Myosaurus

Myosaurus Temporal range:Early Triassic251-247 MA
Restoration ofMyosaurus gracilis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Suborder:	†Anomodontia
Clade:	†Dicynodontia
Family:	†Myosauridae Kitching,1968
Genus:	†Myosaurus Haughton,1917
Type species
†M. gracilis Haughton, 1917

Myosaurus(Greek for “mice-like lizard,” with mys- meaning mouse, and -sauros meaning lizard) is a genus ofdicynodontsynapsids.Myosauruswas a small,herbivoroussynapsid that existed around the earlyTriassic period.All of the fossils found of this species were found in Antarctica and South Africa.Compared to other fossils found from species that existed during this time, theMyosaurusis not common in the fossil record. This is due to a shortage of discovered fossils that possess characteristics unique to theMyosaurus.Notably, under 130 fossil fragments have been found that have been classified asMyosauridae,and almost all have been skulls.These skulls can be classified asMyosaurusbecause this species, unlike other dicynodonts, do not possess tusks or postfrontal teeth.The only species identified in the family Myosauridae is theMyosaurus gracilis,orM. gracilis.It should be recognized that theMyosaurusis almost always referred to as theM. gracilisin scientific research.

History and Discovery

TheMyosauruswas first discovered in the Harrismith Commonage locality, a site found in theLystrosauruszone. This locality is based in South Africa. Around 10 skulls were discovered that could not be classified as Lystrosauridae, and were thus identified as beingMyosaurus.Around 116 specimens later identified asMyosauridaewere also found by W.R. Hammer and J. W. Cosgriff, and these specimens were located in the early Triassic Fremouw Formation of the Cumulus Hills in the Queen Maud Mountains. This location is found in theTransantarctic rangein Antarctica. Almost all the specimens ofMyosaurusfound were preserved in green siltstone.

The skulls were first identified as theEndothiodontidaein 1917, a taxon from the latePermian period.This was in part due to the wide interorbital width that exists in bothMyosauridaeandEndothiodontidaeskulls.Endothiodontidae,though, have both postfrontal and postcanine teeth. The new skulls found had neither. It was also thought that the skulls might have been thought to beCistecephalus.However, it was noted that the wide posterior end of the skull roof was more exaggerated inCistecephalusskulls. Also, theCistecephalusskull lacks the interpterygoid vacuity of which the unidentified skulls had. In 1977, the skulls were later classified asMyosauridae.

Description

Skull

TheMyosaurus gracilisskulls that have been found contain many features that distinguish them from other related species. TheMyosaurusgenerally have smaller skulls compared to their dicynodont relatives.The skulls found in South Africa average to be around 40 millimeters in length, while the skulls found in Antarctica are only marginally larger, averaging to be about 55 mm in length.

Skull Roof

TheMyosaurusskulls were found to have a wide dorsal region, which also creates a large intertemporal region. The pineal foramen is located more posteriorly in the skull.The ophthalmic cavity surrounds the nasal ramus. Compared to other dicynodonts, the nasal ramus is located more anteriorly (toward the mouth).Broad parietals are situated in the lateral plane, but they take up a short region at the posterior end of the skull roof and only reach around to create the dorsal surface of the lower temporal bar.The skulls were found to have a short and narrow pre-orbital region of the skull. However, they do have relatively large orbits.

Palate/Jaw

The large premaxillary portion of the jaw is joined with small ventral exposures to create a secondary palate. The maxillary area is also quite expansive. A palatine foramen is also present and is situated in the space where the pterygoid and ectopterygoid are joined. The palatine foramen is not present in most dicynodont skulls. The ectopterygoid serves as a location where the pterygoid can be joined to both the palatine and maxilla. The pterygoid then splits to outline thechoanaebefore eventually widening toward the anterior part of the palate.

TheMyosaurusalso lacks postfrontal teeth, and their canines protrude from their lower jaw. The lower jaw also exhibits a thick dentary. The dentary envelopes a large mandibular fenestra, another defining characteristic.

Occiput

The occiput portion of theMyosaurusskull contains a small, slit-like hole, and it has been hypothesized by researchers to be a nutrient channel. This would have supplied the skull and existing teeth with the nutrients needed to grow and strengthen.

