Controls on Gut Phosphatisation: The Trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)

Abstract

Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagerstätten. However, the reasons for their fossilisation and their biological implications remain to be thoroughly explored. This is particularly true with arthropods – typically the most diverse fossilised organisms in Cambrian ecosystems – where digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on their biology. Here we describe the phosphatised digestive structures of three trilobite species from the Cambrian Weeks Formation Lagerstätte (Utah). Their exquisite, three-dimensional preservation reveals unique details on trilobite internal anatomy, such as the position of the mouth and the absence of a differentiated crop. In addition, the presence of paired pygidial organs of an unknown function is reported for the first time. This exceptional material enables exploration of the relationships between gut phosphatisation and the biology of organisms. Indeed, soft-tissue preservation is unusual in these fossils as it is restricted to the digestive structures, which indicates that the gut played a central role in its own phosphatisation. We hypothesize that the gut provided a microenvironment where special conditions could develop and harboured a source of phosphorus. The fact that gut phosphatization has almost exclusively been observed in arthropods could be explained by their uncommon ability to store ions (including phosphorous) in their digestive tissues. However, in some specimens from the Weeks Formation, the phosphatisation extends to the entire digestive system, suggesting that trilobites might have had some biological particularities not observed in modern arthropods. We speculate that one of them might have been an increased capacity for ion storage in the gut tissues, related to the moulting of their heavily-mineralised carapace.

Hurdiid radiodontans from the middle Cambrian (Series 3) of Utah

“…They are not interpreted as gut diverticulae, which are often preserved as high relief dark reflective material, as they do not intersect the gut, and are instead associated with the intersection of the body flaps with the cuticularized body, far from the body axis. However it must be noted that euarthropod gut diverticulae are preserved in a variety of ways (Lerosey-Aubril et al, 2012), and the preservation of this musculature is different from musculature reported from some other Burgess-Shale type localities:…”

Exceptionally preserved fossil assemblages through geologic time and space

“…Well-known fossils, which express evidence of multiple taphonomic processes, include the carbonaceous/ pyritized/aluminosilicified fossils of the Ediacaran Gaojiashan Member (Dengying Formation) in South China (Cai et al, 2012); the carbonaceous/aluminosilicified fossils of the Burgess Shale (Orr et al, 1998); the phosphatized/calcified fossils of Libros in Spain (McNamara et al, 2009(McNamara et al, , 2012b; the carbonaceous/phosphatized fossils of some Burgess Shale-type localities (Butterfield, 2002;Lerosey-Aubril et al, 2012); the phosphatized/pyritized/calcified fossils of the Eocene London Clay (Allison, 1988b); and the fossils of the Carboniferous Mazon Creek and Montceau-les-Mines biotas (Cotroneo et al, 2016;Perrier and Charbonnier, 2014). Other examples occur throughout the upper Neoproterozoic and Phanerozoic, such as phosphatized/silicified microfossils within chert nodules of the Ediacaran Doushantuo Formation in South China and carbonaceous/pyritized/ aluminosilcified fossils of the Cambrian Wheeler Formation in Utah (LoDuca et al, 2015;Fig.…”

Arthropod appendages from the Weeks Formation Konservat-Lagerstätte: new occurrences of anomalocaridids in the Cambrian of Utah, USA

“…Internal organs (e.g. digestive structures) are preserved with calcium phosphate (Lerosey-Aubril et al 2012) or more rarely with iron oxides (unpublished data). Exceptionally preserved organisms are otherwise preserved with iron oxides and/or an undetermined material (likely to be silicate) containing O, Si, Al, and Mg. All the specimens described herein are preserved as nearly two-dimensional compression fossils.…”

“…Until recently (Adrain et al 2009, the trilobite fauna had received little scientific attention since Walcott (1916a, b), with the exception of a couple of unpublished Ph.D. theses (Beebe 1990, Peters 2003. This trilobite fauna is remarkable for the high proportion of articulated specimens (Adrain et al 2009) and for the rare but exquisite preservation of digestive structures , Lerosey-Aubril et al 2012. The shelly fauna also includes abundant inarticulate brachiopods (Walcott 1908b, Rowell 1966, Streng & Holmer 2006, hyolithids (Walcott 1908b), sponge spicules, and rare echinoderms (Ubaghs & Robison 1985).…”