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Tracks suggest chase, capture, and after-meal respite

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11:00am, October 27, 2003
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From St. Paul, Minn., at a meeting of the Society of Vertebrate Paleontology

The still life portrayed in a 315-million-year-old set of fossil footprints discovered in southwestern Indiana is a poignant vignette of life, death, and satiation.

The 1.3-meter-long, S-shaped trackway preserves the footprints left by two different creatures. The left and right feet of the animal that made the smaller set of impressions were spaced about 2.3 centimeters apart, says Joe Monks of Purdue University in West Lafayette, Ind. The tracks left by the larger animal have a left-right spacing of about 5.5 cm and straddle those made by the smaller creature all along the trail of prints. Both sets of impressions are scuffed, which, along with the two tight curves, suggests the animals were moving rapidly, says Monks.

At the end of the trackway, the smaller set of footprints disappears, indicating that the larger animal caught and ate the smaller one. Just centimeters away, the ancient siltstone preserves a so-called resting trace, the impression of a creature's belly and its splayed hind limbs. The distance between the rear feet on that resting trace is the same 5.5-cm spacing of the presumed predator's tracks. A postmeal pause is characteristic of reptiles, which often bask in sunlight to raise their body temperatures and thereby accelerate digestion.

Monks says that all the footprints in the Indiana trackway may have been made by Notalacerta missouriensis, an early reptile whose fossils have been identified from other rocks in that formation. The circuitous route and apparent speed of the chase also hint that early reptiles were active predators and possibly cannibals.

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