Riojasaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Melanorosauridae

Genus: Riojasaurus

Species: Riojasaurus incertus

Introduction

Riojasaurus was a 3-ton sauropod from the Late Triassic period. It may have been one of the first large herbivorous (plant-eating) dinosaurs to evolve. Sauropods, which flourished in the Jurassic, were a group of large herbivorous dinosaurs that walked on all fours and had long necks, long tails, and small heads. As a sauropod, Riojasaurus may be a distant relative of sauropod giants such as Apatosaurus and Brachiosaurus, but the exact connection between Riojasaurus and its potential sauropod cousins is widely debated.

To date, Riojasaurus is classified with the prosauropod dinosaurs. Prosauropoda is a group of dinosaurs that were originally believed to be the ancestors of sauropods. Critics argue, however, that prosauropods have too many distinct characteristics for that theory to be valid. They contend that Riojasaurus, with its pillar-like legs and hollow spaces in the vertebrae of its spine, should be removed from the Prosauropoda infra-order and grouped with Sauropoda.

The classification of Riojasaurus and other prosauropods has not yet been resolved, and the case highlights the challenges of dinosaur classification, especially the difficulty of attempting to classify extinct animals from incomplete fossil records that date back more than 200 million years.

Classification

Scientists classify dinosaurs in different ways. The Linnaean system is a hierarchical classification system that narrows organisms into order, genera, and species. In the late twentieth and early twenty-first century, paleontologists moved towards cladistic or phylogenetic classification systems to classify dinosaurs according to their common ancestry and shared physical characteristics.

According to Linnaean taxonomy, Riojasaurus is classified with the prosauropods, one of the most poorly understood and widely debated groups of dinosaurs. Belonging to the infra-order Prosauropoda, the group was originally believed to represent an ancestral connection with the more evolved sauropods, but is now regarded as a parallel lineage. 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 Riojasaurus and Melanosaurus, whose front and rear legs were of nearly equal proportion, indicating that these animals had a quadrupedal (four-legged) posture. Much of the trouble with classifying these dinosaurs is that their remains were buried more than 200 million years ago and only a few partial specimens have been uncovered. A nearly complete Riojasaurus skeleton was unearthed in 1969.

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 latter theory would be the discovery of a common ancestor to both groups. In 2000, a leg bone and other fragments believed to belong to a primitive sauropod dinosaur were discovered in Thailand. The partial skeleton, which was named Isanosaurus, was estimated to be 205 million years old and too incomplete to classify properly. However, many paleontologists believe the discovery confirms that sauropods evolved in Asia. They assert that the Isanosaurus genus refutes any ancestral connection between the prosauropods and sauropods.

Conjecture surrounding the classification of prosauropods leads into further classification disputes. Some paleontologists have classified Riojasaurus in the Melanorosauridae clade, consisting of Riojasaurus, Camelotia, and Melanorosaurus, which is the largest prosauropod dating back to the Triassic and Early Jurassic periods, and to which many scientists feel Riojasaurus is closely related. These animals had heavy-limbs, walked on four-legs, and demonstrated hollow spaces in their vertebrae, as seen in more evolved sauropods. For these reasons, many scientists consider melanorosaurids to be the closest relation to sauropods. In fact, some argue that melanorosaurid dinosaurs should be reclassified and removed from the prosauropod clade to be more accurately grouped with sauropods. Riojasaurus is also sometimes classified as a member of the Massapoda clade, which includes the genus Eucnemesaurus, a sauropodomorph discovered in South Africa. The classification of prosauropods is a fine example that cladistic analyses are often subject to competing scientific opinion and that they are prone to change as new specimens are discovered and as science advances.

Anatomy

Riojasaurus is one of the better known prosauropods. Scientists have unearthed fragments from more than 20 individual specimens, including a complete skull. Riojasaurus weighed up to 3.1 tons (6,950 lbs) and stood up to 5 meters (15 ft) tall. Distinguishing it from most of its prosauropod cousins, Riojasaurus's front limbs were only slightly shorter than its rear limbs, indicating that the animal probably walked on four legs permanently. The legs were incredibly sturdy and elephant-like, consisting of dense bone covered in strong musculature. The vertebrae of the spine, however, included hollow spaces as observed in more evolved sauropods. Hollowing of the vertebrae would have significantly reduced Riojasaurus's body weight. Nonetheless, the weight of Riojasaurus's limbs suggests that it was a slow, lumbering animal. In turn, the legs supported a large, rotund body, a long neck, and a small head. Riojasaurus's jaw sported leaf-shaped, serrated teeth that equipped it for grazing on plant material. As with all prosauropods, Riojasaurus has a large thumb claw on the front hands that may have been used for defense.

Intelligence

With extremely small heads compared with their massive 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. Riojasaurus's low EQ further implies that these animals were docile, lumbering giants. Despite having a low EQ, prosauropod dinosaurs are believed to have had a well-developed olfactory sense. This would have helped Riojasaurus to find food and to avoid predators.

Reproduction and Population

Little is known about Riojasaurus's reproductive behavior or development. Scientists assume that Riojasaurus was oviparous (egg-laying). If an ancestral connection is established between the melanorosaurids and sauropods, this would further confirm that assumption because scientists have identified fossilized remains of sauropod eggs and embryos.

