Dwykaselachus

Dwykaselachus (pronounced dwike-a-selak-us) was an extinct symmoriid chimaera, 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.

Dwykaselachus
Temporal range: Middle Permian
~280 Ma
↓
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Chondrichthyes
Subclass:
Holocephali
Order:
†Symmoriida
Genus:
†Dwykaselachus

Oelofsen, 1986[1]
Species:
†D. oosthuizeni
Binomial name
†Dwykaselachus oosthuizeni
Oelofsen, 1986

Prior to its discovery, symmoriids were thought to be related to sharks, in the family 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 class Holocephali, which includes chimaeras.[2] This implies that the first major radiation of cartilaginous fish after the Devonian extinction was in fact chimaeras, rather than sharks as commonly believed.[3]

History and discovery

Dwykaselachus was first discovered in the 1980, in a nodule of sediments from the Karoo Supergroup by an amateur paleontologist called Roy Oosthuizen. The discovery of Dwykaselachus reveals the origins of Chimaeras, also called ghost sharks, a species live in deep ocean. Chimaeras reliant on cartilage, which doesn't preserve as well as bone, therefore, there’re not enough fossil record to study for this species. Thus, chimaeras are unusual throughout the long span of their fossil records, therefore, it is difficult to understand the origin of this species. Until now, the chimaerpod evolutionary record consisted only isolated specimens of their characteristic hyper-mineralizer tooth plate.[2][4] Fortunately, The Dwykaselachus oosthuizeni fossil provides the initial description based on visible material.

In 2013, co-author Dr. Robert Gess, a researcher in the Geology Department and Albany Museum at Rhodes University in South Africa, CT-scanned[5] the Dwykaselachus skull, and showed a symmoriiform morphology that resembles a 3D-preserved model.[6] At first, the skull was thought to be a symmoriid shark, but after the CT-scanning, the image appeared to show the anatomical structure that mark the specimen as an early chimasera, not a shark.[7]

A research team led by Michael Coates from the University of Chicago Medical center has found that ghosts sharks, also named chimaeras, are descended from a 280 million-year-old fish Dwykaselachus oosthuizeni. The chimaera-like features such as: tell-tale shapes of cranial nerves, nostrils and inner ears suggests that D. oosthuizeni was included in the group symmoriiform.[6] Although the appearance resembles sharks, Dwykaselachus are not actually sharks, they have diverged from one common ancestor.[4]

Modern chimaeras, decedents of Dwykaselachus can grow as long as 1.5 metres (4.9 ft) in length, feature large eyes, bird-like fins and tooth plates used for grinding prey.[6] Marine biologists believe that nearly 50 living species of chimaeras lived in the deep ocean around the world.[2]

Based on published study in Nature, Chimaeraoids were confirmed to be the last major surviving vertebrate group on the tree of life. Chimaeras were considered to somehow related to sharks and rays, but they have very distinct skulls. However, all the fossil records researchers had about Chimaeras were completely chimaera.[8] There were no recognizable differences between the ancestors and the ancestors of the modern sharks apart from their large eyes, tooth plates and grinding prey. The discovery of Dwykaselachus provided visible features of certain primitive 'sharks' as well as of chimaeras.[7]

Description

The computed tomography (CT) analysis of Dwykaselachus shows a symmoriiform morphology that resembles a 3D-preserved model.[9][5] In figure 2, the model with lateral view, exhibits some characteristically chondrichthyans features such as the large hepophyseal 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]

Morphological features

Ethmoid cartilages

As two figures shown below, 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 precerbral fonanelle, a signature of non-chimaeroid chondrichthyans, is either reduced or absent.

