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ARCHOSAURS

ARCHOSAUROMORPHS

       Diapsid reptiles in the Permian split into 2 major lineages: the lepidosaurs which evolved into to snakes, lizards, and a number of extinct reptile groups and the archosauromorphs which includes groups of primitive archosauromorphs and the archosaurs, the “ruling reptiles”.

skull 

      The archosauromorphs possessed a number of derived characteristics: their premaxillary bone extended behind their nostrils, the teeth were in individual sockets rather than in a groove, teeth were no longer found on the palate,  early members had a high quadrate bone which contacted the paraoccipital processes and a long, narrow stapes, the neck was elongated with 7-8 cervical vertebrae,  the humerus lost its epicondylar foramen, the pisiform bone of the wrist was usually lost, and the wrist bones ossified later in life.  The most important archosauromorph modifications involved the ankle (tarsus) and foot: the astragalus and calcaneus articulated along a broad concave-convex surface, the 5th distal tarsal bone was lost, the head of the 5th metatarsal was hooked (serving as a lever to ventroflex the foot) so that it articulated with the 4th distal tarsal, the tarsus was consolidated and the calcaneus extended laterally.  Some of the archosauromorphs held their legs upright as opposed to the ancestral sprawling position. Although this change in body stance was probably a factor in the success of dinosaurs, upright archosaurs coexisted with sprawling ones for millions of years.

      The archosauromorphs evolved from primitive diapsids.  Petrolacosaurus was primitive but modifications of its neck, shoulder, and elongate vertebrae suggest that it might have been a member of the archosauromorph lineage.  The younginoids (Youngina and Thadeosaurus) may be later members of this lineage.

The basal archosauromorphs champsosaurs survived to the Late Cretaceous and even the Early Tertiary. Their narrow jaws and overall morphology probably adapted them for aquatic environments where they fed largely on fish. In the late Cretaceous they could be found in the Arctic (along with other reptiles such as turtles) indicating a warmer climate there (Vandermark, 2007).

champosaurus

 champosaurus

skull

   

      There were a variety of groups of archosauromorphs.  Protoraurs included Protoraurus from the Lower Permian and Tanystropholus, a 3 m organism with a 2 m neck that probably lived near water. Trilophosaurids had a postcranial skeleton resembled that of the archosaurs.  Rhynchosaurs were the most common archosauromorph group, especially during the middle and late Triassic and are known from 5 continents.  They were herbivores and their beaks held rows of cheek teeth which were moved by strong jaw muscles.

 

ARCHOSAURS: THE RULING REPTILES

archosaur

     The archosaurs include the crocodiles, pterosaurs, dinosaurs, birds and thecodonts.  Crocodiles and birds are the only surviving archosaurs.  Thecodonts were a paraphyletic group of archosaurs that gave rise to crocodiles, pterosaurs, and dinosaurs.  The characteristics of archosaurs include an additional opening in the skull in front of the eye, the antorbital fenestra.  This made the skull lighter and perhaps served for muscle attachment or provided room for glands.  Archosaurs were distributed throughout the world and evolved into a number of groups (about ten are recognized). The early archosaurs are classified as the family Proterosuchidae and are known from the Upper Permian.  They were probably ancestral to later archosaurs and possessed a combination of primitive ancestral and advanced characteristics.

PRIMITIVE (ANCESTRAL) CHARACTERISTICS OF PROTEROSUCHIDS

DERIVED CHARACTERISTICS

--supratemporal and postparietal bones in the skull

-- the pineal opening

--side teeth held in shallow sockets

--a pelvis that had a flat puboischiadic plate

--a primitive tarsus and foot

-an opening in the lower jaw (the mandibular fenestrae)

-- some specialization of pelvis and leg

-- an angled orientation of both pubis and ischium

--the head of the femur was more medial as opposed to terminal (they now walked semi-upright as opposed to sprawling)

    

From the primitive archosaurs evolved a number of Triassic groups. In the Late Triassic, the successful groups of reptiles competing with dinosaurs included herbivorous aetosaurs, rauisuchians (some of which could be giant), and phytosaurs. Dinosaurs lived alongside these crurotarsans for 30 million years (Brusatte, 2008). Some crurotarsans were originally classified as dinosaurs (such as Poposaurus, Revueltosaurus, Shuvosaurus, and Teratosaurus) (Brusatte, 2008).

Two lineages, the aetosaurs and the phytosaurs, became extinct by the end of the Triassic. It is not known why the phytosaurs, which are similar to crocodiles in their overall structure, became extinct while the crocodiles thrived (Czerkas, 1990). The phytosaurs were abundant in the Upper Triassic.  They were very similar to crocodiles overall but were a distinct group (for example, their nostrils were far back on the top of the head and they had a  primitive pelvis). Proterochampsa was a very primitive phytosaur whose nostrils had migrated only a short distance from the tip of the snout ( Walker, 1968). The largest phytosaurs could reach lengths of 8-9 meters. After the extinction of the phytosaurs, the previously more terrestrial crocodiles adapted to a more aquatic lifestyle (Russell, 1989)

parasuchus phytosaurs
      Aetosaurs were small headed, armored thecodonts and the only thecodonts which were probably herbivorous.  Some taxonomists have questioned whether the first turtles were descended from anapsid reptiles, or from diapsid reptiles whose temporal skull openings had secondarily fused (as occurred in several groups of dinosaurs such as ankylosaurs and pachycephalosaurs).  Some genetic and anatomical analyses have suggested that turtles should be grouped with diapsid reptiles and within the group of archosaurs(Zardoya, 1998; Liao, 2001; DeBraga, 199; Muller, 2004).  If the first turtles were modified diapsids rather than anapsids, then the armored aetosaurs (whose armor on the stomach is referred to as a plastron—the same term used for that of turtles) might eventually be shown to be related or ancestral to them. 
aetosaur

    The Scleromocidae may be the ancestors of the pterosaurs.

