The Triassic was a key episode in the evolution of the tetrapods. By its end, not only did the dinosaurs come of the scene, but a number of other major groups also arose: the crocodilians, pterosaurs, and the mammals. The Therapsids, which dominated the Permian, continued into the Triassic, but they had already lost many of the niches to the Archosaurs.

I. Archosaurs

A. Thecodonts - the ancestral stock of the crocodilians, pterosaurs, and dinosaurs.

These carnivorous reptiles occupied the niches that contained therapsids in the Permian - preying on other Archosaurs, therapsids, etc. Most were 1-2 m long - fairly large dog-sized.

At the end of the early Triassic smaller thecodonts begin to appear. The first fossil is Euparkeria - about 0.5 m long - it may have been capable of bipedal locomotion. The forelimbs are short, but the hindlimbs are longer and have an unusual flexible ankle. It shows advances in the skull as well - the teeth are in sockets (not fused to the jaw bone) and the eyes are large compared to earlier animals.

The group of thecodonts that includes Euparkeria evolved rapidly in the mid Triassic and two new groups of Archosaurs appeared from them.

1. Crocodylotarsi

There are about 25 species of crocodilians alive today including crocodiles, alligators, and caiman.

The first crocodilians apparently lived much as modern crocodiles and alligators live today - their long thin jaws and conical teeth are perfect for catching fish. They had a long flattened tail that also suggests swimming in the water. Fossil remains often have fish parts in the place where we would expect the stomach to be, but occasional a fossil is found with a small tetrapod in its stomach - suggesting that it was probably seized on the river bank and dragged into the water. Some had a unique modification we don't see in most reptiles today -- they had nostrils just in front of the eyes and completely isolated from the mouth (this allows the animal to eat and breath at the same time).

Some did live completely on land - one group had its jaws modified into a shovel-like structure that it used to dig up roots and edible tubers. There body was covered with dermal bone and thick horny scales armoring the body. But these die out by the end of the Triassic.

The crocodilians represent a small branch of Archosaurs compared with the other lineage --

2. Dinosaurs + Pterosaurs

This is the ancestral stock of the dinosaurs, pterosaurs, and, ultimately, the birds.

Dinosaurs

The earliest Dinosaur is Herrerasaurus from the late Triassic:

The main characteristics that set apart the dinosaurs, are the evolution of fully erect, bipedal locomotion:

1. The forelimbs are reduced in size and the pectoral girdle, which no longer supports the weight of the body becomes smaller and reduced in size.

2. the hindlimbs are relatively long and held directly under the body (erect body posture).

In dinosaurs and the ilium (the upper portion of the pelvic girdle) was greatly enlarged to provide more contact with the vertebral column and thus greater distribution of weight. The other bones of the pelvic girdle (pubis and ischium) become rod-like and spread out. The animals had a strengthened ankle and apparently walked up on their toes rather than flat-footed.

The result of this shift of locomotion? Dinosaurs were fast!

There are two ways to increase speed:

a. Increase the stride length.
b. Make your steps more frequently.

Dinosaurs did both:

a. Increase the stride length: The shift in the position from a semierect posture to an erect posture meant that more of the leg was involved in stepping forward not just in holding the body up. Furthermore, by stepping up on their toes, they made their leg length longer.

b. The increased area of the pelvic girdle indicates massive muscle attachment.

Also this muscle attached proximally - closer to the hip joint. The inertia, or resistance of to movement, of any object is proportional to the distance of the center of the objects mass to the pivot point. By shortening this distance, the inertia is reduced and speed increased.

Two specialized groups of Dinosaurs appeared at the end of the Triassic: the Saurischians (reptile hipped) and Ornithischians (bird hipped).

Reptiles of the Jurassic and Cretaceous

Dinosaurs have many adaptations that made them the successful organisms of the Jurassic

1. erect gait

2. warm-blooded??

