Reptiles: Early Consumers on Land
Evolutionary Novelty: New morphological innovations that define newly established groups:
The amniotic egg: a space capsule for the reptilian embryo. The major evolutionary step to a fully terrestrial existence was accomplished primarily due to innovation in reproduction. As one of their diagnostic features, reptiles have an amniote egg, a reproductive character that eventually allowed them to dominate many available land habits.
Diagnostic features that developed in the gradual evolution of amphibians to reptiles:
Fairly com­mon intermediate forms that combined a blend of typical amphibian and reptilian characters (see figure below). 
In general, early reptiles stabilized a particular style of back­bone construction. 
The lack of an otic notch in reptiles, the ear being situated at the rear of the skull.
Bones of the back part of the reptilian skull are reduced in number and size, a continuation of the trend present from rhipidistian fish to amphibians.
A mounted skeleton of the Permian amphibian SeymouriafromCentral Texas (see image). This small animal, approximately 60 cm (2 ft) long, shows a unique blend of amphibian and reptilian characters but is much too young to have been the direct ancestor of reptiles. Notice that an otic notch is still present in the back of the skull. Other anatomical features resemble those of reptiles.(Courtesy of NationalMuseum of Natural History.)
How to tell early reptiles from others
The most important feature for distinguishing one type of early reptile from another is the structure of the bones in the temple region of the skull, behind the eye, called temporal openings, whichprovide data that is used in subdividing all major reptile groups.
The reptiles of this group are referred to as stem reptiles or anapsidsbecause they are the ones from which the other, more advanced reptiles are thought to have evolved. The only living anapsids are the turtles and tortoises (see figure below)
The synapsids: or mammal-like reptiles have a single temporal opening low on the side of the skull, beneath the postorbital and squamosal bones. They are extinct but very important because mammals evolved from this group of reptiles. 
The diapsids: or ruling reptiles have two openings, one above the other, separated by a bony connection between the postorbital and squamosal bones. They include the dinosaurs of the Mesozoic Era, as well as most living reptiles-the crocodiles, alligators, snakes, and lizards. 
The euryapsids: have a single opening high on the skull, above the postorbital and squamosal bones, a condition derived from their diapsid ancestor. Include the ichthyosaurs and plesiosaurs. Most euryapsids had an aquatic or semiaquatic way of life.
 
Thecodonts: important diapsid group which gave rise to the groups known as dinosaurs before the close of the Triassic. The thecodonts first appeared during the Early Triassic and became extinct at the close of the system. Some the­codonts ran on all four legs (quadrupedal), others exhibited a new bipedal, or two-legged stance. By the end of the Triassic, thecodonts had given rise to both groups of dinosaurs, the Saurischia and Ornithischia
The Saurischia (lizard-hipped): the termcomes from sauria, meaning reptile, and ischia, referring to the ischium bone of the pelvis. These dinosaurs had a pelvis built like that of many other reptiles, hence the name lizard-hipped dinosaurs.From them evolved the other major group of dinosaurs.
The Ornithischia (bird-hipped):which appeared at the end of the Triassic.  This group had a birdlike pelvis.
 REPTILES
Different subclasses are distinguished by skull structure
Anapsids: no holes on side of skull (turtles, extinct primitive [stem] reptiles)
Synapsids:  1 hole on lower side of skull (extinct mammal-like [pelycosaur & therapsid] reptiles). Synapsids ('fused arch'), also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes.The non-mammalian members were traditionally described as mammal-like reptiles, and are sometimes referred to as "proto-mammals" or "stem-mammals".
 
Euryapsids: 1 hole on upper side of skull (most extinct marine reptiles. Examples of euryapsids are: icthyosaur, plesiosaur, nothosaur, placodont. This group of reptiles is included in the informal class  Euryapsida which is considered to be an unnatural, polyphyletic group, as the various members are not closely related. This  group of reptiles is  distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate.
Diapsids: 2 HOLES on side of skull (MOST LIVING REPTILES [lizard, snake, crocodile, tuatara]; extinct DINOSAURS; extinct FLYING [pterosaur] REPTILES, & MOSASAURS [extinct marine reptiles])

 
 

DIAPSIDS: 2 HOLES on side of skull (MOST LIVING REPTILES [lizard, snake, crocodile, tuatara]; extinct DINOSAURS; extinct FLYING [pterosaur] REPTILES, & MOSASAURS [extinct marine reptiles]) 

