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!!
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).
In primitive modern birds it is the male rather than the female which broods the nest: paternal care. These nests are laid by multiple females. In these paternal care cases, the male rather than the female typically watches over the young after they hatch. When plotted against body size, the volume of eggs of nests of deinonychosaurs and oviraptorosaurs more closely resembles the pattern seen in paternal caring birds than in maternal caring or biparental caring birds; this suggests that paternal care may be an ancestral (maniraptoran?) trait that was passed on to primitive avians but modified in more derived birds. However, we do not have evidence yet to see if more basal dinosaurs had paternal care, maternal care (as in crocodilians), or biparental care.
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:
Some evidence of these habits in hatchling dinosaurs:
Parental care of babies for at least several weeks is present in both modern crocodilians and modern birds, implying that this trait was present in archosaurs ancestrally. Birds have long been known to provision (bring food to) their babies; there are cases of crocodilians in captivity doing the same (see video below), but it is not yet known if they do this in the wild. But certainly crocodilian mothers bring their babies to safer places in ponds and lakes where they little ones could hunt.
After baby dinosaurs left the nest, at least seem to have travelled together. Packs/herds of of baby ankylosaurids, ceratopsians, and ornithomimosaurs are known; in at least the case of the ceratopsian Psittacosaurus and the ornithomimosaur Sinornithomimus these herds can include individuals of different ages, including adults. ( The parents may have been with the other baby herds, but were too big to be buried.)
Dinosaur Growth Rate and Skeletochronology
Like many animals, dinosaurs deposited Lines of Arrested Growth (LAGs for short). These are basically growth rings like tree trunks, one per year. Various paleontologists use that information to "age" individual dinosaur fossils. By comparing the age of the dinosaur to its size, and plotting all individuals of the species on a graph, you can figure out the growth pattern and growth rate of that dinosaur species: that is, we can develop a skeletochronology for some species.
Actual rate was 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: all the LAGs bunch up together).
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.
Different types of organisms have different life history strategies: different ways of dealing with how many young are born at a time; how many survive to adulthood; how long they live; etc. There are two general types of strategies that we see commonly in nature:
Non-avian dinosaurs are definitely more r-selected than typical larger placental mammals, but more K-selected than sea turtles.
How did dinosaurs get so big? And why?
The second question is probably easier to deal with. There are several possible advantages:
So how could dinosaurs get so big? Environmental changes in Mesozoic discussed above may have supported giantism:
However, we may be asking the wrong question. The problem might not be "how did dinosaurs get so big?"; it might be "why DON'T placental mammals get so big?" This may in part be due to the lower oxygen level and productivity during the Cenozoic. However, it may have to do with mammalian vs. dinosaurian reproduction:
In contrast, even giant dinosaurs could could lay clutches of a couple dozen eggs a year or more. Thus, they could survive environmental change much more easily than big mammals. So mammals may not be able to reach giant size because they become extremely vulnerable to extinction.
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