Set in stone

The fossilised remains of ichthyosaurs (the name ‘ichthyosaur’ means ‘fish-reptile’) have been known, in one form or another, since the middle of the 18th century – long before the meaning of fossils was either understood or properly appreciated. Now a magnificent specimen found in France will be offered in Paris at Bonhams’ Natural History Sale in December. It was not until 1811, when Joseph and Mary Anning discovered the skull and majority of the skeleton of a well-preserved (and very large) ichthyosaur in Liassic (Early Jurassic) sedimentary rocks on the coast near Lyme Regis, in West Dorset, that these puzzling creatures attracted the attention of scientifically minded investigators. Sir Everard Home attempted to describe an Anning specimen but struggled to understand its anatomy. After much debate, he suggested that it might be a gigantic reptilian equivalent of a type of salamander known as an olm – the distinction between reptiles and amphibians was somewhat hazy at this point in history. Charles Konig, Keeper of the Natural History Collections at the British Museum, proposed the name ichthyosaurus for this fossil in 1818, and the name was adopted by all scientists thereafter.

Mary Anning, one of the most-renowned fossil collectors, continued to collect fine skeletons of ichthyosaurs that came to the attention of two skilled investigators: William Conybeare and Henry de la Beche; they produced well-illustrated descriptions of these remarkable fossil reptiles. Their studies made clear that in prehistoric times there lived reptilian sea creatures with streamlined, dolphin-like bodies, legs modified into turtle-like flippers. Nothing like them was known to be still living in the sea, and this fact opened up the possibility of ancient worlds dramatically different to Regency Lyme Regis, inhabited by extraordinarily exotic creatures. Even more troubling to the Establishment of the time was the implication that fossils could be discovered that had the potential to reveal lost worlds in ‘Deep Time’, a provocative challenge to the biblical account of the history of the Earth.

Within a few more years, Mary Anning had discovered more totally unexpected – and equally intellectually challenging – fossil creatures in the rocks around Lyme, including the bizarre sea creatures named plesiosaurs and leathery-winged flying reptiles known as pterosaurs/pterodactyls. So prolific and varied were Mary Anning’s discoveries that de la Beche was able to sketch a vivid reconstruction of Lyme’s Jurassic World. He titled his sketch Duria Antiquior (‘ancient Dorsetshire’) and it was ‘populated’ by all the creatures that Mary had discovered.

Breath of fresh air

Although clearly fish-shaped, the detailed anatomical structure of ichthyosaur bones reveals that they are in fact reptiles, extinct kin of lizards, snakes, crocodiles and tortoises. This implies they must have breathed air, rather than, as fish do, gulping water and extracting oxygen from the water by means of vascularised gills. Consider this for a moment: how did living ichthyosaurs breathe? Examining the head of this ichthyosaur, the nostril can be seen as a small opening immediately in front of the eye socket – not near the top of the skull.

While swimming, the nostril must have been closed by a muscle-controlled fleshy valve to prevent the lungs from becoming flooded with water. To breathe air, the head would have needed to be physically lifted clear of the water. However, these animals did not have a narrow, flexible neck – that would have ruined the streamlining of the body – so they were unable to tilt their heads. The only realistic mechanism for breathing would have been for these animals to regularly ‘porpoise’ – just as porpoises, dolphins and other members of the whale family do today. The comparison with porpoises (which are, of course, mammals rather than reptiles) goes far further than might have been expected.

In 2018, a Stenopterygius specimen was discovered in such remarkable condition that portions of its skin were preserved. This was found to be smooth (rather than scaly, as might have been expected for a reptile) and, deeper in the skin, was a layer of fatty blubber. Blubber both gives buoyancy and acts as an insulator to keep the body warm. This suggests that these magnificent reptiles had an elevated metabolism and their bodies were naturally warm. This would have meant their muscles worked very efficiently, giving them considerable strength and endurance. When muscles work hard, they need plenty of oxygen, so it is reasonable to assume these ichthyosaurs were obliged to ‘porpoise’ as they swam the Jurassic seas.

In the swim

How did ichthyosaurs swim? They would undulate their bodies from side to side, in the same way as a fish – a dramatic difference from today’s whales, which undulate vertically and use horizontal tail flukes. It is seen in the structure of the fossil tails. The tip of the tail in Jurassic ichthyosaurs has a noticeable kink, where the vertebrae abruptly turn downwards. Some exceptional specimens preserve the shape of the original fleshy ichthyosaur tail fluke and also reveal that there was a shark-like dorsal fin.

While all the power for swimming forward was provided by the deep tail fluke and powerful body muscles, the manoeuvrability of these animals depended on flippers. The shape of the flippers would have reflected their way of life: narrow flippers suggest fast swimmers, whereas broader flippers probably reflect greater manoeuvrability at slower speeds and thus an ability to catch slower-moving but more-evasive prey. The teeth of these ichthyosaurs were rather homogenous: the many simple conical spikes in an elongated snout were ideal for catching and holding slippery prey, such as squid and a variety of fish. Other ichthyosaurs may have had larger and more robust teeth adapted to crunching ammonite shells, or for feeding on larger prey such as plesiosaurs and even sharks.

One unmissable feature common to ichthyosaur skulls is the eye socket. As in the example offered by Bonhams, the eyes must have been truly enormous. Some ichthyosaurs had an eye diameter of around 25cm. A very large eye would have enabled these animals to see in very dim light, which could mean that ichthyosaurs were capable of deep-diving to catch their prey – at depths of up to 500 metres, where light levels are very low. At such depths, the body of an ichthyosaur had to contend with very high hydrostatic pressures. The rigid, bony, sclerotic ring may well have helped to support the eye and prevent distortion of the lens when hunting at great depth.

Baby steps

Such highly specialised swimming reptiles as ichthyosaurs – with their deep, narrow bodies and limbs modified into flippers – would not have been able to return to land to lay their eggs, as turtles do today. Instead, you would expect that they must have given birth to live young while at sea – and so it was. Several ichthyosaurs have been discovered apparently in the act of giving birth, with a small ichthyosaur skeleton apparently hanging, tail-first, out of the birth canal of a mother ichthyosaur. But, gruesomely, these may well have been examples, in fact, of post-mortem abortion: the foetal youngster having been gradually expelled from the mother after she had died and her body became bloated by gas during decay, or squashed out by overlying sediment during burial.

Rare as ichthyosaur teeth

Individual ichthyosaur teeth and bones are found, quite often, in marine rocks of Jurassic age across much of Europe; they have a sparser, but essentially worldwide, distribution in marine rocks dating anywhere between the Late Triassic and the Early Cretaceous. However, complete and well-preserved specimens are rarer. Some of the best-known specimens tend to be flattened in mud-rock deposits, so it is quite rare to find an example, like this one, where the skeleton is not only well-represented but the bones are preserved in three dimensions.

Professor David Norman is Curator of the Sedgwick Museum, Cambridge.