Dwykaselachus
From Wikipedia, the free encyclopedia
| Dwykaselachus Temporal range: (Middle Permian) | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Order: | †Symmoriiformes |
| Genus: | †Dwykaselachus Oelofsen, 1986 |
| Type species | |
| †Dwykaselachus oosthuizeni Oelofsen, 1986 | |
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 nodule of sediments from the Karoo Supergroup. 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 Dwykaselachus oosthuizeni was found.[1]
Prior to its discovery, symmoriiforms were thought to be related to sharks, in the group Elasmobranchii. 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, which includes chimaeras.[2] This implies that the first major radiation of cartilaginous fish after the Devonian extinction was in fact holocephalians, rather than sharks as commonly believed.[3]
Dwykaselachus was first discovered in the 1980, in a nodule of sediments from the Dwyka Group of the Karoo Supergroup by amateur paleontologist Roy Oosthuizen, and originally described by Burger Wilhelm Oelofsen in 1986.[1]
In 2013, co-author Dr. Robert Gess, a researcher in the Geology Department and Albany Museum at Rhodes University in South Africa, CT-scanned[4] the skull of Dwykaselachus, and showed a symmoriiform morphology that resembles a 3D-preserved model.[5] 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.[6]
A research team led by Michael Coates from the University of Chicago 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.[5] Although resembling sharks in appearance, Dwykaselachus was not actually a shark, but rather had diverged from a common ancestor with true sharks in the Devonian.[7]
Description
The computed tomography (CT) analysis of Dwykaselachus shows a symmoriiform morphology with three-dimensional articulation.[8][4] It exhibits some chondrichthyans features such as the large hypophyseal chamber and dorsally projecting endolymphatic duct.[2] 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.[2]
The discovered skull has unusually ethmoid cartilages which include large hemispherical nasal capsules.[3] 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.[3] 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.[3] 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.
