GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
Dinosaur Growth and Behavior

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

Some behaviors to consider:

Important to consider the difference between intraspecific and interspecific displays:

Some examples:

Dinosaur eggs, babies, and growth:
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:

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!!

(Differs from the mammalian condition, where baby elephants etc. are BIG animals!)

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:

Both conditions are found in modern birds (chickens vs. robins, for example) and modern mammals (horses vs. bears).

Some evidence of these habits in hatchling dinosaurs:

Dinosaur growth rate: VERY HIGH compared to typical reptiles, particular in big dinosaurs. Estimates based on bone "growth rings" (and other features) indicate small dinosaurs were full grown at only a couple of years, hadrosaurids (e.g. Maiasaura) at only 7 years for, and only 15-20 years for big sauropods (e.g., Apatosaurus) and theropods (e.g., Tyrannosaurus) 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!).

Unlike typical non-avian reptiles, dinosaurs seem to have determinate growth: reaching a fully adult size, than stopping growth (or at least slowing it WAY down).

Lifespans for dinosaurs seem to be shorter than those of similar sized mammals. For instance, the oldest known Tyrannosaurus individual is only 28, the oldest known sauropod studied so far is only 38, but elephants can live to 70 or more.

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Last modified: 14 July 2006