Ammosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Anchisauridae

Genus: Ammosaurus

Species: Ammosaurus major

Introduction

Discovered by quarry workers in 1884, Ammosaurus represents one of the earliest dinosaur fossils discovered in the United States. The rear end of the 4-meter (13-foot) specimen was uncovered at a time when little was understood about dinosaurs and, more than a century later, the classification of Ammosaurus continues to be problematic. There is debate among paleontologists as to whether Ammosaurus is a variant of Anchisaurus, an early herbivorous dinosaur. For this reason, most texts treat Anchisaurus and Ammosaurus as the same, or synonymous.

This debate carries over into the animal's cladistic classification. Some scientists believe that Ammosaurus is a prosauropod, a primitive, transitional genus that demonstrates how certain dinosaurs evolved from small bipedal (two-legged) animals into giant quadrupedal (four-legged) beasts, such as Brachiosaurus and Apatosaurus. Critics argue, however, that prosauropods have too many distinct characteristics for that theory to be true. The case highlights the complexities of classification, especially the challenge of attempting to classify extinct animals from incomplete fossil records that are millions of years old.

Classification

Under Linnaean taxonomy, Ammosaurus belongs to the order Saurischia, or “lizard-hipped” dinosaurs, which shared a common arrangement of their pelvic girdle inherited from reptile ancestors. It is further classified with the prosauropods, one of the most poorly understood and widely debated groups of dinosaurs. The group was named Prosauropoda because it was believed to represent an ancestral connection to the more evolved sauropods. The prosauropods were characterized by long necks, small heads, teeth designed for herbivorous grazing, and large pear-shaped bodies that were supported by a mainly bipedal (two-legged) posture. However, the prosauropod group also includes genera such as Ammosaurus and Anchisaurus, the forelimb and wrist structures of which suggest that they probably walked primarily on four legs. Much of the trouble with classifying these dinosaurs is that their remains are significantly incomplete.

The cladistic connection between sauropods and prosauropods remains unresolved. Some paleontologists believe that prosauropods represent the ancestral link to sauropods. Others believe that prosauropods have too many distinct characteristics to be sauropod ancestors, in which case Prosauropoda should be regarded as a sister clade to Sauropoda. A key to confirming the sister clade theory would be the discovery of a common ancestor to both groups. However, the theory was seriously challenged by critics in 2000 when a leg bone and other fragments believed to belong to a primitive sauropod dinosaur were discovered in Thailand. The material, named Isanosaurus, is estimated to be 205 million years old. It is too incomplete to classify properly, but many paleontologists believe that it confirms that sauropods evolved in Asia during the Triassic, in which case an ancestral connection between prosauropods and sauropods becomes unlikely.

Debate also surrounds the distinction between Ammosaurus and Anchisaurus. The two genera are largely considered synonyms, with Ammosaurus regarded as a larger individual variant. There are, however, anatomical differences between the two genera, primarily in the pelvis and hind foot. This means that Ammosaurus may yet be reinstated as a valid species. Ammosaurus and Anchisaurus are classified in the Anchisauridae clade, a group of prosauropod dinosaurs that are smaller and lighter than their cousins and equipped with forelimbs that are only slightly shorter than their hind limbs. The classification of Ammosaurus is a fine example that cladistic analyses are often subject to competing scientific opinion and prone to change as new specimens are discovered and as science advances.

Anatomy

Ammosaurus is known from incomplete specimens. When the remains were discovered in a quarry in 1884, only the rear part of the animal was preserved. The front section of the animal had already been quarried and set into a bridge. As a result, the descriptions of Ammosaurus and Anchisaurus combine examples from both genera.

Ammosaurus weighed up to 400 kilograms (882 pounds). It stood 2 meters (6.5 feet) tall and would have grown up to 4 meters (13 feet) long. The animal had a long, flexible neck that supported a small head. The body was rotund and the tail was long and flexible. Distinguishing it from most of its prosauropod cousins, Ammosaurus's front limbs were only slightly shorter than its rear limbs, indicating that the animal was equipped to walk on two legs or on four. The hands on the forelimbs had a multipurpose function; the wrist was strong and able to rotate, which allowed for grasping; and the fingers were able to spread, which provided a strong base for walking. The thumb on each hand was equipped with a large claw that may have been used defensively. Large eyes set to the side of the head would also have provided Ammosaurus with a wide range of vision for spotting predators. Ammosaurus's jaw sported blunt teeth with serrated edges that equipped it for browsing on plant material.

