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The first animals and plants to invade land required water to complete their live cycles. Spore producing plants such as lycopsids, horsetails, and ferns need wet environments so that sperm can swim to and fertilize eggs. Amphibian eggs must be laid in moist conditions that allow the tadpole, fish-like, larval stage to live and develop in an aquatic environment.

Evolutionary Innovations

Evolutionary innovations that appear in the Carboniferous period would free plants and animals from a life cycle tied to water. As we have already discussed, seed plants delivered sperm to eggs via wind carried pollen. Seeds provided plant embryos with an independent life support unit, which provided nourishment and protection from dry conditions. Among tetrapods the reptile-like amphibians would give rise to the amniotes that reproduced on dry land delivering sperm via internal fertilization. Amniotes evolved an independent life support unit for their embryos in the form of the amniotic or cleidoic egg.

Amniotic Egg

Amniotes include reptiles, birds, protomammals, and mammals. Specialized membranes support amniote embryos carried in eggs or inside the female. Amniotes can be identified by specific skull and skeletal structures.

The amniotic egg would allow early amniotes to lay their eggs on land due to two key features. First, the amniotic egg possesses a semipermeable shell that is either calcareous or leathery. The shell allows for gas exchange, but is watertight. Second, the amniotic egg possesses three extraembryonic membranes that lie outside the embryo, the chorion, amnion, and allantois. The amnion surrounds the embryo with water and functions in gas exchange and protection. The chorion surrounds the embryo and yolk sac and also functions in gas exchange and protection. The allantois forms a sac that functions in respiration and waste collection. As the embryo develops the yolk sac is depleted while the allantois fills up.

The amniotic egg allows the embryo to fully develop into a terrestrial hatchling and skip the aquatic larval stage. The materials needed to make these eggs are resource intensive, so amniotes generally lay fewer eggs than fish or amphibians (Benton, 2005, p. 111). The first fossil eggs are from the Triassic, so it is skeletal structures that identify the first amniotes. We will briefly discuss amniote skull structures.

Amniote Skulls

Several amniote evolutionary lines are documented by temporal fenestrae, openings in the skull behind the eye. These holes function to reduce use of bone and provide additional edges for jaw muscle attachment. Anapsid amniotes (subclass Anapsida) have no temporal fenestrae and include turtles and most of the earliest reptiles. Diapsid amniotes (subclass Diapsida) have two temporal fenestrae and include lizards, snakes, crocodiles, dinosaurs, pterosaurs, sphenodonts and the extinct marine reptiles (formerly grouped as the subclass Euryapsid). Euryapsids have one upper temporal fenestra. Synapsid amniotes (class Synapsida and class Mammalia) have one lower temporal fenestra and include protomammals (formerly referred to as mammal-like reptiles) and mammals. Reptiles were the first amniotes and appear in the Carboniferous. The subclass Anapsida and Diapsida are in the reptilian class Sauropsida.

The First Reptiles

The first reptiles (class Sauropsida) were small lizard-sized tetrapods living in damp forests of the mid-Carboniferous feeding on insects and worms. In Nova Scotia there are sedimentary deposits that contain upright lycopod tree stumps. Since 1852 thirty Sigillaria tree stumps have produced abundant tetrapod remains. These tetrapods were living in the rotted out Sigillaria trunks. Hylonomus and Paleothyris are two tetrapods that were preserved in the hollow tree stumps and represent the oldest known amniotes (Benton, 2005, pp 110-111). Reptiles remained relatively small during the Carboniferous and into the Permian. Multiple new families of anapsid type reptiles appear in the Early Permian. We will mention just a few.

Reptiles & Continental Drift

Members of the family Mesosauridae represent the first reptiles to adapt to an aquatic existence, though their ancestors were terrestrial. This family of reptiles appears and goes extinct during the Permian. Mesosaurus was a freshwater species in this group, which acted as a key piece of biological evidence in favor of Alfred Wegener’s theory of Continental Drift and the existence of Pangea. Mesosaurus fossils are found in both South America and South Africa. This animal could not have crossed the Atlantic Ocean thus the continents must have been joined when Mesosaurs was alive (Dixon, 1988, p.65). Mesosaurus was up to 1 meter long with a long jaw, neck, and flat-sided tail. The tail was used for swimming. The jaw was lined with needle-like teeth suited for catching arthropods and fish and then straining water before swallowing (Benton, 2005, p. 115).

Bipedal Tetrapods

Members of the family Bolosauridae are rather rare. However, the bolosaurid Eudibamus, from the Early Permian, has the distinction of being the first known bipedal tetrapod. This slender reptile had a long tail, long hindlimbs, and short forelimbs. The teeth of Eudibamus suggest a diet of tough plant material.

Walking Upright

Members of the family Pareiasauridae were large, heavily built herbivores. Pareiasaurs had their legs placed underneath their body, so they could walk more upright. Pareiasaurus and Scutosaurus were typical members of the family reaching lengths of 8ft.

Glidding Reptiles

Diapsids remained at low diversity during the Early Permian, but underwent an adaptive radiation in the Late Permian. Coelurosauravus (Family Weigeltisauridae) was a small lizard-like diapsid with a very interesting adaptation. The ribs of this weigeltisaurid could fold out forming wing-like structures. This organism could have glided from tree to tree like the living lizard Draco does.


