''Dwykaselachus'' (pronounced dwike-a-selak-us) is an extinct genus of
symmoriiform, a cartilaginous fish that lived in what is now
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
during the Permian period around 280 million years ago. It was first discovered in the 1980s, in a nodule of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
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 perio ...
. ''Dwykaselachus'' was named based on
Dwyka Group
The Dwyka Group is one of four geological groups that compose the Karoo Supergroup. It is the lowermost geological group and heralds the commencement of sedimentation of the Karoo Supergroup. Based on stratigraphic position, lithostratigraphic co ...
, the group of sedimentary geological formation in the southeastern part of Africa. It represents the place where the
type species
In 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 species that contains the biological type specimen ...
''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 sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gil ...
. 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 ("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 ...
, which includes
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 ...
.
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
The Dwyka Group is one of four geological groups that compose the Karoo Supergroup. It is the lowermost geological group and heralds the commencement of sedimentation of the Karoo Supergroup. Based on stratigraphic position, lithostratigraphic co ...
of 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 perio ...
by amateur
paleontologist
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
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, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...
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. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
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 hepophyseal 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 precerbral fontanelle, a signature of non-chimaeroid chondrichthyans, is either reduced or absent.
Classification
The
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analyses establish the importance of the shared similarities between ''Dwykaselachus'' and chimaeroids, suggesting symmoriiforms, including ''Dwykaselachus'', comprise a sister clade to
iniopterygians
Iniopterygiformes ("Nape Wing Forms") is an extinct order of chimaera-like cartilaginous fish that lived from the Devonian to Carboniferous periods (345–280 million years ago). Fossils of them have been found in Montana, Indiana, Illinois ...
and
holocephalans
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 ...
. 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 com ...
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
Permian animals of Africa
Permian cartilaginous fish
Karoo
Fossils of South Africa
Fossil taxa described in 1986