Many new reptile types appeared in the Triassic 

TURTLES (Mesozoic turtles had teeth);

TUATARAS: lizards of New Zealand 

CROCODILES (Cretaceous forms got to be as long as 15 m)

LIZARDS & SNAKES (snakes evolved from lizards by the Cretaceous) 

MARINE & FLYING REPTILES & DINOSAURS (see below) 

THECODONTS (teeth set in sockets) - a MOSTLY BIPEDAL GROUP that GAVE RISE to CROCODILES, LIZARDS, SNAKES, FLYING REPTILES & DINOSAURS 

The Triassic land fauna was dominated by therapsids & thecodonts

MARINE REPTILES 

Several groups with marine adaptations, including paddle-shaped limbs, streamlined bodies, & reproductive adaptations for birth of young at sea.

Euryapsids:PLACODONTS; NOTHOSAURS; PLESIOSAURS (long necks; short, tailless bodies; flippers); ICTHYOSAURS (MOST FISH-LIKE MARINE REPTILE [convergent with dolphins]) 

Anapsids:SEA TURTLES 

Diapsids: MARINE CROCODILES OCCURRED MOSTLY DURING JURASSIC; MOSASAURS (SHORT NECKS; LONG BODIES & TAILS)


 

Diapsid Skull: The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings (temporal fenestrae) posteriorly above and below the eye. 


Synapsid Skull: Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have evolved to provide new attachment sites for jaw muscles.


Anapsid Skull: An anapsid is an amniote whose skull does not have openings near the temples
 

Euryapsid  Skull:  distinguished by a single temporal  fenestra , an opening behind the orbit, under which the post-orbital and  squamosal  bones articulate. 

j: jugal
p : parietal
po : postorbital
q : quadrate
qj : quadratojugal
sq : squamosal
Images source: http://en.wikipedia.org/wiki/

Therapsids: "mammal-like reptiles"  cynodonts : ancestor of Mammalia.

Thecodont ("socket-toothed" reptile), now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period. The group includes the ancestors of dinosaurs (including birds), and ancestors of pterosaurs, and crocodilians, as well as a number of extinct forms that did not give rise to any descendants.

 - Theropods  -  Theropods  are a group of bipedal saurischian dinosaurs. Although they were primarily carnivorous, a number of theropod families evolved herbivory during the Cretaceous System. Theropods first appear during the Carnian stage of the Upper Triassic about 220 million years ago (MYA) and were the sole large terrestrial carnivores from the Lower Jurassic until the close of the Cretaceous, about 65 MYA. Today, they are represented by the 9,300 living species of birds, which evolved in the Upper Jurassic from small specialized coelurosaurian dinosaurs.

 - Tetrapoda: "four feet" thecodonts

Archosauria  (the "ruling reptiles": is a major group of diapsids, differentiated from the other diapsids by the presence of single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics. The ancestral archosaurs probably originated some 250 million years or so ago, in the upper Permian period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and crocodilians exist to provide a glimpse into the past glory of archosaurs.

THE DINOSAURS 

1.Evolved in the Triassic & expanded & diversified in the Jurassic & Cretaceous 

2.Traditionally divided into two orders based on hip structure: Saurischia (lizard-hipped) & Ornithischia (bird-hipped) 

3.Early dinosaurs were mostly Saurichians, which are divided into 2 suborders: (a) Bipedal carnivorous theropods, and (b) giant, 4-footed herbivorous sauropods 

4.The Ornithischians are divided into 5 suborders:Stegosaurs, Ankylosaurs, Ceratopsians, Ornithopods, and Pachycephalosauria

All were herbivorous with the front teeth replaced by a beak & cheek teeth adapted for crushing coarse vegetation 

Ornithopods, ankylosaours & ceratopsians were low browsers and were the dominant herbivores during the cretaceous 

Note: Robert Bakker has suggested that replacement of high browsing sauropods by low browsing ornithischians aided fast growing angiosperms in replacing slow 

growing gymnosperms as the dominant land plant 

Arguments in favor of warm-bloodedness (Endothermy)

Predator to prey ratios; erect stance; richly vascularized bones; growth rates; social behavior & migration of herds; hair on flying reptiles; complete dominance over mammals 

Arguments in favor of cold-bloodedness (Ectothermy) 

Dinosaurs were reptiles & modern reptiles are cold-blooded; erect stance & vascularized bones were responses to large size; large size itself 

Still an open question 

NOTE: it is no longer accepted that dinosaurs were slow & ponderous; dinosaur behavior probably like bird & mammal behavior. 

