ABSTRACT
The Lepidosauria, a clade that includes Rhynchocephalia and Squamata, is a highly diversified group represented today by the Tuatara, Sphenodon punctatus, a rhynchocephalian, and the many limbed and limbless squamates (lizards and snakes). Bone microanatomical and histological features are relatively simple in the rhynchocephalians and the squamates: most often, long bones are tubular and have relatively thick cortices composed of an avascular parallel-fibered tissue with conspicuous cyclic growth marks. The bone histology of large predatory forms, such as monitor lizards (Varanidae) and tegus (Teiidae), differ in several respects from the common condition of other lizards and snakes, especially by the frequent occurrence of well-vascularized cortices. Haversian remodeling is almost nonexistent in rhynchocephalians, lizards and snakes. Among the aquatic Cretaceous lizard group Mosasauroidea, early coastal forms had robust tubular bones and developed osteosclerosis in their ribs as the Aigialosaurs did in their vertebrae. Conversely, large pelagic hydropedal mosasaurs had cancellous amedullary bones. Histologically, all mosasauroids are basically reminiscent of large extant varanids. The case of the strange and heavily pachy-osteosclerotic marine snakelike forms of the Cenomanian is considered. Skeletal microstructures of extinct clades are discussed in functional and adaptive terms.
