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Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin

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Abstract and Figures

Fossilized impressions of soft-bodied organisms are exceptionally rare in coarse-grained strata. Fossilized mass-stranding events of soft-bodied organisms are even rarer. The Upper Cambrian Mt. Simon Wonewoc Sandstone in central Wisconsin contains at least seven horizons characterized by hundreds of decimeter-sized impressions of medusae; these represent one of only two fossilized mass-stranding deposits. Medusae exhibit features nearly identical to those observed in modern scyphozoan strandings, including impressions of subumbrellar margins and gastrovascular cavities. This deposit provides insights about soft-tissue preservation in Phanerozoic marginal marine sediments, and suggests that large soft-bodied pelagic organisms were abundant in Cambrian seas.
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Geology; February 2002; v. 30; no. 2; p. 147–150; 3 figures; Data Repository item 2002010. 147
Stranded on a Late Cambrian shoreline: Medusae from
central Wisconsin
James W. Hagadorn Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California
91125, USA
Robert H. Dott Jr. Department of Geology and Geophysics, University of Wisconsin—Madison, Madison, Wisconsin 53706,
USA
Dan Damrow 1014 West Highway C, Mosinee, Wisconsin 54455, USA
Figure 1. Stratigraphic context (upper left); medusoid horizons are
shown by arrows. Upper inset map indicates location of Mt.Simon–
Wonewoc study exposures near Mosinee (M)and Irma (I).Medusoids
were studied at Krukowski quarry, located within Mosinee outlier.
Lower inset map, cross section: paleogeographic andpaleoenviron-
mental context, measured paleocurrent directions (dashed arrows),
and inferred wind direction (solid arrows in both insets). Note equa-
torial location of study sites within inferred barrier island complex.
ABSTRACT
Fossilized impressions of soft-bodied organisms are exception-
ally rare in coarse-grained strata. Fossilized mass-stranding events
of soft-bodied organisms are even rarer. The Upper Cambrian Mt.
Simon–Wonewoc Sandstone in central Wisconsin contains at least
seven horizons characterized by hundreds of decimeter-sized im-
pressions of medusae; these represent one of only two fossilized
mass-stranding deposits. Medusae exhibit features nearly identical
to those observed in modern scyphozoan strandings, including im-
pressions of subumbrellar margins and gastrovascular cavities.
This deposit provides insights about soft-tissue preservation in
Phanerozoic marginal marine sediments, and suggests that large
soft-bodied pelagic organisms were abundant in Cambrian seas.
Keywords: Scyphozoa, medusae, Cambrian, Mt. Simon, Wonewoc.
INTRODUCTION
Scyphozoan medusae or ‘‘jellyfish’’ are an important carnivorous
component of pelagic ecosystems, often occurring en masse in near-
shore settings during reproductive peaks, storm events, severe tides, or
when they swim into the littoral zone to hunt. During ebbing tides,
enormous strandings can accumulate (Scha¨fer, 1972). Althoughstrand-
ings are common today and isolated fossils indicate that medusae were
an important component of ancient pelagic environments, preservation
of mass strandings is incredibly rare. With the exception of chondro-
phorine hydrozoans, most cnidarian medusae are difficult to preserve
because they have no durable hard parts (Stanley, 1986). When strand-
ed or catastrophically buried, rapid decay processes and intense phys-
ical and biological reworking inhibit preservation of their soft tissues,
especially in coarse-grained marginal marine settings where they are
typically stranded. In contrast, in fine-grained settings where cata-
strophic burial and stagnation inhibit reworking (e.g., konservat-lager-
sta¨tten), isolated medusae are more readily preserved.
Most previously described nonchondrophorine medusae are actu-
ally inorganically formed structures (Cloud, 1960; Plummer, 1980),
trace fossils (Fu¨rsich and Kennedy, 1975; Seilacher, 1984), other meta-
zoans (Conway Morris and Robison, 1988; Landing and Narbonne,
1992), or taxonomically enigmatic forms (Stasinska, 1960; Dzik, 1991;
Runnegar and Fedonkin, 1992; Gehling et al., 2000). Undisputed ex-
amples of nonchondrophorine medusae are virtually unknown from
pre-Devonian strata, and only one other poorly documented Phanero-
zoic deposit contains abundant (.100) medusae impressions (Pickerill,
1982). Thus, genuine jellyfish deposits offer seldom-preserved insights
about soft-bodied pelagic metazoans, preservation in Phanerozoic
coarse clastic rocks, and the nature of marginal marine mass strandings.
