Dinosaurs

Question 1

Are dinosaurs from a monophyletic group or polyphyletic group?

Answer 1

Monophyletic groups - known as the Archosauria

Ancestor originated 250 MYA in the later Permian

Question 2

What are the key features of Archosaurs?

Answer 2

Presence of calcaneal tuber - attachment for lower leg flexor muscles

Two skull openings - antorbital fenestra (probably for increasing skull agility) and mandibular fenestra (probably for increasing jaw strength) although functions are group dependent

Presence of teeth in socket and not fused with the jaw

Reduction of 5th toe

Secondary palate - allows for breathing when eating

Question 3

What are the two main clades of the Archosauria?

Answer 3

Pseudosuchia - includes crocodilians

Ornithodira - includes pterosaurs, dinosaurs and birds

Question 4

Give some basic differences between Pseudosuchia and Ornithodira

Answer 4

Pseudosuchia:

• very high diversity during mid-late Triassic
• Have a semi-erect posture
• More reptile like

Ornithodira:

• Siple hinge-joint ankle
• Very long hindlimbs
• Longer S-shaped neck

Question 5

Give some derived features of the dinosaurs?

Answer 5

Semi-erect to erect posture - limbs are positioned directly below the body (makes them more agile and efficient walkers)

S-shaped neck

Pubis bone - orientated downwards and backwards

Reduced 4th and 5th toe on the hand

Reduced to 3 main toes on the foot

Open acetabulum

Sacrum - made of 3 or more fused vertebrae

Question 6

What are the 2 major clades of the Ornithodira?

Answer 6

Ornithischians and Saurischians

Question 7

What is the hip made up from?

Answer 7

3 major bones - ileum, pubis and ischium

Head of the femur sits in the acetabulum

Question 8

What dinosaurs are bird-hipped and which are lizard-hipped?

Answer 8

Saurichians = lizard-hipped

Ornithischians = bird-hipped

Question 9

Why is the classification of hips strange?

Answer 9

Birds belong in the saurichians and not in the classification of bird-hipped dinosaurs

Found many late-evolving therapods (like birds and velociraptors) had a pubis pointing backwards which gives them bird-hips

Question 10

What is the new hypothesis for dinosaur classification?

Answer 10

Suggested that therapods may not actually be a sister group to the Ornithischians not the sauropods

Maybe reclassify ornithischians (which would no longer be a monophyletic group) into the ornithocelidans

Question 11

What is the problem with studying reproductive behaviour in dinosaurs?

Answer 11

Hard to see reproductive behaviour in fossils - so have to infer from extant taxa

Known diversity of egg-laying dinosaurs is limited to a few groups - sauropods, carnivorous therapods and duck-billed hadrosaurs

Question 12

Why have so many groups of dinosaur’s eggs not been preserved?

Answer 12

Originally thought dinosaurs laid hard-shelled (calcareous) eggs like birds

Did molecular studies and found lots of diversity bewteen eggs so think the calcareous eggs that fossilised evolved at least 3 times but independently - explains the differences

Think ancestral forms had a softer shelled eggs which explains why many dinosaur group’s eggs have not been preserved

There are a lack of fossilised eggs before the cretaceous so these were probably soft-shelled

Question 13

Did dinosaurs have sexual dimorphism?

Answer 13

Yes

Many fossils suggest females were larger - similar to raptors/birds of prey today

Question 14

When did dinosaurs start to diversify? How much diversity do we have on record?

Answer 14

In the Jurassic and early Cretaceous

527 genera have been described but estimate 71% are unknown