Birds are Dinosaurs!

Did you know that birds are actually dinosaurs? 

This point is briefly mentioned in one of the very first Mesozoic Mondays post on saurischian dinosaurs, but little information on the subject is given. Here, I'm going to give a brief overview of the idea, what it means, and some of the evidence we have. 

The similarity between reptile skeletons and birds was noticed as early as the 1800s, with Thomas Huxley (one of the initial proponents of evolutionary theory) pointing out that the early bird Archaeopteryx showed transitional features between the two groups. He compared this fossil to dinosaurs that were known at the time, like Compsognathus, in great detail. However, his ideas were not widely accepted at the time. In the early 1900s, the issue was reviewed again, with Gerhard Heilmann (a Danish scientist) again concluding that theropod dinosaurs showed the most similarity to Archaeopteryx and other birds. However, no clavicles (collar bones) were known from dinosaur fossils, which in his mind meant that they could not be related as birds possess a fused clavicle called the furcula (or wishbone). Although the absence of clavicles does not rule out a relationship between the two (features can be lost and gained throughout evolution), we now know that theropods and possibly all saurischian dinosaurs did indeed possess a clavicle. The debate came around again in the 1960s when famous American palaeontologist John Ostrom named the dromaeosaur Deinonychus, and he noticed the similarities again between it and Archaeopteryx. Ostrom solidified the theory in palaeontology, and brought it to the front as the leading theory on the origin of birds.

Comparison of the hands and wrists of Deinonychus (left) and Archaeopteryx (right) shows the similarities between these two. Image copyright John Conway

Although there were major morphological similarities, there was still the major question of feathers. If Archaeopteryx, an early bird, showed fully developed flight feathers, where did they come from? Features do not just appear in animals fully formed: they need to evolve slowly and gradually. Finally, in the 1990's, several amazingly preserved bird fossils from the Early Cretaceous were found in a region of China, and in 1996, the first 'feathered' dinosaur was described. Sinosauropteryx was initially described as a bird (hence the -pteryx portion of the name, which means wing) [1]. Shortly after, however, its similarity to dinosaurs was discovered, and the importance of the fossil fully understood [2]. Although the skeleton itself is not remarkable in terms of dinosaurian anatomy, it is covered in tiny hair-like filaments from the head to to the tail. These filaments are thought to be 'protofeathers', an early evolutionary stage of feathers. 

Artists impression of Sinosauropteryx by Nobu Tamura. Note the patterned tail

Since Sinosauropteryx, numerous theropod dinosaur fossils have been found with filamentous feathers, mainly from China. These include Caudipteryx which has a cool tail fan, Velociraptor, Yutyrannus (an early tyrannosaur) and Microraptor, an odd 4-winged creature that may have been capable of flying, although this is debated. This means that more and more dinosaurs likely had some kind of feathers or filamentous covering, rather than the scaly appearance we see in the media. Look out for feathered T. rex in any scientifically accurate dinosaur portrayals! There is even some evidence that Psittacosaurus, an early relative of horned dinosaurs had some quills derived from early feathers, although this is controversial [3]. In addition to clarifying the story of bird evolution, these feathers can be useful in understanding colouration of these animals as well. Small structures called melanosomes, which house the colouration pigment melanin, can be preserved. This is where the striped pattern of the tail of Sinosauropteryx comes from. This will be described in more detail next week. 

Although the vast majority of palaeontologists now recognise the dinosaurian origin of birds, like all theories, there are a few holdouts that do not accept it. However, the evidence is overwhelming, and each issue that is brought forth by these people has been countered with fossil or developmental evidence. 

So what does this all mean? Birds are dinosaurs! As birds are directly descended from a group of theropod dinosaurs (the deinonychosaurs), they are considered to be dinosaurs in biological terms. Much in the same way that whales are mammals, as they share numerous characters with mammals, and evolved from terrestrial mammals, millions of years ago. 

I know this was a bit brief, but bird evolution is a topic we could talk about for hours. If you're interested in more, there are numerous websites that cover this topic in more detail!

References:

1. Ji, Q. and Ji, S. 1996. On the discovery of the earliest bird fossil in China (Sinosauropteryx gen. nov.) and the origin of birds. Chinese Geology 233: 30-33. 

2. Chen, P. et al.  1998. An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391: 147-152.

3. Mayr, G. et al. 2002. Britle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus. Naturwissenschaften 89: 361-365.