The Ichthyosaurs

Ichthyosaurs (‘fish lizards’) thrived during the Mesozoic, appearing around 250 mya and surviving (at least in one species) until the Upper Mesozoic. Ichthyosaurs are one of the most notable Mesozoic reptiles, seconded only by the plesiosaurs of Loch Ness fame. Ichthyosaurs resembled modern fish and dolphins and ranged from one to over fifty feet in length (one species reached nearly seventy feet long). Unlike nothosaurs, their limbs were fully transformed into flippers that could contain a wide array of fingers. Some species had a dorsal fin, and most used a more vertical tail fin for powerful propulsive strokes. Ichthyosaurs had short necks, and later species had stiff trunks. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurs had pointed heads and large eyes, which would’ve been useful when diving. Ichthyosaurs were warm-blooded, breathed air, and bore live young.

Temnodontosaurus was a beast

Typical ichthyosaurs had bony rings protecting their eyes, suggesting that they may have hunted at night or at great depths – and their large eyes would’ve been useful for such hunting. Temnodontosaurus, with eyes twenty-five centimeters in diameter, could probably see objects as deep as 1600 meters (one mile!) down. Shockingly, Opthalmosaurus, with even larger eyes, may have been able to go deeper than a mile into the ocean depths. The only living animals with similarly large eyes are the giant and colossal squids. Despite having such robust eyes, they likely had poor hearing, given the nature of their middle ear bones. They may have made up for bad hearing with an acute sense of smell, or they may have possessed electro-sensory organs like those seen in modern sharks, rays, and dolphins; these adaptations could explain the grooves in ichthyosaur palates. Further evidence of deep diving has been the presence of bone necrosis in about twenty percent of Jurassic and Cretaceous remains. Quick ascent from great depths can cause decompression sickness, which can be seen in the resulting bone necrosis. Because this bone necrosis is rare in Triassic species, some paleontologists have argued that the Triassic forms didn’t take to diving like their descendants. Other scientists argue that the lack of bone necrosis doesn’t indicate a lack of deep diving but a lack of rapid ascension: in the Jurassic and Cretaceous, ichthyosaurs were face-to-face with monstrous, fast-moving predators – such as mosasaurs and plesiosaurs – that could attack them in the depths and force them to make a rapid ascent (resulting in the accumulation of bone necrosis). Triassic ichthyosaurs, lacking such predators and dominating the marine food web, could afford to lazily and luxuriously ascend to the surface to breathe. A flaw in this theory is that modern diving animals show bone necrosis not from an accumulation of rapid ascents but as the general degeneration caused by a diving lifestyle. 

a feeding frenzy

Ichthyosaurs were carnivorous and adapted to a variety of lifestyles. Species with pointed snouts were designed for grabbing smaller animals; some forms with protruding jaws may have used their pointy snouts to slash prey like modern swordfish. Early ichthyosaurs were durophagous: their flat convex teeth were designed for crushing shellfish; other early ichthyosaurs may have been suction-feeders, sucking animals into their mouths by quickly opening their short jaws (recent studies, however, indicate that those species thought to be ‘suction feeders’ were more likely ‘ram feeders’ who gathered food by constantly swimming forward with a mouth wide-open). Preserved gut contents tell us that most ichthyosaurs fed on cephalopods (such as squid); others fed on fish and even smaller ichthyosaurs. Some ichthyosaurs were apex predators with large, bladed teeth and adaptations for killing large prey. Ichthyosaurs weren’t averse to scavenging and would eat drowned animals swept out to sea: in 2003 a specimen of the Cretaceous Platypterygius had eaten fish, a turtle, and a land bird. Ichthyosaurs hunted – and were hunted. During the Triassic they had to evade sharks and other ichthyosaurs. In the Jurassic they were hunted by the marine crocodylomorphs and the plesiosaurs (in 2009 a plesiosaur specimen was found with an ichthyosaur embryo in its gut). Another discovery, this one of a large ichthyosaur close to thirty feet long, showed a tail that had been recently bitten off; researchers theorized, given the presence of an ammonite shell in its throat region, that the ichthyosaur was ambushed and attacked, likely by a pliosaur (known from the same habitat), which severed its tail. The ichthyosaur, stripped of its means of locomotion, sank to the depths and drowned (ichthyosaurs, remember, were air-breathers).

an ichthyosaur is about to become a predator's lunch

a pod of dolphin-like ichthyosaurs

Gregarious behavior – or social behavior indicative of a social lifestyle – has been assumed. The assumption comes easy, given ichthyosaurs’ resemblance to dolphins who have a rich social life. Fossil evidence of such behavior, however, isn’t super strong. Though there is evidence of social dimorphism in some species, it isn’t the case across the board (social dimorphism, or slight variations between males and females of the same species, is usually a good indicator of gregarious behavior). Social behavior has also been argued by the fact that ichthyosaurs gave birth to live young who would likely need to be raised to adulthood. Some scientists point to the fossilized remains of an ichthyosaur with bite marks to the snout region as further evidence of gregarious behavior. Analyses of the healed bite marks indicate that they were made by another ichthyosaur of the same species; perhaps these were two males fighting over mates? 