Another important diapsid group, which makes its first appearance in the late Permian, is the archosaurs (Infraclass Archosauromorpha). While some archosaurs had the standard reptilian sprawling gate (limbs directed sideways) others evolved a more erect posture. Archosaurs with erect postures had legs that were placed more directly under the body, an important terrestrial adaptation. A more erect posture increases stride length. Legs tucked beneath the body can support the organisms weight with less stress.

Archosaurs would undergo a great adaptive radiation during the Triassic. One group of archosaurs would give rise to two evolutionary lines one, leading to the crocodilians the other, to pterosaurs, dinosaurs, and birds (Benton, 2005, pp. 138-143). Diapsid reptiles would come to rule the land, sea, and air during the Mesozoic.


Sir Richard Owen (1804-1892), a British comparative anatomist and paleontologist, created the taxon Dinosauria to describe large terrestrial reptiles that walked upright, clearly different from other fossil or living reptiles. He based Dinosauria on the grouping of three taxa including Megalosaurus, Iguanodon, and Hylaeosaurus. The Dinosauria were inaugurated in an 1842 published version of an 1841 lecture given to the British Association for the Advancement of Science in Plymouth (Padian, 1997, p. 175).

Dinosaurs (Superorder Dinosauria "terrible or fearfully great lizards") range from the Triassic to the Cretaceous (to the present if you include birds). The oldest dinosaurs, like Coelophysis, were small bipedal, carnivorous organisms. Dinosaurs diversified into many forms and came to dominate terrestrial faunas during the Jurassic and Cretaceous.

In 1887 Harry Seeley (1839-1904), a British paleontologist, proposed that Dinosauria could be divided into two groups based on their hip structure, braincase, and vertebrae (Padian, 1997, p. 494). Seeley's scheme has persisted to this day. The order Saurischia includes dinosaurs with a lizard-like hip structure. The order Ornithischia includes dinosaurs with hip structures reminiscent of birds. Representatives from both groups appear in the Triassic period, although Ornithischians remained relatively rare until the Jurassic.

Saurischian Dinosaurs

Saurischian dinosaurs have a "primitive" pelvic girdle with the pubis pointing forwards and the ischium back. Saurischians also share an elongate, S-shaped neck, and asymmetrical hands with a distinct thumb (Prothero, 1998, p. 372). Saurischian dinosaurs can be placed into two major groups, the theropods (Suborder Theropoda) and the Sauropodomorphs (Suborder Sauropodomorpha).


The suborder Theropoda ("beast feet") includes the bipedal, carnivrous dinosaurs, which range from chicken size to the 6-tonne giants of the Cretaceous. Theropods have hollow, thin-walled bones. In general, the forth and fifth digits on the hand and foot are reduced. Only three toes on each foot are weight bearing. Most theropods have sharp recurved teeth and claws at the end of each finger and toe (Wagonner, 1995, Theropod Page). Let's take a look at three major theropod groups.


Coelophysoids (Infraorder Coelophysoidea) were the first theropods. Coelophysoids were small, slender, bipedal, dinosaurs with a long tail and long narrow-snouted skull. These small carnivorous dinosaurs range from the Triassic to the Jurassic and were widespread geographically. Coelophysis bauri is New Mexico's state fossil. Ghost Ranch in New Mexico represents a bone bed dominated by specimens of Coelophysis. The victims of this bone bed may have died as a result of drought and were subsequently transported and deposited into a bone bed by flooding.


Ceratosaurs (Infraorder Ceratosauria) were closely related to the coelophysoids. Many ceratosaurs had crests on their skulls (Benton, 2005, p. 191). Dilophosaurus ("two crests"), from the Early Jurassic, had two flat-sided crests oriented vertically on either side of its skull. The crests may have been used in sexual displays. Dilophosaurus was up to 6 meters long and is the creature that kills Dennis Nedry in the film Jurassic Park. There is no scientific evidence to support the poisonous nature of these dinosaurs as depicted in the film. Eubrontes giganteus are believed to be the tracks of Dilophosaurus and are the state fossil for Connecticut. Dinosaur tracks in general are the state fossil for Massachusetts.

("horned lizard"), from the Late Jurassic, was the same size as Dilophosaurus. Ceratosaurus had a pair of horns, possibly used for mating displays, on the nasal bones. Ceratosaurus had small bony plates running down its neck, back, and tail, giving it a serrated crest. Footprints thought to be that of Ceratosaurus found in the Morrison Formation of the western USA suggest these dinosaurs moved in groups.


The remaining theropods and birds belong to the infraorder Tetanurae. Tetanurans first appear in the Jurassic and can be grouped into two major divisions the Carnosauria and the Coelurosauria (Benton, 2005, p. 397).


Carnosaurs are the first well-known tetanurans and were large predators. Megalosaurus ("great lizard") was a 9 meter long carnosaur that ranges from Early to Late Jurassic. Megalosaurus was the first dinosaur to be scientifically named and described in the 1820's. It was one of the creatures that prompted Sir Richard Owen to coin the term Dinosauria in 1841.

