Series: Bulletin of the American Museum of Natural History (AMNH Bulletins) Volume: 194
Proganochelys quenstedti Baur (= Triassochelys dux Jaekel, Stegochelys dux Jaekel) from the late Triassic Norian of Germany is the oldest well-preserved turtle. This paper describes the complete osteology of Proganochelys for the first time. The skull, shell, cervical vertebrae, caudal vertebrae, girdles, and limbs of Proganochelys are compared with primitive amniotes exemplified by Captorhinus, and with more advanced turtles, the pleurodires and cryptodires. The descriptions of Proganochelys are based on seven specimens, including three skull-shell associations. One of these specimens is an almost complete skeleton of what is hypothesized to be a young adult. Another skeleton is interpreted as a younger juvenile, while the remainder are interpreted as older adults.
Proganochelys has the following chelonian synapomorphies:
(1) Bony shell consisting of a carapace formed from costal bones with fused ribs, neural bones with fused thoracic vertebrae, and marginal bones; a plastron formed from interclavicle, clavicle, and five paired bones sutured together; carapace and plastron enclosing shoulder girdle and pelvic girdle.
(2) Quadrate concave posteriorly and exposed laterally on cheek.
(3) Postparietals and postfrontals absent.
(4) Lacrimal bone small.
(5) Maxilla, premaxilla, and dentary edentulous.
(6) Stapes solid, rodlike, without foramen or processes.
Proganochelys is hypothesized as the sister group to all other turtles. It retains the following primitive amniote characters absent in other turtles.
(1) Supratemporal bone.
(2) Lacrimal bone and duct.
(3) Moveable basipterygoid articulation.
(4) Middle ear without bony lateral wall.
(5) Vomer paired.
(6) Paroccipital process of opisthotic attached to braincase only at its distal end.
Proganochelys has the following characters that are interpreted as autapomorphies.
(1) Ventral tubercle on basioccipital.
(2) Tail club.
(3) Phalangeal formula (manus and pes) of 2-2-2-2-2.
Interpretations of the depositional environment of Proganochelys combined with features of the limb morphology suggest that Proganochelys occupied fresh water as a bottom walker but was not exclusively aquatic or terrestrial.
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