Temporal range: 245–90Ma
Middle Triassic - Late Cretaceous
|Diversity of ichthyosaurs|
Well, I had planned to write once every week or so, but, as you can see, that hasn’t gone to plan so far. In theory that should mean that I have a significant amount to write about now, but…
Since last posting, I have (finally) begun my PhD at Bristol University, and after two weeks at university I had finally done a plan for my study; at least the first two years. I have also completed a funding proposal to the Palaeontological Association to allow me to visit some museums. This does mean that over the past seven weeks I have actually written something down! which hadn’t happened in the previous four months. Other than that, most of my time has been spent finding papers to read, and then reading them.
Now is probably a good time to start going through some of the basics of my research project. The title of the project is ‘Ichthyosaurs of the Late Jurassic’, and I will describe the two main constituents of that title in this and the next post; starting with ichthyosaurs.
The name ‘ichthyosaur’ (ICK-THEE-o-SAWR) comes from the Greek ‘ichthyos’, meaning fish, and ‘sauros’, meaning lizard, so they are ‘fish lizards.’ This name was assigned for this group of animals based upon their striking similarity to fish (see below). The group, or clade, to which ichthyosaurs belong is variously called Ichthyopterygia or Ichthyosauria and contains 49 valid genera and 75 valid species. (Maisch 2010). As seen from the photo, ichthyosaurs have an elongate, narrow jaw; occasionally large eyes; four paddle-like limbs, with the forelimbs larger than the hindlimbs, formed by many tessellating finger/foot bones; tail fin defined by a ‘kink’ (apex) it the backbone, with vertebrae on the ventral (frontside) edge (McGowan and Motani 2003).
Ichthyosaurs first appeared in the Lower Triassic (~240 million years ago) as already specialised aquatic animals, probably also with live birth too. This sudden appearance as such a derived animal has led to much debate over ichthyosaurs origins and how they are related to other vertebrates. It has been variously suggested that ichthyosaurs are related to the early amphibians and turtles before more recently settling upon a diapsid (a group comprising , among others, lizards, dinosaurs and birds) origin. Maisch’s (2010) recent attempt to resolve this using two high profile amniote datasets remains places Ichthyosauria within diapsids but is not conclusive on the relationships within.
Triassic ichthyosaurs (e.g. Mixosaurus below, skeletal reconstructions from Sander (2000)) have been the main focus of ichthyosaurs study over the past 30 years as new finds, especially from China, have been uncovered. These earlier ichthyosaurs, although already highly derived, show a definite progression of evolution through the Triassic. The most distinctive differences are having more similarly sized fore- and hindlimbs and more elongate tail with less defined apex.
Following a mass extinction at the end of the Triassic (~200 million years ago), ichthyosaurs with a different form became dominant. These thunniform (literally ‘tuna-shaped’) ichthyosaurs (e.g. Stenopterygius below) include the famous species Ichthyosaurus, from the Lias rocks of Lyme Regis in Dorset, England, and Stenopterygius from the Posidonienschiefer of southwest Germany which have both been found in huge abundances. The thunniform ichthyosaurs had the more characteristic fusiform shape, much larger forelimbs and crescent-shaped tail fin. This morphology is found in all ichthyosaurs until they became extinct. In the Early Jurassic (~200–~170 million years ago) there was a particularly high diversity, although many of these had a very similar body plan, and so disparity was lower compared to the Late Triassic (Thorne, Ruta and Benton 2011).
In the Middle and Late Jurassic (~170–~145 million years ago) the diversity of ichthyosaurs decreased: the Late Jurassic hosts only ~7 species. Many of these are known only from a few specimens, however Ophthalmosaurus (see image below), from the Oxfordian (~160–~155 million years ago) of Europe and America, is know from hundreds of specimens, particularly from the Leeds collection housed dominantly in the Natural History Museum in London. Ophthalmosaurus means ‘eye lizard’ and is so named because it has the largest eye relative to its body size of any known animal. The Cretaceous (144–65.5 million years ago) ichthyosaurs are almost entirely contained within the genus Platypterygius. This ichthyosaur has been found worldwide, from Australia to America to the United Kingdom.
The ichthyosaurs became extinct in the middle of the Cretaceous (~90 million years ago), about 30 million years before the dinosaurs. The reasons for this are unknown and has been hotly debated. At the same time there were many small extinction events occurring. Ichthyosaurs were also not very diverse (only Platypterygius remained). Competition from other groups had increased, particularly with the appearance of mosasaurs, which had a somewhat similar morphology. It is most like that a combination of these caused the ichthyosaur’s demise.