At 12 meters long, 4.6 meters tall, and up to 2 tonnes, Allosaurus ("different lizard") was the largest carnosaur of the Late Jurassic. Joints in the upper and lower jaws allowed Allosaurus to wolf down large chunks of flesh. Allosaurus was equipped with more than seventy teeth up to 7 cm long and three sharp claws up to 25 cm long on each forelimb. Horn-like structures were located above and in front of each eye (Lessem & Glut, 1993, pp. 19-20). Allosaurus is the state fossil for Utah. Saurophaganax maximus is an Allosaur-like theropod that is the state fossil for Oklahoma.


Coelurosaurs are a diverse clade of theropods that are more closely related to birds than to the carnosaurs. Coelurosaurs include the tyrannosaursids (formerly grouped with carnosaurs) of the Late Cretaceous, ornithomimids, and maniraptorans.

The first coelurosaurs appear in the Jurassic and include such dinosaurs as Coelurus ("hollow tail") and Compsognathus ("elegant jaw"). Tyrannosaurids of the Late Cretaceous, like Tryrannosaurus ("tyrant lizard") are among the largest known terrestrial carnivores. Tyrannosaurus measured up to 12 meters long and weighed up to 6 tonnes. Tyrannosaurus had a large skull, over 1.35 meters in length. It's jaw was lined with serrated teeth up to 16 cm long and 2.5 cm wide. It is estimated that Tyrannosaurus had a bite force of up to 13,400 Newtons. Tyrannosaur coprolites contain bones of Triceratops and pachycephalosaurids (Benton, 2005, p. 193). Tyrannosaurs had small, but powerful forelimbs equipped with two clawed fingers. The large powerful hind limbs possessed three large claws. It is estimated that T. rex could achieve speeds of up to 40 km/h.

Ornithomimids of the Early Cretaceous were slender theropods with ostrich-like bodies, small heads, relatively long necks, limbs and fingers. Ornithomimids would reach their greatest diversity during the Late Cretaceous period. Struthiomimus ("Ostrich mimic") from the Late Cretaceous possessed a toothless jaw covered with a keratinous beak. Struthiomimus's anatomy suggests that it was a fast organism, reaching speeds of up to 60 km/h. Their diet consisted of small lizards and mammals.

Maniraptorans are the most derived theropods and include such familiar organisms as troodontids, dromaeosaurids, and birds. Eshanosaurus from the Early Jurassic of China may represent the first known maniraptoran. Maniraptoran theropods from the Early Cretaceous of China, such as Sinosauropteryx, Beipiaosaurus, Protarchaeopteryx, Microraptor, and Caudipteryx, provide evidence that feathers evolved in the earliest coelurosaurs and functioned as insulation and possibly for display. Maniraptoran fossils exhibit an evolutionary progression through different types of feathers from simple bristles to advanced contour feathers. Although contour feathers do appear on some maniraptorans they may not have played a role in flight until the first known bird Archaeopteryx (Benton, 2005, pp 199-201).


The suborder Sauropodomorpha includes both the prosauropods and the sauropods. In general, they are herbivorous quadrupeds with a small head, long neck, large body with legs tucked beneath, and a long counterbalancing tail. The small head contained jaws with lanceolate or spatulate teeth used for stripping vegetation from branches. Food was ground in the gizzard by gastroliths. Their pillar-like legs supported a deep body to accommodate an enormous digestive system. Sauropodomorphs had five-toed, spreading feet. They possessed a large thumb claw that was probably used for digging and no claws on fingers 4 and 5. Sauropodomorphs had large nostrils set higher on the skull than most vertebrates.

Although basal sauropodomorphs were bipedal omnivores the evolutionary trend in this group was towards quadrupeds of increasing size. To accommodate ever larger sizes the vertebrae evolved with increasing bone pneumaticity (hollow spaces). Bone was developed only along the lines of stress. The massive pelvic girdle became firmly fused to the backbone by 4 and later 5 sacral vertebrae. The spaces in these bones may have been filled with air sacs connected to the lungs as in modern birds. These hollowed out bones with air sacs would have reduced weight and enhanced respiratory efficiency (Benton, 2005, p. 202).

("flat lizard"), of the Late Triassic, is the best known and largest prosauropod. Plateosaurus was 8 meters long and could adopt either a bipedal or quadrupedal posture. Gizzard stones have been found in the upper rib cage of specimens. The abundance of reamins suggest a herding behavior for this early dinosaur.

Several Late Jurassic sauropods are familiar to many people. Camarasaurus ("chamber lizard") is the best- known sauropod from North America. This 18 meter long compact browser had a relatively short neck and tail. The forelimbs were only slightly shorter than the hindlimbs. Camarasaurus had a box-like skull with large nasal openings. Camarasaurus had heavy, spoon-shaped teeth that could handle tough plant material. Adults and juveniles have been found in the Morrison formation indicating they traveled in herds. Within these deposits are found isolated piles of polished stones, which may represent regurgitated gizzard stones. The eggs of Camarasurus may have been laid in lines instead of nests.

