Species and Breeds in the Fossil Record. Can We Even Tell?

I was asked this question the other day. No doubt the asker, a personal friend, is not the only person who has ever wondered this:

"OK, I'm reading all of these variations on the species, and the thought of goats 
popped into my head. It would work with dogs too. You have a species, with a 
lot of physical variation in it. All different adult sizes & forms. All of them are the 
same species, though the breeds are distinct.

How much does this apply to the ceratopsids? Or the anklylosaurids? Or any 
other broad body classification? Are they actually truly distinct species, or are 
they breeds of one overarching body formation?"

The gist of this question is, how can we as paleontologists tell one species from another when a single species can have a ton of variation? It's a fair question. If we found a fossilized pack of dogs that included everything from Shih Tzus to Great Danes, we'd probably be able to tell they were related, but would not at all assume they were the same species.

It gets worse when you consider the growth history of animals of a single species or sexual dimorphism. Juveniles of a species can look very different than the adults, as can adult males and females of the same species.

This is a big problem in paleontology. We are dependent upon using morphology (shape) for defining the fossil species we study. Because we cannot observe the fossil organisms in life then we have to make assumptions about the natural variation of shape within a single species. We define this 'acceptable' range of variation whenever we define a new species. Not everyone agrees about how much variation is acceptable, and this can cause problems with interpretations of evolutionary relationships among organisms among other things.

There are some strong opinions about how much variation is 'enough.' Those who prefer to limit the amount of variation per species would be called 'splitters,' as they would split our pack of fossil dogs into many, many species. Those who feel that a single species can have a lot of variation is called a lumper. They would lump our fossil pack into a few, much broader species, though I suspect even a lumper wouldn't put all the world's breeds of dogs into a single species. Some breeds are just bizarre!

Some of the greatest current conflicts in paleontology center around certain species being distinct, or one being the preserved juvenile stage of the other.  For example, there is a great deal of contraversy over whether the famous Triceratops is simply a juvenile Torosaurus. Scannella and Horner in 2010 claimed that Torosaurus and Triceratops were synonymous, whereas Longrich and Field in 2012 argue otherwise. Similarly, there is a lot of argument in paleontology about whether Nanotyrannus is merely a juvenile Tyrannosaurus rex (as initially proposed by Carr in 1999), or if they are distinct species (see Larson, 2013).

We can get at some with life stage by looking at the state of development and fusion of skull bones, which works great for mammals, but maybe not so much for dinosaurs (see last week's post).

Skull morphology may or may not help when trying to distinguish males and females of a single species, or when sorting out diverse breeds within a single species. Whatever we do however, we have to carefully lay out our reasons for naming a new species or synonymizing one species with another. This has to be done in a professional publication, and we have to be ready for people to disagree. That's fine. The truth is, we will probably never know how many of our distict species really were distinct.

But ain't that the fun of it?

Posted By: Penny Higgins, University of Rochester

References Cited:

Carr, T.D. 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 10(3), pp. 497–520.

Larson, P, 2013. The case for Nanotyrannus; pp. 15-53 in J.M. Parrish,  R.E. Molnar, P.J. Currie, P.J., and E.B.Koppelhus (eds.), Tyrannosaurid Paleobiology. Indiana University Press, Bloomington, Indiana.

Longrich, N.R. and D.J. Field. 2012. Torosaurus is not Triceratops: ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy: PLoS ONE 7(2): e32623. doi:10.1371/journal.pone.0032623

Scannella, J.B. and J.R. Horner. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology 10(4), pp. 1157–1168.
Posted: 3/10/2016 11:32:05 AM by pennilynhigginsadmin | with 0 comments
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