This has been a, rather long, summary of ichthyosaur evolution through their existence. Hopefully it has been comprehensible and informative. Soon (and I mean soon), I will write a basic introduction to geological time (explaining the difference between Triassic, Jurassic and Cretaceous). After that I plan to describe my PhD plan in more detail and then go on to more specifics of ichthyosaurs: what we know, what we think we do, and what we simply don’t.
MAISCH, M. W. 2010. Phylogeny, systematics, and origin of the Ichthyosauria—the state of the art. Palaeodiversity, 3, 151–214.
MCGOWAN, C. and MOTANI, R. 2003. Ichthyopterygia. In SUES, H.-D. (ed.) Handbook of Paleoherpetology, Vol. 8. Verlag Dr. Friedrich Pfeil, Munich, 175 pp.
SANDER, P. M. 2000. Ichthyosauria: their diversity, distribution, and phylogeny. Paläontologische Zeitschrift, 74, 1–35.
THORNE, P. M., RUTA, M. and BENTON, M. J. 2011. Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary. Proceedings of the National Academy of Sciences, 108, 8339–8344.
About BenI am a PhD student at the University of Bristol, studying Middle and Late Jurassic ichthyosaurs. Aside from palaeontology, I enjoy listening to, playing and composing (predominantly) classical music. My favourite composers are Jean Sibelius, Arnold Bax and Gustav Mahler.
|Picture a late autumn evening some 160 million years ago,
during the Jurassic time period, when dinosaurs inhabited the
continents. The setting sun hardly penetrates the shimmering surface of a
vast blue-green ocean, where a shadow glides silently among the dark
crags of a submerged volcanic ridge. When the animal comes up for a gulp
of evening air, it calls to mind a small whale--but it cannot be. The
first whale will not evolve for another 100 million years. The shadow
turns suddenly and now stretches more than twice the height of a human
being. That realization becomes particularly chilling when its long,
tooth-filled snout tears through a school of squidlike creatures.
The remarkable animal is Ophthalmosaurus, one of more than 80 species now known to have constituted a group of sea monsters called the ichthyosaurs, or fish-lizards. The smallest of these animals was no longer than a human arm; the largest exceeded 15 meters. Ophthalmosaurus fell into the medium-size group and was by no means the most aggressive of the lot. Its company would have been considerably more pleasant than that of a ferocious Temnodontosaurus, or "cutting-tooth lizard," which sometimes dined on large vertebrates.
Despite these compelling questions, the opportunity to unravel the enigmatic transformation from landlubbing reptiles to denizens of the open sea would have to wait almost two centuries. When dinosaurs such as Iguanodan grabbed the attention of paleontologists in the 1830s, the novelty of the fish-lizards faded away. Intense interest in the rulers of the Jurassic seas resurfaced only a few years ago, thanks to newly available fossils from Japan and China. Since then, fresh insights have come quickly.
Although most people forgot about ichthyosaurs in the early 1800s, a few paleontologists did continue to think about them throughout the 19th century and beyond. What has been evident since their discovery is that the ichthyosaurs' adaptations for life in water made them quite successful. The widespread ages of the fossils revealed that these beasts ruled the ocean from about 245 million until about 90 million years ago--roughly the entire era that dinosaurs dominated the continents. Ichthyosaur fossils were found all over the world, a sign that they migrated extensively, just as whales do today. And despite their fishy appearance, ichthyosaurs were obviously air-breathing reptiles. They did not have gills, and the configurations of their skull and jawbones were undeniably reptilian. What is more, they had two pairs of limbs (fish have none), which implied that their ancestors once lived on land.
Paleontologists drew these conclusions based solely on the exquisite skeletons of relatively late, fish-shaped ichthyosaurs. Bone fragments of the first ichthyosaurs were not found until 1927. Somewhere along the line, those early animals went on to acquire a decidedly fishy body: stocky legs morphed into flippers, and a boneless tail fluke and dorsal fin appeared. Not only were the advanced, fish-shaped ichthyosaurs made for aquatic life, they were made for life in the open ocean, far from shore. These extreme adaptations to living in water meant that most of them had lost key features--such as particular wrist and ankle bones--that would have made it possible to recognize their distant cousins on land. Without complete skeletons of the very first ichthyosaurs, paleontologists could merely speculate that they must have looked like lizards with flippers.
