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Ichthyosaur Article In Science News
Nice cover too: http://www.sciencenews.org/20020824/cover.jpg
Several factors suggest that at least some ichthyosaurs had metabolisms
unlike those of modern reptiles. For example, today's marine iguanas are
still tied to the land. They must climb out of the water and bask in the
sun between feedings to keep their body temperature up and their
biochemistry active, says Ryosuke Motani, a vertebrate paleontologist at
the Royal Ontario Museum in Toronto. Ichthyosaurs couldn't leave the
water, so they must have generated some heat internally. Their large body
mass would also have helped the reptiles maintain a body temperature
higher than the surrounding water, just like modern leatherback turtles
do, Motani notes.
Furthermore, the streamlined shape and the skeletal characteristics of
some ichthyosaurs suggest that these animals cruised efficiently. Using
the same sort of equations with which engineers analyze fluid flow around
boats and aircraft, Motani found that species in the ichthyosaur genus
Stenopterygius had an optimal cruising speed of about 1 meter per second.
That's the same speed range as today's Pacific blue marlin and yellowfin
tuna, which have elevated metabolisms fueled by a diet similar to the
ichthyosaur's. Motani reports his findings in the Spring 2002
Certain skeletal features of the thunniform, or tuna-shaped,
Stenopterygius also hint that the animal was a fast cruiser, says Emily A.
Buchholtz, a vertebrate paleontologist at Wellesley (Mass.) College. The
creature's vertebrae are shaped like hockey pucks, and they're stacked so
close to one another that the spine is essentially unbendable. In the base
of the crescent-shaped tail, the ends of the bones are somewhat rounded,
which suggests there was some flexibility there.
Buchholtz says it's probable that Stenopterygius swam just like a tuna
does, flicking its tail back and forth while holding most of its body
rigid. This so-called oscillatory swimming style would keep the
ichthyosaur more streamlined than an undulating swimmer like, say, an eel.
Buchholtz analyzed the likely swimming modes of various ichthyosaurs in
the March 2001 Journal of Vertebrate Paleontology.
But some ichthyosaurs - especially early species that still had the long
tail, flexible spine and the lizardlike proportions of their landlubber
ancestorsprobably undulated their bodies when they swam. That motion is
less efficient because there's more fluid drag on the body. Therefore,
it's likely that these long, slim ichthyosaurs couldn't swim as fast as
their thunniform cousins, says Buchholtz.
The faster an ichthyosaur could swim, the deeper it could dive on a single
breath to chase its prey. And there's plenty of evidence to suggest
ichthyosaurs foraged at great ocean depths, says Motani. For starters,
some of the more streamlined ichthyosaurs had extremely large eyes.
Temnodontosaurus, which had a body length of about 9 m, had the largest
eyes of any animal known. One specimen's eyes are more than 26 centimeters
across, or larger than a dinner plate. Another thunniform species, the
aptly named Ophthalmosaurus, was only about 4 m long but had eyes more
than 22 cm across, the largest eyes relative to its body size of any known
creature. By comparison, today's champion, the giant squid, has eyes about
25 cm in diameter, and blue whales have eyes 15 cm across, the largest of
any living vertebrate.
Large eyes could house more light-gathering cells and therefore be more
sensitive than small ones. However, two Scottish researchers argue that
ichthyosaurs had outsized eyes not only for overall sensitivity but for
focusing on small, quick prey at great ocean depths. Greater visual acuity
in low light would also enable deep-diving ichthyosaurs to cooperate while
hunting, say Stuart Humphries and Graeme D. Ruxton of the University of
Glasgow. The pair's research appeared in the Feb. 15 Journal of
New high-tech analyses of a particularly well-preserved ichthyosaur skull
taking place nearly a world away from Scotland hint that some of the
ancient marine reptiles may, in fact, have possessed supplementary senses
for detecting prey at short range. Late last year, Benjamin P. Kear, a
paleontologist at the South Australian Museum in Adelaide, and George
Kourlis, a radiographer at the Royal Adelaide Hospital, took CT scans of
the skull of a juvenile ichthyosaur.
The scans showed delicate internal nasal structures that formed from
bones in the reptile's palate and the roof of its skull. These features,
which haven't been seen before in ichthyosaur fossils, may have been
related to the animal's sense of smell, says Kear. The inside of the
fossil's skull bears an imprint of brain lobes that correspond to modern
brain regions dedicated to interpreting sight and smell.
The fossil skull's upper and lower jaws reveal deep channels and grooves
that once held nerves and blood vessels. Although scientists have seen
such anatomy in other ichthyosaur fossils, Kear says that in this fossil,
it's clear for the first time that the channels are associated with bony
cavities that once housed branches of the trigeminal nerve. That major,
three-branched nerve transmits sensations from broad regions of an
animal's face, upper jaw, and lower jaw. The channels could have housed
some elaborate sensory system.
Many people, when asked to name the largest prehistoric predator,
immediately think of Tyrannosaurus rex. Think again. Although some
land-dwelling relatives of T. rex actually were slightly longer than the
tyrant lizard king, a soon-to-be-described ichthyosaur dwarfs them all.
Even a small member of the new species would have matched the size of a
typical blue whale, the largest vertebrate swimming in today's oceans.
The creature's almost complete preservation enables the paleontologists
to confidently peg the ichthyosaur's length at 23 m. The skull alone was
5.8 m long, and each broad, tapered flipper was 5.3 m long. The largest of
the creature's hockey-puck-shaped vertebrae is 27 cm across. The
researchers intend to publish their description of the new species next
year, says Nicholls.