Heterodontosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Heterodontosauridae

Genus:Heterodontosaurus

Species:Heterodontosaurus tucki

Introduction

Heterodontosaurus, a small dinosaur by any comparison, appeared in the Early Jurassic in what is now South Africa. Bipedal and herbivorous, Heterodontosaurus is most remarkable for its unique set of teeth. The dinosaur had not one, but three types of teeth set in its small mouth: sharp incisors at the fore; four large pointed teeth that resembled tusks, two on top and two on bottom; and rows of flat, chiseled teeth in its cheek. Heterodontosaurus would have used these diverse dental tools to cut, slice, and shred vegetation, though some scientists believe that the tusk-like teeth were designed more for defense than food consumption.

Classification

Paleontologists historically have used the hierarchical Linnaean classification system, developed by Carl Linnaeus in 1735, for dinosaur taxonomy. This system comprises different tiers of classification (class, order, family, genus, and species) based on shared characteristics within each grouping. Species, the lowest tier, is based on the most specific shared traits. Additional sub-categories were added later.

Under Linnaean classification, Heterodontosaurus belongs to the ornithischian order of dinosaurs. One of two orders, ornithischians are those dinosaurs with more bird-like skeletal structures (as opposed to the lizard-like saurischians). Specifically, their pelvic, or hip, bones point down and back towards the tail. Heterodontosaurus is sometimes classified under the Cerapoda suborder and the Ornithopoda infraorder. Most ornithopods tended to be bipedal herbivores with beaked mouths.

Dinosaurs similar to Heterodontosaurus were small, primitive dinosaurs that lived in the Early Jurassic. They possessed long tails, the primarily bipedal gait of most ornithopods, and unusually diverse sets of teeth. The genus Heterodontosaurus was named for those teeth.

Scientists may also use another classification system, called phylogenetic or cladistic classification. This system classifies animals in family trees, called cladograms, grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. According to cladistics, Heterodontosaurus belongs to the Ornithopoda clade, or branch, of the dinosaur family tree. Paleontologists have not resolved the ties between Heterodontosaurus and other species sufficiently to form conclusive subclades. However, one suggested cladogram does link Heterodontosaurus with the marginocephalia clade, which comprises dinosaurs with specific skull adornments, such as ridges or frills.

Anatomy

Among the smallest of dinosaurs, Heterodontosaurus had strong legs and arms, nimble fingers, and the characteristic bipedal stance of its family. The small torso flowed back into a long but flexible tail that helped the dinosaur balance, especially when running. Heterodontosaurus had long legs relative to its shorter arms, with feet that ended in three long toes and one short toe. Its five-fingered hands were agile and designed for grasping, though they could also provide support should Heterodontosaurus stoop to a quadrupedal stance to forage for food.

The small head of Heterodontosaurus housed its most distinctive features—the teeth. Heterodontosaurus is known for its three unique types of teeth—sharp frontal incisors, two pairs of long, canine-like tusks, and rows of flat teeth set in its cheek. Paleontologists believe that these teeth would have been adapted to consume both vegetation and small animals. Heterodontosaurus may also have used its cheeks to store food while chewing, as modern mammals do. The short snout ended in a horned beak.

Dinosaurs have generally been regarded as cold-blooded animals, like modern lizards, though the term “cold blooded” itself is deceptive. Modern lizards are ectothermic, meaning that they rely on the sun and other environmental factors to warm their bodies. Though science has traditionally held dinosaurs, specifically non-avian dinosaurs, to be ectothermic, evidence suggests that some might have been endothermic, meaning that they could heat their bodies from within by consuming food. Scientists continue to debate whether Heterodontosaurus and other dinosaurs might have been ectotherms, endotherms or something in between.

Intelligence

Because slow-moving herbivores tended to rank lower on the EQ scale than their faster, carnivorous counterparts. However, Heterodontosaurus was a smaller, faster moving herbivore than the large sauropods, ankylosaurs, and stegosaurs. It seems likely that Heterodontosaurus ranked similarly to other ornithopods, with an EQ of between 0.8 and 1.5.

Reproduction and Population

Though dinosaurs were on the rise in the Early Jurassic, they did not yet dominate the prehistoric landscape. Limited fossil evidence for Heterodontosaurus makes it difficult to conclude how populous the species—or its relatives—were among their habitats.

Most paleontologists believe that dinosaurs were oviparous, meaning that they reproduced by laying eggs, like modern lizards and birds. Insufficient fossil evidence exists to conclude whether or not Heterodontosaurus was an egg layer or to draw any further conclusions regarding its mating, nesting, and parenting behavior. Egg remnants have been found for other ornithopods (including Dryosaurus and Othnielia), suggesting that at least some ornithopods were egg layers. Evidence for later herbivores also indicates that many laid eggs in communal nests and that parents—or other adults of the species—remained with the eggs and tended the hatchlings for a time. However, more fossil evidence must be found to determine the reproductive habits of Heterodontosaurus.

Though only two skulls for Heterodontosaurus have been found, some paleontologists have speculated that the prominent tusk-like teeth appear only in male members of the species.

Diet

Heterodontosaurus subsisted on a plant-based diet. Like other ornithopods, the small dinosaurs was herbivorous, and its unique teeth were well-designed for cutting, grasping, shearing, slicing, and chewing a variety of vegetation. Low to the ground, they would have foraged primarily on ground vegetation and shrubs. However, the presence of canine-like teeth in Heterodontosaurus has also led to debate over the content of the dinosaur's diet. Some scientists suggest that Heterodontosaurus was not strictly herbivorous but was instead omnivorous, feeding on flora but also on small fauna, including insects. Some studies even suggest that Heterodontosaurus represents a transition among ornithopods, a shift from carnivorous feeding habits to herbivorous behavior.

