Biomechanics of the shell of early turtles

The earliest, Triassic (approx. 220–200 million years) turtles are characterized by a large diversity of bony connections between their plastron, carapace, and girdles – pectoral (modified collar bones) and pelvic. This work tests impact of these connections on the mechanical performance of the shell. It turns out that the role of the girdles in stress distribution and strengthening of the shell was relatively minor. Thus, the diverse connections likely result from ecological and locomotor differences between individual species. The co-ossification of the pelvis and the shell in Proterochersis spp. from Poland and Germany might have increased the stability and effectiveness of swimming in those turtles.

Figure: Stress distribution (VMS) in the shells of Triassic (Proganochelys, Proterochersis) and extant turtles (Erymnochelys – side-necked turtle; Trachemys – hidden necked turtle): dark colors indicate the lowest stress, bright colors – the highest. The scenarios simulate biting of various shell regions (arrows) with attached (A) or detached and removed (D) epiplastral processes (clavicles – Proganochelys) and pelvis (Proterochersis, Erymnochelys).