THE POSTCRANIAL MORPHOLOGY OF MEIOLANIA-PLATYCEPS AND A REVIEW OF THEMEIOLANIIDAE

A series of partial shells and disarticulated shell elements is the ba sis for a nearly complete restoration of the carapace and plastron of Meiolania platyceps from the Pleistocene of Lord Howe Island, New Sout h Wales. The carapace is broadly domed, with C-shaped bridge periphera ls, but is not as arched as in living testudinids. A small cervical sc ale Lies on a protuberant nuchal bone that has two midline articulatio n 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 s mall number of disarticulated specimens. The shell bones tend to be th in 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, shall ow troughs. In most specimens the posterior margin is strongly serrate d with no caudal notch. There are 8 costals and 11 peripherals, but th e number of neurals and the presence of a pygal and suprapygals are un known. The first thoracic centrum faces directly anteriorly and the fi rst 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 scap ula. The epiplastra are relatively broad and bear two pairs of scales, about equal in size, with moderate anterior projections. No other pla stral scale sulci are determinable. The entoplastron is a large, ovoid bone with a long posterior median process that separates the hyoplast ra for much of their length on the dorsal surface. The axillary and in guinal buttresses do not extend onto the costals, and contact peripher als 3 and 8, respectively. The hyoplastron and hypoplastron send digit ate processes to the peripherals producing a thin, partially ligamento us 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 contr ast to the squared off end of the anterior lobe. Small articulation fa cets are present on the xiphiplastra for the pubis. The available shel l material of Meiolania platyceps shows a great range of variation. Mo st shells have an extensive peripheral overhang but some specimens hav e 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 shou lder girdle is a stout element with a wide scapular-acromion angle and without a glenoid neck. The coracoid is wide and flat, flaring medial ly. 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 p roximally which contrasts to the narrower condition in nearly all cryp todires, but is similar to Proganochelys. The articular and surface mo rphology, however, is more similar to baenids and other primitive cryp todires. The ulna, radius, tibia, fibula, and femur of Meiolania are s imilar to those bones in Proganochelys and primitive cryptodires, exce pt that in Meiolania they are generally stockier and more robust, with wider ends. The carpus of Meiolania has seven carpal bones: ulnare, i ntermedium, medial centrale, and four distal carpals. The manus formul a is 2-2-2-2-2 with broad, flat unguals. The tarsus of Meiolania has a n astragalocalcaneum showing no sign of sutures or fusion. Two distal tarsals are definitely known, but four were probably present. The peda l formula of Meiolania is 2-2-2-2-0. A revision of the family Meiolani idae recognizes four genera: Niolamia (Eocene, Argentina), Ninjemys (P leistocene, Queensland), Warkalania (Miocene, Queensland), and Meiolan ia (Miocene to Pleistocene, Northern Territory, Queensland, Lord Howe Island, New Caledonia). A PAUP analysis of 22 characters results in on e cladogram: (Niolamia (Ninjemys (Warkalania, Meiolania))). The relati onships of meiolaniids are analyzed using 17 taxa and 40 characters. W ithin the eucryptodires the shortest cladogram in a PAUP analysis is a s follows: (Plesiochelyidae (Xinjiangchelys (Meiolaniidae (((Sinemys, Dracochelys) Ordosemys) (Chelydridae (Chelonioidea (Trionychoidea, Tes tudinoidea))))))). The data matrix consists of 19 cranial characters, 12 vertebral characters, and 9 shell characters. The cervical vertebra e prove to be particularly significant in resolving the extinct eucryp todires. In this analysis the biconcave caudal, ligamentous bridge att achment, and narrow epiplastra are seen to originate within the extinc t eucryptodires. The family level name Sinemydidae is expanded to incl ude Sinemys, Dracochelys, and Ordosemys.