Williams 1 Scott Williams Dr. Parker IFS 2087 Dinosaur Paper 11-7-15 Eoraptor: Discovery, Fossil Information, Phylogeny, and Reconstructed Life Abstract In 1991 Ricardo Martinez found a fossil of a dinosaur in Argentina that had not been seen before. After analysis of the fossil, a team of scientists including Sereno, Forster, Rogers, and Monetta named it Eoraptor in 1993. Eoraptor is thought to be one of the earliest forms of dinosaurs with many theorizing that it is the common ancestor to the more evolved forms of dinosaurs including Theropods and Sauropods. Eoraptor shares a lot of the most basic traits of both of these dinosaur classifications. The most important of these traits is hip structure and the heterdont dentition that Eoraptor possesses. Eoraptor s hip structure makes it a saurischian due
Williams 2 to the shape of its illium, ischium, and pubis. Eoraptor s jaw contains sharp serrated teeth as well as rounded teeth. Therefore, Eoraptor could be a common ancestor of Theropods or Sauropods due to similar lizard hip structure and varied tooth formation. This is part of the reason there is so much debate about what kind of a dinosaur Eoraptor was. Based on this information it is believed that Eoraptor was an omnivore, a fast runner, possibly a scavenger, and it lived in a harsh yet wet climate. Despite controversy about Eoraptor s phylogeny, the organism has given much insight to earlier forms of dinosaurs. Introduction Eoraptor is an important discovery in the dinosaur realm. Discovered in 1991 and named in 1993 it gives insight to how early dinosaurs at that time may have lived. More importantly, Eoraptor could be the common ancestor of both Theropods and Sauropods. If Eoraptor is the common ancestor of both of these dinosaur clades, it can be proven that these types of dinosaurs stemmed from one single ancestor. However, there is a lot of controversy over which dinosaur clade Eoraptor is truly related to. Considering Eoraptor has characteristics of both Theropods and Sauropods it is hard to make an obvious assessment. Luckily paleontologists have found three fully intact Eoraptor skeletons that give insight into its lifestyle. Eoraptor lived in the Late Triassic a time period that predates many dinosaurs. Eoraptor only grew to one meter long and did not weigh a substantial amount. Skeletal structure points to many suggested life habits such as movement and diet as well as phylogenic clues. Results Eoraptor was discovered in Argentina, specifically The Ischigualasto Formation in 1991 (See figure 1.0). The Ischigualasto Formation is known for its fossils. The fossils range from the
Williams 3 Carnian age of the Triassic Period. Early dinosaurs, fossils of plants, primitive reptiles, and other aquatic organisms have been found here (www.dinosaurfact.net Eoraptor Dinosaur 1). (Figure 1.0) The skull of Eoraptor was found on accident by Ricardo Martinez in a single rock (www.dinosaur-world.com Eoraptor lunensis 1). Paleontologists in the area were actually looking for the more commonly known Herrerasaurus. After Eoraptor s discovery, the fossil was studied and named by Sereno, Forster, Rogers, and Monetta in 1993. The fossil information about Eoraptor is impeccable. In the Ischigualasto Formation, a full Eoraptor skeleton was found. Not only was it a full skeleton but it was discovered in pristine condition (See figure 1.1). Bones of the skull, forelimbs, spine, pelvis, as well as those of the hind legs were discovered (www.dinosaurfact.net Eoraptor Dinosaur 1). Initial observations of the skeletal structure that were important were the teeth, the tibia length compared to the femur, and that despite its small size it was not a juvenile specimen.
