Heterodontosauridae
Encyclopedia
Heterodontosauridae is a family
of early ornithischia
n dinosaur
s that were likely among the most basal
(primitive) members of the group. Although their fossil
s 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 hadrosaurid
s. Their diet was herbivorous
or possibly omnivorous
.
itself is known from a complete skeleton. Fragmentary skeletal remains of Abrictosaurus
are known but have not been fully described, while most other heterodontosaurids are known only from jaw fragments and teeth. Consequently, most heterodontosaurid synapomorphies
(defining features) have been described from the teeth and jaw bones. Heterodontosaurus measured just over 1 meter (3.3 ft) in length, while the fragmentary remains of Lycorhinus
indicate an animal up to twice that size.
projected sideways, a feature also present in ceratopsia
ns. As in the jaws of most ornithischians, the anterior
edge of the premaxilla
(a bone at the tip of the upper jaw) was toothless and probably supported a keratin
ous beak (rhamphotheca), although heterodontosaurids did have teeth in the posterior
section of the premaxilla. A large gap, called a diastema
, separated these premaxillary teeth from the those of the maxilla
(the main upper jaw bone) in many ornithischians, but this diastema was characteristically arched in heterodontosaurids. The mandible
(lower jaw) was tipped by the predentary
, a bone unique to ornithischians. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentary bone.
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 carnivora
n mammal
s 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.
Like the characteristic tusks, the cheek teeth of derived
heterodontosaurids were also unique among early ornithischians. Small ridges, or denticles, lined the edges of ornithischian cheek teeth in order to crop vegetation. These denticles extend only a third of the way down the tooth crown from the tip in all heterodontosaurids; in other ornithischians, the denticles extend further down towards the root. Basal forms like Abrictosaurus had cheek teeth in both maxilla and dentary that were generally similar to other ornithischians: widely-spaced, each having a low crown and a strongly-developed ridge (cingulum) separating the crown from the root. In more derived
forms like Lycorhinus and Heterodontosaurus, the teeth were chisel-shaped, with much higher crowns
and no cingula, so that there was no difference in width between the crowns and the roots.
These derived cheek teeth were overlapping, so that their crowns formed a continuous surface on which food could be chewed. The tooth rows were slightly inset from the side of the mouth, leaving a space outside the teeth that may have been bounded by a muscular cheek
, which would have been necessary for chewing. The hadrosaurs and ceratopsians of the Cretaceous Period, as well as many herbivorous mammals, would convergently
evolve
somewhat analogous dental batteries. As opposed to hadrosaurs, which had thousands of teeth constantly being replaced, tooth replacement in heterodontosaurids occurred far more slowly and several specimens have been found without a single replacement tooth in waiting. Characteristically, heterodontosaurids lacked the small openings (foramina) on the inside of the jaw bones which are thought to have aided in tooth development in most other ornithischians. Heterodontosaurids also boasted a unique spheroidal
joint between the dentaries and the predentary, allowing the lower jaws to rotate outwards as the mouth was closed, grinding the cheek teeth against each other. Because of the slow replacement rate, this grinding produced extreme tooth wear that commonly obliterated most of the denticles in older teeth, although the increased height of the crowns gave each tooth a long life.
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 bird
s. 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
tendon
s 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.
Butler et al., 2011
Butler et al. 2010
South African paleontologist Robert Broom
created the name Geranosaurus
in 1911 for dinosaur jaw bones missing all of the teeth. In 1924, Lycorhinus was named, and classified as a cynodont
, by Sidney Haughton. Heterodontosaurus was named in 1962 and it, Lycorhinus and Geranosaurus were recognized as closely related ornithischian dinosaurs. Alfred Romer
named Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus. It was defined as a clade
in 1998 by Paul Sereno
and redefined by him in 2005 as the stem clade consisting of Heterodontosaurus tucki and all species more closely related to Heterodontosaurus than to Parasaurolophus walkeri, Pachycephalosaurus wyomingensis, Triceratops horridus, or Ankylosaurus magniventris.
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. Dianchungosaurus
was once considered a heterodontosaurid from Asia
, but it has since been shown that the remains were a chimera
of prosauropod and mesoeucrocodylia
n remains. José Bonaparte
also classified the South American Pisanosaurus
as a heterodontosaurid at one time, but this animal is now known to be a more basal ornithischian.
