SKULLS

The most complete new specimen described here is IGM 90/11 (fig. 2A–J), which included both a nearly complete, articulated skull and mot of the postcranial skeleton of an adult individual. Most of the posterior elements of the cranium are lost and the edges of bones are slightly broken, but the suturai contacts between bones are clearly visible. The skull is slightly crushed toward the left. In general aspect the skull has large orbits facing outward and upward and deep temporal and cheek emarginations. An isolated portion of the cranium preserves most of the quadrate, otic region bones, and the squamosal (fig. 3A–C). IGM 90/55 (fig. 4A–D) represents a partial skull, preserving most of the basicranium and the parietals. Comparisons are made with the two skulls of Mongolemys elegans previously described (Khosatzky and Mlynarski, 1971; Sukhanov, 2000), as well as with other fossil and extant cryptodires.

The prefrontals of IGM 90/11 lack a midline contact, which is precluded by the long and narrow anterior processes of the frontals. This condition was already noted for Mongolemys elegans (PIN 4693) (Sukhanov, 2000: fig. 17.30A), and is also present in some early Cretaceous eucryptodires including Sinemys gamera Brinkman and Peng, 1993, Ordosemys liaoxiensis Ji ,1995, Ordosemys brinkmania Danilov and Parham, 2007, and Manchurochelys manchoukuoensis Endo and Shikama, 1942. In contrast, all extant cryptodires have a midline contact between prefrontals.

The left frontal is slightly broken at the orbit margin, but from the right frontal it is evident that the frontal makes up part of the orbit margin, as reported in specimen PIN 4693 (Sukhanov, 2000: fig. 17.30A) and observed in most other cryptodires. See Joyce (2007: 11) for discussion of the frontal morphology in other Testudines.

FIG. 2.

Mongolemys elegans paratype IGM 90/11, skull (cranium and lower jaw): A–B, dorsal view; C–D, ventrolateral view, removing the left ramus; E–F, ventral view, with the left ramus in place; G–H, lateral view; I–J, medial view of the left ramus and quadrate. Abbreviations: an, angular; ar, articular; bs, basisphenoid; cor, coronoid; den, dentary; fpc, foramen posterius canalis carotici; fr, frontal; fst, foramen stapedius temporalis; ju, jugal; mx, maxilla; pa, parietal; pf, prefrontal; pmx, premaxilla; po, postorbital; ppe, processus pterygoideus externus; pra, prearticular; pt, pterygoid; qu, quadrate; sa, surangular; scm, sulcus cartilaginous meckelii.

Both parietals are completely preserved anteriorly at the contact with the frontals in IGM 90/11. The right parietal preserves the original lateral margin, indicating that the temporal emargination is very advanced anteriorly, leaving the otic chamber uncovered by the absence of a parietal roof. This seems to also be the condition for the other two skulls of M. elegans previously described in the literature (Khosatzky and Mlynarski, 1971; Sukhanov, 2000), and IGM 90/55.

FIG. 3.

Mongolemys elegans paratype IGM 90/11, isolated left otic chamber-quadrate: A–B, anterior view; C–D, medial view; E–F, posterior view. Abbreviations: ex, exoccipital; faf, fossa acustico-facialis; fnt, foramen nervi trigemini; ha, hiatus acusticus; op, opisthotic; pr, prootic; qu, quadrate; sa, surangular; scm, sulcus cartilaginous meckelii; so, supraoccipital; sq, squamosal.

The postorbitals are in medial contact with the frontals and parietals, and ventrolaterally with the jugals. The posterior end of both postorbitals is lost, but based on the marked posterior extension a potential contact with the squamosal is likely, as mentioned by Sukhanov (2000) for specimen PIN 4693.

