''Dwykaselachus'' (pronounced dwike-a-selak-us) is an extinct genus of

symmoriiform Symmoriiformes is an extinct order of cartilaginous fish. Originally named Symmoriida by Zangerl (1981), the name has since been corrected to Symmoriiformes to avoid confusion with a family. The symmoriiform fossils record begins during the late ...

, a cartilaginous fish that lived in what is now

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 ...

during the Permian period around 280 million years ago. It was first discovered in the 1980s, in a nodule of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s from the

Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a per ...

. ''Dwykaselachus'' was named based on Dwyka Group, the group of sedimentary geological formation in the southeastern part of Africa. It represents the place where the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''Dwykaselachus oosthuizeni'' was found. Prior to its discovery, symmoriiforms were thought to be related to sharks, in the group

Elasmobranchii 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, CT scans of its relatively intact skull showed traits such as brain shape and inner ear structure that are shared with cartilaginous fish from the group

Holocephali Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...

, which includes

chimaeras Chimaeras are Chondrichthyes, cartilaginous fish in the order (biology), order Chimaeriformes (), known informally as ghost sharks, rat fish (not to be confused with rattails), spookfish, or rabbit fish; the last two names are also applied to B ...

. This implies that the first major radiation of cartilaginous fish after the Devonian extinction was in fact holocephalians, rather than sharks as commonly believed.

History and discovery

''Dwykaselachus'' was first discovered in the 1980, in a nodule of sediments from the Dwyka Group of the

by amateur

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Roy Oosthuizen, and originally described by Burger Wilhelm Oelofsen in 1986. In 2013, co-author Dr. Robert Gess, a researcher in the Geology Department and Albany Museum at Rhodes University in South Africa, CT-scanned the skull of ''Dwykaselachus'', and showed a symmoriiform morphology that resembles a 3D-preserved model. At first, the skull was thought to belong a symmoriid shark, but after the CT-scanning, the image appeared to show anatomical structures that mark the specimen as an early relative of chimaeras. A research team led by Michael Coates from the

University of Chicago The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...

Medical center has found that ghosts sharks, also named chimaeras, are related to the 280 million-year-old fish ''Dwykaselachus oosthuizeni''. Chimaera-like features including tell-tale shapes of cranial nerves, nostrils and inner ears suggests that ''D. oosthuizeni'' was included in the group Symmoriiformes. Although resembling sharks in appearance, ''Dwykaselachus'' was not actually a shark, but rather had diverged from a

common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...

with true sharks in the Devonian.

Description

The computed tomography (CT) analysis of ''Dwykaselachus'' shows a symmoriiform morphology with three-dimensional articulation. It exhibits some chondrichthyans features such as the large hypophyseal chamber and dorsally projecting endolymphatic duct. The most visible shared specialization with chimaeroids is the offset between the dorsally prominent mesencephalon chamber and the ventral level of the telencephalon space. Moreover, ''Dwykaselachus'' share the characteristic chimaeroid elevation of the midbrain, relative to forebrain. The discovered skull has unusually ethmoid cartilages which include large hemispherical nasal capsules. The nasal capsules are bridged by an internasal groove. Each capsule roof is shorter than the floor, suggesting that, unlike many sharks, the narial openings were directed slightly dorsally. The capsule wall openings include a canal for the olfactory nerve (nerve I), a foramen for the profundus nerve (nerve V), and an opening in the floor, which resembles the subnasal fenestra of ''Doliodus.'' The braincase roof is mostly complete, leave little space for the fontanelle. Therefore, a precerebral fontanelle, a signature of non-chimaeroid chondrichthyans, is either reduced or absent.

Classification

The

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analyses establish the importance of the shared similarities between ''Dwykaselachus'' and chimaeroids, suggesting symmoriiforms, including ''Dwykaselachus'', comprise a sister clade to

iniopterygians Iniopterygiformes (Originally spelled Iniopterygia and sometimes informally abbreviated as "iniops") is an extinct order of cartilaginous fish known only from the Carboniferous period of the United States. Iniopterygians are characterized by larg ...

and holocephalans. The detailed fossil chondrichthyans data used in Coates et al.’s paper provided strong evidence supporting that hypothesis. Thus, expansion of holocephalian stem membership moves the chondrichthyans crown group divergence to a deeper phylogenetic node. Another study done by Coates ''et al''. showed that phylogenetic analysis confirms ''Gladbachus'' as a stem chondrichthyan. Strong evidence was provided to support chondrichthyans as a crown clade. It suggests that the initial evolutionary radiation of crown chondrichthyans is primarily post-Devonian, forming a significant component of the vertebrate recovery after the end-Devonian extinction. The current analyses focus on the conflicting patterns of character state distributions, implying repeated and

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

of chondrichthyan-like specializations among the earliest total group members. Early chondrichthyan species suggest that the morphological disparity in the early members of the chondrichthyan total group was probably substantially greater than that which is observed.

References

External links

* {{Taxonbar, from=Q30589440 Symmoriiformes Permian animals of Africa Permian cartilaginous fish Karoo Fossils of South Africa Fossil taxa described in 1986