Theropods were quick & agile; large sauropods & ornithischians assembled in social herds; smaller sauropods & ornithischians behaved in a birdlike way.

SUMMARY OF DINOSAUR TAXONOMY

 
Saurischia
Therapoda
Allosaurus
Compsognathus
Deinonychus
Tyrannosaurus
Velociraptor
Coelophysis
Bipedal carnivores. Late Triassic to end of Cretaceous
 
Size from 0.6 to 15 m Long, 2 or 3 kg to 7.3 metric tons. Some smaller genera may have hunted in packs.
Sauropoda
Apatosaurus
Brachiosaurus
Camarasaurus
Diplodocus
Titanosaurus
Giant Quadrupedal herbivores. Late Triassic to Cretaceous, but most common during Jurassic. Size up to 27 m Long, 75 metric tons. Track ways indicate sauropods lived in herds. Preceded in fossil record by the smaller prosauropods
Ornithischia
Ornithopoda
 
Hypsilophodon 
Iguanodon
Parasaurolophus
Some ornithopods, such as Apatosaurus, had a bill-like mouth and are called duck-billed dinosaurs. Size from a few meters Long up to 13 m and 3.6 metric tons. Especially diverse and common during the Cretaceous. Primarily bipedal herbivores, but could also walk on all fours.
Pachycephalosauria
Stegoceras
Stegoceras only 2 m Long and 55 kg, but larger species known. Thick bones of skull cap might have aided in butting contests for dominance and mates. Bipedal herbivores of Cretaceous.
Ankylosauria
Ankylosaurus
Ankylosaurus more than 7 m Long and about 2.5 metric tons. Heavily armored with bony plates on top of head, back, and sides. Quadrupedal herbivore.
Stegosauria
Stegosaurus
A variety of stegosaurs are known, but Stegosaurus, with bony plates on its back and a spiked tail is best known. Plates probably were for absorbing and dissipating heat. Quadrupedal herbi­vores that were most common during the Jurassic. Stegosaurus 9 m Long, 1.8 metric tons.
Ceratopsia
Triceratops
Numerous genera known. Some early ones bipedal, but Later Large animals were Quadrupedal herbivores. Much variation in size; Triceratops to 7.6 m Long and 5.4 metric tons, with Large bony frill over top of neck, three horns on skull, and beaklike mouth. Especially common during the Cretaceous.

 

SAURISCHIAN  DINOSAURS
The saurischians include two dis­tinct groups known as  theropods and sauropods(see table above).
Theropods(beast-footed): Typical genus Tyrannosaurus;bipedal carnivores. Upper Triassic to end of Cretaceous.Size from 0.6 to 15 m long, 2 or 3 kg to 7.3 metric tons. Some smaller genera may have hunted in packs. All theropods were carnivorous bipeds that var­ied from tiny Compsognathus to giants such as Tyrannosaurus and similar but even larger species. Other genera include Allosaurus, Compsognathus, Deinonychus, Velociraptor, and Coelophysis. In 1996, Chinese scientists discovered several theropods  with feathers. No one doubts that these dinosaurs had feathers, and molecular evidence indicates they were composed of the same material as bird feathers.
 Sauropods (reptile-footed): Typical genus  Brachiosaurus;    Giant Quadrupedal herbivores. Upper Triassic to Cretaceous, but most common during Jurassic.  Size up to 27 m long, 75 metric tons.  Track ways indicate sauropods lived in herds. Preceded in fossil record by the smaller prosauropods includes the truly giant,  quadrupedal   herbivorous dinosaurs such as Apatosaurus, Diplodocus, Apatosaurus,Camarasaurus, and  Titanosaurus; the largest land animals ever. Brachiosaurus, a gi­ant even by  sauropod  standards, may have weighed 75 met­ric tons, and partial remains from several areas indicate that even larger sauropods may have existed. Track ways show that sauropods moved in herds. Sauropods were preceded in the fossil record by the smaller Upper Triassic to Lower Jurassic prosauropods, which were undoubtedly related to sauropods but probably not their ancestors. Sauropods were most common during the Jurassic; only a few genera existed during the Cretaceous.
ORNITHISCHIAN DINOSAURS
 
All were herbivorous with the front teeth replaced by a beak & cheek teeth adapted for crushing coarse vegetation
Ornithopods, ankylosaours and ceratopsians were low browsers and were the dominant herbivores during the cretaceous.
Note: Robert Bakker has suggested that replacement of high browsing sauropods by low browsing ornithischians aided fast growing angiosperms in replacing slow growing gymnosperms as the dominant land plant.
 
