viernes, marzo 19, 2004



Placerias was a 1 tonne sturdy animal with tusks for rooting through the soil in search of a range of vegetation, and a beak for uprooting plants during the Triassic. A low-slung, powerful animal (3.5 meters long) resembling a hippopotamus, Placerias was not a dinosaur, but a dicynodont or "two-tooth dog". It was so called because it had toothless jaw margins with only the two canine teeth, near the front. Dicynodonts are therapsids, or mammal-like reptiles, a group that includes the ancestors of modern mammals. The best fossil find was at the Petrified Forest at St Johns, Arizona in the USA, where forty individuals were identified. Some palaeontologists believe there is evidence that Placerias lived in a seasonal environment. Wear patterns on the tusks of other dicynodonts show alternate deep grooves and a smoother polishing effect. In the dry season they would have dug hard earth, which scarred the tusks. In the wet season, the ground would be wet and silty, helping to polish them.

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