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The femoral microanatomy of 155 species of extant amniotes (57 species of mammals, 15 species of turtles, 56 species of lepidosaurs, and 27 species of birds) of known lifestyle is studied to demonstrate a possible link between some basic parameters of bone structure and specific lifestyles, as well as phylogenetic relationships between taxa. Squared change parsimony with random taxon reshuffling and pairwise comparisons reveal that most compactness and size parameters exhibit both phylogenetic and ecological signals. A discriminant analysis produces several inference models, including a ternary model (aquatic, amphibious, terrestrial) that yield the correct lifestyle in 88% of the cases. These models are used to infer the lifestyle of three extinct Permian temnospondyls: Eryops megacephalus, Acheloma dunni, and Trimerorhachis insignis. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 644–655.

As aquatic vertebrates increase in size, hydrofoils, which use lift to generate thrust, are increasingly used as propulsors. One factor affecting the magnitude of the lift force is the area of the propulsor. Resistance to cruising and sprints is mainly due to drag, but inertia is important during maneuvers when animals accelerate or turn. The inertia of the body and entrained water, which is proportional to body volume, resists acceleration. Because a thrust that is proportional to surface area is used to maneuver a resistance that is proportional to volume, acceleration performance and maneuverability are expected to decline with increasing size. This trend is ameliorated to some extent by the high swimming speeds attainable by warm-bodied vertebrates and the reduced resistance to acceleration characteristic of the skeletons of dolphins and ichthyosaurs. Maneuvers are essential for capture of elusive prey and avoidance of predators. As they increase in size, aquatic vertebrates use various means to ensure that their prey are less maneuverable than they. These include consumption of increasingly smaller prey relative to predator body size (culminating in filter feeding by the largest aquatic vertebrates); behaviors to concentrate, disturb, and disorient prey; and ambushing or suction feeding that avoid whole-body acceleration. Advantages of warm muscles are seen in the ability of endotherms to take more maneuverable prey than can ectotherms of the same size. Young stages of large aquatic vertebrates could be especially vulnerable to predators; viviparity or spawning in productive patches provides for rapid growth through vulnerable stages.

The periosteal cortex in the shaft of limb bones is described histologically in three ichthyosaurian genera, Omphalosaurus, Stenopterygius, and Ichthyosaurus. The primary periosteal deposits are composed of typical woven-fibered tissue that was accreted as spongy bone in young individuals, and more or less compact bone in older individuals. During growth, the bone tissue was extensively remodeled with a quantitative imbalance between resorption and redeposition. As a result, the cortex was made cancellous, if previously compact, or still more spongy, if already cancellous. This pattern of remodeling explains why compact cortices are generally lacking in the long bones of ichthyosaurs. The presence of woven-fibered tissue strongly suggests that the limb bones, and probably also the body as a whole, had a rapid postnatal growth in ichthyosaurs, that might have been related to a high, “endotherm-like” metabolic rate. This hypothesis bears on the ecological interpretation of the ichthyosaurs: they could have been capable of sustained, fast swimming and long-range movements, rather than being slow-moving creatures as commonly supposed.