Saturday 17 March 2012

Preserved Trilobite digestive tracts from the Middle Cambrian of Utah.

Trilobites were a group of Arthropods that flourished throughout the Palaeozoic, but died out at the end of the Permian. They are abundant and well studied fossils, but little is known of their internal anatomies.

In a paper published in the journal PLoS ONE on 14 March 2012, a team of scientists led by Rudy Lerosey-Aubril of the Department of Palaeontology and Historical Geology at the Senckenberg Research Institute in Frankfurt am Main, describe the discovery of a large number of Trilobites belonging to three distinct species, found with well preserved digestive tracts in the Middle Cambrian Weeks Formation in Utah.

The Weeks Formation is a conservation Lagerstätten in the House Range of Central Utah, noted for producing well preserved trilobites and other arthropods. It is well known by, and popular with, amateur fossil collectors, but has received little academic attention.

Lerosey-Aubril et al. report the presence of entire preserved digestive tracts in a large number of Trilobites of the species Meniscopsia beebei, Coosella kieri and Genevievella granulatus, as well as a number of non-trilobite arthropods. These reveal the trilobites had simpler digestive systems than modern Chelicerates (the group that includes Trilobites, Horseshoe Crabs and Arachnids).

Photographs (above) and interpretive drawings of two specimens of Meniscopsia beebei. Scale bars 5 mm. Pairs of digestive caeca are numbered from front to rear. From Lerosey-Aubril et al. (2012).

The digestive tracts of the trilobites are phosphatized, a common form of preservation in Cambrian fossils, but not one which is usually so selective; none of these Trilobites show soft-tissue preservation beyond the digestive system, and their exoskeletons, usually the best preserved part of trilobites, are thin and poorly preserved. This form of preservation in a single specimen might not attract much attention, but in a large number of fossils is remarkable, and suggests there was something special about the digestive tracts of these Trilobites.

The most obvious solution to this would be if the Trilobites were ingesting a large amount of phosphates in their diet. Since there are no entire preserved phophatized organisms in the Weeks Formation, this is unlikely to be from a prey item, though it could be from ingested phosphor-rich sediments. However only a single specimen was found with evidence of sediment in its gut, suggesting that they did not generally ingest sediment as part of their normal behavior.

Lerosey-Aubril et al. suggest that the Trilobites may have used their guts as a reserve of phosphate minerals while going through their molt cycle. Modern Crustaceans withdraw minerals from their shells before molting, so it is not unreasonable to assume that more heavily biomineralized Trilobites would have done the same, and that they would have needed a reserve in which to store the absorbed minerals (something some modern Crustaceans also do).

Digestive system of the trilobite Meniscopsia beebei in dorsal, right lateral, and ventral views (from left to right). The foregut and hindgut are in bright pink, the midgut tract in blue-violet, and the midgut caeca/glands in lavender. From Lerosey-Aubril et al. (2012).