Average Dinosaur Individual Size

[DinoLord]

I've recently been thinking and reading a lot about the ecology of Mesozoic ecosystems and a question has emerged: What size was the average individual of a given species?

There seem to be two schools of thought on this:

1. Due to the rarity of fossilization, it is statistically likely that any individual that becomes fossilized represents an average individual of that species. There were likely individuals smaller and possibly even ones quite larger than what we have found.

2. Dinosaurs were r select species that produced many offspring, so the average individual was a juvenile that would most likely not make it to adulthood. I often see this logic used in attempts to explain high large herbivore or carnivore density in certain environments like the Morrison or Hell Creek.

However I see problems with both of these, so I'm not sure what the most correct line of thinking to answer this problem would be. The former seems to fail to take into account possible preservation bias towards larger more robust specimens. The latter relies on the assumption that dinosaurs were indeed r strategists - however given the evidence for parental care in at least a few species of both theropods and ornithischians and the general trend of at least some parental care in extant archosaurs, I'm not sure how safe this assumption is.

Any thoughts on the topic?

[amargasaurus cazaui]

I think there are two things that vastly complicate what you are attempting to sort out here . The first is the idea that sometimes we might find elements from several animals but not a complete single specimen so we build them all together into a single entity..ala the Berlin specimen of Giraffatitan, or we find a vastly incomplete specimen that we add to and infer its precise size from other animals that are similar....take for instance the dinosaur seismosaurus, and its ultimately being considered just a rather large version of Diplodocus . I think another idea that complicates your task here is the one that Horner somewhat first began to work with regarding Triceratops vs Torosaurus. I am unsure he was correct in this instance..but I am also certain there are multiple species in the fossil record that we consider seperate , but which are in reality various stages of a single animals growth window.

[DinoLord]

Good points - those factors certainly add to the uncertainty. However I originally had in mind species whose proportions are known from a relatively complete specimen and that are commonly found, like certain hadrosaurs or ceratopsians. Basically I'm trying to figure out if the Mesozoic landscape was filled with grand and big adult dinosaurs, or if it was dotted with juveniles scrambling around.

[stargatedalek]

not exactly on topic, but Horner's theory regarding torosaurus=triceratops was proven incorrect, since smaller torosaurus fossils were found which overlapped those of triceratops in size
his pachycephalosaurus=dracorex=stygimoloch is still somewhat of an ongoing debate, with stygimoloch generally being considered synonymous with dracorex and pachycephalosaurus inclusion being considered speculative

hope I explained that as I meant to

I think both 1 and 2 are subject to preservation bias, 2 could be that in some conditions large animals preserve better than small ones

[amargasaurus cazaui]

not exactly on topic, but Horner's theory regarding torosaurus=triceratops was proven incorrect, since smaller torosaurus fossils were found which overlapped those of triceratops in size
his pachycephalosaurus=dracorex=stygimoloch is still somewhat of an ongoing debate, with stygimoloch generally being considered synonymous with dracorex and pachycephalosaurus inclusion being considered speculative

hope I explained that as I meant to

I think both 1 and 2 are subject to preservation bias, 2 could be that in some conditions large animals preserve better than small ones

I myself agree with your comment regarding Toroceratops, however I would not tell that to Horner. He is still defending his conclusion and keeps attempting to rebutt the evidence. To be fair there are a few valid points in his argument however I have the paper where his case was dismantled and rebutted solidly leaving little squirming room.

[Dinoguy2]

R selection doesn't necessarily imply no parental care, just relatively limited parental care. This question would also probably have different answers for different groups of dinosaurs just as many strategies exist a,one living archosaurs. There's no physical way an adult sauropod could have cared for babies hundreds of times smaller after leaving the nest, but it's possibly babies could have joined herds of larger juveniles that helped care for them.

But most still would have died. We know for a fact sauropods laid dozens of eggs per clutch, in large nesting grounds. No ecosystem could support hundreds of new sauropods reaching maturity every year. It's ecologically impossible for them to have been k strategists, most of those babies had to die one way or another for the species to survive.