Lifespan of dinosaur species and possible overlaps.

[amargasaurus cazaui]

I was hoping to get some deeper understanding of/input regarding a topic I am noticing in the Safari new for 2018 threads....but which also puzzles me at times when viewing some of the entries in the diorama contest. My question.....we assign
potential dates for various species based on the best dating possible, but often we are working with a single specimen. How do we know if this dinosaur was around 3 million years prior or 8 million?
  Further as Cazaui is the only species assigned to Amargasaurus, would it not be possible that more basal amargasaurids preceded cazaui and more dervived members of the family followed, possibly stretching its temportal footprint even moreso, to some degree?
  I would assume a lot of that discussion would come down to at what point was it more like an Amargasaurus than the animal it is a more derived version of, but the question remains, how do we state positively that one animal did not live alongside another?
  I had read somewhere that most species have a give or take 5 million year lifespan, but in dinosaur terms, there seem to be some contradictions of that term.
I would assume in the case of Amargasaurus, were it found in a more research prone country with more paleos out there looking, further remains might have surfaced, or possibly less derived members of that family.....as with psittacosaurus, which seems to have managed to achieve a 30 million year window.......clarification please!

[Reptilia]

That is something I always wondered myself, sure there's an explanation.

[stargatedalek]

Most species don't last more than a few millions of years, even tens of millions is rare. That being said there are always exceptions, so it's certainly possible to mix species that are only a few million years apart in date, though there are some cases (IE extreme environmental shift) that make that impossible.

That's also a good point about the inevitable ancestral forms of each fossil species, completely reasonable to assume something similar may have existed before the known fossil species (within reason).

[Halichoeres]

This is sort of an evidence of absence/absence of evidence question. Fossilization is, for most terrestrial taxa, so unlikely that you'd be very lucky to find it at two different well-dated time points even if it existed at both times. Marine and freshwater taxa sometimes appear to persist for a very long time, but it's hard to be sure why that is: A) because they are likelier to fossilize and so we are likelier to find them in more than one time point, B) because we probably divide up aquatic taxa less finely in the first place, or C) because there is something inherent in the biology of aquatic organisms that makes cladogenesis (speciation) occur with less frequency. For example, some of the fish I study have 13-million-year-old fossils that are completely indistinguishable from extant specimens.

In dinosaur paleontology in particular, it seems that relatively few specimens are referred to existing taxa; instead, the preference seems to be to erect a new taxon. Some of the reason for that is practical: it makes it more likely that more specimens will be included in phylogenetic analyses, which reduces the likelihood that taxonomy will be misled by generalizing assumptions about nominal species (defining a "species" in the fossil record is a fraught exercise under all but the very best conditions). Some of it is a little more cynical: a new species gets you lots of press, but a new specimen of an existing species gets you less.

For Psittacosaurus in particular, I think that if we had a similar number and variety of any comparable clade of theropods, they would have been named as half a dozen new genera.

Anyway, figuring out exactly how long a taxon existed in the fossil record first requires:
A precise definition of what its taxonomic limits are
The phenomenal luck of finding something matching that definition in two or more temporally separated places
Finding a spot where by all rights you SHOULD find that taxon if it lived there, but don't. This can be construed as weak, circumstantial evidence of absence.

So most of the time it's a sort of guess. It's true that the vast majority of dinosaur taxa aren't known to persist more than 2-3 million years, but some of that is probably an artifact of dinosaur taxonomy. I don't know if that's at all helpful.

[Lanthanotus]

This is sort of an evidence of absence/absence of evidence question. [...]
So most of the time it's a sort of guess. It's true that the vast majority of dinosaur taxa aren't known to persist more than 2-3 million years, but some of that is probably an artifact of dinosaur taxonomy. I don't know if that's at all helpful.

Shortend, but anyway QFE.

If you compare the number of the recent species recognized and the number of all the species we know from fossil records you'll find that there's a hell of a gap. There's no evolutionary or ecological reason reason backed by sufficient evidence to think that diversity has been lower 65 million or 100 million years back in time than it was now. We tend to think that the world back than was of lower diversity because flowering plants haven't been evolved by than and most of the animals we are so familiar with haven't been, but that is just a view biased by our human egocentricity. Sure thing, lot of species didn't exist back than, but hundreds and thousands of other taxa that are now extinct lived back than. The fossil record simply does not show us most of them. Fossils from dense, humid forests for example are almost unknown (if you do not count in coal for itself) as from a lot of other habitats. Small amphibians, reptiles, insects or arthropods were most likely as diverse as they are today, but we will never know if that it true or if it's not, because we just won't find fossils of most of them.

That being said I find it a very bold claim to give an average lifespan for any species, because simply, we only know such a small number of species from the fossil record (compared to the number of species recently living or having lived ever since) that - in addition to what Halichoeres wrote - such claims can only be vagued educated guesses based on the handfull of species that show such a dense record, that anything valuable can be said about their lifespan.

[Neosodon]

Good topic to bring up. It has always bothered me when people say stuff like this species lived from 96 to 93 mya. I prefer when people just name the date the specific fossil is known from or name the general period it is known from like early Jurassic or Cenomanian of the Cretaceous.

Also does anyone know what dinosaur has the biggest time gap between two fossil discoveries of that same spiecies? Psitacosaurus seems to have had a long life span but is it really the longest known of all dinosaurs?

[Dinoguy2]

We can look at high resolution, well sampled formations to get a general idea of how fast dinosaurs evolved and what the turnover rate was. Take a look at charts for the Dinosaur Park or Horseshoe Canyon or Morrison formations and you can see just how fast it was. Most dinosaur species for which we have good constraints lasted about 1 million years before being replaced in its ecosystem or evolving to the point it gets classified as something else. This is in part due to how over split many dinosaur genera are. Centrosaurus apertus lasted less than a million years, but it evolved from and into species we could reasonably call "Centrosaurus", we just choose not to. Look at Psittacosaurus. It has more species lumped into a single genus than any other dinosaur. It also is the longest lived genus. Coincidence? No, because it has more species that have not be given new names, and none of them lived at the same time. Therefore, the dinosaur genus with the longest time span will also be the one with the most species.

Could Amargasaurus exist 6-8 million years prior to known specimens? No. Because such an earlier species would be different enough that scientist would assign it a new name.

how do we state positively that one animal did not live alongside another?

Ecosystems are delicately balanced things, not random collections of cool animals. If you have two similar species in the same ecosystem, that requires an explanation and exploration of niche partitioning etc. Unless we have POSITIVE evidence that two similar species lived together, it's better to assume they don't, rather than have to deal with a potentially unusual ecological situation.

Also, lack of evidence for overlap can be interesting and informative itself. There's no evidence that T. rex and Alamosaurus ever shared the same ecosystem, and past claims for this seem to be based on bad date calibrations. But why? Did tyrannosaurs drive alamosaurs out? Is there something about them that preferred different environment types, so a change that was bad for one was good for the other? It opens up the door for more science. Saying "well I bet they did live side by side and we sit haven't found positive evidence yet" is not helpful or very scientific.

"The fossil record simply does not show us most of them. Fossils from dense, humid forests for example are almost unknown (if you do not count in coal for itself) as from a lot of other habitats."
That's true, but of course we would not expect to find known species in those habitats. Species have habitat preferences, and all the dinosaurs we know of cane from habitats that happened to be good for preservation. If we could go back in time and explore bad preservation habitats, we would find brand new kinds of dinosaurs, not cryptic populations of the same dinosaurs.

[Reptilia]

I find what you say quite logical and reasonable, Dinoguy2. Thanks.