Archosauria

The archosaurs, or "ruling reptiles," branched into an impressive array of important groups, including the pseudo-suchians, crocodilians, pterosaurs, and dinosaurs. The archosaur clade may be defined by two temporal openings in the skull, one (antorbital fenestra) between the eye and the nostril and another (mandibular fenestra) in the hind part of the lower jaw. Many of the other archosaur characters reflect skeletal changes associated with a more upright posture and front-to-back motion in the limbs. It has been pointed out by Robert Carroll that the lineages within the archosaur assemblage were all distinct from one another when they first appeared as fossils.

The living crocodilians are related to the Pseudosuchia, a diverse group of early, sometimes crocodile-like archosaurs that are linked to the true crocodiles mainly on the basis of having the so-called crocodile-normal structure of the tarsus (ankle). The Pseudosuchia is one of the two major clades that branched from the early Archosauria. Pseudosuchians all had extensive external armor composed of bony plates. The group includes the rauisuchids, phytosaurs, and aetosaurs.

The advanced ichthyosaur Ophthalmosaurus from the Upper Jurassic. a restoration. (Illustration by Emily Damstra)

Rauisuchids are Middle and late Triassic reptiles that had a more or less upright stance and grew up to 20 ft (6.1 m) in length. Ticinosuchus, from the early part of the Middle Trias-sic of Switzerland, is one of the best-known forms. In the limb skeleton, the ankle and foot advanced to the level of those of modern crocodiles. Supposedly, the upright stance developed independently of the lineage that led to the dinosaurs. Tici-nosuchus had an armor of two rows of small, bony plates that extended along the trunk and a single row at the top and bottom of the tail. Sharp, piercing teeth set in sockets indicate that this animal was a carnivore.

Phytosaurs occurred abundantly in the late Triassic of North America, India, and Europe. Although they were not true crocodiles, they resembled them in body form and probably had a similar lifestyle. Phytosaurs had a very long snout, with sharp, piercing teeth in sockets along the margin of the jaws. Although the nasal openings were on top of the head, they were set far back on the snout rather than on the tip, as in true crocodiles. Both the trunk and the tail had an extensive covering of dermal armor. A variety of other contemporaneous reptiles have been found in the stomach contents of phytosaurs, thus documenting their carnivorous habits. The fossil record of phytosaurs is confined to the late Triassic.

The aetosaurs formed a distinct group also known only from the late Triassic. Rather than sharply pointed teeth, ae-tosaurs had small, leaf-shaped teeth that suggest a herbivorous diet. The well-known aetosaur Stagnolepus, was a bizarre skeleton is approximately 11.5 ft (3.5 m) long; the top image shows

Eudimorphodon Skeleton

The advanced ichthyosaur Ophthalmosaurus from the Upper Jurassic. a restoration. (Illustration by Emily Damstra)

skeleton is approximately 11.5 ft (3.5 m) long; the top image shows

Eudimorphodon Had Fangs
Eudimorphodon skeleton of the late Triassic rhamphorhynchoid pterosaur. (Illustration by Marguette Dongvillo)

beast with short legs and an upright posture. The body was rotund and the tail massive. The body was covered with large quadrangular plates that formed extensive armor along the back, extended down the sides, covered the belly, and surrounded the tail. The head was small in relation to the body and narrowed into a short rostrum anterior to the teeth, capped by a swollen area that indicated that the animal had a piglike nose. The relationship of aetosaurs to other archo-saurian clades is poorly understood.

Crocodiles are the only surviving giant archosaurs and the top predators in many aquatic environments throughout the world today. A recent survey in Florida reported a male alligator that was 14 ft (4.3 m) long and weighed 946 lb (429 kg).

Much longer crocodiles have been found in the tropics of the Southern Hemisphere. Why crocodiles survived the rounds of extinction that occurred among the other large archosaurs is a matter of conjecture. The earliest crocodilians were terrestrial and included such forms as Gracilisuchus of the Middle Triassic of South America, which walked on two legs, and the four-legged Terrestrisuchus of the late Triassic of Europe, which had an extremely gracile body skeleton and must have been a fast runner. Terrestrisuchus was about the size of a rabbit but was a carnivore that probably scurried in and out of Triassic hiding places looking for mouse-size prey.

The mesosuchians of the Jurassic looked much more like modern crocodiles than the Triassic forms. Several of them became semiaquatic, and some invaded the marine environment. The process of leaving land to become semiaquatic was important in terms of the body changes that took place in these transitional forms. Modern families of crocodiles, Alli-gatoridae (alligators and caimans), Crocodylidae (modern crocodiles), and Gavialidae (the gavial), are known to date back to the late Cretaceous. Alligatorids have a broad snout, with a relatively large number of somewhat blunt teeth. Croc-odylids have a longer, thinner snout, with a significant number of pointed teeth. Gavialids have an elongated snout and needlelike teeth, as do garfish; gavialids are consummate fish eaters. Some Cretaceous crocodiles grew to very large size and probably preyed upon dinosaurs. Sarcosuchus imperator from Africa possibly reached a length of 40 ft (12.2 m), and some from Texas were about that long as well.

