Ischyodus
Name: Ischyodus.
Phonetic: Is-ke-yo-dus.
Named By: Egerton - 1843.
Classification: Chordata, Chondrichthyes,
Holocephali, Chimaeriformes, Callorhynchidae.
Species: I. bifurcatus, I.
brevirostris, I. dolloi, I. egertoni, I. emarginatus,
I. gubkini, I. incisus, I. lonzeensis, I. minor, I.
mortoni, I. planus, I. rayhassi, I. thurmanni, I.
townsendi, I. williamsae, I. yanshini, I. zinsmeisteri.
Diet: Carnivore.
Size: Up to 1.5 meters long, but size does
depend upon the species.
Known locations: Worldwide with fossil sites from
Europe, Russia, North America, Australia and Antarctica.
Time period: Bathonian of the Jurassic through to
Zanclean of the Pliocene.
Fossil representation: Multiple specimens.
Nine
times out of ten Ischyodus in simply referred to as
a prehistoric
Chimaera. In this instance it is meant as being
similar to Chimaera
monstrosa, also known as the rat fish and the rabbit fish,
due to
its physical similarity. Both Ischyodus and Chimaera
monstrosa are
members of the Chondrichthyes, which means that they are relatives of
the sharks and rays. Assuming that Ischyodus
lived like Chimaera do
today, they would have been deep water fish often inhabiting the
depths beyond where sunlight from the surface could penetrate.
The
tail of Ischyodus would have been very thin and
underdeveloped in
relation to its body, just like in the living Chimaera
(hence
another nickname of ‘rat tail’). This meant that Ischyodus
could
not swim fast to chase after prey and instead hunted for slow swimming
or sick and dying fish, or scavenged the carcasses of other marine
animals as they sank down to the bottom of the ocean.
Another
feature that Ischyodus shares with the Chimaera
is a single large spine
that rises up just in front of the dorsal fin. In Chimaera
this
spine connects to a venom sac, which makes it poisonous to predators
that try to eat it. It’s probable, but not known for certain,
that the spine of Ischyodus functioned in the same
way.
Ischyodus
fossils from Europe are usually Jurassic in age and attributed to such
species as I. egertoni and I.
emarginatus, while those of North
America are late Cretaceous (as I. bifurcatus)
to the
Palaeocene. More interesting though is that fossils from Australia
and Antarctica are Eocene to Pliocene in age, and usually attributed
to I. dolloi.
Given
the broad temporal range, it would be interesting to see if Ischyodus
is really extinct and is not just lurking somewhere in the depths of
the ocean just like coelacanths were when the wider scientific
community insisted that they were extinct.
Further reading
- A new species of Ischyodus (Chondrichthyes:
Holocephali:
Callorhynchidae) from Upper Maastrichtian shallow marine facies of
the Fox Hills and Hell Creek formations, Williston Basin, North
Dakota, USA. Palaeontology 48: 709-721., J. W. Hoganson
& J. M. Erickson - 2005.
- A review of the Tertiary fossil Cetacea (Mammalia) localities
in Australia. Memoirs of the Museum of Victoria 61(2):183-208,
E. M. G. Fitzgerald - 2004.
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