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.
Highlights
The Cambrian explosion was a major episode of biological diversification, which was associated with the development of complex marine ecosystems [1]
The unusual morphology of Cambrian metazoans prevents easy comparisons to modern analogues and rarely provides unambiguous clues about their feeding habits. This is 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
In G. granulatus (Figures 3L, M), represented by a single specimen in this study, there are a few crystalline inclusions of about 500 mm that are composed of Si and O, as revealed by energy dispersive X-ray (EDX) analyses
Summary
Background The Cambrian explosion was a major episode of biological diversification, which was associated with the development of complex marine ecosystems [1]. The unusual morphology of Cambrian metazoans prevents easy comparisons to modern analogues and rarely provides unambiguous clues about their feeding habits. This is 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. We describe phosphatised digestive structures of three species of trilobites from the Middle Cambrian Weeks Formation Lagerstatte (Utah) Their exquisite, three-dimensional preservation allows unique details on trilobite internal anatomy to be revealed. 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
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