Data from: Gut evolution in early Cambrian trilobites and the origin of predation on infaunal macroinvertebrates: evidence from muscle scars in Mesolenellus.

Trilobites are particularly common Cambrian fossils, but their trophic
impact on the rapidly evolving marine ecosystems of that time is difficult
to assess, due to uncertainties on how diverse their feeding habits truly
were. Gut anatomy might help to constrain inferences on trilobite feeding
ecology, but preservation of digestive organs is exceedingly rare. Muscle
scars on the glabella, known as ‘frontal auxiliary impressions’ (FAIs),
have been interpreted as evidence of the evolution of a pouch-like organ
with powerful extrinsic muscles (i.e. a crop) in some trilobites. Here we
describe FAIs in Mesolenellus hyperboreus from Cambrian Stage 4 strata of
North Greenland, which represents the oldest example of such structures
and their first report in the Suborder Olenellina. Mesolenellus FAIs
suggest that the crop in trilobites was clearly differentiated from the
rest of the digestive tract, and essentially located under a hypertrophied
glabellar frontal lobe. Reviews of the digestive anatomy of trilobite
sister-taxa and the glabellar morphology of the oldest-known trilobites
suggest that the gut of the trilobite ancestor was an essentially simple
tract (i.e. no well-differentiated crop) flanked laterally by numerous
midgut glands. A crop first evolved in the Cambrian in groups like
olenelloids and (later) paradoxidoids. Using ichnological evidence, we
hypothesize that the emergence of olenelloids yields evidence for the
evolution of predatory inclinations in a group of arthropods originally
dominated by surface-deposit-feeders. By allowing the exploitation of a
rapidly developing food source, infaunal animals, the diversification of
feeding strategies in trilobites might partially explain their
unparalleled evolutionary success.