Utatsusaurus and the origin of ichthyosaurs

Ichthyosaurs, or ‘fish lizards’ to translate their name literally, were marine reptiles that lived in the Mesozoic era, about the same time as dinosaurs on land and pterosaurs in the air. We know of more than 80 species, so ichthyosaurs were by no means all the same. The smallest was around 1.3 metres, the biggest, Shonisaurus, an amazing 23 metres, both from relatively early in the Mesozoic. Their streamlined bodies resembled those of sharks or modern dolphins. The favourite diet of the more corpulent seems to have been the extinct, squid-like belemnites. In absolute terms as well as relative to their body dimensions, ichthyosaurs had eyes as big as those of any animal known, with the record going to the 9-metre-long Temnodontosaurus, whose eyes were 26 cm across.

What makes them recognisable as ‘reptiles’ and not ‘fish’? Most obviously, they had two pairs of limbs, with bones in their flippers that harked back to a time when they had digits, and their shoulder girdle was connected to the skull. These are also characteristics of mammals, but mammals had not yet appeared in the fossil record. The design of their skulls was specifically reptilian. For example, the skull roof had a pair of openings called the upper temporal fenestra. Upper and lower temporal fenestra are characteristic of diapsids, which in the preceding Permian were all reptiles. Presumably, the forbears of ichthyosaurs had lost the lower pair, an inference corroborated to some extent by the skull structure of an early ichthyosaur from Spitzbergen (Maisch & Matzke 2002).

Another indication that they had a terrestrial origin is their lack of gills. Like marine mammals, they had to draw oxygen directly from the atmosphere. This may have been why, when an ichthyosaur embryo uncurled itself in preparation for birth, it knew to orient itself so that it passed out of the womb tail first. Only when its head emerged did it need to breathe, at which point it could take its first gulp of oxygen by immediately swimming to the surface. Several viviparous sharks, which do have gills, also give birth to their young tail-first.

Amongst the oldest known ichthyo- saurs is Utatsusaurus, from the Lower Triassic of Japan. As in terrestrial reptiles, the pelvic girdle was attached to the spine, but not robustly enough to support the body’s weight. Another transitional feature was the equal lengths of humerus and femur. In other ichthyosaurs, including the contemp- orary Chaohusaurus, the humerus was longer than the femur (the front limb was larger than the hind limb) whereas in most terrestrial animals the femur was longer. Utatsusaurus was also closer to terrestrial animals in not having a dorsal fin or a tail fluke. In short, there is much to support the view that ichthyosaurs were former landlubbers that had evolved their design features for life at sea.

From shallow-sea to deep-sea diving

Ichthyosaurs did not stop at the shallows, however. By the end of the Triassic they were venturing far beyond the continental shelf and penetrating the ocean down to hundreds of metres – despite having to surface for air. Their conquest of the marine environment was total.

The earliest known ichthyosaurs were relatively slender, their small vertebrae giving their bodies great flexibility. Comparisons with similarly shaped fish, such as catsharks, suggest that they swam by wiggling their bodies sideways, like modern-day lizards, and were flexible enough as they chased their prey to quickly change direction or suddenly put on speed.

Because it creates more drag and consumes more energy, undulatory swimming is not, however, a good choice for predators in the open ocean, where food is less abundant. The bodies of open-ocean predators such as the great white shark are therefore stiffer, moving little as the tails beat sideways. They have thicker bodies and larger, stubbier vertebrae. Together with a crescent-shaped tail fin, these differences help them to cruise over large distances in search of prey with maximum efficiency.

Remarkably, although unrelated to sharks, ichthyosaurs over time acquired the same adaptations to life in deep water: thicker bodies, larger vertebrae, and a caudal fluke. Through increased conservation of body heat and capacity for oxygen storage, their larger mass had the further advantage of enabling them to plunge down to greater depths. Ocean waters become cooler with depth, and reptiles are cold-blooded: they do not have heat engines and temperature regulation systems to keep their body temperature constant. Consequently ichthyosaurs started off ill-suited to exploiting deep water. On the other hand, their lower metabolic rate and hence oxygen consumption rate allowed them to dive much longer than mammals or birds of the same body mass could have done. In the mid to late Mesozoic, when higher rates of volcanic seafloor spreading meant that oceans were shallower and warmer than they are today, this advantage of lower oxygen consumption outweighed the disadvantage of their cold-bloodedness. A 4-metre Ophthalmosaurus, it has been calculated, could stay submerged for at least 20 minutes, time enough to dive to a depth of 600 metres and back. Their huge eyes were both highly light-sensitive and capable of resolving fine detail, ideally suited to focusing on small, quick prey in the dimmest regions. Around the nose an elaborate system of what may have been electroreceptors, similar to the systems certain sharks have for detecting electric fields, compensated for their inability to see directly in front.

