Marine reptiles: a Museum guide
Around 250 – 65 million years ago, in a time known as the Mesozoic era, a number of reptile groups inhabited the sea. Today, very few reptiles can live in salt water – just a few species of crocodile, turtle, lizard and snake have succeeded.
During the Mesozioc era, a variety of marine reptile groups evolved and produced species of very large sizes. Conspicuous among these were the ichthyosaurs (meaning ‘fish-lizard’) and plesiosaurs (meaning ’primitive lizard). The museum exhibit on marine reptiles shows examples of each, some from nearby localities.
Ichthyosaurs had compact fish-like bodies with paddle-like limbs, of which the hind-pair was reduced in size. In species from the Jurassic period onwards, the tail was a symmetrical crescent-shaped blade. The skull bore a long snout or ‘rostrum’, and the teeth were numerous and simply pointed. The resemblance to modern dolphins suggests a similar adaptation to sustained swimming and their teeth suggest a diet of marine prey such as fish, along with squid and belemnites.
The earliest ichthyosaurs lived in the Triassic period (250 – 205 million years ago), but they reached their greatest abundance in the Jurassic (205 – 135 million years ago). Several species have been found in Great Britain, the best known being from the early Jurassic ‘Blue Lias’ strata around Lyme Regis and Charmouth. The same species are also found within a band of rock running from the south west to the north east, especially around Whitby in North Yorkshire. The specimens we have on display include well-preserved material from Charmouth. The skull at the centre has been carefully removed from its limestone source-rock by use of dilute ascetic acid. It is a particularly fine example of this kind of preparation, and is also unusually good in showing an almost complete and undistorted skull. You can also see on display a partial vertebral column, and an almost complete, small individual.
Vertebrae in ichthyosaurs were simple, narrow spools, paralleling those of some fish. Ichthyosaur vertebrae are relatively common as fossils at sites in Cambridgeshire where they are found in Oxford Clay (Late Jurassic – Early Cretaceous) deposits, the material from which many of the local brick companies make bricks.
Plesiosaurs fall into two body and skull types: plesiosaurs ‘proper’, in which the head is relatively small and the neck relatively long, and ‘pliosaurs’, in which the head is very large and the neck relatively short. The scientific distinction between these two types is not so easy to make, and has been blurred by recent finds.
Both types were formidable predators and were armed with long, sharp teeth often arranged so that upper and lower jaws interlocked. Plesiosaurs are known to have eaten ammonites (spiral-shaped relatives of squid), but pliosaurs, among the largest predatory animals ever known, attacked large prey including other pliosaurs.
Plesiosaurs and pliosaurs, like ichthyosaurs, are quite often found in British Jurassic deposits, especially around Lyme Regis and the ‘Oxford Clay’ of Oxfordshire, Cambridgeshire and N. Yorkshire. The pliosaur pubis in the display here was found at Roswell Pits near Ely, while the specimen showing the lower jaws and vertebral column of a plesiosaur was found at Lyme Regis.
Vertebrae of plesiosaurs frequently (though not always) have the neural arch attached to the vertebral centrum, unlike the usual condition in ichthyosaurs. These vertebrae too are found not infrequently as fossils in local deposits.
Other Mesozoic marine reptiles
Another evolutionary branch of marine reptiles produced some bizarre, armoured and superficially turtle-like forms called placodonts. In these forms, the teeth were adapted into crushing plates, probably for eating molluscs. Still others had pincer-like front teeth, possibly for nipping small animals out of coral reefs. Here we have plaster casts of the skulls of some of these strange creatures.
The small group of reptiles known as ‘nothosaurs’ may be early relatives of plesiosaurs, though they were not all marine, and are found in lake deposits from Switzerland and Germany.
It is not entirely clear where any of these marine reptiles fit into the pattern of reptile evolution, but it is generally agreed that they were ‘diapsids’, thus part of the same evolutionary branch as dinosaurs, crocodiles, birds, lizards and snakes. Some people think that the marine forms are closer to lizards and snakes than to dinosaurs and crocodiles.