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6. DIVING BEHAVIORS OF ICHTHYOSAURS

There are several reasons to believe that some ichthyosaurs, especially Ophthalmosaurus, were deep divers.

Before starting, let me point out that deep diving is not something unusual. Many air-breathing vertebrates dive to significant depths in their daily lives (see figure below). Almost every group of marine air-breathing vertebrates has members that are deep divers (usually species with large body size). It would be surprising if ichthyosaurs turned out to be an exception.

Reason 1: Vision

As you read in the section for the eyes, some fish-shaped ichthyosaurs had very low minimum f-numbers for their eyes. This means that they were adapted to low-light environments, as in cats (cats are nocturnal, and can see in the dark much better than we can). If you put a cat in a submarine and turn all the lights off, it is expected from calculation that it can detect preys at a depth of 500 meters or more in the majority of oceans (some oceans are clearer than the other, so the oceans are usually divided into five types according to their clarity).

The f-number of an Ophthalmosaurus eye was similar to that of a cat eye. But an Ophthalmosaurus eye was much larger than a cat eye, meaning that it contained more photoreceptive cells and hence had an enhanced visual capacity. Ophthalmosaurus could probably have outperformed a cat in seeing at depths.

In conclusion, the eye data support the hypothesis that some ichthyosaurs were deep divers.

Reason 2: Body Mass and Aerobic Diving Limit

It is known among air-breathing diving animals that heavier species can dive longer on average (see figure below). Reptiles are exceptional since their low oxygen consumption rate allows them to dive much longer than mammals or birds of the same body mass. Using the graph below, it is possible to estimate the average diving duration of a given mammal or bird from their body mass. We can use the same relationship to estimate at least how long a given ichthyosaur could stay submerged. This assumes the physiology of mammals and birds for ichthyosaurs, but it is ok since assuming a reptilian physiology would give much longer diving duration.

A calculation suggests that a 4-meter Ophthalmosaurus could stay submerged for about 20 minutes or more. The cruising speed of Ophthalmosaurus has been estimated to be about 2.5 m/s or more, but even when assuming a very conservative speed of 1 m/s, an Ophthalmosaurus can reach 600 meters deep and come back to the surface in 20 minutes.

As discussed in the section on Vertebrae, fish-shaped ichthyosaurs had much thicker body trunk than their lizard-shaped predecessors. Not only did this result in stiffer body trunk, but also heavier body mass. A simple calculation suggests that fish-shaped ichthyosaurs were about six times heavier than lizard-shaped ichthyosaurs of the same body length. Heavier body weights enable longer diving durations, as mentioned earlier in this section, whereas stiff bodies are useful for efficient swimming, as mentioned in the section for Swimming. So, it seems that these to nicely came together for fish-shaped ichthyosaurs.

In conclusion, diving duration data from living deep divers support the hypothesis that at least some fish-shaped ichthyosaurs were a deep diver.

Reason 3: Bone Structure

Long bones of land vertebrates have dense sheaths that increase their mechanical strength for supporting the body on land. For marine vertebrates, which have help from buoyancy to support their body, this dense cortex layer has totally different meanings. For those animals with large lungs, the layer is usually thickened to increase the total body weight to counteract the buoyancy from the lung. For many deep divers, which have lungs that flatten during diving, the layer is spongy and hence less heavy to make up for the lack of buoyancy from the lung. The spongy cortex layer is unique to cetaceans and deep-diving pinnipeds and turtle among living vertebrates. French biologists have clarified that fish-shaped ichthyosaurs also had spongy cortex layer. So it is likely that these ichthyosaurs were deep divers.

Reason 4: Squid Eaters

Squid-eating diets are common among living deep divers. Most fish-shaped ichthyosaurs were squid eaters, as stated in the section on Diets. Then, it seems reasonable that some of them did dive deep to catch the preys, as in living squid eaters. Squids in the Mesopelagic zone are slow, as in the other organisms in the same habitat. Epipelagic organisms with higher activity rates can take advantage by getting down to the Mesopelagic zone.

The life in the ocean is largely controlled by the amount of light. It is a known fact that a layer of organisms called DSL (or the false bottom) goes up and down in the ocean at daily basis, following a certain level of light. It is at 300 to 500 meters deep during the day, but could come to the surface at night. At least some living deep divers change their diving depth accordingly, and ichthyosaurs, being visual rather than acoustic animals, probably did the same.

Pointless Objections

I have heard some false objections against the deep diving habits of ichthyosaurs. Some are quite funny. I will list a few for clarification.

Ichthyosaurs may have been obligatorily nocturnal.--Well, it is a possibility, but quite unlikely. Sleeping in the broad sunlight is the last thing you would want to do in the ocean. Also, ichthyosaurs would need very low metabolic rates to keep from drowning while asleep. It is more likely that ichthyosaurs were active both during the day and at night, as in living thunniform swimmers, changing the diving depth according to the light level, as discussed earlier in the Squid Eater section.

Rocks containing Ophthalmosaurus fossils indicate shelf environment.--It is true, but pointless. That's where we find whale fossils too. Living deep divers come inshore frequently, so why not Ophthalmosaurus? We have so little of truly oceanic sediments from the Jurassic anyway.

Having a sclerotic ring does not mean deep diving.--This is the most pointless objection I've heard so far. The answer is, of course not. Who said it did anyway? See the Eye section for the function of the sclerotic ring.

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Last updated on November 15, 2000