True crocodylians first appeared in the Early Jurassic, but the line of crurotarsan archosaurs leading to the crocodylians included several diverse groups that lived during the Triassic Period. Of these, only basal crocodylomorphs—including a line of small to large, swiftly moving "proto-crocodylians" known as the sphenosuchians—survived the two mass-extinction events that closed the Triassic Period.
Crurotarsans arose during the Early Triassic; by the Middle Triassic, they dominated terrestrial ecosystems as the top predators. Some forms were also herbivorous. The crurotarsans diverged into several distinct lines of archosaurs, only one of which led directly to true crocodylians. All crurotarsans are united by a suite of anatomical features that hint at the ancestry of crocodylians: a heavily built skull, a long snout that narrowed near the end, conical or laterally compressed teeth, a short but muscular neck, and a long tail. Some crurotarsans had wide bodies and a sprawling posture, but some lines were more lightly built and nearly erect in their posture. A few converged on a body plan similar to that of theropod dinosaurs, with bipedal posture, long legs, and long necks. The backs of the crurotarsans often were armor plated, with rows of bony scutes reminiscent of today's crocodiles. The plant-eating varieties of crurotarsans were especially well protected; their backs and sides were thoroughly paved with tight-fitting rows of bony shingles, some of which bore spikes.
Fossils of crurotarsans have been found over a widespread geographic range, including Triassic rocks of North America (Arizona, New Mexico, North Carolina, and Texas); Europe (Scotland, Wales, Germany, Switzerland, and Italy); Central Asia (India); Africa (Morocco, Madagascar, and South Africa); South America (Argentina and Brazil); and possibly China and Thailand. The names of crurotarsans often contain the root word suchus, meaning "crocodile" in Greek. This attests to the crocodylian relationships of these archosaurs. An overview of the main groups of crurotarsans and their chief representatives follows.
Phytosaurs (Late Triassic Epoch). These were superficially crocodile-like archosaurs with long snouts and narrow jaws equipped with small, sharp teeth for snagging fish. Unlike true crocodylians, phytosaurs had nostrils on the tops of their skulls, just in front of the eyes, rather than at the end of the snout, and were not heavily armored. Well-known phytosaur specimens include Parasuchus (India), Mystriosuchus (Italy), and Rutiodon (Europe and North America), each of which was about 10 feet (3 m) long. Some specimens of Rutiodon have been reported that are considerably larger than this.
Ornithosuchians (Late Triassic Epoch). Superficially resembling carnivorous dinosaurs, the ornithosuchids had an erect posture and could have walked on their hind legs. Walking on all fours was probably their customary way of moving around, however. Orni-thosuchus (Scotland), one of the best known members of this group, had a skull that is so strikingly similar to that of later dinosaurs that without any other skeletal evidence it would be difficult to prove that it actually belonged to a different line of archosaurs. The presence of armored, crocodilelike plates on its back; a short, flat pelvis that is weakly connected to its spine; and five toes on its hind feet instead of four clearly link it to the crurotarsans, however. This is not to dispute the dominance of these predators in the world of the Late Triassic, a time when the earliest dinosaurs probably were no match for such large and towering ornithosuchids as Ornithosuchus and Riojasuchus (Argentina). Both animals were up to 13 feet (4 m) long and weighed several tons.
Aetosaurs (Late Triassic Epoch). Among the first lines of herbivorous archosaurs was that of the aetosaurs. These small-headed, armor-plated animals superficially resembled the armored dinosaurs from the latter half of the Mesozoic. Aetosaurs had short, sprawling legs and stout bodies that were covered on the top, the sides, and sometimes on the bottom with a flexible grid of bony plates. For additional insurance against predators, some aetosaurs also had a series of short spikes around the perimeter of the torso. Desmatosuchus (Texas) measured about 16 feet (5 m) long and had two curved, hornlike spikes jutting to the sides from its shoulders. Staganolepis (Scotland, Brazil, Poland, New Mexico, Arizona, and Utah) was about 10 feet (3 m) long, with a long, broad body, a piglike snout, and a particularly heavy and deep armor-plated tail. Aetosaurs were equipped with peglike teeth for stripping greens from plant stems and roots.
Rauisuchians (Middle to Late Triassic Epoch). Perhaps the most formidable line of large predatory archosaurs were the rauisuchians. Primarily quadrupedal but capable of locomoting bipedally, the rauisuchians were among the largest carnivores that dominated in the second half of the Triassic Period. Think of them as crocodylians with shorter, rounder heads and a more upright stance that allowed them to run and chase down their prey. Ticinosuchus (Switzerland)
was about 10 feet (3 m) long. Some other rauisuchians were smaller, with lighter-weight bodies and longer necks; among these were Vyt-chegdosuchus and Dongusuchus, from the Middle Triassic of Russia. The most characteristic members of this group, however, grew to proportions comparable to those of later mid- to large-sized dino-saurian predators. At about 30 feet (9 m) long, Saurosuchus was as large as another, less well-known rauisuchian, Fasolasuchus (Argentina), which may have been as long as 37 feet (11 m) and weighed 4 tons—t rue dinosaurian proportions. Remains of rauisuchians are widespread. In addition to those found in Argentina, Switzerland, and Russia, other important specimens have been discovered in Brazil (Prestosuchus); Germany and Poland (Teratosaurus); India (Tikisuchus); and the American Southwest (Postosuchus).