("deceptive lizard") was once known as Brontosaurus ("thunder lizard"). Othniel Charles Marsh (1831-1899), an American paleontologist, described two specimens as separate animals; Apatosaurus in 1877 and Brontosaurus in 1879 (McIntosh, 1990, p. 349). Later, it was realized that the two specimens represented separate species of the same genus. Apatosaurus was the first genus given, so by taxanomic convention it is the name used. Apatasaurus was 21 meters long and up to 30 tonnes. For almost one hundred years the skull of Camarasaurus was used as a model for Apatosaurus. The relatively small head of Apatosaurus was found in 1975. The teeth of Apatosaurus were all at the front of the mouth and served to rake foliage off branches. Like most sauropods Apatosaurus had five toes on each foot. The front foot had a large claw on the thumb, while each back foot had 3 claws. Each ankle joint had thick wedges of weight-bearing cartilage. The long whip-like tail in many sauropods like, Apatosaurus, may have been used for defense as well as a counter balance to the long neck.

("arm lizard") was one of the tallest and largest sauropods, designed for reaching high in the trees. Brachiosaurus was up to 25 meters long, 16 meters tall, and weighed in at 80 tonnes. Unlike other sauropod families Brachiosaursids had front legs that were longer than the hind legs. The neck of Brachiosaurus accounted for half of its height. The humerous was enormous accounting for over 2 meters of the animal's height and giving it the name "arm lizard". Brachiosaurus had a relatively short tail and the whole body sloped down from its highest point at the shoulders, not unlike the modern giraffe. The vertebrae were marvals of engineering with large hollowed out spaces on the sides making a lightweight framework that provided maximum strength along the lines of stress with a minimum amount of bone (Dixon, 1998, p. 129). Pleurocoelus is a brachiosaurid and the state dinosaur for Texas. The Cretaceous-aged sauropod, related to Brachiosaurus, Astrodon johnstoni is the state dinosaur for Maryland.

Ornithischian Dinosaurs

Ornithischian dinosaurs have a pelvic girdle in which the pubis runs back parallel to the ischium. There is also a prepubic process pointing forwards. Ornithischians were all herbivorous dinosaurs and possessed a predentary bone, which is a beak-like bone in front of the lower jaw. The predentary bone is matched with the premaxilla or the rostral (in ceratopsians) in the upper jaw. These bones helped Ornithischians clip vegetation. Ornithischians possessed cheek teeth that are inset into the jaw, suggesting they had fleshy cheeks for holding food (Prothero, 1998, p. 372).

Ornithischian dinosaurs can be divided into two major groups. The suborder Cerapoda includes ornithopods (Infraorder Ornithopoda), pachycephalosaurs (Infraorder Pachycephalosauria), and ceratopsians (Infraorder Ceratopsia). The suborder Thyreophora includes the ankylosaurs (Infraorder Ankylosauria) and stegosaurs (Infraorder Stegosauria).


Ornithopods ("bird feet") were the most diverse and successful group of ornithischians and included the heterodontosaurids, hypsilophodontids, iguanodontids, and hadrosaurids.

Ornithopods evolved complex chewing mechanisms making them unique among reptiles. Two different solutions to chewing can be seen in the jaw structures of Ornithopods. Heterodontosaurids possessed a ball and socket joint that allowed the lower jaw to rotate, creating a shearing action between the cheek teeth. All later ornithopods possessed pleurokinetic hinges in the upper jaw, which allowed the sides of the upper jaw to flap in and out, creating a lateral shearing action between the cheek teeth. Ornithopods were the most successful herbivores during the Cretaceous because of their ability to chew (Benton, 2005, p. 207).

("lizard tooth") was the second dinosaur ever described. Dr. Gideon Mantell, an English amateur geologist, described Iguanodon from some teeth in 1822 and credited their discover to his wife Mary Ann Mantell. Iguanodon's hand is unusual, digit 1 is reduced to a thumb spike, and digits 2 and 3 have small hooves. Iguanodon could walk both bipedally and on all fours. The thumb spike of Iguanodon was first believed to be a horn positioned on the snout.

, Megalosaurus, and Hylaeosaurus were the first dinosaurs to be represented as three-dimensional restorations. They were part of London's Sydenham Park built to showcase the glass and iron structure named Crystal Palace, which had been featured at the 1851 Great Exhibition held in London. Twelve guests dined inside the incomplete mould of Iguanodon at a New Year's dinner party on December 31, 1853. Richard Owen supervised the restorations, which suffered from misinterpretations and incomplete information (Sarjeant, 1997, pp, 161-164).

Hadrosaurs ("sturdy lizard") or duck-billed dinosaurs were the most successful ornithopod clade. Hadrosaurs had long rows of grinding cheek teeth arranged in closely packed batteries. Plant material was ground with a sideways shearing movement as the pleurokinetic hinge pushed the cheeks in and out with each bite. The jaws also moved forward and backward providing additional grinding action (Benton, 2005, p. 209).

