Old Earth Ministries Online Dinosaur Curriculum
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Lesson 53 - Heterodontosaurs
Heterodontosauridae ("different-toothed lizards") is a family of early ornithischian dinosaurs that were likely among the most basal (primitive) members of the group. Although their fossils are rare, they lived around the globe beginning in the late Triassic Period, and a few late-surviving species persisted into the Early Cretaceous.
Heterodontosaurids were fox-sized dinosaurs less
than 2 meters (6.6 ft) in length, including a long tail. They are known
mainly for their characteristic teeth, including enlarged canine-like tusks
and cheek teeth adapted for chewing, analogous to those of Cretaceous
hadrosaurids. Their diet was herbivorous or possibly omnivorous.
Length: 6.5 feet
Date Range: 215 - 133 Ma, Late Triassic to Early Cretaceous Period
|Cast of the type specimen Heterodontosaurus tucki (Picture Source)|
Skull and teeth
Both Abrictosaurus and Heterodontosaurus had very large eyes. Heterodontosaurids are named for their strongly heterodont dentition. There were three premaxillary teeth. In the Early Jurassic Abrictosaurus, Heterodontosaurus, and Lycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines of carnivoran mammals and is often called the caniniform or 'tusk'. A lower caniniform, larger than the upper, took the first position in the dentary and was accommodated by the arched diastema of the upper jaw when the mouth was closed. These caniniforms were serrated on both the anterior and posterior edges in Heterodontosaurus and Lycorhinus, while those of Abrictosaurus bore serrations only on the anterior edge. In the Early Cretaceous Echinodon, there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla, and Fruitadens from the Late Jurassic may have had two lower caniniforms on each dentary.
The postcranial anatomy of Heterodontosaurus tucki has been well-described, although H. tucki is generally considered the most derived of the Early Jurassic heterodontosaurids, so it is impossible to know how many of its features were shared with other species. The forelimbs were long for a dinosaur, over 70% of the length of the hindlimbs. The well-developed deltopectoral crest (a ridge for the attachment of chest and shoulder muscles) of the humerus and prominent olecranon process (where muscles that extend the forearm were attached) of the ulna indicate that the forelimb was powerful as well. There were five digits on the manus ('hand'). The first was large, tipped with a sharply curved claw, and would rotate inwards when flexed; Robert Bakker called it the 'twist-thumb'. The second digit was the longest, slightly longer than the third. Both of these digits bore claws, while the clawless fourth and fifth digits were very small and simple in comparison. In the hindlimb, the tibia was 30% longer than the femur, which is generally considered an adaptation for speed. The tibia and fibula of the lower leg were fused to the astragalus and calcaneum of the ankle, forming a 'tibiofibiotarsus' convergently with modern birds. Also similarly to birds, the lower tarsal (ankle) bones and metatarsals were fused to form a 'tarsometatarsus.' There are four digits in the pes (hindfoot), with only the second, third, and fourth contacting the ground. The tail, unlike many other ornithischians, did not have ossified tendons to maintain a rigid posture and was probably flexible. The fragmentary skeleton known for Abrictosaurus has never been fully described, although the forelimb and manus were smaller than in Heterodontosaurus. Also, the fourth and fifth digits of the forelimb each bear one fewer phalanx bone.
Tianyulong from China appears to preserve filamentous integument which has been interpreted to be a variant of the proto-feathers found in some theropods. These filaments include a crest along its tail. The presence of this filamentous integument has been used to suggest that both ornithischians and saurischians were endothermic.
Heterodontosauridae includes the genera Abrictosaurus, Lycorhinus, and Heterodontosaurus, all from South Africa. While Richard Thulborn once reassigned all three to Lycorhinus, all other authors consider the three genera distinct. Within the family, Heterodontosaurus and Lycorhinus are considered sister taxa, with Abrictosaurus as a basal member. Geranosaurus is also a heterodontosaurid, but is usually considered a nomen dubium because the type specimen is missing all its teeth, making it indistinguishable from any other genus in the family. More recently, the genus Echinodon has been considered a heterodontosaurid in several studies. Lanasaurus was named for an upper jaw in 1975, but more recent discoveries have shown that it belongs to Lycorhinus instead, making Lanasaurus a junior synonym of that genus.
