The Mosasaur‘s kinky tail
For centuries scientists routinely straightened the tails of Mosasaur fossils in their reconstructions. But a recent re-examination changed overnight the way they see the sea-going lizards
On 6 April 1821 – a little more than two decades before their countryman Richard Owen would coin the term “Dinosauria” – the English naturalists Henry de la Beche and William Conybeare presented a report on a peculiar group of fossil animals to their fellows in the Geological Society of London. One of the subjects of their paper, the long-necked marine reptile Plesiosaurus, made its academic debut that night, but the others were already familiar to the scholars in attendance. Called Ichthyosaurus, these fossil creatures seemed to have been cobbled together out of equal parts fish and crocodile, and even during this era of pre-evolutionary palaeontology, de la Beche and Conybeare could not help but place Ichthyosaurus in what they believed to be a graded series of forms between fish, the newly discovered Plesiosaurus, and crocodiles.
At the time of their report, de la Beche and Conybeare did not have much to work with. Popular accounts of the marine reptile had made Ichthyosaurus famous, yet a significant portion of its skeleton remained unknown. The tireless efforts of one of the first expert fossil collectors – Mary Anning, of “She sells seashells on the seashore” fame – provided naturalists with more complete specimens, showing the various species of Ichthyosaurus to be crocodile-like reptiles with straight, tapering tails. Restorations remained true to the animal’s “fish lizard” moniker, and when Richard Owen examined an Ichthyosaurus with a kink in the distal part of its tail, he came up with a series of scenarios by which the tail of the dead individual may have become bent. (My personal favorite: that part of the tail had become bloated with gas during decomposition and pulled the spinal column out of place.)
But Owen, as well as the various scientists and artists who had reconstructed Ichthyosaurus with a straight tail, was wrong. Exceptionally well-preserved ichthyosaur specimens discovered in the 1890s from Holzmaden, Germany, exhibited dark-coloured “halos” – created by bacteria that ate away at the carcasses as they laid on the bottom of the Jurassic seas – which represented the body shapes of these animals. Not only did Ichthyosaurus have a fleshy dorsal fin, but the downward tailbend was not a pathology – it was a normal feature which supported a large tail in the shape of a crescent moon.
Re-examined in this light, it became clear that even specimens preserved without soft-tissue impressions had vertebrae near the end of their tails that were wider at the top than at the bottom; a sure sign of a downward-kinked tail that supported a large caudal fin.
The image of Ichthyosaurus changed overnight. The piscivorous predator was not a big amphibious lizard with paddles where its hands and feet should be; it was a streamlined, fusiform creature which more closely resembled a shark than any lizard. By the close of the 19th century, the issue was settled, but spectacular specimens continue to change what we thought we knew about prehistoric life.
One such skeleton, found in the middle of Kansas in the 1960s, sat in storage for years, but a re-examination has caused scientists to reconsider what they thought they knew about another marine reptile – a mosasaur called Platecarpus.
Many books and documentaries cast mosasaurs among the many “also-rans” that lived alongside the dinosaurs between 98 and 65 million years ago.
A genus or two – usually Mosasaurus and Tylosaurus – get mentioned now and again, but the larger swath of mosasaur diversity is rarely elucidated. These marine reptiles, which were much more closely related to today’s Komodo dragons than any dinosaur, were the fiercest predators of the Cretaceous seas, with many species occupying a range of habitats from near-shore to the open ocean. Most were not streamlined speed hunters like the ichthyosaurs, but instead looked like seagoing lizards; they were ambush predators that propelled themselves out of their hiding places with their long tails.
Among the most common of these marine predators was the species Platecarpus tympaniticus (named by the notorious “bone sharp” Edward Drinker Cope in 1869), and one century after it was first described an unusually complete specimen was collected from the well-known Niobrara Chalk in Kansas – a formation representing a time when a shallow sea covered much of western North America.
Shortly after it was excavated in the 1960s, the Platecarpus skeleton (known as LACM 128319) was stored in the collections at California’s Natural History Museum in Los Angeles County. For one reason or another, it sat there, undescribed for decades, but in August of this year a team of palaeontologists led by Johan Lindgren of Sweden’s Lund University at long last published a report on the specimen in the journal PLoS One.
Not only did it retain traces of soft tissues – including skin impressions and a reddish residue on its ribs that may be the remnants of its heart or liver – but its tail contained a distinctive set of vertebrae that were wider at the top than at the bottom. Platecarpus, just like Ichthyosaurus, had a downward-kinked tail that probably supported at least a modest tail fin.
Specimen LACM 128319 was not the first mosasaur skeleton to show signs of a tail fluke. In 2007, Lindgren led a different set of colleagues in describing the skeleton of a specialised form of mosasaur found in California called Plotosaurus (a specimen of which has also been found sporting soft-tissue impressions). The end of its tail sported a modified portion of vertebrae that looked extremely similar to the tail arrangement of sharks (just flipped down inside of up).
Along with a streamlined body that was deep from top-to-bottom, Plotosaurus was a mosasaur adapted to cruising in the open ocean – it was a mosasaur built like an ichthyosaur.
The skeleton of Platecarpus was not as specialised for pelagic life as that of Plotosaurus, but the examination of the new specimen shows that it was an intermediate form between the early, lizard-like mosasaurs and the last, highly streamlined types.
What is curious, however, is that the new specimen of Platecarpus represents yet another case of a marine reptile that independently evolved a downward tailbend. Ichthyosaurs did, some seagoing crocodiles (such as Geosaurus) did, and now we know that some mosasaurs did. Putting this in an even wider context, sharks have the same kind of tail, but their spinal column kinks upward and the fleshy part of their tail is below. In marine reptiles it is the other way around – with the spinal column bent downward – and perhaps there is some kind of shared evolutionary constraint, inherited from their last common ancestor, that caused the tails of marine reptiles to consistently bend downward when evolving this kind of propulsion.
As yet, such an evolutionary constraint has not been identified, but if it could be discerned, such a quirk of natural history might help us better appreciate how contingency and constraint shape evolution’s grand pattern.
Brian Switek blogs at brianswitek.com
A vegetarian crocodile seems impossible like a lion eating fruits only, but it existed in a very ancient past: it was the Chimaerasuchus and we can neither consider it as an ephemeral presence, given that it lived in the middle Cretaceous, from 125 to 100 million years ago. Its name witnesses the many problems palaeontologists had to describe correctly this animal: “Chimaerasuchus“, in fact, means “Chimera crocodile” and Chimera was a famous symbolic monster of Greek mythology, formed by parts of different animals: here.