''Plesioselachus'' is an extinct genus of Late

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

(

Famennian The Famennian is the later of two faunal stages in the Late Devonian epoch. The most recent estimate for its duration is that it lasted from around 371.1 to 359.3 million years ago. An earlier 2012 estimate, still used by the International Commis ...

)

cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...

with uncertain classification, which contains only one species, ''P. macracanthus'' from the Waterloo Farm lagerstätte in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

. Known from a single incomplete articulated skeleton and some isolated remains, it is characterized by having a long dorsal spine with length about one third of body length.

Description

''Plesioselachus'' was originally classified as

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...

, however a redescription placed it under Chondrichthyes since it lacks sufficient data to consider what subgroup it belongs to. The holotype specimen, which has a preserved length of shows most of postcranial elements while lacking most of the head. It is originally considered that preserved lower jaw ( Meckel’s cartilage) and upper jaw ( palatoquadrate), however these materials are more likely to belong to ceratohyal and

hyomandibula The hyomandibula, commonly referred to as hyomandibular one(, from , "upsilon-shaped" (υ), and Latin: mandibula, "jawbone"), is a set of bones that is found in the hyoid region in most fishes. It usually plays a role in suspending the jaws ...

. The

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

is fully preserved, and extends for toward the

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

. The packed neural arches are similar to those of symmoriid and stethacanthid holocephalians. The holotype has a long dorsal fin spine. It is slightly recurved, and morphologically close to ones of ctenacanth elasmobranchs. Originally, it is considered that dorsal fin have a triangular basal plate, however that material probably belongs to ''Bothriolepis africana'' instead. There are no traces of a second dorsal fin or fin spine, nor of an anal fin – such absences might result from incomplete preservation. The caudal fin is incompletely preserved. ''Plesioselachus'' is one of the few Devonian chondrichthyan that preserves a pectoral skeleton. The left

scapulocoracoid The scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula. The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions. The scapula is commonly known as the ''shoulde ...

is preserved in the holotype, and some paratype specimen show detailed morphology of its scapulocoracoids. The morphology of scapulocoracoid is comparable with that of symmoriid and stethacanthid holocephalians, except lacking an articular ridge or crest on pectoral radials. Broader proportion from lateral view is also comparable with xenacanthid elasmobranch. The faint impression of a pectoral fin is visible. Pelvic fin and girdle are not preserved, material once considered may belong to phaeophyte axes instead. Simple rhombic scales are visible on the caudal fin, as well as a few on the ventral trunk region. Lack of scales may be a taphonomic artifact, although it is also a common character among early cartilaginous fish. Some specimens, such as long isolated fin spine, show ''Plesioselachus'' could grow larger than the holotype specimen, up to .

References

{{Taxonbar, from=Q116940071 Late Devonian cartilaginous fish Devonian cartilaginous fish of Africa Devonian South Africa Fossils of South Africa Fossil taxa described in 1999