Anaschisma

''Anaschisma'' is an extinct genus of large temnospondyl amphibians. These animals were part of the family called Metoposauridae, which filled the crocodile-like predatory niches in the late Triassic. It had large skull about long, and possibly reached long. It was an ambush hunter, snapping up anything small enough to fit in its huge jaws. It was very common during the Late Triassic (Norian age) in what is now the American Southwest.

History of discovery

''Anaschisma'' was erected by Branson (1905) for two metoposaurid skulls from the Popo Agie Formation of Wyoming. The type species, ''A. browni'', was coined for the skull UC 447, while a second nominal species, ''A. brachygnatha'', was erected for the skull UC 448. Moodie (1908) considered ''A. brachygnatha'' a junior synonym of ''A. browni'', although Branson and Mehl (1929) retained the two species as distinct.Branson, E. B. & Mehl, M. G. 1929. Triassic amphibians from the Rocky Mountain region. The University of Missouri Studies, 4, 154–253. Colbert and Imbrie (1956) synonymized ''Anaschisma'' with the Newark Supergroup genus ''Eupelor'' but retained it as a valid ''Eupelor'' species endemic to the Popo Aggie Formation. Chowdhury (1965) synonymized ''Anaschisma'' with ''Metoposaurus'' and sunk all North American metoposaurids from the Chinle and Dockum into ''browni''.

Hunt (1989) recovered ''Anaschisma'' as an advanced or highly derived form. Some specimens attributed to ''Anaschisma'' from the Redonda Formation were renamed '' Apachesaurus'' by Hunt (1993).

''Koskinonodon''

The genus ''Koskinonodon'' was formerly named ''Buettneria'' by Case in 1922, but in 2007, B.D. Mueller realized that the name '' Buettneria'' had already been given to a bush cricket from the Republic of Congo by Karsch (1889), so he made the genus ''Koskinonodon'' the earliest available unpreoccupied name for the temnospondyl. Lucas ''et al.'' (2007), however, petitioned the ICZN to suppress Karsch's name in favor of Case's name, citing evidence that the amphibian name is much more well known and widely used (the authors cited 75 uses of the name in scientific literature and books over the last 85 years), and that the bush cricket name had been seldom used in technical literature. However, ICZN Opinion 2255 issued in 2010 rejected the petition.

Known ''Koskinonodon'' fossils have been found in the United States, especially the Chinle Formation of Petrified Forest National Park and the ''Placerias'' Quarry in Arizona, the Garita Creek Formation of central New Mexico (the quarry at Lamy), the Petrified Forest Member of northern New Mexico, the Bluewater Creek Formation of western New Mexico, the New Oxford Formation of Pennsylvania, the Tecovas Formation of western Texas, and the Popo Agie Formation of the Chugwater Group of Wyoming.Romer, A. S. 1947. Review of the Labyrinthodontia. Bulletin of the Museum of Comparative Zoology, 99, 1–368.

Synonymy of ''Koskinonodon'' with ''Anaschisma'' started when Romer (1947) proposed that ''Anaschisma'' was a senior synonym of ''Buettneria'', ''Koskinonodon'', and ''Borborophagus''. This was followed by Gee ''et al.'' (2019), where they redescribed the holotypes of the two nominal ''Anaschisma'' species, and then synonymized ''Koskinonodon'', ''Buettneria'' and ''Borborophagus'' with ''Anaschisma''. The species ''B. bakeri'' which has long been assigned to the various synonyms of ''Anaschisma'', was moved to its own genus, '' Buettnererpeton'', in 2022.

Description

The skull of ''Anaschisma'' differs from the skulls of other closely related organisms in a few key ways. It is wider overall and features eye sockets that are very anterior and forward oriented, more so than its contemporaries. Also, the skull has prominent slime canals, which are used for transporting mucus, as well as large external nares. In addition, the upper jaw is relatively weak and thin, used only for holding teeth. Their large jaws could have held many teeth at once, maybe even over 100 on each side of the upper and lower jaws, but the actual number varies constantly over the animal's lifetime due to natural causes such as fighting, eating, disease, etc. The sheer size of the skull is one of the most defining traits for the genus ''Anaschisma''.