STRATIGRAPHIC AND PALEOENVIRONMENTAL
CONTEXT
The Mt. Simon and Wonewoc Sandstones are known for spectac-
ular preservation of the tire-track–sized trail Climactichnites (Yochel-
son and Fedonkin, 1993) and record interfingering shallow-marine, flu-
vial, and eolian facies (Dott et al., 1986; Mahoney et al., 1997). In
north-central Wisconsin, isolated erosional outliers of these units reflect
deposition of shoreface to shallow-subtidal marine sands in a silici-
clastic sediment-dominated barrier island complex along the southern
margin of the Wisconsin dome (Driese et al., 1981; Haddox and Dott,
1990; Runkel et al., 1998). In western Wisconsin it is possible to dis-
tinguish between the Mt. Simon and Wonewoc on the basis of litho-
logic, facies, and biostratigraphic criteria, especially where formations
are separated by fossiliferous mudstones of the Eau Claire Formation
(Fig. 1). However, in our study site exposures the Eau Claire is absent;
therefore we can only assign the strata to the uppermost Mt. Simon
Sandstone or basal Wonewoc Sandstone.
In the Krukowski flagstone quarry, at least seven flat-lying planar
bed surfaces contain hundreds of medusae impressions preserved in
convex and concave epirelief (Figs. 2 and 3). Medusae occur on planar
and rippled, medium-grained quartz sandstone surfaces, many ofwhich
are overlain by thin, silty sand layers. Medusae are underlain bycoarse,
148 GEOLOGY, February 2002
Figure 2. Fossil and sedimentary structure distribution within Kru-
kowski quarry. Upper inset: size histogram of medusoids on two of
best-exposed bedding planes, measured as maximum inner diameter
of convex ring surrounding former medusoid carcasses. Lowerinset:
ripple-crest orientations at Krukowski and nearby Nemke quarry. Rip-
ples suggest paleowave oscillation that conforms approximately with
regional paleocurrent patterns in western Wisconsin.
bed-parallel laminations or, more rarely, low-angle ripple laminations
(Appendices DR1 and DR2
1
). Several mud layers in the quarry are
characterized by deep polygonal cracks. Although red oxidized mud
and sand intraclasts occur within ripple troughs, no suspect microbial
structures (Hagadorn et al., 1999) were noted. Dominant sedimentary
structures are planar bedding and symmetrical ripples with straight
crests and tuning-forked junctions; small channels, low-angle cross-
stratification, ladder ripples, and asymmetric ripples also occur. Dom-
inance of planar lamination implies swash and backwash in a beach
environment, whereas ripples require a greater depth slightly seaward
of the beach (Komar, 1998). Small ripple spacings (1.5–4 cm) and
coarseness of the rippled sandstones (0.2–1 mm) indicate waves with
periods of only a few seconds but with vigorous orbital motions. In
western Wisconsin the Mt. Simon Sandstone has evidence of a major
tidal influence with a range of ;1–2 m (Driese et al., 1981). Hyolithid
impressions and a moderately diverse marine ichnofauna (Fig. 2; Ap-
pendix DR3; see footnote 1) suggest that many beds were deposited
1
GSA Data Repository Item 2002010, Appendices 1–4, Supplementary
sedimentologic, taphonomic, and paleoecologic information, is available from
Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, editing@
geosociety.org, or at www.geosociety.org/pubs/ft2002.htm.
under marine conditions. However, medusae impressions co-occur with
putative marginal marine ichnofauna, including Climactichnites and
Protichnites (Yochelson and Fedonkin, 1993; MacNaughton et al.,
1999). Together, these features are consistent with an intermittently
exposed intertidal and shallow-subtidal setting that was probably lo-
cated in a shallow lagoonal area with limited wind fetch (P.D. Komar,
1999, written commun.). Its location within a possible sandy barrier
island system on the flank of the Wisconsin dome may have further
restricted the environment, and severe tropical storms (Dott, 1974) pro-
vide a plausible mechanism for medusoid stranding.