Intelligence

With extremely small heads compared with large bodies, creatures in the Sauropodomorpha suborder have the lowest EQ of all dinosaurs. Their EQ score is less than 0.5 (Hopson), compared with 1.0 for a crocodile, up to 2.0 for theropods, and 5.8 for dromaeosaurids. Despite having a low EQ, prosauropod dinosaurs are believed to have had a well-developed olfactory sense. This would have helped Ammosaurus to find food and to avoid predators.

Reproduction and Population

Little is known about Ammosaurus's reproductive behavior or development. Scientists assume that Ammosaurus was oviparous (egg-laying). With the discovery of fossilized remains of sauropod eggs and embryos, scientists were able to confirm that the giant genera had been oviparous, but little is understood about prosauropod reproduction due to a lack of material. If Ammosaurus was oviparous, scientists can only speculate as to whether it provided parental care of its eggs (brooded) or if it was too large for such behavior. It is also not known if dinosaurs cared for their young after hatching. If Ammosaurus exhibited the social behaviors of its sauropod descendants, then it may have lived in herds. Herding behavior may imply that the young were adopted into adult groups, but because little is understood about Ammosaurus's behavior, conclusions about its reproductive habits are based on suppositions.

Diet

Ammosaurus had blunt teeth with serrated edges that equipped it for grazing on plant material. The jaw structure was not developed for grinding, which suggests that these animals cropped plants and swallowed the vegetation without chewing. This may have led Ammosaurus to use its keen sense of smell to locate mainly soft, succulent vegetation. The animal's diet likely consisted of low level and mid-sized plants, such as gymnosperms (non-flowering seed-bearing plants), which included tree ferns, conifers, and cycads. Ammosaurus was also capable of rearing on its hind legs to grasp higher foliage with its multipurpose hands. Digestion was aided by the presence of gastroliths (stomach stones) in the animal's stomach. These small stones, which were swallowed by the animal, helped to grind up food as it passed through the digestive system.

After identifying the remains of a small animal in the stomach of a prosauropod fossil, some scientists propose that prosauropods may have been omnivorous (eating plants and meat). Critics argue, however, that prosauropod teeth and jaw structures were poorly designed for meat eating. They suggest that Ammosaurus and its prosauropod cousins may have been opportunistic feeders that supplemented an herbivorous diet with small animals and insects when possible.

Behavior

Ammosaurus and its more lightly built synonym, Anchisaurus , were probably capable of moving swiftly when required. Fossilized foot prints attributed to Ammosaurus suggest that the animal exhibited an erect, bipedal (two-legged) gait, as well as a quadrupedal (four-legged) posture, while computed tomography (CT) scans of Anchisaurus skulls indicate that it was solely bipedal. Other studies suggest it was among the first sauropodomorphs to become quadrapedal. Certain anatomical adaptations suggest that Ammosaurus was somewhat equipped to deal with predatory attacks.

Large thumb claws on the forelimbs may have been used defensively, while Ammosaurus's large eyes located on the sides of its head and its good sense of smell would have helped the animal to avoid predators. Comparatively small nostrils indicate that sight was probably the animal's most important sense. This may have made Ammosaurus highly alert and cautious.

The behavior of all prosauropods, including Ammosaurus, is largely unknown because this group of dinosaurs is not as well known as are some other dinosaur groups. Scientists can only guess if Ammosaurus lived in social groups, like its sauropod descendants, or if it preferred a solitary existence. Its comparatively lighter frame, moderate level of agility, and alertness may have equipped the animal to survive in solitary circumstances, whereas larger, slower sauropods may have relied on herding behavior to provide protection to the group from predators.

