By Ryan Marek*1
A literal translation of ichthyosaur is ‘fish lizard’, yet ichthyosaurs were neither fish nor members of the lizard family; they were a group of highly successful marine reptiles who lived from the early Triassic period to the late Cretaceous period, around 248 million to 90 million years ago. Ichthyosaurs were among the first diapsids (a group of animals which have evolved two holes on each side of their skulls, includes birds and all reptiles except turtles) to evolve a thunniform (fish-like) body plan as an adaptation to life in the sea (Fig. 1). They were also the first air-breathing vertebrates to evolve this body plan; the only other group to do so was the whales and dolphins, millions of years later. Most ichthyosaurs after the Triassic had the fish-like body plan, but they ranged in length from less than a metre to 21 metres in the colossal Shastasaurus. They also gave birth to live young, with the first evidence for this appearing in the fossil record 248 million years ago. Ichthyosaurs have been studied for a long time, making their first appearance in the scientific literature in 1814. They are important in the history of palaeontology, with famous palaeontologists such as Mary Anning, William Buckland and William Conybeare all working on them. Two hundred years after they were discovered, we have learnt much about the anatomy and biology of ichthyosaurs, but their evolutionary origins and relationships, functional morphology and sensory biology remain elusive.
Classification and relationships
Ichthyosaurs evolved from terrestrial reptiles, but their most recent common ancestor has not yet been discovered. However, a small short-snouted and possibly amphibious ichthyosaur-like animal found in China brings us a step closer to understanding the terrestrial origins of this aquatic group. The immediate outgroup (i.e. closely related to ichthyosaurs, but not actually ichthyosaurs themselves; for example gorillas are the outgroup to humans and chimps) of ichthyosaurs is the hupehsuchians, a short-lived clade of marine reptiles (early Triassic) whose overall shape resembles that of ichthyosaurs.
Ichthyosaurs have been classed as diapsids, yet their position in this large group is still debated. The evolutionary relationships between ichthyosaurs are clearer (Fig. 2). There are two distinct clades : the more advanced groups form the clade Parvipelvia (‘little pelvis’), whereas the more primitive (basal) Triassic groups form the ‘nonparvipelvians’. It was the parvipelvians that evolved the thunniform body plan that is so iconic of ichthyosaurs; earlier, nonparvipelvian ichthyosaurs were more lizard-like in appearance, with less pronounced tail and dorsal fins. A recent discovery of a basal thunnosaurian from Iraq has enabled researchers to compile the most comprehensive parvipelvian evolutionary tree to date. Some researchers think this confirms that post-Triassic ichthyosaurs fall into two main clades, Neoichthyosauria and Thunnosauria, although others disagree.
The most obvious features of ichthyosaurs are a long, thin snout and large eye sockets, along with a tail fluke that was supported by vertebrae only in the ventral half. This often earns them a superficial comparison to modern dolphins, yet they are not mammals. The differences between the fish-like body plan of the parvipelvians and the lizard-like body plan of the nonparvipelvians played a fundamental part in swimming mechanisms, as will be discussed in the ‘Palaeoecology’ section.
Eye size is another feature that differs between the groups, with parvipelvians having much larger eyes than their predecessors. The differences in anatomy have led researchers to conclude that over their evolutionary history, ichthyosaurs became better adapted to life in the open ocean. All ichthyosaurs had paired flipper-like appendages, rather than walking limbs, and many (except very early species) had both dorsal and tail fins.
Ichthyosaurs had the largest relative eyeball size of any known vertebrate. The large eyeballs (which meant that ichthyosaurs could see very well) and the presence of symptoms consistent with Caisson disease (decompression sickness or ‘the bends’, a condition that affects divers) have led researchers to think that many post-Triassic ichthyosaurs could dive to depths of up to 600 metres. Some studies have found osteological adaptations to deep diving, such as a lightening of the skeleton in a variety of ichthyosaurs. Early ichthyosaurs that were lizard-like in appearance probably swam by wriggling their bodies from side to side, like eels. Later ichthyosaurs with a more fish-like body shape and a pronounced tail fin swam like tuna, with most of the movement in the tail. This allowed them to swim at higher speeds over longer distances.
