Ichthyosaur Lifestyle: Diet, Locomotion, and Deep Diving

Introduction to Ichthyosaurs

Ichthyosaurs are a family of marine reptiles that existed during the same era as the dinosaurs. They swam the seas from 245 million years ago to 90 million years ago, during the same period that dinosaurs ruled the land. Ichthyosaur fossils were discovered in the late nineteenth century, before the first dinosaur fossils were found, and they subsequently captured the imaginations of scientists and laypeople alike. It was not until the recent discovery of new specimens in Japan and China that a wider interest in the ichthyosaur was revived. [1]

The ichthyosaur is a fish-shaped tetrapod of the diapsid family. A tetrapod is a four-limbed vertebrate. Diapsids are classified by having two openings in the skull. There is substantial fossil evidence supporting the conclusion that ichthyosaurs were descended from terrestrial, or land-dwelling, reptiles. One such indicator is that ichthyosaurs were air-breathers, much like cetaceans. [2]

Early ichthyosaurs did not resemble fish; they were much more like lizards with fins. These lizard-shaped ichthyosaurs later moved out to the deep sea and adapted to their habitat by becoming fish-shaped, just as dolphins and whales would millions of years after them. As a result, the ichthyosaur became heavily specialized for life in the water. Ichthyosaurs ranged in size from less than the length of a human arm to fifteen meters long, and often larger — the largest specimen on record measures an astonishing 23 meters. They developed a long snout filled with teeth that resembled those of many modern dolphins, such as the Indus River dolphin and the Ganges River dolphin. [3] 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 Ichthyopterygia. Ichthyosaurus is the name of one of these species. Most people continue to use the term ichthyosaur rather than the unwieldy Ichthyopterygia.

Dolphins are fish-shaped mammals of the cetacean order, which also includes all species of whale. Like ichthyosaurs, cetaceans are descended from terrestrial ancestors that returned to a life at sea. The oldest cetacean fossils date to 50 million years ago. [4] One of these early species, known as the Zeuglodon or Basilosaurus, resembled a giant sea serpent more than a fish. [5] Just like the ichthyosaur, cetaceans first developed a snake- or eel-like body shape before fully adapting to the fish-shape that proves so effective in the water. There are over forty species of dolphin in the world, and they can be found virtually anywhere, from tropical regions to the Arctic.

Sharks are fish that have swum the oceans since before the arrival of cetaceans. One of the earliest and most impressive ancestors of the modern shark is the immense Megalodon. The Megalodon was essentially an enlarged version of the great white shark, measuring up to fifty feet in length. There are over four hundred species of shark throughout the world. Like dolphins, sharks can be found virtually everywhere, though being fish they tend to favor warmer waters.

Physical Adaptations for Ocean Life

The purpose of this paper is 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 comparison. Since the ichthyosaur resembles so many other aquatic species — both air-breathers and non-air-breathers — it stands to reason that the ichthyosaur filled a very similar ecological niche in its day.

The ichthyosaur was built for life in the ocean. Its body shape is hydrodynamic, reducing drag as it swims forward. It had vertical tail fins like those of sharks, which it used to propel itself forward. The ichthyosaur likely used its dorsal and pectoral fins for stability and steering, just as modern fish and dolphins do. While some scientists have speculated that ichthyosaurs used their pectoral fins for propulsion, this seems unlikely given that the fins are not particularly robust.

The ichthyosaur also had extremely large eyes that enabled it to see well and therefore hunt successfully. The large eyes indicate that the reptile could see exceptionally well in the dark ocean depths. [6] These eyes have low f-numbers, which — as in cats — allowed the animal to see well in low light. The f-number is a measure of how well a creature can see in dim conditions. Humans have f-numbers around 2.1 or higher, typical of a diurnal animal. Cats have an f-number of about 0.9, while owls are around 1.1. In ichthyosaurs, f-numbers have been estimated at between 1.1 and 1.3, derived from equations used in comparative ophthalmology. [7] In addition, the sheer size of the ichthyosaur's eyes means that it had far more visual receptors than the smaller eyes of cats, allowing for even greater visual acuity.

Another recent discovery is that ichthyosaurs had delicate internal nasal structures formed from bone. [8] These structures suggest that ichthyosaurs may have possessed senses beyond sight and hearing with which they hunted.

Ichthyosaurs underwent a complex evolutionary process that transformed their feet into flippers. [9] Several observations can be made about the evolution of forefin skeletons. The lower arm bones grew progressively shorter along the family tree, though there are exceptions. The finger bones also became shorter and eventually disk-shaped. The number of finger bones increased early in the evolutionary history. The thumb disappeared at one point, and additional digits later appeared on both sides of the remaining ones. The final result was a fin similar to that seen in seals today.

The backbone structure of the ichthyosaur is well suited for swimming. The vertebrae of lizard-shaped ichthyosaurs were shaped like film canisters. Fish-shaped ichthyosaurs underwent a thickening of the spine that eventually changed the vertebrae into the shape of hockey pucks. [10] This same thickening effect 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-shaped vertebrae. These two basic shapes indicate the swimming method used by the creature.

The eyes of the ichthyosaur also feature a sclerotic ring that probably helped maintain the eye's shape. Sclerotic rings are more common in species with non-spherical eyes. [11]

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

Diet and Feeding Behavior

The diet of the ichthyosaur consisted of fish, squid and other cephalopods, mollusks, and small turtles. The fossilized contents of ichthyosaur stomachs show that their primary food source was cephalopods — squid-like creatures. [12] These are also common food sources among contemporary cetaceans and sharks. Recently, fossilized vomit from an ichthyosaur was discovered — the first such fossil ever found. Contained within it were the shells of mollusks the ichthyosaur had eaten. Apparently unable to digest the shells, the ichthyosaur expelled them. [13]

Modern squid-eating whales typically hunt at depths ranging from 100 to 1,000 meters, with some individuals diving as deep as 3,000 meters in search of food. [14] These are toothed whales, also known as Odontoceti. Estimates based on current knowledge give the ichthyosaur a comparable diving depth, ranging from 600 meters to as much as 1,500 meters. [15]

Fossils found alongside ichthyosaurs — as well as within their stomachs — indicate that ichthyosaurs were deep-diving predators that preferred open waters. [16] The animals associated with them belong to open-sea environments rather than the ocean floor, confirming that ichthyosaurs hunted in open water rather than along the bottom.

Conclusion

Ichthyosaurs were probably deep divers. Diet, bone structure, body mass, and vision all indicate that ichthyosaurs engaged in prolonged dives deep beneath the ocean surface in order to obtain squid — a habit shared by cetaceans of all sizes.

With access to the open seas, ichthyosaurs were probably highly migratory. This explains why ichthyosaur fossils can be found all over the world, a characteristic shared by both dolphins and sharks.

Ichthyosaurs reproduced viviparously, a trait shared with large sharks and all cetaceans. Giving birth to live young is a markedly different reproductive strategy from hatching eggs, and it offers an additional dimension to our understanding of ichthyosaur lifestyle.

While many questions about these fascinating reptiles remain unanswered, the evidence at hand has provided genuine insight into the life of the ichthyosaur. With continued study, even more will be learned about these ancient sea dragons.