Five Classes of Ornithischians are recognized: Ornithopoda, Pachycephalosauria, Anky­losauria, Stegosauria, and Ceratopsia (see table above).
Ornithopoda(bird-footed): Typical genus Maiasaura  ("good mother dinosaur"). Some ornithopods, such as Apatosaurus, had a bill-like mouth and are called duck-billed dinosaurs. Size from a few meters long up to 13 m and 3.6 metric tons.  Especially diverse and common during the Cretaceous.  Primarily bipedal herbivores, but could also walk on all fours. Consist of several subgroups, including the familiar duck-billed dinosaurs or  hadrosaurs. Hadrosaurs  (duck-billed dinosaurs) were especially numerous during the Cretaceous, and several species had head crests that may have been used to amplify bellowing, for sexual display, or for species recognition. All ornithopods were herbivorous and pri­marily bipedal, but they had well-developed front limbs that allowed them to walk in a  quadrupedal  fashion, too. The Upper Cretaceous ornithopods Miasaura ("good mother dinosaur") nested in colonies and used the same nesting area repeatedly where 2 m diameter nests were placed 7 m apart or about the length of an adult. Some nests contain juveniles up to 1 m long, which is much larger than when they hatched, so they probably stayed in the nest area where adults protected them and perhaps fed them. The fact that these animals lived in vast herds is demonstrated by the fossils of an estimated 10,000 individuals in a single bone bed in Montana. Apparently they were overcome by toxic gases from a volcano and later buried in flood deposits. Other genera included in this class are Hypsilophodon ('high-ridge tooth'), Iguanodon, and Parasaurolophus View some specimens of Maiasaura
 
Pachycephalosauria: Typical genus: Stegoceras; only 2 m Long and 55 kg, but larger species known. Thick bones of skull cap might have aided in butting contests for dominance and mates. Bipedal herbivores of Cretaceous.The most distinctive feature of the pachycephalosaurs is their thick-boned, dome-shaped skull. The traditional view of these as animals that butted heads for dominance or mates has been challenged. Now some paleon­tologists note that the thick skull bones are found only in ju­veniles but not in adults. In any case, pachycephalosaurs were bipedal herbivores that varied from 1 to 4.5 m long. Their fossils are known only from Late Cretaceous-aged rocks.
Ceratopsia (horned dinosaurs):Typical genus Triceratops. Numerous genera known. Some early ones bipedal, but Later Large animals were Quadrupedal herbivores. Much variation in size; Triceratops to 7.6 m Long and 5.4 metric tons, with Large bony frill over top of neck, three horns on skull, and beaklike mouth. Especially common during the Cretaceous.The fossil record of  ceratopsians  (horned dinosaurs) shows that small Lower Cretaceous animals were the ancestors of large Upper Cretaceous genera such as Triceratops. Triceratops and related genera with huge heads, a large bony frill over the neck, and a horn or horns on the skull were very common in North America. Fossil track ways show that these large, quadrupedal  herbivores moved in herds. Furthermore, bone beds with fossils from a single species indicate that large numbers of  ceratopsians  perished quickly, probably during river crossings.
 
 Stegosauria: Typical genus Stegosaurus. A  variety of stegosaurs are known, but Stegosaurus, with bony plates on its back and a spiked tail is best known. Plates probably were for absorbing and dissipating heat. Quadrupedal herbi­vores were most common during the Jurassic.   Stegosaurus 9 m long, 1.8 metric tons.The most distinctive features of Stegosaurus include being a  medium-sized, herbivorous quadruped from the Jurassic System, are a spiked tail, almost certainly used for defense, and plates on its back. The exact arrangement of these plates is uncertain, although they are usually depicted in two rows with the plates on one side offset from those on the other. In any case, most paleontol­ogists think the plates functioned to absorb and dissipate heat.
 
Ankylosauria: Typical genus Ankylosaurus, more than 7 m Long and about 2.5 metric tons.  Heavily armored with bony plates on top of head, back and sides. Quadrupedal herbivore. Were quadrupedal  herbivores and more heavily armored than any other dinosaur. Bony armor protected the animal's back, flanks, and top of the head. The tail of some species such as Ankylosaurus ended in a bony club that undoubtedly could de­liver a crippling blow to an attacking predator.
 
