#gluteus

Despite the caption, these are specimens of Gluteus minimus (source)

Species:G. minimus

Etymology: Presumably named for the fossils’ resemblance to butts

Age and Location: Devonian of Iowa

Classification:Eukarya: Metazoa: ?Bilateria: incertae sedis

Thousands of these fossils, also known as “horse collars,” have been collected, and every single one of them is asymmetrical in the same direction. Because most organisms are symmetrical, fossils tend to be either symmetrical or come in mirror-image pairs, so that alone makes these strange. Some invertebrates, especially mollusks, areasymmetrical, however, though even in asymmetrical mollusks different individuals are mirror images of one another.

These fossils are solid, with internal structure, so this is not the external appearance of some bivalve or brachiopod. In fact, no animal hard part ever found bears a plausibly close resemblance to these fossils. One surface has growth lines, which provides the only clear evidence of any sort of biology for whatever organism produced these. Of all the various unlikely options, perhaps the most plausible is that it is an extremely unusual gastropod shell of some kind, but the microstructure of the shell is nothing like that of a mollusk, so these remain a total mystery.

Sources:

Davis RA., Semken HA. 1975. Fossils of uncertain affinity from the Upper Devonian of Iowa. Science 187:251–254.