Habitat and Other Life Forms

Ammosaurus remains have been discovered in Connecticut, and an Anchisaurus specimen was located in Western Massachusetts. In the Early Jurassic period, the climate and geography of the northeastern United States was significantly different than it is today. The climate was warm, with minimal seasonal variances. The area known today as Connecticut was then a semi-arid, sandy landscape that was subject to strongly contrasting seasons that included sporadic droughts and drenching rains. Cyclical rock deposits of sandstone and shale (sedimentary rock formed from clay) indicate that the area featured intermittent rivers, floodplains, lakes, and dunes.

Ammosaurus shared its environment with a number of other dinosaur genera, but most are only known from fossilized footprints; skeletal remains are much less common. Trackways suggest that the eastern coast of the United States during the Early Jurassic was populated with multiple genera of prosauropods, small ornithischians, such as Anomepus (known only from its footprints), and carnivorous theropods, such as Podokesaurus.

Research

The remains of Ammosaurus were discovered by quarry workers in 1884. By the time scientist Othniel Charles Marsh was called to the quarry site, however, the front part of the skeleton had already been cut away and encased in a bridge. In the late 1960s, an attempt was made to locate the quarried stone. The search was narrowed to a single bridge where fossil markings were subsequently discovered.

In addition to Anchisaurus major (later Ammosaurus major), Marsh assigned two other skeletons from the same quarry to different species: Anchisaurus colurus and Anchisaurus solus. Their names and relationship would be debated throughout the twentieth century and into the twenty-first. A. solus was deemed a juvenile of A. major in the 1970s, for instance. In 2010 paleontologist Adam Yates concluded the three quarry skeletons, along with a fragmentary Western Massachusetts specimen, all belong to the species Anchisaurus polyzelus, which was a basal sauropodomorph more nearly related to the Melanorosaurus clade than the plateosaurid and massospondylid prosauropods. Yates argued that supposed differences between the quarry specimens could be attributed to factors such as individual size variation and loss of bone matter and damage after death.

Fossil hunting is often made difficult on the East Coast of the United States because the area is densely populated. Nonetheless, the area has made important contributions to the understanding of dinosaurs through examples of fossilized trackways (footprints). These trackways have been instrumental in developing scientists’ understanding about dinosaur posture, locomotion, and behavior.

It often occurs that scientists discover behavioral traces of an extinct organism without discovering any remains of its skeleton. They may discover, for example, fossilized footprints, bite marks preserved in fossilized timber, fossilized burrows, or preserved biological secretions. This evidence can help scientists to understand what the organism may have looked like and how it may have behaved. These fossils are called “trace fossils” and the study of them is called “ichnology.” A zoological name can be assigned to a trace fossil, in which case the taxon is known as an ichnotaxon. For example, Anomepus, also discovered in Connecticut, is an ichnotaxon, as it is the name assigned to the animal's footprints and not to the animal itself.

Analyses in the 2010s found other sauropodomorphs from the American South were not closely related to Anchisaurus/Ammosaurus. Although Seitaad ruessi, unearthed from Utah, had a similarly narrow pelvis, it differed from Anchisaurus/Ammosaurus in the shape of its toes. The 2010 study by Joseph Sertich and Mark Loewen also bolstered the hypothesis that Anchisaurus/Ammosaurus was closer to the sauropods than to prosauropods. Likewise, there was debate among paleontologists as to whether the Sarahsaurus aurifontanalis remains found in Arizona in the late 1990s were more related to Ammosaurus or Massospondylus, a prosauropod known from South Africa. In their 2018 analysis, Adam Marsh and Timothy Rowe concluded it was a massospondylid and, thus, the early North American sauropodomorphs could not all be grouped into a single clade.

Bibliography

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Marsh, Adam D., and Timothy B. Rowe. “Anatomy and Systematics of the Sauropodomorph Sarahsaurus Aurifontanalis from the Early Jurassic Kayenta Formation.” PLOS ONE, vol. 13, no. 10, 10 Oct. 2018, p. e0204007, doi:10.1371/journal.pone.0204007. Accessed 29 Apr. 2020. ‌

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Yates, Adam M. “A Revision of the Problematic Sauropodomorph Dinosaurs from Manchester, Connecticut and the Status of Anchisaurus Marsh.” Palaeontology, vol. 53, no. 4, 19 July 2010, pp. 739–52.