Waukesha Biota

The Waukesha Biota (also known as Waukesha Lagerstätte, Brandon Bridge Lagerstätte, or Brandon Bridge fauna) refers to a Konservat-Lagerstätte ( a fossil site that preserves soft bodied remains) of Early Silurian (Telychian to Sheinwoodian) age found within the Brandon Bridge Formation in Waukesha County and Franklin, Milwaukee County, Wisconsin. It is known for the exceptional preservation of its diverse, soft-bodied and lightly skeletonized taxa, including many major taxa found nowhere else in strata of similar age. The sites discovery was announced in 1985, thus leading to a wide plethora of discoveries being made. This biota is one of the few well studied lagerstättes from the Silurian, making it very important in our understanding of Silurian faunas. Some of the taxa found here are not easily classified into known animal groups, showing that there is a great amount of studying that needs to be done on this site. Other taxa that are normally common in other Silurian deposits are rare here, but Trilobites are still quite common in the area. This site is also important because it shows that while Wisconsin's paleontology is often overlooked, it still provides exceptional insights concerning the history and diversity of life on Earth.

History and significance

Prior to the discovery of the Waukesha Biota, very little was known about the soft-bodied animals that were sure to have lived in the Silurian seas. Thanks to the famous Burgess Shale of the Canadian Rockies, scientists knew that the seas were already teeming with soft-bodied and lightly skeletonized animals as far back as the Cambrian, yet relatively few were known to be preserved in later Paleozoic rocks. This is because the fossil record is greatly skewed toward biomineralized skeletal remains. The announcement of the Waukesha Biota in 1985 thus marked a major turning point in our knowledge of Silurian life. In his popular book, '' Wonderful Life'', Stephen Jay Gould compared the "anatomical disparity" of the Burgess Shale with that of only five other occurrences in the world –one of which was the Waukesha Biota, which he referred to as the "Brandon Bridge fauna"– and none of the others were Silurian. Other “soft-bodied” Silurian sites have turned up in more recent years, but none have produced the high diversity of soft-bodied animals observed in this biota.
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. Currently fossils from the biota are on display in the UW–Madison Geology Museum.

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

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.

The exceptional preservation of non-biomineralized and lightly skeletonized 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 predators 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

Green algae

Carbonized fossils of a noncalcified dasycladalean alga of the family triploporellaceae have been found at the Waukesha site and were given the name of ''Heterocladus waukeshaensis''.

Arthropods

The fossil record of the Waukesha Biota is dominated by arthropods, both in number of fossils and number of species. This is even true when excluding trilobites and ostracods, which in most Early Paleozoic marine biotas are the source of the vast majority or all of the arthropod fossils. This is due to preservational conditions which protect soft and lightly skeletonized tissues from decomposition but select against preservation of biomineralized skeletal remains. Many of the arthropods include trilobites, crustaceans, chelicerates, and some of questionable taxonomy.

Chelicerate

File:20200814 Venustulus waukeshaensis.png, '' Venustulus'', a Synziphosurine arthropod that is distantly related to modern horseshoe crabs.
File:Fossil specimen of Venustulus (Synziphosurina) from the Waukesha lagerstätte.jpg, A fossil of Venustulus from the Waukesha Biota.

Well preserved fossils of a chelicerate (the grouping of arthropods including arachnids, sea spiders, etc) have been found in this biota and belong to the Synziphosurina (A extinct group distantly related to horseshoe crabs). This species was named '' Venustulus waukeshaensis''. The genus name ''Venustulus'' derives from Latin and means "charming", while the species name ''waukeshaensis'' is derived from the town of Waukesha. The creature was possibly blind (due to the lack of any noticeable eyes) and is known from eight specimens, three of which are almost complete and the other five only being the prosoma and opisthosoma of the arthropod. The five-six pair of appendages seen in ''Venustulus'' contrasts the seven usually seen in other synziphosurans, possibly suggesting that this creature represents a more derived form than other members of its grouping. This arthropods taxonomy is not fully understood, but is believed to belong to the family Weinberginidae (alongside '' Weinbergina'', '' Legrandella'' and ''Willwerathia)''. Interestingly it seems there could be another possible synziphosuran from the same deposits as ''Venustulus'', but the fossils are too fragmentary to consider it a unique genus.

