Liaochelys jianchangensis

Zhou, Chang-Fu, 2010, A new eucryptodiran turtle from the Early Cretaceous Jiufotang Formation of western Liaoning, China, Zootaxa 2676, pp. 45-56 : 46-52

Liaochelys jianchangensis gen. et sp. nov.

( Figs 2–4 )

Etymology. Chinese Pinyin ‘Liao’, abbreviation of Liaoning Province, ‘chelys’ Greek ‘turtle’; the specific epithet refers to the type locality.

Holotype. PMOL-AR00160 ( Figs 2 – 3 ), a nearly complete skeleton exposed in dorsal view, including a partial skull, two isolated cervical vertebrae, a whole shell, disarticulated caudal vertebrae, and appendicular skeleton. The fossil is deposited at the Paleontological Museum of Liaoning ( PMOL), Shenyang Normal University.

Referred specimen. PMOL-AR00140 ( Fig 4 ), a large turtle skeleton, which is mostly disarticulated, with exception of the skull.

Type locality and horizon. Both fossils are from the Early Cretaceous Jiufotang Formation (Aptian; Chang et al. 2009) of Lamadong, Jianchang County, Huludao City, western Liaoning Province (40º42´8ʺN; 119º41´13ʺE; Fig 1 ).

Diagnosis. As a primitive eucryptodire Liaochelys jianchangensis has a low domed shell and a ligamentous connection between the plastron and carapace as in Ordosemys spp., sinemydids and "macrobaenids". It is distinguished from these taxa by the following unique combination of characters: nares open dorsally, nasals present, prefrontals contact one another along the midline, postorbital-squamosal contact present, parietal and squamosal separated, crista supraoccipitalis short, foramen palatinum posterius medium sized, cervical scute present, vertebral scutes wider than long, first vertebral wider than nuchal, preneural absent, eight neurals present, first peripheral-costal contact present, third costals strongly expanded distally, medial contact of eighth costals, two suprapygals present of which the posterior one is much larger than the anterior one central, lateral, and posterior fontanelles present, xiphiplastra stout.

Description. Skull: The skull is preserved in the holotype, but its preorbital region is damaged ( Figs 2–3 ). In contrast, the skull is well preserved and dorsally exposed in PMOL-AR00140 ( Figs 4 – 5 ). It has a subtriangular outline with a narrow snout and expanded postorbital portion. The skull is about 51 mm long from the tip of the rostrum to the posterior end of the crista supraoccipitalis and is 43 mm wide across the postorbital bones. The external nares open more dorsally with an oval outline, as in Dracochelys bicuspis . The orbit has a large exposure in dorsal view, implying that it faces more dorsally than laterally. The upper temporal emargination is moderately developed and so the processus trochlearis oticum is fully exposed in dorsal view. This extent is less than that of Sinemys spp. and Manchurochelys manchoukuoensis , but larger than that in Ordosemys spp. As in Ordosemys spp., and Sinemys lens , the crista supraoccipitalis is short and fails to reach the distal level of the squamosals.

As in Kirgizemys spp. (including Hangaiemys spp.; Danilov et al. 2006), the nasals are small bones wedged between the prefrontals along the midline. Anteriorly, the nasals enclose the external nares with the prefrontals, maxillae and premaxillae, different from Ordosemys liaoxiensis and Sinemys spp., in which the nasals exclude the prefrontals from the external nares ( Brinkman & Peng 1993a; Brinkman & Wu 1999; Tong et al. 2004). Behind the nasals, as in D. bicuspis and "macrobaenids", the prefrontals contact one another along the dorsal midline in L. jianchangensis , different from the medial separation of the prefrontals in Ordosemys spp., M. manchoukuoensis and Sinemys spp. ( Brinkman & Peng 1993b; Brinkman & Wu 1999; Tong et al. 2004; Zhou 2010). Posteriorly, the prefrontals contact the frontals along a V-shaped suture, the apex of which points anteriorly, and form with the latter the dorsal rim of the orbit. The frontal contacts the prefrontal anteriorly, the postorbital laterally, and the parietal posteriorly. Medially, the frontals contact one another along their entire length. The parietal is the largest element of the cranial roof with an expanded anterior portion and a posterior portion that narrows along the crista supraoccipitalis. The parietal contributes to the upper temporal emargination with a slender lateral process along the postorbital. However, the process fails to contact the squamosal laterally. A parietal-squamosal contact is common in primitive eucryptodires, except for the sinemydids Sinemys spp. and M. manchoukuoensis , in which the postorbital and parietal are isolated distally from the squamosal ( Brinkman & Peng 1993a; Zhou 2010).

