During the early development of turtles and other amniotes, the parabasisphenoid, or basisphenoid s.l., is formed by at least two centers of ossification: the endochondral basisphenoid s.s. and the dermal parasphenoid. This fusion is usually so dramatic that the two elements cannot be distinguished from each other in the adult stage. Here, we describe the basicranium of two species of Mesozoic turtles from Europe, Plesiochelys etalloni and Pleurosternon bullockii, partly using micro-CT (computer tomography) scans, and show that in both taxa para- and basisphenoid remain distinguishable throughout life. We also identify the extent that each of the two elements has contributed to the formation of the braincase floor. Because the structure of the parabasisphenoid determines the course of the internal carotid artery into the skull, our findings allow us to discuss the early evolution of the carotid pattern and the turtle basicranium in new detail. By surveying the main patterns of carotid circulation in extinct and extant turtles, we bring new evidence to the idea that it was largely the ossification of the parasphenoid that, along with the closure of the interpterygoid vacuity and the posterior extension of the pterygoids, shaped the internal carotid patterns as seen in modern turtles.

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