April 5. More trilobites

Trilobites continued a remarkable diversification during the Silurian. Some, such as Staurocephalus murchisoni, had these big bulbous things like noses at the front of their bodies. It’s actually the glabella, which in turn is the central, axial portion of the cephalon, or head. The sketches here give you an idea how bizarre it looks. 

Staurocephalus murchisoni (left); Deiphon forbesi (right)

Many trilobites have a simple dome-like glabella, slightly raised above the side parts of the cephalon, or head, where the eyes are. The glabella apparently covered and contained some of the trilobite’s forward digestive organs – the stomach, which was above and forward of the mouth. Exactly how it worked isn’t clear, at least not to me after checking around – if any trilobite specialist hears this and can enlighten us, please contact me at rigibson@earthlink.net. But at a first pass, it seems that the glabella is more or less the stomach, or maybe part of the intestine. It’s been speculated that larger glabellas indicate more complex food sources, which some infer to mean that the trilobite in question might have been carnivorous – but that seems to be a bit of a stretch to me.

How and why did these bulbous glabellas evolve into knob-like protrusions extending away from the animal’s body? Beats me.

—Richard I. Gibson

See also:

Richard Fortey, Lifestyles of the trilobites (PDF)
The drawings above are from a 19th century textbook.