Today, dinosaurs are alive in the form of birds. This means that similarities in behaviour should not be shocking, even if they pertain to the odd dietary habit of eating rocks! Termed gastroliths, these are rocks that birds eat today and bird-like dinosaurs ate millions of years ago to aid their digestion. As all birds have toothless, beaked jaws, it is necessary for them to swallow sharp gastroliths, which they store in their gizzard (a specialized stomach) to help them grind up food (Wedel, 2007). Once these rocks are ingested, they are rigorously broken down due to repeated contact with each other, leading to different levels of visible polish when examined (Wedel, 2007). Moreover, the breakdown of gastroliths over time results in smoothened rocks (Figure 1), which can be found in regurgitation and excrement of present-day birds, and fossilized with bird-like dinosaurs.
(Wings and Sander, 2007a).
Gastroliths have been found in the rib cages of toothless theropods, dinosaurs with hollow bones and three-toed limbs; these include ornithomimosaurs (Figure 2), commonly known as “ostrich dinosaurs” (Wedel, 2007). To validate whether ostrich dinosaurs really ate rocks, palaeontologist Oliver Wings let some ostriches, present-day birds known to use gastroliths, feed in an area with an ample distribution of rocks (Wings, 2003). After measuring regurgitated and excreted rock weight, it was found that gastroliths made up between 20 % and 50 % of stomach contents and about 1 % of body mass (Wings, 2003). As similar proportions of gastroliths to body weight were calculated for ostrich dinosaurs, it is highly likely that these ostrich precursors used gastroliths to aid their digestion. In addition, rocks have also been found with sauropods, long-necked herbivorous dinosaurs. Although it is plausible that sauropods would also benefit from voluntarily swallowed gastroliths, Wings and Sander (2007b) have found evidence that the gastrolith phenomenon is limited to the clade of dinosaurs that evolved into birds. According to their findings, the degree of polish was low in rocks found with sauropod fossils and high in those found with bird-like dinosaur fossils, where only a high degree of polish (due to intense breakdown, Figure 1) would indicate use as gastrolith (Wings and Sander, 2007b). Therefore, the paleoenvironmental evidence suggests that only bird-like dinosaurs, such as ornithomimosaurs, swallowed gastroliths. Maybe that’s why, out of all animals, we have only observed birds eating rocks today!
Although the 200 million years that made up the Mesozoic era are most popularly associated with dinosaurs, these times were not only the dawn of the dinosaurs but also that of many plants, such as cycads (Figure 3). It is known that cycads, seed plants that have stout and woody trunks with crowns of stiff evergreen leaves, were one early plant type that dominated vegetation during the Mesozoic era (Stanberg, Stevenson and Hill, 2012). As cycads are composed of rough textures, it is no wonder that the difficulties this may have posed to toothless dinosaurs like ornithomimosaurs resulted in an adaptation involving gastroliths (Stanberg, Stevenson and Hill, 2012). Having eased digestion for toothless, bird-like dinosaurs, the gastrolith adaption from 200 Ma has been conserved until today and is observed in ostriches, chickens, and many other birds.
Birds, like ostriches, swallow rocks to compensate for lack of teeth, and their dinosaur ancestors, ornithomimosaurs, did the same. Throughout the Mesozoic, even the toughest plants such as cycads took a toll against herbivorous dinosaurs, thanks to gastroliths!
References
Engh, B., 2018. Ornithomimosaur. [image online] Available at: <http://dontmesswithdinosaurs.com/?p=2087> [Accessed 13 Jan. 2019].
South African Cycad Species Protection Project, 2012. Cycad. [image online] Available at: <https://www.cycads.org.za/> [Accessed 19 Jan. 2019].
Stanberg, L., Stevenson, D. and Hill, K., 2012. The Cycad Pages. [online] The Cycad Pages. Available at: <http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/foss.html> [Accessed 12 Jan. 2019].
Wedel, M., 2007. Gastroliths. [online] University of California Museum of Paleontology. Available at: <http://www.ucmp.berkeley.edu/taxa/verts/archosaurs/gastroliths.php> [Accessed 12 Jan. 2019].
Wings, O., 2003. The function of gastroliths in dinosaurs – new considerations following studies on extant birds. Journal of Vertebrate Paleontology, 23(3 suppl.), p.111A.
Wings, O. and Sander, P.M., 2007a. Granite breakdown. [image online] Available at: <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2197205/>[Accessed 12 Jan. 2019].
Wings, O. and Sander, P.M., 2007b. No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and functionin ostriches. Proceedings of the Royal Society B: Biological Sciences, 274(1610), pp.635–640.