The assemblage of diapsid reptiles with socketed (thecodont) teeth, which includes the crocodiles, two orders of dinosaurs and pterosaurs, can be grouped as a Subclass Archosauria, or merged with the squamates and others in a Subclass Diapsida. The term diapsid refers to the two temporal openings which are located in the upper and lower fenestra in the temporal region of the skull.  

The earliest known diapsid, Petrolacosaurus from the late Carboniferous, was about 60-70cm long and is thought to have been a very agile and terrestrial animal.

• Note that the birds are clearly derived archosaurs, and in a phylogenetic classification should be included, but with their expanded brains and slight jaws (beaks), the diapsid nature of their skull has been lost

Diapsids can be split into two groups:

• Lepidosauromorpha: 

this group includes the tuatara, squamates and the marine tetrapods such as the plesiosaurs (see lecture 11). They possess a dorsal temporal opening but may lose the lower (quadratojugal) bar, so the lower temporal fenestra is no longer evident.

• Archosauromorpha:

this group includes the birds, pterosaurs, crocodiles and dinosaurs. The archosaurs are distinguished by the presence of an antorbital (in front of the eye) fenestra. Other characteristics include:

• a deep skull
• the orbit of the eye is not circular but instead is shaped like an inverted triangle
• teeth are laterally compressed
• trend towards an upright stance, many  develop bipedalism
• ventral side of the femur shaft, the fourth trochanter, has a caudiofemoral muscle inserted on to it.
• have modified the structure of their pelvic girdles for this, and can be classified on this basis. 

Order Crocodilia.  

The skull is diapsid and akinetic, but highly derived; the elongate snout with a long secondary palate, and pterygoid fenestra rather than the antorbital fenestrae seen in other archosaurs, are very specialised. 

The body form seems more “conventional”, lizard-like, than that of dinosaurs, but they can adopt a semi-erect posture, and the pelvic girdle and muscles are modified accordingly.

The few (20 or so species) of crocodiles, alligators and caimans, plus the birds, give us much evidence about the behaviour of dinosaurs, and an interesting ecology and conservation problem of their own. 

The pelvic girdle of an Alligator

The pelvic girdle is reduced in size because it is no longer required for weight support. The ilium is enlarged and has expanded contact with the vertebral column for increased area of attachment of locomotor muscles.

(PIFE = pubo-ischio-femoralis externus)

(pictures courtesy of Dr. Derek Yalden, University of Manchester)

Commercial farming for meat and leather has done much to reverse the declines due to commercial overhunting.  Standard practice is to return some well-grown young of each brood to the wild, at an age when they are relatively safe from predatory lizards and birds.  

• The archosauria also includes the dinosaurs which are split into two orders; saurischia and ornithishcia.

Order Saurischia. “Reptile-hipped” dinosaurs. 

Includes big quadrupedal herbivores of the suborder Sauropoda.  

Sauropods include the largest land vertebrates ever (Brachiosaurus, Brontosaurus, Diplodocus), but had tiny heads and mouths, no chewing teeth; how did they eat enough to survive?  

Bipedal carnivores, of the Suborder Theropoda, both large forms (e.g. Tyrannosaurus) and small ones (e.g. Compsognathus). 

The Theropoda include the ancestors of birds.

Upright limb posture essential with large size, but also more efficient mechanically; with a large ventrally-directed pubis and ischium, and muscles attached to them. 

The pelvic girdle of an Allosaur 

(pictures courtesy of Dr. Derek Yalden, University of Manchester)

The earliest prosauropods are from the late Triassic, latest sauropods are late Cretaceous

Anchisaurus and Plateosaurus

• gastroliths - stones swallowed by dinosaurs and lodged in the muscular gizzard, assist in grinding like some birds
• long necks and browse on plant material
• serrated teeth
• largest terrestrial vertebrate ever existed
• 30 metres
• 100,000kg!

Brontosaurus and Diplodocus                                                  

• long necks
• teeth at front of mouth
• not powerful jaw muscles

Camarasauroids

• shorter tails
• no whip like extension like diplodocus
• chisel like teeth indicating they fed on abrasive material

Theropods

• predators (or scavengers?) of large prey
• fast moving
• bipedal

Large Theropods

• Tyrannosaurus 15m long
• small front legs
• small fingers on each hand
• 15cm long teeth
• serrated teeth and powerful jaw muscles

Small Theropods

• includes birds
• Dromeosaurs e.g. Deinonychus 2m claw on 2nd toe of hind foot, and long stiff tail
• claw used in hunting

All herbivorous, with complex cheek teeth to chew vegetation. 

Some bipedal, most quadrupedal. 

