Milwaukee Formation

Early interest in the Milwaukee Formation was mainly of a commercial or other practical nature. Strata apparently of what would later be known as the Milwaukee Formation[2] were utilized as early as the 1670s, when French visitors used “pitch” (natural asphalt) from the rock exposures on the shore of Lake Michigan near Milwaukee to patch their boats.[3] The rock was later used to produce lime or building stone, and in the 1840s, Wisconsin's first resident scientist, Increase A. Lapham, noted the natural cement potential of certain of its layers. When calcined (heated to high temperatures) and crushed, cement was produced, without requiring additives or further processing. During that time, Lapham also collected fossils from those strata, which caught the attention of James Hall, Thomas Chrowder Chamberlin and other noted American paleontologists and geologists who went on to study the biota. In 1876, the Milwaukee Cement Company began manufacturing natural cement, and soon became one of the country's largest natural cement producers. Quarrying methods at that time required many hands to load rock into the dump cars destined for the kilns. This hands-on method was the perfect way for fossils to be detected. Several wealthy businessmen and professionals interested in acquiring large fossil collections recognized this and paid the quarry workers to set aside the best fossils. This continued until about 1911, when the cement rock was depleted, Portland Cement replaced natural cement, and the quarries were closed. Many thousands of high quality fossils were obtained this way between 1876 and 1911 for the collections of Thomas A. Greene, Edgar E. Teller, and Charles E. Monroe, and these can now be studied in numerous museums around central and eastern United States and Canada, especially the Milwaukee Public Museum, the Greene Geological Museum (University of Wisconsin-Milwaukee), the National Museum of Natural History, the Museum of Comparative Zoology, the Buffalo Museum of Science, the Field Museum of Natural History, Williams College, and the Royal Ontario Museum.[1]

The fossils found during and after that time indicate that the Milwaukee Formation contains one of the richest and most diverse biotas in North America coming from a single formation of its age (late Givetian), with about 250 species coming from around 100 families, at least sixteen phyla, and four kingdoms. The formation preserves fossils of marine and terrestrial organisms, including placoderm fish that were the size of great white sharks, some of the first trees, and fungi that may have reached the height of a two-story building.[1]

Lower jaw of the arthrodire placoderm fish Eastmanosteus pustulosus

Bark from a lycopod tree showing leaf scars

The large fungus Prototaxites milwaukeensis

The Milwaukee Formation is considered to be Givetian in age (~385 mya), as determined by using its biota to correlate the strata with certain portions of the Cedar Valley Group to the west. This age determination is also consistent with the formation's placement above the Givetian Thiensville Formation and below the Frasnian to Famennian Antrim Shale.[1]

The Milwaukee Formation lies disconformably between the Thiensville Formation and Antrim Shale. It was apparently deposited in relatively shallow, subtidal, normal marine conditions.[1] It is divided into three members.[4]

The fossils of the Milwaukee Formation are preserved in many different ways. Most of the shelly fossils in the Berthelet Member are preserved as internal molds and external molds, whereas most of those in the Lindwurm Member are permineralized to some degree. Many fossils in all three members are formed by replacement. The most frequent replacement minerals are marcasite followed by pyrite, which are generally weathered to ferrous sulfate or limonite, respectively. Chert is a common replacement mineral in certain layers of the North Point Member. Plant fossils in the Milwaukee Formation are usually coalified to some degree.[1]

Geodized brachiopod fossil lined with calcite with a single crystal of sphalerite

Fin spine of the ptyctodont placoderm Gamphacanthus, showing color patterns

Other taphonomic phenomena often found in the Milwaukee Formation are geodization of shelly fossils, corals, bryozoans, and echinoderms, and preservation of color patterns in certain brachiopods, trilobites, and fin spines of fish.[6][1]

The most distinctive feature of the Milwaukee Formation is the diversity of its biota, as measured by the total number of species. The most species-rich group of the Milwaukee Formation's biota is the Brachiopoda, having around 46 named species and sub-species,[7] followed by the Bivalvia, which has around 43.[8] Those plus species and sub-species from the following groups of marine and terrestrial organisms comprise a total of approximately 250 species: agglutinated foraminifers, conulariids, rugose corals, tabulate corals, tentaculitids, microconchids, cornulitids, hyoliths, gastropods, rostroconches, nautiloids, actinoceratoids, ammonoids, polychaetes, phacopid trilobites, proetid trilobites, ostracods, phyllocarids, crinoids, blastoids, edrioasteroids, dendroid graptolites, conodonts, arthrodire placoderms, ptyctodont placoderms, rhenaniform placoderms, petalichthyiform placoderms, chondrichthyans, acanthodians, sarcopterygians, fungi, cladoxylopsids, and lycopods.[1]

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  The conulariid scyphozoan Conularia milwaukeensis
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  The tabulate coral Aulopora living as an epibiont on the brachiopod Strophodonta
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  The tentaculitoid Tentaculites bellulus
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  The bryozoan Sulcoretepora
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  The bryozoan Eridotrypa
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  The linguliform brachiopod Barroisella
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  The rhynchonelliform brachiopod Tylothyris
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  The rhynchonelliform brachiopod Schizophoria
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  Striatochonetes and other brachiopods
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  The rhynchonelliform brachiopod Cranaena
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  The gastropod Platyceras
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  The bivalve Mytilarca cingulosa
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  The nautiloid cephalopod Gyronaedyceras eryx
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  The nautiloid cephalopod Acleistoceras whitfieldi
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  The phacopid trilobite Eldredgeops? norwoodensis
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  The proetid trilobite Crassiproetus
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  The spiny, stalked crinoid Melocrinites nodosus spinosus
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  Crown of the crinoid Taxocrinus telleri
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  Theca of the blastoid Hyperoblastus
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  Edrioasteroid, possibly Krama or Agelacrinites, with dissociated ambulacral plates
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  Fin spine from the placoderm Eczematolepis
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  "Tooth" plates from the ptyctodont placoderm Ptyctodus ferox
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  Tooth from the sarcopterygian fish Onychodus
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  Bark from a possible cladoxylopsid tree
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  Archaeosigillaria or related lycopod