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Monday, 12 January 2009

The Dinosaurs of Bristol Zoo

As everyone knows, about 65 million years ago there was a catastrophic mass extinction, which resulted in the extinction of the dinosaurs and the rise of the mammals. There is only one problem, it is not actually true. In fact, there are dinosaurs all over the world today, including your back gardens, and one of the largest surviving species are at Bristol Zoo, the cassowaries.

Ok, let me explain. It has been known for some years that birds evolved from dinosaurs, but recent discoveries from China have produced beautifully preserved fossil specimens showing feather and soft part impressions, and has enabled a much clearer picture to be formed of the origin of birds and their relationship with their closest relatives.

One of the most significant discoveries is that feathers, at least of the hair-like forms found on cassowaries and emus, actually developed at an early stage, and were universal in the later bipedal carnivorous dinosaurs. Large species are known from fossil skin impressions to have had leathery or scaly skin, but this is simply because large animals do not need to retain body heat and do not need insulation. Small species, and possibly juveniles of large species, probably looked very bird-like in life. For example, the early tyrannosaur Di long (about the size of one of our Dwarf Crocodiles), had distinct feather impressions on the tail of the fossil that was found, and after examination of the arm bones it became clear that Velociraptor had feathers on its arms as the bones had the attachment points for large feathers.

The care of the young was also very bird-like in these small dinosaurs. A fossil has been found of a raptor still on its nest incubating its eggs (it was caught in a sandstorm), and at least some post-hatching nest care is very likely.

So if these dinosaurs looked liked birds, what more can we discover from their descendants?

Scattered over most of the southern hemisphere are a group of mostly large, flightless birds (and some recently extinct species) which together belong to the group known as the ratites. This group, including the Ostrich, the two South American Rheas, the several species of Kiwi, the Emu and the three species of cassowaries, together make up the oldest branch on the family tree of modern birds, and probably split off from the lineage leading to more advanced species well before the end of the age of dinosaurs. In size they are in fact towards the top end of the more bird-like therapods (the Velociraptor in real life stood about 1 metre tall and probably ate animals no bigger than rabbits or chickens)

In fact, you could look at our cassowary and the only distinctive feature that sets it apart from later therapods is the loss of the tail, which like all birds is reduced to a set of fused bones supporting the tail feathers. Otherwise, you could put them in a line up with Cretaceous dinosaurs and not notice a major difference.

The question has been asked in the past as to how the ratite species became so widespread, and there are two scenarios proposed. One is that the common ancestor of all ratites was flightless and the various species evolved as the continents drifted apart. The other is that the common ancestor could fly became flightless later.

It is at this point that a group of pheasant or quail-like birds of South America enter the picture. The tinamous have several features in common with the ratites, and have long been recognised as a side branch of the same stock. Recent genetic work however has identified them as belonging to the same branch that gave rise to the cassowaries, emu and kiwis, and this implies that these birds at least dispersed by flying and became flightless later. It might seem surprising that the nearest relatives of the cassowary live in the Andes, but before continental drift split the continents and climate change made Antarctica freeze over all the southern continents were joined in a temperate and well vegetated southern supercontinent called Gondwana. Many animals ranged over the whole supercontinent, which is why today both South America and Australia have marsupials.

One other interesting fact about the ratites – most of them (including the tinamous) have a nesting system where the male does a great deal, or in some species all, of the incubation and chick care. This must date back to the origins of the group, which means that if we found a fossil ratite from over 100 million years ago we could make a good guess as to how it looked after its young.

Which I think is pretty cool.

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