Gravicalymene are often confused with their better known “cousins”, Flexicalymene. Many amateurs confuse the two, and I have seen a few Gravis improperly labeled as Flexis. Probably, the easiest method to distinguish the two is the shape of the glabellar furrow. In Gravicalymene, the furrow has a distinctive bell shape; in Flexicalymene, the furrow is more trapezoidal or triangular shaped. But sometimes those distinctions aren’t apparent.
This Flexicalymene retorsa was collected from the Arnhiem Fm near Cincinnati Oh. It appears to have the classic “bell” shape of the Gravis, but look at the furrow on the left side of the image-the trapezoidal shape is seen.
Here is a Gravicalymene trucatus(?) that was collected from the Millersburg Mb of the Lexington Limestone, in Fayette Co. Ky., for comparison:
This is a molt that was collected beneath a stromotoporoid. The bell shape is there, but to avoid any confusion, the pygidium of Gravicalymene has five interpleural furrows, Flexicalymene has 6 or 7.
Evidence seems to suggest(published soon???) that trilobites would seek out safe havens during ecdysis. Hence, the critters found beneath sponges, bryozoa, and other structures.