Despite looking like fish, their anatomy shows they were once land-lubbers. They had two pairs of limbs, with digit-like bones rather than rays or spines in their flippers, and a shoulder girdle connected to the skull. The roof of the skull had a pair of openings called fenestra: a hallmark of reptiles.
Another indication of their terrestrial origin is their lack of gills. Like marine mammals, they had to draw oxygen from the atmosphere. When an ichthyosaur embryo in the womb uncurled itself in preparation for birth, it instinctively oriented 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.
Amongst the oldest known ichthyosaurs was Utatsusaurus, from the Lower Triassic of Japan. As in terrestrial reptiles, the pelvic girdle was attached to the spine, but no longer robustly enough to support the body’s weight. Another transitional feature were the equal lengths of humerus and femur. In more advanced ichthyosaurs, including the contemporary Chaohusaurus, the humerus was longer than the femur (the front limb 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. On several fronts, the view that ichthyosaurs were former land-dwellers that evolved adaptations for life at sea is well supported.
Few specimens are preserved with embryos. One pregnant Chaohusaurus – found north of the city of Chaohu, China – made the news because the trove included no fewer than 80 skeletons. In this case the embryos left the womb head-first, as with most land animals, indicating that a tail-first exit was a later adaptation.
The earliest fossil ichthyosaurs were small (around 1 m long) and relatively slender. Comparisons with similarly shaped fish, such as catsharks, suggest that they swam by wiggling their bodies sideways, like modern-day lizards. As they chased their prey, their small vertebrae gave them the flexibility to accelerate or change direction quickly. But they did not stop at the shallows. By the end of the Triassic ichthyosaurs they were venturing far beyond the continental shelf and, despite having to surface for air, penetrating the ocean down to hundreds of metres. Their mastery of the pelagic environment quickly equalled that of any animal.
Because undulatory swimming creates more drag and consumes more energy, it is a poor choice for predators in the open ocean, where food is less abundant. Predators such as the great white shark therefore have stiffer bodies, moving little as the tails beat sideways. They have thicker bodies and larger, stubbier vertebrae. Together with their crescent-shaped tail fins, 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: thicker bodies, larger vertebrae, and a caudal fluke. By increasing both heat conservation and oxygen storage, their larger mass enabled them to plunge to greater depths. Oceans become cooler with depth, and reptiles are cold-blooded: they do not have heat engines and temperature regulation systems to keep their body at a constant temperature. Consequently the first ichthyosaur reptiles were ill-suited to exploiting deep water. Later species appear to have put on fat and evolved an ability to keep their bodies warm. Although greater volcanism on the seafloor meant that oceans were shallower and warmer than today, below the surface layer they would still have been chilly. Nonetheless, it has been calculated that a 4-metre Ophthalmosaurus could stay submerged for at least 20 minutes, time enough to dive down to 600 metres and back. Their huge eyes were highly light-sensitive and capable of resolving fine detail, ideally suited to focusing on small, quick prey in the dimmest regions. As well as additional sensor cells around the nose, an elaborate system of what may have been electro-receptors similar to the systems certain sharks have for detecting electric fields compensated for their inability to see directly in front.
The identity of the land reptile that evolved the ichthyosaur form is not known. The founding population was apparently too small for individuals to stand much chance of being fossilised and the lineage acquired its marine adaptations too rapidly. By the time the fossil record pulls the curtain back, 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, with ichthyosaurs kings of the open ocean as well as the continental shelf, the rate of evolution slowed. They underwent minor variation, not radical innovation. The final goal of life in the deep seas had been attained. Ichthyosaur diversity waxed and waned as sea temperatures fluctuated. When average temperatures were high, species flourished; few occur in sedimentary rocks from cooler times. The warmth of sea and land could be the principal explanation why reptiles and dinosaurs generally dominated the Mesozoic. Mammals came into their own as temperatures fell.
