(Medusae) stranded on an ancient Cambrian shore
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. 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
In the February
2002 issue of Geology,
Hagadorn, Dott and Damrow publish a paper titled: "Stranded
on a Late Cambrian shoreline: Medusae from central Wisconsin".
The implications are of some note to paleontology since such huge
Scyphozoan medusae (jellyfish) were likely to have been one of the
largest and fiercest predators in the early Paleozoic.
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 quarry
contains different layers with densely populated impressions the
likely result of a mass-stranding of groups of madusae on a Paleozoic
shoreline. Such strandings frequently still occur on Earth, but
in the Cambrian there were no land predators. The fossils exhibit
features nearly identical to those observed in modern scyphozoan
strandings. Some event in the absence of shore erosion may have
enabled the stranded jellyfish to be buried and these fossil imprints
to form. It was during the Cambrian that animals with hard shells
first appeared, enabling a rich fossil record to begin. Trace fossils
such as these soft-bodied jellyfish are exceedingly rare.
medusae jellyfish fossils have so far been found in seven layers
in the quarry, representing some 12 vertical feet of rock and corresponding
to a span of time of about one million years. 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
Trace or body fossils?
In a layer below the madusae are other unique
and amazing fossils called Climactichnites, though whether they
are track fossils or body impressions of a large Cambrian soft-bodied
animal is yet 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 that the motorcycle-like
tracks were made by a slug-like organism that secreted prodigious
mucus as it moved over the shallow sand flats. 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 additional 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.
arthropod trackways, Diplichnites
The May 2002 issue of Geology presents a
by MacNaughton, et. al. (First steps on land: Arthropod
trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada.
Robert B. MacNaughton, Jennifer M. Cole, Robert W. Dalrymple, Simon J. Braddy,
Derek E.G. Briggs, Terrence D. Lukie, pages 391-394.) describing Arthropod trackways
that might push back the record of the first arthropod landfall by as
much as 40 million years. The trackways occur in an inactive quarry in the Nepean
Formation (upper formation of the Potsdam Group in Ontario), 20 km northeast of
Kingston that may date to the late Cambrian. The tracks occur in laminations that
appear wind-rippled, indicating that the strata were deposited in an eolian (sand)
dune field, probably in a marginal-marine setting.
undescribed, and on a layer some ten feet below the madusae in the Wisconsin quarry
are incredible trackway fossils (Diplichnites)
that may push the date of arthropods on land still further back
into the Cambrian. This layer is of rippled, medium-grained quartz sandstone surfaces
of an ancient Cambrian tidal shoreline. Some areas have an incredible density
of tracks, and tracks that are highly crisscrossed. It is believed these are the
tracks of Cambrian, soft-bodied arthropods and are called Diplichnites. There
is a broad distribution of spacing of individual tracks ranging up to 4 1/2 inches.
Hence, some of these jointed-legged, creatures could have been of considerable
slab shown in the picture is one of the most interesting found in the quarry (click
the picture to view more pictures). It measures 38 x 52 inches at its maximum
width and height. There are two distinct types trackways on the slab. The widest
dimples are called Diplichnites and the other is undescribed.
another layer of the quarry yields jellyfish that are smaller, but have tenacles
in the familiar radial symmetry of Cnidarians, as shown
in the picture. Jellyfish were some of the most ferocious preditors of the Cambrian
marine environment, and may well have munched on the arthropods that left the
trackways known as Diplichnites, described
Shadows Theme Park at Fossil