The roots of the euryapsid family tree are largely uncertain, although the design of the euryapsid skull, with its single temporal fenestra, looks very much like the skull of a diapsid without the lower opening. There is consensus among paleontologists that the euryapsid skull was probably a modification of the diapsid pattern.
Regardless of similarities in their skulls, most euryapsids arose during the Mesozoic as a diverse and plentiful group of reptiles that had returned to an aquatic lifestyle. Just as the dinosaurs conquered life on land, so, too, did the euryapsids become the dominant vertebrates in the oceans of the world. All of these marine reptiles remained air breathers but adapted paddlelike limbs and elongated and streamlined bodies that enabled them to swim well. Some developed needlelike teeth for snaring fish from the water, while others had teeth and bony chewing surfaces better suited for crushing hard-shelled crustaceans and other invertebrates.
Most of the euryapsids did not rise to prominence until the middle of the Mesozoic Era, an era beyond the timeframe discussed in March Onto Land. All four euryapsid groups are described here, however, with some additional notes on those members that could be found during the first half of the Triassic Period.
Placodontia (Early to Late Triassic): These early euryapsids were bottom-feeding reptiles that resembled armor-plated walruses. Of all the euryapsids, the placodonts were the least specialized for life in the water. The placodonts were small- to medium-sized creatures; most were only from 0.5 to 6.5 feet (0.15 to 2 m) long. These animals would have been dwarfed by other marine predators of the time. The placodonts' limbs were stumpy and not paddlelike; this design suggests that the animals stayed in shallow, near-shore waters and probably moved about by moving along clumsily on the sea floor.
Placodonts are rare in the fossil record. The first evidence of placodonts consisted of curious, black, rounded teeth that sometimes were discovered in limestone quarries in Germany during the nineteenth century. One collector, a man named Georg Münster (1776-1844) took an interest in these "beans," as the quarry diggers called them. In 1830, Münster expressed the opinion that these were teeth from ancient fishes. It was not until 1858 that the examination of a skull of one of these creatures revealed that it was a reptile and not a fish. The first placodonts to be described in great detail were Placodus ("flat tooth") and Cyamodus ("bean tooth"), in 1863.
Placodonts have been placed in two families based on the presence of body armor. The Placodontoidea, or unarmored placodonts, had only a hint of external body armor in the form of a bony knob on top of each vertebra. The Cyamodontoidea, or armored placodonts, developed a broad, turtlelike shell on their backs (the carapace); some forms had armor plating on the neck, skull, and tails as well.
Some members of the Cyamodontoidea also had an armor-plated underside similar to the plastron of turtles.
Placodus is the best-known member of the unarmored placodonts. It measured up to 6.5 feet (2 m) long and had a small head with strong jaws that seem to have been adapted for crushing hard-shelled mol-lusks. The front of the Placodus jaw had blunt, forward-pointing teeth. The back of the jaws had wide, flat, tilelike teeth. To capture prey such as a clam or mussel, Placodus would use its protruding front teeth to dig the bivalve mollusk out of the seafloor or to yank it from a rock to which it might have attached itself. The placodont then would position the bivalve between the large, flat teeth in the roof and floor of its mouth and use its powerful jaws to crush prey. Once the prey was crushed, the placodont would spit out the shell and swallow the soft parts of the animal that had been hidden inside.
Among the armored-plated placodonts were several members that resembled, but were not directly related to, turtles. Cyamodus, from the Middle Triassic of Germany, had a wide but short skull with a greatly reduced snout and face. Its front teeth were small, and it had a wide, flattened body with extensive armor that consisted of a carapace made up of hexagonal and rounded plates.
Nothosauria (Early to Late Triassic): Nothosaurs were better adapted to aquatic life than were placodonts. They had elongated bodies with long necks and tails and had powerful paddles for swimming. Although nothosaurs became extinct by the end of the Triassic, they appear to have been the ancestral stock from which the more streamlined and successful plesiosaurs arose, late in the Triassic. Nothosaur remains are limited almost exclusively to ancient ocean deposits in Europe, although a few remains have been found in China and Israel.
Most nothosaurs were only about three feet (1 m) long, although several larger kinds existed that measured about 10 to 13 feet (3 to 4 m). The neck and tail of a nothosaur were often about the same length, as in Nothosaurus and Ceresiosaurus ("Lake Lugano lizard"). Tails became shorter in many of the later plesiosaurs, giving them better speed and maneuverability in the water.
