DINOSAURS

Question 1

What group did dinosaurs evolve from?

Answer 1

The Archosauria
(a monophylectic group)

Question 2

What are the synapomorphies of archosaurs?

Answer 2

• Presence of calcaneal tuber - bone projecting from ankle - flexor muscle is attached
• 2 skull opening - one in lower jaw, hypothesised that it was where jaw muscles attached.
• Second skull opening is between eye socket and nostrils - hypothesised: adaptation for reducing skull weight
• Presence of teeth in sockets - roots that are not fused to the jaw. Thecodont.
• Reduction in the 5th toe
• Secondary palate (allow them to breath whilst eating)

Question 3

What are the 2 major clades within archosauria?

Answer 3

1. Pseudosuchia
2. Ornithodira

Question 4

What were the Pseudosuchia?

Answer 4

• Reptiles & crocodilians
• High diversity during mid - late triassic
• Only extant group crocodiles
• Quadrupeds and some facultative bipeds

Question 5

What were the Ornithodira?

Answer 5

• Pterosaurs, dinosaurs + primitive relatives
• Non-avian dinosaurs and birds
• Simple hinge-joint ankle
• Very long hindlimbs
• Longer s-shaped neck

Question 6

Which was more prominant, Pseudosuchians or dinosaurs when dinos first appeared?

Answer 6

• Pseudosuchians
• Ruled by reptiles

Question 7

What were the shared derived features of dinosaurs?

Answer 7

• The semi-erect posture (more upright)
• Limbs positioned directed below the shoulders
• Pubis bone - downwards and backwards
• Hip joint
• S-shaped neck
• Reduction in 4th and 5th digit on hand
• Reduction to 3 main toes on foot

Question 8

What are the characteristics of the dinosauria hip?

Answer 8

• Open acetabulum (hip socket)
• Sacrum - fused vertebrae

Question 9

What are the two major clades within dinosauria?

Answer 9

1. Ornithischians
2. Saurischians

Question 10

What is the clade saurischians comprised of ?

Answer 10

• Sauropodomorphs
• Theropods (including birds)

Question 11

How did two groups evolve based on the study of the hip joint ?

Answer 11

• Difference is in the pubis bone
• In Ornithischian the pubis is aligned and parallel with the ischium - facing backwards by tail
• In Saurischians pubis points forward and flares into keel

Question 12

How is the hip positioned in the Ornithischians?

Answer 12

pubis is aligned and parallel with the ischium - facing backwards by tail

Question 13

How is the hip positioned in the Saurischians?

Answer 13

pubis points forward and flares into keel

Question 14

Which group are ‘Lizard Hip Dinosaurs’?

Answer 14

Saurichians

Question 15

Which group have ‘Bird Hips’?

Answer 15

Ornithischians

Question 16

Which hip dinosaur group did modern birds evolve from?

Answer 16

Lizard Hip Dinosaurs
Bird hipped dinosaurs did not include birds

Question 17

How has dinosaur phylogeny been reworked?

Answer 17

• Evolutionary tree was shuffled about and a new phylogeny hypothesis was established including new clade Ornithocelidans. WIP
• This would mean birds evolved from bird-hip dinosaurs

Question 18

Features of reproduction in dinosauria

Answer 18

• Study extant dinosaurs and infer behaviour of dinosaurs
• All extinct dinosaurs laid eggs (this is known)
• Hard-calcium carbonate shell - was widely accepted to be the only type of eggs that dinosaurs laid.
• Did they develop from soft egg shells? Leathery (like turtle eggs)
• Fossil bias - most are from cretaceous - were the earlier eggs soft and so unpreserved
• Sexual dimorphism - larger females?
• Male competition - all very hypothetical

Question 19

Diversity of dinosaurs

Answer 19

• More is known about the extinction of the dinosaurs that how they diversified
• 700 - 800 groups
• Only around 5% excavated from triassic rocks
• Birds are the only group of dinosaurs that survived from the cretaceous period

Question 20

What is the most accepted hypothesis for the extinction for non-avian dinosaurs?

Answer 20

Asteroid in the yucatan peninsula

Question 21

Were dinosaurs declining before the extinction event?

Answer 21

• End of cretaceous was a changing time (environmental etc)
• Study suggested that dinosaurs were declining about 10 million years before the asteroid caused extinction

Question 22

What drove the delcine in dinosaurs before the asteroid?

Answer 22

• Abiotic factors (climate, temperature, sea level etc)
• Biotic factors (diversity of ferns, evolution of angiosperms etc)
• Diversity dependence (interaction between carnivores and herbivores)

Two factors were at play that explain decrease

1. Temperature - global cooling climate
2. Density of herbivorous dinosaurs

Question 23

Birds evloved in the triassic, diversified in the jurrasic and are the only group of dinosaurs to survive the end of the cretaceous period