Thylacocephalans

File:Thylacares brandonensis S12862-014-0159-2-1.jpg, alt=, Fossil specimen UWGM 1748 of '' Thylacares brandonensis''
File:Fossil of Thylacares from the Waukesha biota site in Wisconsin.jpg, Fossil specimen of ''Thylacares''
File:Thylacares brandonensis chasing after prey.png, Life reconstruction of ''Thylacares'', chasing a flatworm

'' Thylacares brandonensis'' is the name given to one of the oldest known unequivocal Thylacocephalans, which were an enigmatic group of possible crustaceans having a pair of raptorial appendages and a bivalved shield that enclosed most of the body, and survived until the upper Cretaceous. This species is distinguishable from other thylacocephalans by its smaller raptorial appendages and compound eyes. All of the known specimens are kept in the UW geology museum. The body is fully encased in a bivalved shell, with only the eyes protruding on stalks. ''Thylacares'' means "Small pouch" in reference to the small eyes and stomach seen in the species. The species' trunk is composed of about 22 segments. Originally this genus was thought to have been the oldest known thylacocephalan, however recently two new genera from the czech republic, ''Pseudoprotozoea'' and ''Bohemiacaris'' from the upper Ordovician now hold this record. This creature represents one of the few known thylacocephalan species from the Silurian, with one of the few being the Scottish genus '' Ainiktozoon''.

Mandibulate

alt=Reconstruction of Acheronauta stimulapis based on "Morph A" specimens, 220x220px, Reconstruction of '' Acheronauta stimulapis'' based on "Morph A" specimens
Fossils of a vermiform arthropod have been known from the biota since 1985. In 2022 these fossils were properly described and given the name of ''Acheronauta stimulapis''. The paper describing this creature found it to be a possible basal mandibulate (the grouping of arthropods including insects and crustaceans). This creature had a long worm like body, a shielding carapace covering its head, a pair of antennae coming off the head, and a set of appendages coming from below the carapace. Currently it is thought that this arthropod forms a previously unknown clade with the Devonian stem-group arthropod ''Captopodus'', and the thylacocephalans. ''A. stimulapis'' has been recognized as one of the most common animals of the biota, with over 20 specimens being known. ''Acheronauta'' derives from the latin word ''Acheronta'', which is a Latinizing of the Greek river ''Acheron'', which itself is another name for the River of Woe, a river of the Greek underworld. The last part of the genus name ''-nauta'' means "sailor". This name is a reference to the harsh environments of Silurian Waukesha that allowed for this site to be preserved. The species name ''Stimulapis'' derives from the latin words ''Stimulas'' which means sting, and ''apis'' which means bee. The specific epithet translates to ‘sting like a bee’, which according to the authors of the paper is in honor of the American boxer Muhammad Ali.

Marrellomorphs

In 2015, paleontologists announced the discovery of a marrelomorph, a bizarre grouping of arthropods that first originated in the Cambrian whose taxonomic relationships are currently uncertain, from the biota. About five specimens are known with each having a varying level of completion, with some preserving the cephalic and trunk regions of the body. Certain characteristics of the fossils suggest a possible relation with "'' Furca mauretanica''" from the Fezouata biota in Morocco, and a cladogram preformed seemed to confirm this thought. Appendages are rare, but some of the specimens seem to preserve antennae and trunk appendages. This discovery is significant as the only other known Silurian marrellomorph is '' Xylokorys chledophilia'' from the United Kingdom. This means that this creature was the first known post-Cambrian marrellomorph from the United states, and North America in general.

Phyllocarids

Phyllocarids are a group of primitive crustaceans that first appeared in the Cambrian period and have survived to the modern day. Three species of phyllocarids are part of the Waukesha Biota, consisting of ''Ceratiocaris macroura'', ''C. papilio'', and ''C. pusilla''. Although these bivalved crustaceans are not new to science, their fossils preserve the delicate appendages and further show the diversity of the lightly biomineralized fossils of the Waukesha Biota.

Myriapods

Several fossils appearing to be myriapods (the group that includes millipedes and centipedes) are part of the Waukesha Biota.

Other arthropods

File:Meroperyx.jpg, The styginid trilobite '' Meroperyx''
File:Dalmanitid trilobite.jpg, An unnamed Dalmanitid trilobite; the most abundant arthropod of the biota
Other kinds of arthropods having varying degrees of biomineralization of their skeletal elements are also present in the Waukesha Biota, including leperditicopid ostracods (crustacea), and about thirteen species of trilobites, several of which are new. Some trilobite taxa preserve their digestive tracts. An unnamed dalmanitid is commonly found with its dorsal exoskeleton intact and sometimes seen in such numbers that they cover large surfaces of sediment.