Hadrosaurs all have similar skeletons and skulls; however, many possessed various shaped crests. Parasaurolophus ("by lizard crest") was a highly derived hadrosaur of the Late Cretaceous. Parasaurolophus could walk on all fours as well as on two legs. The 9 meter long, two tonne hadrosaur had a head equipped with a curved horn-like crest up to 1.8 meters long. The crest had two hollow passages that ran from the nostrils back to the tip of the crest and curved back down to the throat region. It is believed that hadrosaur crests may have acted as resonating chambers. It is common to find several species of hadrosaurs in the same formation, so they probably roamed in mixed groups. Hadrosaurs were the dominant herbivores towards the end of the Mesozoic and one can imagine the reverberating sounds of dinosaurs with different shaped crests filling the air in ancient North American and Mongolian forests of the Late Cretaceous.

The hadrosaur Maiasaura peeblesorum is the state fossil for Montana. Maiasaura nests on Egg Moutain provide evidence that this dinosaur was nest bound as a hatchling and required parental care. The ends of the hatchling leg bones are not fully formed and the egg shells are found in pieces. Jack Horner and Robert Makela, American paleontologists, found Maiasaura. Horner named the dinosaur Maiasaura ("good mother lizard") because of the evidence that it took care of its hatchlings. Hadrosaurus foulkii is the state fossil for New Jersey.


Pachycephalosaurs ("thick head lizard") are the dome-headed dinosaurs. These bipedal herbivores range in size from 1 to 5 meters long. Fossils of thickheaded dinosaurs are restricted to the Cretaceous. In one specimen of Pachycephalosaurus wyomingensis the skull was 22 cm thick. The thickened skull bones of Pachycephalosaurs suggest to many that they engaged in a head-butting behavior not unlike moder day bighorn sheep (Sues, 1997, p. 512).


Ceratopsians ("horned faces") are a diverse group of ornithischians from the Late Cretaceous. Ceratopsians have a triangular shaped skull when viewed from above and a beak-like rostral bone on the upper jaw that meets with the predentary bone on the lower jaw. Ceratopsians evolved neck frills and horns. Later forms also had skeletons adapted for galloping.

("three horn face") is the best-known horned dinosaur. Triceratops was 8 meters long and weighed in at 4.5 tonnes. The brow horns of Triceratops reached lengths of 1 meter and the neck frill up to 2.5 meters wide. The teeth of Triceratops were elongated blades designed for shearing. This herbivore did not chew, it may have browsed on fibrous plant material like cycad or palm fronds. Triceratops is the state dinosaur for Wyoming and the state fossil for South Dakota.


Stegosaurs are known mainly from the Late Jurassic. Stegosaurs have small heads, massive bodies, and rows of alternating bony plates that extend down either side of the backbone. Their heavy tail is equipped with pairs of long, sharp spikes. Stegosaurs hindlimbs are much longer than their forelimbs, which hint at a bipedal ancestry.

(roof lizard) is the state fossil for Colorado and is the largest and most well known of the stegosaurs. Stegosaurus was up to 9 meters long and weighed in at over 2 tonnes. Stegosaurus was not equipped to chew food, so it probably utilized a gizzard to help grind food, as did many other herbivorous dinosaurs. The Stegosaurus brain was the size of a walnut. A cavity in the hip vertebrae of Stegosaurus housed a nerve ganglia to help control the hindquarter movements. In some big-hipped dinosaurs this nerve ganglia or sacral "brain" was larger than the brain (Dixon, 1988, p. 156). The back plates of Stegosaurus show evidence of extensive vascularization and probably functioned to thermoregulate body temperature. They may have also been used for sexual and deterrent displays (Benton, 2005, p. 217).


Ankylosaurs arose in the Mid-Jurassic and diversified during the Early Cretaceous. Hylaeosaurus ("belonging to the forest lizard") is the earliest known primitive ankylosaur (nodosaur) and was named by Gideon Mantell in 1832. It was the third kind of dinosaur to be named and one of the specimens considered by Richard Owen when erecting the term Dinosauria. Nodosaurs are primitive ankylosaurs without tail clubs. Ankylosaurus ("curved lizard") is the largest known ankylosaur and survived to the end of the Cretacous. Ankylosaurus was up to 10 meters long and weighed in at 3.6 tonnes. The body was broad (up to 5 meters wide) and squat supported by powerful legs. Ankylosaurs had a massive bony club at the end of their tail, which would have made a formidable weapon.

Dinosaur Physiology

Paleontologists continue to examine evidence concerning the physiology of dinosaurs. Debate continues as to whether or not dinosaurs were endothermic (“warm-blooded”) organisms, maintaining a constant body temperature from a high metabolism or ectothermic (“cold-blooded”), relying on external, environmental sources of heat. Robert Bakker provided multiple lines of evidence in favor of viewing dinosaurs as endotherms. Benton (2005, pp. 219-223) outlines the evidence.

Paleoclimate & Dinosaur Distribution

Dinosaur remains are found in both northern and southern Polar Regions. Dinosaurs above latitudes of 60 degrees would need to migrate if they were ectotherms. The dinosaur fossils found in Alaska and Southern Australia are ambiguous concerning physiology because they could have been migrating seasonally.