The membership of Heterodontosauridae is well-established in comparison to its uncertain phylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians. Due to supposed similarities in the morphology of the forelimbs, Robert Bakker proposed a relationship between heterodontosaurids and early sauropodomorphs like Anchisaurus
, bridging the orders Saurischia
and Ornithischia. The dominant hypothesis over the last several decades has placed heterodontosaurids as basal ornithopods. However, others have suggested that heterodontosaurids instead share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs), a hypothesis that has found support in a number of recent studies. The clade containing heterodontosaurids and marginocephalians has been named Heterodontosauriformes
. Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurids are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family.
s. 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 States. Described in print in 2009, this material was placed in the genus Fruitadens
. Heterodontosaurid teeth lacking a cingulum have also been described from Late Jurassic and Early Cretaceous formations in Spain
and Portugal
. The remains of Echinodon were redescribed in 2002, showing that it may represent a late-surviving heterodontosaurid from the Berriasian
stage of the Early Cretaceous in southern England. Dianchungosaurus
from the Early Jurassic of China is no longer considered a heterodontosaurid; though one late surviving Asian form is known (Tianyulong
).
Most heterodontosaurid fossils are found in geologic formation
s 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 reptile
s, 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 anteater
s. These forelimbs may have also functioned as digging tools, perhaps for root
s and tuber
s.
The length of the forelimb compared to the hindlimb suggests that Heterodontosaurus might have been partially quadruped
al, 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.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of early ornithischia
Ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...
n dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s that were likely among the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
(primitive) members of the group. Although their fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are rare, they lived around the globe beginning in the late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
Period, and a few late-surviving species persisted into the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
.
Heterodontosaurids were fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
-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
Analogy (biology)
An analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution....
to those of Cretaceous hadrosaurid
Hadrosaurid
Hadrosaurids or duck-billed dinosaurs are members of the family Hadrosauridae, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper...
s. Their diet was herbivorous
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
or possibly omnivorous
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
.
Description
Among heterodontosaurids, only HeterodontosaurusHeterodontosaurus
Heterodontosaurus is a genus of small herbivorous dinosaur with prominent canine teeth which lived in the Early Jurassic of South Africa. It was similar to a hypsilophodont in shape, and ate plants, despite its canines.Heterodontosaurus is currently known from specimens of the SAFM from South...
itself is known from a complete skeleton. Fragmentary skeletal remains of Abrictosaurus
Abrictosaurus
Abrictosaurus is a genus of heterodontosaurid dinosaur from the Early Jurassic Period of what is now southern Africa. It was a small bipedal herbivore or omnivore, approximately 1.2 meters long, and weighing less than 45 kilograms .This dinosaur is known from the fossil remains of only...
are known but have not been fully described, while most other heterodontosaurids are known only from jaw fragments and teeth. Consequently, most heterodontosaurid synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
(defining features) have been described from the teeth and jaw bones. Heterodontosaurus measured just over 1 meter (3.3 ft) in length, while the fragmentary remains of Lycorhinus
Lycorhinus
Lycorhinus is a genus of heterodontosaurid ornithischian dinosaur hailing from the Early Jurassic strata of the Elliot Formation located in the Cape Province, South Africa....
indicate an animal up to twice that size.
Skull and teeth
Both Abrictosaurus and Heterodontosaurus had very large eyes. Underneath the eyes, the jugal boneJugal
The jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or Zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species....
projected sideways, a feature also present in ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
ns. As in the jaws of most ornithischians, the anterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
edge of the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
(a bone at the tip of the upper jaw) was toothless and probably supported a keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
ous beak (rhamphotheca), although heterodontosaurids did have teeth in the posterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
section of the premaxilla. A large gap, called a diastema
Diastema (dentistry)
Diastema is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...
, separated these premaxillary teeth from the those of the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
(the main upper jaw bone) in many ornithischians, but this diastema was characteristically arched in heterodontosaurids. The mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw) was tipped by the predentary
Predentary
The predentary is an 'extra' bone in the front of the lower jaw, which extended the dentary . It is found in the fossilised remains of ornithischian dinosaurs, which were herbivorous. The predentary coincided with the premaxilla in the upper jaw. Together they formed a beak-like apparatus used to...
, a bone unique to ornithischians. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentary bone.
Heterodontosaurids are named for their strongly heterodont
Heterodont
The anatomical term heterodont refers to animals which possess more than a single tooth morphology. For example, members of the Synapsida generally possess incisors, canines , premolars, and molars. The presence of heterodont dentition is evidence of some degree of feeding/hunting specialization...
dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
. There were three premaxillary teeth. In the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
Abrictosaurus, Heterodontosaurus, and Lycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines of carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
n mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s 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
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
Echinodon, there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla, and Fruitadens from the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
may have had two lower caniniforms on each dentary.