PALEOBIOLOGIC INTERPRETATION
Examination of part-counterpart specimens and bed-surface and
sole impressions yielded no evidence that discoidal structures from the
Krukowski quarry were formed by inorganic processes. X-ray radio-
graphic, serial slab, and petrographic study of overlying and underlying
strata from part-counterpart specimens revealed no postdepositional
disruption or collapse of overlying or underlying laminations, most of
which are bed parallel (Appendices DR1, DR2, DR4; see footnote 1).
The lack of postdepositional deformation of surface structures (i.e.,
adjacent and overlying ripples), absence of geometrically symmetrical
spheroidal structures, and low synoptic relief also exclude inorganically
formed structures such as sand volcanoes, monroes, evaporite pseu-
domorphs, concretions, or gas escape structures. The absence of evi-
dence for sediment excavation, upward migration during sediment ac-
cretion, or organism burrowing or resting activities precludes
interpretation of these structures as discoidal trace fossils such as Ber-
gaueria. Ichnofabric indices (Droser and Bottjer, 1986) are typically
0–1 throughout the quarry, and abundant sediment laminations suggest
minimal postdepositional bed disruption by burrowing organisms. Al-
though some modern molluscan egg cases can make discoidal impres-
sions when washed ashore, experimental taphonomic data and similar-
ities with modern medusae mass-stranding events suggest that the
Wisconsin structures are most likely impressions made by radially sym-
metric soft-bodied organisms, such as cnidarian medusoids.
Hydrozoa and Scyphozoa are the principal cnidarian classes that
have life cycles dominated by a medusoid form. Assigning the Wis-
consin fossils to a taxon within these classes requires uniformitarian
assumptions and the presence of diagnostic structures such as the ve-
lum, lappets, or septate gastrovascular cavity. Because such diagnostic
characters are absent, and misleading taphonomic artifacts are common
(e.g., radial or crossed desiccation cracks in central portions of fossils),
we do not formally describe fossils herein. However, the fossils’ lack
of typical chondrophorine pneumatophore features, such as concentric
ring-shaped corrugations or radial structures, suggests that they are not
chondrophorine hydrozoans.
MODE OF BURIAL
How are jellyfish stranded? During mass strandings, most jellyfish
settle on the surface with their subumbrella down, and they commonly
pump their bells in an attempt to escape from stranding (Scha¨fer, 1941,
1972; Bruton, 1991; Rozhnov, 1998). This escape behavior often com-
pounds their fate, because they pump much of their stomach and in-
ternal cavities full of sand, thus exacerbating their fate throughextreme
sand loading. After prolonged exposure, their carcasses shrink in size
and interspaces between umbrellar lobes increase, sometimes leaving
a thin film of dehydrated mesoglea that contains symmetry similar to
the original animal (Bruton, 1991). After further decay, the only struc-
ture often preserved within the umbrellar center is a convex sediment
mound formed either through pulsing of the dying jellyfish or decom-
position of the sediment-laden gastrovascular cavity (Nathorst, 1881;
Scha¨fer, 1941; Norris, 1989). Around these sediment mounds, concave
rings can form when jellyfish repeatedly contract their bell marginupon
touching the substrate, excavating a moat-like circular depression near
GEOLOGY, February 2002 149
Figure 3. Rippled bedding
plane with abundant me-
dusae impressions at me-
ter 7.15 in Fig. 2 (A, B).
Both bed surfaces (A, B,
E, G) and bed soles (C, D,
F) were examined, includ-
ing part-counterpart spec-
imens (Appendix DR4;
see footnote 1). Although
a wide range of morphol-
ogies exists, probably
reflecting varying behav-
ioral, morphologic, and
taphonomic parameters,
most medusae impres-
sions have a central con-
vex mound of sediment
surrounded by a convex
sediment ring. This mound
is sometimes character-
ized by quadripartate or ra-
dial cracks (C–E, G) or
contains two distinct con-
centric rings (E, G), per-
haps outlining margins of
collapsed gastrovascular
cavity. Carcass impres-
sions are often elongated
parallel to current directions (B, C, F, G) or exhibit rill marks on downcurrent side of sediment rings deposited around carcass margins (upper
medusoid in B), suggesting minor poststranding reorientation of carcasses. In some cases, convex rings have asymmetrically steepened edges
(G), perhaps reflecting organism escape behaviors. First few layers of sediment deposited around and over medusae impressions often exhibit
multiple generations of ripples (C); together with absence of ripples within central sediment mound, these features suggest survival of intact
carcasses through multiple tidal cycles. Width of field of view: A (foreground)—5 m; B—52 cm; C—45 cm; D—70 cm; E—55 cm; F—22 cm; G—
34 cm.
