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Giant feathered dinosaurs!!

October 2, 2012

But first an aside or two …

The last decade has borne witness to discovery of a spectacular array of feathered dinosaur fossils that have added to our understanding of origins and possible function of feathers, but have also generated some interesting questions about avian and dinosaur evolution.

It has been noted for a long time that birds are probably related in some way to dinosaurs. For example, in the 19th Century many, including luminaries like Thomas Henry Huxley, noted that the morphology of birds mirrored that of some dinosaurs.1  Dinosaurs are a monophyletic assemblage  (i.e. descended from a single common ancestor) that originated sometime in the Triassic, probably about 230 million years before present. Bird origins are suggested to be in the Triassic some time before the famous reptile-bird mosaic species, Archaeopteryx lithographica lived (approx. 150 million years before present). What was most beguiling about the best preserved Archaeopteryx fossils was the feathers, including the flight feathers with a narrow leading edge and wider trailing edge that implied flying ability. The point is that many consider Archaeopteryx to be the first bird with all avian diversity descended from it (a bit of an overstatement to be sure as this could be a side branch to the main avian lineage). Feathers, then, were a unifying attribute of birds, in the parlance of cladistics a synapomorphy or shared derived feature.

This view became less tenable with some spectacular discoveries from fossil beds in China in the 1990s, notably from the Yixian Formation in Liaoning in northeastern China, beginning with a small carnivorous dinosaur, Sinosauropteryx.

Many of these creatures appeared to be reasonably small (e.g. Sinosauropteryx was about a metre long). However in April 2012 Chinese researchers (Xu et al. 2012. A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature 484, 92–95) revealed the largest feathered dinosaur ever, a relative of the fearsome Tyrannosaurus rex. Yutyrannus huali, while not as large as some of its cousins, attained lengths of some 9 metres and weights of over 1000 kg. Important to the discussion here it had long filamentous feathers.

What are the ramifications of such finds? Well certainly it challenges our conceptions of feather function and evolution. But perhaps more relevant to some of the major themes of this course, is that contemporaneous with the “avian” descendents of Archaeopteryx or something akin to it in the lower Cretaceous, were feathered dinosaurs. So it would seem that feathers are not synapomorphies of birds sensu stricto and that feathers are a feature shared with other dinosaur lineages.

Footnote:

1. While birds continue to enjoy the status of vertebrate Class, the reality is that they are indeed allied with dinosaurs and thus should be subsumed within the Class Reptilia (or more radically we should break apart this traditional category all together).

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