Name:
Ophthalmosaurus
(Eye lizard).
Phonetic: Of-fal-moe-sore-us.
Named By: Harry Govier Seeley - 1874.
Synonyms: Apatodontosaurus, Ancanamunia,
Baptanodon, Paraophthalmosaurus, Yasykovia.
Classification: Chordata, Reptilia,
Ichthyosauria, Ophthalmosauridae, Ophthalmosaurinae.
Species: O. icenicus (type),
O.
natans, O. saveljeviensis, O. yasykovi.
Diet: Piscivore, possibly a specialist in deep
water squid.
Size: Up to 6 meters long.
Known locations: England - Oxford Clay Formation.
France. Greenland, Mexico - La Caja Formation. USA, Wyoming - Sundance
Formation.
Time period: Bathonian through to Tithonian of the
Jurassic.
Fossil representation: Lots of specimens of
individuals of all ages which make Ophthalmosaurus one of the best
known ichthyosaurs.
The
large number of remains relating to Ophthalmosaurus
has not only
indicated that it was a common ichthyosaur
of the late Jurassic, but
they have formed the basis to a lot of research about ichthyosaurs in
general. Perhaps one of the most important areas is that concerning
live birth in marine reptiles, something which has been proven in
other ichthyosaur genera such as Platypterygius,
but in
Ophthalmosaurus is proved by the total discovery of
over fifty
different pregnant females. Further to this it seems that
Ophthalmosaurus at least gave birth to multiple
young at the time with
numbers of pups inside a mother’s body being anything between two and
eleven. As such it is very likely that other genera of ichthyosaurs
also gave birth to litters of several individual pups.
The
sources of the inspiration of the name Ophthalmosaurus
are the large
eyes. Not only did they take up most of the orbital cavity, but
Ophthalmosaurus possibly had the proportionately
largest eyes amongst
the known sea creatures of the late Jurassic seas. These large eyes
were
obviously to provide Ophthalmosaurus with vision in
low light levels,
and while the two main theories are nocturnal activity or deep water
hunting, current fossil evidence provides much stronger support for
the
latter.
The
eyes of Ophthalmosaurus were supported by bony
growths called scleral
rings, features that in themselves are very common, but the scleral
rings associated with Ophthalmosaurus are extremely
well developed.
Beyond the obvious large size of the eye these rings would have
supported the eye against the crushing effect of the water that would
have been at a much higher pressure than the water nearer the surface.
Additionally examination of the joints of Ophthalmosaurus
has revealed
damage to the bones caused by decompression sickness (better known
as ‘the bends’). This is where the body moves from higher to
lower pressure levels faster than the body can adjust which causes
gases that are dissolved in the blood stream to be released. This
indicates that Ophthalmosaurus would have had to
dive very deep in
order for it to be able to sustain these kinds of injuries.
The
reasons why an ichthyosaur like Ophthalmosaurus
would dive into deep
water are varied, one of which is predator evasion. The late
Jurassic seas saw large pliosaurs
like Liopleurodon
and Pliosaurus
as
the dominant predators of other marine reptiles and Ophthalmosaurus
may
have dived deep in order to avoid them, the bends injuries being
caused as a result of this avoiding action. However the large eyes of
Ophthalmosaurus suggest that deep diving into low
light layers was a
matter of routine rather than escape, otherwise it would have had
eyes similar to other near surface dwelling ichthyosaurs. The deeper
down in the water you go the more water there is for the surface light
to pass through which means the ocean can soon become so dark that all
you would be able to see is black. These dark layers however are
usually abundant in creatures like squid however, and by focusing
upon a source of prey like this, ophthalmosaurid ichthyosaurs could
co-exist with other types of ichthyosaurs that were dedicated fish
hunters.
A
possible further adaption to deep water are the disc-shaped vertebrae
which are closely packed together, reducing the amount of space
available for gases to build up between them. More in depth study by
palaeontologists has suggested that Ophthalmosaurus
may have been able
to dive well beyond five hundred meters deep, and possibly stay down
for as much as twenty minutes at a time.
Ophthalmosaurus
is the type genus of the ophthalmosauridae group which includes other
big eyed ichthyosaurs that are similar in form to Ophthalmosaurus.
This however has led to some confusion between which genera deserve to
be kept separate from Ophthalmosaurus, but still
in the same group,
and which actually represent other previously described genera.
Mollesaurus periallus that was once classed as a
synonym of
Ophthalmosaurus has since been re-examined and
confirmed as
representing a distinct genus. Undorosaurus is
also once again
accepted by the majority of researchers as being valid, although some
still list it as a synonym. One species of Ophthalmosaurus,
O.
chrisorum, has since been redescribed as its own genus,
Arthropterygius.
Out of all the ophthalmosaurid
ichthyosaurs,
Acamptonectes
and Mollesaurus
are thought to be two of the most
closely related to Ophthalmosaurus.
Further reading
- On the pectoral arch and fore limb of Ophthalmosaurus,
a new
ichthyosaurian genus from the Oxford Clay. - Quarterly Journal of the
Geological Society of London 30:696-707. - H. G. Seeley - 1874.
- The osteology and taxonomy of the fossil reptile Ophthalmosaurus.
-
Proceedings of the Zoological Society of London 126(3):403-448. - R. M.
Appleby - 1956.
- First discovery of the ichthyosaur Opthalmosaurus
in the Kimmeridgian
of the USSR. - Paleontological Journal 112-114. - B. M. Efimov - 1991.
- The first definite record of a Valanginian ichthyosaur and its
implications on the evolution of post-Liassic Ichthyosauria. -
Cretaceous Research 32 (2): 155–163. - V. Fischer, A. Clement, M.
Guiomar & P. Godefroit - 2011.
- A new Barremian (Early Cretaceous) ichthyosaur from western Russia. -
Journal of Vertebrate Paleontology 31 (5): 1010–1025. - Valentin
Fischer, Edwige Masure, Maxim S. Arkhangelsky & Pascal
Godefroit - 2011.
- New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous
Demonstrate Extensive Ichthyosaur Survival across the
Jurassic–Cretaceous Boundary. - PLoS ONE 7 (1): e29234. - Valentin
Fischer, Michael W. Maisch, Darren Naish, Ralf Kosma, Jeff Liston,
Ulrich Joger, Fritz J. Krüger, Judith Pardo Pérez, Jessica Tainsh
& Robert M. Appleby - 2012.