their bell margin (Kornicker and Conover, 1960; Bruton, 1991). Con-
vex rings also form around these mounds as sediments accrete around
umbrellar margins. In subsequent sedimentation events, ripples abrupt-
ly terminate at these ring margins (Scha¨fer, 1941; Linke, 1956; Kor-
nicker and Conover, 1960; Hamada, 1977). In coarse-grained sedi-
ments, jellyfish tissues cannot be fossilized; however, impressions of
their carcasses and penultimate behaviors can be preserved when bur-
ied by damp sediments that are subaerially exposed or intermittently
wetted (Scha¨fer, 1941, 1972; Kornicker and Conover, 1960; Thiel,
1971), like the sediments that characterized Mt. Simon environments.
Like extant occurrences, the Wisconsin medusae are characterized
by flat-topped to rounded central mounds of sandy to silty sediment,
the latter of which sometimes contain quasiradial cracks (Fig. 3; Ap-
pendix DR4; see footnote 1). Although these cracks are likely desic-
cation features, Nathorst (1881) and Rozhnov (1998) suggested that
such cracks may mimic the tetramerous or quadripartate manubrial,
gonad, or oral structures of extant medusoids. Few of the Wisconsin
medusae impressions have well-developed concave rings, suggesting
little poststranding bell contraction among the preserved population.
However, nearly all specimens exhibit convex rings, many of which
have rill marks on downcurrent margins and are associated with thin
ripple-marked bed surfaces (Fig. 3; Appendix DR4; see footnote 1). In
sediment layers immediately overlying the initial stranding layer, rip-
ples are absent within this ring, suggesting rehydration and/or survival
of the carcass through multiple tidal cycles (Scha¨fer, 1941).
The following scenario for medusoid stranding, decay, and burial
is hypothesized. (1) Jellyfish were blown or swam into a shallow em-
bayment. (2) The tide ebbed and escape behaviors were initiated, in-
cluding ingestion of sediment. As the tide waned, some jellyfish con-
tracted their bells atop the sediment, forming convex depressions under
their outer umbrellar margins. The majority of jellyfish were dead or
did not pulse, so they formed no depressions near their umbrellar mar-
gins. Many carcasses became oriented with their long axes parallel to
the current direction. Ripples formed in adjacent sediments, but not in
carcass centers. Carcasses may have been subaerially exposed. (3) The
tide returned and sediment was occluded around carcasses, forming
convex rims around carcass margins. During ebb tide, some rill marks
formed in or near downcurrent parts of rims. Ripples formed in this
second sedimentation event have a slightly different orientation and
amplitude compared to those of step 2. (4) The tide ebbed and me-
dusoids were subaerially exposed. As the carcasses desiccated, their
guts collapsed, depositing mounds of sediment, and quasiradial cracks
formed in the center of the mounds, perhaps after multiple episodes of
exposure and rehydration. Sediment in the carcass center may have
been reworked by short-period waves generated by winds acting on
ponded water. (5) The tide returned, depositing more sediment, some-
times completely burying the entire medusoid carcass and marginal
sediment rims. Elsewhere, only the carcass center and areas outside of
sediment rims were covered, and tops of sediment rims were reworked,
with ripples of yet another orientation and amplitude. (6) After one or
more tidal cycles, the entire carcass was covered.
SIGNIFICANCE
How do these medusae compare to the only other medusae-strand-
ing deposit? The Wisconsin medusae are broadly similar to the un-
named medusae of the Upper Cambrian King Square Formation of
New Brunswick, Canada (Pickerill, 1982, 1990). Despite taphonomic
overprinting, fossils from both deposits have marginal ridges and fur-
rows, which separate a convex central region. The most striking dif-
ferences between these deposits are that Wisconsin medusae lack radial
canals, have greater surface relief, and are much larger (mean diameter
of ;25 cm; Fig. 2) than the New Brunswick medusae (;7 cm).
