Massospondylus

• Kingdom: Animalia
• Phylum: Chordata
• Class: Reptilia
• Order: Saurischia
• Family: Massospondylidae
• Genus:Massospondylus
• Species:Massospondylus carinatus

Introduction

Massospondylus was a large herbivorous dinosaur that lived in Early Jurassic Africa and North America. A member of the prosauropod group, Massospondylus was one of the first dinosaurs discovered in Africa and a forerunner of one of the most successful lineages of herbivores in the Jurassic period.

In the Triassic period, the continents were united in a single land mass known as Pangaea. This massive supercontinent split apart during the Jurassic, giving rise to two smaller supercontinents, Gondwana in the south and Laurasia in the north. Dinosaurs first appear in the Triassic period, spreading across the world and diversifying into a multitude of species.

The prosauropods were one of first types of dinosaurs to evolve. (Scientists debate whether prosauropods were a precursor and eventual contemporary of sauropods, or if prosauropods were a sister group.) Relatives of Massospondylus first emerged during the Triassic period, spreading throughout the globe. By the Jurassic, the prosauropods were one of the most common groups of dinosaurs, comprising more than 90 percent of the fossils uncovered from some sites.

Classification

Scientists use two primary classification systems to place organisms into different categories. Linnaean taxonomy, or “systematics,” classifies organisms based on overall physical similarity, while “cladistics” places organisms into groups called “clades” that represent the evolutionary relationships between species.

Taxonomic analysis places Massospondylus in the order Saurischia based on the “lizard-like” organization of its pelvis (though the longstanding differentiation between Saurischian and Ornithischian dinosaurs has come under much debate). From there it is placed in the suborder of long-necked, herbivorous dinosaurs called Sauropodomorpha. The infraorder Prosauropoda is reserved for Sauropodomorphs with small heads and long necks that were semi-bipedal.

Cladistic analysis also uses the clade Sauropodomorpha based on the small head, elongated neck, and leaf-shaped teeth. Cladists are still evaluating evidence to determine whether prosauropods belong together in a definitive clade. Most experts place Massospondylus within an informal group called “basal sauropods,” including those early sauropods that were bipedal, had ancestral grasping hands, and often enlarged thumb spikes.

In animal classification, a genus is associated with a "type species" for defining purposes. The type species for Massospondylus is Massospondylus carinatus.

Anatomy

Massospondylus was a medium-sized dinosaur, reaching lengths of 4 to 6 meters (13 to 19.6 feet) and standing roughly 1.8 meters (5.9 feet) at the hip. Paleontologists estimate its weight at between 130 and 150 kilograms (286.6 and 330.6 pounds). The animal had a relatively slim profile with a long, flexible tail and neck. Massospondylus had a small head with sharp cutting teeth.

The animal may have had cheeks to prevent the loss of food when eating, a hypothesis based on the discovery of arterial pathways in the jaws and skull.

Massospondylus had relatively small eyes and nostrils. The legs were sturdily built, ending in five-toed feet with blunt, hoof-like claws. The front arms were muscular, with smaller fingers and an enlarged thumb claw that may have been used for either feeding or defense. Though it was originally believed to walk on all fours, later examinations revealed that Massospondylus was primarily bipedal, as it was seemingly unable to rotate its wrists sufficiently to bring its palms in line with the ground.

Intelligence

Animal intelligence can be estimated by a measurement called the encephalization quotient (EQ), based on brain size relative to body size. Sauropods in general tend to fall into the low range of the EQ scale, between 0.1 and 0.8, making them likely one of the least intelligent groups of dinosaurs. Sauropods had small heads relative to their body size and generally had correspondingly small brains. Paleontologists believe that sauropods were similar in intelligence to small lizards or amphibians.

Reproduction and Population

Paleontologists have found eggs from Massospondylus in South African fossil beds and have conducted a detailed examination of the fossilized embryos, finding that Massospondylus gave birth to relatively small young without teeth. This suggests that it may have been necessary for adults to feed the newly hatched young, a level of care that has been debated in regard to dinosaurs. In addition, paleontologists have suggested that juvenile Massospondylus were quadrupedal and probably developed bipedal movement as they matured.