Predator-Prey Ratios

Warm-blooded organisms consume 10 times as many calories as cold-blooded organisms. Thus, there is a metabolic cost for being an endotherm, one must consume greater amounts of food. The need to consume more food affects predator-prey ratios. One hundred antelope could be used to sustain one warm-blooded lion or ten cold-blooded crocodiles (Benton, 1984, p. 151). Robert Bakker found that present day spiders and the Permian carnivore Dimetrodon had predator-prey ratios of 25% or more. Fossil mammals and dinosaurs have predator-prey ratios of between 1 and 5%. Benton (2005) points out that these ratios seem to vary with the size as well as thermoregulatory state (p. 220).

Erect Gait & High Speeds

The erect stance, like that found in dinosaurs, is today found only in living endotherms (birds and mammals). Fossil trackways suggest that dinosaurs could achieve high speeds, especially small bipedal forms.


Haemodynamics is the study of blood flow or circulation. Long-necked dinosaurs, like Brachiosaurus, would need an efficient, strong heart to push well-oxygenated blood to the brain. Many argue you would need a four-chambered heart like a mammal for this kind of a job and that the three-chambered heart of most reptiles would not suffice. Other scientists point out that possessing a four-chambered heart is not necessarily evidence of endothermy; crocodilians are ectothermic and possess a four-chambered heart.

Bone Histology

Dinosaur bones are highly vascular, unlike modern reptiles. Many dinosaurs have haversian canals like that found in mammals. However, some small birds and mammals with high metabolic rates lack haversian canals. Fibrolamellar bone found in many dinosaurs is a compact, fast-growing bone formed without growth rings. This bone is also found in large, fast growing mammals and some birds. However, lamellar-zone bone that grows slowly, forming growth rings is also found in some dinosaurs and modern reptiles.

Growth Rates

Dinosaurs, like large mammals, were fast growers achieving adult size in only a few years; even large sauropods reached maturity in 10 to 15 years. Modern reptiles grow slowly.


Benton (2005) points out that Bakker was ridiculed when he suggested that some dinosaurs might have feathers (p. 221). It is now known that feathers evolved in theropods. The insulation provided by feathers is an indication of endothermy.

Core & Peripheral Temperatures

Oxygen isotope ratios of O-18/O-16 in peripheral versus core bones can be correlated to body temperature. Analysis of dinosaur bones seems to be consistent with endotherms, while crocodilians from the same time period are consistent with present day ectotherms.


Modern birds and mammals have thin nasal bones called turbinates that function to recapture water from exhaled air. Modern ectotherms and dinosaurs lack turbinates.

Size & Body Temperature

Some scientists argue that large sauropods were homeotherms, they maintained constant body temperatures by virtue of being large.

Many of these arguments taken alone can be regarded as ambiguous concerning the possibility of endothermy in dinosaurs. As a young person interested in science, what impressed me most about Robert Bakker’s case for warm-blooded dinosaurs was the fact that he employed multiple lines of evidence. For example, dinosaurs with low predator to prey ratios are also the ones with fast growing bone and high average walking speeds (Bakker, 1986, p. 392). I am sure that paleontologist will find new ways to test ideas about dinosaur metabolism in the future.

Winged Reptiles

Pterosaurs (Order Pterosauria "winged reptile") were the first vertebrate fliers and range from the Late Triassic to the Late Cretaceous. Pterosaurs are not dinosaurs, but they are close relatives. These flying reptiles had wings of skin connecting the entire length of the body and thigh to the forelimbs, which extended out some distance due to a greatly elongated fourth finger. Pterosaurs occupied many of the same habitats dominated by living birds and ranged from sparrow size to giants with 12-meter wingspans. Pterosaurs are divided into two groups. Rhamphorhynchs (suborder Rhampho-rhynchoidea) range from the Late Triassic to the Late Jurassic. Pterodactyles (suborder Pterodactyloidea) range from the Late Jurassic to Late Cretaceous (Padian, 1997, p. 614). This classification will most likely change as the Rhamphorhynchoidea is a paraphyletic group.

Rhamphorhynchs ("beak snouts") were the first group of pterosaurs to evolve. Eudimorphodon ("true dimorphic tooth") appears in the late Triassic and exhibits many of the characteristics of the group. Eudimorphodon’s wings, like all pterosaurs, were made of membranous skin attached along the length of the elongated fourth finger and connecting back along the body to the level of the thigh. Wing-like skin also attached the wrist bones and neck. In addition to the elongated fourth finger the hand was equipped with three short grasping fingers. The foot had five long toes. A vertical, diamond-shaped flap of skin adorned the tip of the tail and was probably used as a rudder in flight. Eudimorphodon had two types of specialized teeth for capturing and consuming fish.

A recently discovered pterosaur, Darwinopterus modularis, from the Middle Jurassic of China represents a transitional fossil linking primitive long-tailed pterosaurs and more advanced short-tailed pterodactyls (Lu, Unwin, Jen, Liu, & Ji, 2009, p. 1). The genus Darwinopterus (“Darwin’s Wing”) is for Charles Darwin, honoring the 2009 anniversaries of his birth (200 years) and the publication of On the Origin of Species (150 years). The species name modularis is Latin for “composed of interchangeable units”.