The jugal is exposed between the maxilla and postorbital and contributes to the posteroventral corner of the orbit. Medially, the postorbital is a large element. It contributes to the posterior margin of the orbit and forms posteriorly the temporal bar with the parietal and squamosal. Along the orbital rim, the postorbital bears a medial process. A similar condition is seen in M. manchoukuoensis and D. bicuspis : the medial process is strongly developed, and clearly limits the contribution of the frontal to the orbital rim ( Gaffney & Ye 1992; Zhou 2010). The medial process is also present in Ordosemys sp. ( Brinkman & Wu 1999) but seems to be absent in O. liaoxiensis ( Li & Liu 1999; Tong et al. 2004). As the major element of the temporal bar, the postorbital has a broad suture with the squamosal posteriorly. However, the contact with the quadratojugal is uncertain due to compaction of the specimen. The squamosal forms the posterolateral margin of the skull with a rounded ridge. The ridge extends posteriorly beyond the level of the crista supraoccipitalis. Medial to the ridge, the squamosal bears a deep longitudinal trough.

The premaxilla and maxilla have a limited exposure in dorsal view. The premaxillae are small, positioned at the tip of the snout, and form the ventral rim of the external nares together with the maxilla. Through the external nares, the palatal portion of the premaxilla is partially exposed. The premaxillae seem to form a small central foramen along the ventral midline, just below the anterior rim of the nares. The maxilla is a large element and contacts the premaxilla anteriorly, the prefrontal dorsally, and the jugal posteriorly. Through the orbit, the palatal portion of the maxilla is partially exposed. The maxilla contacts the palatine medially and forms with the latter a moderately sized foramen palatinum posterius as in Ordosemys spp. and which is in contrast to the large foramen, considered as a derived character, present in D. bicuspis and Sinemys spp. (e.g. Danilov et al. 2006).

The palatoquadrate is not exposed in dorsal view except for the palatine and quadrate. The palatine has a small exposure within the orbit. It contacts the maxilla laterally and, in combination with the maxilla, encloses the foramen palatinum posterius. The quadrate is partially exposed within the upper temporal fossa. The quadrate forms the processus trochlearis oticum with the prootic medially and contacts the opisthotic posteromedially, the squamosal posteriorly, and the postorbital anterodorsally. The presence of a quadratejugal contact cannot be determined due to crushing.

From the braincase, the supraoccipital, exoccipital, prootic, and opisthotic are exposed in dorsal view. The supraoccipital contacts the prootic and opisthotic bilaterally, the parietals anteriorly, and the exoccipitals posterolaterally. As in S. lens and O. liaoxiensis , the crista supraoccipitalis extends posteriorly but does not reach the distal level of the squamosals. The exoccipital has a small exposure on the right side. It contacts the supraoccipital medially and the opisthotic anterolaterally. The prootic is located between the parietal medially and quadrate laterally and forms with the latter the processus trochlearis oticum. Posteriorly, the prootic contributes to the rim of the foramen stapedio-temporale with the supraoccipital medially, opisthotic posteriorly, and quadrate laterally. The opisthotic forms the posterior margin of the skull with the supraoccipital and exoccipital medially and the squamosal laterally.