Crocodylomorpha (Late Triassic to present). Crocodylomorphs include the crocodylia and their extinct relatives. Subgroups within the crocodylomorphs include Eusuchia (including Crocodylia); Mesosuchia ("primitive crocodiles"); Protosuchia ("protocrocodiles"); and Thalattosuchia ("sea crocodiles"). The Crocodylia are restricted to only the living members of the crocodylomorphs and their immediate relatives.
The roots of the living Crocodylia date from the Middle Triassic and include some players that at first glance would not appear to be crocodylians at all. Some, such as Gracilisuchus (Middle Trias-sic, Argentina), were merely 12 inches (30 cm) long and may have walked on their slender hind limbs. The skull of Gracilisuchus had a mouth lined with small, pointed teeth and an overbite reminiscent of crocodylians. Another ancestral crocodylian from the Late Triassic of Wales was Saltoposuchus. This slender animal was even more lightly built than Gracilisuchus, although longer at about 1.6 feet (0.5 m) long. Saltoposuchus can be described as a slender, lizardlike animal with upright legs; these probably made it one of the swiftest crurotarsans.
Both Gracilisuchus and Saltoposuchus were most likely insect eaters, and it takes some imagination to picture them as being at the root of the crocodile family tree. What unites them with croco-dylians are the structures of their skulls, the vertebrae of the neck, and ankle structure. In the case of Saltoposuchus, its skull, although tiny, had begun to acquire traits that were recognizably crocodyl-ian: a long skull that was two-thirds snout and that was somewhat flattened on top, nostrils placed at the very front of the snout, and robust teeth in the upper jaw that overhung the lower jaw.
In 2003, paleontologist Hans Dieter-Sues reported the discovery of a new specimen of early crocodylomorph, this one from Late Triassic rocks of North Carolina. Named Dromicosuchus ("swift crocodile"), the animal was closely related to Saltoposuchus but was somewhat longer, at about 4 feet (1.2 m). Interestingly, this specimen appears to have perished while struggling with a larger rauisu-chian archosaur whose bones were found on top of Dromicosuchus. Several missing neck bones and bite marks suggest that the larger reptile had taken a chunk out of Dromicosuchus before both were suddenly buried by a mudslide.
True crocodiles once were a more widely abundant and varied group than they are today. Although current crocodilians include the crocodiles, the alligators, the gavials, and the caimans, their numbers consist only of 8 genera and 21 species. The fossil record suggests that crocodylomorphs varied considerably during the Mesozoic and were found in many different habitats, not just in the tropical waters occupied by current species. In comparison with the 8 living genera, there are about 150 known genera of fossil crocodylomorphs.
One line of proto-crocodylians to survive the transition from the Late Triassic to the Early Jurassic was the group known as the sphenosuchians. Heavier and more crocodilelike than Gracilisu-chus and Saltoposuchus, this line is best represented by Sphenosu-chus (Early Jurassic, South Africa). One of the larger of the early crocodylomorphs, Sphenosuchus measured about 5 feet (1.4 m) long and has a skull that is more like the skulls of modern croco-dylians than were the skulls of other early members of this ancient
clan. The sphenosuchians were widely distributed by the Late Triassic Epoch; similar forms are known from locations as widely separated as Texas and China.
Protosuchus ("first crocodile"), from the Early Jurassic of Arizona, is one of the best understood of the early crocodylomorphs. This animal is known from a nearly complete skeleton described by Edwin H. Colbert (1905-2001) and Charles Mook in 1951. Protosuchus measured about 3.5 feet (1 m) long and had many of the hallmark traits of crocodylians. The skull was not as long as those normally associated with crocodylians, but it was flat on top and had nostrils placed at the very end. The eyes were aimed outward and forward rather than mostly upward as in modern crocodyl-ians. The front of the jaw featured a jutting premaxilla—the most forward portion of the upper j aw—with a battery of teeth; this is another trait that is seen in later crocodylians. Protosuchus was also preserved with significant evidence of body-armor plates on its back and stomach. It had more armor, in fact, than modern crocodylians, which suggests a strong carryover from the heavily armored anatomy of earlier archosaurs. The gait of Protosuchus was pictured by Colbert and Mook as somewhat more upright than that of modern crocodylians but nonetheless sprawling.
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