Mastodonsaurids were generally large amphibians, with some length estimates ranging from 3 to 6 meters. Some genera, like Mastodonsaurus[1] and Cyclotosaurus[2] were specially large, reaching at least 4 meters or more. Other genera like Parotosuchus and Paracyclotosaurus[3] only reached 2 meters or more in length, but it's still large compared to most modern-day amphibians.
Distinguishing features
Some mastodonsaurids including Parotosuchus were covered in a scaly skin, unlike the smooth skin of modern-day amphibians, and probably moved with an eel-like motion in the water.[4] Another peculiar mastodonsaurid is Sclerothorax, with unusual features including its elongated neural spines. The neural spines are tallest at the front of the spine. Sclerothorax also has a very large pectoral girdle; the interclavicle bone is longer than the skull, meaning that Sclerothorax has the proportionally largest interclavicle of any temnospondyl. The body of Sclerothorax is covered by small scale-like bony ossicles that would have been embedded in the dermis in life. The skull of Sclerothorax is short, wide, and roughly rectangular in shape. The snout is wider than the cheek region. The bones along the midline of the skull roof are also very wide relative to those of other temnospondyls.[5]
There is a lot of known information about the skulls of mastodonsaurids, for example, Stanocephalosaurus, one of the most well-known mastodonsaurids, has a lot of data about its skull. The skull of Stanocephalosaurus has key features characteristic to other temnospondyls, with the most apparent being the flattened shape of its skull roof. The dorsal as well as ventral surfaces of the skull of Stanocephalosaurus have well ornamented honeycombed patterns throughout. Palatal views of skulls in this genus show narrow interpalatal vacuities, as well as paired palatal tusks alternately functioning on the vomer and palatine, with the vomerine tusks directed backwards. This animal shows an increased expansion of the pterygoids and palatines, as well as narrow posterior nostrils. The vomer also bears two vomerine rows: a transversal denticle row and a longitudinal denticle row that runs along the inner margin of the choana. The palatines and ectopterygoids are broader than in other temnospondyls, which is somewhat common within mastodonsaurids. Stanocephalosaurus also has a long, triangular skull which is much longer than broad, and has an elongated preorbital region continuously narrowing anteriorly to an obtuse rounded snout. This type of skull is very similar to that of Mastodonsaurus, which is also triangular in shape.[6] The Stanocephalosaurus has at least 100 straight and conical teeth on the premaxilla and maxilla combined, gradually increasing in size towards the front.[7]
While there is a lot of known information about Stanocephalosaurus, some of the skull regions which were previously inaccessible or too poorly preserved on Stanocephalosaurus have been observed with X-ray micro-CT scans, including the otic capsule, delta groove of the exoccipital, as well as parts of the arterial and nervous system. Air pockets around the stapes of Stanocephalosaurus have been hypothesized to act as resonance chambers, meaning that the spoon-shaped inner ear bone could be related to underwater hearing. This trait is possibly associated with early tetrapod evolution, which probably also appears on other members of this family and can act as a link to anuran tympanum evolution.[8]
The narrow head and elongated snout of Stanocephalosaurus suggests that stress levels during biting are slightly higher than temnospondyls with a wider and shorter skull. Its skull also has an elongated preorbital region compared to other mastodonsaurids. [9]
The vertebrae of Stanocephalosaurus are rhachitomous, with a neural arch and a bipartite centrum that is divided into a large, unpaired wedge-shaped intercentrum and smaller paired pleurocentra. In anterior and posterior views, the intercentrum is a dorsally half-ring, surrounding the persistent notochord from ventral and lateral sides. Lateral and ventral surfaces of the intercentrum are smooth, suggesting a continuation of cartilage due to the unfinished medial surface. The posterodorsal margin of the intercentrum also shows a parapophysis for articulation with the capitulum of the ribs.[10]
Skeleton of Mastodonsaurus, with a close up view of the skull
Another well-known mastodonsaurid in terms of fossil knowledge is Mastodonsaurus, notably from numerous skull specimens found in Germany. The large, oval eye sockets are midway along the skull. The jaws are lined with conical teeth, two large tusks project up from the end of the lower jaw, fitting through openings on the palate and emerging out from the top of the skull when the jaw is closed. Based on recent research, the body of Mastodonsaurus was more crocodile-like in shape than in earlier reconstructions, with a longer trunk section and a longer tail. The greatly reduced limb bones have joints that are poorly developed.[1][11]
