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Ichthyosaurs

Past Perceptions

Ichthyosaur fossils have been known of since 1699 when partial remains were uncovered in Wales. It was not however until 1811 when the first complete specimen was unearthed that people had any conception of what they actually looked like. This first complete ichthyosaur fossil was discovered by Mary Anning in the cliffs near the Dorset town of Lyme Regis (Torrens, 1995). Anning was just twelve when she made the discovery with her brother Joseph. Their specimen was sold to the German naturalist Charles Konig as a stone crocodile. He proposed the name Ichthyosaurus (Latin for ‘fish-lizard’) for the new creature (Rudwick, 2008).

Figure 1. One of Mary Anning's ichthyosaurs. This skull belongs to the species Temnodontosaurus platydon although it was initially assigned as Ichthyosaurus platydon by Richard Owen.

The first attempt at classifying the fossils was made by Sir Everard Home who initially proposed them to be a type of fish with duck-billed platypus affinities (Home, 1814). He later revised his classification dubbing them Proteo-Saurus; a transitional form between salamanders and lizards. In 1821 a number of ichthyosaur specimens found by Anning and others were analysed in a paper by two members of the Geological Society of London; Henry De la Beche and William Conybeare. They were the first to recognise that the fossils represented a new type of marine reptile. One of the key features used to make this interpretation was the location of the temporal fenestrae in the skulls. This opening in the skull is situated behind the eye orbit, above the articulation of the post-orbital and squamosal bones. Synapsids, a group which includes mammals, have an opening which is also situated behind the eye orbit but is located below the post-orbital-squamosal articulation. Fishes have no such opening.

Figure 2. A classic 19th century depiction of an ichthyosaur and a plesiosaur locked in battle. Note the water jets spouting from the ichthyosaur's 'blowholes', features originally thought to be possessed by the group. This is now known to be inaccurate.

Although correctly interpreted as marine reptiles, early reconstructions of ichthyosaurs were based solely on the animal’s preserved hard skeletal structure. It wasn’t until specimens were discovered with soft tissue preservation in the Posidonia Shale of Germany that their full external anatomy was revealed (Figure 4). The exquisitely preserved specimens showed a clear dorsal fin which had no skeletal support. Another feature not realised until this discovery was that of the tail. The vertebral column was thought to run straight resembling the tail of a lizard. Soft tissue preservation showed a clear double lobed tail with the vertebrae running at an angle down into the lower (ventral) lobe. This is the opposite of animals such as sharks whose vertebrae run into the upper (dorsal) lobe. This type of tail found on sharks is referred to as being ‘heterocircal’. As ichthyosaur’s vertebrae ran into the lower lobe their tails are referred to as ‘reverse heterocircal’. Up until this discovery all reconstructions had quite logically never included a dorsal fin or lobed tail. Henry De la Beche’s own watercolour Duria Antiquior (Figure 3) painted in 1830 clearly shows ichthyosaurs with no dorsal fins and straight tails. This can also be seen in Édouard Riou’s drawings for Jules Verne’s 1864 book ‘Journey to the Centre of the Earth’, in which ichthyosaurs are depicted as morphologically similar to crocodiles and locked in battle with plesiosaurs. Another example of this interpretation can still be found at the Crystal Palace Dinosaur Court in London. This sculpture park was commissioned in 1852 and among the dinosaurs can be found ichthyosaurs with straight tails and no dorsal fins basking on rocks! It seems some early enthusiasts believed they had life modes similar to that of a modern walrus.

Figure 3. Duria Antiquior painted by Henry De la Beche. This was the first depiction of a known prehistoric scene and is considered to be the first piece of art in a genre now known as palaeoart.


