Living dinosaurs (birds) and their closest living relatives (crocodilians) share many derived features of reproduction; these are probably synapomorphies of Archosauria and so would be expected to be found in all extinct dinosaurs:

• Eggs with a hard calcium carbonate shell (as opposed to the leathery shells of lepidosaurs, turtles, egg-laying mammals)
• Nests built of vegetation (rather than buried in sand)
• Vocal communication between parents and offspring prior to hatching
• Some degree of parental care (typically shorter term in crocs than birds)

Fossil dinosaur eggs and nests found in the rock record, as have embryos of most major dinosaur clades.

ALL non-avian dinosaur eggs are basketball-sized or smaller: NO dinosaur hatched from eggs the size of people!!

ALL dinosaurs came from small babies!

Some dinosaur nests associated with covered mats of vegetation: probably helped to keep warm (as in croc nests).

Some dinosaurs (maniraptorans) found in “brooding position” over nests; unlikely to be found in dinosaurs which are too large (i.e., tyrannosaurids, hadrosaurids, sauropods, etc.) or lacking feathers (non-coelurosaurs).

Most primitive modern birds are ground nesters; suggests that tree nesting did not evolve until well into the modern bird (Aves) radiation.

Dinosaurs tend to have nests of about a dozen or so eggs each: more than found in modern birds, less than in (for example) turtles. This is regardless of size: troodontids to titanosaurids!

Implies that unlike placental mammals, dinosaurs could produce a dozen or so offspring a year regardless of size; among placental mammals, larger body size means LONGER gestation periods.

Two main potential life habits upon hatching:

• Precocial: able to move around easily shortly after birth.
• Altricial: nest-bound, wholly dependant on parents for food.

Some evidence of these habits in hatchling dinosaurs:

• Some baby hadrosaurines have poorly developed joint surfaces in the legs (unable to move well), but have worn teeth (were feeding): suggests altricial.
• Some other baby dinosaurs (such as troodontids) have fully formed joints prior to hatching: would have been able to move from day 1.

Dinosaur growth rate: VERY HIGH compared to typical reptiles, particular in big dinosaurs. Estimates (based on bone “growth rings”, and other features) indicate only 3-7 years for big ceratopsids and hadrosaurids to reach adult size, and only 10 years for big sauropods (e.g., Apatosaurus) to reach adult size.

(In contrast, big crocodilians from the Late Cretaceous seem to have taken 50 years or so to reach the same size as big hadrosaurs).

Since most animal populations stay generally stable over time, more baby dinosaurs died before reaching adult size than in typical populations of modern birds or mammals (imagine herd of antelope where every female produced a litter of 12 every year!).

Dinosaur behavior:
Four main sources of information for forming behavior hypotheses:

• Analogy to modern animals (i.e., horns in modern mammals compared to horns in ceratopsids)
• Phylogenetic distribution of behaviors in modern animals (i.e., behaviors shared by crocs & birds would be expected in all dinosaurs)
• Biomechanics to determine whether certain behaviors were feasible under laws of physics
• Sedimentary record where unusual preservation suggests certain behaviors (i.e., bone beds made up almost exclusively of centrosaurines suggests these dinosaurs lived in herds)

Some behaviors to consider:

• Displays: sexual, territorial, defensive
• Combat: as in display
• Locomotion: adaptations for weight bearing vs. speed
• Feeding: different types of carnivory; different herbivorous specializations

Important to consider the difference between intraspecific and interspecific displays:

• Intraspecific: within a species
• Interspecific: between species

Some examples:

• Crests in lambeosaurines: do not develop until adulthood; males & females different
• Horns in ceratopsians: as above
• Arctometatarsus vs. the spreading feet of sauropods and big thyreophorans
• Typical theropod teeth vs. tyrannosaurids vs. spinosaurids
• Simple dentition of primitive plant eaters vs. dental batteries

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