Some paleontologists have suggested that dinosaur success stemmed, at least partially, from the high metabolic rates provided by being warm blooded (homeothermic). This would enable continuous high levels of activity. In support of this idea is the following evidence:

- microscopic bone structure shows high density of haversian canals similar to those found in mammals.

- fossils found very far north where homoiothermy would be necessary to survive (not as cold as now but still a problem).

- descendants (birds) are warm blooded.

- speed and agility suggests high metabolic rates.

- predator-to-prey ratios the same for living warm blooded mammals and not cold blooded reptiles (which can exist on fewer prey).

- oxygen isotope measurement indicates that most dinosaurs measured had a stable temperature throughout their life. (This is a new technique. When living things combine oxygen with phosphorus or carbon to form bone or shell the ratio of the two isotopes -O16 or O18 - depends on the temperature. The technique has been applied for decades to the carbonate shells of marine microfossils to determine the temperature of ancient oceans. Instead of trying to measure absolute body temperature, researchers look for the hallmark of warm-bloodedness: temperature stability as indicated by the same reading from all parts of all bones.)

BUT

- haversian bone structure may be related to rapid growth rate, not homoiothermy.

- Continental drift may have displaced fossils far extreme latitudes than the living animals were found.

- warm bloodedness could have originated in birds (its not in the ancestors of the dinosaurs after all).

- predator-prey ratios very suspect since fossil record isn't complete.

- Caution about relying on oxygen isotopes - calcium phosphate of fossils may not be from bone of animal but ground water may have dissolved the original mineral and replaced it uniformly with different phosphates.

Jurassic and Cretaceous Dinosaurs
During the Jurassic, the supercontinent Pangea was at its most extensive, with few inland seas or large estuaries. The terrestrial flora was dominated by mosses, ferns and gymnosperms. In the Cretaceous, the first flowering plants appeared (something like a magnolia). They radiated and reached today's dominance by the end of the Cretaceous.

Two specialized groups of dinosaurs appeared about 200 million years ago:

The Saurischians (reptile hipped) and
Ornithischians (bird hipped)

The saurischians retained the pelvic hip seen in Triassic dinosaurs: the divergent rod-like pubis and ischium:

In the ornithischians, the pubis extends back to brace the ischium and the forward part of the ilium extends forward for the attachment for bigger muscles to move the hindlegs.

1. Saurischian Dinosaurs
There are two groups of Saurischian Dinosaurs - Herbivores (called sauropods) and Carnivores (called theropods).
a. Sauropods

Earliest sauropods were bipedal and browsing herbivores. In the Jurassic they evolved into quadrupeds of large size (10-20 feet in length) and then to truly gigantic lengths of 60-80 feet (the longest is Seismosaurus of new mexico 35 m or 110 feet long). Although its sometimes difficult to estimate weight, many were at least 50 tons and two new partial skeletons from Colorado (Ultrasaurus and Supersarus) may approach 80 tons. These sauropods were the largest land animals to ever evolve. With an increase in size they reverted secondarily to moving on all four limbs (but maintained erect body posture - they couldn't have stood upright otherwise).

Early reconstructions of them show them as ponderous swamp-dwellers, we now know this isn't the case. Early workers thought that sauropods would have difficulty supporting their weight suggested that they spent much of their life buoyed up by water. Their long necks were interpreted as snorkels. But, based on biomechanics, this is unlikely. pressure from even a small depth would have kept the rib cage from expanding adequately. Therefore, they walked on land and probably held their long necks erect for browsing off the tops of trees. The evidence comes from fossilized stomach remains which indicate a woody diet of a type of plant not found in swamps and the fact that many fossils are found in deposits from dry grasslands. Furthermore, an early Cretaceous walkway found in Texas indicates that they traveled in migrating, socially organized herds.

But here is a problem: the head of a sauropod is relatively small and their teeth are weak and blunt. How could the eat and chew enough plant material to keep their large bodies going?

