Askeptosaurus italicus

Klein, N., Sander, P. M., Liu, J., Druckenmiller, P., Metz, E. T., Kelley, N. P. & Scheyer, T. M., 2023, Comparative bone histology of two thalattosaurians (Diapsida: Thalattosauria): ASKeptoSaURUS ItalICUS from the Alpine Triassic (Middle Triassic) and a Thalattosauroidea indet. from the Carnian of Oregon (Late Triassic), Swiss Journal of Palaeontology (15) 142 (1), pp. 1-20 : 5

Askeptosaurus italicus

Vertebrae

Te transverse sections of the two sampled caudal vertebrae of Askeptosaurus italicus (PIMUZ T 4839_vert_a &_b) are highly laterally compressed. Te original morphology of the centra is collapsed, resulting in a thin, elongated rod-like shape of the vertebral sections

( Fig. 1A). In addition, numerous large parallel and smaller intercalated cracks run in a roughly lateromedial direction across the entire vertebrae. Tose cracks are widened and filled by an opaque (i.e., black) mineral phase

( Fig. 1A–G). Furthermore, isolated patches of opaque mineral growth also occur in the periosteal and endosteal tissue ( Fig. 1A–G). Te transition (i.e., neurocentral suture) between centrum and neural arch cannot be identified. Te heavily compressed centrum still allows the identification of a rather thin compacta that encompasses a formerly spongious area. that Te spongious region consists of (now collapsed) secondary trabeculae made of lamellar bone permeated by rare local patches of calcified cartilage ( Fig. 1C). Te slender neural spine is compressed as well, but the original, more compact structure is better preserved here than the spongious structure of the centrum ( Fig. 1B). A rhombic medullary region filled by endosteal bone is surrounded by an avascular and highly organized coarse parallel-fibred tissue ( Fig. 1A, B). Te rest of the compact cortex consists of a poorly vascularized to avascular coarse parallel-fibred tissue that is locally less ( Fig. 1E) or more strongly organized ( Fig. 1G).

Te collagenous scaffolding of the parallel-fibred bone is so coarse and thick that it locally forms a honeycomb-like pattern due to the crossing of the single thick collagen fibers ( Fig. 1E). Locally, Sharpey’s fibers enter the cortex at high angles, and the opaque mineral phase basically follows this arrangement ( Fig. 1D, F, G) (see Konietzko-Meier & Sander, 2013 for similar diagenetically affected fibers). Only very few small, mainly longitudinal primary osteons ( Fig. 1F) and simple longitudinal vascular canals are identified. Te latter as well as osteocyte lacunae are largely obscured by the opaque mineral phase ( Fig. 1F, G). Te cortex is in general less organized in PIMUZ T 4839_vert_b when compared to PIMUZ T 4839_vert_a. However, the latter shows a lesser amount of calcified cartilage. Due to the heavily compacted shape of the vertebrae, overall vertebral compactness cannot be estimated. In the outer cortical layer, bone compactness is nearly 100%.