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Chondrichthyans - Holocephalans

There are about 34 species still living of the subclass Holocephali. Some are the elephant shark, rabbitfish and chimaerids. They share many similarities with their relatives the sharks and rays, having a cartilaginous internal skeleton and reproduce by internal fertilisation, and the males have claspers near the pelvic fin and a median clasper on the face.

 There are differences between them and the chondrichthyans, they have a large soft  operculum covering their gill arches, and the upper jaw is fused to the braincase, increasing the power available for their crushing plate dentition, unlike the other chondrichthyans. They have long bodies with whip-like tails, swimming by fluttering their pectoral fins and slow sideways movements of the tail. Included in this group are the living chimaerids and many fossil forms that come in many bizarre shapes, one of which is the iniopterygians. They are often found at great depth on the abyssal plains.

Fossil holocephalans have shown that they split from the sharks, their common ancestral form with the sharks being in the Devonian. An early radiation of the holocephalans is indicated by phylogenetic analyses, their shark-like ancestral forms appearing similar to Falcatus and Damocles (Coats & Sequeira, 2001). The cladogram is made less robust by insufficient data being available for other forms. This has led to the suggestion that they may be a sister group of the sharks (selachians).

The early history of holocephalans was virtually unknown before the discoveries in the Bear Gulch Limestones in Montana. According to one of the early theories by Tor ōrvig of the Swedish Museum they were suggested to have arisen from the Ptyctodontid placoderms. As ptyctodontids have been long accepted as a monophyletic group this theory didnít have many adherents (Goujet & Young, 1995). It has been found from the fossil record that the holocephalans split early from the main line of Devonian sharks, whose basal ancestors include forms such as stethacanthids that includes Akmonistion (Coats & Sequiera, 2001). In the past it has been believed that the holocephalans and sharks diverged from a common ancestral form.

A wide diversity of fossils of holocephalans has been recovered from the Bear Gulch Limestone that were complete. According to Long there have been more than 40 new types of holocephalans found in this site over the past 3 decades, with new types being found almost every field season. There were simple eel-like forms such as Harpagofutator; a form in which elaborate spined appendages trailed off the face of males that have been presumed to have a function is mating. Fossils of 2 newly born foetuses of Delphyodontos have been found that suggest these fish had evolved the process of live birth. This evidence has been interpreted as indicating that interuterine [intrauterine?] competition had been developed by 340 Ma (Lund, 1980).

One of the Holocephalans from the Bear Gulch Limestone of Montana is Echinochimaera meltoni. It had a body form similar to that of modern holocephalans, a long whip-like tail, anal fin close to the tail, a large head with large eyes, and a long bony spine in front of each dorsal fin. There were brush-like structures on the first dorsal spine of the males, and several feathery spines over the eyes. There was a single eye spine and broader first dorsal fin in females.

Also from the Bear Gulch Limestone are some unusual holocephalans that include the petalodont sharks and cochliodont sharks. Belanstea Montana is a deep-bodied form with large feathery pelvic and dorsal fins. Identification of these sharks is mostly by the dentition of a few broad crushing teeth, suggesting a diet of invertebrates with hard shells. The cochliodontiforms were a diverse group that are mostly known from their teeth and spines. A feature of these fish is knobbly crushing plates that are strongly convex. Included among these forms are Cochliodus, Deltodus and Sandalodus. Throughout Europe and North America these are well known in the deposits from the Carboniferous. There are believed to have been up to 12 tooth plates that have been described as brick-like, in species of Psammodus, the tooth plates fitting together tightly to form crushing pavements. There are thought to have been 2 crushing tooth plates in each jaw of some copodontids, such as Copodus.

Included among the menaspiform fishes were some that were well-preserved that were fishes with deep heads and they had bony plates on their head. They also had head spines that were well-developed, that in some forms, such as Menaspis armata, projected strongly outward. Deltoptychius was found in deposits of Carboniferous age at Bearsden in Scotland. Scales covered its body and the head was encased in armour of layered dentin with thickly ornamented surface sculpturing. It had a short tapering tail, the back part being covered with stout spiny scales.

Chimaerids lived on the bottom, feeding mostly on shellfish and crustaceans. They first appear in the Carboniferous, becoming a successful group in the Mesozoic. They reached their peak in the Early Cretaceous. The characteristic crushing tooth plates of fossil forms such as Edaphodon and Ischyodus are found throughout the world. Many species are known from both these genera. Some of these crushing plates indicate fish up to 3 m or more. The living chimaerids are mostly small deep-water fish. In these fishes the typical dentition includes a lower jaw formed of large paired mandibular tooth plates. On each side of the upper jaw were palatine and vomerine plates.

In New Zealand and some other countries species of chimaerid are fished and sold as flake. It can be seen from the fossil record that genera of living chimaerids, such as Callorhynchus and Chimaera display very little change from their first appearance in the fossil record in the Mesozoic.

The typical chimaeriform body was already present in Echinochimaera meltoni from Carboniferous deposits of Montana, with a long whip-like tail and close to it an anal fin. It had a large head with large eyes, and each dorsal fin had a large bony spine in front of it. There is a brushlike structure on the first dorsal spine and over the eyes, several feathery spines. In females the dorsal fin is broader and there is a single eye spine.

An unusual ray-like fish found in Jurassic deposits of Europe was Squaloraja. A long triangular rostrum extended out from the front of its face, rostral cartilage supporting it, which was more than double the length of the braincase. In the upper jaw were 2 pairs of tooth plates and a single pair in the lower jaw. Myriacanthids were another group of unusual chimaerids that possessed elongated rostra. This group is represented by forms such as Acanthorhina that was found in Jurassic deposits in Germany.

A group of small, bizarre-looking fish, the iniopterygians, that are represented by 5 known genera have all been found in deposits from the Late Carboniferous of North America. Previously they were classified as a completely separate group of chondrichthyans, but are now believed to be holocephalans that have become highly specialised. All the known genera had stout pectoral fin spines projecting high up on the shoulder girdle, though some, such as Promexyele had much larger pectoral spines. In some forms such as Polysentor, the upper and lower jaws are free, though the upper jaw is fused to the braincase in most. In the front of the lower jaws there are symphysial tooth whorls, the dentition consisting of either simple cones or in some cases teeth that were wider and had smaller lateral cusplets.

An iniopterygian skull inside a rock nodule that was 300 Ma has been imaged in 3-D with a synchrotron particle accelerator. The brain was found to have fossilised inside the skull (Pradel et al., 2009).

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
  2. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, 2nd ed, University of New South Wales Press.

 

 

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Last Updated 28/11/2010

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading