fossils

The evolution of ichthyosaurs, the mighty "fish-lizards" of the Mesozoic

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Vasika
Udurawane

Writer
They arose millions of years before the first dinosaurs and dominated the seas in a vast array of finned fish-like and dolphin-like forms. Some were hardly the size of a small porpoise while some were as vast and gentle as today’s biggest whales and still others terrorized their kin with vicious jaws and teeth. They were the spectacular ichthyosaurs, among the most specialized of all marine reptiles.

During the Mesozoic Era, it was not merely the dinosaurs that dominated the Earth. The seas around them were home to a wide variety of oceanic reptiles, not closely related to the huge creatures on land. Some of these reptiles used four flippers to cruise through the water while others developed their bodies even further. They are known today as the ichthyosaurs, and history of their discovery and study stretches back far beyond that of dinosaur paleontology. Among the first fossils of extinct beasts from the Age of the Dinosaurs were those of odd ocean creatures, uncovered from the rocks of Lyme Regis in Dorset, England.


These creatures were first brought to light by none other than pioneering fossil hunter Mary Anning, then just a girl, in the year 1811. Soon, more of the ancient animals were found, and curiosity began to spread around both lay and scientific circles. For the longest time, it was thought that the Earth had been created by a divine power, and that the world and its species were just a few thousand years old. The idea of extinction was not familiar to the scientific community of the era. Of course, the presence of an arguably fish-like yet somehow long-jawed and reptilian animal in the cliffs of Dorset began to raise questions. Was it a fish? Was it a crocodile? Perhaps a missing link between salamanders and lizards? Or what about a type of platypus, an animal which had just been found alive? One researcher named Karl Dietrich Eberhard Koenig coined the new name “Ichthyosaurus” in 1817. The name means “fish-lizard”, and was quite evocative although this new title was not scientifically published until eight years later. Much later, in 1835, the order Ichthyosauria was named by Henri Marie Ducrotay de Blainville and both names stuck firmly.


The animals were reptiles, of course. Yet they were reptiles unlike any other alive on Earth. More ichthyosaur remains were discovered all over Europe, and not just from England’s Lyme Regis. It was soon realized that the planet was older than had been presumed, with the first Ichthyosaurus fossils dating back to the Early Jurassic, more than 190 million years old. Both the 18th and 19th centuries were filled with a number of early paleontological innovations and new discoveries, including some unique finds that allowed us to reconstruct not just the bodies but also the lifestyles of the ichthyosaurs.


Fossils in Germany’s Posidonia Shale, also an Early Jurassic site, showed mother ichthyosaurs that died in childbirth. This ichthyosaur, named Stenopterygius, was three meters long, and was also one of the commonest ichthyosaurs found here, with more a hundred specimens known to date. Many of the mothers had died while attempting to give birth to their offspring, succumbing to the pain, shock and blood loss. The portraits of the ancient mothers are still caught in the limestone blocks, and are perfectly preserved to this day. Their young are quite large, although whether or not they cared for their offspring is a matter of debate.


Stenopterygius
by Julio Lacerda


The German Stenopterygius remains also showed that most ichthyosaurs had cylindrical and streamlined bodies that were suited for a fast life in the open ocean. Skin impressions from Holzmaden show us tails that terminated in a half moon-shaped caudal fin. This fin was oriented vertically like a fish’s tail, and not like a dolphin’s fluke. Their backs bore a dorsal fin too, just like that of a shark, while the young were born tail-first like those of a dolphin. The need to crawl out of the water to give birth had thus been eliminated and they could evolve their unique, fish-like shape. This robust body was the perfect design for a highly specialized marine animal, and it was design that worked in the long run. After all, the ichthyosaurs were among the oldest of all the Mesozoic marine reptiles. Not only were their fossils pioneering discoveries in the field of paleontology but in life they also took to the seas far before any dinosaurs appeared on land.


The earliest ancestors of these fish-lizards date back to over 248 million years ago. This was during the Early Triassic, and the planet was just starting to recover from the mightiest natural calamity it had ever faced. Just a few million years earlier, a massive extinction had wiped out nearly three-quarters of life on Earth, including a wide variety of unique reptiles and some mammalian relatives. A few enterprising animals though, managed to cling on and among these were the ancestral ichthyosaurs.


Cartorhynchus
by Julio Lacerda


One such animal was described on the basis of remains found in China just two years ago. It was named and published in the journal Nature by a team comprising of world-renowned ichthyosaur specialist Ryosuke Motani and colleagues. The creature’s name was Cartorhynchus and it took the world by storm. It was a small animal around 40 centimeters long, and it had a short face and strong limbs. The animal did not have the massive eyes of the Lyme Regis beasts, although its short face might suggest some degree of specialization toward a certain food type. Little Cartorhynchus might have been a suction feeder early in the Triassic. It may have had an expandable throat cavity and jaw muscles for rapid opening and closing of its mouth, thus allowing it to suck in tiny prey rapidly. This degree of specialization is not common in animals this primitive. Its limbs were still shaped like paddles though, but it was able to climb up on land unlike its evolutionary cousins. According to Motani’s conclusions, it might not have been an actual ancestor of the later ichthyosaurs per se. It was likelier to have been a close cousin that shared a common ancestor with the later fish-lizards. This proto-ichthyosaur still did not have the unique dorsal and caudal fins of its future relatives. Instead of behaving like a whale or dolphin, it was probably more like an otter or a small seal. Of course as the ichthyosaurs proper spread out they became bigger and less terrestrial.


