Term Paper: Lifestyle of the Ichthyosaurs

Pages: 11 (3108 words)  ·  Bibliography Sources: 1+  ·  Level: College Senior  ·  Topic: Animals  ·  Buy This Paper

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[. . .] Locomotion

Fish-shaped ichthyosaurs resemble mackerel sharks and these characteristics suggest thunniform or tuna-like swimming.

Mackerel sharks, of which the Great White is an example, swim by holding the body still and moving the tail. Some fish undulate the entire body in order to swim, like eels do. Early, lizard-shaped ichthyosaurs seemed to swim in an undulatory way. Fish-shaped ichthyosaurs, it is believed, swam in the thunniform way, with the body being still.

In his 1996 paper, Peter Cowen introduces what he terms Carrier's Constraint. Referring to the earlier work of Carrier (1987,1991), Cowen explains that amphibians and reptiles cannot breathe while they run. This is evident in the behavior of lizards that would scurry very quickly over a short distance but come to a rest a short distance away. The ambulatory gait of reptiles requires the same set of muscles used in breathing. These same reptiles can breathe while they walk, but must pause for breath between steps. Amphibians and reptiles have a three-chambered heart. This arrangement is efficient for short bursts because it does not send blood to lungs that cannot breathe while it runs, thereby saving more energy for running. Early tetrapods had this sprawling gait as well and since breathing and locomotion use the same muscle groups, both could not be done efficiently at the same time. Erect stance and bipedal posture are also both adaptive responses to carrier's constraint. Both allow an animal to breathe while running.

Undulatory swimming suffers from the same problem. An air-breathing creature that swam by undulation would not be able to breathe while it was swishing its tail. Its breathing would be restricted to times when the spine is straight, allowing both lungs to draw air evenly.

Thunniform swimming overcomes this restriction, but there is some debate as to whether or not ichthyosaurs were thunniform swimmers. The spine of the Ichthyosaur is thickened to allow for larger body mass, which aids in the retention of oxygen for deep diving. These two characteristics are also found in the family of mackerel sharks, and this family is also composed of thunniform swimmers. However, Cowen maintains that there is insufficient evidence in the fossil record that suggests the Ichthyosaur's spine is rigid.

According to Cowen, if Carrier's Constraint is real it must apply to all air breathing tetrapods. Aquatic air-breathers still fall under carrier's constraint if they have laterally flexing fish-like motion, which ichthyosaurs do.

There are three possible solutions to this according to Cowen. Firstly, he brings up the possibility that there was a stiffening of the spine that doesn't show up in the fossil records. While possible, he dismisses this as unlikely. Second, that ichthyosaurs simply had poor stamina and were not capable of sustained swimming over long distances. Thirdly, Cowen posits that ichthyosaurs overcame the limitations of Carrier's Constraint by "porpoising" through the waves the way that dolphins do today. With the elimination of the first option, Cowen concludes that either ichthyosaur lacked the capacity for sustained swimming or resorted to leaping through the waves.

Fish shaped ichthyosaurs are largely believed to have swum using their tail only, which is also known as thunniform swimming. Mackerel sharks, of which the Great White is an example, swim the same way. Lizard-shaped ichthyosaurs swam by undulatory motions, in which it moved the entire body back and forth in the same way that eels and catsharks swim. The ichthyosaurs had a style of swimming that lends itself well to hunting in open seas. Large sharks and toothed whales follow this same behavior.

Undulatory swimming is less energy efficient than thunniform, or tail swishing. While it confers quick acceleration and maneuverability, which is good for coastal waters, undulatory swimming is not as beneficial for those creatures that must swim long distances for food.

Deep Divers

Most living aquatic species claim deep divers within their ranks. There are four main reasons to think that ichthyosaurs were deep divers as well. Those reasons are vision, body mass & aerobic diving limit, bone structure, and diet of squid.

The ichthyosaur is the proud possessor of the largest eyes the world has ever known. As was stated earlier, these large eyes allow for a correspondingly large number of visual receptors, therefore increasing visual acuity. Ichthyosaur eyes also have low f-numbers, which allow them to see well in the dark. The f-number of the ichthyosaur is comparable to both cats and owls, two of the most famous nocturnal predators of today.

