Lesson 11: Dinosaur Origins

Question 1

Recall: What are fenestrae?

Answer 1

Additional openings in the skull that do not house sensory organs. Usually, fenestrae provide an open area for large muscles to fill. The number and arrangement of fenestrae are key characters that are used to help classify amniotes into their major lineages.

Question 2

What are anapsids?

Answer 2

Amniotes that completely lack fenestrae are called anapsids. Modern turtles are one example, but anapsids are relatively rare today, and were more common earlier in the history of amniotes.

Question 3

What are synapsids?

Answer 3

Synapsids are amniotes with one fenestra on each lateral side of their skull. All mammals are synapsids and so were our close reptilian ancestors, like the famous sail-backed synapsid Dimetrodon.

Question 4

What is the Dimetrodon?

Answer 4

Although it is commonly misidentified as dinosaur, Dimetrodon is more closely related to you and me than it is to any dinosaur. Dimetrodon lived during the Permian period, so it was millions of years older than the first dinosaurs.

Question 5

What are diapsids?

Answer 5

Amniotes with one set of fenestrae on the lateral sides of their skulls (laterotemporal fenestrae) and one set on the top surfaces of their skulls (supratemporal fenestrae).

Question 6

What are Lepidosauromorphs (or lepidosaurs)?

Answer 6

Diapsids with no additional fenestrae. Modern lepidosaurs include lizards, snakes, and tuataras.

Question 7

What are Archosauromorphs (or archosaurs)?

Answer 7

Diapsids with an additional fenestra in front of each orbit (the antorbital fenestra) and an additional fenestra on the rear of the lower jaw (the mandibular fenestra). Crocodilians, birds, dinosaurs, and the extinct flying reptiles called pterosaurs are all archosaurs.

Question 8

Note that some lineages of archosaurs, such as modern crocodilians, have secondarily lost their antorbital fenestra and some, like the pterosaurs, secondarily lost their mandibular fenestra. Does this mean that crocodilians or pterosaurs lose their status as archosaurs?

Answer 8

No. “Archosaur” is a name applied to the evolutionary lineage. As long as the ancestors of crocodilians and pterosaurs had the characters that define an archosaur (and they did), crocodilians, pterosaurs, and all other such descendants will be classified as part of this evolutionary group.

Question 9

Dinosaurs, pterosaurs, and a few of their close relatives belong to a special group of archosaurs, and are known as avemetatarsalians. What are they?

Answer 9

Avemetatarsalians are characterized by having ankles that flex like a hinge, while other archosaurs have ankles that rotate like a ball-and-socket. This adaptation gave avemetatarsalians stiffer ankles, which were better able to safely support their weight while running and were better suited to locomotion on upright (non-sprawling) limbs.

Question 10

The archosaurs are thus divided into two main lineages: the ___ archosaurs, which include today’s living crocodiles, their ancestors, and many unusual extinct groups that we’ll cover in greater detail shortly, and the ___ archosaurs, which include dinosaurs and their immediate ancestors, pterosaurs, and birds.

Answer 10

Pseudosuchian and avemetatarsalian.

Question 11

In the Permian (299 to 252 million year ago), all the world’s landmass was part of the supercontinent Pangaea. This single continent had an arid interior, with rapidly fluctuating temperatures and climates. The first group of amniotes to evolve large body size and to dominate the ecological roles were…?

Answer 11

Reptile-like synapsids, including Dimetrodon, became common and thrived for millions of years.

Question 12

Gradually, these early synapsids become more mammal-like. Give examples.

Answer 12

Gradually, these early synapsids become more mammal-like. Late in the Permian, large sabertoothed synapsids, called gorgonopsids, were the top predators, and synapsids, like the tusked dicynodonts, were the top herbivores. There was a diverse array of small and mediumsized synapsids, including the cynodonts. Cynodonts would go on to evolve into true mammals, and the early forms looked a little like short-legged dogs.

Question 13

What is the general consensus for the end-Permian mass extinction?

