Ichthyosaurs were the most marine-adapted type
of all the reptiles. When they first make their appearance
in the fossil record in the Triassic, they
are already recognizable as ichthyosaurs adapted
to an aquatic existence. For this reason, their ancestry
has remained a mystery since their discovery
in the eighteenth century. Ichthyosaurs ranged
in size from less than a meter to over ten meters in
length. Early ichthyosaurs had already modified
their forelimbs and hindlimbs into winglike structures.
The limbs in ichthyosaurs had hyperphalangeal
conditions, whereby there was an increase in
the usual number of phalanges in each finger.
Some ichthyosaurs expanded the width of the
wing by adding extra rows of fingers, a condition
known as hyperdactyly. The tails had reevolved a
fin that either extended from the middorsal surface
to the midventral surface or formed a semilunate
tail. In these ichthyosaurs, the body plan
already was porpoiselike, with a barrel-shaped
thorax and tapering tail. Along these lines, icthyosaurs
reevolved a dorsal fin. The skull, with its
beaklike rostrum, had a narrow snout with large
orbits, with sclerotic plates for maintaining the
shape of the eyes. The nostrils had migrated backward
to lie in front of the orbits and the temporal
region was reduced. The skull behind the large
eyes wasmuchreduced. The ichthyosaurs retained
one large temporal fenestra (bone opening) that
housed the pseudotemporalis muscle. The other
main jaw adductor, the pterygoideus, originated
on the palate. Based upon the temporal fenestral
pattern, these forms were classified as either their
own class of reptiles or were considered to be a
member of the Euryapsida. Evidence from the
skull and reproductive strategy point to another
ancestry. The ichthyosaur skull shows the loss of
the lower temporal opening of the diapsid skull
condition. In addition, ichthyosaurs were live
bearers of their young, with a placenta-like structure.
The only reptilian group with the loss of the
lower temporal fenestra and loss of the eggshell
is the diapsid Lepidosauria. All other reptilian
groups use the eggshell for the main source of calcium
for the skeletal development of the embryo,
and thus the eggshell cannot be eliminated in their
reproductive cycle.
Types of Ichthyosaurs
Ichthyosaurs are typed according
to their mode of locomotion
or by the morphology and
shape of the pectoral flippers.
The Neoceratodus type, or lungfish
type, is so called because
their straight tails resembled
the tails of lungfish. These
types had equally sized pectoral
and pelvic flippers. It is considered
that these forms had a
flexible body with some degree
of tail undulation possible
for locomotion. The winglike
forelimbs, with their hyperphalangy
condition, gave them
a hydrodynamic shape, and
may have been their main propulsion
system, acting as wings. Jurassic ichthyosaurs
saw the development of a tunalike semilunate
tail fin. These types, known as Inia type,
named after the Amazon dolphin, were considered
to be faster and more maneuverable swimmers
than the Neoceratodus type. The hind limbs
are reduced in these forms, and it is considered by
some that the forelimbs were used as wings and
the tail was used as a steering device. Others consider
that the semilunate tail was used in a form of
swimming used by tunas, known as carangiform
swimming, using rapid movements of the tail.
The vertebral column here extended into the lower
lobe of the tail. A cartilage ray extended into the
leading edge of the upper lobe of the tail, as seen in
beautifully preserved tail found in the upper Jurassic
deposits around Solnhofen, Germany. These
forms also had a well-developed dorsal fin to
aid in the prevention of rolling during swimming.
It is thought that these ichthyosaurs had a more
rigid body and thus had less drag on the body
during swimming. Another ichthyosaur form, the
Leptopterygius type, had well-developed hind limb
flippers and pelvic girdle, and slightly reduced
pectoral flippers. Undulation of the tail was considered
the main propulsion system of these ichthyosaurs.
Finally, in the Mixosaurus type, undulation
of the body seems to have been the mode of
locomotion. In this body form, a series of elongated
neural spines supported the tail fin. There seem to
be two opposing views of ichthyosaur swimming
speed and prey capture. Some authors believe that
ichthyosaurs were high-speed swimmers, while
others feel that ichthyosaurs were slower moving
but highly maneuverable, and perhaps capable of
short, high-speed swimming bursts.
Much is known about the diet of ichthyosaurs
through the preservation of stomach contents.
Many ichthyosaur species relied heavily on squid
and, to a lesser degree, fish. There are reports of
pterosaur remains preserved in the guts of ichthyosaurs
as well.
Surprisingly, much is also known concerning
the reproductive habits of ichthyosaurs. Because
of their wing limbs and their round-girthed bodies,
it is unlikely that ichthyosaurs came out of the
water. How then did they give birth? The answer
to this question lay in some of the most spectacular
fossils of ichthyosaurs housed in the Stuttgart
Museum in Germany. Female ichthyosaurs were
preserved in the act of giving birth, whether from
problems arising in the birthing process or possibly
from poisonous dinoflagellate blooms that
killed the ichthyosaurs during the birthing process.
In some of these fossils the preserved offspring
can be seen lying in a placenta-like structure
expelled from the female’s body. Other baby
ichthyosaurs are often seen in the abdominal area
of the ichthyosaur, and were previously thought
to be the result of cannibalism. However, it is more
likely that these were developing fetuses.
Classification:
Kingdom: Animalia
Phylum: Chordata
Subphylum: Craniata
Class: Reptilia
Subclass: Lepidosauria
Order: Ichthyopterygia
Family: Ophthalmosauria
Genus and species: Ophthalmosaurus icenicus
Geographical location: Oceans of the Mesozoic
Era
Habitat: Marine
Gestational period: Unknown
Life span: Unknown
Special anatomy: Limbs transformed to flippers
(wings) that employ hyperphalangy to elongate
flipper; reevolve dorsal fin and tail fluke
in some; smooth skin without scales; rostrum
elongate with conical labyrinthodont teeth;
live-bearing
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