Ichthyosaur

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.