The palaeohistology of the basal ichthyosaur Mixosaurus (Ichthyopterygia,Mixosauridae) from the Middle Triassic: Palaeobiological implications

Here, we provide the first bone histological examination of an ontogenetic series of the basal ichthyosaur Mixosaurus encompassing postnatal to large adult specimens. Growth marks are present in sampled humeri, a femur, a fibula, as well as in other skeletal elements (gastral ribs). Ontogenetic changes are traceable throughout stylo- and zeugopodial development, but interior remodelling and resorption deleted part of the internal growth record in the primary cortex. Mixosaurus humeri started as flat structures consisting of a core of endochondral woven bone and residual calcified cartilage, whereas growth continued by deposition of periosteal fibrolamellar and parallel-fibred bone. Unlike the fast-growing post-Triassic ichthyosaurs that lack growth marks, microstructural and life history data are now becoming available for a basal ichthyosaur. The high growth rate of Mixosaurus may indicate that higher metabolic rates characterised small, non-thunniform ichthyosaurs, as had been suggested already for post-Triassic, cruising forms.