Key Distinguishing Characteristics of the Skull

A skull feature observed in most dicynodonts is tusks. However, one of the distinguishing characteristics of theMyosaurusis its lack of tusks. Their maxillary area is expansive and larger than other dicynodonts. Because tusks are not a feature of the skull, there is no structure taking up room and crowding the maxillary area.

Another unique feature of theMyosaurusskull is the shortened beak. Their keratinous upper beak, although present, is greatly reduced in size and does not protrude as far as its dicynodont relatives. This is due in part to the distinct trigeminal canal patterns in theMyosaurusskull.

Classification

TheMyosaurus gracilisbelongs to theKistecephliaclade, a subclade of the greaterAnomodontiaandDicynodontiaclades. TheMyosaurusis in the Myosauridae family, of which it is its only member. TheMyosaurusis most closely related to theCistecephalusandKawingasaurustaxa, which are both members of the Cistecephalidae family.

Paleobiology

Burrowing

Since there have been so few fossils found of theMyosaurus,there is no actual evidence that verifies that this taxa was a burrower.However, based on the types of burrows found in both South Africa and Antarctica made by theirLystrosaurus/ dicynodont relatives, it is widely hypothesized that theMyosauruswere burrowers as well.Most of this evidence derives from Antarctica where there may have been large environmental fluctuations during the Triassic period, a possible repercussion of thePermian-Triassic extinction event.The burrows would have provided shelter from hot and cold temperatures because of their insulation qualities. They also may have been utilized by tetrapods to gain shelter from predators.Two types of burrows were made during the Triassic period. They were classified by their size by scientists as Type L (large) and Type G (giant) burrows.Tetrapodswere hypothesized to have made Type G burrows; these burrows match the size and shape of Triassic tetrapods. The size of the burrows is closely correlated with the size of the creator. The stone deposits in these burrows also match the age of the tetrapod fossils found around it. The Type G burrows in Antarctica range from 8–19 cm in diameter. This size would have accommodated theMyosauruswell because it would have been the appropriate size to fit in these burrows.

Further evidence that supports the idea theMyosaurusmight have had a fossorial lifestyle is that the Myosaurus was close relatives toCistecephalusandKawingasaurus,both members of the family Cistephalidae. These taxa were known to be burrowers as well.

Feeding Mechanism

Dicynodont Skull

There are so few fossils from theMyosaurusto justify any evidence of feeding mechanisms unique to this taxonomic group. However, theMyosaurusis classified as a part of the dicynodont clade. Luckily, there is extensive research on the mechanism of chewing in dicynodont taxa. TheMyosaurus,like its dicynodont relatives, was herbivorous.Consequently, the dicynodont jaws were highly advanced for the purpose of feeding to break down the fibrous structure of plants.

The keratinous beak possessed by dicynodonts and theMyosauruswas likely used for cutting plants. This is because the large surface area of the beak would have provided a sharp tool for the taxa to cut plants on. The jaw itself functioned as a good shearing and grinding instrument. The elongated articular paired with the short quadrate allowed theMyosaurusto take an orthal (vertical) bite. This would have provided them with the ability to break the plants from their stalks. Grinding ability was created by the palatines’ ability to rub against the dentary groove located in the lower jaw.

Dicynodonts likely used their tusks to grub, or dig in the ground for food. However, since theMyosaurusdid not have tusks, grubbing was likely not a component of their feeding behavior.

Paleoecology

Image of the Queen Maud Mountains in Antarctica

Myosaurusskulls have been found in both South Africa and Antarctica. In South Africa, only a about 10 skull fragments were found in theKaroo Basinof theLystrosauruszone at the Harrismith Commonage locality. This area, however, is extremely species rich, primarily filled with dicynodont therapsids.However, the most fossils of theMyosaurushave been found in Antarctica in the Fremouw Formation of the Cumulus Hills in theQueen Maud Mountains.This location lies between the Shackleton and McGregor Glaciers.This site is also diverse, containing mostlyMyosaurus,Lystrosaurus,andThrinaxodonfossils.Fragments ofMyosaurushave been primarily found in greensiltstonein these regions. This stone is formed in layers and dates back to the Early to Late Triassic periods.

See also

• List of therapsids