Massospondylus was too large to brood its eggs by sitting on the nest and may therefore have covered its eggs in vegetation, which gave off heat as it decomposed and shielded the eggs from the sun.

Paleontologists have discovered significant variation in the skull shape of Massospondylus specimens, suggesting that the species may have displayed sexual dimorphism, a phenomenon in which males and females of the same species have different physical attributes. Alternatively, these differences may represent variation between individuals.

Diet

Though for some time paleontologists suggested prosauropods may have been omnivores because of their pointed, generalized teeth, contemporary paleontologists agree that Massospondylus were primarily herbivores. Most of the plants in Early Jurassic Africa were either gymnosperms, like conifers and cycads, or pteridophytes (“spore-bearing” plants like ferns). Massospondylus probably fed primarily on leaves and fibrous plant matter, swallowing its food whole with limited chewing.

Paleontologists have found gastroliths (stomach stones) in close association with some fossils of some prosauropods. Like some modern birds and mammals, prosauropods like Massospondylus may have swallowed these stones and other materials to aid in digestion. Within the stomach, muscular contractions ground plant matter against the gastroliths, speeding digestion.

Behavior

Massospondylus lived in an environment surrounded by predatory dinosaurs. The large thumb spikes on the animal's hands may have been used to defend against predators, swiping with the arms to inflict wounds. Alternatively, some have suggested that the animal's thumb spikes may have been used in competitive contests with other Massospondylus, competing for mates, food, or territories. They may also have been used for digging or to strip vegetation.

Massospondylus was heavy for a biped and was probably not capable of achieving significant speeds. The long neck of Massospondylus evolved to enable the species to feed on the leaves of mid-sized trees. Many later sauropod species are believed to have been herd animals, and this behavior might apply to some prosauropods as well. Traveling in groups may have been one of the primary ways that prosauropods protected themselves from predators.

Habitat and Other Life Forms

Early Jurassic Africa was a diverse habitat ranging from tropical forest and wet flood plains to dry arid zones with little vegetation. Where plants flourished, tall conifer trees and a wide variety of cycads were among the most abundant types. There were a variety of insects, reptiles, amphibians, and small mammals living alongside the dinosaurs. Birds had not yet evolved and the skies were dominated by pterosaurs, flying reptiles that lived alongside the dinosaurs throughout the Mesozoic.

Prosauropods were common in Jurassic Africa, and the group was represented by a variety of species. There were also true sauropods in Africa like Vulcanodon, which was similar in size to its prosauropod cousins. A variety of ornithopod herbivores like Fabrosaurus lived in the area, along with a few species of the smaller heterodontosaurids, like Heterodontosaurus and Geranosaurus.

Predatory species included the dilophosaurids, bipedal hunters distinguished by their skull crests, including species like Dracovenator. There were also smaller predators like Syntarsus, another genus that, like Massospondylus, appeared in both Gondwana and Laurasia. There were also a number of unusual crocodilian predators in Jurassic Africa, like the small terrestrial Protosuchus, an ancestor of modern crocodiles that hunted small reptiles, amphibians, and mammals.

Research

The first specimens of Massospondylus were studied by paleontologist Richard Owen in 1854 after they were discovered by fossil hunters in South Africa. Later specimens were discovered in Zimbabwe and in Arizona in the United States, proving Massospondylus to be one of the few dinosaur genera with species present in both Laurasia and Gondwana in the Early Jurassic.

In 1978, researchers discovered eggs attributed to Massospondylus, but the fossilized embryos within the eggs were not examined until 2005, by University of Toronto researcher Robert Reisz and colleagues. Reisz's study has been important to ideas about how quadrupedal movement evolved in sauropods. In 2009, researchers working in South Africa discovered a new species of prosauropod that appears to be transitional in its movement, alternating between bipedal and quadrupedal stances, providing new information about the evolution of the group.

Debate about the relationship between prosauropods and sauropods continues. Questions remain about why prosauropods tend to be dominant vertebrates, with great diversity, in the Late Triassic and Early Jurassic, but decline in numbers during the Norian Age (222–205 million years ago), and then go extinct about 183 million years ago. While prosauropods were in decline, sauropods seemed to increase in diversity and numbers during the same period. Scientists continue to investigate whether the two were competing for the same resources, and if sauropods had an adaptation that gave them an evolutionary advantage.

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