Symmoriiformes is an extinct order of

holocephalians Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primit ...

. Originally named Symmoriida by Zangerl (1981), it has subsequently been known by several other names. Lund (1986) synonymized the group with Cladodontida, while Maisey (2008) corrected the name to Symmoriiformes in order to prevent it from being mistaken for a family. The symmoriiform fossils record appear at the beginning of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

. Most of them died out at the start of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

, but ''

Dwykaselachus ''Dwykaselachus'' (pronounced dwike-a-selak-us) is an extinct genus of symmoriiform, a cartilaginous fish that lived in what is now South Africa during the Permian period around 280 million years ago. It was first discovered in the 1980s, in a ...

'' is known from the

Artinskian In the geologic timescale, the Artinskian is an age or stage of the Permian. It is a subdivision of the Cisuralian Epoch or Series. The Artinskian likely lasted between and million years ago (Ma) according to the most recent revision of the ...

Kungurian In the geologic timescale, the Kungurian is an age or stage of the Permian. It is the latest or upper of four subdivisions of the Cisuralian Epoch or Series. The Kungurian lasted between and million years ago (Ma). It was preceded by the Artin ...

of South Africa. Teeth described from the

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretaceou ...

of France and Austria indicate that members of the family

Falcatidae Falcatidae is a family of Paleozoic holocephalians. Members of this family include ''Falcatus'', a small fish from the Bear Gulch Limestone of Montana. The family first appeared around the start of the Carboniferous, and there is some evidence ...

might have survived until the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

; however, these teeth were also argued to be more likely neoselachian teeth.

Fossil distribution

Fossil evidence of Symmoriida have been found at Bear Gulch,

Fergus County, Montana Fergus County is a county in the U.S. state of Montana. As of the 2020 census, the population was 11,446. Its county seat is Lewistown. The county was founded in 1885 and named for James Fergus, a Montana politician who was instrumental in cr ...

, Bethel Quarry,

Pike County, Indiana Pike County is a county in the southwest portion of the U.S. state of Indiana. As of the 2010 United States Census, the population was 12,845. The county seat is Petersburg. It contains the geographic point representing median center of US popu ...

, Kinshozan quarry, Alaska,

Gifu Prefecture is a prefecture of Japan located in the Chūbu region of Honshu. Gifu Prefecture has a population of 1,991,390 () and has a geographic area of . Gifu Prefecture borders Toyama Prefecture to the north; Ishikawa Prefecture to the northwest, F ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, Bashkortostan,

Russian Federation Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...

and possibly also

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

Classification

The symmoriiformes have been assigned to Cladoselachii by Goto et al. (1988), to Elasmobranchii by Williams (1998), and to

Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...

by Sepkoski in 2002 and by Maisey in 2008. The uncrushed braincase of ''

'' indicates that symmoriiforms are members of

Holocephali Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primiti ...

, as much of the internal anatomy, including the otic labyrinth and brain space configuration are similar to those of

chimaeras Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. At o ...

Gallery

References

*http://palaeo.gly.bris.ac.uk/benton/vertclass.html {{Taxonbar, from=Q139057 Symmoriiformes Prehistoric cartilaginous fish orders