Homalonotids: Large Weird Trilobites

Fear has many eyes and can see things underground. –Miguel de Cervantes

Trimerus, Rochester Shale, Silurian Period, New York
Trimerus delphinocephalus, Rochester Shale, Silurian Period, Middleport, New York. This homalonotid trilobite occurs alongside such familiar forms as Dalmanites and Calymene. At this scale of observation the exoskeleton looks smooth, but under a hand lens it is covered in tiny pustules. Specimen is 17.3 cm long.

Homalonotids are well-known fossils of Silurian and Devonian age from around the world. Despite occurring in deposits alongside other more typical-looking trilobites, they have a number of unusual features.

Many specimens show “indistinct trilobation,” giving them a streamlined torpedo-like appearance. They generally lack spines, although some “Burmeisteria armatus” (aka “Elvis”), widely faked and composited specimens from Morocco, apparently have short, stout spines. Such streamlining (in all but Elvis) could be used to make a case for a burrowing lifestyle.

Dipleura dekayi, Devonian Period, New York
Dipleura dekayi, Skaneateles Formation, Devonian Period, Hamilton County, New York. Here trilobation is nearly absent. Specimen is 16 cm long.

What gives pause to the notion of burrowing, however, is the pitted orange-peel texture exhibited by some species. There are a variety of types of pores and canals, often associated with bumps or pustules, that perforate the exoskeletons of trilobites. Interpretations of the functions of these structures vary and include openings for the diffusion of oxygen, a chemosensory function, secretion, and most often setae (hair-like filaments or bristles) that could have had a protective or sensory function.

Dipleura detail, Devonian Period, New York
Orange-peel skin: Dipleura dekayi detail showing pitted surface texture. Was this trilobite covered in hair-like filaments?

Presence of bristles over the surface of the body would seem to be at crossed purposes with a burrowing lifestyle where smoothness would most helpful. Perhaps the pores of such animals as Dipleura just allowed easier diffusion of oxygen through the shell to the gills below and are unrelated to setae. Maybe a secreted slime layer flowed through the pores and allowed easy movement through a gritty substrate. Or perhaps they were for setae–but allowed a buried, often immobile, animal to sense prey or predators in the surrounding sediment. We will likely never know.

In the seascape of my imagination, though, homalonotid trilobites like Dipleura were covered in hairs like giant asp caterpillars wandering the seabed. Perhaps, like asps, these trilobites, too, were venomous–offering up the most unpleasant possible mouthful for any passing monster cephalopod or placoderm.

"Homalonotus," Devonian Period, Morocco
“Homalonotus,” Devonian Period, Morocco. This specimen has a smooth exoskeleton, unlike the Dipleura above–there is no reason to think that this animal was hairy. Axial length of pygidium is 19mm.

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

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