Paleohistology of Cretaceous-Paleogene pan-trionychian turtle shells

At the end of the Cretaceous, a bolide impact wiped out ~75% of life on Earth, but turtles show minimal gross anatomical changes. Herein, we examine the shell histology from trionychid turtles 2 million years before and 8 million years after the extinction event. We collected over 25,000 semi-quantitative and quantitative measurements and statistically compared them against latitude, stratigraphic position, lithologic context, ontogeny, phylogeny, and K/Pg survivorship to better understand the various ways in which each respective variable influences histology. We find that trionychids from mudstones and higher in section were larger and older. Traits hypothesized to be biomechanically relevant, like the plywood-like structure and suture margins, showed minimal change across the boundary, but shells in northern Danian deposits do appear to be selected for biomechanical resistivity. Turtles like Helopanoplia and Gilmoremys had well-vascularized external cortices with deep ornamentation pits devoid of Sharpey's fibers, which likely enclosed a dense vascular capillary bed. These turtles also have more intact primary cortical tissues and smaller medullary regions, meaning that they remodeled their shell infrequently compared with other turtles. Because the shell is used as a calcium storage reservoir to combat metabolic and respiratory blood acidosis, we suggest that the vascular capillary bed nestled among ornamentation may have aided in cutaneous respiration, which in turn lessened reliance on shell remodeling. Helopanoplia and Gilmoremys are among the few trionychids to go extinct at the end of the Cretaceous, suggesting that this adaptation was maladaptive during and after the extinction, though we lack the chronological resolution required to infer intermediate selective mechanisms. Paleocene taxa generally show subtle ornamentation with uniform Sharpey's fiber distribution and abundant remodeling. Specimens from ~9500 years after the K/Pg extinction are only modestly more remodeled compared with later Paleogene specimens, suggesting that freshwater ecosystems had almost fully recovered by this interval.