While originally known only from the Early Jurassic of southern Africa, heterodontosaurid remains are now known from four continents. Early in heterodontosaurid history, the supercontinent Pangaea was still largely intact, allowing the family to achieve a near-worldwide distribution. The oldest known remains are a jaw fragment and isolated teeth from the Laguna Colorada Formation of Argentina, which dates back to the Late Triassic. These remains have a derived morphology similar to Heterodontosaurus, including a caniniform with serrations on both anterior and posterior edges, as well as high-crowned maxillary teeth lacking a cingulum. The most diverse heterodontosaurid fauna comes from the Early Jurassic of southern Africa, where fossils of Heterodontosaurus, Abrictosaurus, Lycorhinus, and the dubious Geranosaurus are found.
Undescribed Early Jurassic heterodontosaurids are also known from the United States and Mexico, respectively. In addition, beginning in the 1970s, a great deal of fossil material was discovered from the Late Jurassic Morrison Formation near Fruita, Colorado in the United
Most heterodontosaurid fossils are found in geologic formations that represent arid to semi-arid environments, including the Upper Elliot Formation of South Africa and the Purbeck Beds of southern England. It has been suggested that heterodontosaurids underwent seasonal aestivation or hibernation during the driest times of year. Due to the lack of replacement teeth in most heterodontosaurids, it was proposed that the entire set of teeth was replaced during this dormant period, as it seemed that continual and sporadic replacement of teeth would interrupt the function of the tooth row as a single chewing surface. However, this was based on a misunderstanding of heterodontosaurid jaw mechanics. It was thought that heterodontosaurids actually did replace their teeth continually, though more slowly than in other reptiles, but CT scanning of skulls from juvenile and mature Heterodontosaurus shows no replacement teeth. There is currently no evidence that supports the hypothesis of aestivation in heterodontosaurids, but it cannot be rejected, based on the skull scans.
While the cheek teeth of heterodontosaurids are clearly adapted for grinding tough plant material, their diet may have been omnivorous. The pointed premaxillary teeth and sharp, curved claws on the forelimbs suggest some degree of predatory behavior. It has been suggested that the long, powerful forelimbs of Heterodontosaurus may have been useful for tearing into insect nests, similarly to modern anteaters. These forelimbs may have also functioned as digging tools, perhaps for roots and tubers.
The length of the forelimb compared to the hindlimb suggests that Heterodontosaurus might have been partially quadrupedal, and the prominent olecranon process and hyperextendable digits of the forelimb are found in many quadrupeds. However, the manus is clearly designed for grasping, not weight support. Many features of the hindlimb, including the long tibia and foot, as well as the fusion of the tibiofibiotarsus and tarsometatarsus, indicate that heterodontosaurids were adapted to run quickly on the hindlegs, so it is unlikely that Heterodontosaurus moved on all four limbs except perhaps when feeding.
The short tusks found in all known heterodontosaurids strongly resemble tusks found in modern musk deer, peccaries and pigs. In many of these animals (as well as the longer-tusked walrus and Asian elephants), this is a sexually dimorphic trait, with tusks only found in males. The type specimen of Abrictosaurus lacks tusks and was originally described as a female. While this remains possible, the unfused sacral vertebrae and short face indicate that this specimen represents a juvenile animal, while a second, larger specimen of Abrictosaurus clearly possesses tusks. Therefore, it is possible that tusks are found only in adults, rather than being a secondary sexual characteristic of males. These tusks could have been used for combat or display with members of the same species or with other species. The absence of tusks in juvenile Abrictosaurus could also be another characteristic separating it from other heterodontosaurids as well, as tusks are known in juvenile Heterodontosaurus. Other proposed functions for the tusks include defense and use in an occasionally omnivorous diet.
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