Iowa’s Enigmatic “Horse Collars”

I love museum collections. They are truly wondrous places that document the curiosities of nature as well as the stories of the people who have tried to better understand those tales. So you can imagine my delight when University of Iowa paleontologist Christopher Brochu dropped me off in the college’s fossil repository last Friday for some independent exploration. As Brochu led me into the study space, passing by mastodon bones laid out on a cart and a titanothere skull nestled in a corner passage, I started making a mental list of all the doors I was going to pry open just to see what was inside.

One cabinet door stood out from the rest. Most of the fossil storage containers bore scientific descriptors of their contents. Pleistocene Carnivora. Placoderms. Cretaceous Reptilia. But the label on one low door read “Horse Collars.” I had never heard of these before. I asked Brochu what was behind the little white door. He replied that didn’t know, either, and having other duties to attend to, left me among the fossils.

I didn’t open the door right away. My affection for megafauna got the better of me and I started pulling doors for the oversized mammals and other Pleistocene fossils, snapping photos of the giant beaver skulls, American mastodon limbs, and remnants of other beasts. But once I got my fossil mammal fix, I walked over to the horse collar cabinet. I couldn’t leave without peeking inside.

Just from the name, I expected the horse collars to be something like a U-shaped portion of turtle shell. Or maybe something like the “sand collars” I sometimes saw on the Jersey shore. But what I found was a drawer full of tiny vials containing individually-numbered black specks. On top lay a 1975 scientific paper with the mundane title “Fossils of uncertain affinity from the Upper Devonian of Iowa.” Not quite so exciting as a sabercat skull, but I read the abstract to make sure I wouldn’t miss anything before shutting up the shelf.

The paper starts, “Thousands of specimens of the enigmatic fossil Gluteus minimus…” I laughed in disbelief. This couldn’t be real, right? I’m not a prehistoric encyclopedia, but I figured that I would have heard a fossil named after a butt muscle which itself resembles a tiny posterior. But a quick check with my smartphone showed that paper was legit. In 1975, in the pages of Science, no less, paleontologists Richard Davis and Holmes Semken, Jr. really did name the curious fossils Gluteus minimus.

A snapshot of the Science paper describing Gluteus minimus, depicting the holotype.
A snapshot of the Science paper describing Gluteus minimus, depicting the holotype.

The tiny fossils weren’t a new discovery. In 1902, geologist Stewart Weller collected the earliest known specimens from an eastern Iowa site dating back to over 360 million years ago. Other experts picked up additional specimens over the years, building a collection of over 3,000 individual fossils, and the bitty specimens became known as “horse collars” before Davis and Semken, Jr. gave them a formal name.

But no one really knows what Gluteus minimus truly was. Davis and Semken, Jr. studied thousands in preparation for their paper and concluded that the bi-lobed fossils couldn’t confidently be assigned to any group of organisms. They could be fish scales, fish teeth, a sort of invertebrate called a brachiopod, or something else entirely. Over a century after their discovery, and almost 40 years since the species was named, Gluteus minimus is still an enigma.

I can’t hazard any better guesses. The few minutes I spent marveling at the small mysteries didn’t give me any clues or flashes of insight. I made sure all the little fossils were back in their vials and put the paper on top, carefully closing the drawer and cabinet door to not disturb the resting fossils any more than I already had. Someday, someone’s going to come along, open that door, and perhaps begin to figure out what these fossils truly were. Until then, the Gluteus minimus will quietly wait.


Davis, R., Semken, Jr., H. 1975. Fossils of uncertain affinity from the Upper Devonian of Iowa. Science. 187, 4173: 251–254. doi:10.1126/science.187.4173.251.

9 thoughts on “Iowa’s Enigmatic “Horse Collars”

  1. Very strange. I’m a little puzzled as to the name “horse collars,” though. I think “tiny butt muscles” would be much more descriptive!

  2. Zach Miller: Horse Collars are U-shaped… but a lot bigger than these. Probably a bit of an in-house joke.

    David Peters: That seems like a useful suggestion. Note that these are not split: with thousands of specimens to go on, I’d guess this is the unbroken shape. … I don’t know my paleoichtyology: did Devonian fishes have otoliths comparable to modern ones?

    Tom Kapitany: The abstract says “apatitic” constitution: so very likely made out of a mineral like that in vertebrate bone. What are brachiopod shells made of?

  3. To answer my question about the composition of brachiopod shells… apparently it depends on the brachiopod: some use calcium carbonate (like mollusk shells) and others use… apatite.
    But these objects look solid: not divided into an upper and lower valve.

  4. B, C and D look like internal casts of brachiopods. A looks like the shell of a brachiopod, but since it isn’t being called that, I’m guessing it is a solid piece. And then there is the black color, which no brachiopod I’ve run into has. Weird. B, C and D also look like something I have seen before, but not in jet black.

  5. I was thinking ‘brachiopod steinkern’ before seeing any of the previous comments. The colour isn’t necessarily an issue: on the south coast of NSW (Australia) there are intertidal exposures of Permian brachs where the calcite/aragonite(?) shells are weathered away rapidly, leaving external impressions and internal moulds (steinkerns) in hard blackish rock. I never studied invert fossils particularly, but that’s what these tiny bums remind me of. One of the things they remind me of.

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