A small portion of the right quadrate is visible in dorsal view at its contact with the most ventrolateral margin of the parietal, as in specimen PIN 4693, IGM 90/55, and some testudinoids, particularly the geoemydids Cuora amboinensis Daudin, 1802, Rhinoclemmys pulcherrima Gray, 1855, Rhinoclemmys areolata Duméril et al., 1851, Rhinoclemmys funereal Cope, 1875, Rhinoclemmys puntularia Daudin, 1802, Rhinoclemmys rubida Cope, 1870, and Geocle-mys hamiltonii Gray, 1831, and the trionychid Trionyx triunguis Forskal, 1775. The foramen stapedio-temporale is located at the suture between the quadrate and the parietal (as observed in specimen PIN 4693 and IGM 90/55) as a result of a highly reduced dorsal exposure of the prootic, a condition common in testudinoids. On the ventral surface of IGM 90/11, small portions of the right quadrate are visible in two different regions: at its dorsomedial contact with the parietal and at the anteroventral contact with the pterygoid. The most ventral portion of the left quadrate is articulated to the left ramus of the lower jaw and preserved in separation from the rest of the skull (fig. 21–J). The left otic chamber including most of the quadrate, squamosal, prootic, exoccipital, opisthotic, and supraoccipital is preserved as an isolated element from the skull (fig. 3A–F). Medially, the prootic clearly preserves the foramen nervi trigemini and the fossa acustico-facialis. The hiatus acusticus is large, located between the opisthotic, exoccipital, supraoccipital, and the prootic. The processus articularis of the quadrate is long, projected ventrally as in specimen PIN 4693 and all other cryptodires.

FIG. 4.

Mongolemys elegans IGM 90/55, partially preserved skull. A–B, ventral view; C–D, dorsal view. Abbreviations in addition to those in figures 2 and 3: fbs, foramen caroticum basiphenoidale.

Both pterygoids are partially preserved in IGM 90/11, but highly crushed. The left pterygoid preserves the foramen posterius canalis carotici interni, as well as the processus pterygoideus externus, which is very short, as in all testudinoids. A very small portion of the basisphenoid and the prootic are also visible on the ventral surface. IGM 90/55 preserves most of the posteriomedial portions of both pterygoids, which contact the basioccipital posteriorly and the parietals and quadrate laterally. The basisphenoid has a pentagonal shape, exhibiting a small foramen caroticum basisphenoidale at the lateral suturai contact with the pterygoid, which is also present in the specimen PIN 4693. In the stem cryptodires Sinemys lens Wiman, 1930, and Xinjinagchelyidae, the foramen carotium basisphenidale makes part of large fossa located between the basiphenoid-pterygoid suturai contact (see Kear and Lee, 2006: char. 36). All testudinoids lack a ventral expression of the foramen carotium basiphenoidale at the basisphenoid-pterygoid contact.

LOWER JAW

The lower jaw of IGM 90/11 is preserved in articulation with the cranium. On the dentary the sulcus cartilaginis meckelii is deep and ends posteriorly with the opening for the foramen intermandibularis medius. Both dentaries are fused at the symphysis, and the angle between the rami is around 60°. In medial view (fig. 2I–J) the coronoid is very high due in part to crushing. The foramen intermandibularis caudalis is visible in the prearticular, located very close to the suture with the angular. The area of the triturating surface that is visible in dorsomedial view is smooth and very narrow, lacking ridges or toothlike processes. In all of these aspects it resembles the lower jaw of geoemydids and some emydids (e.g., Glyptemys muhlenbergii, specimen UF 85274). In other emydids (e.g., Trachemys scripta, observed in NCSM 74304 and 74305), the triturating surface is wide with elongate shallow fossae. In the dermatemyidid Dermatemys mawii (observed in USNM 66669), the triturating surface is very wide with strong toothlike processes.

CARAPACE

IGM 90/11 (fig. 5A–D) includes a nearly complete shell (articulated carapace and plastron). The carapace lacks only some of the left anterior peripherals and some portions of the costals. There is no evidence of general crushing, indicating the shell had a moderately domed shape as in the holotype of Mongolemys elegans. The size of the carapace (22 cm long, 17.5 cm wide, 7 mm average thickness) indicates a potential subadult individual. There are no keels and the sculpturing of the dorsal surface is principally branching sulci.

FIG. 5.

Mongolemys elegans paratype IGM 90/11, articulated shell (carapace and plastron): A–B, carapace, dorsal view; C–D, carapace, ventral view; E–F, plastron, ventral view; G–H, plastron, dorsal view. Abbreviations: abd, abdominal scale; anl, anal scale; axb, axillary buttress; c, costal; ce, cervical scale; en, entoplastron; ep, epiplastron; fem, femoral scale; gul, gular scale; hum, humeral scale, hyo, hyoplastron; hyp, hypoplastron; if, inframarginal scale; ing, inguinal buttress; isc, ischium; lpp, anal scale m, marginal scale; n, neural; p, peripheral; pi, pleural scale; pub, pubis; py, pygal scale; s, suprapygal scale, v, vertebral scale; xip, xiphiplastron. Missing portions of bone in black, scales in dotted lines. Labels in bold indicate scales.

