Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
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Chapter 9 - The Triassic Period
Lesson 47: Species In-Depth: Ichthyosaur
Ichthyosaurs (Greek for "fish lizard" - ιχθυς/ichthys meaning "fish" and σαυρος/sauros meaning "lizard") were giant marine reptiles that resembled fish and dolphins. Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared approximately 245 million years ago and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct. During the middle Triassic Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphins and whales.
Chapter 9 - The Triassic Period
Ichthyosaur - Stenopterygius quadriscissus
They were particularly abundant in the Jurassic Period, until they were replaced as the top aquatic predators by plesiosaurs in the Cretaceous Period. They belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' - a designation introduced by Sir Richard Owen in 1840, although the term is now used more for the parent clade of the Ichthyosauria).
Ichthyosaurs averaged two to four metres in length (although a few were smaller, and some species grew much larger), with a porpoise-like head and a long, toothed snout. Built for speed, like modern tuna, some ichthyosaurs appear also to have been deep divers, like some modern whales (Motani, 2000). It has been estimated that ichthyosaurs could swim at speeds up to 40 km/h (25 mph). Similar to modern cetaceans such as whales and dolphins, they were air-breathing and also were viviparous (some adult fossils have even been found containing fetuses). Although they were reptiles and descended from egg-laying ancestors, viviparity is not as unexpected as it first appears. All air-breathing marine creatures must either come ashore to lay eggs, like turtles and some sea snakes, or else give birth to live young in surface waters, like whales and dolphins. Given their streamlined bodies, heavily adapted for fast swimming, it would have been difficult for ichthyosaurs to scramble successfully onto land to lay eggs.
According to weight estimates by Ryosuke Motani a 2.4 meter (8 ft) Stenopterygius weighed around 163 to 168 kg (360 to 370 lb), whilst a 4.0-meter (13 ft) Ophthalmosaurus icenicus weighed 930 to 950 kg (about a ton).
Although ichthyosaurs looked like fish, they were not. The earliest reconstructions of ichthyosaurs omitted the dorsal fin, which had no hard skeletal structure, until finely-preserved specimens recovered in the 1890s from the Holzmaden lagerstätten in Germany revealed traces of the fin. Unique conditions permitted the preservation of soft tissue impressions.
Ichthyosaurs had fin-like limbs, which were possibly used for stabilization and
Apart from the obvious similarities to fish, the ichthyosaurs also shared parallel developmental features with dolphins, lamnid sharks, and tunas. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecological niche.
For their food, many of the fish-shaped ichthyosaurs relied heavily on ancient cephalopod kin of squids called belemnites. Some early ichthyosaurs had teeth adapted for crushing shellfish. They also most likely fed on fish, and a few of the larger species had heavy jaws and teeth that indicated they fed on smaller reptiles. Ichthyosaurs ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Typical ichthyosaurs have very large eyes, protected within a bony ring, suggesting that they may have hunted at night
History of Discoveries
The genus had first been described in
1699 from fossil fragments discovered in Wales.
The earliest ichthyosaurs, looking more like finned lizards than the familiar fish or dolphin
These very early proto-ichthyosaurs, which are now classified as Ichthyopterygia rather than as ichthyosaurs proper, quickly gave rise to true ichthyosaurs sometime around the boundary between the Early Triassic and Middle Triassic. These later diversified into a variety of forms, including the sea serpent like Cymbospondylus, which reached 10 meters, and smaller more typical forms like Mixosaurus. By the Late Triassic, ichthyosaurs consisted of both classic Shastasauria and more advanced, "dolphin"-like Euichthyosauria (Californosaurus, Toretocnemus) and Parvipelvia (Hudsonelpidia, Macgowania). Experts disagree over whether these represent an evolutionary continuum, with the less specialised shastosaurs a paraphyletic grade that was evolving into the more advanced forms, or whether the two were separate clades that evolved from a common ancestor earlier on.
During the Carnian and Norian, shastosaurs reached huge sizes. Shonisaurus popularis, known from a number of specimens from the Carnian of Nevada, was 15 meters long. Norian shonisaurs are known from both sides of the Pacific. Himalayasaurus tibetensis and Tibetosaurus (probably a synonym) have been found in Tibet. These large (10 to 15 meters long) ichthyosaurs probably belong to the same genus as Shonisaurus. While the gigantic Shonisaurus sikanniensis, whose remains were found in the Pardonet formation of British Columbia by Elizabeth Nicholls, reached as much as 21 meters in length - the largest marine reptile known to date.
These giants (along with their smaller cousins) seemed to have disappeared at the end of the Norian. Rhaetian (latest Triassic) ichthyosaurs are known from England, and these are very similar to those of the Early Jurassic. Like the dinosaurs, the ichthyosaurs and their contemporaries the plesiosaurs survived the end-Triassic extinction event, and immediately diversified to fill the vacant ecological niches of the earliest Jurassic.
The Early Jurassic, like the Late Triassic, was the heyday of the ichthyosaurs, which are
Ichthyosaurs were still common in the Middle Jurassic, but had now decreased in diversity.
Ichthyosaurs seemed to decrease in diversity even further with the Cretaceous. Only three genera are known, Caypullisaurus, Maiaspondylus, and Platypterygius, although they had a worldwide distribution. This last ichthyosaur genus became extinct during the Cenomanian-Turonian extinction event early in the Late Cretaceous (as did the pliosaurs). Interestingly, less hydrodynamically efficient animals like mosasaurs and long-necked plesiosaurs flourished. It could be that the ichthyosaurian over-specialisation was a contributing factor to their extinction, possibly being unable to 'keep up' with the fast swimming and highly evasive new teleost fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasaurs proved superior.
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Source Page: Ichthyosaur