67.2 Friday, Jan. 6 Functional morphology of the crocodilian and avian integument: Implications for the evolutionary origin of feathers in dinosaurs HOMBERGER, D.G.*; DUBANSKY, B.H.; Louisiana State Univ., Baton Rouge; Louisiana State Univ., Baton Rouge email@example.com
The integuments of crocodilians and birds differ fundamentally in their structure. Crocodilian skin resembles fossilized dinosaur skin in having non-imbricating plate-like or tuberculate scales. Hence, the skin of alligators with its hard-cornified plate-like scales, expandable interscale skin that is rich in elastic fibers, a subcutis (Fascia superficialis) that is composed of obliquely helical collagen fibers, and a complete absence of dermal musculature is an appropriate extant model for the skin of dinosaurs. In contrast, the skin of birds has a very thin, scale-less epidermis, and the adipose dermis and subcutis, which are separated by an elastic membrane, form a hydraulic skeleton for the follicles of contour feathers. A complex layer of dermal musculature underlies the entire skin and forms feather muscles that move contour feathers and sheets of apterial muscles that adjust the tension and placement of feather tracts. Furthermore, the feather muscles comprise not only arrector muscles (like those that move the hairs of mammals and the imbricating scales of lepidosaurians), but also stronger depressor muscles that are derived from the arrector muscles. Biomechanical models and experiments have shown that contour feathers perform their aerodynamic function only if they are integrated into a hydraulic skeleto-muscular apparatus. Hence, feathers could obviously not have evolved from a scaly integument that is devoid of dermal musculature and is moved only passively and by elastic resilience as in alligators and probably dinosaurs.