SUMIDA, S.S.*; DEVLIN, K.R.: Features of Transitions between Vertebrate Major Groups of Terrestrial Vertebrates

The advent of cladistic analysis has provided more rigorous means of defining lineages. Interpretations of the stratigraphic range or nature of the transition depend on the use of crown-clade or node-based approaches. In seeking common patterns, we restrict ourselves to transitions that: (1) characterise what have traditionally been referred to as “classes” of vertebrates; and (2) reflect, as least in part, significant environmental changes. The beginning and end of the water to land transition (origin of tetrapods and origin of amniotes), and the dinosaur to bird transition may each be characterised by the exploitation of a new niche (or niches), be they physiographic, physiological or both. Most transitions provided profound new opportunities for habitat use, food acquisition, and predator avoidance. Features of transitions between major groups may be obvious from fossils, only indirectly evident, or not at all discernible. Respective examples include: the development of weight-bearing limbs in basal tetrapods (obvious), addition of a slow-opening phase to prey capture in basal amniotes (indirect evidence from morphology), and potential refinement of endothermy related to pre-existing insulating structures (primitive feathers inherited from maniraptoran theropod dinosaurs) in basal birds. This last transition is a matter of some recent media attention, but no controversy about bird origins actually exists, as all phylogenetic analyses exclude Longisquama from close relationship with birds, its possession of “feathers” remains contentious, and many the arguments to support its supposed relationships to birds are those previously used to reject a theropod relatiionship with birds. Supported by the CSU Tyrannosaurus project.