CT scans of an exceptionally preserved ichthyosaur (Hauffiopteryx typicus) skull have revealed the anatomy of its brain, and back up previous ideas about how well ichthyosaurs could see. With such large eyes, the brain of an ichthyosaur has large optic lobes (Fig. 3) — the area of the brain responsible for processing vision. Hauffiopteryx (and possibly many other ichthyosaurs) had an enlarged cerebellum, the area of the brain responsible for motor control. Unexpectedly, it also had an enlarged olfactory region, the region of the brain that processes smells. This shows that ichthyosaurs were highly agile, fast-moving predators that used a combination of vision and smell to find prey at depth and in shallower waters.
All ichthyosaurs have similar overall skull shape, with a thin, elongated snout. This has implications for their diet, because such thin jaws cannot bite very hard without breaking. Many ichthyosaurs must therefore have eaten organisms much smaller than themselves, such as small fish and cephalopods. Various species of ichthyosaur have a range of different tooth shapes to aid prey capture. These are used to assign ichthyosaurs into ‘feeding guilds’. For example, ichthyosaurs with thin, needle-like teeth are members of the ‘pierce’ guild and are best adapted to catching and eating small fish, whereas ichthyosaurs with more rounded, robust teeth are members of the ‘crunch’ guild, and can catch and eat hard-shelled prey. Some ichthyosaurs, such as Thalattoarchon, had shorter, more robust skulls and some researchers think that these species ate large prey. Suction feeding has been suggested for ichthyosaurs such as Shonisaurus and Shastasaurus, yet remains contentious.
The ichthyosaur fossil record is both well studied and abundant, with 102 valid species. Everything from individual teeth and vertebrae to almost-whole body fossils are found on a regular basis by palaeontologists and members of the public. Ichthyosaurs have been found around the world, including in England, Germany, Tibet, China and the United States.
Ichthyosaur diversity was drastically reduced after an extinction event at the end of the Triassic period (around 201 million years ago). Researchers once thought that ichthyosaurs never fully recovered from this event. However, recent work has suggested that there may have been hotspots of ichthyosaur diversity, for example in Western Europe in the late Cretaceous, even as diversity decreased in areas such as Australia and the United States.
Spectacular ichthyosaur fossils can be found in the Holzmaden deposit in southern Germany, where soft-tissue outlines can be seen with stunning detail and show tail and dorsal fins (Fig. 4). Also found in this deposit are adult specimens that were preserved giving birth to live young, an incredible rarity in the fossil record of any organism. Despite such finds, the ichthyosaur fossil record suffers from one major problem: compression. The vast majority of specimens (particularly from the early Jurassic, 199 million to 176 million years ago) are squashed sideways, because when an ichthyosaur died, the orientation of its tail meant that it lay on its side after death. This makes it hard to see anatomical details in the fossils, and has hindered studies on ichthyosaur evolution. It has also hindered work using digital-reconstruction techniques, which can now quantify form and function in extinct animals, such as bite force, jaw function and ways of moving.
After more than 200 years of study, some involving important figures in the history of palaeontology such as Mary Anning, there is still much unknown about this hugely successful clade of marine reptiles. From terrestrial beginnings, ichthyosaurs quickly adapted to life in the water, and over the course of their 160-million-year history evolved to be masters of the open ocean, with functional, hydrodynamic and neuroanatomical adaptations that would enable them to be fast, agile and efficient ocean-faring hunters. With the latest techniques now being applied to ichthyosaurs, we can expect to learn even more about their functional morphology and sensory biology, and to further understand how terrestrial vertebrates can infiltrate and dominate ocean environments.
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1University of Liverpool –address–