Flying Reptiles
First flying reptiles - gliders 
Pterosaurs - active flyers with maneuvering ability, wings were skin stretched between elongated 4th finger, sides of the body & rear limbs.
Pterosaurs were warm-blooded - fine hair covers well preserved pterosaurs 
Paleozoic insects were the first animals to achieve flight, but the first among vertebrates were pterosaurs, or flying reptiles, which were common in the skies from the Upper Triassic until their extinction at the end of the Cretaceous. Adaptations for flight include a wing membrane supported by an elongated fourth finger, light, hollow bones; and development of those parts of the brain that con­trolled muscular coordination and sight. Because at least one pterosaur species had a coat of hair or hair like feathers, possi­bly it, and it is likely that all pterosaurs, were endotherms.
Pterosaurs: Are generally depicted in movies as large, aggressive creatures, but some were no bigger than to day's sparrows, robins, and crows. However, a few species had wingspans of several meters, and the wingspan of one Cre­taceous pterosaur was at least 12 m. Nevertheless, even the very largest species probably weighed no more than a few tens of kilograms.Experiments and studies of fossils indicate that the wing bones of large pterosaurs such as Pteranodon were too weak for sustained flapping. These comparatively large animals probably took advantage of rising air currents to stay airborne, mostly by soaring but occasionally flapping their wings for maneuvering. In contrast, smaller pterosaurs probably stayed aloft by vigorously flapping their wings just as present-day small birds do.
PTERODACTYLOIDS - advanced pterosaurs with no tails & some had enormous wingspans: - Pteranodon = 7 m; Quetzalcoatlusnorthropi = 15.5 m 
Mesozoic Marine Reptiles
Several types of Mesozoic reptiles adapted to a marine en­vironment, including turtles and some crocodiles, and the Triassic mollusk-crushing placodonts. Best know exaples of marine reptilesareichthyosaurs, plesiosaurs and mosasaurs. 
Ichthyosaurs:Used their powerful tail for propulsion and maneuvered with their flipper like forelimbs. They had nu­merous sharp teeth, and preserved stomach contents reveal a diet of fish, cephalopods, and other marine organisms. It is doubtful that ichthyosaurs could come onto land, so females must have retained eggs within their bodies and given birth to live young. A few fossils with small ichthyosaurs in the ap­propriate part of the body cavity support this interpretation.
Plesiosaurs:Belonged to one of two subgroups: short-necked and long-necked. Most were modest-sized animals 3.6 to 6 m long, but one species found in Antarctica measures 15 m. Short-necked ple­siosaurs might have been bottom feeders, but their long­necked cousins may have used their necks in a snakelike fashion, and their numerous sharp teeth, to capture fish. These animals probably came ashore to lay their eggs.
Mosasaurs:They were Upper Cretaceous marine lizards related to the present-day Komodo dragon or monitor lizard. Some species measured no more than 2.5 m long, but a few such as Tylosaurus were large, measuring up to 9 m. Mosasaur limbs resemble paddles and were used mostly for maneuvering, whereas the long tail provided propulsion. 
Crocodiles, Turtles, Lizards, and Snakes
Crocodiles:  They develop since Jurassic  time,  crocodiles had become the most common freshwater predators. All crocodiles are amphibious, spend­ing much of their time in water, but they are well equipped for walking on land. Overall, crocodile evolution has been conservative, involving changes mostly in size from a meter or so in Jurassic forms to 15 m in some Cretaceous species.
Turtles: Have been evolutionarily conservative since their appearance during the Triassic. The most remarkable feature of turtles is their heavy, bony armor; turtles are more thoroughly armored than any other vertebrate animal, liv­ing or fossil. Turtle ancestry is uncertain. One Permian ani­mal had eight broadly expanded ribs, which may represent the first stages in the development of turtle armor.
Lizards and snakes:Are    closely related, and lizards were in fact ancestral to snakes. The limbless condition in snakes (some lizards are limbless, too) and skull mod­ifications that allow snakes to open their mouths very wide are the main difference between these two groups. Lizards are known from Upper Permian strata, but they were not abundant until the Upper Cretaceous. Snakes first appear in the Cretaceous, but the families to which most living snakes belong differentiated since the Early Miocene. One Lower Cretaceous genus from Israel shows characteristics intermediate between snakes and their lizard ancestors.