by Rachel Narducci and Megan Higbee Hendrickson
During the morning hours of Thursday, August 13, 2015 a 6th and 7th grade science teacher at the Academy of the Holy Names, Megan Higbee Hendrickson, discovered a right partial Mesohippus mandible, including the 4th premolar to the 3rd molar, eroding out of the Chadron Formation in Northwestern Nebraska directly beside an impressive titanothere skeleton. How much more perfect could it get! The Chadron Formation has been defined, faunally, as the time during which titanothere’s and Mesohippus had overlapping ranges by Wood et al. 1941, and is largely known for producing vast quantities of fossils from these two species. However, these definitions ultimately do not work as they cause confusion to the boundaries; Mesohippus is known from older strata and titanothere’s exist in strata younger than the Chadron Formation. The boundaries of the Chadron Formation are now faunally defined by the first appearances of different, less abundant, taxa. But that doesn’t mean titanothere’s and Mesohippus remains are any less plentiful within the strata.
Mesohippus was a ‘medium dog-sized’ perissodactyl (odd-toed ungulate) that lived from the late Eocene into the Oligocene and was common in the grasslands of North America. Mesohippus means “middle horse” because it is a transitional form between the smaller unfamiliar horses of the Eocene (Hyracotherium) and the larger recognizable horses of today. It is also the first of the three-toed horses. The older horses of the Eocene had four toes, while the modern horses of today have only one. The premolars of Mesohippus also became more “molariform”; possibly because they were adapting a more limited diet of leaves; abandoning a more diverse diet including fruit. The image of the mandible in this article displays a great example of how the 4th premolar is “molariform”; it looks almost identical to the 1st molar.
I was thrilled to receive an invitation to attend the Nebraska Badlands Collaborative Field Conference. Since my first experience collecting fossils in Panama with the GABI-RET program, my love of fossils and thirst to learn has become stronger with each field experience I’ve had. To say I learned a great deal about geology on this trip would be an understatement. As a teacher, I am constantly watching my students for the “Ah ha!” moments, the moment when the content suddenly becomes clear to them. That moment is exactly how I describe my Nebraska experience to my students. Being a native Floridian, my geology knowledge has come almost solely from books. Working in the Badlands and being able to see four mammal ages in the rocks was truly incredible. Working along side scientists who have a desire to foster interest in their field creates a constant buzz of excitement among all of the participants at each of the field localities.
Our second field location in Nebraska was called Sand Creek Flats North and was comprised of fossils from the Chadronian Land Mammal Age. As groups split up and began their hunt, I found myself chatting with Sue Hutchins. She was familiar with the area where we would be looking and wanted to show me where a titanothere skeleton was exposed in the side of the hill. Knowing this could be a great photograph to share with my students I happily followed her to the fossil. While the rest of the group remained admiring the titanothere, I began to wander away from the group taking pictures. The surface of the ground was very bright white and I noticed a dark brown line in the dirt. Upon closer inspection I realized that what I was looking at was a mandible with four teeth, I had found Mesohippus. After applying some glue and doing a little digging I removed the mandible, bagged it, and am anxiously awaiting the photographs from its preparation.
The 2015 PCP PIRE/GABI-RET/FOSSIL Project Collaborative Conference in the panhandle of Nebraska was an absolutely amazing experience. Working as an assistant preparator in the vertebrate and invertebrate prep labs at the Florida Museum of Natural History means that I receive fossils, clean and glue them, and then pass them off to be cataloged into the collection. Actually being able to talk to educators and see how they can pass this knowledge of geology and paleontology onto their students was really eye-opening.
Once we safely received the fossils collected during this conference in the lab I located the Mesohippus mandible that Megan discovered and carefully unwrapped it from the protective toilet paper into a box. There were about 10 smaller fragments that we picked up in the field (just in case they were a part of it) along with the bulk of the mandible, which was coated in B-72. B-72 is a plastic based glue mixed with acetone to dilute it to varying levels of thickness (in the field we used a relatively thicker mixture). I then placed the mandible on top of plastic wrap in a sand box. We use sand boxes, rather than flat surfaces, so that we are able to stand a fossil up to allow the glue to completely dry and so that the sand will give way with any pressure exerted onto the fossil during preparation. Looking under a microscope, I began using acetone with a paintbrush to reverse some of the B-72 in order to remove the surrounding matrix. I also used a sharp carbide handheld tool to reach tiny areas and to remove harder concretions of matrix. Using these tools, I cleaned the individual pieces of the mandible as much as possible before gluing them back together. To attach these pieces I used super glue, instead of B-72, because at this point we do not want the glue to be as reversible.
It is also helpful to understand that even with careful prep work in a lab setting fossils will typically not look ‘perfect’. The pressure exerted on fossils as they are overlain with strata can be immense, causing deformation, so that they are not the same shape as they were originally. Because of this fragments of fossils might have attachment points but will typically not fit perfectly back together. Also, as fossils erode out of the ground, rain or other disturbances can wash away or destroy some of the fragments. With that being said it is extremely exciting to find a decent fossil in the field, and to be able to remove it, rebuild it, and identify it!