       The Erythrosuchids were large and reached up to 5 meters in length.

CROCODILES

      The crocodilians are members of group known as Crurotarsi which separated from the archosaur group Ornithodira which would evolve into dinosaurs, birds, and pterosaurs.  The Crurotarsi include Parasuchia, Ornithosuchidae, Prestosuchus, and Suchia (crocodilians).  The family Ornithosuchidae includes Ornithosuchus, Riojasuchus, and Venaticosuchus (Sereno, 1991a).  Crocodilians have a strong skull.  Their antorbital fenestrae became smaller in fossil groups and eventually closed in the lineage which led to modern crocodiles.  Powerful jaw muscles gave their mouths a wide gape and rapid jaw closure.

A)    Sphenosuchians and Protosuchians

Sphenosuchus was one of the earliest crocodiles.

skull

gracilisuchus

      These first groups of crocdiles are known from the Late Triassic and Early Jurassic.  Grasilisuchus, the earliest known crocodile, was a bipedal archosaur about 30 cm long.  It was first classified as a thecodont and most of its skeletal features link it with the thecodonts.  Its foot and cheek strongly support its association with crocodiles.  Sphenosuchus was more similar to later crocodiles but still retained primitive characteristics such as dermal armor.  Protosuchians retained long limbs and were probably terrestrial.  They were more similar to modern crocodiles that the sphenosuchids were (Carroll, 1988). Stegomosuchus, Protosuchus, and Notochampsa form a family of the most primitive, ancestral crocodiles from the Late Triassic ( Walker, 1968).

protosuchus terrestrisuchus

terrestrisuchus

B)    Mesosuchians

The first mesosuchians are first known from clearly aquatic (marine) deposits and later specimens are known from semi-aquatic environments.  Extinct marine crocodiles are depicted below.

crocodiles

About 70 genera are known (and therefore they make up the largest assemblage of crocodiles).  They were intermediate between earlier groups and modern crocodiles.  Some could reach considerable sizes; the giant crocodile Deinosuchus could reach a length of 15 meters, about the same length as Tyrannosaurus and possessed a larger skull (Czerkas, 1990). There is evidence that Deinosuchus outcompeted, and perhaps even preyed on, Albertosaurus, a tyrannosaur from the southeastern U.S. in the Cretaceous. Theriosuchus may be ancestral to the eusuchians.

crocodile

The Cretaceous group of crocodiles known as Notosuchia were unique in possessing different types of teeth: incisiform front teeth, long canine-like teeth, and cheek teeth whose wear facets indicate occlusion and thus some degree of chewing (Lecuona, 2008; Czerkas, 1990).

The Late Cretaceous Brazilian crocodile, Armadillosuchus, was an unusual terrestrial form which possessed armadillo-like body armor and some ability to chew. It probably had a more omnivorous diet than most crocodilians and may have been capable of burrowing to some degree (Marinho, 2009).

C)    Eusuchians

big croc
      Eusuchians, the group which includes modern crocodiles, first appeared in the Late Cretaceous.  Almost all modern genera of crocodiles are known are known from the Cretaceous.  Purussaurus, known from the Amazon 8 million years ago, stood 8 feet tall, measured 39 feet long and weighed 10-12,000 kg.  As such, it was more massive than Tyrannosaurus rex.  An incomplete fossil of what was probably even a larger specimen of Purussaurus has been found.  They might have preyed on large South American rodents that could reach the size of small cattle (Monastersky, 1991).

Diverse lineages of crocodilians independently evolved alligator-like snouts (including "duck-billed" crocodiles) and long, thin snouts for capturing fish.

skulls

skulls

croc skull

croc skeleton

Modern crocodiles share a number of traits which support their relatively recent origin from a common ancestor. They can control their breathing through valves which can close the nostrils (to prevent the entry of water while diving), the formation of a secondary palate, and folds (a basihyal throat valve) which can close off their mouth (both of which allow them to breathe while they were eating or while their mouth was open while under water). They possess a four chambered heart (although it is structured differently from that of mammals and does allow some mixing of blood between the two sides of the heart), efficient lungs, brains which are more advanced than other modern reptiles, a greater capacity for learning, and lingual salt glands. While some rarely use their legs to support their body weight (such as the gharial and saltwater crocodile), others can travel far distances from water and the Australian freshwater crocodile can actually gallop. Crocodiles possess plates of bone called osteoderms in their skin beneath their thick epidermal scales. They are present along the back and some species have them underneath as well.