Sea dragons: big news about ichthyosaurs, which cruised ancient oceans while dinosaurs ruled the land.
Ichthyosaurs swam through prehistoric seas for more than 150 million years, almost as long as their dinosaur cousins ruled the land. While some of the creatures retained the lizardlike proportions of their ancestors, others were as sleek as porpoises and probably had a lifestyle similar to that of those modern mammals. Analyses of ichthyosaur fossils are shedding new light not only on their body structure, but also on what they ate and how they may have homed in on their prey. Fossils still being teased from the rock strongly hint that the largest predator ever on our planet may well have been an ocean-dwelling ichthyosaur.
OUT TO SEA The bones inside an ichthyosaur's flippers betray the creature's descent from land animals. What in ancestral species had been the upper leg bone became short, broad, and flat in ichthyosaurs. The bones of the ankles and feet also took on a paddle shape, and individual digits were closely packed within a streamlining envelope of soft tissue. Such flippers wouldn't have supported the animal's weight on land, says Larry D. Martin, a vertebrate paleontologist at the University of Kansas in Lawrence.
Many ichthyosaur fossils come from layers of limestone formed out of ocean-floor ooze that was particularly good at preserving fine details of the creatures it entombed. In some cases, it even recorded the outlines of soft tissues. These relics reveal that some ichthyosaur species had smooth skin, dorsal fins, and a vertical, crescent-shaped tail.
The fine-grained sediments that encased the ancient reptiles also preserved evidence of the creatures' stomach contents, giving paleontologists insight into the creatures' diet. Many ichthyosaur species ate prodigious amounts of belemnites, extinct relatives of squid that had long, torpedo-shaped internal skeletons and tough hooks on their arms. Some ichthyosaur remains contain hundreds of belemnite shells and thousands of their hooks, says Martin.
What's more, Martin notes, ichthyosaur diversity waxed and waned with the planet's climate. When average worldwide temperatures were high, many species of the ancient reptiles flourished. Sediments laid down during global cool spells record few ichthyosaur species.
Although the first ichthyosaurs and dinosaurs evolved at about the same time, these reptiles didn't go extinct together. Ichthyosaurs gradually disappear from the fossil record of about 90 million years ago, a full 25 million years before mass die-offs wiped out the dinosaurs.
LEAPIN' LIZARDS? 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 Paleobiology.
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 ancestors--probably 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.
What's more, an undulatory mode of swimming may have had detrimental effects on an ichthyosaur's breathing, says Richard Cowen, a biologist at the University of California, Davis. Among today's air-breathing animals that flex their torso side-to-side when they walk--lizards and salamanders, for example--none can run and breathe at the same time.
If an ichthyosaur undulated its body while swimming, it probably couldn't go more than 100 m or so at full speed without taking a breath, says Cowen, and it would need to pause when it came to the surface for air.
One way around this limitation would have been to adopt a swimming style known as porpoising. By leaping from the water as they took a breath, just as porpoises and other aquatic mammals often do today, ichthyosaurs could inhale enough oxygen to sustain a high speed while chasing prey, escaping predators, or traveling long distances.
Even if thunniform ichthyosaurs could hold their bodies perfectly rigid when swimming, Cowen, speculates they may have porpoised, anyway. Dolphins, killer whales, and even penguins adopt this swimming style when migrating long distances. It enables sea animals, while taking breaths during leaps, to avoid the fluid drag that is greatest just below the water's surface.
Buchholtz agrees that thunniform ichthyosaurs didn't have to porpoise when they swam but that they probably had the speed and the body shape to do it. "It's neat to think that they might have," she adds.
GO DEEP 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 icthyosaurs 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 Experimental Biology.
In their report, the scientists pointed out that some modern mammals, such as seals, forage at depths up to twice those estimated for Ophthalmosaurus, yet seals don't have large eyes. However, many of those mammals depend on other senses to pinpoint their prey during the last moments of the chase, Humphries says. For example, seals have whiskers that can detect subtle changes in water flow caused by fleeing prey, and some toothed whales use sonar to detect and home in on their victims.
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.
For instance, the ichthyosaur might have had electroreceptors in the skin of its face and jaws, says Kear. Those sensor cells might be akin to the cells lining the heat-sensitive pits in some snakes or the ones that some fish and sharks use to detect electric fields emitted or disturbed by prey. The young ichthyosaur, like many members of related species, could have used such a system to detect prey directly in front of it, says Kear. Although the reptile had large eyes, they pointed sideways, and the animal therefore could not see straight ahead.
The fossil that Kear analyzed had been extracted from a limestone nodule unearthed from 110-million-year-old sediments near Hughenben, Queensland. It was only 5.6 m long, but adults of the species--the sole Australian member of the worldwide genus Platypterygius--grew to a maximum length of about 8 m. Kear's juvenile had a mouth packed with 200 stout, conical teeth whose shape suggests that the ichthyosaur crushed its prey. Each tooth was about 4 cm long, 1 to 2 cm of which would have protruded from the animal's gums. And, says Kear, the ancient reptile was stricken with an ailment seen all too often in kids these days, but never before in ichthyosaurs: One of the teeth had a cavity.
Kear and his colleagues are now working with the partial remains of a pregnant female Platypterygius. The abdominal cavity of that fossil, besides containing the skeleton of a fetus, is yielding new clues about the adult's diet. The stomach contents consisted not of belemnites, but of fish and hatchling turtles. The 6-cm-long turtles had been crushed and swallowed whole. Because the turtle remains didn't show any signs of being digested, Kear concludes that the ichthyosaur died soon after its [Text unreadable in original source.] meal.
BIG KAHUNA-SAURUS 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 nearly complete fossil of the 210-million-year-old aquatic behemoth was found eroding from the streambed of the Sikanni Chief River in northern British Columbia in 1991, says Elizabeth Nicholls of the Royal Tyrrell Museum in Drumheller, Alberta. Excavations were difficult because the river floods the bone site for part of the year. Also, the large blocks of limestone that held the fossil--some of them weighing 4,000 kg--had to be helicoptered out of the remote location.
The fossil is missing about 2 m of backbone. The part that's gone, which held the creature's hind limbs, was scoured away by the river, says Nicholls. She and her colleagues have removed the rock surrounding the fossil's skull, front limbs, and tail, and they're working their way from each end toward the center of the fossil. Some of the bones from the tips of the front flippers, found in sediments nearby, had been scattered by strong currents that swept the ocean bottom where the corpse originally fell. Only the skull was crushed during fossilization.
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.
Even though this specimen's length easily tops the previous record for ichthyosaurs--held by a 15-m-long Shonisaurus--there's evidence that its relatives got much larger. Isolated fossil vertebrae of other animals from the same species, taken from this site and others in British Columbia, are 36 cm across. Without knowing which part of the ichthyosaur's spine these isolated bones came from, the length of the animal that grew those bones can't be estimated, Nicholls says.
The eating habits of these giants remain mysterious. Members of the newly found species swam with an undulatory motion and presumably were too slow to pursue prey, says Nicholls. However, the shape of the long, slender, toothless snout suggests the animal wasn't a filter feeder either. Such a filtering configuration couldn't process enough seawater to nourish the bulky beast. She suggests that the ichthyosaur was an ambush predator, lunging at its meals as they swam past.
Because the scientists haven't yet exposed the creature's body, they don't know if its last meal has been preserved. Whatever--and however--the ichthyosaur ate, Nicholls says, it likely was the largest predator that ever lived.
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|Date:||Aug 24, 2002|
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