The Early Jurassic landscape was dominated by ferns as well as gymnosperms, such as cycads and palms. However, ferns would have made up much of the ground cover and other low-lying vegetation and were likely a major component of the Heterodontosaurus diet. Paleontologists have also suggested that Heterodontosaurus used its agile hands to dig roots and tubers from the ground to eat.

Behavior

Only one complete skeleton and two skulls have been uncovered for Heterodontosaurus. Though this provides thorough anatomical evidence for study, it does not provide much in the way of behavioral information. Though some scientists have speculated that the tiny herbivores traveled in small herds, these theories are largely unfounded. Many herbivorous species have been found to be gregarious herd animals, as well as migratory, but insufficient evidence exists to date to draw firm conclusions about the social behavior of Heterodontosaurus. If the tusk-like teeth existed only in male members of the species, then this feature might support conclusions that Heterodontosaurus were more interactive, and that the teeth served to attract mates, combat competitors, and denote social ranking.

The presence of these teeth has also been suggested as being primarily for defense against predators. More likely, when faced with attack, Heterodontosaurus probably relied on its strong and fast legs to carry it away. Though primarily bipedal, Heterodontosaurus likely assumed a quadrupedal stance (stooping on all four limbs) when foraging for food.

Habitat and Other Life Forms

Heterodontosaurus lived during the Early Jurassic period, at a time when dinosaurs were becoming more numerous and the landscape was beginning to undergo great change. Up until that time, the modern continents known today instead formed one giant landmass, or supercontinent, known as Pangaea. This supercontinent is believed to have been located around the equator and had a largely tropical and subtropical climate.

However, by the beginning of the Jurassic period, the earth's tectonic plates began to shift and Pangaea began to break apart. By the Middle Jurassic, the supercontinent would be split into two large landmasses known as Gondwana and Laurasia. Thus far, Heterodontosaurus fossils have only been found in what is now modern Africa, making it a resident of the emerging Gondwana. Large inland seas began to separate the landmasses, lowering temperatures and increasing precipitation in some regions. This milder climate fueled more extensive plant growth, which in turn supported an increase in animal life.

Plate tectonics contributed to the diversification of flora and fauna across the land. Ferns and gymnosperms, such as conifers and cycads, covered much of the landscape, and fauna included, in addition to dinosaurs, amphibians, reptiles, cynodonts, small mammals, insects, and fish. Great forests began to grow, resplendent with conifers, cycads, and sequoias. The lush greenery encouraged population growth among herbivorous dinosaurs, which in turn led to larger numbers of their predators, the carnivores.

Other herbivorous dinosaurs that lived during the Early Jurassic in what is now Africa include Lesothosaurus, Massospondylus and Vulcanodon.

Research

In 1962, Alfred W. Crompton and Alan Charig unearthed the first fossil for Heterodontosaurus in the Upper Elliot Formation of South Africa. However, this first find yielded only a skull. More than a decade later, in 1976, a complete skeleton—with skull—was unearthed. This fossil proved to be one of the best preserved skeletons in the dinosaur record. Then, in 2008, University of Chicago student Laura Porro discovered additional Heterodontosaurus fossils, including the skull of a juvenile dinosaur, in a collection housed at the Iziko South African Museum. The fossils in this collection date from excavations conducted in the 1960s.

In 1975, a closely related contemporary, Abrictosaurus, was unearthed in the Upper Elliot Formation of South Africa and Lesotho. Another close relation from the same time period, Lycorhinus angustidens, was discovered decades earlier in 1924, in the same South African formation. The other known heterodontosaurid genus, Echinodon, was found in England in 1861. This first fossil of the family was actually the geologically youngest, dating from the Early Cretaceous.

In 2016, the 1976 skeleton—which is encased as a fossil and unable to be removed—was X-rayed over five days at the European Synchrotron Radian Facility in order to create a non-invasive reconstruction of the skeleton. The created image allowed researchers a view a 3d depiction of the full skeleton for the first time.

Bibliography

Alleyne, Richard. "One of the World's Smallest Ever Dinosaurs Is Discovered." Telegraph, 24 Oct. 2008, www.telegraph.co.uk/news/science/science-news/3353829/One-of-the-worlds-smallest-ever-dinosaurs-is-discovered.html. Accessed 1 June 2020.

Carpenter, Kenneth. Eggs, Nests, and Baby Dinosaurs. Indiana UP, 1999.

Cox, Barry, et al. Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. Simon & Schuster, 1999.

"Dinobase." University of Bristol, dinobase.gly.bris.ac.uk. Accessed June 2010.

European Synchrotron Radiation Facility. "Researchers Scan Most Complete Heterodontosaurus Skeleton Ever Found." Phys.org, 29 July 2016, phys.org/news/2016-07-scan-heterodontosaurus-skeleton.html. Accessed 1 June 2020.

Jerison, H. J. “Brain Evolution and Dinosaur Brains.” American Naturalist, vol. 103, 1969, pp. 575–88.

"Paleoportal." UC Museum of Paleontology, University of California, Berkeley, ucmp.berkeley.edu/. Accessed June 2010.

Richardson, Hazel. Dinosaurs and Prehistoric Life. DK Publishing, 2003.

"Tiny Juvenile Dinosaur Fossil Sheds Light On Evolution Of Plant Eaters." University of Chicago Medical Center, 24 Oct. 2008. Science Daily, www.sciencedaily.com/releases/2008/10/081023222248.htm. Accessed 1 June 2020.

Weishampel, David B., et al, editors. Dinosauria. University of California Press, 2007.

Wright, Rebekah A. "Were Dinosaurs Bird Brains." University of Arizona Geosciences, 2002, www.geo.arizona.edu/~rwright/abs2002.html. Accessed June 2010.