ECOPHYSIOLOGICAL INFLUENCES ON THE METAPLASTIC TISSUES OF END-CRETACEOUS PAN-TRIONYCHIAN TURTLE SHELLS

Most aquatic turtles use their shell as a calcium reservoir to combat blood acidosis during anoxia. However, modern soft-shelled turtles like Apalone employ extrapulmonary respiration to avoid the metabolic processes that induce acidosis. This reliance on normoxic water is reflected in their unremodeled bone histology, a trait traceable in fossil trionychids. Although most turtles survived the K/Pg extinction, some taxa, such as Helopanoplia and Gilmoremys, vanished. To assess the selective significance of extrapulmonary respiration, this study analyzed the paleohistology and ornamentation of 200 Pan-Trionychian shells across the K/Pg boundary.

Shell texture was quantified using Surface Topographic Analyses, including Orientation Patch Count Rotated and Relief Index, while over 25,000 osteohistological data points (e.g., zonal thickness, porosity, vascular orientation) were analyzed. These metrics were statistically compared against variables like latitude, stratigraphy, lithology, ontogeny, phylogeny, and K/Pg survivorship.

Distinctly ornamented shells had pits lacking Sharpey’s fibers, suggesting dense cutaneous capillary beds, and exhibited minimal remodeling, indicating reliance on cutaneous respiration. Unlike Apalone, which utilizes cloacal/buccopharyngeal respiration, these turtles relied heavily on cutaneous respiration. Trionychids with pronounced ornamentation and minimal remodeling went extinct at the K/Pg boundary, likely due to normoxic dependence becoming a liability in anoxic freshwater systems following photosynthesis collapse.

Recovery was rapid; trionychids from ~9,500 years post-extinction show only modest remodeling differences compared to later Paleogene specimens, indicating near-complete freshwater ecosystem recovery by this time. This study highlights how shell morphology is deeply shaped by physiological and ecological factors beyond ontogeny and phylogeny.