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
heterodontosaurids were also unique among early ornithischians. Small ridges, or denticles, lined the edges of ornithischian cheek teeth in order to crop vegetation. These denticles extend only a third of the way down the tooth crown from the tip in all heterodontosaurids; in other ornithischians, the denticles extend further down towards the root. Basal forms like Abrictosaurus had cheek teeth in both maxilla and dentary that were generally similar to other ornithischians: widely-spaced, each having a low crown and a strongly-developed ridge (cingulum) separating the crown from the root. In more derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
forms like Lycorhinus and Heterodontosaurus, the teeth were chisel-shaped, with much higher crowns
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
and no cingula, so that there was no difference in width between the crowns and the roots.
These derived cheek teeth were overlapping, so that their crowns formed a continuous surface on which food could be chewed. The tooth rows were slightly inset from the side of the mouth, leaving a space outside the teeth that may have been bounded by a muscular cheek
Cheek
Cheeks constitute the area of the face below the eyes and between the nose and the left or right ear. They may also be referred to as jowls. "Buccal" means relating to the cheek. In humans, the region is innervated by the buccal nerve...
, which would have been necessary for chewing. The hadrosaurs and ceratopsians of the Cretaceous Period, as well as many herbivorous mammals, would convergently
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
evolve
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
somewhat analogous dental batteries. As opposed to hadrosaurs, which had thousands of teeth constantly being replaced, tooth replacement in heterodontosaurids occurred far more slowly and several specimens have been found without a single replacement tooth in waiting. Characteristically, heterodontosaurids lacked the small openings (foramina) on the inside of the jaw bones which are thought to have aided in tooth development in most other ornithischians. Heterodontosaurids also boasted a unique spheroidal
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
joint between the dentaries and the predentary, allowing the lower jaws to rotate outwards as the mouth was closed, grinding the cheek teeth against each other. Because of the slow replacement rate, this grinding produced extreme tooth wear that commonly obliterated most of the denticles in older teeth, although the increased height of the crowns gave each tooth a long life.
Skeleton
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 chestPectoralis major muscle
The pectoralis major is a thick, fan-shaped muscle, situated at the chest of the body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female...
and shoulder
Deltoid muscle
In human anatomy, the deltoid muscle is the muscle forming the rounded contour of the shoulder. Anatomically, it appears to be made up of three distinct sets of fibers though electromyography suggests that it consists of at least seven groups that can be independently coordinated by the central...
muscles) of the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
and prominent olecranon process
Olecranon
The olecranon is a large, thick, curved bony eminence of the forearm that projects behind the elbow.It is situated at the upper end of the ulna, one of the two bones in the forearm...
(where muscles that extend the forearm
Triceps brachii muscle
The triceps brachii muscle is the large muscle on the back of the upper limb of many vertebrates. It is the muscle principally responsible for extension of the elbow joint .-Terminology:...
were attached) of the ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
indicate that the forelimb was powerful as well. There were five digits on the manus
Manus (zoology)
The manus is the zoological term for the distal portion of the fore limb of an animal. In tetrapods, it is the part of the pentadactyl limb that includes the metacarpals and digits . During evolution, it has taken many forms and served a variety of functions...
('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
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
was 30% longer than the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, which is generally considered an adaptation for speed. The tibia and fibula of the lower leg were fused to the astragalus
Talus bone
-External links:* *...
and calcaneum of the ankle, forming a 'tibiofibiotarsus
Tibiotarsus
The tibiotarsus is the large bone between the femur and the tarsometatarsus in the leg of a bird. It is the fusion of the proximal part of the tarsus with the tibia.A similar structure also occurred in the Mesozoic Heterodontosauridae...
' convergently with modern bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. Also similarly to birds, the lower tarsal
Tarsus (skeleton)
In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...
(ankle) bones and metatarsals
Metatarsus
The metatarsus or metatarsal bones are a group of five long bones in the foot located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the medial side : the first, second, third, fourth, and fifth...
were fused to form a 'tarsometatarsus
Tarsometatarsus
The tarsometatarsus is a bone that is found in the lower leg of certain tetrapods, namely birds.It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsal and metatarsal bones...
.' There are four digits in the pes
Pes (zoology)
The pes is the zoological term for the distal portion of the hind limb of tetrapod animals. It is the part of the pentadactyl limb that includes the metatarsals and digits . During evolution, it has taken many forms and served a variety of functions...
(hindfoot), with only the second, third, and fourth contacting the ground. The tail, unlike many other ornithischians, did not have ossified
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
s 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
Phalanx bones
In anatomy, phalanx bones are those that form the fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. Phalanges are classified as long bones.The phalanges do not have individual names...
.