The nuchal has a convex anterior edge as in the holotype and the specimen figured by Danilov et al. (2002: fig. 1). It lacks a nuchal emargination, which is present but variably developed in other lindholmemydids (Danilov, 1999a). The holotype of M. elegans and the holotype of Haichemys ulensis, figured by Sukhanov and Narmandakh (2006: fig. 2), share a nuchal with nearly parallel lateral margins. This feature was one of the putative diagnostic characters used by Sukhanov and Narmandakh (2006) to define the family Haichemydidae, which includes only the species H. ulensis. There are eight neurals in IGM 90/11. The neurals exhibit the formula (number of sides) 4-6-6-6-6-6-6-6, as in all other lindholmemydids and H. ulensis. Suprapygal 1 is trapezoidal in shape, tapering anteriorly. Suprapygal 2 is hexagonal and very elongated. Eight pairs of costals (completely separated medially by the neurals and the suprapygals), 11 pairs of peripherals, and the pygal together complete the carapace, as in all other lindholmemydids, and most other cryptodires. The cervical scale is very small, rectangular in shape, and slightly smaller than in ZIN. N° RN, T/M–46.1 (M. elegans holotype), and other lindholmemydids. Vertebral scale 1 is the widest of the five vertebral scales, pentagonal in shape, and slightly wider than long. Laterally it covers the most posteromedial portion of peripheral 1, as in the holotype and specimen figured by Danilov (2001 This characteristic is exclusive of M. elegans within lindholmemydids, and is also present in H. ulensis. In all other lindholmemydids and testudinoids vertebral 1 does not cover peripheral 1. The most posterior margin of vertebral 4 reaches the suprapygal 1, which in the holotype and all other lindholmemydids ends over neural 8. Four pairs of pleural scales and twelve pairs of marginals complete the scales covering the bony carapace. The contact between pleural and marginals occurs over the peripherals, as in the holotype and all other lindholmemydids. In testudinoids the pleuro-marginal contact generally overlaps the suturai contact between the costals and peripherals.

On the ventral surface of the carapace, the axillary and the inguinal buttresses and their scars on the costals are well preserved, but weakly developed. The right axillary buttress is out of its original position and placed over the suturai contact between peripheral 1 and 2. The left axillary buttress is still in its original position, indicating a weak development onto costal 1, only reaching about 1/3 of its total length. The development and projection of the axillary buttress onto costal 1 has been used for differentiation within lindholmemydid genera (Danilov, 1999a) defined three different states: (1) weakly developed (1/3 of the costal length) for Mongolemys elegans, (2) moderately developed (1/2 of the costal length) for Gravemys and Khodzhakulemys, and (3) strongly developed (2/3 of the costal length) for Lindholmemys and Hongilemys. The inguinal buttress in IGM 90/11 is weakly developed and restricted to costal 5. Variation is seen in the holotype: on the left side the inguinal buttress is restricted to costal 5, but on the right side the inguinal buttress is located between costal 5 and 6. The former condition is present in Gravemys barsboldi (Danilov, 2003), and in most testudinoids, with some exceptions as for example Chinemys reevesi figured in Joyce and Bell (2004: fig. 110). An inguinal buttress contacting costals 5 and 6 was mentioned by Claude and Tong (2004) as a diagnostic character of Lindholmemydidae. However, based on the new specimens described here, it is clear that there is variation even within individuals and that the most common state in lindholmemydids, as in testudinoids, is an inguinal buttress restricted to costal 5. At the medial region of costal 1, thoracic rib 1 is clearly defined, and resembles the condition for M. elegans figured in (Danilov et al., 2002: fig. 2). The last two thoracic vertebrae and the first two sacrals are visible, and exhibit a relative flat dorsal surface and short lateral ribs. The contact between the marginals and the visceral surface is clearly defined for the entire carapace, and corresponds to a shallow sulcus which deepens slightly at the anterior portion of the carapace, as in the holotype.