Diversity (number of species at any one time) apparently declined in the course of the Jurassic, though not as much as was once thought. The last ichthyosaur passed away during the Cenomanian-Turonian extinction in the Late Cretaceous, as did the pliosaurs, another group of marine reptiles. By contrast, plesiosaurs and the newly evolved mosasaurs continued to flourish to the end of the period. What caused the extinctions is unclear, but it may have been connected with the extrusion of huge volumes of lava in the Pacific, creating the Ontong Lava Plateau.
Ichthyosaurs are a classic example of how creationist and evolutionist ideologues both adopt positions at variance with the evidence. The Answers in Genesis organisation asks:
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.
They say the same about the sirenians, when even a cursory knowledge of the facts shows that this is not a tenable view. The evidence for ichthyosaur evolution is clear. The fossils are restricted to the Mesozoic (and particular species to defined periods within the Mesozoic), species become progressively less terrestrial in nature, and there is a trend from near-shore to deep-sea. The more than 80 species of ichthyosaur are not scattered through the fossil record without pattern.
It is also worth considering 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 rare cases, even carbonised skin impressions to be preserved. Fine-grained sediment takes time to settle. In the case of the Posidonia Shale enclosing the famous Holzmaden ichthyosaurs (illustrated in the photograph at the top), 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 see that a land reptile evolved into a marine reptile and think, ‘this confirms that all life evolved into existence, without design or purpose.’ The logic is that any substantial degree of change in a species constitutes evidence against creation because created species would have had fixed forms; they would have been unable to adapt and diversify. Consequently, adaptation and diversification indicate that all apparent design in the natural world is illusory.
But the logic rests on straw-man conceptions. There is no reason to think that the earth was designed to last forever, changeless and incorruptible. The very idea of a species is man-made: in reality there are only populations of individuals, and even individuals are not the same. Although, somehow, individuals recognise and are attracted to their own kind, what defines the kind can change over time. Indeed, since individuals are never exactly the same as their parents, the kind is bound to change, and while that is partly because of random gene shuffling between the sexes, it may also be the consequence of programmed change, initiated as populations explore new environments.
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, encased in a streamlined mitten; the streamlining 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 electro-receptors to help them close in on their prey. The question is, are these changes – to say nothing of the entire preceding history of the lineage – well explained on the basis that, in the course of genetic miscopying, slight advantages repeatedly passed into the whole population as animals with the mutation left more offspring than those without? The history of ichthyosaur DNA cannot be retraced, so we can only consider the question in the light of what seems most likely – likely as judged by the phenomena themselves, not a preconceived belief about the nature of reality. And the phenomena, surely, suggest a co-ordinated sequence of changes directed towards a pre-determined end. The more scientific approach is to attribute large-scale evolutionary change to the intrinsic cause of the organism’s genome, rather than to ‘chance’ entering and leaving the scene like some deus ex machina.
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’. Did these ‘remarkable’ innovations evolve from nothing? In the mind of a scientist, evolution from nothing should be a scientifically well-founded inference, not a quasi-religious presupposition. Gould’s position is to believe that matter itself is a miracle-worker, the essentially pantheistic view of Epicurus. In reality, ‘just the right place’ and ‘just the right hydrological design’ point to processes that were not accidental.
In performing extraordinary feats of adaptation ichthyosaurs were far from alone. Other reptiles that adapted to life at sea include mesosaurs, nothosaurs, pistosaurs, placodonts and thalattosaurs. One branch of the pistosaurs gave rise to the plesiosaurs. Mosasaurs were a further group, originating from varanid lizards in the mid Cretaceous and evolving to become, by the end of the period, ferocious sea monsters up to 10 metres long.
The evolution of reptiles from terrestrial to marine is part of the story of recolonisation following global destruction and extinction at the end of the Hadean. At that time, all land animals were wiped out – all but the few pairs saved in the purpose-built, ocean-going lifeboat we know as the ark. It was from them that the ever changing earth was recolonised. The survivors were genetically pre-programmed to adapt to new environments, often in revolutionary ways. While 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, the penguins, gave up the power of flight in return for an ability to swim, in the process colonising the coldest and most hostile continent 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 regulatory networks in the genome. However, by the end of the Mesozoic most of the marine reptiles had died out. Today only iguanas, snakes and turtles remain from that pioneering age.