Williams 4 (Figure 1.1) This full skeleton discovery indicated many things about Eoraptor. First of all, Eoraptor was a very small dinosaur. It grew to one meter in length and weighed near ten kilograms. Perhaps most indicative of Eoraptor s lifestyle was the research done on its teeth. Eoraptor s jaw contained both serrated and rounded teeth (See figure 1.2). This is called heterodont dentition. Presumably the sharp serrated teeth would be used to tear apart meat while the rounded teeth would be used to chew plant life. According to dinosaurfact.net because of this Eoraptor was either an omnivore or had omnivore ancestors. Whether or not Eoraptor itself was omnivorous, it definitely evolved from an omnivorous ancestor. Eoraptor shared these dental characteristics with Theropods and Sauropods. The sharp serrated teeth were attributed to Theropods and the rounded teeth were attributed to Sauropods. As of 2011 there has been a lot of argumentation over what Eoraptor should be classified as. Many firmly believe that Eoraptor is the common ancestor of only Theropods. Some believe that Eoraptor is the common ancestor of both Theropods and Sauropods. Although Sereno, the one who studied and named Eoraptor, has
Williams 5 most recently classified it as a basal sauropod. Although it does not exhibit the specific distinctions which make a dinosaur a sauropod, it does have similar characteristics to sauropods. One of the main distinctions is the fact that Eoraptor is a probable omnivore. This means that it ate vegetation as well as meat. Eoraptor s consumption of vegetation is a hint at the ancestry to Sauropods. Although, as an omnivore, Eoraptor would also have eaten meat. However, the recent discovery of Eodromaeus, a strict carnivore from the same time period, has led scientists to think Eodromaeus is the ancestor to Theropods rather than Eoraptor. Still, there is no denying there are distant characteristics of both clades in the organism. (Figure 1.2) Eoraptor was a primitive dinosaur that exhibited upright obligate bipedalism. This meant it could only be bipedal. Some dinosaurs had the choice of bipedalism or quadrupedalism but Eoraptor did not have such a choice. Another important physical characteristic of Eoraptor is its tibial length compared to it femoral length. The fact that Eoraptor s tibia was longer than its femur meant that it was a sprinting animal. Eoraptor was fast and had claws that were able to grasp. It was fast and lean. This means that Eoraptor could most likely chase down whatever it ate. Considering its small stature Eoraptor could have been a scavenger as well. Eoraptor also had claws that could grasp vegetation off of bushes and trees meaning it could have been herbivorous as well as carnivorous.
Williams 6 Eoraptor had a partially open acetabulum and five digits on its forelimbs (Martinez et al. 2012), two characteristics incredibly important to classification. These characteristics confirm that Eoraptor is in fact a dinosaur and not some other form of archosaur. Eoraptor also had three sacral vertebrae another common characteristic of dinosaurs (Weishampel et al. 2004 32). The hips are very important to the dinosaur taxonomy of Eoraptor as well. Since it is known that Eoraptor is in fact a dinosaur, it has to be put into one of the two clades the Saurischians or the Ornithiscians. These are distinguished as Lizard Hipped and Bird Hipped. Eoraptor had hips composed of an ilium, ischium, and pubis (See figure 1.3). What distinguishes Saurischians (among other major characteristics; including a grasping hand, asymmetrical fingers, and a long, mobile neck) is the pubis that points downward and forward at an angle to the ischium (Smith, The Saurischian Dinosaurs 1). Eoraptor is clearly a Saurischian judging by this information and its skeletal structure. (Figure 1.3) Many scientists point out the disparity in overall fossil size in comparison to the eye socket size. Based on general growth, large eyes and a small body would be viewed as a juvenile trait. However, Sereno came to the conclusion that Eoraptor was in fact adult due to the fused bones in its vertebrae. The fact that Eoraptor was not a juvenile specimen is important as well. If Eoraptor had in fact turned out to be juvenile it could have been an already found species in a
Williams 7 different state of maturity. However, since it was fully matured, there is no doubt that it was its own species of dinosaur. Eoraptor lived roughly 225 million years ago (USA Today, 225,000,000-Year-Old Dinosaur found. 1993. 5) in the Triassic period. The Triassic period itself is divided into three different sections, the Early, Middle, and Late Triassic. The Early Triassic did not have any dinosaurs as they are thought of today. Most of the Triassic was dominated by archosaurs that were early relatives of dinosaurs. It was not until the Late Triassic that dinosaurs such as Eoraptor would appear. The Late Triassic had three different ages within it, the Carnian (228 Ma 216 Ma), the Norian (216 Ma 204 Ma), and the Rhaetian (204 Ma 200 Ma) (Hamilton, 2015 1). Eoraptor lived during the Carnian which was thought to have a harsh and desert like climate. Although many say the Carnian also experienced monsoon like storms (Kazlev, White, 2015 1). The Ischigualasto Formation in Argentina, where Eoraptor was discovered, is one of the largest Carnian deposits in the world. This area is commonly referred to as the Valley of the Moon due to its topography (See figure 1.4) (Speer 1) and influenced Sereno, Forster, Rogers, and Monetta in giving Eoraptor its full name. Eoraptor lunensis meaning dawn thief from the moon.