Some fortunate fossil finds have helped up solve this question: several smooth stones found in the area of the body just above the stomach. In other words - the mouth simply raked the food in and a large gizzard ground the plant material, not the teeth. Many of the gizzard stones are brightly colored (relative to the other stones laying around) but why dinosaurs preferred bright stones isn't known.

Many sauropods had greatly enlarged nasal passages located in front of the brain case high on the skull. These may have been part of a system to cool the blood before it flowed to the brain. If blood is too warm it can damage the brain cells. If they were warm blooded, the large fermenting stomach may have added too much heat, necessitating cooling before it gets to the brain.

Another problem is their long necks. Lifting the head high would have placed a great deal of strain on the heart:

The blood pressure required to pump blood to the brain 30 or more feet up in the air would have seemingly placed the animals at sever risk for stroke or some other type of respiratory disaster. First, the heart would have to be more efficient than the usual three-chambered reptilian heart. This has lead some scientists to speculate that dinosaurs had 4 chambered hearts like their descendants the birds.

Furthermore, the cardiac compartment in the rib cages of dinosaurs suggests that the heart was enormous. If there are all of these problems associated with large body size, why did the sauropods get so large? Perhaps to escape predation. And boy did they have some predators to escape:

b. Theropods
These carnivorous saurischian dinosaurs remained bipedal and ranged from animals 3-4 feet in length to gigantic specimens as long as 40 feet. They had grasping hands of four or fewer fingers on relatively small forearms (in some the forearms are very small). Although they retained the anatomical body plan and ecological characteristics of ancestral dinosaurs, theropods include some of the most highly advanced dinosaurs (and the living birds).Their heads (and brains, by the way) were relatively large and armed with sharp, blade-like teeth. The hindlegs were very large and powerful. These hindlegs were often armed with 4 toes with sharp claws and occasionally with a claw on the back that could be used to slice open prey when they were jumped on. A large muscular tail helped balance the animal and offset the weight of the large head.

The smallest forms probably ate insects, larger forms, like Tyrannosarus rex, ate whatever they wanted.

There are 4 major groups of Theropods:

i. Coelosaurs - these are the earliest theropods; throughout the mesozoic they remained lightly built and agile.

ii. Carnosaurs - derived coelosaurs that accentuated strength and power at the expense of mobility and agility. They had extremely large heads and teeth and very short forelimbs.

Tryannosarus from the Cretaceous is the best-known carnosaur. It weighed 6-7 tons, stood 20 feet (6m) high, and its length was about 40 feet (12m). This mass suggest that they could run for only short distances and must have depended upon impact from the head and jaws for killing. The lower jaw had an additional joint that allowed the animal to increase the gap of its mouth.

iii. Deinonycosaurs (Jurassic Park) - derived from coelosaurs and include some of the most impressive carnivores that ever evolved. They were fast and agile but also could be large. The braincase is often large and, because the eyes faced forward, they had binocular vision. The backbone was held horizontal with the tail extending straight back (and it is reinforced). The arms are strong and armed with deep claws (sometimes the had is as large as the rest of the arm). The foot had a modified 2nd toe with a large sickle-shaped claw. These animals could have pounced on a prey held it with the strong forearms, then disemboweled it with the sickle-shaped claw. Fossil footprints suggest that they often hunted in packs, thus enabling them to hunt large prey. The picture below includes some artistic liscence - we do not know if dinosaurs had fur or modified scales covering their bodies (all the same it IS a cool picture):

An enormous deinonycosaur is known from the Cretaceous rocks of Mongolia. The fossil is just a shoulder, forelimb and hand with slashing claws - but the forelimb is 2.4 m (7 feet) long.

iv. Ornithomimids - called the ostrich dinosaurs because of their long thin necks. These had large brains and, unlike other theropods, they didn't have slashing claws but instead long fingers that were positioned such that we conclude they were highly dextrose in manipulating objects. they apparently ate small lizards and mammals.

