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. 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.
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.
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 species. If this is the case, then most likely the ichthyosaur also reproduced through internal fertilization, although there is no current literature to substantiate or repudiate this claim.
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.
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.
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.
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