Modern Interpretations

The exceptional level of preservation in German specimens has allowed highly accurate reconstructions of early Jurassic ichthyosaurs. This has revealed a striking morphological similarity to modern dolphins. This similarity is far more than superficial; it is a perfect example of convergent evolution. The specimens from Germany and England however, which are all from the early Jurassic (201-174 million years ago) only represent a small fraction of the species now known to have existed. Fossil ichthyosaurs have now been found all over the world from Australia to the Arctic. They are also known from sediments ranging in age from the early Triassic to the late Cretaceous; a period spanning around 160 million years (Montani, 2009). This is a vast span of time meaning that they must have been very successful in their life-modes. During this time they radiated into a huge diversity of forms from small fast swimming predators to giants the size of whales.

Figure 4. One of the fantastically preserved ichthyosaur specimens of the genus Stenopterygius from the Posidinia Shale of Germany. Note the exquisite detail of the skin showing the outline of the original animal. Copyright © University of Glasgow/Hunterian Museum (taken with permission from www.hmag.gla.ac.uk/Neil/reprods).

Although the early interpretations of Jurassic ichthyosaurs were inaccurate owing to their lack of dorsal fin and straight tail this is now known to be roughly how the first ichthyosaurs would have looked. As the first ichthyosaurs evolved from a lizard-shaped reptile the earliest forms still bore lizard like features (Montani, 2009). Their tails were straight and they had no dorsal fin. They may have looked remarkably similar to the original interpretations of the Jurassic ichthyosaurs first described in the 19th century.

Ichthyosaur’s feeding habits have also been analysed from various evidence sources giving us a good knowledge of their ecology. The most direct evidence comes from fossil specimens with preserved stomach contents. These often include the hooks of cephalopods such as squid and belemnites. Fossilised coprolites also show clear remains of fish scales and some have even been found with large bones of other ichthyosaurs in suggesting possible predation or scavenging. Morphological aspects also give us clues as to their diet. Most species had jaws full of long, pointed teeth which were likely used to puncture and crush prey. The relative size of these teeth and the robust jaws in which they were set suggest many species had high bite forces. Modern computer modelling and biomechanics will allow palaeontologists to learn this information from well preserved fossils. Advanced ichthyosaur’s streamlined shape and powerful tails suggest that they could swim at very high speeds. Their eyes were some of the largest known of all animals ever to have existed with some measuring 26cm across. These features suggest that many species were fast active hunters that pursued their prey in very low light levels such as at great depths.

Figure 5. A Modern computer generated image of an advanced ichthyosaur. Of all of the marine reptiles ever to have existed they were likely the best adapted to an aquatic life mode.

Ichthyosaur locomotion has now been analysed in great detail for many known genera. The earliest ichthyosaurs most likely swam using simple lateral body undulations much like a modern eel (Sander, 2000). This action, known as anguiliform locomotion, has been extrapolated from their similar morphology to modern swimming reptiles such as marine iguanas which swim using the same motion. The biggest evolutionary changes which enhanced locomotion in ichthyosaurs came from the modification of the tail and a change in overall body shape. The more advanced ichthyosaurs of the late Jurassic and early Cretaceous evolved thunniform shapes analogous to some modern fast swimming predatory fish species. From comparative studies of fish like tuna and detailed analysis of ichthyosaur skeletal structure and musculature, models can be made of swimming action. It is likely that these ichthyosaurs only moved their tails rather than their entire bodies. For this they used their incredibly strong posterior trunk muscles which moved the tail in a sinuous motion. The tail would have been held at an angle to the direction of its movement. This has been shown to generate a higher degree of lift than drag with the result of effectively pulling the animal forward through the water (Sander, 2000). Interestingly, unlike other extinct marine reptile groups, ichthyosaurs most likely did not use their paddles for locomotion. It is thought that their paired fins (Figure 6) were used primarily for balance and manoeuvrability thus keeping them level in the water column as they swam and allowing fast turning.

Figure 6. A well preserved complete specimen of the genus Leptonectes. Note the enormous front paddles which would have given the animal great stability and manoeuvrability in the water.


Thecodontosaurus illustration courtesy of Richard Deasey.
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