Controls on Gut Phosphatisation: The Trilobites from the Weeks Formation Lagersta ¨tte (Cambrian; Utah) Rudy Lerosey-Aubril 1 *, Thomas A. Hegna 2,3 , Carlo Kier 4 , Enrico Bonino 4 , Jo ¨ rg Habersetzer 5 , Matthieu Carre ´ 6 1 Department of Palaeontology and Historical Geology, Senckenberg Research Institute, Frankfurt am Main, Germany, 2 Department of Geology and Geophysics, Yale University, New Haven, Connecticut, United States of America, 3 Department of Geology, Western Illinois University, Macomb, Illinois, United States of America, 4 Back to the Past Museum, Puerto Morelos, Quintana Roo, Mexico, 5 Department of Palaeoanthropology and Messel Research, Senckenberg Research Institute, Frankfurt am Main, Germany, 6 Institute of Evolutionary Sciences, University Montpellier II, Montpellier, France Abstract Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagersta ¨ tten. 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 Lagersta ¨tte (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. Citation: Lerosey-Aubril R, Hegna TA, Kier C, Bonino E, Habersetzer J, et al. (2012) Controls on Gut Phosphatisation: The Trilobites from the Weeks Formation Lagersta ¨tte (Cambrian; Utah). PLoS ONE 7(3): e32934. doi:10.1371/journal.pone.0032934 Editor: Carles Lalueza-Fox, Institut de Biologia Evolutiva - Universitat Pompeu Fabra, Spain Received October 20, 2011; Accepted February 1, 2012; Published March 14, 2012 Copyright: ß 2012 Lerosey-Aubril et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was financially supported by the Senckenberg Research Institute, the Back to the Past Museum, Yale University, and the University of Montpellier II (ISEM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: leroseyaubril@gmail.com Introduction Background The Cambrian explosion was a major episode of biological diversification, which was associated with the development of complex marine ecosystems [1]. During this ecological revolution, the diversification of feeding strategies no doubt played a major role, but the importance of this diversification remains difficult to assess. Indeed, the unusual morphology of Cambrian metazoans prevents easy comparisons to modern analogues and rarely provides unambiguous clues about their feeding habits. This is particularly true with arthropods, typically the most diverse fossilised organisms in Cambrian ecosystems [2,3], which commonly lack the kind of appendage specialization that characterises feeding strategies in modern arthropods. Fossilised digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on the biology of primitive arthropods [2,4]. Despite being internal organs, their fossilisation is unusually common in Cambrian Lagersta ¨ tten. However, the reasons for this preservation and the biological implications remain to be thoroughly explored. Here we describe phosphatised digestive structures of three species of trilobites from the Middle Cambrian Weeks Formation Lagersta ¨ tte (Utah). Their exquisite, three-dimensional preservation allows unique details on trilobite internal anatomy to be revealed. In addition, similarly- preserved enigmatic structures under the pygidium, probably representing as-yet unknown organs, are reported for the first time. The unusual preservation of the digestive structures is thoroughly explored, in an attempt to reveal anatomical and systematic controls on gut fossilisation. Exploring the possible relationships between the anatomy and function of the gut and its preservation may permit novel inferences on aspects of Cambrian metazoan biology which are otherwise inaccessible. Geological setting The House Range of central Utah is home to three Cambrian Konservat-Lagersta ¨ tten: the Marjum Formation, the Weeks Formation, and the Wheeler Shale. The Weeks Formation has received the least scientific attention of the three while paradox- ically being well-known to amateur palaeontologists for its well- PLoS ONE | www.plosone.org 1 March 2012 | Volume 7 | Issue 3 | e32934