Skeletal reconstruction of Paracyclotosaurus
Paracyclotosaurus is yet again, another well-known mastodonsaurid, and just like most genera of the family, it had various species. This mastodonsaurid has at least three species, including its type species, which is P. davidi, along with two other: P. crookshanki and P. morganorum. The completeness of the skeleton of this animal led to an accurate description. The small distortion of the head can be overlooked, and the complete preservation of the skull and jaws makes its shape certain at a length of 60cm. The right corner of the head is essentially undistorted and shows that the skull at its point of greatest depth is almost exactly as high as the lower jaw. The lower jaw also has its symphysis the same height as the skull above it. Therefore, at its point of greatest height, the head was essentially of oval section, 22cm high and 44cm wide. The nostril, orbit and tympanic membrane are all directed upward, but whether the eye could be projected upward above the head to give a horizontal view similar to a frog, is difficult to determine.[3]
With very similar features to Cyclotosaurus, Subcyclotosaurus is another good example of mastodonsaurid. Its skull is characterized by the small tabular without any trace of a "horn", but with a round lappet that approaches the squamosal flange lateral to the tympanic membrane, failing to meet it by about its own width. The occiput between the otic notches is proportionately wide, a reflection of the small size of the skull. The skull is otherwise of normal mastodonsaurid structure, being specifically more similar to Parotosuchus than to any other genera of this family. The skull also has a small internasal vacuity between the dorsal processes of the premaxillae and lateral lines are often shown as continuous grooves with well-defined borders. A deep groove on the maxilla begins immediately behind and lateral to the nostril and passes straight back to the lachrymal, on which bone it turns outward and forward and ends abruptly. Another groove appears to begin on the maxilla, immediately lateral to that described above. It passes back just above the insertion of the teeth for the full length of the bone. The supraorbital groove begins abruptly on the dorsal surface of the premaxilla, immediately passes on to the nasal, and extends back on that bone close to its suture with the lachrymal. It then comes on to the prefrontal, passing on to the frontal where that bone enters the orbital border. Then as a well-defined groove it surrounds the hinder part of the orbit, turns vertically on to the jugal, and then backward to cross the point where jugal, quadratojugal and squamosal meet, continuing over the squamosal to pass back on to the body.
Xenotosuchus is a member of this which is not very well known, and only a few fragments have been discovered from this animal. This genus was originally a species of Parotosuchus. Like many mastodontosaurids, it was a large animal with a large head. Its amphibian life history meant that the distinct shape of the skull roof would change from a generalized tadpole-like skull to the distinct adult shape.[12] The head bones are covered in large pits and grooves, indicating extensive dermal armour on the head. Both the upper and lower jaw had tusks, those of the upper jaw being situated on a second row of teeth on the vomer and palatine bone. Contrary to related forms like Mastodonsaurus and Stanocephalosaurus, the tusks of the lower jaw were of moderate size and did not penetrate the premaxilla.[13]
Like the mentioned Xenotosuchus, the genus Wetlugasaurus is also one of the smaller and less known mastodonsaurids.[14] It had a skull that measured around 22 centimetres (8.7in), and reached a total length of at least 1 metre (3ft 3in), and although it is pretty large compared to modern-day amphibians, Wetlugasaurus is still very small compared to most of the other mastodonsaurids.
Paleobiology
Mastodonsaurids were piscivorous, mainly preying on small and mid-sized fish, although they also ate land-living animals, such as small archosaurs. Some evidences, including the fossils of some smaller temnospondyls bear tooth marks made by mastodonsaurid-like animals.
It is very likely that mastodonsaurids caught their food like the living Giant Salamander, waiting until the prey came near, then quickly opening their enormous mouths and swallowing them prey whole. The most probable food would be small fishes, up to some 15cm. long, which swam in shoals.
