The timeless swimmer

A group of researchers, led by a young paleontologist from the University of Liege have discovered and studied Malawania anachronus, a fossilized specimen of ichthyosaur, that is more than 100 million years old. What is so particular about this specimen? It belongs to a group of ichthyosaurs that were believed to have been extinct for more than 180 million years. The discovery proves that ichthyosaurs remained diversified for much longer than was previously thought, and challenges theories that have been posited about their extinction. We take a dive with a timeless marine reptile.

A little more than a year ago, Valentin Fischer and his team, at the Department of Geology of the University of Liege, revealed that the Baracromia, a group of ichtyosaurs that appeared during the lower Jurassic, had survived a presumed extinction at the end of the Jurassic, and went on to diversify during the Cretaceous period. (Read The false extinction of ichtyosaurs). They challenged previous theories about the extinction of the ichthyosaur and suggested that a significant part of the group had survived for more than some 50 million years.  

Today, research conducted by this team has enabled them to go a step further and study the characteristics of Malawania anachronus (literally, the out of time swimmer, in Kurdish and Latin), by examining the fossil remains of this ichthyosaur that also lived during the Cretaceous period. To their surprise the researchers discovered that this ichthyosaur belonged to a completely different group which became separated from the Baracromia during an evolutionary radiation dating back to the end of the Triassic (200 million years) and was closer to the species Ichthyosaurus communis. The analysis was published in the journal Biology Letters (1).

“What this study reveals, is that another group, showing different characteristics than the Baracromia, survived several presumed extinctions of marine species during and at the end of the Jurassic period. Before Malawania was studied, ichthyosaurs of the Cretaceous were believed to be a group that had not greatly diversified, stemming from a single small radiation. Today, we can attest to the fact that there was a much greater diversity of ichthyosaurs during the Cretaceous because 70 million years of evolution separate Malawania from its contemporaries. As a comparison, an equal number of years of evolution separate the whale and the bat”, explains a delighted Valentin Fischer.

Another striking particularity is the fact that Malawania shows morphological traits that are very similar to its ancestor which is 70 million years older. This reveals a stasis that is unusual for a pelagic reptile. Conversely, its distant cousins the Baracromia, evolved steadily so that they no longer resembled their Jurassic ancestors.

A rigorously selective evolution…

Malawania is therefore a strange evolutionary exception. “Animals, and in particular, marine reptiles, tend to evolve steadily, notably, under the influence of a very strong selective pressure in this ecological environment”, explains the palaeontologist. Initially, ichthyosaurs, like all marine reptiles, descended from a land animal. Their arrival into this new environment “obliged” them to evolve rapidly with regard to hands, the spinal column and metabolism… The ichthyosaurs then diversified, occupying several levels of the food chain.

The ichthyosaurs were originally coastal animals, feeding on small animals or algae in the case of some species. The ichthyosaurs specialized in deep and rapid swimming, becoming first-rate pelagic hunters.  Other characteristics also changed. Living further and further away from the coasts and having lost the faculty to move on dry land, the ichthyosaur had to adopt a metabolism that resembled that of warm-blooded animals. Unable to find a sunbaked rock in the middle of the ocean to warm itself in the manner of reptiles on land, it depended on its muscular activity to warm its body. Because of this, the animal had a more voracious appetite than that of other reptiles in order to satisfy its energy needs. Gradually, coastal types of Ichthyosaur disappeared and the animal adopted the wider oceans as its habitat. By now, it had become very different to its land-dwelling ancestor and other marine reptiles that were its contemporaries.

But then, even the Baracromia, which were already quite “sophisticated”, continued to evolve during the Jurassic and Cretaceous periods. Some of the Opthalmosaurinae, for example, acquired the biggest eyes known in the animal kingdom with pupils that were nearly 10 centimeters in diameter, enabling them to see into the depths and detect prey up to 30 meters further away than the other ichthyosaurs (2). Thanks to the study of these pupils but also to the study of histology, and the detection of the “Caisson’s disease”, it was deduced that certain representative examples of the species could see and dive to depths of 500 meters, while others remained much closer to the surface. The different generations of ichthyosaurs constantly diversified and specialized.

“And then we came across Malawania, which displayed characteristics that were undoubtedly primitive, which made it the last known representative of a line that was confined to the early Jurassic”, said the surprised researcher.

A less than ordinary morphological stasis

“These stases are rare”, continues Valentin Fischer. “They can often be explained by the fact that an animal finds a refuge zone which changes little during geological periods like a shallow sea that is not affected by great ocean changes. This would typically be a stable zone, without any major new conditions that might force a species to evolve”. But in the case of Malawania, it is difficult to establish a theory that might explain such a long stasis. Malawania is an oceanic species, the ichthyosaurs that are its contemporaries evolved greatly. “Yet at the same time, it is the only known representative of the species and it is not perfectly preserved, so we cannot make firm deductions about its habits or its biotope. Malawania was found in the Middle-East; we do not yet know if that means that its line was confined to this region or not, although that is unlikely for such a long period, lasting several tens of millions of years”.

From a general point of view, although the fossil record of the ichthyosaurs is not so bad in the northern and southern latitudes of our planet, very few specimens have been found in the tropical zones of that time (Middle-East, Africa…). There is therefore a need for a considerable amount of exploration work to be carried out in this region which the discovery of Malawania could initiate, and new discoveries in these regions could provide a better understanding the evolution of this very new group represented by the timeless swimmer. “One thing is certain, the discovery of such a lineage implies that there is an enormous amount of information that we don’t yet know. We need to find a large number of representatives of sub-groups and different eras which are the missing links required to complete this new chain”.

