A GASTROLITH CLUSTER IN THE STERNAL AREA OF THE SAUROPOD DINOSAUR MOABOSAURUS UTAHENSIS

Gastroliths are a contentious topic. Many workers consider them to be ventifacts or fluvially polished stones, except in rare cases when they are found in specific positions within skeletons. In some formations, highly polished chert pebbles are often abundant.

Here, we report the rare discovery of a mass of gastroliths from BYU’s MayRae Dinosaur Quarry in the Yellow Cat Mbr of the Cedar Mountain Fm near Arches National Park. The mass consists of over 100 stones in a tight, oval cluster 1250 x 800 mm, located adjacent to a pair of articulated sternal plates, an ulna, and several dorsal ribs, all pertaining to the sauropod, Moabosaurus utahensis. The close association of this mass with sternal plates and other bones indicate soft tissues, including part of the digestive tract, were present at the time of burial.

The average gastrolith size is 62 mm in maximum dimension; the largest measuring 105 mm and the smallest 24 mm. Most of these gastroliths exhibit a high degree of polish, with one bearing two grades of polish, with only high points on a “flat” surface being rounded, suggesting the stone fractured while in the gizzard. Some, however, are relatively rough with small pits as a function of their lithology. Marks on the gastroliths include mm-long, shallow, “chattered” linear grooves (attributed to contact with rough gastroliths), small crescentic cracks (impact marks), and dull concavities with radiating fractures (post depositional sediment compaction-related strain features).

All the gastroliths consist of chert derived from late Paleozoic marine carbonates (based on fossils in the chert), sourced from the rising Sevier Plateau and transported into the basin via major fluvial complexes, such as the Buckhorn Conglomerate.

The MayRae Quarry extends over 120 m and gastroliths are somewhat common in the bonebed. They, like the bones in the quarry, are hydrologically “out of place” in the quarry’s mudstone matrix. We propose that these isolated, polished stones with gouge scratches and crescentic fractures were utilized by sauropods, passed through their digestive track, and accumulated over millions of years. The erosion of the mudstone units leaves behind these cobbles as a lag on the slopes. We conclude that the highly polished chert cobbles are true gastroliths. Not simply stream polished stones.