The early lack of evidence so confused scientists that they proposed almost every major vertebrate group--not only reptiles such as lizards and crocodiles but also amphibians and mammals--as close relatives of ichthyosaurs. As the 20th century progressed, scientists learned better how to decipher the relationships among various animal species. On applying the new skills, paleontologists started to agree that ichthyosaurs were indeed reptiles of the group Diapsida, which includes snakes, lizards, crocodiles and dinosaurs. But exactly when ichthyosaurs branched off the family tree remained uncertain--until paleontologists in Asia recently unearthed new fossils of the world's oldest ichthyosaurs.
The first big discovery occurred on the northeastern coast of Honshu, the main island of Japan. The beach is dominated by outcrops of slate, the layered black rock that is often used for the expensive ink plates of Japanese calligraphy and that also harbors bones of the oldest ichthyosaur, Utatsusaurus. Most Utatsusaurus specimens turn up fragmented and incomplete, but a group of geologists from Hokkaido University excavated two nearly complete skeletons in 1982. These specimens eventually became available for scientific study, thanks to the devotion of Nachio Minoura and his colleagues, who spent much of the next 15 years painstakingly cleaning the slate-encrusted bones. Because the bones are so fragile, they had to chip away the rock carefully with fine carbide needles as they peered through a microscope. As the preparation neared its end in 1995, Minoura, who knew of my interest in ancient reptiles, invited me to join the research team. When I saw the skeleton for the first time, I knew that Utatsusaurus was exactly what paleontologists had been expecting to find for years: an ichthyosaur that looked like a lizard with flippers. Later that same year my colleague You Hailu, then at the Institute for Vertebrate Paleontology and Paleoanthropology in Beijing, showed me a second, newly discovered fossil--the world's most complete skeleton of Chaohusaurus, another early ichthyosaur. Chaohusaurus occurs in rocks the same age as those harboring remains of Utatsusaurus, and it, too, had been found before only in bits and pieces. The new specimen clearly revealed the outline of a slender, lizardlike body.
Utatsusaurus and Chaohusaurus illuminated at long last where ichthyosaurs belonged on the vertebrate family tree, because they still retained some key features of their land-dwelling ancestors. Given the configurations of the skull and limbs, my colleagues and I think that ichthyosaurs branched off from the rest of the diapsids near the separation of two major groups of living reptiles, lepidosaurs (such as snakes and lizards) and archosaurs (such as crocodiles and birds). Advancing the family-tree debate was a great achievement, but the mystery of the ichthyosaurs' evolution remained unsolved.
From Feet to Flippers
But examination of fossils ranging from lizard- to fish-shaped--especially those of intermediate forms--revealed that the evolution from fins to feet was not a simple modification of the foot's five digits. Indeed, analyses of ichthyosaur limbs reveal a complex evolutionary process in which digits were lost, added and divided. Plotting the shape of fin skeletons along the family tree of ichthyosaurs, for example, indicates that fish-shaped ichthyosaurs lost the thumb bones present in the earliest ichthyosaurs. Additional evidence comes from studying the order in which digits became bony, or ossified, during the growth of the fish-shaped ichthyosaur Stenopterygius, for which we have specimens representing various growth stages. Later, additional fingers appeared on both sides of the preexisting ones, and some of them occupied the position of the lost thumb. Needless to say, evolution does not always follow a continuous, directional path from one trait to another.
Backbones Built for Swimming
The new lizard-shaped fossils have also helped resolve the origin of the skeletal structure of their fish-shaped descendants. The descendants have backbones built from concave vertebrae the shape of hockey pucks. This shape, though rare among diapsids, was always assumed to be typical of all ichthyosaurs.
Undulatory swimming enables predators to thrive near shore, where food is abundant, but it is not the best choice for an animal that has to travel long distances to find a meal. Offshore predators, which hunt in the open ocean where food is less concentrated, need a more energy-efficient swimming style. Mackerel sharks solve this problem by having stiff bodies that do not undulate as their tails swing back and forth. A crescent-shaped caudal fin, which acts as an oscillating hydrofoil, also improves their cruising efficiency. Fish-shaped ichthyosaurs had such a caudal fin, and their thick body profile implies that they probably swam like mackerel sharks.
Inspecting a variety of shark species reveals that the thicker the body from top to bottom, the larger the diameter of the vertebrae in the animal's trunk. It seems that sharks and ichthyosaurs solved the flexibility problem resulting from having high numbers of body segments in similar ways. As the bodies of ichthyosaurs thickened over time, the number of vertebrae stayed about the same. To add support to the more voluminous body, the backbone became at least one and a half times thicker than those of the first ichthyosaurs. As a consequence of this thickening, the body became less flexible, and the individual vertebrae acquired their hockey-puck appearance.