Ichthyosaurs likely emerged during the Triassic (though some scientists speculate, given the sudden diversity of Triassic ichthyosaurs, that they may have emerged as far back as the Upper Permian). It’s believed that ichthyosaurs, like modern whales and dolphins, developed from terrestrial land-animals that returned to the sea. While dolphins and whales evolved from land mammals, ichthyosaurs evolved from land reptiles (though a minority of scientists argue that they came from land amphibians). Because there are so many dissimilarities between the earliest ichthyosaurs and their hypothesized ancestor, ichthyosaur origins continue to be hotly debated. What is generally known (or believed) is that the ichthyosaurs, despite sharing the Mesozoic oceans first with nothosaurs and then with plesiosaurs, comes from a wholly different lineage than those two Sauropterygian groups (the mosasaurs, who coexisted with ichthyosaurs for a time, also had a unique lineage from aquatic lizards). A boom to ichthyosaur origins took place in 2014 when a small basal ichthyosauriform from the Early Triassic was found in China; this creature had characteristics suggesting a semi-aquatic rather than fully-aquatic lifestyle, and it’s been hailed as a missing ‘transitional link’ between land-dwelling reptiles and ‘true’ ichthyosaurs.

Utatsusaurus

The earliest ichthyosaurs emerged in the Olenekian and Anisian stages of the early Triassic (at least according to most scientists). These early forms include Chaohusaurus, Grippia, and Utatsusaurus. Their diversity suggests an earlier origin than the fossil record has told us, which is why some scientists place their emergence as far back as the Late Permian (though most hold to an Early Triassic emergence due to the fact that the Permian-Triassic Extinction rendered the oceans anoxic and would’ve likely killed off any ichthyosaurs with a Permian start-date). These early ichthyosaurs looked more like finned lizards than the fish- and dolphin-like species of the later Mesozoic. Their bodies were elongated and they likely propelled themselves through the water by undulating their entire trunk. Their pectoral girdles and pelves were robustly built, and their vertebrae possessed the interlocking processes used to support the body against the force of gravity (all of which are seen in terrestrial animals). That they weren’t semi-aquatic is evidenced in the fact that their limbs had been completely transformed into flippers. These, like later ichthyosaurs, were likely warm-blooded and viviparous (giving birth to live young). Because these early ichthyosaurs, despite their variations, have such a distinct built when compared to later ichthyosaurs, some paleontologists class them as ‘proto-ichthyosaurs’ and classify them as the Ichthyopterygia. 

a pair of Cymbospondylus

These Early Triassic forms gave rise to ‘true ichthyosaurs’ sometime around the early Middle Triassic. These ichthyosaurs, like their predecessors, displayed a wide range of variation. There was Cymbospondylus, for example, which resembled a thirty-foot sea-serpent, compared to the more ‘proper’ (albeit smaller) Mixosaurus. The Mixosauria were fish-like with a pointed skull, a shorter trunk, a more vertical tail fin, a dorsal fin, and short flippers. Mixosauria’s sister group, Merriamosauria, diversified into the large and classic-looking Shastasauria and the dolphin-like Euichthyosauria. Euichthyosaurs had more narrow front flippers with a reduced number of fingers, and they include species such as Californosaurus and Toretocnemus. Then there were the Parvipelvia, who had a reduced pelvis (hence the name), and they included species such as Hudsonelpidia and Macgowania. 

the wide-bellied Shonisaurus hanging with smaller ichthyosaurs

In the Late Triassic, the Shastosaurs reached epic sizes. Shonisaurus was fifty feet long; Himalayasaurus was thirty feet long; and Shastasaurus sikanniensis may have reached up to seventy feet in length (if this is correct, it was the largest marine reptile known). It was during the Late Triassic that ichthyosaurs reached the peak of their diversity, dominating numerous ecological niches. Some were top predators while others fed on smaller prey. Some may have specialized as suction or ram feeders. Towards the end of the Late Triassic, variability began to decline. The giant species disappeared, perhaps due to increased competition by sharks, ray-finned fishes, and the emerging plesiosaurs.