PLASTRON

The plastron of IGM 90/11 (fig. 5E–H) is nearly complete, missing only the lateralmost portion of the right hyoplastron. The anterior lobe is wider than long, the posterior lobe is longer than wide, and the bridge is longer than either lobe. In all of these aspects the plastron resembles that of the holotype of M. elegans and Specimen A. The anterior lobe has a nearly straight anterolateral margin, same as in the holotype and also in Gravemys barsboldi, G. hutchisoni, and Elkemys australis (Danilov, 2003; Danilov et al., 2012), these three having a shorter anterior lobe. In contrast, Hongilemys kurzanovi (Sukhanov, 2000) has a rounded anterior lobe margin. The anteriormost portion of the plastron is unknown for Lindholmemys elegans, L. martinsoni, and Amuremys planicostata. The posterior lobe in IGM 90/11 has a slightly rounded posterior edge and lacks an anal notch, as in the holotype, Specimen A, and Lindholmemys elegans. In contrast, G. hutchisoni, G. barsboldi, and E. australis have a tapering posterior lobe, with a deep V-shaped anal notch. The posterior lobe is unknown for A. planicostata and L. martinsoni, and partially preserved anteriorly for Ho. kurzanovi. The presence of an anal notch has been considered apomorphic for Testudinoidea (Claude and Tong, 2004; Joyce, 2007). However, the absence of an anal notch in the lindholmemydids mentioned above and the presence of an anal notch in the lindholmemydids Gravemys spp. and Elkemys australis (see next paragraph) together indicate that presence/absence of this feature cannot be reliably used to separate lindholmemydids from testudinoids.

IGM 90/11 preserves all the plastral scales. The gulars are transversely elongated and contact just the anteriormost corner of the entoplastron as in the holotype, Specimen A, and Ho. kurzanovi. The gulars of Gravemys spp. and “Mongolemys” trufanensis, figured by Danilov (2003: fig. 6), cover the anteriormost portion of the entoplastron. This is also the condition in most testudinoids, with some exceptions as, for example, Rhinoclemmys punctularia (observed in specimens CRI 2871, 2702, and 2514). As in all other lindholmemydids and extant testudinoids the humeropectoral sulcus does not reach the entoplastron. The pectoral scales are slightly smaller than the abdominals, but not so marked as in all other lindholmemydids. The length of the femorals at midline is slightly longer than that of the anals as in the holotype and Specimen A, whereas the femorals are much longer than the anals in all other lindholmemydids. Within testudinoids this character is variable, and in some cases proportions between right and left femoral/anal scales can be very different within a single individual, as observed in Trachemys scripta (specimens NCSM 74304 and 74308). Three inframarginal scales are present and restricted to the hyoplastron and hypoplastron as in the holotype and Specimen A, a characteristic that is exclusive of Mongolemys elegans within lindholmemydids (Danilov, 2003). In all other lindholmemydids the inframarginals extend to overlap the peripherals and, in the particular case of Gravemys spp., “Mongolemys” trufanesis, Paragravemys erratica, and Elkemys australis there are four inframarginals. In IGM 90/11, as in the holotype and Specimen A, the inframarginal 2 is hexagonal in shape, slightly larger than inframarginal 1, and much shorter than inframarginal 3.

On the dorsal surface of the plastron, both pubes are preserved in place, showing a long medial contact between each other and a lateral pubic process, which is ventrolaterally projected, and located far behind the anteriormost tip of the bone. In all these aspects the pubis resembles that of most freshwater testudinoids. A small fragment of the left ischium is preserved and vertically oriented, but no additional features can be described for this bone. Both epiplastra have very incipient epiplastral lips, in contrast to almost all testudinoids which have very thick and marked epiplastral lips.

FIG. 6.

Mongolemys elegans paratype IGM 90/11, long bones. Left femur: A, ventral view; B, posterior. Right humerus: C, ventral view; D, dorsal view. Left tibia: E, dorsal view; F, ventral view. Left fibula: G, dorsal view; H, ventral view.

LONG BONES

Four long bones were found inside the rock sediments that were infilling the shell of IGM 90/11, including a partial right humerus, left femur, left tibia, and left fibula (fig. 6A–H). The distal end and approximately 20% of the diaphysis of the right humerus is preserved, including the ectepicondylar foramen. In ventral view the capitulum and trochlea are medially positioned. The left femur is completely preserved, as well as the left tibia and left fibula. In all aspects the forelimb and hind-limb bones of M. elegans IGM 90/11 resemble those of freshwater testudinoids, and other freshwater turtles in general (e.g., podocnemidids).