Body mass allows for more oxygen to be stored for long dives and also is more efficient in oxygen use than smaller creatures. Fish shaped ichthyosaurs are estimated to be six times heavier than lizard-shaped ichthyosaurs of the same length. Essentially, being larger allows the animal to hold more air, which in turn equates to a longer dive time. This thickened body mass is also a trait shared with many large sharks and cetaceans.

The bone structure of ichthyosaurs also allowed for deeper dives. Terrestrial animals have a dense sheath surrounding their bones, since the bones must be strong to support the creature's body mass. Ichthyosaurs and other aquatic animals instead have a porous, spongelike layer encasing their bones. These pores allow for more oxygen storage. Additionally, the lack of dense matter assists the animal in returning to the surface for air, which is of course essential to the aquatic air-breathing species like cetaceans and ichthyosaurs

Lastly, ichthyosaurs are known to have been squid eaters due to the fossilized remains found within their stomachs. Modern squid-eating whales routinely dive to depths between a hundred and a thousand meters to hunt for squid. These same whales have been known to dive as deep as three thousand meters in some instances. Since squid rarely can be found near the surface, deep-dives became necessary in the constant hunt for food.

Deep divers need to conserve energy and oxygen, so a streamlined, hydrodynamic body is found in modern deep divers as well as Ichthyosaurs. Deep divers also develop heavy body mass to aid in diving. Heavier bodies can store more oxygen, thereby allowing for longer dives. The thickened spine and girth of fish-shaped ichthyosaurs are evidence of this. A fish-shaped ichthyosaur is estimated to weigh six times more than a lizard-shaped ichthyosaur of the same length.

Calculations based upon the capabilities of today's air-breathing aquatic animals estimate that the opthalmosaurus could hold its breath for twenty minutes. Given its estimated swimming speed, this allowed for dives from 600 meters to possibly 1500 meters in depth.

The bone structure of the ichthyosaur also supports the deep dive theory. Four limbed landwalkers have a dense outer shell on their bones in order to support the animal's body weight. Ichthyosaurs instead possess a porous, spongy layer that aids in lightness, which is especially important for returning to the surface. The pores of t6he bones also store more oxygen for the dive. Modern deep divers like cetaceans and seals share this attribute.

Conclusion

In conclusion, it can be said that as an air-breather, the ichthyosaur has more in common with cetaceans than with sharks. It is hard to imagine that the ichthyosaur's lifestyle was much different than dolphins or whales.

An ichthyosaur swam by swishing its tail in a lateral fashion like fish. It had dorsal fins and flippers that it would use to maneuver. Likely, its body remained still and only its tail moved.

Ichthyosaurs were probably deep divers. Such factors as diet, bone structure, body mass and vision indicate that ichthyosaurs engaged in prolonged dives deep beneath the surface of the water in order to obtain a meal of squid. This is a habit shared by cetaceans of all sizes.

Having access to the open seas, ichthyosaurs were probably heavily migratory. For this reason, fossils of ichthyosaurs can be found all over the world. This is another characteristic that appears in dolphins and sharks as well.

Ichthyosaurs reproduced viviparosly. This is a trait shared with large sharks and all cetaceans. Giving birth to live young is certainly a different experience than hatching eggs, so that provides an extra perspective into the lifestyle question.

While there are many questions that cannot be answered about these fascinating reptiles, the evidence at hand has given us some insight into the life of the ichthyosaur. With luck and patience, even more will be learned of these sea dragons.

Bibliography

Motani, Ryosuke. "Rulers of the Jurassic Seas." Scientific American, Dec 2000. Pages 52-59.

Cowen, Richard. "Locomotion and Respiration in Marine Air-breathing Vertebrates." Evolutionary Biology. 1996.

Motani, Ryosuke. "Ryosuke Motani's Ichthyosaur Page." 2000. Berkeley University. 29 Apr 2004. http://www.ucmp.berkeley.edu/people/motani/ichthyo/index.html

Orndoff, Richard L., Wieder, Robert W., Filkorn, Harry F. "How the West was Swum." Natural History. June 2001.

Perkins, Sid. "Sea dragons: Big News about ichthyosaurs, which cruised oceans while dinosaurs ruled the land." Science News. Aug 2002.

Dyer, Nicole. "Jurassic Puke." Science World. 8 Apr 2002.

Sylvestre, Jean-Pierre. Dolphins… [END OF PREVIEW]

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