What is that black stone?

Combining two  prime interests—paleontology and geology—I visited Howard R. Feldman, professor and chairman of Geology and Environmental Sciences at Touro College in Manhattan. His passions are the geology of the Mohonk, NY area plus the paleontology of Israel, Egypt and Jordan where he collects Mesozoic brachiopods. Since paleontologists in and of Israel are themselves almost extinct, I was delighted to make Dr. Feldman’s acquaintance.

For clarity’s sake, let us define Mesozoic and brachiopods.  The Mesozoic  is that geologic period  from 251 million years ago (MYA) to 65.5 million years ago and is divided into three smaller periods—the Triassic (251-199 MYA), the Jurassic (199-145MYA), and the Cretaceous (145-65MYA). To add perspective, dinosaurs began evolving in the Triasssic and became extinct at the end of the Cretaceous. Brachiopods are  marine animals that superficially resemble clams, I was, however,  quickly corrected. Clams are mollusks while brachiopods are Lophophorates because they have a specialized feeding organ called a lophophore. That being said, the easy way to tell them apart is that brachiopod shells are symmetric through the midline while the top and bottom shells are not identical to each other exactly opposite of how clam shells are configured.

But I digress. While looking at the various treasures Dr. Feldman has in his study/workshop, he pulled out what appeared to be a polished black stone and asked if I had any idea what I was looking at. He added that no one had ever identified it correctly and he was willing to offer me some hints. I thought it could only be an effort in futility but without getting any direct hints I was willing to play 20 questions. The outstanding characteristic was that it was highly polished. However, the polishing was not caused by geologic processes such as water, wind, sand, nor was it caused by human activity. All of a sudden  I remembered  “gizzard stones” which some modern animals still retain.  These stones are actually gravel or small rocks that are swallowed whole and remain in an animal’s stomach where they act  as teeth to help grind down fibrous  or tough foodstuffs that the animals swallow without significant chewing. Crocodiles, herbivorous birds, seals and sea lions still have gizzard stones today... But how can one prove that such a stone—recovered from ancient times —is equivalent to something seemingly similar found  among today’s animals.

 In this case, Dr. Feldman explained that the gastrolith—from gastro meaning stomach and lithos meaning stone—was found nestled within the chest cavity of a fairly complete dinosaur skeleton. The other distinguishing characteristics are that the stone should be different of geologic type than  other rocks in the excavation area. The polishing itself is also diagnostic because the action of other gastroliths in the stomach —there can be several present at one time —and tough  vegetation leaves the higher surfaces well polished while the depressions are not. Those who used rock tumblers as kids recognize this polishing pattern. However, because I went the “20 questions route” Dr. Feldman was not impressed enough to anoint me “the first guest to figure this out.” Pity.

Despite some scientific concern that these stones are not gastroliths, let us assume for the sake of discussion that they are. Why are gastroliths necessary now and what role did they play back in prehistory? It turns out that the question is a bit more complicated than it sounds. Digestion is not a simple process and animals have many different ways of dealing extracting nutrients. Hunger causes foraging and eating  and fortunately, digestion takes place out of sight.  There are many factors affecting digestion including what kind of food is eaten and how fibrous it is, is the food swallowed whole or chewed, and what kind of teeth does the animal possess?

For today, suffice it to say that there were huge, herbivorous dinosaurs whose teeth were not designed for chewing, only for stripping leaves off the local plants.  The plant material would go from mouth to esophagus to gizzard where the gastroliths would help the stomach grind up the plant fibers. These stones were taken up by the animals during browsing and were eliminated later either by regurgitation or fecal passage as they became too worn to function further. The ground-up foodstuffs would then empty into another digestive chamber where bacterial fermentation would continue the breakdown of the food into nutrients that could be absorbed by the intestines. 

Whenever we travel I love to collect interesting stones but I have never yet found a gastrolith. Maybe I’ll be lucky one day!

Sura Jeselsohn  lives in Riverdale. Point of View is a column open to all.