The species name focuses on a key aspect of this transitional fossil. Darwinopterus does not exhibit characteristics intermediate between basal pterosaurs and derived pterodactyloids; rather, it possesses the body and limbs of a primitive pterosaur and the head and neck of a pterodactyloid. This mosaic of primitive and derived characteristics provides evidence for modular evolution. Natural selection may have acted on modules instead of individual characteristics. In the first phase of pterosaur evolution the head elongated, teeth were reduced, the braincase enlarged, and the neck became more flexible. In the second phase, body and limbs modifications improved locomotion on the ground (Lu et al., 2009, p. 6).

A genetic basis for modular selection may ultimately be found in the study of regulatory genes or homeotic genes. Regulatory genes such as Hox genes are found in all multicellular organisms and control the development of body plans and organ systems. A small change in Hox genes can produce a significant change in body plan (Prothero, 2007, p. 102).

Pterodactyls ("winged finger") are the best-known flying reptiles. Pterodactyls had the same general structure as the rhamphorhynchs; however, the tail was shorter, the neck longer and the skull more elongate. Pteranodon ("wing toothless") is one of the largest and best-known pteranosaurs. Pteranodon had a wing span of up to 8 meters and was probably a glider. A crest on the back of the head doubled the skull length. The crest may have acted as stabilizer during flight, although it was sexually dimorphic. The jaws of Pteranodon were toothless, which is unusual for a pterosaur. Pteranodon may have fed on fish like the modern Pelican. The cervical vertebrae had pneumatic foramen that served to reduce weight and increase respiratory efficiency.

Pterosaur wings were 1 mm thick and made of several layers including striated muscles, collagenous fibers, dermis, and epidermis. The membranes of several species were reinforced with parallel stiff fibers, termed actinofibrils. The actinofibrils embedded in the wing helped to ensure a stable aerodynamic shape and proper folding when not in use. The fact that Pterosaurs could fly and were covered with insulation (hair) is strong evidence that they were endothermic or warm blooded (Wellnhofer, 1991, p. 40).

When walking, pterosaurs used all four limbs with legs in the middle and hands a short distance in front and to the side, wing tips (formed from the elongated fourth finger) slanted upwards on either side of the head. The largest known flying vertebrate of all time is Quetzalcoatlus from the upper Cretaceous of Texas. Quetzalcoatlus is known from a single wing that measures 12 meters (Benton, 2005, pp. 224-229).

Marine Reptiles

A variety of marine predators hunted fish and cephalopods in the Mesozoic seas. Pterosaurs and crocodilians fed on fish near the surface. The main reptilian predators were the ichthyosaurs, sauropterygians and finally the mosasaurs.


Ichthyosaurs (Infraclass Ichthyosauria) range from the Triassic to the Cretaceous. Ichthyosaurs and sauropterygians have a euryapsid type skull. Euryapsids have a single upper temporal opening and use to be placed in their own subclass. It is now believed that the euryapsid skull pattern arose from diapsids that lost the lower temporal opening (Benton, 1997, p.641).

Ichthyosaurs possessed streamline bodies not unlike tuna, had long snouts and large eyes. Ichthyosaurs filled a niche similar to modern day dolphins and ranged in lengths from 1 to 16 meters. Ichthyosaurs were air-breathing reptiles that propelled themselves quickly through the water with a powerful tail. These reptiles were so specialized for aquatic life that they gave birth to live young in their marine habitat. The stomach contents of ichthyosaurs include fish scales, hooklets from the arms of cephalopods and in one specimen, from the late Cretacous, a primitive turtle hatchling and bird.

Ichthyosaurs had the largest known eyeballs of any animal. The 9 meter long Temnodontosaurus had a 300 mm diameter eyeball. These eyes must have been used for detecting prey in low light conditions (Benton, 2005, pp. 246-247). Ichthyosaurs may have been ectotherms that maintained a body temperature higher than their surroundings by high levels of exercise, such as modern tuna do today. The cruising speed of Ichthyosaurs is estimated to have been around 5.4 km/h (Benton, 2005, p. 247). The ichthyosaur Shonisaurus popularis is the state fossil for Nevada. This 50 foot long ichthyosaur preyed on cephalopods while cruising in the Triassic seas. The remains of 37 specimens were uncovered from what is now Berlin-Ichthyosaur State Park in Nevada.


Sauropterygians (Superorder Sauropterygia) range from the Triassic to the Cretacous and include the placodonts and nothosaurs of the Triassic and the plesiosaurs and pliosaurs of the Jurassic and Cretaceous.

Placodonts were the least specialized reptilian swimmers; some evolved turtle-like shells. The evolution of these turtle-like shells is a good example of convergent evolution as placodonts and turtles evolved from different reptilian lines. Many placodonts specialized in eating shellfish. Nothosaurs had streamlined bodies with long necks and tails. Most nothosaurs had webbed feet, while others had flippers. Nothosaurs ate fish.

Plesiosaurs (order Plesiosauria) make their first appearance during the Triassic, but do not become well established until the Jurassic. Plesiosaurs can be divided into two major groups. Plesiosaurs (suborder Plesiosauroidea) have long necks, short heads and paddle-like flippers. Plesiosaurs fed on fish and cephalopods. Pliosaurs (suborder Pliosauroidea) had short necks, long heads, and paddle-like flippers. Pliosaurs fed on fish, cephalopods, sharks, ichthyosaurs, and even plesiosaurs.

