Lifestyle of Ichthyosaurs Term Paper

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Lifestyle of the Ichthyosaur

Ichthyosaurs are a family of marine reptiles that existed during the same era as the dinosaurs. It swam the seas from 245 million years ago to 90 million years ago, during the same period of time that dinosaurs ruled the land. Ichthyosaur fossils were discovered in the late nineteenth century, before the first dinosaur fossils were discovered and subsequently captured the imaginations of scientists and laymen alike. It wasn't until the recent discovery a few years ago of new specimens in Japan and China that a wider interest in the ichthyosaur was revived.

The ichthyosaur is a fish-shaped tetrapod of the diapsid family. A tetrapod is a four limbed vertebrate. Diapsids are classified as having two openings in the skull. There is much fossil evidence to support the conclusion that ichthyosaurs were descended from terrestrial, or land-dwelling, reptiles. One such example is that Ichthyosaurs were air-breathers like cetaceans.

Early ichthyosaurs didn't resemble fish; they were much more like lizards with fins. These lizard-shaped ichthyosaurs later moved out to the deep sea and adapted to fit their habitat by becoming fish-shaped just like dolphins and whales would millions of years after them. As a result, the Ichthyosaur is heavily specialized for life in the water. Ichthyosaurs ranged in size from less than the length of the human arm to fifteen meters long, or quite often bigger (the largest find on record is an amazing 23m). They developed a long snout filled with teeth that resembled many modern dolphins, such as the Indus River dolphin and the Ganges River dolphin. They also had large eyes surrounded and protected by large sclerotic rings.

Ichthyosaurs encompass eighty species. The word ichthyosaur is used to describe an entire class of Reptilia properly known as Ichthopterygia. Ichthyosaurus is the name of one of these species. Most people still refer to the group as Ichthyosaur instead of the unwieldy Ichthyopterygia.

Dolphins are fish shaped mammals of the cetacean order, which also includes whales of all species. Like ichthyosaurs, cetaceans are descended from terrestrial ancestors that returned to the life of the sea. The oldest cetacean fossils are from 50 million years ago. One of these early species, known as the Zeuglodon or Balisaurus, resembled a giant sea serpent more than a fish. Just like the Ichthyosaur, cetaceans first developed a snake or eel-like body shape before fully adapting to the fish-shape which works so well in the water. There are over forty species of dolphin in the world and they can be found virtually anywhere, from tropical regions to arctic.

Sharks are fish who have swum the ocean since before the arrival of the cetaceans. One of the earliest and most impressive ancestors of the modern day shark is the immense Megalodon. The Megalodon is essentially the same as the Great White shark if much bigger, measuring up to fifty feet in length. There are over four hundred species of shark throughout the world. Just like the dolphins, sharks can be found virtually everywhere; although being fish they tend to be found in warmer waters.

It is the purpose of this paper to examine and speculate upon the lifestyle of the ichthyosaur. Because of the many similarities the ichthyosaur shares with the dolphin and the shark, the lifestyles of these two modern animals will be used for the purpose of comparison. Since the ichthyosaur looks so much like other aquatic species, both air-breathers and non-, it stands to reason that the ichthyosaur filled very much the same ecological niche in its day.

Well Adapted to Ocean life

The ichthyosaur was built for life in the ocean. Its shape is hydrodynamic, reducing drag as it swims forward. It had vertical tail fins like sharks do, which it used to propel itself forward. The ichthyosaur likely kept itself stable with its dorsal and pectoral fins, just likes modern fish and dolphins. It also would have used them to steer through the water. While some scientists have speculated that it actually used its pectoral fins for propulsion, this seems unlikely, as the fins are not very robust.

The ichthyosaur also had extremely large eyes that enabled them to see well, and therefore hunt successfully. The large eyes of the ichthyosaur indicate that the reptile would see exceptionally well in the dark ocean depths. These eyes have low f-numbers, which like cats, allows the animal to see well in the dark. The f-number is an indication of how well a creature can see in low light.

