The Uniquely Spiny Thysanopeltis

Always remember that you are absolutely unique. Just like everyone else. –Margaret Mead

Thysanopeltis, Devonian Period, Morocco
Thysanopeltis sp., Hamar Laghdad Formation, Devonian Period, Morocco. Trilobite is about 80 mm long.

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.

Thysanopeltis: detail of margin of pygidium, Devonian Period, Morocco
Thysanopeltis sp.: detail of spiny margin of pygidium, Hamar Laghdad Formation, Devonian Period, Morocco.

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.

Pygidium of Platyscutellum, Devonian Period, Morocco
Pygidium of Platyscutellum, AM Limestone Formation, Devonian Period, Morocco. Platyscutellum is not a common trilobite and has a row of small spines down the axial lobe, but like most other scutellids no marginal pygidial spines at all. Pygidium is 45 mm across at its widest.

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.

Scabriscutellum furciferum, Devonian Period, Morocco
Scabriscutellum furciferum, Hamar Laghdad Formation, Devonian Period, Morocco. Some specimens of Scabriscutellum have small stout spines projecting from the tops of the eyes and occipital ring, but this one does not. S. furciferum is a common species with no marginal spines. Specimen is 40 mm long.

©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

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