Williams 8 (Figure 1.4) While the area of the Ischigualasto Formation today is very barren, it is thought to have been filled with vegetation and water in the Late Triassic Carnian Age (Speer 1). From this information one can speculate about Eoraptor s actual life habit. Eoraptor was small in an age of dinosaurs and other archosaurs that could dominate it. There were larger reptiles such as Herrerasaurus, Coelophysis, and Postosuchus. All of these would have easily been able to kill the small Eoraptor. However, most think Eoraptor was a dinosaur with a fairly high level of intelligence (www.dinosaur-world.com Eoraptor lunensis 1). This suggests that it knew its limits when it came to food. It is unlikely that Eoraptor would have tried to eat a large carnivorous animal like Herrerasaurus. Instead, Eoraptor probably would have gone around scavenging already dead animals. Another theory is that Eoraptor quickly ate the young of other dinosaurs and archosaurs. Although most probable is that it fed on smaller aquatic animals (www.dinosaurfact.net Eoraptor Dinosaur 1). This makes sense considering watery the environment Eoraptor would have been living in. Discussion There is a lot of controversy over Eoraptor s phylogeny. Many think Eoraptor is strictly a Theropod ancestor. Some think Eoraptor could be an ancestor to both Theropods and Sauropods. While others, including Paul Sereno, think Eoraptor is more closely related to only Sauropods. Paleontologists are reconsidering the phylogenic relationship of Eoraptor to Theropods and Sauropods because of the discovery of Eodromaeus. Eodromaeus is a dinosaur from around the same time period and was a larger strictly carnivorous organism. Eoraptor was
Williams 9 not strictly carnivorous or strictly herbivorous. It had the capacity to eat both meat and vegetation. Most likely, Eoraptor was omnivorous which is why many think Eoraptor is the ancestor of Sauropods since they ate vegetation. However, despite Eoraptor s eating habits, Eoraptor is constructed much more like a Theropod. Sauropods are large creatures that need a lot of power in their step to move such copious amounts of weight. Therefore their femur would be longer to produce such power. However, Theropods were fast and agile animals. Their femur did not need to be as long. Eoraptor had a longer tibia than it did femur just like a Theropod. Another thing to take into account is that Eoraptor would have been standing on its toes, a common stance for fast sprinting dinosaurs like Theropods. Eoraptor has many characteristics of both Theropods and Sauropods. This is also why some decide not to put Eoraptor in either group. Instead many classify Eoraptor as simply an early Saurischian. From this skeletal information Eoraptor s suggested life habits can be gathered. Eoraptor would have been a force to be reckoned with if it had only been bigger. Yet Eoraptor was only one meter long and only weighed about ten kilograms. Looking at other organisms at the time like Coelophysis and Herrerasaurus, Eoraptor would have been a puny pitiful creature. However, some theorize that Eoraptor had a fairly high level of intelligence (www.dinosaurfact.net Eoraptor lunensis 1). Therefore Eoraptor would have known not to mess with creatures much bigger than it. Some think Eoraptor was a scavenger due to its size and its ability to sprint fast. This would have given it enough time to sneak in, eat, and run away before anything saw it. However, there is another theory that Eoraptor would quickly run to other creature s nests, devour their young, and then run away making Eoraptor a predator. Perhaps the most accurate assumption is that Eoraptor s diet actually consisted of aquatic life.
Williams 10 Eoraptor lived in an area considered wetlands. If Eoraptor lived in this environment, it would not be surprising if it ate small fish or something similar. Eoraptor could have quickly attacked animals that were distracted by feeding or animals that were either cleaning themselves or drinking from water. If Eoraptor was a small fast sprinter, it could have easily fed in any of the aforementioned ways. Eoraptor also had arms with five digits on its hand. The digits were not shrunken down to the point where they were useless either. Eoraptor had apposable digits that were able to grasp. Therefore Eoraptor could have also been grasping at things to eat such as vegetation. Conclusion Eoraptor could have either been a Theropod or a Sauropod. It displays characteristics of both types of dinosaurs. There is a lot of fossil information available about Eoraptor due to the discovery of three different intact skeletons. Some of this information includes hip structure, varied teeth, long tibia, five digits on the hand, and the fact that Eoraptor is in fact an adult specimen. At most Eoraptor should be classified as an early Saurischian because of its hip structure. Eoraptor is thought to have been omnivorous because of its teeth. Eoraptor was also a fast sprinting dinosaur because of its tibia length. The five digit claw would have allowed it to grasp at vegetation and eat it. And most importantly Eoraptor was an adult. If it had not been an adult it could have been an already discovered dinosaur in a different state of maturity.
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