GENERAL STRUCTURE

For all three ontogenetic stages of M. elegans, the bone structure of the carapace has three well differentiated layers or bone tissue types, which have been characterized for very large number of Testudines by Scheyer (2007), Scheyer and Sanchez-Villagra (2007), whose terminology we adopt for the descriptions below. In M. elegans, the external and internal cortices are similar in thickness and the cancellous bone layer is the thickest of the three for both the plastron and the carapace.

EXTERNAL CORTEX: The bone surface of the external cortex of juveniles (fig. 9A) is rough, with low vascularization expressed by a low density of widely spaced primary osteons embedded in a matrix of interwoven structural collagenous fiber bundles. Additionally, Sharpey's fibers are very abundant with an almost vertical orientation, indicating the proximity of this costal bone to the next one. In adult stages the external cortex remains rough, exhibits far fewer primary osteons, and shows an increase in the development of reticular bone (fig. 9G, J). In contrast to that of the carapace, the external cortex of the plastron (fig. 9F) exhibits higher levels of vascularization with larger and more abundant primary osteons.

FIG. 8.

Mongolemys elegans, additional material. IGM 90/30, small articulated shell: A–B, dorsal view; C–D, ventral view. IGM 90/22–23, one juvenile and one hatchling associated shells: E–F, dorsal view. IGM 90/31, nearly complete plastron of a juvenile: G–H, ventral view. IGM 90/51, complete carapace and caudal vertebrae series: I–J, ventral view. IGM 90/54, partial carapace showing the complete series of thoracic vertebrae: K–L, ventral view. IGM 90/41, complete right hyoplastron from a juvenile specimen: M, ventral view; N, dorsal view. Abbreviations: abs, axillary buttress scar; brg, bridge; cav, caudal vertebra. (See fig. 4 for remaining abbreviations).

FIG. 9.

Mongolemys elegans, bone macrostructure and microstructure. Photographs of thin sections, all were taken in transmitted light, 4Ox otherwise is indicated. IGM 90/42, hatchling-juvenile specimen. A, external cortex and part of the cancellous bone of a costal; B–C, internal cortex and part of the cancellous bone of a costal; D, internal cortex and part of the cancellous bone of the hypoplastron; E, cancellous bone of the hypoplastron; F, external cortex of the hypoplastron. IGM 90/12, adult specimen. G, general view of the costal, showing the external, internal, and cancellous bone layers, photograph take in transmitted light, 1Ox. H, internal cortex of a costal; I, cancellous bone of a costal; J, external cortex of a costal. K–L, osteocytes in the cancellous bone, both taken in transmitted light, 63x oil lens. Abbreviations: cb, cancellous bone; ec, external cortex; ic, internal cortex; os, osteocyte; po, primary osteon; rb, reticular bone; sf, Sharpey's fiber; so, secondary osteon.

CANCELLOUS BONE: In juveniles the cancellous portion of the shell is highly vascularized, with abundant secondary osteons. These secondary osteons increase in size in adult stages (fig. 9G–I). The secondary osteons in juveniles and adults are composed of an average of seven lamellar bone layers. In the adult stage, the erosion cavities are large, and some of them form elongated channels (filled principally by quartz in this sample). Also in the adult stage at least two generations of secondary osteons are clearly identifiable by cement lines. Cancellous bone layers of the plastron (fig. 9D, E) exhibit the same characteristics described above for the carapace.

INTERNAL CORTEX: The internal cortex is similar in the juvenile and adult stages. It is dominated by avascular parallel-fibered bone (fig. 9B–C). In the adult stage these fibers are slightly rotated relative to one another, creating a succession of light and dark bands (fig. 9G–H) that is characteristic of lamellar bone (Scheyer, 2007). Internal cortex layers of the plastron (fig. 9D) exhibit the same characteristics described above for the carapace.