Classification

Phylogenetic info

The phylogenetic analyses establishes the importance of the shared similarities between Dwykaselachus and chimaeroids, suggesting symmoriiforms, includes Dwykaselachus, as sister clade to iniopterygians and holocephalans.[10] The detailed fossil chondrichthyans data used in Michael et al.’s paper provided strong evidence supporting that hypothesis.[11] Thus, expeansion of holocephalian stem membership moves the chondrichthyans crown group divergence to a deeper phylogenetic node.[3]

Another study done by Michael showed that phylogenetic analysis confirms Gladbachus as a stem chondrichthyans. Strong evidence was provided to support chondrichthyans is a crown clade.[8] 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.[7]

The current analyses focus on the conflicting patterns of character state distributions, implying repeated and convergent evolution of chondrichthyan-like specializations among the earliest total group members.[12] Early chondrichthyan species suggests that the morphological disparity in the early members of the chondrichthyan total group was probably substantially greater than that which is observed.[8]

Geology and paleoenvironment

Very little is known about the paleoenvironment of Dwykaselachus and its evolutionary origins, but since Dwykaselachus is in the group of chondrichthyans, they are aquatic and probably share some similarity with their descendants, chimasera.

Chimaeroids lives in deep ocean and possess an extensive battery of sense organs that allow them to detect information about the external environment in order to mate, locate food and avoid predators.[11] They are known in various parts of the world as ratfish, rabbit fish, ghost sharks, et. Because chimaeras are found in deep water, they were long considered rare. But as marine biologists gained the technology to explore more of the ocean, they are now known to be widespread, but their numbers remain uncertain.[4]

References
  1. B.W. Oelofsen (1986). "A fossil shark neurocranium from the Permo-Carboniferous (lowermost Ecca Formation) of South Africa". In T. Uyeno; R. Arai; T. Taniuchi; K. Matsuura (eds.). Indo-Pacific Fish Biology: Proceedings of the Second International Conference on Indo-Pacific Fishes. Ichthyological Society of Japan. pp. 107–124. ISBN 978-4930813121.
  2. "Dwykaselachus oosthuizeni: Permian Fossil Reveals Origins of Chimaeras | Paleontology | Sci-News.com". Breaking Science News | Sci-News.com. Retrieved 2018-05-21.
  3. Coates, Michael I.; Gess, Robert W.; Finarelli, John A.; Criswell, Katharine E.; Tietjen, Kristen (2017). "A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes". Nature. 541 (7636): 208–211. doi:10.1038/nature20806. PMID 28052054.
  4. Turner, Susan. "Miller, R.F., Cloutier, R. & Turner, S. 2003. The oldest articulated chondrichthyan from the Early Devonian. Nature, 425, Oct. 2nd, 501-504". Cite journal requires |journal= (help)
  5. "Fossil reveals origin of chimaeroid fishes". www.enca.com. Retrieved 2018-05-21.
  6. "Bizarre Ghost Sharks Evolved From An Even Weirder Species Of Ancient Fish". Gizmodo Australia. 2017-01-08. Retrieved 2018-05-21.
  7. "280 million-year-old fossil reveals origins of chimaeroid fishes". www.sciencedaily.com. 2017-01-04. Retrieved 2017-01-20.
  8. Coates, Michael I.; Finarelli, John A.; Sansom, Ivan J.; Andreev, Plamen S.; Criswell, Katharine E.; Tietjen, Kristen; Rivers, Mark L.; Riviere, Patrick J. La (2018-01-10). "An early chondrichthyan and the evolutionary assembly of a shark body plan". Proc. R. Soc. B. 285 (1870): 20172418. doi:10.1098/rspb.2017.2418. ISSN 0962-8452. PMC 5784200. PMID 29298937.
  9. Pskhun (2017-01-06). "Species New to Science: [PaleoIchthyology • 2017] Dwykaselachus oosthuizeni • A Symmoriiform Chondrichthyan Braincase and the Origin of Chimaeroid Fishes". Species New to Science. Retrieved 2018-05-21.
  10. Lisney, Thomas J. (2010-12-01). "A review of the sensory biology of chimaeroid fishes (Chondrichthyes; Holocephali)". Reviews in Fish Biology and Fisheries. 20 (4): 571–590. doi:10.1007/s11160-010-9162-x. ISSN 0960-3166.
  11. Bangert, Berthold; Stollhofen, Harald; Lorenz, Volker; Armstrong, Richard (1999-07-01). "The geochronology and significance of ash-fall tuffs in the glaciogenic Carboniferous-Permian Dwyka Group of Namibia and South Africa". Journal of African Earth Sciences. 29 (1): 33–49. doi:10.1016/S0899-5362(99)00078-0. ISSN 1464-343X.
  12. Didier, D.A. (2012). Biology of Sharks and Their Relatives.
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