Fossil evidence of ichthyosaurs was discovered in limestone quarries near the town of Street in the early 1800's. The limestone was largely used to build houses during the boom period of the town, driven mainly by the efforts of one local Quaker family, the Clarks*. Without their industry, the ichthyosaurs of Street may never have been discovered.

Morphology

Ichthyosaurs were very similar to modern day dolphins. There are a few differences in form – the ichthyosaurs didn’t have a blow hole, and had four lateral fins as opposed to the dolphin’s two. They were also reptiles, unlike dolphins, which are mammalian. One of the most obvious immediate differences is the ichthyosaur's enormous eyes, in some species up to 23cm across. The eye and outward form are the keys to understanding the ichthyosaur's eating habits and environment.

Although there were over 80 species of ichthyosaur the body-plan doesn’t vary much other than eye size and body length. There are differences in vertebrae, however, between early ichthyosaurs and their later descendents. In the early ichthyosaurs, the vertebrae were longer and less numerous. This would suggest a serpentine method of swimming particularly well suited to shallower waters. In later species, the vertebrae were more compact and greater in number, suggesting a stiffer swimming action, similar to that of modern sharks. This action maximises speed whilst minimising energy expenditure. In an environment in which food could be sparse, this would be the difference between evolutionary success and extinction*. This adaptation worked, as the ichthyosaurs managed to survive over a period stretching from 250 mya*, to about 90 mya.

The smallest ichthyosaurs were approximately three feet long, and the longest so far discovered is over forty feet*. This variation exemplifies the widespread success of these creatures, which are found all over the world. For the purposes of this entry though, we will be concentrating on the ophthalmosaurs of the late Jurassic*. This is a medium sized and wide-spread ichthyosaur which displayed characteristics which can be found in almost all ichthyosaurs.

Environment

A creature adapted to shallow water environments is restricted to eking out its existence in its chosen niche. A deep-water adapted creature, however, has fewer restrictions. The very earliest ichthyosaur (Utatsusaurus) was restricted to a shallow coastal environment by its swimming technique – undulatory, and non-energy efficient in an environment in which sustenance could be scarce. But its descendent, ophthalmosaurus, could take advantage of food-rich times (spawns) in shallow coastal waters, and also (as we will see) dive off the continental shelf to hunt in deep sea environments. Ophthalmosaurs could have the best of both worlds, and therefore increase their chances of survival as a species.

Beside reefs of coral, there lay two other major food sources below the Jurassic waves. These were silicaceous sponge reefs, which grew in deeper water than coral reefs, and further out still, microbial reefs, which could grow at much greater depths than the corals or silicaceous reefs. Although not a food source for a hungry ophthalmosaur, these sponge and microbial ‘meadows’ were harvested by a large array of creatures, and supported significant populations of prey animals – a solid base for a food web.

Reproduction

Moving into shallow waters at certain times of year would have been unavoidable, as ophthalmosaur young were quite small. Ophthalmosaurs gave birth to live young. Several noteworthy fossils have been discovered which show the creatures in birth, with the ‘pup’ at mid point in the birth canal. The other important thing which these fossils demonstrate is that they gave birth tail first. The advantage of this being that the pup, if the birth was a difficult one, would not drown. Once ejected, the pup would have to swim quickly to the surface to take it’s first breath. Owing to size, the pup would be unable to feed on the same sources as adults, so it makes sense that they were delivered in shallow waters, possibly lagoons or coral reefs. These options would provide cover, and a large array of smallish prey for juvenile ichthyosaurs. Lagoons could also bar entry for any (marine) predator larger than the parent ophthalmosaur.

Feeding habits

We know that ophthalmosaurs had large eyes, and a stiff swimming action. These characteristics suggest specialist feeding activities. The eyes of ophthalmosaurs, and indeed, all ichthyosaurs, are the largest of any vertebrate, living or dead. We can tell the eye size due to the fact that ichthyosaurs had a 'sclerotic ring', a torus of bone that supported the eye. In ichthyosaurs, this ring is highly developed, and fossilises well. One of the invertebrate animals we know lived in the same water as ophthalmosaur were squid. Giant squid are the only animals in the same environment that have eyes comparable to opthalmosaurs. They can in fact exceed the eye sizes of ophthalmosaurs. The tentacles of squid have hooks or barbs at the tips, with which to secure their hold on prey. These barbs can frequently be found in fossilised ophthalmosaurs.

Both the large eye and fossil evidence indicate that ophthalmosaurs fed on squid, and had to have an impressive diving range in order to reach their prey, which often inhabit deep waters. It has been calculated, using comparison techniques with similar modern animals, that ophthalmosaurs could hold their breath for approximately 20 minutes. The same calculation indicates that it would be possible for ophthalmosaurs to reach depths of 1,500 ft, and return to the surface in this time. Another animal that inhabited this depth range is the ammonite*. These creatures could deftly manipulate their air chambers to sink rapidly in an attempt to avoid capture. Shell fragments found in ichthyosaurs coprolites* suggest that this tactic was not always successful. Fish bones and scales also found in coprolites show wide dietary habits.

Predators

The late Jurassic seas contained a number of creatures that could pose a threat to ophthalmosaurs. Liopleurodon was the largest recorded (80ft) and had the biggest jaws of any creature in existence. Liopleurodon was a short-necked plesiosaur, and could eat practically anything it turned its mind to. Evidence suggests liopleurodon fed on shark.

Mososaur were common. These grew in excess of 50 ft and were voracious predators. The base of the lower jaw was well cushioned by a cartilaginous material that absorbed shock effectively. Their teeth have been found embedded in turtle shells, and jaw fractures which have later healed are common, suggesting they attacked their prey at high speeds. They swam in an undulatory manner, and resembled a cross between an alligator, a shark and a moray eel.

