8 further suggests that the inguinal buttress only reached peripheral 7, but it is unclear if it reached the overlying costals as well. Previous authors did not discuss the contacts of the bridge explicitly.

Plastral scutes

There is disagreement in the number of plastral scales for Peltochelys duchastelii . All authors agree that this turtle possesses paired extragulars, humerals, pectorals, abdominals, femorals, and anals, but while Hummel (1929) and Meylan (1988) suggest the presence of a single, median gular, PØrez-García (2011) reconstructs the presence of highly asymmetric, paired gulars. In our opinion, the gular–gular sulcus highlighted by PØrez-García (2011) resembles one of many cracks that run through the anterior plastral lobe. We therefore side here with Hummel (1929) and Meylan (1988). Our observations otherwise fully agree with those of previous authors: the midline sulcus is sinuous along its full length, the extragulars are triangular elements that do not cover the entoplastron and hyoplastron, the gular is a broad element that laps onto the anterior tip of the entoplastron, the humeral–pectoral sulcus is located far behind the entoplastron, the abdominal contributes to the inguinal notch, and the anals are clearly restricted to the xiphiplastra ( Fig. 1 View Figure 1 ). At least three pairs of inframarginals are present, of which the first contacts the pectoral, the second covers the hyoplastral– hypoplastral suture and contacts the pectoral and abdominal, and the third contacts the abdominal.