Paleoecology
Mastodonsaurids were aquatic animals that rarely left water and may have been completely unable to leave the water, as large quantities of bones have been found that suggests that many species have died en masse when pools dried up during times of drought.[15] They mostly inhabited swampy pools and fed mainly on fish, whose remains have been found in fossilized coprolites.[15]
Temnospondyli or temnospondyls is a diverse ancient order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian and Triassic periods, with fossils being found on every continent. A few species continued into the Jurassic and Early Cretaceous periods, but all had gone extinct by the Late Cretaceous. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including freshwater, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are amphibians, many had characteristics such as scales and large armour-like bony plates (osteoderms) that generally distinguish them from the modern soft-bodied lissamphibians.
Mastodonsaurus is an extinct genus of temnospondyl from the Middle Triassic of Europe. It belongs to a Triassic group of temnospondyls called Capitosauria, characterized by their large body size and presumably aquatic lifestyles. Mastodonsaurus remains one of the largest amphibians known, and may have exceeded 6 meters in length.
The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecological niche to modern crocodilians prior to the diversification of pseudosuchian archosaurs.
Eryosuchus is an extinct genus of capitosauroid temnospondyl from the Middle Triassic of northern Russia. It was a very large predator: the largest specimen known could reach up to 3.5 m (11.5 ft) in length, with a skull over 1 m long.
Sclerothorax is an extinct genus of temnospondyl from the Early Triassic of Germany. It is distinguished from other temnospondyls by its short and very wide skull and the elongated neural spines that form a ridge along its back. Sclerothorax is a basal member of Capitosauria, a large clade of temnospondyls that lived throughout the Triassic.
Intasuchus is an extinct genus of temnospondyl amphibian from the Middle Permian of Russia. It is known from a single species, Intasuchus silvicola, which was named in 1956. Intasuchus belongs to the family Intasuchidae and is probably its sole member, although other taxa such as Syndyodosuchus and Cheliderpeton have been assigned to the family in the past. Intasuchus most likely belongs to the group Archegosauroidea, Permian relatives of the large, mostly Mesozoic temnospondyl clade Stereospondyli.
Saharastega is an extinct genus of basal temnospondyl which lived during the Late Permian period, around 251 to 260 million years ago. Remains of Saharastega, discovered by paleontologist Christian Sidor at the Moradi Formation in Niger, were described briefly in 2005 and more comprehensively in 2006. The description is based on a skull lacking the lower jaws.
Laidleria is an extinct genus of temnospondyl that likely lived between the Early to Middle Triassic, though its exact stratigraphic range is less certain. Laidleria has been found in the Karoo Basin in South Africa, in Cynognathus Zone A or B. The genus is represented by only one species, L. gracilis, though the family Laidleriidae does include other genera, such as Uruyiella, sister taxon to Laidleria, which was discovered and classified in 2007.
Eocyclotosaurus is an extinct genus of mastodonsauroid temnospondyl from the Middle Triassic (Anisian). The name Eocyclotosaurus means "dawn round-eared lizard". It is characterized as a capitosauroid with a long and slender snout, closed otic fenestra, and small orbits. It measured over one metre and had a 22 cm skull.
Rhinesuchidae is a family of tetrapods that lived primarily in the Permian period. They belonged to the broad group Temnospondyli, a successful and diverse collection of semiaquatic tetrapods which modern amphibians are probably descended from. Rhinesuchids can be differentiated from other temnospondyls by details of their skulls, most notably the interior structure of their otic notches at the back of the skull. They were among the earliest-diverging members of the Stereospondyli, a subgroup of temnospondyls with flat heads and aquatic habits. Although more advanced stereospondyls evolved to reach worldwide distribution in the Triassic period, rhinesuchids primarily lived in the high-latitude environments of Gondwana during the Guadalupian and Lopingian epochs of the Permian. The taxonomy of this family has been convoluted, with more than twenty species having been named in the past; a 2017 review recognized only eight of them to be valid. While several purported members of this group have been reported to have lived in the Triassic period, most are either dubious or do not belong to the group. However, at least one valid genus of rhinesuchid is known from the early Triassic, a small member known as Broomistega. The most recent formal definition of Rhinesuchidae, advocated by Mariscano et al. (2017) is that of a stem-based clade containing all taxa more closely related to Rhinesuchus whaitsi than to Lydekkerina huxleyi or Peltobatrachus pustulatus. A similar alternate definition is that Rhinesuchidae is a stem-based clade containing all taxa more closely related to Uranocentrodon senekalensis than to Lydekkerina huxleyi, Trematosaurus brauni, or Mastodonsaurus giganteus.
