Tuesday, October 2, 2012

My Pet Dinosaur

Were I allowed by some chance of time and nature - and my tolerant wife - to keep a pet dinosaur, then this particular dinosaur would be high in the running for my choice. I have, however, no name to offer it, because science has not given it one. The reason for this is that it is known only by its tracks: not a single fossilized bone has been discovered which can be associated with these tracks.

Such fossils are known as trace fossils, because they are the indirect traces which an organism has left behind, rather than being actual fossilized remains. But trace fossils can still tell us much. If they are animal tracks, how was it walking - on two legs or on four? How fast was it travelling? Was it alone, or in a group? And of course tracks will also tell us with reasonable certainty whether it was herbivorous or carnivorous, and, depending upon the strata in which the tracks were found, how long ago it lived.

Tracks which cannot be associated with a specific animal are given their own name, and that is the case with our little dinosaur here. The dinosaur's name remains unknown. The tracks have been named by science as Atreipus, after their 19th century discoverer, Atreus Wanner. These little footprints (my drawing below, about life-size) have been known from eastern America's Connecticut Valley for some time, and we can deduce that they were made by a small herbivore walking on all fours: the prints of both fore- and hind feet are clearly visible. In fact, the tracks are so small that this dinosaur's body size could not have been much larger than that of a domestic cat's. It's style of locomotion is described as being 'habitually quadrupedal', which is just a way of saying that walking on all fours was the usual thing for it to do.

Reconstructing an animal from its tracks alone is a major challenge in itself, but these delicate, almost dainty tracks drew me to them, and I wanted to know how this little dinosaur might have appeared in life. The skin patterns and colours of my painted reconstruction above are conjectural, but the feet and limbs - long hind legs and shorter front legs - and the walking stance, are highly probable, and are what the Atreipus tracks themselves indicate. At some time in the very early Jurassic, around 205 million years ago, this unknown little dinosaur wandered over what is now the Connecticut Valley, living out its life, and in its wake leaving these modest tracks as our only record of its passing.

Paul E. Olsen and Donald Baird: The ichnogenus Atreipus and its significance for Triassic biostratigraphy: in K. Padian (ed.), The Beginning of the Age of Dinosaurs, Faunal Change Across the Triassic-Jurassic Boundary, Cambridge University Press, New York, 1986, p. 61-87.

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