Modern crocodilians were much more widespread in the world in the early Tertiary (ca. 65-34 mya) than they are today, their decline probably resulting from climatic deterioration in the Cenozoic era. Crocodilians are the only living reptiles that give true parental care to their young, including nest guarding, helping the young exit eggs by cracking the eggs with their jaws, carrying the young from the nest to the water in their mouths, and protecting them in the water for a time. Vocal communication between the parents and the young also occurs.

The Ornithodiran evolved a neck that could be folded into an S shape and a narrow, compressed foot. One branch of the Ornithodira, the Pterosauria (flying reptiles) of the Mesozoic, had a very long, much enlarged fourth finger on the hand, which supported a wing membrane. The other branch of the Ornithodira, which included Lagosuchus and the dinosaurs, had elongated lower legs (tibiae and metatarsals) as well as a thigh bone (femur) with the head turned inward so that it could fit into a deep socket in the hip girdle (pelvic girdle). This socket allowed the legs to support the large hip girdle, which in turn supported the body.

Pterosaurs were the first vertebrates to evolve true wing-flapping flight. They are known from the late Triassic and had become quite diverse by this time. The fact that pterosaurs had a foramen in the skull in front of the eyes (an-torbital foramen), legs arranged mainly straight under the body, and a dinosaur-like ankle joint indicates their close relationship to Lagosuchus and the dinosaurs. But the fact that they had a hooked fifth metatarsal bone and an long fifth finger suggests that they diverged from the early archosaurs before the dinosaurs (which had the fifth finger reduced or lost) had split off.

Pterosaurs ranged from very small forms to by far the largest creatures that have ever flown. They are divided into two major groups, the primitive rhamphorhynchoids and the more advanced pterodactyloids. Rhamphorhyncoids are first known from the late Triassic and were dominant throughout the Jurassic. They had a short face, a short neck, and a long tail. Some rhamphorhyncoids were as small as English sparrows. From the beginning, rhamphorhyncoids had evolved various specialized characters for flight, including a sternum (breastbone) that had a boatlike keel that supported the wing-flapping muscles, as in birds. Other birdlike bones included the scapula (shoulder blade), cora-coid (upright shoulder girdle bone), and humerus (upper arm bone), all of which were specialized to contribute to active, flapping flight.

The pterodactyloids appeared in the late Jurassic and lasted until the end of the Cretaceous. These reptiles had a much longer face, longer neck, and shorter tail than the rham-phorhynchoids. The skull was highly modified in the Cretaceous genus Pteranodon, which had a long, bladelike rostrum (snout) in front of the eye and an almost equally long bladelike projection in back of the eye. On the other hand, Ptero-daustro, from the Upper Cretaceous of Argentina, had practically no skull at all behind the eye, but it did have large, elongate, upwardly curved jaws. The lower jaw had very long, closely packed teeth that are thought to have strained small invertebrate animals from the water in the same manner as baleen in whales. Quetzalcoatlas, from the late Cretaceous of Texas, was the largest animal ever to fly, with a wing span of more than 35 ft (10.7 m). Before the discovery of Quetzalcoatlas, British zoologist J. Z. Young suggested that Pteran-odon, with its 23-ft (7-m) wing span, was probably the largest animal that could possibly fly. Robert Carroll pointed out that Quetzalcoatlas was obviously far heavier than Pteranodon. It has been suggested that Quetzalcoatlas, might have fed upon the dead bodies of dinosaurs, like some gigantic vulture. Pterosaurs were quite different from birds, in that the wing was composed of a membrane, something like the one in bats. The difference is that in pterosaurs the membrane was supported entirely by a long, robust fourth finger. Unlike pterosaurs, bats have a wider wing membrane, and it attaches to the rear limbs.

Lagosuchus, from the Middle Triassic of South America, provides a structural link between early archosaurs and dinosaurs. Lagosuchus was only about 1 ft (0.3 m) long and had a very lightly built skeleton, with long delicate limbs; this was a humble ancestor of the gigantic animals to come. In Lago-suchus the posterior limbs were much longer than the anterior ones. Moreover, the tibia (lower leg bone) was much longer than the femur (upper leg or thigh bone). The pelvic girdle (hip girdle) was composed of three bones forming a tri-radiate (three-pronged) structure. Thus, the long ilium was directed forward, the long ischium was directed backward, and the short pubis sat on top of the other two pelvic girdle bones. In the tarsus (ankle) there was a hinge between the upper and lower tarsal bones. All of these features (reflecting changes in the limbs and limb girdles, mainly the pelvic girdle and hind limbs) made Lagosuchus the most dinosaur-like of any of the primitive archosaurs.

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