The Waukesha Biota (also known as Waukesha Lagerstätte, Brandon Bridge Lagerstätte, or Brandon Bridge fauna) is an important fossil site located in Waukesha County and Franklin, Milwaukee County within the state of Wisconsin. This biota is preserved in certain strata within the Brandon Bridge Formation, which dates to the early Silurian period. It is known for the exceptional preservation of soft-bodied organisms, including many species found nowhere else in rocks of similar age. The site's discovery was announced in 1985, leading to a plethora of discoveries. This biota is one of the few well studied Lagerstätten (exceptional fossil sites) from the Silurian, making it important in our understanding of the period's biodiversity. Some of the species are not easily classified into known animal groups, showing that much research remains to be done on this site. Other taxa that are normally common in Silurian deposits are rare here, but trilobites are quite common.
History and significance The discovery of the Waukesha Biota was first published in 1985 by paleontologists D. G. Mikulic, D. E. G. Briggs, and Joanne Kluessendorf. At the time this site was one of only several known that preserved soft-body parts in fossils. Examples of other sites of this type known at the time were the famous Cambrian aged Burgess shale in British Columbia, and the Carboniferous aged Mazon Creek fossil beds in northern Illinois. This was the only one of its kind known from the Silurian, meaning it was instrumental in the study of early Paleozoic soft-bodied organisms. Since then other Lagerstätten from the Silurian (like the Eramosa lagerstatte) have been found, but none have the same faunal diversity that the Waukesha Biota has. The exceptional preservation of the fossils of the Waukesha Biota thus provides a window to a significant portion of Silurian life that otherwise may have been undetected and therefore unknown to science.
Stratigraphy and depositional environment
Most of the Waukesha Biota is preserved within a layer of thinly-laminated, fine-grained, shallow marine sediments of the Brandon Bridge Formation consisting of mudstone and dolomite deposited in a sedimentary trap at the end of an erosional scarp over the eroded dolomites of the Schoolcraft and Burnt Bluff Formations. A separate thin bed containing the biota is also present about above the interval. Fossils of unambiguous, fully terrestrial organisms are lacking from the Waukesha Biota. Most of the Waukesha Biota fossils were found at a quarry in Waukesha County, Wisconsin, owned and operated by the Waukesha Lime and Stone Company. Other fossils were collected from a quarry in Franklin, Milwaukee County, owned and operated by Franklin Aggregate Inc. That quarry lies south of the quarry in Waukesha. The Franklin fossils were from blasted material apparently originating from a horizon and setting equivalent to that of the Waukesha site. Its biota is similar to that from the Waukesha site, except that it lacks trilobites.
Taphonomy Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The taphonomy of the Waukesha Biota is unusual in preserving few of the kinds of animals that typically dominate the Silurian fossil record, including in other strata of the same two quarries. Fossils of corals, echinoderms, brachiopods, bryozoans, gastropods, bivalves, and cephalopods are rare or absent from the Waukesha Biota, although trilobites are diverse and common. This is because of the preservation bias this site has where soft bodied and lightly skeletonized organisms preserve more often than hard shelled organisms. The complete opposite when compared to the taphonomy of most other fossil sites.
The exceptional preservation of non-biomineralized and lightly sclerotized remains of the Waukesha Biota is generally attributed to a combination of favorable conditions, including the transportation of the organisms to a sediment trap that was hostile to scavengers but favorable to the production of organic films that coated the surfaces of the dead organisms, which inhibited decay, sometimes enhanced by promoting precipitation of a thin phosphatic coating, which is observed on many of the fossils.
Biota
Alga One genus of dasycladalean alga is known from the lagerstätte. AlgaGenusPhylumHigher taxonAbundanceNotesImagesJimaodanusAlgaDasycladalesA genus of dasycladalean alga of the family Triploporellaceae. This genus is notable for showing that dasycladalean algae underwent a massive radiation during the early Paleozoic. Originally named as Heterocladus.
Hemichordata Many of the hemichordates are members of the group Graptolithina. HemichordataGenusPhylumHigher taxonAbundanceNotesImagesDesmograptusHemichordataGraptolithinaA graptolite of the order Dendroidea that is characterized by its unique branching pattern and conical rhabdosomes.DictyonemaHemichordataGraptolithinaA graptolite of the order Dendroidea. These colonial organisms are characterized by a conical, net-like structure. The colonies (known as rhabdosomes) are branched and may vary from almost discoidal to almost cylindrical. They were stationary planktonic suspension feeders.OktavitesHemichordataGraptolithinaA graptolite of the order Bireclinata that formed a spiral-like pattern.Thallograptus?HemichordataGraptolithinaA graptolite of the order Dendroidea that formed branching structures, and often incrusted other organisms.