A false start

This encounter with Malawania was somewhat incongruous. Its “young age” seemed so improbable that nearly 60 years passed before it was given the attention that it really deserved. In 1952, a group of British petroleum geologists stopped to chat on a mule-path in Kurdistan, in Northern Iraq. When one of them happened to look down, he noticed some ribs and other fragments of bone at his feet. Subsequently it was decided to bring the fossil back to the London museum, on the supposition that the slab came from the nearby rocks which contained sediments dating back as far as the Jurassic.

An English expert, Robert Appleby, decided to study the specimen. Given its morphology and the location in which it was found, there could be no doubt that the animal evolved during the Jurassic. Appleby decided to write a paper to record this discovery that did not seem too important at the time. However, his paper was rejected due to imprecisions in the methods for dating the rock. The morphology of the animal and the location of the fossil were not sufficient proof. There was always the possibility that it had been brought there from another site. During the 1970s, an expert from Cambridge took a sample and after dissolving the rock in acid, he studied the remaining microfossils and pollen. The conclusion was certain, the fossil dated from the Cretaceous. However, Appleby then thought that the wrong sample had been delivered. He rewrote an article in which the ichthyosaur was still considered to be from the Jurassic period. The paper was rejected again. The researcher ceased working on the bone samples at the end of the 1970s.

It was only in 2003, after the death of Appleby, that his widow contacted two English researchers known to Valentin Fischer, to ask them if they could take up her husband’s unfinished research and publish it. The young Belgian researcher then became involved, and took an interest in this timeless fossil. After a series of statistically reliable tests, there was no longer any room for doubt. Malawania had indeed lived during the lower Cretaceous, between the Hauterivian and the Barremian and was a representative example of a primitive group of ichthyosaurs.

A major reshuffling of the cards

Ichthyosaurs therefore did not become extinct at the end of the Jurassic period, unlike many other marine reptiles and they maintained a diversity that was as wide as during their major evolutionary radiations. Current understanding of the group makes it possible to establish that in all likelihood they became extinct “suddenly” (over a relatively “short” geological period, in the order of a few million years) during the Cenomanian age (95 million years ago, in the middle of the Cretaceous period).  What was the reason for this?

Certainly, life on our planet has had to endure many mass extinctions, some of which were violent. There was one at the latter end of the Jurassic, and another, more well-known extinction which occurred 65 million years ago at the end of the Cretaceous. After a meteorite crashed into the Earth, many species (non-avian dinosaurs, groups of tortoises and crocodiles, pterosaurs…) disappeared quite suddenly, paving the way for small mammals to propagate, evolve and thrive up to the present day. Ichthyosaurs, which were not affected by the Jurassic extinction, disappeared some 30 million years earlier, at a time when, on the face of it, very little was happening from a geological and paleontological point of view.

It was easier to find convincing hypotheses when it was believed that ichthyosaurs were in decline during the Cretaceous. A harmless factor for a thriving species can be fateful for a species that is already becoming extinct. “These theories only invoked biological factors.  First, there was the idea of competition with other faster animals that reproduced more quickly, like fish, or a reduction in the diversity of cephalopods, which was the principal food of the ichthyosaurs.  However, it seems that the species of ichthyosaurs that lived during the Cretaceous had varied diets and they remained healthy for millions of years after the appearance of fishes and other groups of sea reptiles. These theories were no longer satisfactory for a group that was revealed to be so diverse”.

Another explanation needed to be found, and this was the subject of the last chapter of Valentin Fischer’s thesis (3). With the help of researchers who are specialized in this period, he painstakingly went through the literature dealing with what happened 95 million years ago and this proved to be a revelation. “We noticed that most of the marine groups were affected by something at this precise time. There were extinctions and explosions of diversity across the food chain and in a relatively short space of time”.

There was a great reorganization of ecosystems, which correlated with global warming that affected the temperature, salinity and movement of the oceans. “Studies have made it possible to estimate that ocean levels rose to 200 meters above the current level, and the surface water was sometimes 15 to 25 degrees hotter. There was no ice anywhere at the poles. Clearly, this change favored or disadvantaged the groups that lived in the oceans at that time. It is in this context that the extinction of the ichthyosaurs must be placed”.  Yet this is only the first part of the solution to the puzzle. At the current time, it is impossible to determine what finally brought about the extinction of the ichthyosaurs. Was it solely because of the temperature, the extinction of another species, or some other cause? “Such a shake-up involves so many variations of simultaneous factors that it is difficult to know precisely what subset of factors worked against the ichthyosaurs”.

The fact remains that the ichthyosaurs disappeared at this time, leaving no descendants, after one hundred and fifty million years of existence, closing a chapter in the history of the Earth. It was a chapter in the true sense of the word as even their appearance remains a mystery. The first fossils studied, dating from the beginning of the Triassic, show that they had already become aquatic animals, bearing no resemblance to their land ancestors. No one knows where to place the animal within the genealogy of reptiles.

Outside evolution and outside time… Why did Malawania evolve so little in relation to its contemporaries? What were its real faculties, its habits, and its environment? What happened during the 70 million years that separate it from its best-known cousin? More broadly, what caused the sudden disappearance of an animal that moved seamlessly through a time period of several tens of millions of years? Analysis of this specimen opens up a new branch in the study of ichthyosaurs. In particular, it raises new questions and provides specialists with the key to new avenues of research on the animal.

(1)Fischer V, Appleby RM, Naish D, Liston J, Riding JB, et al. (2013) a basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs. Biology Letters 9: 20130021.

(2) Fischer V, Arkhangelsky MS, Uspensky GN, Stenshin IM, Godefroit P (2013) a new Lower Cretaceous ichthyosaur from Russia reveals skull shape conservatism within Opthalmosaurinae. Geological Magazine In press.

(3) Fischer V. (2013) Origin, biodiversity, and extinction of Cretaceous ichthyosaurs, Doctoral thesis, University of Liege.