Proganochelys quenstedtii, Baur, 1887, Baur, 1887

Szczygielski, Tomasz, Klein, Nicole, Słowiak-Morkovina, Justyna & Scheyer, Torsten M., 2023, Limb histology of the Triassic stem turtles Proterochersis porebensis Szczygielski & Sulej, 2016 and Proganochelys quenstedtii Baur, 1887 with insights into growth patterns of early turtles, Comptes Rendus Palevol 22 (32), pp. 635-665 : 644

HUMERAL HISTOLOGY OF PROGANOCHELYS QUENSTEDTII

Most of the humeral cross section of SMF 09-F2 (about 77% of the lateromedial diameter) is formed by a spongious inner medullary region ( Fig. 2J, K), which is typical for all turtles and independent of their respective life style (e.g. Nakajima et al. 2014). The cortical thickness varies between approximately 1.8 mm and 3 mm. The “pure” cortex (i.e., without any erosion or remodeling) is difficult to measure due to scattered erosion cavities and secondary osteons (see below). The medullary region consists of endosteal bone and irregularly formed medium-sized and small erosion cavities ( Figs 2J, K; 8G, H). The medullary region grades into a perimedullary region ( Figs 2J, K; 8 C-F) where erosion cavities and endosteal bone are intermixed or grade into, respectively, large secondary osteons. Smaller scattered secondary osteons reach far into the outer cortex. Only the outer third of the cross section displays primary cortex ( Figs 2J, K; 8 A-F). This periosteal tissue consists generally of in overall low vascularized parallel-fibered tissue, locally even grading into lamellar tissue. The inner preserved cortex is subsequently followed or partially intermixed with the perimedullary region. It shows less organized and less strongly vascularized parallel-fibered tissue. Small longitudinal and few reticular to radial simple vascular canals, as well as some longitudinal primary osteons occur, which are not ordered in a clear pattern. The number of osteocyte lacunae is low and they remain small and flattened throughout. At the dorsolateral bone side, the highly organized, poorly vascularized outer cortex is followed by local regions of less organized parallel-fibered tissue, indicating an area of temporary faster growth ( Fig. 8A, B). This area is barely recognizable as a slight rugosity on the bone surface. It likely represents a kind of localized pathology, potentially a small surface exostosis ( Rothschild et al. 2012). The outer cortex contains 6-7 growth marks, indicated by thin annuli in the inner cortex and LAGs (lines of arrested growth) or multiple closely spaced rest lines in the outermost cortex. This resembles, at least locally, an outer circumferential layer (OCL)/ external fundamental system (EFS; Ponton et al. 2004), but is not deposited all around the outermost cortex of the cross section. Growth marks in the inner cortex, which would represent earlier ontogenetic stages, are already lost due to remodeling. Extensive secondary remodeling is also revealed along the bone, including the area of the ossification center, by CT data ( Fig. 7I, J). There is no medullary cavity proximal or distal to the thin-sectioning plane. Based on the increase in tissue organization and the nearly avascular condition and increase of the number of growth marks in the outer cortex, growth rate was clearly reduced, and the specimen likely represents an adult individual close to or already fully-grown. This late ontogenetic stage of SMF 09-F2 is also supported by humerus and carapace lengths that are in the upper range of other known individuals of Proganochelys quenstedtii ( Gaffney 1990; Scheyer et al. 2022).