As the Triassic wore on, the sleek and somewhat eel-like shapes of the primitive ichthyosaurs became more commonplace and they had reached their heyday midway through the period. Some developed huge heads and massive, cutting teeth while others shed the short snouts of Cartorhynchus and its kin to become narrow and speedy fish-eaters. Still other primitive animals fed on a wide range of food, crushing small and tough-shelled creatures with strong teeth. None of them had the well-developed fin configuration of their later cousins, though at the time they were among the most advanced and successful marine reptiles. They were the only marine reptiles of their time that did not need to crawl out of the water. All Triassic ichthyosaurs were live-bearers that navigated the deep waters of the Panthalassic Ocean while the other turtle- and lizard-like reptiles around them probably stuck to the shallows. At the time this single ocean surrounded the Earth’s only continent, the famous Pangaea and the early sea reptiles could roam far and wide across its watery depths. And all the while, new reptiles had been evolving, and even fish were becoming more streamlined.


Yet ichthyosaur diversity never showed any signs of waning. Instead, the cousins of Cartorhynchus kept pushing on until they had given rise not just to some early cylindrical dolphin-like forms but also to the biggest reptile ever to inhabit the seas. This veritable leviathan was the mighty Shastasaurus, an immense animal that existed between the Middle and Late Triassic. It gives its name to a whole family of Triassic ichthyosaurs, the shastasaurids. Fossils of the largest one have been found in the Pardonet Formation of British Colombia in rocks up to 210 million years old. These remains were named in 2004 as a new species in the genus, called Shastasaurus sikanniensis. Fully grown, these massive ichthyosaurs stretched 21 meters in total length, and are the only extinct marine reptiles that come close to matching the largest modern whales in size. Two other species were described, one from China and another from Europe, each as big as a killer whale. Despite its length though, Shastasaurus was very narrow and lightly built. It had a slim physique and somewhat narrow torso.


Even the animal’s head was very small and light in comparison to its monumental body. It was slim and cylindrical and had a tail built for slow and sustained cruising rather than for swimming at great speeds. The animal also probably lacked a large tailfin like all Triassic ichthyosaurs, and its flippers too were very small. And just like modern whales, the largest animal in the Triassic oceans fed on some of the smallest and most unremarkable of animals. Shastasaurus’ small and specialized head meant that it was feeding on a wide variety of tiny fish and squid as opposed to going after large and active prey. Its skull was short-snouted, almost calling to mind the short skull of Cartorhynchus in terms of adaptations. It also had a large tongue to help it strengthen this suction effect. Evidence of this comes from fossilized hyoids, the bones in the throat that support the tongue muscles. This huge and powerful tongue might have strengthened the vacuum effect by a great degree, and would have allowed it to gather up a large quantity of food. This large size was repeated many more times among the ichthyosaurs, with Shastasaurus’ relatives reaching comparable dimensions and even later ichthyosaurs doing the same.


However, for all their size and seemingly indestructible nature, the whale-like ichthyosaurs would not last long. The end of the Triassic brought the best days of these gentle giants to a crashing halt, and very few continued the trend of vastness into the opening days of the Jurassic. For the most part, the diversity in body forms had been whittled down to a streamlined, torpedo-shaped body. The jaws were mostly long and toothy, while the eyes were large and there was now both a tailfin and a dorsal fin. This is the type of animal that Mary Anning found at Lyme Regis, and the kind of animal that also set the standard for all ichthyosaurs to come. Her English Ichthyosaurus, and the German Stenopterygius, both as big as dolphins, were of this nature. They were light, small and speedy hunters which targeted ever-evolving schools of fish and squid. Their old Panthalassic world was also no more, for Pangaea had broken into two separate supercontinents with the Tethys Ocean in between.


Temnodontosaurus
by Julio Lacerda


Early in the period there were both big and predatory temnodontosaurids and the long-snouted leptonectids among the odder ichthyosaurs, while the thunnosaurians, like Ichthyosaurus itself, set the standard for the majority. The name Thunnosauria means “tuna lizard”, and they were indeed somewhat like a big, open-water fish. This was the shape of the things for millions of years to come, and the relatives of Stenopterygius would carry on for millions of years.


Studies of Stenopterygius show us that they were incredibly fast, with speed being the norm for the family as a whole. As time passed, they grew ever-sturdier and faster, becoming more specialized for a lifestyle of chasing after swift prey. However, even this asset was not enough for them anymore. Even though there were still giants in the ichthyosaurs’ ranks, their place would be usurped by a whole host of new Jurassic predators. Among these were the plesiosaurs, which swam by using all four flippers rather than just a finned tail. These animals consisted of both long-necked fish-eaters and generalists, and massive oceanic monsters with huge heads and incredibly powerful jaws. The latter groups were big predators that took on prey either somewhat smaller than themselves or similar to their own body sizes.


