Krukowski Quarry Black Berry Hill

The Earliest Evidence of Terrestrial Colonization

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Scyphozoan Medusae (jellyfish) Fossils Climactichnites Arthropods - Protichnites and Diplichnites Stromatolites

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Near a sleepy rural burg named Mosinee, some 200 miles northwest of Milwaukee is a commertial flagstone quarry that may help fill in some pieces in the Cambrian puzzle, those associated with the enroachment of life to land. Over the last decade this quarry has yielded some amazing body fossils and ichnofossils. The Krukowski stone Quarry dates to the late Cambrian some 510 million years ago. The sediments are believed to have been deposited in a tropical barrier island system that sat near the equator (about 10 degrees south latitude). The rocks comprise medium- to course-grained quartz arenite in either the Mount Simon or Wonewoc Formation of the Elk Mound Group. In the February 2002 issue of Geology, Hagadorn, Dott and Damrow published a paper titled: "Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin", described what is the oldest terrestrial habitat on earth yielding fossils.

Scyphozoan Medusae (jellyfish) Fossils

Among the fossils and trace fossils found in this quarry are numerous circular impressions in layers of rippled sandstone where there once was a shallow tropical seashore during the Cambrian. Hagadorn, et. al. describe an exceedingly rare event, the largest mass stranding in the fossil record of Cnidarian medusae (Class Scyphozoa – jellyfish). Hagadorn, et. al. state that the quarry's features are "consistent with an intermittently exposed intertidal and shallow-subtidal setting that was probably located in a shallow lagoonal area with limited wind fetch . . . . within a possible sandy barrier island system on the flank of the Wisconsin dome may have further restricted the environment, and severe tropical storms provide a plausible mechanism for medusoid stranding". Preservation is as epirelief or hyporelief fossils. The fossils record the jellyfishes' final behavior to escape back to the sea that resulted in their ingesting of sand, thus leaving a sand mound trace fossil. According to Hagadorn, the jellies from the Mosinee quarry are not just large for the Cambrian, but are the largest jellyfish in the entire fossil record. The only comparable fossil site occurs in New Brunswick, Canada – not only are New Brunswick jellyfish fossils younger, but they are some 8-fold smaller. Some of the Krukowski medusae are some 50 cm in diameter. At this size, it is easy to conjecture that such medusae rivaled the dreaded Anomalocarids as the reigning top predators of the early Paleozoic.

Far more rare in the quarry are bedding planes with small jellyfish fossils that like their large medusoid counterparts exhibit evidence of struggling subsequent to mass standing and prior to their demise. Additionally, some of these markedly smaller jellyfish fossils reveal subtle impressions of tentacles in the familiar radial symmetry of Cnidarians. These fossils are not yet described in the peer-reviewed literature.

Climactichnites

Climactichnites fossils have remained a mystery since their discovery in Canada by Sir William Logan in 1859. Whether they are track fossils or body impressions of a large Cambrian soft-bodied animal is still shrouded in mystery. Climactichnites is a unique Upper Cambrian band-like fossil described as a trace found in sandstone formations throughout the northeastern and north-central US and southeastern Canada. It has been conjectured (Yochelson, 1993) that the motorcycle track-like fossils were made by a slug-like organism (i.e., a gastropod of order Opisthobranchia) that secreted prodigious mucus as it moved over the shallow sand flats filtering the sand for microbes. In contrast, in a poster at the Geological Society of America Annual Meeting (November 5-8, 2001), Damrow, et. al. propose that Climactichnites are equally likely to be fossils of body impressions of a gelatinous zooplankter that floated into shallow water where they were deposited gently across the extensive tidal sand flats. Shown in the picture is a large section with many overlayed Climactichnites. That they are overlayed is suggested as evidence that they are body impressions. It is interesting to note that when you first walk into the fossil section of the Museum of Natural History in downtown Washington you see a large plate of Climactichnites, a testament as to how long its mytery has persisted.

In his recent thesis, Getty (2007) has revisited the vexing enigma of Climactichnites bases on new knowledge of the role of microbial mats in Cambrian sediments. Specifically, microbial mats may have been abundant through the Cambrian, mediating the preservation of Climactichnites (and other foosils of the Krukowski quarry), but were no longer prevalent from the Ordovician, where Climactichnites are no longer appears. Experiments confirmed that the trackways are similar to those formed by modern gastropods. If so, the uniqueness of Climactichnites is in its preservation, not its origins. Climactichnites from the quarry are found in association with sand stromatolites. Ichnofossil preservation was thus was possibly aided by a microbial mat which bound the substrate and resisted bioturbation. Getty concludes that the the Climactichnites trackmaker was an "elongate, bilaterally symmetric, dorsoventrally flattened, soft-footed animal with a muscular anterior used during locomotion". Most of the animals were between 1.4 and 18 cm wide and 3.2 to 41 cm long, though one trackway measured some 29 cm wide, implying an animal 67 cm long. The ventral surface of the animal may have had glands that secreted mucus during track formation, and the dorsal surface may have been naked or may have borne sclerites. The trackmaker may have made some trackways in subaerial conditions and, if so, Climactichnites may mean that a mollusks may be included with arthropods as first terrestrial pioneers.

Arthropods - Protichnites and Diplichnites Trackways

The most exciting fossils of the Krukowski quarry are trackway ichnofossils of amphibious arthropods - creatures making the first footprints on land. Both Protichnites and Diplichnites ichnogenera are prominently represented. A number of sedimentary features, adhesion surfaces, large ripples, and raindrop imprints support the hypothesis that subaerial exposure occurred at or near the time the tracks were made, and that the arthropods had voluntarily ventured out of the water. What creatures made the prints and what were they doing out of the water? The discovery of arthropod body fossils in 2003 (not yet published) provides clues. These arthropod fossils contained in dessication zones are not so well articulated, but suggest multi-legged telson-bearing creatures having morphological characteristics of Aglaspid or Euthycarcinoid origins. Prior to discovery of the arthropod fossils, both horseshoe crabs and eurypterids were suggested as the Protichnites track makers of the quarry. The Diplichnites exhibit only foot prints, and lack the telson drag marks. This does not rule out telson bearing creatures made Diplichnites, but does suggest other trackmakers such as such as a primitive marine Myriapod-like creatures may have been present.

Though rare, ichnogenus Rusophycus has been found in the quarry. Rusophycus was placed in the ichnogenus Cruziana by Seilacher in 1970, but the term is still widely used by ichnologists for a putative resting place rather than trackway of a trilobite or other arthropod.

Stromatolites

Microbial sedimentary structures are abundant in the Krukowski quarry and are often found in association with the other fossils. These include elephant skin, sand peloids (i.e., bacterially mediated precipitation), domal sand stromatolites, and sand chips. Such structures may underlie and overlie medusae stranding surfaces, and mats of these microbes may well have been the principle reason the ichnofossils were not destroyed by wave action, erosion, and bioturbation.

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