Diapsids: the archosaurs

Question 1

What are the 2 extant groups of archosaur?

Answer 1

1. Crurotarsi (crocodiles)

2. The Aves (birds)

Question 2

How many extant species of crocodile are there?

Answer 2

23-26.

Question 3

What 3 recognisable groups does the genus crocodillus contain?

Answer 3

Crocodiles, alligators and gharials.

Question 4

What was tichinosuchus?

Answer 4

An early croc: 5-6m long terrestrial predator with a double row of armoured scutes to support the spine.

Question 5

How many extant species of bird are there?

Answer 5

10,000.

Question 6

Define ornithordira.

Answer 6

Clade containing the pterosaurs, dinosaurs and their descendants.

Question 7

Birds are the only living ornithodirans. True or false?

Answer 7

True.

Question 8

Archosauria is composed of crurotarsi plus ornithodira. True or false?

Answer 8

True.

Question 9

List 5 main synapomorphies of archosaurs.

Answer 9

1. Committed to egg laying
2. No palatal teeth as in lepidosaurs
3. Sharp, serrated, ‘laterally compressed recurve teeth’
4. Antoribital fenestrae
5. Lateral mandibular fenestrae

Question 10

Why are the teeth of crocodiles characteristic?

Answer 10

They are non-serrated and conical. This is a derived state.

Question 11

Define antorbital fenestrae.

Answer 11

Openings in front of the eye towards the snout.

Question 12

Define lateral mandibular fenestrae.

Answer 12

Holes in the side of the lower jawbone.

Question 13

What is the point of antorbital fenestrae?

Answer 13

Lightens the skull.

Question 14

What is the point of lateral mandibular fesnetrae?

Answer 14

For jaw muscle attachment.

Question 15

Archosaurs can assume any of 3 stances. What are they?

Answer 15

1. Sprawling gait
2. Semi-erect
3. Erect

Question 16

Explain a sprawling gait.

Answer 16

The femur is parallel to the ground with the tibia and fibula at a right angle.

Question 17

Which archosaurs have a sprawling gait and why?

Answer 17

Crurotarsans: it provides stability and facilitates swimming.

Question 18

Explain an erect stance.

Answer 18

Limbs are directly below the body to support the weight.

Question 19

Which archosaurs have an erect stance?

Answer 19

Larger terrestrial species and birds.

Question 20

Did erect stance evolve in large animals?

Answer 20

No, it evolved in smaller animals. Size came later.

Question 21

List 3 early archosaur groups.

Answer 21

1. Rhynchosaurs: Terrestrial herbivores, Triassic
2. Proterosuchids: quadrapedal predators, Permian
3. Erythrosuchids: large aquatic predators, early Triassic

Question 22

Archosaurs were globally distributed until global cooling reduced their geographic radius. True or false?

Answer 22

True.

Question 23

Define the choanae.

Answer 23

Internal nares that open into the back of the throat.

Question 24

How do the choanae allow crurotarsans to be effective predators?

Answer 24

Crocodiles and mammals are the only groups to have a secondary bony palate. This separates their nasal passages from their mouth. The choanae allow them to breathe at the surface even if their mouth is open underwater.

Question 25

How does the secondary palate help crocodiles in prey capture?

Answer 25

It reinforces the skull against torsion when the croc rips chunks of flesh off its prey.

Question 26

The choanae are in the same place in all archosaurs. True or false?

Answer 26

False: you can see them moving further back towards the throat in more recent species.