FOSSILIZED DIGESTIVE TRACTS OF ARTHROPODS AND WORMS FROM THE WAUKESHA LAGERSTÄTTE, SILURIAN OF WISCONSIN, USA

The macrobiota of the Waukesha Lagerstätte (Brandon Bridge Formation, Silurian) of southeastern Wisconsin is dominated by an assortment of biomineralizing, lightly skeletonized, and non-biomineralized invertebrates including articulated trilobites and other euarthropods, lobopods, worms, conulariids, and graptolites. Synsedimentary early diagenesis leading to exceptional preservation took place in an environment influenced by mat-stabilized carbonate muds, magnesium flux and possibly dysoxic microenvironments in sediment. Early diagenesis occurred on a sufficiently rapid time scale that digestive systems of an array of invertebrates have been preserved. Digestive systems preserved by phosphate are known from three trilobite taxa, a new dalmanitid, Arctinurus, and Meroperyx. A contrasting style of preservation is expressed in the exoskeletons of most trilobite specimens; they tend to be partly decalcified, evidently through dissolution in sediment, and perhaps under the influence of microbial mats. Phosphatized gut tracts are also recognized in two new arthropods and a xenusiid lobopod. Some specimens of these taxa have retained appendages. Fossilized guts are evident in at least two worm taxa, a polychaete annelid and a palaeoscolecid, and in both instances appear to be sediment-filled. Diagenesis of gut tracts was probably initiated within days of the time that remains entered the sedimentary environment. The process seems to have been completed before membranes connecting major skeletal sclerites could disarticulate, and in most trilobites it was completed following disarticulation of the appendages.