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

Abstract

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.

Caught in the act: priapulid burrowers in early Cambrian substrates

“…There has been a consistent trend in the literature to associate treptichnids or other Cambrian burrows with priapulids (e.g. [8,9,33,40,62]). This assignment has, however, not hitherto rested on a particularly firm basis.…”

Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period

“…However, there are limited (and perhaps contentious) examples of gut pyritization 47 (but see also ref. 48 ) as well as gut-content pyritization 46 . The consistent geopetal nature of the pyritized soft-tissue structures observed here supports the notion that they were originally centrally located structures in vivo, rather than adjacent to the exterior tube wall.…”

A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources

“…The Cambrian colonization of infaunal and meiofaunal niches is thought to have been increasingly enabled by the oxygenation of marine sediments during the Cambrian agronomic revolution [47,53] and these niches were evidently well established by the mid-Cambrian in the Burgess Shale ecosystem and elsewhere [24]. The inferred ecology of Cambroraster indicates that the evolution of large nektobenthic consumers, alongside smaller carnivores like trilobites [54], occurred in tandem with the radiation of these prey, in line with hypotheses emphasizing the catalysing effects of escalation during this radiation [55]. As large and abundant nektobenthic carnivores, hurdiids like Cambroraster likely had a considerable impact on the local benthic community through both predation and bioturbation.…”