Australia: The Land Where Time Began |
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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 Ørvig
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 didn’t 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).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |