What A Bone Can Tell

Due to their excellent preservation, the plateosaur finds from Frick, Switzerland have provided some interesting insight in the evolutionary biology of dinosaurs.

Until some twenty or thirty years ago, there was a general consensus that dinosaurs, a group descended from reptiles, were cold blooded just as present day reptiles are. This view changed drastically in the last couple of decades and it was stipulated that at least the farthest evolved dinosaurs could have been warm blooded.

> Were plateosaurs cold blooded? Copyright painting by Raúl Martín.

In the last ten years or so, the osteology (bone structure) of many dinosaurs has therefore been studied in order to find out more about the physiology of dinosaurs (dinosaur bones preserved well enough to display microscopic structures are still rather rare). The microscopic bone structure of cold blooded and warm blooded animals differs in that cold blooded animals have a lamellar bone structure mirroring the seasonally different growing rates due to changing ambient temperatures. Warm blooded animals on the other hand have a more uniform bone structure, mirroring steady growth rates throughout the year.

It turned out that dinosaurs have a bone structure that ressembles the one of present warm blooded animals such as birds or mammals rather than that of cold blooded reptiles. The interesting point is that judging from their bone structure, apparently even some of the oldest dinosaurs already displayed a warm blooded physiology.

In the last years, the new paradigm therefore was that dinosaurs probably descended from reptiles which (like the predescessors of mammals) had already evolved a warm blooded physiology.

A study by Dr. Martin Sander on the bone structure of the plateosaurs from Frick which was published in the december 2005 issue of “Science“ for the first time revealed apparently cold blooded bone structures in dinosaurs.

> Microscopic cross section through a plateosaur bone. Copyright photo: Dr. M. Sander.

So the Frick plateosaurs could proove that a warm blooded physiology was actually developed independently several times in evolution; in mammal-like reptiles and mammals but also in different groups of early dinosaurs. It seems that some of the earliest dinosaurs such as plateosaurus were rather cold blooded after all.

On a related note it is interesting that the importance of these bones was only discovered recently. When the bone structure of the Frick plateosaurs was first fotographed some ten years ago (at the time I also took some photos from the bones I found at “my“ site), it was not surprising to see an apparently cold blooded structure in these ancient dinosaurs. At the time the bone structures of dinosaurs was largely unknown and many questions on bone structures had yet to be answered in present animals. So then, we were rather disappointed in what we saw. Only after some ten years of studying bone structures and discovering that any other preserved dinosaur bones (including triassic forms) looked warm blooded did we actually understand that our original find was interesting after all...