MESOZOIC AVIAN BONE MICROSTRUCTURE - PHYSIOLOGICAL IMPLICATIONS

We report on the bone microstructure of the Late Cretaceous birds Pata gopteryx deferrariisi and members of the Enantiornithes. These birds r epresent the most primitive birds ever studied histologically. The occ urrence of growth rings indicating alternating periods of slowed and f ast growth suggests that these basal birds had slower growth rates, an d differed physiologically from their modern relatives. Our findings a lso call into question previous ideas suggesting that nonavian theropo ds developed a full avian degree of homeothermic endothermy, which was later inherited by birds. On the contrary, our findings suggest that birds developed classic endothermy relatively late in their phylogenet ic history.