“How do you know if you have found a new species?”

This is certainly among the top ten questions I am asked as a paleontologist. The idea of discovering a species never before seen by anyone else in the world is undoubtedly one of the attractions of my field. Those who aren’t paleontologists often want to know how it is possible to recognize a specimen as something new.

Sometimes it is easy, sometimes it isn’t. And then there is Scenario C.

For example, when my friend and collaborator, Federico Anaya, first showed me the lower jaw of a medium-sized notoungulate that we later named Hemihegetotherium trilobus, I immediately knew I was looking at something new. There were lots of specimens of this animal, and all of them had a last molar with three distinct lobes quite unlike the two-lobed last molars of closely-related species. H. trilobus differed from other species in several additional ways, but this easy-to-identify feature would have been enough in and of itself to recognize it as something new. I usually describe a situation like this as a “slam dunk”.

Unfortunately, recognizing a new species is seldom a slam dunk. For example, I found a very nice lower jaw of a different type of notoungulate at a site in Bolivia in 2007. In size and overall structure, it resembled a previously-named species from southern Argentina called Protypotherium attenuatum. But it also differed in some more subtle respects, such as the proportions of some of the teeth. Did it represent a different species or not? With only one specimen, we weren’t sure whether the differences were simply individual variation or a characteristic of a different species. Therefore, we referred it to a previously-named species using the designation “cf.”, which essentially (but not literally) means that it compares favorably: Protypotherium cf. attenuatum. This is the scientific way of hedging one’s bets: the specimen probably represents an old species, but there is a chance it does not.

Hopefully these two examples illustrate that a paleontologist has to use their judgment to determine whether a specimen represents a new species. But this isn’t the only time when judgment (opinion) is required. It also plays an important role in deciding whether or not a species that undoubtedly is new should be given a name.

You may now be asking yourself: “Why would anyone pass up the opportunity to name a new species?”

One reason is that naming a new species could make life difficult for future paleontologists.

The thing about naming a new species is that you have to designate one specimen to represent it. That specimen is known as the holotype. When any later researcher wants to understand the unique characteristics of a particular species, they start by studying the holotype. Other specimens can provide additional examples of those features, but fundamentally, the holotype is the one that counts.

Choosing a holotype can be tricky when dealing with fossils because specimens vary greatly in their completeness. If you have a complete skeleton, it makes a fine holotype. But what if you only have two specimens, an upper jaw and a lower jaw? Which do you choose? What if you have a complete jaw with poorly-preserved teeth and a well-preserved jaw with only two molars? In those cases, the decision is based on which specimen better illustrates the feature(s) that distinguish the new species. In the case of Hemihegetotherium trilobus, we chose a lower jaw rather than an upper jaw because the most important feature had to do with the shape of a lower tooth.

Which leads me to Scenario C. What happens when you find a fossil mammal specimen that clearly represents something new but doesn’t preserve teeth, the parts used most often to recognize a mammal species? 

A situation like this arose with a former undergraduate student of mine several years ago. We were studying a partial skull of a small marsupial that preserved the bones of the snout and palate but only two, very heavily worn teeth; the remaining teeth fell out sometime between when the animal died and when this fossil was discovered. Although we were able to recognize that is represented a new species based on the proportions of its snout, the spacing of its tooth sockets, and other features, we decided not to name it because it did not include a single well-preserved tooth. We thought that having an essentially toothless holotype would make it too difficult for future paleontologists to determine with certainty whether other fossils that might be discovered – such as a partial jaw with a few teeth – pertained to this species or not. We also thought it would make it difficult to compare this new species to previously-named ones, nearly all of which have been distinguished based on characteristics of their teeth.

Was this the right decision? That is hard to say. Perhaps a specimen of this species will eventually be discovered with well-preserved teeth. If so, it could be made the holotype of a new species in the future. But maybe not. The fossil record is nothing if not unpredictable. Maybe this will be the only specimen of this animal that is ever found. In the mean time, we and other researchers are left having to refer to this new species by its museum catalog number – UF 27881 – rather than a scientific name. The main downside to this is that it is easy for a non-expert to miss that it represents a new species. After all, it doesn’t have a unique name.

Not creating a new species based on a fragmentary specimen is certainly justified. It isn’t possible to “upgrade” a poor holotype if a better specimen is discovered later on. A holotype is always a holotype. Thus, a poor holotype causes problems forever. Many early paleontologists coined new names based on poor holotypes, and later paleontologists have had to spend a lot of time dealing with these decisions. In some extreme cases, dozens of species have later been recognized as pertaining to a single valid species. Even when only two names refer to the same species, it can cause confusion among both scientists and the public. For a good example, how about when the animal called Brontosaurus had its name changed to Apatosaurus?

So what is a paleontologist to do?

They must use their judgement. Ay, there’s the rub.

Science is very seldom black and white. Some new species are a slam dunk, but most are not. A paleontologist has to play the odds and make a judgment based on the material they have available. It is always fun naming a new species. But it isn’t always the right thing to do.

References:

• Bengtson, P. 1988. Open nomenclature. Palaeontology 31:223-227.
• Croft, D. A., and F. Anaya. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology 26:387-399.
• Croft, D. A., F. Anaya, D. Auerbach, C. Garzione, and B. J. MacFadden. 2009. New data on Miocene Neotropical provinciality from Cerdas, Bolivia. Journal of Mammalian Evolution 16:175-198.
• Engelman, R. K., and D. A. Croft. 2014. A new species of small-bodied sparassodont (Mammalia, Metatheria) from the middle Miocene locality of Quebrada Honda, Bolivia. Journal of Vertebrate Paleontology 34:672-688.