MICROTOMOGRAPHIC STUDY OF OLENELLOID TRILOBITE GUTS FROM THE LOWER CAMBRIAN ROSELLA FORMATION IN BRITISH COLUMBIA

Trilobites are iconic fossils of the Palaeozoic era, but their internal anatomy is poorly understood, especially their digestive system, which is partially documented in c. 0.2 percent (c. 50 species) of all described species. Fossilized trilobite guts have been chiefly recovered from Lower Palaeozoic conservation deposits in North America and South China. Typically, they consist of a simple, undifferentiated intestine flanked laterally by numerous pairs of metamerically disposed digestive glands. The presence of a crop in some trilobites (e.g., phacopids) can be indirectly inferred from the observation of a large glabellar frontal lobe, sometimes bearing muscle scars, but direct evidence for this remains debatable. A good understanding of the mode of preservation of putative internal organs – and its limitations – is crucial to reconstruct the internal anatomy of trilobites.

We report the discovery of phosphatized guts in 25 trilobites from the McDame biota, a Cambrian Age 4 Burgess Shale-type fossil assemblage recovered from the Rosella Formation in northern British Columbia. The specimens, which belong to two olenelloid species, were investigated using X-ray computed microtomography, allowing the visualization of the 3D organization of the gut remains. A few individuals exhibit a noticeably enlarged gut tract under the frontal glabellar lobe, but the shape of this crop is obscured by its close association with the hypostome. Despite a fossilization mode (phosphatization) favorable to their preservation, the only evidence for putative digestive glands in these trilobites is the rare presence of digitiform structures projecting from the gut tract immediately posterior to the crop. In the post-cephalic region, the gut solely consists of a thick intestine running up to the tip of the pygidial axis. This atypical gut architecture likely relates to the evolution of carnivorous, possibly predatory aptitudes in olenelloids, a previously proposed hypothesis that finds support in the discovery of a small bivalved carapace in the crop of a single Rosella trilobite.

Our microtomographic investigations also revealed a diversity of shelly fossils and variously sized burrows in the vicinity of the trilobite carcasses, suggesting that this technique could provide new insights into the environmental conditions and taphonomy of this formation.