Always remember that you are absolutely unique. Just like everyone else. –Margaret Mead
Spines are a persistent preoccupation of the trilobite enthusiast. Scutellids, by and large, are not known for significant spininess, although the group is among the most ornamented. Members bear every conceivable form of prosopon including pustules, terrace lines, and pygidial ribs. There are spiny exceptions, however, like Weberopeltis from the Silurian of Russia, Kolihapeltis from the Devonian of Morocco—and of course, Thysanopeltis.
In the case of each spiny scutellid, though, the arrangement of spines is very different. Weberopeltis has long marginal spines projecting backwards from the pygidium as extensions of pygidial ribs, as well as spike-like spines projecting backwards from the glabella and occipital ring. Kolihapeltis has large spines projecting backwards from the tops of the eyes and the occipital ring of the cephalon—but no marginal spines around the pygidium. Thysanopeltis is unique in the scutellid group and unusual among all trilobites in having numerous small spines fringing the pygidium.
In imagining the purpose of the marginal spines of Thysanopeltis it’s logical to consider the case of enrollment. Clearly an enrolled Thysanopeltis would have a well projected “zone of weakness” between the cephalon and pygidium, a “picket fence” if you will. Why this trilobite needed such a feature and other scutellids did not is, of course, completely unknown. Absent a breakthrough in our understanding in the functional morphology of the trilobite exoskeleton, all we can do is enjoy the fantastic diversity of our favorite arthropods.
©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.