Answer 13

About 70% of all terrestrial vertebrate species, and 90-95% of all marine species, went extinct in a short span of time.
Single greatest extinction event ever, and it took millions of years for Earth’s ecosystems to recover.

Question 14

What are possible causes of the end-Permian mass extinction?

Answer 14

Huge lava deposits, known as the Siberian Traps, formed at this time. The volcanic eruptions that formed these deposits may have been ongoing for 200 000 years or more! These long-lasting eruptions must have released large quantities of volcanic gases into the atmosphere, leading to a greenhouse effect and increased global temperatures. Increased global temperatures may have also resulted in the melting of frozen chemicals called methane hydrates deep in the ocean, which in turn would have contributed to more global warming and even more melting of methane hydrates, and so on. Global temperatures may have increased by at least 6 degrees right at the end of the Permian. Some scientists have hypothesized that the extinction may have been brought about by a comet or meteorite impact, although a crater from such an impact has yet to be found.

Question 15

What happened at the beginning of the Triassic?

Answer 15

Cynodonts and dicynodonts were among the synapsids that succeeded in rebounding, and it is during the Triassic that the first true mammals appeared. However, at the same time, a new group of diapsids, the archosaurs, also began to diversify and grow. Gradually, large archosaurs became more abundant, while large synapsids became less abundant

Question 16

(In Triassic) The first widely successful group of archosaurs was a lineage that would later go on to evolve into modern crocodilians. These crocodile-line archosaurs are called …?

Answer 16

Pseudosuchians. The pseudosuchians of the Triassic include the often huge and slender-snouted phytosaurs, which were semiaquatic predators like their distant crocodile relatives; the heavily armored and herbivorous aetosaurs; the rauisuchids and prestosuchians, which were terrestrial predators with upright limb posture; and the poposauroids, some of which were sail-backs and demonstrate convergent evolution with the earlier Dimetrodon.

Question 17

The oldest record of dinosaur-like archosaurs comes from footprints that have been dated at roughly 250 million years old. Describe these early dinosaur-like archosaurs.

Answer 17

The earliest dinosaur-like archosaurs were small and bipedal and looked a lot like the true dinosaurs, but they lacked some of the specializations that characterize true Dinosauria, such as a hip socket with a hole through it – for this reason, we call these animals dinosauromorphs. Some early dinosauromorphs were quadrupedal, like the silesaurids.

Question 18

The best record of early dinosaur bones comes from 228 million year old fossil beds of Argentina. Eoraptor, Eodromaeus, Herrerasaurus, and Panphagia are examples of early carnivorous saurischian dinosaurs (small at under a meter in length), and Pisanosaurus is an early herbivorous ornithischian dinosaur (about a size of a tiger). But…?

Answer 18

These dinos were rare components of their ecosystems.

Question 19

As the Triassic drew to a close, dinosaurs were gaining ground. Describe common ones.

Answer 19

• Coelophysis was a wolf-sized Triassic theropod that has been found in large bonebeds in New Mexico. Coelophysis appears to have been one of the most common predators of its time and place.
• Prosauropods evolved late in the Triassic and also were hugely successful. Plateosaurus is the best known of the prosauropods and would have weighed more than three tons.

Question 20

In the Triassic, ___ were record breakers, the largest herbivores that had ever evolved up to that time.

Answer 20

Prosauropods.

Question 21

Describe how dinos came to dominate their environment.

Answer 21

After the end Permian extinction, pseudosuchian archosaurs diversified and became very common. The earliest dinosaurs coexisted alongside more primitive dinosauromorphs for some time, and many pseudosuchians had evolved dinosaur-like body plans.

At the end of the Triassic another mass extinction event of unknown cause occurred. It hit many of the thriving archosaur groups hard . . . but not dinosaurs.

Question 22

Is the story of the rise of dinosaurs one of dinosaurs conquering and defeating other groups or one of chance and opportunity?

Answer 22

Chance and opportunity.