Sea Mink Stories: Part 2

In the second installment of Sea Mink Stories, we explore ecomorphology... what will thousand-year-old sea mink bones tell us about this extinct creature's daily life?

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The lab work of the summer is officially in full swing. We (my research advisor, Alexis Mychajliw, and I) have returned to Vermont after a weeklong road trip in Maine (see Blog 1). We arrived at Middlebury College with two boxes of zooarchaeological specimens from Dr. Arthur Spiess at the Maine Historic Preservation Commission, all to be cataloged and measured by me, the HEDGE lab’s summer research assistant. These specimens come from three shell middens along the coast of Maine. A variety of animals are represented in our sample: there are many deer, beaver, rodents and seals mixed in with the less common porpoise, puffin, great auk, bear, river otter, and turtle. Of the 900+ elements I recorded, 385 belong to various species of mink, 80+ of which I was going to measure. I measured some of those elements in 20 ways - needless to say, my summer’s work was cut out for me. 

My workspace at Middlebury College. 

A defining circumstance of paleoecology is that the specimens from a paleontological site represent only some what was alive at the time of study due to chance and preservation biases (i.e., taphonomy). This means that researchers can’t go back 2000 years to find a perfect sample size of complete sea minks and measure every bone we’re interested in studying; we work with fragments. When I pick up one of the many Ziploc bags in our sample, I may find the distal end of a mandible, the lingual surface of a molar, or an ambiguous premolar. Whatever I can measure, I measure. Like a game of telephone, the sea mink whispers into my ear and I pass on the story I heard. 

An almost complete left mandible from a sea mink. Photo by O. Olson. Specimen from the Maine Historic Preservation Commission. 

We can then bring our taxa to life, so to speak, by comparing them with studies from living animals that do have all the details recorded. After decades of research on the relationships between the size, shape, and ecology of animals, ecomorphologists have developed equations that can predict how an animal moved and what it might have eaten based on its morphological attributes. Some of these equations tell us the available space for muscles to insert on a bone, which is indicative of relative size and strength of limbs1. Or that the shearing blade of a tooth is much bigger than the grinding area of the same tooth, telling us that the animal ripped through flesh more often than they ground up hard food matter2.

The American mink skull (left) is much smaller and shallower than the American marten skull (right). Photo by O. Olson.

For example, sea otters (Enhydra lutris) (Mustelidae cousins of our mink friends) have short snouts and a great bite force, which they use to open the hard shells of molluscs3. In contrast, river otters (Lontra canadensis) have longer snouts and a weaker but faster bite - good for catching quick fish as they swim. With the size and shape of jaw bones, we can estimate the bite force of sea minks and compare them with those of American minks. Maybe, like the sea otter, sea minks had to bite into tougher prey. Maybe their teeth had a large grinding area to chew up big codfish bones. 

The snout of the sea otter (left) is shorter and more robust than that of the river otter (right). Sea otter photo source: https://www.inaturalist.org/observations/88595455. River otter photo source: https://www.inaturalist.org/observations/88652403

Long bones tell us a different story. Whereas a tooth can describe how an animal ate, a radius can describe how the animal moved through its world. Consider the difference between the radius of a seal and a deer. The seal slips through the water, paddling efficiently with its short, wide forelimbs. On land, the deer tiptoes quietly through the forest until it hears the rustle of a bear, cueing it to bound away on long, thin legs. Both radii are suited for the setting and activities of each respective animal. The differences between two mink species radii will be subtler than those between two distantly related mammals, but we may find some parallels. Maybe the surfaces where muscle attached on the will be proportionally bigger than those on the American mink. These differences could mean that the sea mink had a more powerful kick than the American mink when swimming in ocean currents using wide, strong paddles. 

An American mink swimming. How would a sea mink swim differently? Photo source: https://www.inaturalist.org/observations/10385898 

These pieces of anatomical information along with historical accounts of sea mink will help us paint a picture of the sea mink and what life it must have led. The picture might also help us answer questions about its extinction, about its relationships to humans, and the niche it left behind for other small, furry carnivores. Perhaps this is why I was so giddy when I saw my first sea mink bones: I could imagine the sea mink scurrying in and out of the rugged Maine coastline, as I had seen squirrels do countless times growing up. On the other hand, the animals that left behind million-year old fossils are unimaginable to me. Giant lizards with feathers and tiny arms? Ridiculous. This extinction is close behind us - so close that I can see and feel the weight of its loss. A piece of Maine was forgotten, only to be brought back with fragments of bone and stories of old trapping days on the northeast coast. 

The next phase of this project will be to build the life story of the sea mink. What can I tell my neighbors in Maine when I go home after all of this is done? Where will they see themselves in the story of the sea mink? In the coming months, I will travel and talk with fellow Mainers about the journey the sea mink and I have shared: about my work and the fascinating tale of the sea mink.

1. Samuels, J. X., Maechen, J. A., & Sakai, S. A. Postcranial morphology and the locomotor habits of living and extinct carnivorans. J. Morphol. 274, 121-146 (2013).  

2. Friscia, A. R., Van Valkenburgh, B., & Biknevicius, A. R. An ecomorphological analysis of extant small carnivorans. J. Zoo. 272, 82-100 (2006). 

3. Timm-Davis, L. L., DeWitt, T. J., & Marshall, C. D. Divergent skull morphology supports two trophic specializations in otters (Lutrinae). PLoSONE 10, e00143236. doi:10.1371/journal (2015). 

Olivia Olson

Student, Middlebury College