Cladoselache
Name:
Cladoselache
(Branch toothed shark).
Phonetic: Clay-do-sel-ah-kee.
Named By: Dean - 1894.
Classification: Chordata, Chondrichthyes,
Cladoselachiformes, Cladoselachidae.
Species: C. clarki, C. elegans, C.
fyleri, C. kepleri, C. magnificus, C. mirablis, C.
newmani, C.pattersoni.
Diet: Carnivore/Piscivore.
Size: Up to 1.8 meters long.
Known locations: USA.
Time period: Fammenian of the Devonian.
Fossil representation: Over a hundred specimens,
some of which include impressions of the cartilaginous skeleton and
soft tissue.
Cladoselache
is often hailed as the first true shark
form to enter the fossil
record, and although Cladoselache has a number of
features that are
different to the sharks that we know today, you can still see a basic
fusiform body plan. One thing that makes Cladoselache
stand out
however is the lack of 'claspers'. These are two fleshy
projections that are present on the underside of modern sharks and
serve the purpose of sperm transfer during reproduction. The exact
method of reproduction used by Cladoselache is
still a matter of debate
because no hard evidence exists to show us how it happened. Another
thing that separates Cladoselache from other sharks
is the overall lack
of scales, save for small areas around the mouth, eyes and edges of
the fins.
The
mouth of Cladoselache was not under slung like in
today’s sharks but
instead more closely resembled the mouths of other fish. The jaw
joint appears to have been quite weak, but was supported by powerful
muscles, something that would have enabled Cladoselache
to tackle
larger prey. The number of gill slits varies between five and seven
depending upon the specimen (modern shark forms only have five,
with a few late surviving ancient forms having six). The eyes were
positioned near the front of the head suggesting that Cladoselache
was
a visually orientated predator. There were two dorsal fins, also
like in modern sharks, with the front dorsal fin being a little bit
bigger than the rear one. These dorsal fins were reinforced with fin
spines on their front edges to help keep them erect against the flow of
water as Cladoselache swam forward. Sharks like
the later Hybodus
would retain these spikes although they would become quite a bit more
developed.
Not
only can we see enough to accurately recreate the outside appearance of
Cladoselache, but in some fossils the internal
workings of the
muscles and even some internal organs like the kidneys can also be
identified. This has brought valuable insights into the early
evolution of sharks with palaeontologists being able to glean some
information on their similarity to modern sharks.
The
high level of preservation seen in Cladoselache
fossils recovered from
the Cleveland Shale has even revealed the kind of prey they ate. By
far the most popular choice were ray-finned fishes, but Cladoselache
also shows signs of being a generalist by also including significant
amounts of eel like Conodonts and even arthropods of the extinct
Thylacocephala. One specimen of Cladoselache even
had the remains of
what appear to be another shark inside of it.
Cladoselache
had an exceptionally streamlined body plan with a broad caudal fin
where the epichordal (top) lobe is almost the same size as the
hypochordal (bottom) lobe, features that would have provided
efficient and fast locomotion through the water. The pectoral fins of
Cladoselache were also proportionately large
revealing that
Cladoselache had to deal with the same problem that
sharks do today.
Because of their body shape, if the pectoral fins were not present,
a shark would nose-dive towards the bottom as it swam. This is why
the pectoral fins in sharks act like hydrofoils to counter this
effect, and if the fins are proportionately bigger the countering
effect is greater. From this it can then be implied that the reasons
for this increase in pectoral fin size is to better cope with faster
swimming speeds.
These
adaptations meant that Cladoselache was a fast and
agile predator, a
notion that is further supported by the presence of prey that have been
preserved inside of Cladoselache fossils. Prey
are usually in an
orientation that suggests they were swallowed tail first, indicating
that Cladoselache actively chased after prey
instead of using ambush
tactics. The teeth are also adapted for seizing prey instead of
slicing through flesh. This is taken from the observation that the
teeth are smooth, not sharp, and have the presence of several
cusps, or points connected to a single tooth that would have
increased the amount of grip Cladoselache had upon
its prey. This
along with fossil evidence means that prey was swallowed whole.
However
despite the above adaptations combined with an almost two meter
length, Cladoselache was no way near being close
to the top of the
food chain. That place was reserved by the bony arthrodire placoderms
like the ten meter long Dunkleosteus,
which were the top predators of
the Devonian seas. However Cladoselache had an
edge over these
potential predators in being both faster and more agile, making it
easier for it to stay out of the way.
Further reading
- Contributions to the morphology of Cladoselache (Cladodus).
Journal
of Morphology 9:87–114. - B. Dean - 1894.
- The dermal tubercles of the Upper Devonian shark Cladosclache.
-
Annals and Magazine of Natural History 11: 367–368. - A. S. Woodward
& E. J. White - 1938.
- The paired fins and shoulder girdle in Cladoselache,
their morphology
and phyletic significance; pp. 111–123 in J. P. Lehman (ed.), Probl�mes
Actuels de Pal�ontologie: �volution des Vert�br�s. Colloque
international du Centre National de la Recherche Scientifique, 218,
Paris. - S. E. Bendix-Almgreen - 1975.
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