Proto-feathers
Tianyulong
Tianyulong is a genus of heterodontosaurid ornithischian dinosaur. The only species is T. confuciusi, whose remains were discovered in Jianchang County, Western Liaoning Province, China...
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.
Classification
South African paleontologist Robert Broom
Robert Broom
Professor Robert Broom was a Scottish South African doctor and paleontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow...
created the name Geranosaurus
Geranosaurus
Geranosaurus is a genus of ornithischian dinosaur from the Early Jurassic. It is known only from a jaw bone and limb elements discovered in South Africa. Because of the limited remains, it is considered a nomen dubium...
in 1911 for dinosaur jaw bones missing all of the teeth. In 1924, Lycorhinus was named, and classified as a cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
, by Sidney Haughton. Heterodontosaurus was named in 1962 and it, Lycorhinus and Geranosaurus were recognized as closely related ornithischian dinosaurs. Alfred Romer
Alfred Romer
Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...
named Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus. It was defined as a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
in 1998 by Paul Sereno
Paul Sereno
Paul Callistus Sereno is an American paleontologist from the University of Chicago who discovered several new dinosaur species on several continents. He has conducted excavations at sites as varied as Inner Mongolia, Argentina, Morocco, and Niger...
and redefined by him in 2005 as the stem clade consisting of Heterodontosaurus tucki and all species more closely related to Heterodontosaurus than to Parasaurolophus walkeri, Pachycephalosaurus wyomingensis, Triceratops horridus, or Ankylosaurus magniventris.
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
Nomen dubium
In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
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. Dianchungosaurus
Dianchungosaurus
Dianchungosaurus is an extinct genus of mesoeucrocodylian crocodylomorph from the Early Jurassic of China. It was previously considered a dinosaur, but it was recently reclassified as a mesoeucrocodylian by Paul Barrett and Xing Xu . It is probably the same animal as the informally named...
was once considered a heterodontosaurid from Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, but it has since been shown that the remains were a chimera
Chimera (paleontology)
In paleontology, a chimera is a fossil which was reconstructed with elements coming from more than a single species of animal. A now classic example of chimera is Protoavis.-List of paleontological chimeras:*Brontosaurus*Lametasaurus...
of prosauropod and mesoeucrocodylia
Mesoeucrocodylia
Mesoeucrocodylia is the name of the clade that includes Eusuchia and the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, and continues to the present day....
n remains. José Bonaparte
José Bonaparte
José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...
also classified the South American Pisanosaurus
Pisanosaurus
Pisanosaurus is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton...
as a heterodontosaurid at one time, but this animal is now known to be a more basal ornithischian.
The membership of Heterodontosauridae is well-established in comparison to its uncertain phylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians. Due to supposed similarities in the morphology of the forelimbs, Robert Bakker proposed a relationship between heterodontosaurids and early sauropodomorphs like Anchisaurus
Anchisaurus
Anchisaurus is a genus of basal sauropodomorph, and was an early herbivorous dinosaur. Until recently it was classed as a member of Prosauropoda. The name comes from the Greek αγχι/agkhi...
, bridging the orders Saurischia
Saurischia
Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...
and Ornithischia. The dominant hypothesis over the last several decades has placed heterodontosaurids as basal ornithopods. However, others have suggested that heterodontosaurids instead share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs), a hypothesis that has found support in a number of recent studies. The clade containing heterodontosaurids and marginocephalians has been named Heterodontosauriformes
Heterodontosauriformes
Heterodontosauriformes are a group of herbivorous dinosaurs within the order Ornithischia. The taxon was named by Xu et al. in 2006 to group the suborder Marginocephalia with the family Heterodontosauridae , after the discovery of Yinlong revealed the two to be closely related....
. Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurids are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family.
Distribution
While originally known only from the Early Jurassic of southern Africa, heterodontosaurid remains are now known from four continentContinent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s. Early in heterodontosaurid history, the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
Pangaea
Pangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
was still largely intact, allowing the family to achieve a near-worldwide distribution
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
. The oldest known remains are a jaw fragment and isolated teeth from the Laguna Colorada Formation
Laguna Colorada Formation
The Laguna Colorada Formation is a geological formation in South America. It dates back to the Late Triassic.-Vertebrate fauna:Dinosaur tracks geographically located in the Provincia de Santa Cruz....
of Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
, which dates back to the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
. 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
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
comes from the Early Jurassic of southern Africa, where fossils of Heterodontosaurus, Abrictosaurus, Lycorhinus, and the dubious Geranosaurus are found.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
, respectively. In addition, beginning in the 1970s, a great deal of fossil material was discovered from the Late Jurassic Morrison Formation
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
near Fruita
Fruita, Colorado
The City of Fruita is a Home Rule Municipality located in the western part of Mesa County, Colorado, in the United States. It is part of the Grand Junction Metropolitan Statistical Area and within the Grand Valley...
, Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
in the United States. Described in print in 2009, this material was placed in the genus Fruitadens
Fruitadens
Fruitadens is a genus of heterodontosaurid dinosaur. The name means "Fruita tooth", in reference to Fruita, Colorado , where its fossils were first found. It is known from partial skulls and skeletons from at least four individuals of differing biological ages, found in Tithonian rocks of the...
. Heterodontosaurid teeth lacking a cingulum have also been described from Late Jurassic and Early Cretaceous formations in Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
and Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
. The remains of Echinodon were redescribed in 2002, showing that it may represent a late-surviving heterodontosaurid from the Berriasian
Berriasian
In the geological timescale, the Berriasian is an age or stage of the Early or Lower Creteceous. It is the oldest or lowest subdivision in the entire Cretaceous. It spanned between 145.5 ± 4.0 Ma and 140.2 ± 3.0 Ma...
stage of the Early Cretaceous in southern England. Dianchungosaurus
Dianchungosaurus
Dianchungosaurus is an extinct genus of mesoeucrocodylian crocodylomorph from the Early Jurassic of China. It was previously considered a dinosaur, but it was recently reclassified as a mesoeucrocodylian by Paul Barrett and Xing Xu . It is probably the same animal as the informally named...
from the Early Jurassic of China is no longer considered a heterodontosaurid; though one late surviving Asian form is known (Tianyulong
Tianyulong
Tianyulong is a genus of heterodontosaurid ornithischian dinosaur. The only species is T. confuciusi, whose remains were discovered in Jianchang County, Western Liaoning Province, China...
).
Paleobiology
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...
s that represent arid to semi-arid environments, including the Upper Elliot Formation
Upper Elliot Formation
The Upper Elliot Formation is a geological formation dating to roughly between 200 to 190 million years ago and covering the Hettangian to Sinemurian stages. The Upper Elliot Formation is found in South Africa and Lesotho and is a member of the Stormberg Group. It consists mainly of limestone,...
of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
and the Purbeck Beds of southern England. It has been suggested that heterodontosaurids underwent seasonal aestivation
Estivation
Aestivation is a state of animal dormancy, characterized by inactivity and a lowered metabolic rate, that is entered in response to high temperatures and arid conditions...
or hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
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 reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, but CT scanning
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
of skulls from juvenile and mature Heterodontosaurus shows no replacement teeth. There is currently no evidence that supports the hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
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 anteater
Anteater
Anteaters, also known as antbear, are the four mammal species of the suborder Vermilingua commonly known for eating ants and termites. Together with the sloths, they compose the order Pilosa...
s. These forelimbs may have also functioned as digging tools, perhaps for root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s and tuber
Tuber
Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to survive the winter or dry months and provide energy and nutrients for regrowth during the next growing season and they are a means of asexual reproduction...
s.
Quadruped
Quadrupedalism is a form of land animal locomotion using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet"...
al, 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
Musk deer
Musk deer are artiodactyls of the genus Moschus, the only genus of family Moschidae. They are more primitive than the cervids, or true deer, in not having antlers or facial glands, in having only a single pair of teats, and in possessing a gall bladder, a caudal gland, a pair of tusk-like teeth...
, peccaries
Peccary
A peccary is a medium-sized mammal of the family Tayassuidae, or New World Pigs. Peccaries are members of the artiodactyl suborder Suina, as are the pig family and possibly the hippopotamus family...
and pigs
Suidae
Suidae is the biological family to which pigs belong. In addition to numerous fossil species, up to sixteen extant species are currently recognized, classified into between four and eight genera...
. In many of these animals (as well as the longer-tusked walrus
Walrus
The walrus is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family and Odobenus genus. It is subdivided into three subspecies: the Atlantic...
and Asian elephants
Asian Elephant
The Asian or Asiatic elephant is the only living species of the genus Elephas and distributed in Southeast Asia from India in the west to Borneo in the east. Three subspecies are recognized — Elephas maximus maximus from Sri Lanka, the Indian elephant or E. m. indicus from mainland Asia, and E. m....
), this is a sexually dimorphic
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
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
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
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
Secondary sex characteristic
Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive...
of males. These tusks could have been used for combat or display
Display (zoology)
Display is a form of animal behaviour, linked to survival of the species in various ways. One example of display used by some species can be found in the form of courtship, with the male usually having a striking feature that is distinguished by colour, shape or size, used to attract a female...
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.