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