The Breaks local fauna and North American Land Mammal “ages”

Many years ago, I wrote a dissertation. That was the final step toward my ultimate goal of becoming a ‘real’ paleontologist. My research was strictly biostratigraphy: putting rocks and fossils in order by age from oldest to youngest. Specifically, I was looking at mammal fossils from the middle of the Paleocene, about 60 million years ago. I worked mostly with isolated mammal teeth, almost all of which were small enough to be mounted on the heads of pins. I spent about two years doing little more than looking at little tiny teeth though the microscope, taking notes and measurements and identifying them. I had more than 2000 teeth to go through. In the end, I identified about 800 of them into 72 unique species. It was a big deal.

The teeth came from any of 136 localities found in a few square miles in the Hanna Basin of Wyoming, in a little area of badlands called “The Breaks.” I spent a couple of summers walking out the rock units and tying all the localities together, in order. They spanned about 500 meters of thickness in the Hanna Formation, a ~3000 meter thick rock unit known to span from the middlish Paleocene into the early Eocene. Why vertebrate fossils only occupy 500 meters of thickness of the Hanna Formation is its own interesting geochemical puzzle, which later became my Master’s thesis in chemistry. I’ll post on that some other time.

Once I had all the mammal fossils identified and the localities in order, I created some charts to show the distribution of the fossils over time. I already knew, based upon some important species present (Plesiadapis, Tetraclaenodon, Pronothodectes, Phenacodus), that the rocks represented both the Torrejonian and Tiffanian North American Land Mammal “ages,” or NALMas. I knew that somewhere in those 500 meters of rock was the boundary between the two NALMas. My research project was to characterize that boundary.

What are North American Land Mammal ‘ages’?

North American Land Mammal ‘ages’ (NALMAs) are divisions of time, specific to North America, based upon mammal species present.

The individual ages are usually defined by the first appearance of one species. Since I was looking for the transition from the Torrejonian to the Tiffanian, the species I was looking for was Plesiadapis praecursor. The presence of Plesiadapis is considered indicative of the Tiffanian.

Other species are considered characteristic of specific NALMas. For example, Tetraclaenodon (a the time) was considered an index taxon for the Torrejonian. If you had Tetraclaenodon, you were in the Torrejonian. Phenacodus, which evolved from Tetraclaenodon, was present only in the Tiffanian (or so they said).

Most NALMas can be subdivided into smaller parts, each with their own Characteristic species. The Tiffanian has six parts, Ti1-Ti6, and the Torrejonian three, To1-To3.

The Breaks

With so many localities, so many specimens, and so many species, I was in the unique position of studying whether this notion of index species for adjacent NALMas was valid. It’s hard to imagine a whole community of mammals suddenly being replaced (or evolving rapidly into) a whole new community of mammals. I had some of the key players in my fauna. I could find out.

I had intentionally done all of my identifications without knowing the stratigraphic position of the fossils. I didn’t want to get caught in the circular argument of making identifications based upon some preconceived notion of the age of the locality, then using the identification to assign an age to the locality. Yes, it’s happened before. I didn’t want to fall into that trap. In doing so, I made some important discoveries:

• The transition from the Torrejonian into the Tiffanian was not instantaneous. I estimate to took about a half-million years or so. There were species considered ‘index’ taxa for both the Torrejonian and the Tiffanian living side-by-side for quite a while. I called this the ‘Overlap Zone.’
• It turns out that using Tetraclaenodon and/or Phenacodus to assign an age is really bad, because unless you’ve got the exact right tooth, you can’t tell the difference between these two genera. In fact, I’m not convinced that they’re different genera at all.
• I did decide to stick with using the first appearance of Plesiadapis as a marker for the beginning of the Tiffanian. I can’t think of a better way to do it.

In April is the Blogging from A to Z challenge. For giggles, I’ve decided to use Paleocene mammalian species as the theme. As much as possible, I’ll be dipping into my old notes and using species from The Breaks. If there isn’t a representative species from The Breaks, then I’ll go with a North American species. For a few letters, I’ve had to broaden the scope and consider species from other continents. I’m hoping that not only will this be a fun way to revisit my doctoral research, it might also lend readers an idea of what the practice of paleontology is really like.