Limnarchia is a clade of temnospondyls. It includes the mostly Carboniferous-Permian age Dvinosauria and the mostly Permian-Triassic age Stereospondylomorpha. The clade was named in a 2000 phylogenetic analysis of stereospondyls and their relatives. Limnarchia means "lake rulers" in Greek, in reference to their aquatic lifestyles and long existence over a span of approximately 200 million years from the Late Carboniferous to the Early Cretaceous. In phylogenetic terms, Limnarchia is a stem-based taxon including all temnospondyls more closely related to Parotosuchus than to Eryops. It is the sister group of the clade Euskelia, which is all temnospondyls more closely related to Eryops than to Parotosuchus. Limnarchians represent an evolutionary radiation of temnospondyls into aquatic environments, while euskelians represent a radiation into terrestrial environments. While many euskelians were adapted to life on land with strong limbs and bony scutes, most limnarchians were better adapted for the water with poorly developed limbs and lateral line sensory systems in their skulls.
Cryobatrachus is an extinct genus of temnospondyl amphibian from the Early Triassic of Antarctica. The type species is Cryobatrachus kitchingi. It is known from a partial skull and an imprint of the skull roof, both found in the Fremouw Formation of the Transantarctic Mountains at about 85° south latitude and described in 1974. Many small bone fragments have also been identified, although they cannot be attributed with certainty to C. kitchingi. Cryobatrachus has been classified in the family Lydekkerinidae, as it is similar in appearance to the genus Lydekkerina from South Africa.[a] Because only a small number of features distinguish it from other lydekkerinids, Cryobatrachus kitchingi has more recently been considered a nomen dubium, meaning that its distinction from other better-known species may be unwarranted.
Rhineceps is an extinct genus of temnospondyl amphibian in the family Rhinesuchidae. Rhineceps was found in Northern Malawi in Southern Africa known only from its type species R. nyasaensis. Rhineceps was a late Permian semi-aquatic carnivore that lived in streams, rivers, lakes or lagoons. Rhineceps is an early divergent Stereopondyl within the family Rhinesuchidae, which only existed in the late Permian (Lopingian) and failed to survive the Permian-Triassic extinction unlike other stereospondyl families.
Wellesaurus is an extinct genus of mastodonsauroid temnospondyl. They were amphibious carnivores that lived in freshwater environments.
Xenotosuchus is an extinct genus of mastodonsaurid temnospondyl within the family Mastodonsauridae known from the Triassic of South Africa. The genus is based on a skull originally described as Parotosuchus, an animal which it resembled in general build and habit.
Stanocephalosaurus is an extinct genus of large-sized temnospondyls living through the early to mid Triassic. The etymology of its name most likely came from its long narrow skull when compared to other temnospondyls. Stanocephalosaurus lived an aquatic lifestyle, with some species even living in salt lakes. There are currently three recognized species and another that needs further material to establish its legitimacy. The three known species are Stanocephalosaurus pronus from the Middle Triassic in Tanzania, Stanocephalosaurus amenasensis from the Lower Triassic in Algeria, and Stanocephalosaurus birdi, from the middle Triassic in Arizona. Stanocephalosaurus rajareddyi from the Middle Triassic in central India needs further evidence in order to establish its relationship among other Stanocephalosaurs. Like other temnospondyls, Stanocephalosaurus was an aquatic carnivore. Evidence of multiple species discovered in a wide range of localities proves that Stanocephalosaurus were present all across Pangea throughout the early to mid Triassic.
