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What Can Be Learned from a Fossil Tooth? an Inquiry Into the Life & Diet of a Small Theropod Dinosaur

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What Can Be Learned from a Fossil Tooth? an Inquiry Into the Life & Diet of a Small Theropod Dinosaur What Can Be Learned From A Fossil Tooth? An Inquiry Into The Life & Diet Of A Small Theropod Dinosaur Karolyn Hamlin The Hell Creek and Lance Formations of the animals alive at the time, paleontologists Montana, North Dakota, South Dakota, and strive to understand why the K-Pg extinction Wyoming (Fig. 1) are valuable sources of seemed to affect dinosaurs so heavily Maastrichtian age (Late Cretaceous) fossils. compared to other organisms. Diverse The Maastrichtian is the last age before the vertebrate remains have been found at these well-known Cretaceous-Paleogene (K-Pg) sites, including fish, sharks, amphibians, extinction that marked the demise of the non- mammals, reptiles, and birds. The phrase avian dinosaurs. Studying the fossils of these “non-avian dinosaurs” hints that birds are formations is not only key to understanding considered dinosaurs, the avian dinosaurs, and the effects the K-Pg extinction had on this paper will focus on the group that biodiversity, but understand the biodiversity includes birds, their close ancestors, and near trends leading up to it. Through understanding cousins: the theropods. these trends as well as the characteristics of . Figure 1: Map of the Hell Creek and Lance Formations. (Modified from Johnson, 1992) 53 Figure 2: Recreation of the Late Cretaceous Western Interior Seaway. The Hell Creek and Lance Formations cover areas that were very near water, making fish available as prey animals. (Blakey, 2006) If you have ever been to a museum with a mouths that flat teeth for grinding are useful dinosaur exhibit, you may have seen a for consuming fibrous plant material, and recreation of a large theropod such as sharp teeth for puncturing or shearing are Tyrannosaurus rex or Allosaurus. Indeed, good for eating meat. Beyond these very fossil remains of Tyrannosaurids have been general types, teeth can be grouped for more found in the Hell Creek and Lance specific functions. For carnivores, one very Formations (Longrich, 2008). However, not important function is the method of seizing all theropods are as massive or as well and disabling prey. It has been suggested that documented as T. rex. Two such enigmatic the straight, isosceles triangle shape of taxa are Richardoestesia isosceles and Richardoestesia isosceles teeth would be Paronychodon, which are known only from suitable for catching and eating fish (Sankey teeth. Interestingly, even though only teeth 2008, Longrich 2008). Because of these have been found, these two taxa compose observations, we chose to focus on over half of all small theropod teeth from characteristics of piscivorous, or fish-eating, Maastrichtian microsites (Sankey, 2008). vertebrates. What were these creatures? Why were they Judy Massare’s “Tooth Morphology and so abundant? To answer these questions, we Prey Preference of Mesozoic Marine must first consider what can be learned from Reptiles” established ‘feeding guilds’ for a fossil tooth. ancient marine predators by relating records PREVIOUS STUDIES of stomach contents to the overall shape, robustness, and wear of their teeth. A guild is Teeth can offer numerous clues about an “a group of species that exploit the same animal’s diet. We know from our own 54 class of environmental resources in a similar contrasted. From this research, Massare was way” (Root, 1967) without considering able to make inferences about the diets and taxonomy. Therefore, a feeding guild for ecological niches of the Mesozoic marine predators would include animals that reptile taxa for which no stomach content consume the similar prey items in similar data was available. ways, without necessarily being related. To determine these guilds, Massare used both METHODS quantitative and qualitative methods. For the Quantitative: Using data sets from Sankey, quantitative, two types of ratios were 2008, we calculated basal width to crown measured from the Mesozoic marine reptile height ratios of Richardoestesia isosceles teeth as well as the teeth of modern marine teeth and compared them to ratios from predators such as dolphins and whales for Massare, 1987. Because Richardoestesia comparison. First was the crown height (the isosceles teeth have ovoid cross sections, we height of the tooth not including the root) to averaged the values of cross-sectional gullet width (the distance between the jaws) thickness (CST, Fig. 3) and fore-aft to basal ratio to demonstrate the size of the tooth length (FABL, Fig. 3) for the basal width relative to the size of the mouth. Second was values. We calculated ratios from the data set the crown height to basal width (the width at from specimens where the height, CST, and the bottom