The plesiosaurs were also live-bearing marine reptiles and were only the first to challenge the ichthyosaurs for dominance of the Jurassic oceans. Even crocodiles had started colonizing the oceans. A few remained as they were, armored beasts with crushing jaws that could climb back on land to lay their eggs. Some others became more fish-like, with fins and flippers that enabled to exploit open ocean environment. Some of them were fish-eaters while some gigantic ones tackled similar-sized reptiles with their slicing teeth. Fish too were beginning to grow much swifter, and less heavy than their ancient cousins. A new lineage of quick-moving ray-finned fish had begun to emerge and these proved to be a perfect challenge for the already speedy ichthyosaurs. Stenopterygius too would soon find itself replaced by more robust animals, with the final ichthyosaur stragglers breathing their last breaths halfway through the Cretaceous. These last of the ichthyosaurs had survived a number of catastrophes, including the extinction event at the end of the Jurassic, which further cut down their number and diversity. They were called the ophthalmosaurids, and their ancestry lies in the middle of the Jurassic Period, after Anning’s famous ichthyosaurs were long since dead.


The later ophthalmosaurids took their streamlining and hunting prowess to the extreme. All of these creatures were muscular and robust, and a few had very large eyes and front paddles. Their family name comes from the term “eye lizard”, a reference to these giant, rounded peepers which were strengthened by a tough sclerotic ring like those of many other ichthyosaurs. They were both fast and deep-diving, and managed to cross the Jurassic-Cretaceous border without much trouble. Earlier it was presumed that they had all died out save for one or two genera, an idea hit upon due to the rarity of fossils from the very earliest Cretaceous. New finds though, are beginning to overturn this old theory. Not only did numerous ichthyosaurs cross over from the Jurassic after having evolved halfway through this period, but they positively thrived during the Early Cretaceous.


For a long time, most of these Cretaceous ichthyosaurs were thought to be part of a single genus called Platypterygius. It was once a wastebasket taxon, a dumping ground for many late ichthyosaurs which were later split apart after further clarification that they were separate animals. Fossils as far apart as Russia, Ukraine, America and Australia were once part of this seemingly gigantic genus, which is now limited to a few species. One of the best-known species existed during a time when Australia was closer to the South Pole and was splitting away from Antarctica, with a shallow sea known as the Eromanga Sea existing between the two landmasses. It was a rather cold southern seaway filled with a number of fish, squid, ammonites and marine turtles. Living alongside Platypterygius were both macropredatory plesiosaurs and their long-necked cousins. During the Early Cretaceous these creatures dominated the oceans alongside an array of massive and modern-looking sharks as well as beautifully streamlined bony fish which were also becoming speedy hunters. Despite this diversity of carnivores, the ichthyosaurs were not yet done with their time. Platypterygius was a big animal, more than seven meters in length, bigger than a great white shark. It was a powerful and long-jawed predator capable of tackling almost any kind of small prey. It is one of the best-known of all late ichthyosaurs, with even pregnant females being known in the fossil record and also the final one to go extinct.


The last of them disappeared more than 30 million years before the extinction that killed the dinosaurs, and the exact reasons behind this death were puzzling for several years. These days, scientists may have some answers that might help to solve the riddle, including those relating to the global climate. They were living through a time of massive climatic shifts as the continents broke up even further, changing the former shallow seaways around the world. One particular event that happened midway through this period was the Cretaceous Thermal Maximum, which occurred due to a higher rate of global warming. The Early Cretaceous world seems to have been somewhat cool for the most part. This may not have been the only reason though, at least not according to a study by Dr. Valentin Fischer, of the University of Liège, Belgium, and the University of Oxford, UK. He and his team have uncovered the startling idea that ichthyosaurs were simply slower to evolve during their latter days. The ophthalmosaurids represented just the one ecological type that worked in the long run while all others had died out earlier.


The team found out that there were at least seven or so ecological morphs during the Early Jurassic, during the peak of the group’s success. Not only were there fast swimmers and big predators but also swordfish-like hunters and nearshore-dwellers that had a generalized diet. But as time went by, these were cut down to just two, and finally to one, the advanced and generalized ophthalmosaurids represented by Platypterygius. This bizarrely slow evolutionary rate was not helped by the increasing diversity of other efficient marine hunters and the ever-present climatic disruptions. The last of these wonderful marine predators died out, unable to keep up with the changing Cretaceous world. Their reign lasted more than 168 million years and while they did not see the asteroid that killed the dinosaurs, their line gave rise to some of the biggest, strangest and most adaptable of all swimming reptiles.

Image Credit: Julio Lacerda

3986b46bb4f35aa1ff4d42167a12e0fc

Vasika
Udurawane

Writer


http://sulc.us/fktvz
http://www.eartharchives.org/articles/the-evolution-of-ichthyosaurs-the-mighty-fish-lizards-of-the-mesozoic/