Pelvis with pubis lying back alongside ischium (as in birds, but evolved in parallel), but often with an extra anterior prong (prepubis) giving a tetraradiate appearance. Same functional implications as for saurischians - upright stance, better mechanics.  

The pelvic girdle of Stegosaurus

   (pictures courtesy of Dr. Derek Yalden, University of Manchester)

• Different structures of the pelvis to saurischians, as shown above.
• Cheeks and horny beaks rather than teeth at the front of the mouth
• Batteries of cheek teeth at the back of the jaws.

There are 3 groups:

1. Thyreophora

• armoured dinosaurs
• includes the stegosaurs from the Late Jurassic: Stegosaurus:

• quadrupedal,  possess spines on the back and tail
• N. American, 6 metres, front legs shorter
• 2 pairs of spines on tail

Kentrosaurus:

• African, 2.5m, several pairs of spikes down back of tail

• function of plates of stegosaurs? protection, heat exchange
• small skull
• may have used stones in the muscular gizzard to digest plant material

Ankylosaurs

• Jurassic - Cretaceous
• quadrupedal
• 2-6m
• heavily armoured - osteoderms - bones in the skin and fused together
• short tails - lump of bone at the end, used as a club
• some had spikes

2. Ornithopoda

• appeared at the end of the Jurassic
• 1 - 3 m
• bipedal form
• 4 toes on hind feet
• specialised teeth for grinding plant material

Iguanodon:

• from England, learly Cretaceous, and one of the first dinosaurs ever discovered (albeit giant teeth only, at first)
• very large at 10m
• possessed large heads and elongated snouts
• beaks were toothless but had serrated edged teeth in rear of jaw
• 1st digit (thumb) on each hand is a spine thought to be a defensive weapon

Ouranosaurus: mid Cretaceous, African = sail highly vascularised may have been used for heat exchange

Hadrosaurs

• duck-billed dinosaurs; toothless at front, many teeth at the back
• most derived during mid-Cretaceous
• 10 metres long and weighed 10,000 kg
• could form replacement teeth so it was possible to have several thousand teeth in its mouth at       any one time!
• herbivorous and frugivorous - coincided with the radiation of angiosperms

3. Marginocephalia

• most specialised herbivores
• pachycephalosaurs were bipedal and thick skinned
• had enormous bone on the skull roof, 25cm thick
• heads may have been used as battering rams

4. Ceratopsia

• quadrupedal horned dinosaurs
• Triceratops had powerful jaw-closing muscles with a frill over the neck and three horns.
• other ceratopsians had 0, 1 or 3 horns, smaller or less complete frills.
• late dinosaurs, Cretaceous.

The diagram below provides a summary of the changes in the reptilian pelvic girdles and how the orders are distinguished by the differences in structure. In Sphenodon and lizards, with a sprawled stance, the femur is horizontal and many of the muscles are "press-up" muscles. In dinosaurs, with their upright stance, the femur is vertical, the muscles mostly pull backwards and forwards, and the pelvic girdle is accordingly restructured. The early archosaur Euparkeria and the crocodylian Alligator show intermediate conditions.

 (pictures courtesy of Dr. Derek Yalden, University of Manchester)

The biology of the dinosaurs and the reason(s) for their extinction at the end of the Cretaceous have generated much discussion and some research. New fossils of nests with embryos in the eggs, and of adult Oviraptor incubating, show that parental care was well developed. Trackways of herds of mixed sizes/ages indicate the same. 

Were dinosaurs also warm blooded, like birds? (See lecture 14) If they were incubating, that must seem likely, but they could have been sheltering eggs from the sun or sand. The very large sauropods could not eat enough to have sustained endothermic metabolism, given likely estimates of bite size and nutrient values. Smaller dinosaurs could have been more active, and the more bird-like their anatomy, the more plausible a bird-like physiology becomes. 

Extinction was not a sudden demise of all the dinosaurs at the end of the Cretaceous; they had been declining for several million years before, and only a few (12 species?) survived to the end. An asteroid impact on the Yucatan Peninsula certainly caused a lot of damage, but many endotherms, which ought to have been vulnerable (with their higher metabolic rates) survived, and plants seem to have been little affected. There is even a possibility that a few dinosaurs did survive, additional to the feathered theropods we call birds. (See lecture 13 and lecture 15)

We have missed out one main group of dinosaurs - the flying dinosaurs. In the next lecture we will discuss Pterosaurs and also other flying vertebrates; the birds and bats

Further Reading:

Carroll, R.L. (1988) Vertebrate Palaeontology and Evolution. Chapters 14 – 15: pages 261 - 325

Pough, F.H., Janis, C.M. & Heiser J.B. (2002) Vertebrate Life.  Chapter 14: pages 367 – 407