Birds are dinosaurs: Simple answer to a complex problem

Auk, The, Oct 2002 by Feduccia, Alan

The real challenge presented by those early Cretaceous Chinese fossils is to determine exactly what is being preserved in those unique lake sediments. The so-called "hairy devil" pterosaur Sordes (Upper Jurassic lake deposits of Kazakhstan) is preserved in similar lacustrine deposits and preserves structures remarkably similar, if not identical, to dino-fuzz (Wellnhofer 1991). Yet, the finest-grained lithographic limestone, the Archaeopteryx bearing Solnhofen beds, do not preserve dino-fuzz on the small coelurosaur Compsognathus, a very close ally of the early Cretaceous Chinese Sinosauropteryx, which exhibits a halo of that material.

One must explain also why all theropods and other dinosaurs discovered in other deposits where integument is preserved exhibit no dino-fuzz, but true reptilian skin, devoid of any featherlike material (Feduccia 1999), and why typically Chinese dromaeosaurs preserving dino-fuzz do not normally preserve feathers, when a hardened rachis, if present, would be more easily preserved.

There are clearly two different taphonomic phenomena in the early Cretaceous lacustrine deposits of the Yixian and Jiufotang formations of China, one preserving dino-fuzz filaments, as in the first discovered, so-called "feathered dinosaur" Sinosauropteryx (a compsognathid), and one preserving actual avian feathers, as in the feathered dinosaurs that were featured on the cover of Nature, but which turned out to be secondarily flightless birds. The only other preservation of feathers on those Chinese fossils is in the abundant beaked bird Confusiusornis, other enantiornithines, and in a newly described seed-eating bird, Jeholornis prima, which exhibits some dromaeosaurid-like features in the tail and secand claw (Zhou and Zhang 2002). There are also asymmetric flight feathers preserved on the wing and near the hind limbs of a dromaeosaurid (Norell et al. 2002, Czerkas et al. 2002). Given the now substantial evidence that certain taxa once thought to be dinosaurs (e.g. Caudipteryx, Protarchaeopteryx, and the Oviraptosauria; Maryanska et al. 2002) are most likely secondarily flightless birds, and the new hypothesis that certain dinosaurs were secondarily flightless descendants of Mesozoic birds (Paul 2002), we must now carefully consider the possibility that there may have been a number of radiations of secondarily flightless Mesozoic birds that evolved morphologies quite similar to theropod dinosaurs.

This view is now endorsed by Czerkas et al. (2002) who describe the Chinese early Cretaceous Cryptovolans pauli, characterized by the presence of asymmetric, primary flight feathers, avian hand, and sternum, but with typical dromaeosaur-like teeth, a hypertrophied second sickle claw, and dromaeosaurlike stiffened tail. Such a specialized, derived tail is virtually the same in rhamphorhynchoid pterosaurs of the Jurassic.

In a series of papers, Lowe (last paper 1944) proposed that ratites and small coelurosaurian dinosaurs shared a common ancestor, and that the ancestors of ratites never acquired flight. Of course, we now know that this is wrong (Feduccia 1999), but one can only ponder how the postcranial remains of any ratite would be identified if recovered from the early Cretaceous of China. Too, given the view that "the smallest dinosaur is the bee hummingbird ... found only in Cuba," (Norell et at. 1995), it follows that any bird, flighted or flightless, discovered in Mesozoic deposits, would be classified as a dinosaur.