The shell of the oldest true turtle (Testudinata) branch (Proterochersidae) from the
Late Triassic (Norian) of Poland and Germany was built in its anterior and posterior
part from an osteodermal mosaic which developed several million years after the plastron,
neurals and costal bones. We provide the most detailed description of the shell composition
in proterochersids to date, together with a review of the shell composition in other
Triassic pantestudinates. A scenario of early evolution of the turtle shell is proposed
based on new data, and the possible adaptive meaning of the observed evolutionary
changes is discussed. These observations are consistent with the trend of shell simplification
previously reported in turtles. Several aspects of proterochersid shell anatomy are
intermediate between Odontochelys semitestacea and more derived turtles, supporting
their stem phylogenetic position. Three additional ossifications were sutured to xiphiplastra
and pelvis in Proterochersis spp. and at least in some individuals the nuchal bone
was paired. The peripherals, suprapygals, and pygal bone are most likely to be of
osteodermal origin and homologous to the proterochersid shell mosaic.