Shell: The carapace of PMOL-AR00160 is an oval and flat dome with a maximum length of 172 mm along the midline and a maximum width of 142 mm across the fourth costal bones ( Figs 2–3 ). The posterior half is constricted and tightly curved as in O. liaoxiensis , M. manchoukuoensis , and D. bicuspis ( Brinkman 2001; Tong et al. 2004; Zhou 2010), but different from Ordosemys leios and most "macrobaenids", in which the curvature is more gentle (e.g. Brinkman & Peng 1993b; Parham & Hutchison 2003; Fig. 6 ). The surface of the carapace is ornamented by numerous tiny pits and grooves. A shallow midline depression is present along the neural region. The scute sulci are lightly impressed on the carapace. The cervical scute is located anterior to the first vertebral scute. All five vertebral scutes are hexagonal and much wider than they are long, as in Ordosemys spp. and Judithemys sukhanovi . By contrast, the vertebral scutes of D. bicuspis only slightly wider than long and the 2nd–4th vertebral scutes of S. lens , M. manchoukuoensis , and other "macrobaenids" are longer than wide (e.g. Brinkman & Peng 1993a; Zhou 2010). The first vertebral contacts the first two pairs of the marginals anterolaterally, while the fifth vertebral contacts the last two pairs of the marginals. The other three vertebrals are broadly separated from the marginals. As in most "macrobaenids", the first vertebral is wider than the nuchal. Its sulcus crosses the nuchal and the first peripherals anteriorly, the first costals posterolaterally, and the first neural posteriorly. The second vertebral is well enlarged. Its sulcus crosses the first three costals bilaterally and the third neural posteriorly. The third vertebral is comparable to the second in size. It covers the 3th–5th costals and neurals. The fourth vertebral is slightly reduced in size, but it covers four (5th–8th) costals and neurals. The last vertebral is much reduced in width. Its sulcus crosses anterolaterally the last neural and costals, and posteriorly the last peripherals and pygal. By contrast, in most "macrobaenids", the posterior sulcus of the fifth vertebral more-or-less crosses the suprapygal. As in sinemydids and "macrobaenids", the pleurals appear to partially overlap the anterior and posterior peripherals. Twelve pairs of marginal scutes cover the peripherals of the carapace.

A nuchal emargination is weakly developed, in contrast to the deep emargination of D. bicuspis ( Brinkman 2001) . The nuchal bone is trapezoid-like, much wider than it is long. The nuchal is slightly narrower than the first vertebral scute, in contrast the strongly-widened nuchal of Ordosemys spp. and Wuguia spp. Behind the nuchal, there are eight neural plates, which are subrectangular, irregular in outline, and decrease posteriorly in size. The eighth neural is greatly reduced, resulting in a midline contact of the eight costals behind the neural. Two suprapygals are present with a subtriangular shape. As in S. lens and M. manchoukuoensis , the first suprapygal is smaller than the second, different from the condition seen in O. leios , D. bicuspis , and other "macrobaenids" where the suprapygals are subequal. Distally, the pygal forms the caudal end of the carapace, which has a slight emargination. The subrectangular pygal has a long marginal edge that is distinctly longer than the adjacent peripherals.

Bilaterally, eight pairs of costal plates are present. The second costal plate is pinched towards its distal end whereas the succeeding third costal plate is strongly expanded distally and has a long sutural contact with the 4th–6th peripherals. The same condition is also apparent in the larger referred specimen, PMOL-AR00140. This contrasts with the condition apparent in Ordosemys spp., sinemydids and "macrobaenids" where the third costals align with the adjacent peripherals (e.g. Brinkman & Peng 1993a, b; Parham & Hutchison 2003; Vandermark et al. 2009; Zhou 2010). The eighth costal has a middle contact with its opposite, as in Wuguia efremovi ( Danilov & Sukhanov 2006) and in some individuals of Xinjiangchelys latimarginalis (Peng & Brinkman 1993). The distal ends of the 5th–8th costals are visible in dorsal view, and have a sutural contact with the adjacent peripherals.

Eleven pairs of the peripherals form the margin of the carapace together with the nuchal and pygal. The first peripheral is subtriangular, and has a distinct contact with the first costal bone, as in most sinemydids and "macrobaenids", but different from the limited contact in O. leios and the completely separated condition in O. liaoxiensis and D. bicuspis ( Brinkman & Peng 1993b; Brinkman 2001; Tong et al. 2004). In contrast, the succeeding five peripherals are subrectangular, except for the third which has a small medial process. The lateral insertion of the costal ribs into the peripherals is visible in dorsal view and creates the illusion that the posterior peripherals produce medial processes. These medial processes are present from the 7th to the 11th peripheral and gradually increase in height posteriorly.

The plastron of PMOL-AR00140 is partially preserved with the hyoplastra, hypoplastra and xiphiplastra. The restored plastron ( Fig 7 ) shows a cruciform outline, which is longer than it is wide. As in "macrobaenids" generally, the plastron has a narrow bridge and triangular anterior lobe. As in Ordosemys spp., sinemydids and other "macrobaenids", the axillary and inguinal buttresses lack sutural connections to the carapace, implying a ligamentous attachment between the plastron and carapace. As in O. liaoxiensis , a central fontanelle, two lateral fontanelles, and a posterior fontanelle are present. The posterior fontanelle is located between the hypoplastra and xiphiplastra, as is characteristic of O. leios and O. liaoxiensis ( Brinkman & Peng 1993b; Tong et al. 2004). The posterior lobe is short and subrectangular with a rounded terminal. The xiphiplastron is stout, in contrast to the elongate condition in Ordosemys spp., sinemydids and other "macrobaenids". The plastral scutes are indistinct on these disarticulated plastral elements.