Rapid evolution and diversification, evolutionary slow-down and extinction

The identity of the land reptile species that evolved the ichthyosaur form is not known. The lack of eligible fossils suggests that the population was too small for an individual to stand much chance of being fossilised, and that the lineage acquired its marine adaptations rapidly, by leaps and bounds. By the time the fossil record opens a window on what was going on, in the early Triassic, most of the story had already been played out, though evolution was still proceeding apace. By the late Triassic ichthyosaurs had diversified into the full range of body form and size, with the biggest reaching the size of a blue whale. Shonisaurus’s flippers alone were more than 5 metres in length.

Thereafter, in the Jurassic, ichthyosaurs dominated the open ocean as well as the continental shelf and the rate of evolution slowed, consisting of minor variation rather than radical innovation: their final goal of life in the deep seas had been reached. Ichthyosaur diversity waxed and waned, it seems, with global sea temperatures. When average temperatures were high, species flourished; sediments deposited during cool spells preserve few species. The warmer temperatures of sea and land could indeed be the principal explanation why reptiles and dinosaurs generally were dominant in the Mesozoic. Mammals came into their own as temperatures cooled.

Diversity (number of species at any one time) declined in the course of the Jurassic. Cretaceous deposits have revealed only one genus. This last became extinct during the Cenomanian-Turonian extinction event, as did another group of marine reptiles called the pliosaurs. By contrast, plesiosaurs and the newly evolved mosasaurs continued to flourish to the end of the Cretaceous. What caused the extinction is unclear, but very probably it was connected with the extrusion of huge volumes of lava in the Pacific, creating the Ontong Lava Plateau. A similar event may have caused the extinction of several ichthyosaur species at the end of the Early Jurassic.

Evolution by design

Ichthyosaurs are a classic example of how creationists and evolutionists both adopt positions at variance with the evidence in order to maintain their views. Answers in Genesis, for example, ask:

Did they really evolve from land reptiles? If we look to the fossil record for clues, the answer has to be a resounding ‘No’! … The claim that the ichthyosaurs evolved from terrestrial reptiles simply does not stand up to scientific examination. … Ichthyosaurs did not evolve from anything.

Even a cursory knowledge of the data reveals that this is not a tenable view. Like the idea that ichthyosaur fossils derive from animals buried during Noah’s Flood, it imposes on the world a preconceived idea of what the truth should be. So the article passes over the fact that the fossils occur only in the Mesozoic (and the fact that particular species are restricted to defined periods within the Mesozoic), that the older species are more terrestrial in nature, and that there is a trend from near-shore to deep sea. The more than 80 species of ichthyosaur were not all separately created and then scattered without pattern through the fossil record. And this is before one begins to consider the sedimentary circumstances of burial. One of the reasons we know so much about ichthyosaur anatomy is that many were buried in fine-grained sediment, allowing, in a few cases, even carbonised skin impressions to be preserved. Fine-grained sediment takes time to settle, albeit not thousands of years. In the case of the Posidonia Shale and its famous ichthyosaurs (illustrated in the photograph above), the environment appears to have been a stagnant basin. Oxygen-starved conditions prevailed on the soupy sea floor, interrupted by interludes of ‘a few weeks to a few years’ when the surface was colonised (Schmid-Röhl & Röhl 2003). Such colonisations included bivalves attaching themselves to ammonite conchs whose upper surfaces were still partly exposed. Bivalve larvae had time to locate a hard surface above the mud, settle, and grow to adult size.

Those who believe that life must have come into being by itself look at the evidence that a land reptile evolved into a marine reptile and think, ‘this confirms the view that all life evolved into existence, without design or purpose.’ But the thought needs unpacking. The logic is that any substantial degree of change in a species constitutes evidence against creation (because – isn’t this what creationists say? – God would have created species fixed in form) and evidence for the theory of evolution (because if there is no God, life must have come into existence by steps). Any evidence of change in the fossil record is therefore evidence that life did come into existence by steps – ichthyosaurs included.

