The early composition and evolution of the turtle shell (Reptilia, Testudinata)

Abstract

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.

Peer Review #2 of "Shell variability in the stem turtles Proterochersis spp. (v0.1)"

“…Another possible solution is seasonal hypercalcifiaction and decalcification of sutures or shell bones that could increase the rigidness of the shell mosaic (see Szczygielski & Sulej, 2019) but also allow seasonal growth -similar mechanism of de-ossification was reported locally in the mid-section of plastron in males of some modern turtles during mating season (Wibbels, Owens & Rostal, 1991;Wyneken, 2001;Pritchard, 2008). Finally, the growth might have occurred mainly by the means of bone remodeling.…”

“…5C,9D,S5I, is the largest complete shell found thus far (approx. 49 cm of carapace length) and thus is treated as an adult, but some fragmentary specimens, such as ZPAL V.39/8 (see Szczygielski & Sulej, 2019), ZPAL V.39/57 ( Figs S1N, S3B), ZPAL V.39/60 ( Fig. S1O-P), and ZPAL V.39/63 indicate that this species could have reach even larger sizes.…”

“…Most specimens of Proterochersis spp., however, are fully ankylosed, regardless of their size. Even if the prevalence of ankylosed specimens in Poręba and localities around Stuttgart may be a preservation or sorting artifact (e.g., the unankylosed specimens were typically destroyed by currents or small fragments of disarticulated unankylosed shells were buried elsewhere; see Szczygielski & Sulej, 2019), the fact that ankylosis occurred even in relatively small specimens with juvenile features (e.g., ZPAL V.39/2, ZPAL V.39/34, ZPAL V.39/66) is more troubling. Many of these small specimens are well-preserved and it is PeerJ reviewing PDF | (2018:06:29084:3:0:NEW 7 Nov 2018)…”

The origin of the turtle body plan: evidence from fossils and embryos

“…() noted that the dorsal migration of the cleithra and their eventual fusion into the nuchal, along with the loss of the scapular rami of the clavicles, led to a wide separation of these originally closely associated bones and reconfiguration of the shoulder girdle in turtles. Finally, Szczygielski & Sulej () hypothesized that the peripherals, suprapygals and the pygal are most likely to be osteodermal in origin.…”

“…205–215 Ma). They include Proganochelys quenstedti , Proterochersis robusta and Keuperotesta limendorsa from Germany (Baur ; Fraas ; Jaekel , ; Gaffney , ; Szczygielski & Sulej , ), Proterochersis porebensis from Poland (Szczygielski & Sulej ) and Palaeochersis talampayensis from Argentina (Rougier et al . ; Sterli et al .…”