Vertebral column: The vertebral column is represented in the holotype by two isolated cervicals and at least 22 isolated caudals. One cervical is damaged and is located against the posterior margin of the skull. The other is exposed in lateral view. It bears a single transverse process located on the anterior portion of the centrum. The centrum is opisthocoelous and keeled ventrally. The neural spine is reduced. Two cervical ribs are preserved in PMOL-AR00140, but are isolated from the associated cervicals. The rib is small and has a triangular shape.

The proximal caudal vertebrae are not preserved. The remaining caudals are opisthocoelous and the neural spines are strongly reduced. The transverse processes are present in the most anterior five caudals, but absent in the succeeding caudals. Several small chevrons are present but poorly exposed.

Appendicular skeleton: Only parts of the left humerus, ulna and radius are exposed. In contrast, the hindlimbs are well exposed on both sides, except for the proximal portions of the femora, which are covered by the shell. The tibia and fibula are 28 and 27 mm long, respectively, but the former is distinctly more massive than the latter. The tarsus is well exposed on the right side, but somewhat displaced from its original position. The tarsus is composed of six circular elements. Of these, the astragalocalcaneum is the largest element, and situated adjacent to the distal end of the tibia. Metatarsals I–IV are elongated. Metatarsal I is more robust but shorter than the other metatarsals, while metatarsal IV is more slender than the remaining metatarsals. The fifth metatarsal is large and subcircular. The phalanges are well preserved on the both sides, showing a formula of 2-3-3-3-2+. The first digit is the shortest. The third and fourth digits are elongated and subequal in length. The phalanges decrease distally in length along the digits.

Figures

FIGURE 1. Map showing the type locality (marked by an asterisk; 40 º 42´8 ʺN; 119 º 41´13 ʺE) of Liaochelys jianchangensis gen. et sp. nov. in Lamadong, Jianchang County, Huludao City, western Liaoning Province, China.

FIGURE 2. Liaochelys jianchangensis gen. et sp. nov. (PMOL-AR 00160, holotype), from the Early Cretaceous Jiufotang Formation of Lamadong, Jianchang City, western Liaoning Province, China.

FIGURE 3. Line drawings of Liaochelys jianchangensis gen. et sp. nov. (PMOL-AR 00160, holotype), from the Early Cretaceous Jiufotang Formation of Lamadong, Jianchang City, western Liaoning Province, China. Abbreviations: as, astragalocalcaneum; c 1 - c 8, costal plates 1 – 8; cave, caudal vertebrae; cs, cervical scute; cve, cervical vertebrae; lfe, left femur; lfi, left fibula; lh, left humerus; lt, left tibia; lu, left ulna; m 1 - m 12, marginal scutes 1 – 12; mtV, metatarsal V; nu, nuchal; p 1 - p 11, peripheral plates 1 – 11; pdI-pdV, pedal digits I – V; ps 1 - ps 4, pleural scutes 1 – 4; py, pygal; rfe, right femur; rfi, right fibula; rt, right tibia; sp 1 - sp 2, suprapygals 1 – 2; v 1 - v 5, vertebral scutes 1 – 5; 1 - 8, neural plates 1 – 8.

FIGURE 4. Liaochelys jianchangensis gen. et sp. nov. (PMOL-AR 00140, paratype), from the Early Cretaceous Jiufotang Formation of Lamadong, Jianchang City, western Liaoning Province, China.

FIGURE 6. Carapaces of Ordosemys leios (A), Liaochelys jianchangensis gen. et sp. nov. (B), and Manchurochelys manchoukuoensis (C). (A) and (C) modified from Brinkman & Peng (1993 b) and Zhou (2010). Scale bar equals 20 mm.

FIGURE 7. Plastron of Liaochelys jianchangensis gen. et sp. nov. (PMOL-AR 00140, paratype), from the Early Cretaceous Jiufotang Formation of Lamadong, Jianchang City, western Liaoning Province, China, in ventral view. The line drawings are reversed from their opposites. Abbreviations: ab, axillary buttress; cf, central fontanelle; hyo, hyoplastron; hyp, hypoplastron; ib, inguinal buttress; lf, lateral fontanelle; pof, posterior fontanelle; xi, xiphiplastron.