United States
£ GBP
A series of partial shells and disarticulated shell elements is the basis for a nearly complete restoration of the carapace and plastron of Meiolania platyceps from the Pleistocene of Lord Howe Island, New South Wales. The carapace is broadly domed, with C-shaped bridge peripherals, but is not as arched as in living testudinids. A small cervical scale lies on a protuberant nuchal bone that has two midline articulation facets on its ventral surface for the 7th and 8th cervical vertebrae. The bones of the shell are usually fused with sutures present in a small number of disarticulated specimens. The shell bones tend to be thin in the central areas with the peripherals and other marginal areas thicker. The bone surface is coarsely textured with irregular grooves, pits, and foramina.
The scale sulci are usually poorly defined, shallow troughs. In most specimens the posterior margin is strongly serrated with no caudal notch. There are 8 costals and 11 peripherals, but the number of neurals and the presence of a pygal and suprapygals are unknown. The first thoracic centrum faces directly anteriorly and the first thoracic rib reaches the axillary buttress on the third peripheral. In the plastron the epiplastra meet on the midline and bear a short median process, apparently not homologous to that in Proganochelys and Kayentachelys, that bifurcates dorsally and articulates with the scapula. The epiplastra are relatively broad and bear two pairs of scales, about equal in size, with moderate anterior projections. No other plastral scale sulci are determinable. The entoplastron is a large, ovoid bone with a long posterior median process that separates the hyoplastra for much of their length on the dorsal surface.
The axillary and inguinal buttresses do not extend onto the costars, and contact peripherals 3 and 8, respectively. The hyoplastron and hypoplastron send digitate processes to the peripherals producing a thin, partially ligamentous contact with numerous fontanelles. Mesoplastra are absent. A large, irregular, median fontanelle is present at the junction of hyoplastra and hypoplastra. The posterior plastral lobe tapers strongly in contrast to the squared off end of the anterior lobe. Small articulation facets are present on the xiphiplastra for the pubis. The available shell material of Meiolania platyceps shows a great range of variation. Most shells have an extensive peripheral overhang but some specimens have almost no overhang. The shell sulci are usually broad and indistinct but in some specimens they are finely incised with raised edges. The limbs and girdles are completely known in Meiolania platyceps and are represented by both articulated and disarticulated specimens. The shoulder girdle is a stout element with a wide scapular-acromion angle and without a glenoid neck. The coracoid is wide and flat, flaring medially.
The pelvis has large but widely separated thyroid fenestrae and a small epipubic process. The ilium flares slightly dorsally. There are two sacral ribs and, usually, the rib of the first caudal is fused to the second sacral. The humerus in Meiolania is expanded distally and proximally which contrasts to the narrower condition in nearly all cryptodires, but is similar to Proganochelys. The articular and surface morphology, however, is more similar to baenids and other primitive cryptodires. The ulna, radius, tibia, fibula, and femur of Meiolania are similar to those bones in Proganochelys and primitive cryptodires, except that in Meiolania they are generally stockier and more robust, with wider ends. The carpus of Meiolania has seven carpal bones: ulnare, intermedium, medial centrale, and four distal carpals. The manus formula is 2-2-2-2-2 with broad, flat unguals. The tarsus of Meiolania has an astragalocalcaneum showing no sign of sutures or fusion. Two distal tarsals are definitely known, but four were probably present. The pedal formula of Meiolania is 2-2-2-2-0. A revision of the family Meiolaniidae recognizes four genera: Niolamia (Eocene, Argentina), Ninjemys (Pleistocene, Queensland), Warkalania (Miocene, Queensland), and Meiolania (Miocene to Pleistocene, Northern Territory, Queensland, Lord Howe Island, New Caledonia).
A PAUP analysis of 22 characters results in one cladogram: (Niolamia (Ninjemys (Warkalania, Meiolania))). The relationships of meiolaniids are analyzed using 17 taxa and 40 characters. Within the eucryptodires the shortest cladogram in a PAUP analysis is as follows: (Plesiochelyidae (Xinjiangchelys (Meiolaniidae (((Sinemys, Dracochelys) Ordosemys) (Chelydridae (Chelonioidea (Trionychoidea, Testudinoidea))))))). The data matrix consists of 19 cranial characters, 12 vertebral characters, and 9 shell characters. The cervical vertebrae prove to be particularly significant in resolving the extinct eucryptodires. In this analysis the biconcave caudal, ligamentous bridge attachment, and narrow epiplastra are seen to originate within the extinct eucryptodires. The family level name Sinemydidae is expanded to include Sinemys, Dracochelys, and Ordosemys.