Many species of shark roamed the ocean then, as they do today. Other than variations in fin shape and size, these successful predators have changed very little. These would probably only pose a threat to the smaller species of ichthyosaurs.

A reconstruction

We join the residents of Street about halfway through Saturday morning. They’re trolling the shops as usual, pausing here and there to peer in at the odd bargain or two. Considering perhaps a little brunch at a fast food restaurant, mulling over what the rest of the weekend might bring. For the purpose of this entry, let’s assume that they are not paying much attention to the ground beneath their feet.

Let’s have a wider look at the town before turning the clock back a little. The town of Street is for all intents and purposes connected to the town of Glastonbury.

When you enter Street you may notice an odd creature adorning the ‘Welcome to Street’ sign. This is an ichthyosaur. The remains of ichthyosaurs were discovered here in the early 1800’s. A nearby limestone quarry yielded several perfect specimens as the rock was being quarried for building materials. One of the buildings constructed from the limestone was the Clark's shoe factory. Clark's shoes began in Street, as the business empire of the Clark family, renowned local Quakers. Clark's Village – a shopping village – provides a lot of support for the town. The college – Strode – provides excellent educational facilities and a theatre. But. . .

Turn back time about 160 million years. Now the shoppers are trudging across the bottom of an ocean. The nearest land is 5 miles behind them, part of a chain of carboniferous tropical islands that will eventually become the Mendip hills. They are moving along sea-bed, their feet kicking up clouds of chalky dust that swirl in the eddies they leave behind. This is calcite. It has been borne to this spot by off-shore currents. It originated in a crinoid meadow. Crinoids are filter-feeding animals that look like plants; a stalk attaches them to the seabed and a crown of petals sits at the top. The petals are constructed of a thin layer of calcite. When crinoids die, this calcite drifts to the ocean floor. It can build up in layers hundreds of feet in depth. After a long, long while these layers turn into limestone. This is the material which will become handy building blocks for their houses. If one of the shoppers obligingly looks down, they will see the remains of a living, breathing creature that may one day be a distinctly unusual shape in the stone above their porch. And if they look up. . .

We’re five miles out, so there’s nothing much up there at the moment. It’s still morning, but there’s not much light to see by. So if we turn the troupe round and set them marching for the islands which will become the Mendips...

A mile or two off-shore and things become a little more interesting. The water isn’t quite as deep, so it’s a little lighter. There is consequently more life. There’s an enormous coral reef ahead, which would block further progress to the island, but we don’t want to go there yet. The shoppers are trudging through the crinoid meadow that produced the calcite drift, and now there are larger creatures moving around. Fish are everywhere in enormous shoals. Plesiosaurs are hunting them, and sharks are milling around, ready to take advantage of anything missed. Ichthyosaurs have also gathered in large numbers, following the shoals of fish to their spawning grounds. The fish have attracted other, larger predators – mososaurs. The ichthyosaurs are fleeter than the mososaurs, and the fish are providing an easy meal for the big predators. The ichthyosaurs are safe for the moment.

The ichthyosaur that brought us here had spent the night in a shallow lagoon. She has given birth, which is a taxing experience for any animal. Others have been there over the past few days, and the pups have formed themselves into a protective pod, forming ties that will last them a lifetime. But the mother is tired. When she left the lagoon at dawn she was in need of food. Not all her wits were about her. Using the last reserves of her energy she dives through a shoal of fish, and collides with a mososaur coming the opposite way. The big creature had its mouth shut when it impacted into our ichthyosaurs’s ribcage. She wasn’t cut, but a couple of ribs were stove in and, more seriously, all the wind was driven from her lungs. She didn’t have the strength to reach the surface and take another breath; she drowned.

As she is carried out to sea, she bloats with gases, which keep her buoyant. Creatures nibble at her flesh, a passing shark takes a bite. After a few days she sinks to the bottom, her corpse disturbing the crinoid dust a few miles out from the island. There is not much visible life down here initially, but within hours of her arrival, hagfish are feeding off her. Hagfish have been around for nearly four hundred million years at this point. They are also known as slime eels for a very good reason; put one in a bucket of sea water and agitate it; all the water will turn into a thick gel. After six months or so she is nothing but bones – bones covered in a dusting of calcite – sea lily petal dust.

Our shoppers have followed her out, and now stand around the skeleton disconsolately. We’ll fast forward. As we do so, our ichthyosaur disappears under layer after layer of calcite which continues to build up, yard after yard, gradually forming limestone. The land rises, the ocean recedes. Evidence of this ocean remains in the form of ‘Mendip potatoes’. These can be found on the edges of fields in the area. If you spy a small reddish nodule with a potato shape and crack it open, a beautiful rose pattern will be exposed. These were formed when the ocean lapped against the red Triassic sandstone of the Mendips and the salt in the water was soaked into the matrix. When the ocean receded, and the remaining water was evaporated in a hot climate, the nodules of salt, lime and sandstone were left behind. Fast forward again, about 160 million years. We’re almost home, but not quite. We’re in the 1940’s. Two more ichthyosaurs are discovered when foundations are being dug for an air-field. There is no time to get them out, so they are hastily covered, and the cement of the air-strip is poured over their bones. The cement consists largely of lime which comes from. . . The air-strip is complete, and now man can take to the air in flying machines so that they may murder each other. The ichthyosaurs remain where they are.

Fast forward. The shoppers are safely returned. Some consider a little brunch at a fast food restaurant, to mull over what the rest of the weekend might bring.

For images of the creatures mentioned in this entry, please click the links below.