Some other taxa do not readily fit within existing arthropod groups, such as the very common “Butterfly animal”, a small enigmatic arthropod that has stumped paleontologists since it was discovered along with the biota in 1985. Despite this animal being named after a butterfly, it is not actually an insect. This creatures nickname derives from two large wing like projections that extended from the sides of its body. Whether the extensions flapped like wings or were a carapace shield protecting the arthropod's body is uncertain. The largest known specimens have a width of around 20 centimeters (7.8 inches) in width. One of the more supported hypotheses that paleontologists have made on this arthropods taxonomy is that it is some kind of crustacean, with some suggesting it could be related to phyllocarids, but whether these similarities are just superficial is currently up in the air.
File:Parioscorpio fossil.png, Multiple specimens of the enigmatic arthropod '' Parioscorpio venator''
File:20211029 Parioscorpio venator.png, Reconstruction of ''P''. ''venator'' as a non-scorpion arthropod
File:20211104 Parioscorpio raptorial appendages mobility.gif, The suggested movement of the raptorial appendages of ''P. venator''
Another enigmatic arthropod from the Waukesha Biota is ''Parioscorpio'' ''venator'' (Wendruff et al., 2020). Having a pair of sub-chelate great appendages, multiramous anterior trunk appendages, filamentous fan-like rear trunk appendages, and a lack of antennae, this peculiar species has been classified over the years under different arthropod groups. In 1985 when it was first discovered, it was thought to have either been a remipede or branchiopod crustacean. In that same year, the arthropod was given the unofficial nickname of the "Hellgrammite animal" due to its close resemblance to a Hellgrammite, which is the larvae of modern day dobsonflies. In 2020 when it was first described it was interpreted as the oldest known scorpion, hence its name meaning “Progenitor scorpion”. In 2021 it was reinterpreted as either an arthropod of uncertain placement, or a cheloniellid (a grouping of arthropods close in relation to trilobites). Further study appears to have reaffirmed a possible affinity with the Cheloniellida, however this interpretation was denied in a later study, making this taxon enigmatic again. In 2014, scientists described ''Latromirus tridens'', a supposed cheloniellid from the biota, but because the naming of the animal did not meet the standards of the ICZN, its name was considered a nomen nudum. Later on it was revealed that many of the fossils of ''L. tridens'' were actually those of ''P. venator'', possibly rendering ''L. tridens'' as a junior synonym. Specimen UWGM 2439, once described invalidly as a fossil of ''Latromirus'', still has a possibility to be a distinct cheloniellid, but could also be a specimen of ''Parioscorpio''.

Other arthropods are either not yet described or cannot be confidentially classified. One example of this is the specimen UWGM 2339, which is an arthropod appendage discovered in the biota. Currently it is thought to be the grasping appendage of a Radiodont, a group of primitive stem-group arthropods that thrived in the oceans of the Cambrian, with some Ordovician and Devonian specimens.Van Roy, P.; Briggs, D. E. G. (2011). "A giant Ordovician anomalocaridid". Nature 473 (7348): 510–513. doi:10.1038/nature09920. edit If this classification is correct, this would not only be the first radiodont known from Wisconsin, it is also would be the first one known from Silurian rock layers.

Lobopods

Several unnamed species of lobopods (worm-like panarthropods with annulated appendages) are known from the Waukesha Biota. One of these was originally thought to be a myriapod arthropod until further study revealed key lobopod characteristics.

"Worms"

Many exquisitely preserved vermiform fossils, some preserving digestive tracts, are part of the Waukesha biota. These include annelids, such as an aphroditid polychaete, a spiny polychaete, and an annelid that appears to have a sucker disc at one end. If this proves to be a leech, it would be the first unequivocal fossil leech ever recorded. Another worm-like taxon in the biota is identified as a palaeoscolecid. Scolecodonts, which are the fossilized jaws of polychaete worms, have also been found in this biota.

Hemichordates

Several genera of the hemichordates known as graptolites, including ''Oktavites'', ''Desmograptus'', ''Dictyonema'', and ''Thallograptus''?, are reported from this biota.

Chordates

A fossil of ''Panderodus,'' a genus of conodont animal, which were primitive, eel-like chordates now known to be the source of certain small, phosphatic, tooth-like structures long-used in the dating of strata and now known as " conodont elements," has also been found. An assemblage of conodont elements were found associated with this fossil. The fossil reveals that this conodont was a macrophagous predator (meaning it fed on large prey items.) It also showed that this creature had a more tapering, larger body compared to most other conodonts. A certain groove on the elements of ''Panderodus'' has been suggested to accommodate the flow of venom, meaning that this genus may have been one of the first venomous animals. The Waukesha Biota was only the second occurrence to produce a conodont animal. Numerous, well-preserved chordate fossils that may also be conodont animals were later found in the Waukesha Biota at Franklin Quarry.

Other animal taxa

The Waukesha Biota includes fossils of many fragmentary, unidentifiable, nonbiomineralized and lightly biomineralized skeletal elements that may belong to additional taxa. It also includes fossils of conulariids and certain biomineralized taxa that are common in typical Silurian marine occurrences, but are rare or uncommon here, including tabulate corals, brachiopods, cephalopods, and echinoderms. This is due to the unique preservation bias the site has where it favors the fossilization of lightly skeletonized and soft bodied creatures.

See also

* Blackberry hill, a Cambrian aged lagerstätte found in central Wisconsin
* Milwaukee formation, a Devonian aged fossil site located in eastern Wisconsin
* Ludlow group, a group of lagerstättes of Silurian age found in the British isles

References

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Lagerstätten
Paleontology in Wisconsin
Prehistoric fauna by locality
Silurian United States
Sheinwoodian
Telychian