C U L T U R A L H E R I T A G E , A R C H A E O L O G Y A N D P A L A E O N T O L O G Y

S C I E N T I F I C H I G H L I G H T S

1 5 4 H I G H L I G H T S 2 0 2 3 I

X-ray microtomography shines a light on a trilobite s last supper

Trilobites are fossil arthropods (animals related to spiders, crabs and insects) that were important members of early marine ecosystems. Until now, their diet has been unknown, but X-ray microtomography of the trilobite Bohemolichas has enabled the identification of shell fragments in its gut, deepening our understanding of this ancient creature.

Animals first rose to prominence during the Cambrian Explosion about 520 million years ago. A major component of these earliest animal faunas were the trilobites, which looked rather like large woodlice. They diversified rapidly into different sizes and shapes, and remained abundant for about 100 million years, before going into slow decline and finally disappearing some 250 million years ago. Clearly, if we want to understand these early animal- dominated ecosystems, we need to understand what the trilobites were doing, but this presents a problem: we don t know what they ate. The best evidence for the diet of an extinct animal is fossilised gut content, but despite millions of trilobite fossils having been found over the past 200 years or so since the group was first recognised in the fossil record, not a single describable gut content had been discovered.

The challenge with fossil gut contents is not just to find them, but to study them without destroying the fossil. Typically, they consist of small, chewed-up pieces of whatever the animal was eating, scattered through the rock in the area where the gut used to be; it is almost impossible to extract them without destroying both the fossil and the gut contents.

High-resolution synchrotron X-ray microtomography provides the ability to image small fossil objects inside blocks of rock without extracting them, thus enabling fossils to be studied without destroying them. Following previous work imaging early fossil fishes at the ESRF [1], a specimen of the trilobite Bohemolichas, which showed some small shell fragments scattered along the body axis, looking like a possible gut fill, was scanned using synchrotron microtomography at beamline ID19.

The results of the scan enabled researchers to visualise and identify almost every shell fragment in the gut fill, giving them the first-ever comprehensive menu of a trilobite (Figure 126). It turns out to have been a voracious and indiscriminate feeder, ingesting whatever small animals it came across on the sea floor that were small enough to swallow whole or fragile enough to crush with its rather weak mouthparts (Figure 127). The contents include bivalves, ostracods (small bivalved crustaceans that still exist today), stylophorans (extinct echinoderms related to sea urchins and starfish) and hyoliths (cone-shaped invertebrates with no living relatives).

The speed of food intake, which left the entire gut packed with shelly debris, seems excessive and raises the question of why the trilobite was behaving this way. A possible clue is given by an odd kink in the skeleton, which looks like it is starting to pull apart. Like all arthropods, trilobites grew by periodically shedding their hard exoskeleton. Some modern arthropods help the process along by inflating their gut with water or air, creating pressure that cracks and pops off the old skeleton; it may be that Bohemolichas was over-feeding for the same reason and was just starting to shed its exoskeleton when it was entombed and killed by an underwater mud slide.

Fig. 126: 3D rendering of trilobite with gut content (colour-coded for different food species) in ventral view. Scale bar 10 mm.