The logic is faulty, however, since there is no reason to suppose that the Creator, if he existed, would have created only fixed forms. The very idea of a species is man-made: in reality there are only populations of individuals, and in the case of sexually reproducing organisms even individuals are not the same. By some mysterious knowledge individuals recognise and are attracted to their own kind, whether male and female look alike or very different, but that does not mean that what defines the kind cannot change over time. Since individuals are never exactly the same as their parents, the kind is bound to change, and if that is the consequence of random gene shuffling between the sexes, it may also be the consequence of non-random gene shuffling, of programmed change, initiated as populations explore new environments. Evidence for the theory of evolution would have to be evidence that radical change such as the transformation of a land-dwelling animal into an ichthyosaur was capable of happening – and did happen – by random mutation.

We have touched on the changes that can readily be seen or inferred from fossilised bones and skin: the change from foot to flipper as finger bones shortened, became more numerous and more packed together, all encased in a stream-lined mitten; the stream-lining of the whole body, from head to tail; the thickening of the trunk, enabling the diving animal to store more oxygen and body heat; the stiffening of the spinal column; the development of a stiff dorsal fin and tail fluke; the change in eye design, enabling ichthyosaurs to pick out prey in the dark; the acquisition of electroreceptors to help them close in on their prey. The question is, are these changes satisfactorily explained on the basis that the genetic code (as it had evolved up to that point) was miscopied, each time passing an advantage from one individual to the whole population as those that had the mutation left more offspring than those that did not (the idea of natural selection)? Since the history of the DNA cannot be retraced, we can only come to an answer on the basis of what seems likely – likely as judged by the phenomena themselves, not a preconceived belief about the nature of reality. And the phenomena themselves, surely, suggest a co-ordinated sequence of changes directed towards a pre-determined end. This also commends itself as the more scientific interpretation: large-scale evolutionary change is presumed to have an intrinsic cause, attributable to the organism’s genome, rather than to ‘chance’ entering and leaving the scene like some deus ex machina.

Stephen Jay Gould expresses the wonder of it all:

This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. The evolution of these forms was all the more remarkable because they evolved from nothing — the ancestral terrestrial reptile had no hump on its back or blade on its tail to act as a precursor.

In the Cenozoic, sea-going mammals with terrestrial ancestors – dolphins and killer whales – also converged ‘on a dorsal fin and tail in just the right place and with just the right hydrological design’. But is the miraculous nature of these innovations ‘all the more remarkable because they evolved from nothing’? In the mouth of a scientist, evolution from nothing should be a scientifically well-founded inference, not an unfounded quasi-religious belief. Gould’s position is to believe that matter itself is a miracle-worker, the essentially pantheistic view of Epicurus, and one that the evidence does not support. ‘Just the right place’ and ‘just the right hydro- logical design’ point to processes that were not accidental: to evolution by design, not ex nihilo.

The recolonisation story

In performing extraordinary feats of adaptation ichthyosaurs were far from alone. Other reptiles that managed to adapt their terrestrial bodies to the demands of life at sea include mesosaurs, nothosaurs, pistosaurs, placodonts and thalattosaurs. One branch of the pistosaurs may have given rise to the plesiosaurs. Mosasaurs were a further group, originating from varanid lizards in the middle Cretaceous and evolving to become, by the end of the period, ferocious monsters up to 10 metres long.

The evolution of reptiles from terrestrial to marine is part of the story of recolonisation following an extinction event at the end of the Hadean. According to a tradition that parallels the geological record, the then land animals were all wiped out, all except a few representatives that were saved in a purpose-built, ocean-going ark. It was from them that the still convulsing earth was recolonised. The survivors were programmed to adapt to new environments, often in revolutionary ways. At the same time as some diapsids were exploring the sea, another group (the pterosaurs) was acquiring the ability to fly, as membranes developed between the digits and transformed their forearms into wings. Certain groups of mammal later underwent similar transformations, becoming whales, dolphins, manatees, seals, otters, moles, bats. One group of birds (penguins) gave up their ability to fly in return for an ability to swim, in the process colonising one of the coldest and most hostile places on Earth.

Where reptiles and mammals were concerned, the transition was invariably from land to sea – the opposite of the direction taken, in the Darwinian story, when fish became tetrapods. The evolution was horizontal, not vertical: from complex to complex, controlled by signalling and gene regulatory networks that were complex. In today’s world iguanas, snakes and turtles still testify to the remarkable ability of certain land reptiles to live off the sea.