Nothosaurs were fish eaters; this becomes all the more evident when their jaws and teeth are examined. The front of the nothosaur snout was somewhat broad and spoon shaped. The upper and lower jaws were filled with long, curved, needlelike teeth that were widely spaced. The teeth interlocked when the jaws were closed; this made them excellent tools for clamping down on fish. The feet of the early nothosaurs, such as Nothosaurus and Lariosaurus, were webbed but still very reptilelike. (The name Lariosaurus comes from the Latin name for Lake Como, in Italy, Lacus Larius.) Later nothosaurs, including Ceresiosaurus, developed longer toes and more paddlelike feet. The forelimbs of nothosaurs were often larger and stronger than their hind limbs. This suggests that the forelimbs had a role in swimming, providing forward thrust and helping the animal to negotiate turns effectively.
Ichthyosauria (Early Triassic to Middle Cretaceous): Ichthyo-saurs—the "fish lizards"—were one of the first fossil creatures to attract scientific attention. An illustration of one was published as early as 1699, but the bones were believed to be those of a fish. Everything understood at the time about fishes and reptiles would certainly make these swimming creatures appear, to most observers, to be fish. The fish theory prevailed for more than a hundred years, even after several more fossil remains were discovered. It was not until 1814 that the reknowned French anatomist Georges Cuvier (1769-1832) revealed the reptilian nature of ichthyosaurs by comparing the features of their skull and skeletal elements to those of other vertebrates. This was a startling conclusion at the time, 28 years before the word "dinosaur" was invented to describe the remains of several land-going extinct creatures.
Ichthyosaurs were dolphinlike in appearance but unrelated to those marine mammals. Ichthyosaurs have been found in many parts of the world, and they thrived throughout most of the age of dinosaurs. Some measured as small as six feet (1.8 m) long, but the group also included the largest ancient marine reptile of all—an ichthyosaur that measured an astounding 77 feet (23 m) long. That is longer than a sperm whale.
All ichthyosaurs had four side fins. Two, called forefins, were in front, next to the chest; the other two, called hindfins, were in the rear, next to the hip. The forefins were always larger than the hindfins. Later ichthyosaurs from the Jurassic also had a dorsal fin on the back, similar to that seen on a shark. Ichthyosaurs propelled themselves either by undulating the body or by waving the tail from side to side. Fins provided the animal with control over steering and balance, allowing it to stay upright, go up and down, and make turns.
Ichthyosaurs achieved their greatest diversity and numbers during the Jurassic Period.
Plesiosauria (Late Triassic to Late Cretaceous): Joining the ichthyosaurs in dominating the Mesozoic oceans were the plesiosaurs, a widespread and successful group of long- and short-necked predators that fed on fish, squid, and other swimmers.
Plesiosaurs are classified in two groups, based on the relative lengths of their necks. The long-necked plesiosaurs, although streamlined, may not have been skilled at high-speed chases of prey because movement of their heads may have caused the animals to change course. Instead, long-necked plesiosaurs may have hunted using an ambush style, waiting for fish to pass and then lashing out to snare them with their wicked teeth. The ambush technique also would have permitted the plesiosaur to conceal its body behind a rock or other obstacle, out of the view of approaching fishes. While intriguing, the ambush technique seems unlikely to some scientists because of the huge body size of these animals.
The short-necked plesiosaurs fell into several families, including the massive pliosaurs. Short-necked plesiosaurs were fearsome creatures—the top predators in their underwater world. With their large heads and streamlined bodies, pliosaurs were capable of quick movement and of sustained chases at high speed. Unlike the long-necked plesiosaur ambush hunters, pliosaurs probably were pursuit predators, chasing down large prey until they could snatch them in their jaws. Pliosaurs had sturdy, pointed teeth that were cone-shaped and abrasive, capable of cutting through thick flesh and even bones. The animals' mouths were huge. This made any sea creature of the time a potential meal, including the long-necked plesiosaurs, ichthyosaurs, giant squid, sharks, and the largest fish of the time. Pliosaurs probably filled an ecological niche similar to that of killer whales today. Among the largest pliosaurs were Krono-saurus (Australia), which was 42 feet (13 m) long, and Liopleurodon (England, Germany, France, and Russia), which was about 40 feet (12 m) long.
One reason that reptiles were able to become fully terrestrial was that they had developed the amniotic egg, which could be laid and fertilized outside of the water. While many marine reptiles probably continued this strategy of leaving the water to lay their eggs, the fishlike ichthyosaurs were clearly not capable of crawling onto land to complete this critical chore. Ichthyosaurs solved this problem by evolving the ability to retain their "eggs" inside the body and then give birth to live young when the eggs hatched. Several remarkable fossil specimens of ichthyosaurs either show unborn babies in the mothers' body cavities or show a baby ich-thyosaur emerging tail first from the birth canal, its head still inside the mother.
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