A PROPOSED METHOD FOR RECOGNIZING SELECTED PALEOGASTROLITHS USING A SCANNING ELECTRON MICROSCOPE

Gastroliths are stomach stones associated with certain extant and extinct animals (e.g. birds, reptiles and mammals). Paleogastroliths, as defined in this study, are stomach stones associated exclusively with extinct animals (e.g. dinosaurs and plesiosaurs). Definitive recognition of paleogastroliths is problematic and controversial. Polished, rounded stones found within skeletal remains of dinosaurs and plesiosaurs are the strongest evidence for genuine paleogastroliths. This study examined proposed dinosaur paleogastroliths using a Scanning Electron Microscope (SEM) to determine if any surface textures occur on the stones that could be used to support the interpretation as genuine paleogastroliths. If unique surface textures exist, these could be used as a technique for identification of proposed paleogastroliths outside of skeletal remains.

Proposed paleogastroliths from the skeletal remains of a Seismosaurus were obtained and examined using an SEM. The samples are mostly highly polished stones composed dominantly of chert, impure chert, and quartzite. The stones were observed at magnifications that ranged from 100x to 400x. The Seismosaurus stones exhibited a micropitted texture, with the cherty stones being significantly smoother than the quartzite stones. All of the cherty samples contained sets of parallel to sub-parallel, curved, polish grooves. In places, the sets of polish grooves crosscut one another. Polish grooves were not noted on either the impure chert or quartzite samples. We suggest that the sets of polish grooves formed as a result of muscle manipulation in the stomach; small particles lodged in the muscle acted in much the same manner as a polishing cloth on the stones.

In conclusion, sets of polish grooves were observed on all of the cherty samples and not noted on the impure chert and the quartzite samples. Therefore, we suggest that the presence of sets of polish grooves on cherty stones found within skeletal material is probable evidence for genuine paleogastroliths. Consequently, the recognition of sets of polish grooves on suspected paleogastroliths outside of skeletal remains would also provide supporting evidence for genuine paleogastroliths.