of the crown) ratio for measuring FABL were available and certain. Specimens the robustness of the tooth. Qualitatively, where some measurements were not listed or wear patterns were examined to further the teeth were chipped (making the actual determine robustness, as well as to make height uncertain) were excluded in the inferences about what kind of surfaces those calculations. Because Richardoestesia teeth were striking. These ratios and isosceles is only known from teeth, we were characteristics of taxa with known diets unable to calculate the crown height to gullet (either from observation of the modern width ratio. predator’s feeding behavior or fossilized stomach contents) were compared and Figure 3: Diagram of measurements of a ziphodont tooth. Ziphodont teeth are characterized by the presences of denticles, like the serrations of a knife, as well as recurvature and lateral compression. A = Fore-Aft Basal Length (FABL). B = Crown Height. C = Recurvature. D = Cross-Sectional Thickness (CST). E = Denticle Width. F = Denticle Height. (Sankey, 2008) 55 12 Qualitative: We compared the overall tooth Qualitative: Richardoestesia isosceles teeth shape and wear patterns of Richardoestesia are elongated triangles with slightly rounded isosceles to the tooth morphology feeding yet acute apices. The triangular silhouettes of guilds presented in Massare 1987. the teeth (Fig. 4) were most consistent with the Smash guild, and not dissimilar from the RESULTS General and Pierce II guilds (Fig. 5). Quantitative: Values were available and However, all of the guilds had teeth with complete for twenty-two specimens from the circular cross sections, most of which came Richardoestesia isosceles data set. The from conical teeth. Richardoestesia isosceles averaged basal width to crown height ratio teeth differ considerable in this respect, as was 2.9. This ratio falls within the ranges for they exhibit the lateral compression of the General, Pierce II, and Smash feeding ziphodont dentition characteristic to guilds described in Massare (1987). theropods. Figure 4: Small theropod teeth from the Lance and Hell Creek Formations. 1-10, Richardoestesia isosceles; 11-23, Paronychodon lacustris Morphotype A; 32-34, Paronychodon lacustris Morphotype B; 24-31 are bird teeth. (Sankey, 2008) 56 Figure 5: Tooth morphology-prey type guilds. (Modified from Massare, 1987.) Richardoestesia isosceles and fit the Smash, General, and Pierce II morphologies and had similar shape ratios. DISCUSSION of piscivorous vertebrates are being examined The basal width to height ratio of for comparison at UC Berkeley. In addition to Richardoestesia isosceles corresponds to the previous methods, we are looking for tooth morphology feeding guilds called qualitative information on the tooth placement General, Pierce II, and Smash. The General and orientation within the jaw, as well as guild and Pierce II guild are similar in that more detailed accounts of tooth wear. they consist of long yet not delicate teeth Although we do not have any jaw bones for suitable for piercing. They are different in that Richardoestesia isosceles, we plan to see if the Pierce II guild has smooth teeth and the there is consistent tooth orientation and wear General guild has teeth with longitudinal patterns across fish eating vertebrates. In ridges with wear indicative of both piercing addition to this, a trip to the University of and smashing actions. However, Massare Washington, Seattle is planned. Many of the notes that this distinction may not be very teeth identified as Richardoestesia isosceles significant. The Smash guild is characterized from the UC Berkeley collections are in their by small teeth with rounded tips. possession. While there, more detailed Unfortunately, because Richardoestesia accounts of tooth wear will be recorded. This isosceles is only known from isolated teeth, will be compared with the data collected at comparing tooth size relative to the size of the UC Berkeley to potentially make further conclusions. jaw is not possible. CONCLUSIONS Stable Isotope Analysis. Stable isotope analysis is useful for studying aquatic Because of the similarity in relative shape, ecosystems. The basic principle is that the Richardoestesia isosceles teeth would be higher the trophic level (the higher up in the suitable for prey similar to the prey of food chain), the higher the number of heavy metriorhynchids, teleosaurs, plesiosaurs, isotopes in the animal’s body. These heavy pliosaurs, and ichthyosaurs. This includes isotopes can still be detected even after small fish and other small aquatic life. Given millennia. The concept is similar to carbon its tooth shape and environment, it is very dating. The reason this method is good for possible that Richardoestesia isosceles was a aquatic ecosystems is because of the large fish-eater. number of trophic levels within them. In a FURTHER RESEARCH terrestrial
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