Other more minor skull traits characterize ''Anaschisma'' as well: elongation of the lachrymal, shortening of the prefrontal, reduction of the interclavicle, and the most characteristic is the center lachrymal entering margin of the orbit. While the shortened prefrontal is a characteristic of the family Metoposauridae, it is shorted even more in ''Anaschisma''. The interclavicle is reduced in the way that it has many hexagonal pits as well as grooves and ridges. The skull of ''Anaschisma'' is also covered in this reticulate ornamentation. Some researchers believe that ''Anaschisma'' has a shorter posterior process of the interclavicle, which may discriminate it from other closely related species, while others believe that there is not enough information to make that distinction. ''Anaschisma'' also has ossified opisthotics, the more posterior of the bones surrounding the inner ear.

''Anaschisma'' had sharp, pointy teeth for catching and killing prey. They had marginal teeth as well as larger teeth on the palate, specifically palatine and ectopterygoid teeth. They had two basic types: large with shallow grooves and small with deeper grooves. These grooves run lengthwise down the teeth and aided the animals in catching prey. The teeth are elongated labio-lingually at their base and opposite, mediodistally, at their tip. In the middle, they are not elongated either way but instead circular. These dental adaptations enhanced the ability of ''Anaschisma'' to capture prey; the teeth are optimized for piercing prey and not allowing it to escape, resisting the bending force applied by the struggling organism, and propagating cracks in the hard parts of the object, such as bone, allowing for easier eating and digestion.

''Anaschisma'' had a wide, wedge-shaped, powerful tail to assist it with swimming, hunting and likely defense. It was not long like the crocodiles of today, but more likely short and strong to enable it to quickly spring up from hiding and capture prey before it escapes. The legs of ''Anaschisma'' display a sprawling stance and short legs with 4-digits on the front and 5 on the back limbs. ''Anaschisma'' likely spent a lot of time motionless, waiting for prey, which these short legs were likely an adaptation to.

Paleobiology

The hunting style of ''Anaschisma'' involved lying at the bottom of a shallow swamp, waiting for a fish, crustacean, smaller amphibian, or even a young phytosaur to wander by. When it spotted prey, it used its huge jaws to engulf and consume them. A few particular adaptations suggest ''Koskinonodon'' had this aquatic lifestyle. First, they had lateral lines formed by the sensory sulci. These are useful for detecting changes in water pressure made by the swimming motions of nearby organisms. Their sprawling limbs were also adapted for water. They would not move quickly or efficiently on land, although they may have done it to find another water pool with more food or other resource. Mass graves have been found, thought to be a result of a group of these animals gathering together in a withering water pool during a drought and all perishing because the water was never replenished.

Paleoecology

They lived mostly in the late Triassic; by the time the Jurassic began, most temnospondyls, ''Anaschisma'' included, were gone. It is likely that they went extinct during the Triassic-Jurassic extinction event, along with the majority of other large amphibians, the class of Conodonts and 34% of all marine genera. It is unknown what caused this mass extinction; theories include huge volcanic eruptions, climate change, oceanic acidification, and even another asteroid impact. It is known, however, that over half of the species living on Earth at that time went extinct from this event. It lived alongside many other smaller amphibians, and its fossils are also commonly found with phytosaur fossils. It was named in 1931 by Case. The best conditions for fossilization occur in river valleys or floodplains where deposition is occurring, and this animal likely lived in similar shallow, swampy habitats. As it follows, ''Anaschisma'' is famous for having extremely well preserved fossils, and they are often found in groups.

See also

* Prehistoric amphibian
* List of prehistoric amphibians

References

{{Taxonbar, from=Q4751797

Trematosaurs
Prehistoric amphibian genera
Triassic temnospondyls of North America
Norian genera
Triassic amphibians of North America
Fossil taxa described in 1905
Taxa named by Edward Branson