COPROLITES OF PREDATOR AND PREY DINOSAURS IN THE AGUJA FORMATION OF TRANS-PECOS TEXAS

Twenty-four coprolites were recovered from the surface of a ridge in the Upper Shale Member of the Aguja Formation west of the Big Bend National Park, Texas. All specimens were partially exposed in a single horizon of muddy siltstone two meters above a rich bonebed. None appears to have been disturbed, and all are well-preserved in outward appearance. Specimens range in weight from 5.6 g to 4.8 kg and in volume from 2.0 ml to 1.8 l. The coprolites were mostly in groupings of two to four specimens separated by several meters and scattered over an area of approximately 150 m². The largest coprolite was found in isolation. Other fossils discovered on the coprolite horizon were sections of gymnosperm logs, a tree stump, and two poorly-preserved, hadrosaur, caudal centra.

All of the coprolites contain impressions of leaves and small sticks on their surfaces. Most contain sticks that penetrated the surface. The impact surfaces are flattened to a minor degree. One coprolite appears to have folded over a branch. Some appear pinched off. A surprise was the discovery of a smaller coprolite nestled precisely into a groove atop a large coprolite. The much larger coprolite, the first extruded, is highly vesicular while the smaller one lacks this texture. Thin sections of most of the coprolites reveal ingested leaves and woody material. One thin section contains bone.

This discovery places herbivores and carnivores on the same surface within a short time frame. Such mingling is consistent with the discovery of grooves and punctures inflicted by crocodiles and carnivorous dinosaurs on ornithischian bones in the underlying bonebed. Ingestion of woody materials evidenced in the coprolites may indicate a dearth of nitrogen-rich food. The presence of only minor flattening on the impact surfaces suggests ejection of coherent rather than more fluid masses. Of unknown significance is the presence of the highly vesicular texture in the initial stage of one movement as this suggests a primary gaseous phase followed by extrusion that was much less so.