After the Triassic-Jurassic Extinction, plesiosaurs had a head-start in diversification and overwhelmed the ecological niches that ichthyosaurs had dominated – but this didn’t mean ichthyosaurs were squeezed out entirely. There were still many large ichthyosaurs that reached up to thirty feet in length. Because most early ichthyosaur discoveries were species from the Jurassic, some of these early Jurassic species have become household names, such as Ichthyosaurus. Nevertheless, the ichthyosaurs of the Early Jurassic weren’t as varied as they had been in the Late Triassic; they didn’t grow as large, and suction/ram feeders and durophagous species (those who subsisted on shellfish) disappeared. Early Jurassic ichthyosaurs were streamlined, dolphin-like forms. An odd-ball of the time was the Thunnosauria, a group of ichthyosaurs that adapted thunniform locomotion, propelling themselves with the end of the tail only, which had a vertical tail fin. Another odd-ball group were the Eurhinosauria, who were specialized forms with very elongated and pointy snouts. The fossil record gets shady in the Middle Triassic, which isn’t necessarily problematic (the Middle Jurassic tends to have a spotty fossil record). When we get to the Late Jurassic, there are indications that a further decrease in diversity took place in the shadows of the Middle Jurassic. All ichthyosaurs belonged to an off-branching line of the thunnosaurs, the Opthalmosauria. The most famous of these Late Jurassic ichthyosaurs was Opthalmosaurus; these had monstrous eyes and likely hunted in dark, deep water. 

the wide-eyed Opthalmosaurus

Though ichthyosaurs had worldwide distribution in the Cretaceous, there is an apparent continuation of decreasing diversity. For decades it was believed all fossils referred to a single genus, Platypterygius. Traditionalists have placed ichthyosaur extinction in the Early Cretaceous about 95 million years ago, making them the first to go among the other Mesozoic reptiles such as the plesiosaurs and the mosasaurs. Two theories emerged to explain their disappearance: first, maybe they were just unlucky; second, perhaps they just didn’t have what it took to compete with other Cretaceous aquatic animals, such as the mosasaurs and plesiosaurs. Fast-swimming and highly-evasive ray-finned fishes could dominate feeding grounds and easily escape their more cumbersome ichthyosaur predators, and the ambush strategies of the mosasaurs were superior to those of the ichthyosaurs. Or perhaps, some argue, they became too specialized: the more specialized a creature becomes, the more at risk it is of extinction, as environments are in a continual state of flux. ‘Evolutionary stagnation’ has been the death-knell for many species throughout our planet’s history. But maybe, some paleontologists speculate, we got it all wrong. Recent scientists have argued that fragmentary remains attributed to Platypterygius may in fact represent more diverse species. In 2012 it was shown that at least eight ichthyosaur lineages spanned the Jurassic-Cretaceous boundary; why would all but one go extinct? The next year, 2013, a thunnosaurian ichthyosaur, Malawania, was discovered. These discoveries have led many scientists to postulate that ichthyosaurs didn’t decline in the Early Cretaceous but actually diversified as more coastlines opened up due to the plodding but continuous break-up of the continents.

Nevertheless, ichthyosaurs did die out well before their Mesozoic counterparts. Recently this has been viewed as a two-stage process. The first extinction event eliminated two of the three ichthyosaur ‘feeding guilds’: the ‘soft-prey specialists’ and the ‘generalists,’ leaving only an apex predator group. The second extinction event took place during the Cenomian-Turonian boundary event (in the early stages of the Upper Cretaceous). This ‘anoxic event,’ in which the oceans suffered a decrease in available oxygen, led to the demise and eventual wipe-out of the apex ichthyosaurs. Though Platypterygius survived into the later Cretaceous, it disappeared around 93 million years ago, well before the mosasaurs and plesiosaurs (both of whom would meet their maker in the Cretaceous-Tertiary Extinction). This two-stage process of ichthyosaur demise is attributed not to competition in the ocean but to environmental factors, such as changes in migration, food availability, and birthing grounds. Concurrent with the ichthyosaur demise was a variety of other marine extinctions, hinting at ecological factors: microplankton, ammonites, and reef-building bivalves suffered greatly. When the ichthyosaurs disappeared, mosasaurs diversified into larger forms that filled the ecological niches left vacant.