Subcyclotosaurus is an extinct genus of carnivorous mastodonsaurid temnospondyl. It is known from a single partial skull from the Hawkesbury Sandstone formation in Australia.
Vigilius is an extinct genus of brachyopid temnospondyl amphibian from the Triassic of Arizona. It is known from the single type species Vigilius wellesi.
Rhachitomi is a group of temnospondyl amphibians that includes all temnospondyls except edopoids and dendrerpetontids. It was established as a clade name by German paleontologist Rainer R. Schoch in 2013, although the name had first been established in 1919 by British paleontologist D. M. S. Watson to encompass an evolutionary grade of temnospondyls leading to the group Stereospondyli. American paleontologist Alfred Romer used the term in a similar sense, grouping most Permian and Triassic temnospondyls under Rhachitomi. A similar name that appeared earlier in the scientific literature is Rachitomi, which was named by American paleontologist Edward Drinker Cope in 1882. Rachitomi was commonly used in the late nineteenth and early twentieth century to include early amphibians such as Eryops and Archegosaurus that had rhachitomous vertebrae. Many early tetrapods have vertebrae that are split into two parts below the notochord: a pleurocentrum and an intercentrum. In rhachitomous vertebrae, the intercentrum is large and semicircular, while the pleurocentrum divided into two smaller paired elements. Schoch defined Rhachitomi as a node-based taxon to include four major and well-supported clades of temnospondyls: Dvinosauria, Eryopidae, Stereospondyli and a clade formed by Zatracheidae and Dissorophoidea. Not all members of Rhachitomi have rhachitomous vertebrae; the largest subgroup, Stereospondyli, lacks pleurocentra. Below is a cladogram from Schoch's analysis showing the placement of Rhachitomi within Temnospondyli:
Manubrantlia was a genus of lapillopsid temnospondyls from the Early Triassic Panchet Formation of India. This genus is only known from a single holotype left jaw, given the designation ISI A 57. Despite the paucity of remains, the jaw is still identifiable as belonging to a relative of Lapillopsis. For example, all three of its coronoid bones possessed teeth, the articular bone is partially visible in lateral (outer) view, and its postsplenial does not contact the posterior meckelian foramen. However, the jaw also possesses certain unique features which justify the erection of a new genus separate from Lapillopsis. For example, the mandible is twice the size of any jaws referred to other lapillopsids. The most notable unique feature is an enlarged "pump-handle" shaped arcadian process at the back of the jaw. This structure is responsible for the generic name of this genus, as "Manubrantlia" translates from Latin to the English expression "pump-handle". The type and only known species of this genus is Manubrantlia khaki. The specific name refers to the greenish-brown mudstones of the Panchet Formation, with a color that had been described as "khaki" by the first British geologists who studied the formation.
↑ Howie, A.A. (1970). "A new Capitosaurid Labrinthodont from East Africa". Palaeontology. 13: 210–153.
↑ Brown, Barnum; Bird, Roland T. (Roland Thaxter) (1933). "A new genus of Stegocephalia from the Triassic of Arizona". American Museum Novitates (640). hdl:2246/2062.
↑ Schoch, R.R. (2015). "9. Amphibien und Chroniosuchier des Lettenkeupers"[Amphibians and chroniosuchians from the Lettenkeuper](PDF). Palaeodiversity Supplement 1 Special Issue: Der Lettenkeuper – ein Fenster in die Zeit Vor den Dinosauriern[The Lettenkeuper – a Window on the Time before the Dinosaurs] (in German): 203–230.
↑ Steyer, J. Sebastien (2003). "A revision of the early Triassic "capitosaurs" (Stegocephali, Stereospondyli) from Madagascar, with remarks on their comparative ontogeny". Journal of Vertebrate Paleontology. 23 (3): 544–555. doi:10.1671/1740. S2CID85667716.
Damiani, Ross J. (2000). "Early Triassic mastodonsaurids (Temnospondyli, Stereospondyli) from Western Australia, with remarks on mastodonsauroid palaeobiogeography". Alcheringa: An Australasian Journal of Palaeontology. 24 (4): 299–305. doi:10.1080/03115510008619533. S2CID140541478.
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