Porifera Poriferans, also known as sea sponges, are rare in this locality, with only one specimen known. Porifera Genus / TaxonPhylumHigher taxonAbundanceNotesImagesTo be determinedPoriferaTo be determined 1 specimen (known as UWGM 3126)One specimen of a poriferan, or sea sponge, has been found in this locality. It has a mesh-like skeleton consisting of longitudinal strands. These characteristics suggest a possible placement with either the vauxiids or the anthaspidellids.
Cnidaria The cnidarians of the site are mainly represented by conulariids, but coral are also known. CnidariaGenus / TaxonPhylumHigher taxonAbundanceNotesImagesMetaconulariaCnidariaStaurozoaA genus of conulariid, which are a somewhat enigmatic group that are now considered possible members of the group Staurozoa. In this image, figures A and B show specimens of Metaconularia.ConulariaCnidariaStaurozoaConularia is the type genus of the conulariid group, and the main species found at this site is Conularia niagarensis.SphenothallusCnidariaStaurozoaA problematic genus lately attributed to the holdfasts (attachment points) of conulariids. This fossil is found throughout rocks of the Paleozoic.TabulataCnidariaAnthozoaFossils of undetermined tabulate corals are rare in this biota, but their poor preservation prevents precise identification.
Echinodermata Echinoderm fossils are rare at the site, but crinoids have been found here. EchinodermataGenus / TaxonPhylumHigher taxonAbundanceNotesImagesCrinoideaEchinodermataCrinozoaCrinoid fossils are rare at the site, but at least one complete decalcified specimen is known from the site.
Brachiopoda Like many of the hard shelled organisms known from this site, the brachiopods found here are poorly preserved and rare. BrachiopodaGenus / TaxonPhylumHigher taxonAbundanceNotesImagesRhynchonellidaBrachiopodaRhynchonellidaFossils of articulate brachiopods that have been found at the site have been tentatively assigned to the still living group Rhynchonellida.OrbiculoideaBrachiopodaLingulidaInarticulate brachiopods from this site have been confidentially assigned to the genus Orbiculoidea.
Cephalopoda Normally common in Silurian deposits, nautiloid cephalopods are known from only a handful of specimens from the Waukesha biota. CephalopodaGenusPhylumHigher taxonAbundanceNotesImagesTo be determinedMolluscaKionoceratinaeA cephalopod shell found at the site has been interpreted as belonging to the family Kionoceratinae based on its structure and appearance.To be determinedMolluscaTo be determinedAnother cephalopod shell is known, but due to it being an internal mold, and being poorly preserved, it has been not assigned to any specific families.
"Worms" Multiple soft bodied fossils of "worms" and other vermiform animals are known from the site. "Worms"GenusPhylumHigher taxonAbundanceNotesImagesMacromyzon AnnelidaHirudineaA singular holotype specimen (UWGM 7056)A stem-group hirudinean annelid, and the oldest known leech. This animal was most likely not hematophagous and probably fed on live prey. To be determinedAnnelidaPolychaetaMany fossils of multiple polychaete worms are known from the site, and represent many different groups. Some fossils represent members of the family Aphroditidae, and others have bodies covered in spines.To be determinedPalaeoscolecidaTo be determinedSpecimens of vermiform nature from the site have been found and interpreted as members of the palaeoscolecida grouping (which were extinct ecdysozoan worms resembling armoured priapulids). These are some of the most common worms from this locality, with up to 30 fossils known. To be determinedLobopodiaTo be determinedLobopodians are not "traditional" worms, but are instead panarthropods with stubby legs. Some of these fossils were at one point attributed to myriapods. Over 40 specimens are known, which show that the animal was around 4 cm long, and possessed short legs. Based on known fossil material, it seems that there are multiple different species known. To be determinedUncertainCycloneuralia At least one specimen An annulated worm. It was thought that it might represent a leech, which if true would make this specimen the earliest known leech in the fossil record, although available characters are limited so it is hard to identify. Shcherbakov et al. (2020) considered it to be a cycloneuralian, possibly a member of the family Ancalagonidae. The specimen was found with a circular structure at one end interpreted as a sucker; however, Braddy, Gass & Tessler (2023) reinterpreted the purported suckered end as a breakage, and consider the specimen to be a molt of a member of Cycloneuralia of uncertain affinities, possibly a priapulid. Its large size, at up to 24-mm wide suggests a predatory lifestyle.
Arthropoda Arthropods dominate the fauna of the Waukesha biota in both number of specimens and diversity. A wide variety including crustaceans, trilobites, chelicerates, and less familiar groups like the thylacocephalans and cheloniellids are known. Also found are enigmatic arthropods whose taxonomy has puzzled paleontologists since the sites discovery.