OSTEOCYTE MORPHOLOGY

Morphological aspects of osteocyte lacuno-canicular networks, as well as the variations in density of osteocytes per volume of bone and according to bone tissue type, can be characterized for M. elegans. Osteocytes in the carapace and plastron of M. elegans show exquisite preservation of three-dimensional morphology hitherto unreported for fossil turtles, including the morphology of the main cell bodies as well as the network of channels for the filopodia. In most cases the filopodial channels reach a third level of ramification. Due to differential mineralization, the osteocytes also exhibit a high variety of colors, which include black, brownish, and orange (fig. 9K). The osteocytes located at the internal cortex are particularly elongated and oriented parallel to the lamellar bone. Those located in the large secondary osteons of the cancellous bone are arranged in a concentric way pattern, thus also maintaining an orientation parallel to the lamellar bone layers. Osteocytes in the external cortex are much less elongated than in the internal cortex, and in most cases they are semicircular. In most aspects the relationships of osteocytes to bone layers in M. elegans conform to the patterns exhibited by other turtles as recently described by Cadena and Schweitzer (2012).

OSTEOCYTES DENSITY PER VOLUME OF BONE (VOD)

The density per volume of bone (VOD) was calculated for each of the three layers of bone, in both juvenile and adult stages, using the same thin sections described above. The result of the VOS analysis are shown in table 2 and figure 11, the raw data for the analysis including thickness of the bone thin sections used to calculate volume of bone, areas, and osteocyte counts are available in appendix 2.

SIMILARITY BETWEEN MONGOLEMYS ELEGANS AND HAICHEMYS ULENSIS

Haichemys ulensis, the single member of Haichemydidae (Sukhanov, 2000; Sukhanov and Narmandakh, 2006), shares two main apomorphies that diagnose Mongolemys elegans: (1)three inframarginal scales, which do not onlap the peripherals, and (2) vertebral 1, which reaches the posteromedial portion of the peripheral 1, causing the nuchal sides to be almost parallel (present in the holotype of M. elegans, but variably parallel or subparallel in other specimens). Variation in the inclination of the lateral sides of the nuchal is common among individuals of the same species in testudinoids. Ontogenetic variations occur, with the nuchals slightly more inclined in adults of, for example, Rhinoclemmys puntularia. Based on the evidence described above, we consider it most likely that all specimens of H. ulensis represent hatchings of M. elegans. However, a conclusive test of the validity of Haichemydidae will be possible only with the discovery of associated skull-shell specimens.

TABLE 2.

Thin sections and their corresponding bone, ontogenetic stage, and VOD (volumetric osteocytes density) for each of the three layers, values given in osteocytes (O) per mm³ of bone.

MONGOLEMYS ELEGANS AND THE HIGHER LEVEL AFFINITIES OF LINDHOLMEMYDIDAE

The Lindholmemydids Mongolemys elegans, Gravemys hutchisoni, Gravemys barsboldi, Hongilemys kurzanovi, Lindholmemys elegans, Lindholmemys martinsoni, and Paragravemys erratica as well as the fossil testudinoids Palaeoemys hessiaca Schleich, 1994, Achilemys cassouleti Claude and Tong, 2004, and Pseudochrysemy gobiensis Sukhanov and Narmandakh, 1976, were added to the character-taxa matrix of Joyce et al. (2011) (see appendix 1).

An initial analysis was run including all taxa and using the same parameters as specified by Joyce et al. (2011): all characters were considered reversible and assigned equal weight, characters (7, 29, 35, 37, 49, 68, 69, 74, 75, 77, 80, 94, 107, 129, 142, and 143) ordered. A heuristic search was performed using PAUP version 4.0a125 (Swofford, 2002) for 1000 replicates, with multistate characters considered polymorphic. A strict consensus of 964,275 most parsimonious trees (fig. 10A) with tree length (TL) = 449, consistency index (CI) = 0.434, retention index (RI) = 0.802, and rescaled consistency index (RC) = 0.384 shows a large polytomy inside Cryptodira involving testudinoids and lindholmemydids.

In the second analysis, we excluded all lindholmemydids except Mongolemys elegans and the three fossil testudinoids that lacked associated skull and shell material. All search parameters were the same as for the first analysis. A strict consensus tree was obtained from 270 most parsimonious trees (fig. 10B), with TL = 447, CI = 0.480, RI = 0.798, and RC = 0.382. Mongolemys elegans is recovered as pantestudinoid, supporting the attribution made by Danilov and Sukhanov (2001) and the result obtained in the phylogenetic analysis of Sterli (2010). Pantestudinoidea is supported here by the presence of an axillary buttress reaching costal 1 (char. 95), and Testudinoidea is supported by the presence of an weak anal notch (char. 96) and two pairs of inframarginal restricted to the axillary and inguinal regions, allowing contact between plastral and marginal scales (char. 109).