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Humans have f-numbers around 2.1 or higher, which is normal in a diurnal animal. Cats have an f-number of about.9, while owls are about 1.1. In ichthyosaurs, the f-numbers have been estimated to be between 1.1 to 1.3. These ranges have been produced by equations used in comparative opthamology. In addition, the sheer size of the ichthyosaur's eyes means that it had many more receptors compared to the smaller eyeballs of cats, for instance, thus allowing for even more accurate vision.

Another recent discovery is that Ichthyosaurs had delicate internal nasal structures formed from bones. These structures indicate that Ichthyosaurs may have possessed senses other than sight and hearing with which it hunted.

Ichthyosaurs underwent a complex evolutionary process that transformed its feet into flippers. There are several observations that can be made about the evolution of forefin skeletons in ichthyosaurs. The lower arm bones became shorter and shorter along the family tree, although there are exceptions. The finger bones also became shorter and shorter, and eventually became disk-shaped. Additionally, the number of finger bones increased early in the evolution. The thumb disappeared at one point, and then additional digits appeared on both sides of the remaining digits. The final result was the same sort of fin that we see in seals today.

The backbone structure of the ichthyosaur is built for swimming. Vertebrae of lizard-shaped ichthyosaurs were the shapes of film canisters. Fish shaped ichthyosaurs underwent a thickening of the spine that eventually changed the vertebrae into the shape of hockey pucks. This same thickening effect of the spine can be seen when comparing shark species. Smaller, slender sharks have film canister shaped vertebrae, while thick-bodied sharks like the Great White have hockey puck vertebrae. These two basic shapes indicate what method the creature used to swim.

The eyes of the ichthyosaur have a sclerotic ring that probably helped maintain the shape of the eye. Sclerotic rings are more common in species with non-spherical eyes.

The fossils of ichthyosaurs can be found all over the world. With the open sea as their range, they spread far and wide across the planet. This proliferation is shared by dolphins and sharks, which both have representative species in most parts of the world. In particular, the evidence suggests to us that the ichthyosaur kept to the open ocean instead of the coastal regions, where competition for food was much more intense.

Diet

The diet of the ichthyosaur was composed of fish, squid and other cephalopods, mollusks and small turtles. The fossilized contents of ichthyosaur stomachs show us that their main food source was cephalopods (squid-like creatures). These are also common food sources amongst contemporary cetaceans and sharks. Recently, fossilized vomit from an ichthyosaur was discovered, the first fossilized vomit to be found. Contained within it were the shells of mollusks that the ichthyosaur had eaten. Apparently unable to digest the shells, the ichthyosaur vomited them up.

Modern squid eating whales hunt in the usual range of 100 meters deep to 1000 meters. Some go as deep as 3000 meters in search of food. Whales of this variety are toothed whales, also known as Odontoceti. Estimations based upon current knowledge give the ichthyosaur a comparable diving depth. The estimates range from 600 meters to as much as 1500 meters.

Fossils found with ichthyosaurs (as well as in their stomachs) show that the ichthyosaurs were deep diving predators that preferred open waters. The animals found with them are those of the open seas, not bottom dwellers. The conclusion therefore is that ichthyosaurs hunted in open water instead of the ocean floor.

Reproduction

Despite being air-breathing marine reptiles, Ichthyosaurs did not reproduce by laying eggs. They instead gave birth to live young. Fossils have been found of ichthyosaurs with fetuses within the body.

Both sharks and dolphins reproduce through internal fertilization, which is a common trait amongst higher animals. All cetaceans are viviparous, and generally give birth to singular calves, which are nourished within a placenta. Sharks reproduce in one of three ways, depending upon species. About forty percent of shark species are oviparous, or egg-laying sharks. The other sixty percent are viviparous, and are either placental or aplacental. Aplacental species have young that are not connected to the mother by an umbilical cord and instead are nourished by the yolk sack of their temporary egg or sometimes eating new eggs that are deposited within the birth sack.

The current research states that the ichthyosaur is a viviparous.....

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