BackgroundDuring their evolution in the Late Cretaceous, mosasauroids attained a worldwide distribution, accompanied by a marked increase in body size and open ocean adaptations. This transition from land-dwellers to highly marine-adapted forms is readily apparent not only at the gross anatomic level but also in their inner bone architecture, which underwent profound modifications.Methodology/Principal FindingsThe present contribution describes, both qualitatively and quantitatively, the internal organization (microanatomy) and tissue types and characteristics (histology) of propodial and epipodial bones in one lineage of mosasauroids; i.e., the subfamily Mosasaurinae. By using microanatomical and histological data from limb bones in combination with recently acquired knowledge on the inner structure of ribs and vertebrae, and through comparisons with extant squamates and semi-aquatic to fully marine amniotes, we infer possible implications on mosasaurine evolution, aquatic adaptation, growth rates, and basal metabolic rates. Notably, we observe the occurrence of an unusual type of parallel-fibered bone, with large and randomly shaped osteocyte lacunae (otherwise typical of fibrous bone) and particular microanatomical features in Dallasaurus, which displays, rather than a spongious inner organization, bone mass increase in its humeri and a tubular organization in its femora and ribs.Conclusions/SignificanceThe dominance of an unusual type of parallel-fibered bone suggests growth rates and, by extension, basal metabolic rates intermediate between that of the extant leatherback turtle, Dermochelys, and those suggested for plesiosaur and ichthyosaur reptiles. Moreover, the microanatomical features of the relatively primitive genus Dallasaurus differ from those of more derived mosasaurines, indicating an intermediate stage of adaptation for a marine existence. The more complete image of the various microanatomical trends observed in mosasaurine skeletal elements supports the evolutionary convergence between this lineage of secondarily aquatically adapted squamates and cetaceans in the ecological transition from a coastal to a pelagic lifestyle.

Actualistic interpretation (on the basis of disorders documented in life) of pathology in modern turtles, and across the phylogenetic spectrum, allows confident diagnosis of disease in fossil turtles. Fortunately, those diseases that affect bone do so in a manner that appears independent of phylogeny and even of geologic chronology. To date, most pathologies have been reported as isolated observations. Therefore the latter have been characterized and placed within the framework of recognized diseases and activities. Toward this end, we surveyed the literature, assessing reports in terms of contemporary definitions. Epidemiologic studies of pathology in turtles are rare, with the exception of frequency of anomalies and of avascular necrosis and recognition of diving behavior. Notation of avascular necrosis in 50% of Cretaceous turtles, limitation to marine turtles, and its gradual reduction over geologic time present a unique window into turtle behavior and their development of strategies that minimize its occurrence.

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