FIG. 10.

Phylogenetic hypotheses for the relationship of Mongolemys elegans among cryptodires. A, strict consensus tree including all characters and taxa as in appendix 1. Taxa outside of Cryptodira were excluded for the purpose of this figure. Tree length (TL): 449, Consistency index (CI): 0.434, Retention index (RI): 0.802, Rescaled index (RC): 0.384. B, strict consensus after excluding all lindholmemydids except Mongolemys elegans and the three fossil testudinoids (Palaeoemys hessiaca, Achilemys cassouleti and Pseudochrysemy gobiensis). TL = 447, CI = 0.480, RI = 0.798, and RC = 0.382. Bootstrap values from 1000 heuristic search replicates (left) and Bremer support values (right) are provided above or below the branches they pertain too.

FIG. 11.

Type of bone layer vs. log-transformed value for volumetric osteocytes density (lnVOD(#0/mm3)) for adult, juvenile, and hatching specimens of Mongolemys elegans. Abbreviations: cb, cancellous bone; ec, external cortex; ic, internal cortex.

Our phylogenetic results reaffirm that Lindholmemydidae remains problematic in the context of higher-level turtle phylogeny because all the taxa included in this group, except for Mongolemys elegans, lack complete skeletons, which contributes to a lack of resolution in phylogenetic analyses. Although Mongolemys is supported as a member of Pantestudinoidea, more work is needed to determine whether Lindholmemydidae is monophyletic. The new material described here actually reveals one previously unrecognized cranial similarity between M. elegans and some testudinoids. The presence of a contact between the quadrate and the parietal, which are well exposed dorsally at the roof of the otic chamber and encapsulate the foramen stapedius temporalis, is shared by M. elegans and the Geoemydidae (one of the largest groups of Asian testudinoids). In other testudinoids, however, the prootic generally prevents contact between the parietal and quadrate at the dorsal roof of the otic chamber, suggesting this feature may be convergently acquired.

BONE MICROSTRUCTURE IN MONGOLEMYS ELEGANS

Bone macrostructure provides additional support for the phylogenetic placement of Mongolemys elegans. When this species was originally described by Khosatzky and Mlynarski (1971), it was attributed to Dermatemydidae based on the presence of inframarginal scales. Bone macrostructure of D. mawii and other dermatemydids shows a particular trabecular arrangement in the cancellous bone that is highly organized and apomorphic among turtles (Scheyer, 2007). M. elegans lacks the pattern of cancellous bone organization seen in dermatemydids and instead resembles the diploe structure of external-internal cortices and interior cancellous bone of testudinoids, being particularly similar to the Testudinoid histotype III (Scheyer, 2007) that is seen in Trachemys scripta as well as geoemydids. The bone macrostructure organization of M. elegans shows several features such as similarity in thickness of the external and internal cortices and low vascularization that coincides with ecological category number II of Scheyer (2007), which is typical of semiaquatic to mainly aquatic turtles.

For all three ontogenetic stages studied in Mongolemys elegans specimens, the VOD values proved to be higher in the external cortex than in cancellous bone and the internal cortex (fig. 11). Implications of these variations in VOD values for the different cortices and layers of the bone of the turtles is currently under investigation (Cadena, unpubl. data). An important ontogenetic trend in M. elegans is the notable reduction in the VOD values for all three layers between hatchling and adult stages. A considerably reduction in osteocyte density with the increase of body mass has been documented in mammals, frogs, lizards, and snakes, with implications for general relationships between of metabolic rate and body mass in vertebrates (Cullinane, 2000). Thus, VOD may be useful in the study of fossil organisms, for example, to aid body-mass estimation of large vertebrates when only fragmentary bones are recovered, to establish variations in metabolic rates in fossil taxa with similar body mass, or even infer changes in metabolic rates during ontogeny or through evolution of a particular lineage. Our data from VOD in M. elegans show that turtles also follow the general vertebrate pattern of osteocyte density decreasing over ontogeny.