Plesiosaurus (early Jurassic) used its flippers to quickly maneuver and its long neck to snatch swimming fish. Muraenosaurus (late Jurassic) had a neck and flippers that were longer than Plesiosuarus. The elongation of the neck and flippers along with an increase in the rigidity of the body was the evolutionary trend in plesiosaurs.

Interestingly, pliosaurs exhibit an almost opposite evolutionary trend, an increase in head size, a shortening of the neck, and lengthening of the paddle-like flippers. Plesiosaurs forelimbs were always longer than their hindlimbs, while pliosuars evolved to have hindlimbs longer than the forelimbs. Pliosuars were built for speed and maneuverability.

Macroplata (early Jurassic) was an early pliosaur that still had a fairly long neck, making it look like a plesiosaur. Liopleurodon (late Jurassic) exhibits the classic pliosaur look, large streamlined body with a heavy head and short neck. It almost has a whale-like appearance (Dixon, 1988, pp 76-77).


Mosasaurs (Order Squamata) became important shallow marine predators in the Late Cretaceous. Mosasaurs ("Meuse river lizard") were air-breathing lizards adapted for a marine life that looked somewhat like a crocodile with flippers. Mosasaurs have an elongate body, deep tail, paddle-like fins, and large skulls lined with sharp conical teeth. Mosasaurs ranged in length from 2 to 10 meters and ate fish and other marine animals. Mosasaurs were so fully adapted to a aquatic life that they gave birth to live young in their marine habitat. Platecarpus and Plotosaurus are two mosasaurs found in the Late Cretaceous chalk deposits of Kansas. Ammonite shells have been found with mosasaur tooth marks.

Mosasaurus hoffmani
was one of the largest and most derived mosasaur marine reptiles. A 1-meter long mosasaur jaw found in 1786 was known as the "Beast of Maastricht" named for the town in Holland where it was found. Napoleon's troops occupied Holland and brought the jaw to Paris in 1795, where it was studied by the great French anatomist George Cuvier (1769-1832). The jaw is still housed in the Natural History Museum in Paris. The Beast of Maastricht was important because it encouraged scientists to consider and debate the possibility of extinction, which was a very controversial idea at the time (Palmer, 1999, pp. 120-121).

Mass Extinction

The largest mass extinction at the end of the Permian period provided reptiles with the opportunity to become the dominant vertebrate life forms on Earth. Roughly, one hundred and eighty-six million years later the second largest mass extinction would take away Mesozoic reptilian dominance and usher in the Cenozoic, an age for mammals.

Diapsid reptiles underwent great adaptive radiations during the Triassic and Jurassic. Reptiles dominated the land in the form of dinosaurs, crocodilians, and lizards, the air in the form of pterosaurs, and the marine environment in the form of ichthyosaurs, plesiosaurs, and mosasaurs. Modern forms of diapsid and anapsid reptiles (lizards, snakes, turtles, and birds) would diversify from the Late Cretaceous to the present. Although successful to the present day the mass extinction event at the end of the Cretaceous would mark an evolutionary switch in tetrapod evolution from diapsid dominated faunas to mammals (Benton, 2005, p. 247).

At the end of the Cretaceous, 65 million years ago, 85% of all species would go extinct, making this event second only to the Permian mass extinction (Hooper Museum, 1996). Sixteen percent of marine families and 18% of terrestrial vertebrate families would go extinct (Siegel, 2000). Dinosaurs, pterosaurs, marine reptiles, ammonoids, and belemnoids would go extinct. Many organisms were already on the decline during the Late Cretaceous.

Scientists at the University of California at Berkeley including Luis and Walter Alvarez, Frank Asaro, and Helen Michel discovered an iridium anomaly in a fine-grained clay layer in several K-T (Cretaceous/Tertiary) boundary sites around the world (now the Cretacous/Paleogene boundary). The group recognized that iridium is abundant in stony meteorites and proposed that the fallout from a meteorite on the order of 10 kilometers could explain the anomaly and possibly the extinction event. Subsequently, a crater was found beneath the Gulf of Mexico off the Yucatan Peninsula during exploration for oil. The Chicxulub crater is of the right size and age. Volcanic activity may also act as a source of iridium. The Deccan Traps in India represent a large terrestrial flood basalt. Ironically, the Deccan Traps would have been positioned on the opposite side of the Earth at the time of the Chicxulub impact. There is also evidence for climatic changes as well as floral and fauna changes leading up to these events (Stanley, 1987, pp. 133-171).

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes ichthyosaurs (order Ichthyosauria), plesiosaurs (order Plesiosauria), mosasaurs (family Mosasauridae), and pterosaus (order Pterosauria) under the class Reptile.

The Science Olympiad Fossil List places dinosaurs in the clade Dinosauria. Dinosauria is broken down into the order Saurischia (Lizard hipped) and Ornithischia (Bird hipped). Within the Saurischian category the following genera are listed: Allosaurus, Apatosaurus, Coelophysis, Deinonychus, Plateosaurus, Velociraptor, and Tyrannosaurus. Within the category Ornithischians the following genera are listed: Iguanodon, Stegosaurus, Triceratops, and Parasaurolophus.




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