Fossilized appendages and guts in three phacopid trilobites from the Tremadocian lower Fezouata Konservat-lagerstätte (Lower Ordovician, Morocco)

Abstract

The Fezouata Shale has yielded remarkably diverse assemblages of non-biomineralizing and biomineralizing organisms of Early Ordovician age. Overall, some 160 genera have been documented, about a quarter of which are essentially soft-bodied. Exceptional preservation in the Fezouata Shale occurs in two main separated intervals, of late Tremadocian and mid-Floian age respectively. These intervals are best regarded as distinct Konservat-lagerstätten, since they contain different faunas and are separated in time by c. 9 Ma. Both assemblages are dominated by arthropods, including a diverse trilobite fauna (c. 25 genera). Here we report the discovery of fossilized appendages and guts in 31 trilobite specimens from the lower Fezouata Konservat-lagerstätte belonging to the genera Anacheirurus, Bavarilla, and Prionocheilus (Order Phacopida). The preservation of appendages is generally poor, especially for their proximal parts covered by the dorsal exoskeleton. However, when appendages project from below the dorsal carapace, they exhibit delicate structures, such as annuli boundaries and lateral spines of antennae, or podomere boundaries, clustered endites and terminal claws of endopods. Lamellate exopods are rarely preserved, and when they are, their morphologies are largely obscured by the dorsal carapace. Appendages are typically preserved as thin, yellowish films of iron oxides, interpreted as oxidized pyrite. Among preserved gut structures, Anacheirurus displays a parallel-sided, sediment-filled digestive tract, running under the entire trunk axis while the tract of Bavarilla (wide and sediment-filled) is associated with 10 pairs of digestive glands, which are atypically preserved as a sediment infill. The new examples of fossilized ‘soft’-parts of trilobites from the lower Fezouata Lagerstätte complement previous observations in Megistaspis and Symphysurus, two taxa also belonging to the pilekiid-bavarillid biofacies typical of lower shoreface settings. Despite notable differences in size and affinities, the preservational styles are essentially the same in all specimens, suggesting that only a limited number of taphonomic pathways is responsible for the preservation of trilobite labile parts in these horizons. In addition to the work on the preserved soft parts, the morphological variability of preserved hard parts is currently also under study; in time, this will result in detailed systematic (re)descriptions of the taxa at hand.