Scientists do not definitively know how Riojasaurus reproduced, but the very large size of these dinosaurs suggests that parental care of the eggs, or brooding, would have been difficult. Alternatively, it is possible that adults protected the nesting ground from a distance. It is not known if dinosaurs cared for their young after hatching, but it's likely that Riojasaurus was a herding animal. This implies that the young may have been adopted into adult groups. Scientific investigation of Riojasaurus bones indicates that these animals probably grew quickly after hatching.

Diet

Riojasaurus's jaw sported leaf-shaped, serrated teeth 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 without chewing. This may have led Riojasaurus to use its well-developed olfactory sense to locate mainly soft, succulent vegetation. Digestion was probably 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.

Because Riojasaurus could weigh more than 3 tons (6,600 pounds), its energy requirements meant that it spent the majority of each day grazing. Riojasaurus's high food demand also suggests that these dinosaurs roamed over a wide geographic area. The animal's diet likely consisted of mid-level and tree-sized plants, such as gymnosperms (non-flowering seed-bearing plants) that included ferns, cycads, and Ginkgo.

Some scientists have proposed that the quadrupedal (four-legged) prosauropods, such as Riojasaurus, may have been capable of rearing on their hind legs to reach higher foliage. Proponents of this idea suggest that the animal used its rear legs and tail as a tripod. Critics, however, argue that the large size of these animals, coupled with thick, heavy legs, would have made balancing difficult.

Behavior

Riojasaurus was very likely a docile animal that moved slowly in herds, with an average walking speed of approximately 5 kilometers (3 miles) an hour. Assuming that Riojasaurus exhibited behaviors that were similar to those of its sauropod relatives, it is likely to have sometimes moved in mixed groups. It is not known if Riojasaurus had a simple or complex social structure or whether the species preferred a social or solitary existence. Scientists do know that their large size prevented them from walking quickly. Traveling in numbers, therefore, may have provided protection from predators, especially for the young, and may have helped them to safely cover great distances in search of the large food intake required for their size. Defensively, Riojasaurus was equipped with a large thumb claw on its forelimbs, which it may have used to protect itself from predators. The animal is also likely to have had a good sense of smell, which may have helped it to avoid predation.

Riojasaurus remains have only been discovered in South America, but the genus is believed to be closely related to Eucnemesaurus, a Late Triassic sauropod discovered in South Africa with which Riojasaurus shares a number of leg similarities. The discovery bolsters the theory that the continents were once joined in a single land mass known as Pangaea. Prosauropod dinosaurs are thought to have originated in Africa, from whence they migrated with great effectiveness. Prosauropod remains have been discovered on every continent.

Habitat and Other Life Forms

The climate of the Late Triassic period was warm and humid, contributing to lush forests and fertile plains. Angiosperms (flowering plants) were yet to evolve, but there was an abundance of gymnosperms (non-flowering, seed-bearing plants). These plants included tree ferns, conifers, cycads, and Gingkoes. Riojasaurus lived in modern-day Argentina in South America. The area looked much different in the Triassic period than it does today; South America remained joined with Antarctica, Africa, and North America, and the Andes Mountains had yet to be formed. The geography consisted of low-lying marsh areas, fertile plains, and forested hills. Riojasaurus very likely ranged the plains and inland lake shores in exclusive or mixed herds. Riojasaurus shared its environment with other prosauropods, as well as primitive carnivorous (meat-eating) dinosaur species, such as the 6-meter-long Herrerasaurus and the smaller Eoraptor, both theropods, Pisanosaurus, the oldest known ornithischian dinosaur, and Sillosuchus, a primitive crocodilian.

Research

Argentinean paleontologist José F. Bonaparte discovered the first Riojasaurus fossil in 1969 in the Los Colorados Formation, located in the La Rioja province in western Argentina. One of the most prolific dinosaur discoverers and researchers in the history of paleontology, Bonaparte is largely responsible for stimulating interest in paleontology in Argentina, a region that now accounts for around 10 percent of dinosaur species discovered worldwide. The Los Colorados Formation in western Argentina is a sedimentary rock formation that has produced the remains of numerous prosauropod genera. Fossils dating from the Triassic period are exceptionally rare and valuable for the information they provide scientists about how dinosaurs may have evolved. Today, the western border of Argentina is defined by the rugged Andes Mountain Range, which spans the length of the South American continent, but the range did not begin to form until Riojasaurus had been extinct for nearly a million years.

Researchers David Button, Paul Barrett, and Emily Rayfield studied examined skull and jaw features among sixty-seven sauropodomorphs and, in a 2017 Paleobiology paper, reported functional convergences between Riojasaurus and diplodocids or diplodocines, Yunnanosaurus, Atlasaurus, rebbachisaurids, and dicraeosaurids. Riojasaurus is distinguished from other prosauropod groupings because of the keratin sheath it is strongly presumed to have possessed. It may have had differently shaped teeth as well, suggesting greater specialization for plant eating than other prosauropods of its time.

Another comparative study, published in PLOS ONE in 2017, found that Riojasaurus and other sauropodomorphs matured sexually before they finished growing overall. Differences in their femur structure were also thought to indicate phylogenetic differences in movement among prosauropods and sauropods.

Bibliography

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Cerda, Ignacio Alejandro, et al. “Novel Insight into the Origin of the Growth Dynamics of Sauropod Dinosaurs.” PLOS ONE, vol. 12, no. 6, 27 June 2017, p. e0179707, doi:10.1371/journal.pone.0179707. Accessed 29 Apr. 2020. ‌

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