2. Ornithischian Dinosaurs

The second major group of dinosaurs were all herbivores. Biologically, these dinosaurs are perhaps the easiest for us to understand because their biology and ecology is about the same as an elephant's, rhinoceros', or hippopotamus'. All replaced their front teeth with horny pads or beaks and had massive batteries of grinding molars. The exact classification of these dinosaurs isn't clear. We will consider four groups, one of which remains bipedal but the other three secondarily return to quadruped locomotion (moving on all fours).

a. Hadrosaurs - Duckbilled dinosaurs

These dinosaurs retained bipedal locomotion. These dinosaurs are called duckbilled dinosaurs because their mouth is flattened like a duck's beak. They had hundreds of tightly packed rows of teeth. Although bipedal, they had a flattened tail and this suggested to early paleobiologists that duckbill dinosaurs lived around and in water feeding of the touch aquatic vegetation. However, more recent fossil finds of mummified hadrosaurs shows conifer needles and twigs in the stomach - this suggests tree browsers. At any rate, they were successful - it has been estimated that 75% of the terrestrial vertebrate biomass in the Cretaceous was duckbill dinosaurs.

These had a diversity of bony crests on the top of their heads. Some of these crests were solid bone but others contained greatly elongated nasal passages. Generally , males had the largest crests while juveniles lacked them altogether. What were they used for??

i. Puffing up as visual cues.
ii. Resonating sound for calling mates or offspring.
iii. Head butting in ritualistic fighting

b. Stegosaurs - Plated dinosaurs

These animals are noted for two feature:

i. two rows of erect diamond-shaped plates extending along their vertebral column and spikes on the end of the tail.

ii. very small heads.

For quite a long time biologists though that these plates served as armored protection for the vertebral column. The plates, however, are relatively thin and covered with grooves from blood vessels, which is not what one would expect of armor. Recent wind tunnel experiments with models of stegosaurs suggest that the plates serve as heat radiators (not for basking and soaking up heat), from which we may infer that they functioned for thermoregulation. The inferred presence of extensive blood vessels is further support for this hypothesis. Why would an animal like Stegosaurus need to radiate heat? It probably built up a great deal of heat in its large plant fermenting gut.

These dinosaurs were extinct by the early to mid-Cretaceous. They were replaced ecologically by the armored dinosaurs

c. Anklosaurs - Armored dinosaurs

Covered with bony plates, supposedly for protection. The tails of some with impressive spikes. These would have been powerful defensive weapons.

d. Ceratopsians - Horned dinosaurs

These dinosaurs have an impressive head-shield and often spikes over the eyes or on the snout. A well-known example is Triceratops. The function of the head shield isn't known. It was thought to be protective of the vulnerable neck region, but it isn't particularly thick and is perforated with large sinuses. Maybe its used in display. We simply haven't figured it out.

Baby Dinosaurs

Fossilized dinosaur nests have been found in cretaceous rocks from Montana, Alberta, and Mongolia. The Montana nests are particularly well studied - these are carefully constructed bowls of mud or sand lined with vegetation. In each nest the eggs were arranged in a neat pattern so that they would not roll around. The eggs were elongated ellipsoids. The narrow end was buried in the sediment. Baby dinosaurs exited through the top leaving the bottom portion buried.

Many of the nests are clustered together at close regular intervals and have been reused several times - this suggests large nesting colonies such as we see today in many birds.

There is good evidence for long-term parental care by dinosaurs. One nest of hadrosaur babies found in Montana contains young that are too big to have just hatched. And their teeth show wear. A large adult dinosaur is fossilized nearby. It is easy to imagine a caring parent and offspring killed and buried together.

In the hills of Mongolia's Gobi Desert, scientists have discovered a unique fossil of a carnivorous dinosaur nesting on its eggs like a brooding bird, revealing for the first time how Earth's most fearsome parents may have tenderly cared for their young.

The 80-million-year-old fossil suggests that the nesting behavior so common among birds today may have originated long before modern feathers and wings, reinforcing the intimate evolutionary link between birds and dinosaurs. It suggests they share complex behavior as well as important anatomic features.