Drawn to the Deep
The ichthyosaurs' invasion of open water meant not only a wider coverage of surface waters but also a deeper exploration of the marine environment. We know from the fossilized stomach contents of fish-shaped ichthyosaurs that they mostly ate squidlike creatures known as dibranchiate cephalopods. Squid-eating whales hunt anywhere from about 100 to 1,000 meters deep and sometimes down to 3,000 meters. The great range in depth is hardly surprising considering that food resources are widely scattered below about 200 meters. But to hunt down deep, whales and other air-breathing divers have to go there and get back to the surface in one breath--no easy task. Reducing energy use during swimming is one of the best ways to conserve precious oxygen stored in their bodies. Consequently, deep divers today have streamlined shapes that reduce drag--and so did fish-shaped ichthyosaurs.
Characteristics apart from diet and body shape also indicate that at least some fish-shaped ichthyosaurs were deep divers. The ability of an air-breathing diver to stay submerged depends roughly on its body size: the heavier the diver, the more oxygen it can store in its muscles, blood and certain other organs--and the slower the consumption of oxygen per unit of body mass. The evolution of a thick, stiff body increased the volume and mass of fish-shaped ichthyosaurs relative to their predecessors. Indeed, a fish-shaped ichthyosaur would have been up to six times heavier than a lizard-shaped ichthyosaur of the same body length. Fish-shaped ichthyosaurs also grew longer, further augmenting their bulk. Calculations based on the aerobic capacities of today's air-breathing divers (mostly mammals and birds) indicate that an animal the weight of fish-shaped Ophthalmosaurus, which was about 950 kilograms, could hold its breath for at least 20 minutes. A conservative estimate suggests, then, that Ophthalmosaurus could easily have dived to 600 meters--possibly even 1,500 meters--and returned to the surface in that time span. Bone studies also indicate that fish-shaped ichthyosaurs were deep divers. Limb bones and ribs of four-limbed terrestrial animals include a dense outer shell that enhances the strength needed to support a body on land. But that dense layer is heavy. Because aquatic vertebrates are fairly buoyant in water, they do not need the extra strength it provides. In fact, heavy bones (which are little help for oxygen storage) can impede the ability of deep divers to return to the surface. A group of French biologists has established that modern deep-diving mammals solve that problem by making the outer shell of their bones spongy and less dense. The same type of spongy layer also encases the bones of fish-shaped ichthyosaurs, which implies that they, too, benefited from lighter skeletons.
The size of their eyes also suggests that visual capacity improved as ichthyosaurs moved up the family tree. These estimates are based on measurements of the sclerotic ring, a doughnut-shaped bone that was embedded in their eyes. (Humans do not have such a ring--it was lost in mammalian ancestors--but most other vertebrates have bones in their eyes.) In the case of ichthyosaurs, the ring presumably helped to maintain the shape of the eye against the forces of water passing by as the animals swam, regardless of depth.
The diameter of the sclerotic ring makes it possible to calculate the eye's minimum f-number--an index, used to rate camera lenses, for the relative brightness of an optical system. The lower the number, the brighter the image and therefore the shorter the exposure time required. Low-quality lenses have a value of f/3.5 and higher; high-quality lenses have values as low as f/1.0. The f-number for the human eye is about 2.1, whereas the number for the eye of a nocturnal cat is about 0.9. Calculations suggest that a cat would be capable of seeing at depths of 500 meters or greater in most oceans. Ophthalmosaurus also had a minimum f-number of about 0.9, but with its much larger eyes, it probably could outperform a cat.
Many characteristics of ichthyosaurs--including the shape of their bodies and backbones, the size of their eyes, their aerobic capacity, and their habitat and diet--seem to have changed in a connected way during their evolution, although it is not possible to judge what is the cause and what is the effect. Such adaptations enabled ichthyosaurs to reign for 155 million years. New fossils of the earliest of these sea dwellers are now making it clear just how they evolved so successfully for aquatic life, but still no one knows why ichthyosaurs went extinct.
Loss of habitat may have clinched the final demise of lizard-shaped ichthyosaurs, whose inefficient, undulatory swimming style limited them to near-shore environments. A large-scale drop in sea level could have snuffed out these creatures along with many others by eliminating their shallow-water niche. Fish-shaped ichthyosaurs, on the other hand, could make a living in the open ocean, where they would have had a better chance of survival. Because their habitat never disappeared, something else must have eliminated them. The period of their disappearance roughly corresponds to the appearance of advanced sharks, but no one has found direct evidence of competition between the two groups.
Scientists may never fully explain the extinction of ichthyosaurs. But as paleontologists and other investigators continue to explore their evolutionary history, we are sure to learn a great deal more about how these fascinating creatures lived.
