Palaeohistology of <i>Macrospondylus bollensis</i> (Crocodylomorpha: Thalattosuchia: Teleosauroidea) from the Posidonienschiefer Formation (Toarcian) of Germany, with insights into life history and ecology

The Posidonienschiefer Formation of southern Germany has yielded an array of incredible fossil vertebrates. One of the best represented clades therein is Teleosauroidea, a successful thalattosuchian crocodylomorph group that dominated the coastlines. The most abundant teleosauroid, Macrospondylus bollensis, is known from a wide range of body sizes, making it an ideal taxon for histological and ontogenetic investigations. Previous studies examining thalattosuchian histology provide a basic understanding of bone microstructure in teleosauroids, but lack the taxonomic, stratigraphic, and ontogenetic control required to understand growth and palaeobiology within a species. Here, we examine the bone microstructure of three femora and one tibia from three different-sized M. bollensis individuals. We also perform bone compactness analyses to evaluate for ontogenetic and ecological variation. Our results suggests that (1) the smallest specimen was a young, skeletally immature individual with well-vascularized-parallel-fibered bone and limited remodeling in the midshaft periosteal cortex; (2) the intermediate specimen was skeletally immature at death, with vascularized parallel-fibered bone tissue interrupted by at least 10 LAGs, but no clear external fundamental system (EFS), and rather extensive inner cortical bone remodeling; and (3) the largest specimen was skeletally mature, with parallel-fibered bone tissue interrupted by numerous LAGs, a well-developed EFS, and extensive remodeling in the deep cortex. Macrospondylus bollensis grew relatively regularly until reaching adult size, and global bone compactness values fall within the range reported for modern crocodylians. The lifestyle inference models used suggest that M. bollensis was well adapted for an aquatic environment but also retained some ability to move on land. Finally, both larger specimens display a peculiar, localized area of disorganized bone tissue interpreted as pathological.