Ophiacodon

O. mirus restored with a terrestrial lifestyle.

Ophiacodon most likely lived on land, but paleontologists have sometimes thought that it was semi-aquatic. An aquatic habitat for Ophiacodon was first proposed by paleontologist E. C. Case in 1907, although he later dismissed the idea. Anatomical features suggesting that it spent much of its time in the water include broad claws that seemed to be adaptations for paddling, thin jaws and numerous small teeth that seemed to be adapted for eating fish, and weakly developed bones, which are seen in many other secondarily aquatic tetrapods. In 1940, paleontologists Alfred Romer and Llewellyn Ivor Price proposed that hindlimbs with a greater length than forelimbs was another aquatic adaptation of Ophiacodon, supposedly because the hindlimbs would have been used to propel it through water. Several of these features are no longer thought to be evidence of an aquatic lifestyle; for example, broad claws are seen in most early tetrapods, even those that are known to have been almost exclusively terrestrial, and the long hindlimbs of Ophiacodon would not have been an effective means of propulsion because the feet were still relatively small and had little surface area over which to form a paddle. Analysis of the vertebrae of Ophiacodon indicate that it was most likely terrestrial and spent little time in water.[1] A paleobiological inference model for the femur[8] likewise suggests a terrestrial lifestyle for Ophiacodon,[9] even though the rather thick cortex might also suggest amphibious, rather than truly terrestrial habits.

Skeletons of Ophiacodon show a fast growth pattern called fibrolamellar bone (FLB), suggesting at least partial warm-bloodedness. The FLB pattern is also found in birds and mammals.[10]