This iconic reptile swam the seas for 150 million years. Then the climate changed
This swimming, dolphin-like reptile ruled the waters for more than 150 million years, spanning the early Triassic period to the late Cretaceous. But it mysteriously vanished about 90 million years ago — long before the cataclysmic, asteroid-driven extinction event that wiped out so many other species at the end of the Cretaceous period. Scientists have proposed a variety of explanations, but there’s been little concrete evidence to sway the vote in any direction.
But now, a group of scientists believe they’ve shed a little more light on what happened to the ichthyosaurs. Their research, which was published Tuesday in the journal Nature Communications, suggests that climatic change during the late Cretaceous period, which likely caused great upheaval in marine ecosystems at the time, was the nail in the ichthyosaur’s coffin.
To conduct their analysis, the researchers combed through museum collections and literature describing ichthyosaur fossil findings to put together a comprehensive, up-to-date dataset of the many different species of ichthyosaur that existed over the millions of years the animal was on the Earth. They then conducted a special kind of analysis that investigates the links between all the different species and the way they evolved over time.
In general, the results painted a picture of a diverse group of organisms that maintained a high degree of species richness with many different traits, such as differently shaped teeth or different body sizes, until close to the end of their 157-million-year reign. The analysis also indicated that ichthyosaurs suffered not one, but two extinction events — an initial event that greatly reduced their numbers and diversity during the late Cretaceous period, and then a second event several million years later that finished them off.
These results are somewhat at odds with previous theories that had been proposed about the ichthyosaur’s fate. Scientists had postulated that ichthyosaurs had either been out-competed for resources by other marine predators, or there was a major extinction event among the animals the ichthyosaurs relied on for food, essentially leaving them to starve.
The researchers observed that the competition hypothesis doesn’t quite fit with our knowledge of the types of marine organisms that shared the waters with ichthyosaurs. They were unable to pinpoint an animal that might have had the ability to outcompete the ichthyosaurs and also existed at the same time and in the same places.
And the theory about the disappearing prey doesn’t make complete sense either, considering how diverse the ichthyosaurs were as a group. This theory suggested that ichthyosaurs relied almost exclusively on a specific type of squid-like organism, known as the belemnite, for food.
But given how diverse the ichthyosaurs were as a group, according to the new analysis, the researchers feel that it’s unlikely they would have all focused on just one type of animal. So although the loss of the belemnites may have been a factor in the ichthyosaur’s eventual disappearance, it probably doesn’t fully explain what happened. And the fact that there appeared to be two extinction events — the second one terminal — also remained somewhat of a mystery.
So the researchers conducted more tests to find out what kinds of environmental changes might best explain the ichthyosaur’s demise. These tests, known as correlation tests, essentially perform a mathematical analysis to find out how well a given theory matches up with what was known about the way ichthyosaur diversity changed over time.
It turns out that changes in the climate during the late Cretaceous period were the strongest predictors of the ichthyosaur’s extinction. Specifically, climate volatility — that is, extreme variations in the climate during certain parts of the late Cretaceous — are thought to be the culprit. Scientists have evidence, through geological and fossil findings, that there was great environmental upheaval around the time the ichthyosaurs first started to decline, marked by fluctuating temperatures and sea levels and chemical changes in the ocean, such as the amount of oxygen and carbon dioxide in the water.
All of this variability is believed to have caused profound changes in the marine ecology during that time, with many different types of animals rising and falling in dominance. The ichthyosaurs’ first extinction event happened in the midst of all these changes.
By the time the ichthyosaurs went extinct for good, the marine landscape had settled into a strange period — the culmination of all the climate swings that had characterized the previous few million years — marked by high temperatures, a lack of polar ice and low oxygen levels. At this point, a number of other animals also declined or went extinct alongside the ichthyosaurs.
Plankton and certain types of molluscs, cephalopods and reef animals, for instance, also began to disappear around the same time, while other animals rose to dominance in their place. “As such, the abrupt yet staggered extinction of ichthyosaurs thus appears as just a facet of a much broader series of biotic events,” the authors note in the paper.
So it seems that the disappearance of one of the most famous ancient swimmers was not so special after all, but was likely part of a much bigger set of changes occurring on the planet at that time, largely driven by climatic changes and their effects on the ocean ecosystem. The findings may help to close the door on an old and well-debated mystery, while also serving as a reminder of the acute influence climate change has on the Earth and all its inhabitants.
Chelsea Harvey is a freelance journalist covering science. She specializes in environmental health and policy.