Microanatomy and histology of frontal bones in two species of Albanerpeton sensu lato (Lissamphibia: Albanerpetontidae) from the Upper Cretaceous Oldman Formation in southeastern Alberta, Canada

Albanerpetontids are an extinct clade of salamander-like lissamphibians characterised by features such as fused frontals and sculptured skull roof bones. Here we provide a histological analysis of the fused and sculptured frontals in two Late Cretaceous species of the type genus Albanerpeton sensu lato that differ in body size: moderate-sized 'Al.' gracile and larger-sized 'Al.' nexuosum. Despite general similarities in microanatomy/histology (no sutural trace remains between left and right frontals; sculpture formed by appositional growth of bone; predominance of parallel-fibred bone; and lack of growth marks, secondary osteons, and Sharpey's fibres) and the presence of sub-ventrolateral crest canals (possible albanerpetontid synapomorphy), 'Al.' nexuosum differs from 'Al.' gracile in exhibiting: a distinct three-layered diploe structure; bone remodelling; a higher degree of vascularisation; and more numerous osteocytic lacunae. We suggest that (1) histological differences between 'Al.' gracile and 'Al.' nexuosum are related to differences in absolute body sizes and relative development of sculpture and robustness of frontals and (2) the sub-ventrolateral crest canals in albanerpetontids are homologous to the canalis arteriae orbitonasalis in modern anurans. Two features (internal vascularisation and lack of Sharpey's fibres) support a recent suggestion that albanerpetontids may have relied entirely on cutaneous respiration.