The Wisconsin medusae are the largest known from the fossil
record, including many specimens .50 cm in diameter. Although this
size is not atypical for modern medusae, fossil medusae are rarely
larger than 10 cm. Based on the uniformitarian assumption that ancient
medusae were nektonic carnivores (Lipps et al., 2000), could these
150 GEOLOGY, February 2002
fossils join the anomalocaridids (Whittington and Briggs, 1985) as the
largest Cambrian predators?
CONCLUSIONS
These fossils indicate that abundant populations of large cnidarian
medusae lived in Cambrian seas, and that they have been getting
stranded en masse for the past 510 m.y. Given the abundance of Phan-
erozoic strata representing marginal marine facies and the key role
these pelagic organisms played in associated ecological regimes, why
are no post-Cambrian medusae strandings known? Moreover, why do
the only two known mass strandings occur in the Late Cambrian?
Exceptional obrution- or stagnation-style preservation processes
typical of Phanerozoic lagersta¨tten cannot be invokedfor the Wisconsin
deposit; these medusae were exposed to ‘‘normal’’ Phanerozoic taph-
onomic processes, including wave-dominated and subaerial burial pro-
cesses. Yet lack of typical Proterozoic-style microbial sedimentary
structures and fossils precludes invoking Neoproterozoic-style preser-
vational processes. Although it is possible that mucous may have aided
preservation of the medusae and Climactichnites trails in the quarry,
evidence for such features is lacking. Thus, we hypothesize that pres-
ervation of medusoid carcasses stems from: (1) lack of erosional scour-
ing of fossiliferous layers by poststranding tides, waves, and wind; (2)
minimal scavenging by terrestrial and intertidal organisms; and (3) lit-
tle or no postburial bioturbation. Given the early Paleozoic onset of
terrestrial bioturbation and the onslaught of scavenging that accom-
panied the Cambrian explosion, these Late Cambrian stranding events
represent a unique window in geologic time, where Phanerozoic soft-
bodied organisms could be preserved in a setting not yet dominated
by Phanerozoic-style taphonomic feedback mechanisms.
ACKNOWLEDGMENTS
We thank the Krukowski family for quarry access; M. Droser, L. Gersh-
win, P.D. Komar, J. Lipps, G. Stanley, B. Waggoner, and an anonymous re-
viewer for discussions and comments on this manuscript; and A. Friedrich and
K. Hagadorn for translations. Hagadorn thanks J. Kirschvink, K. Nealson, and
the Division of Geological and Planetary Sciences for postdoctoral support.
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Printed in USA
1
APPENDICES FOR GSA DATA REPOSITORY
Appendix 1: Several specimens were CT-scanned and subsequently slabbed and thin-
sectioned to examine the nature of sedimentary structures underlying and overlying medusae
impressions. For example, we serially sectioned a large slab from horizon at 6.70 m in Krukowski
Quarry, which contained a large medusoid impression in its center and a small impression at far left
(A). Ten slices were made through the center of larger specimen (B, C) to examine the nature of
sediments underlying the medusoid impression. All slabs (1-11) exhibit parallel lamination, low-
angle inclined lamination, or layers of unlaminated sediment bounded by laminations on upper and
lower surfaces. Similarly, overlying sediments (e.g., Appendix 4A-C) are characterized by similar
bed-parallel sedimentary features, and no post-depositional bed disruption. Based on these
observations an inorganic mode of formation for medusae impressions is rejected. Slab in A is 145
cm wide, and fields of view in images 1-11 are approximately 50 cm wide.
Appendix 2: Magnified view of slab number 7 from Appendix 1, illustrating bed-parallel
laminations underlying the entire medusoid impression. Note absence of sedimentary structures in
sediment which makes up the convex central mound of the medusoid (white arrows), suggesting
deposition of this sediment without wave/tidal influence. These features are consistent sediment
which may have been deposited from collapse of gastrovascular cavities and/or that was reworked
during post-stranding contraction of scyphomedusae bells. Field of view is 22 cm wide.
Appendix 3: Contextual biostratigraphic and paleoenvironmental information from the
Krukowski quarry, including data from bed soles (A,C-F) and bed surfaces (B). Fossils which are
most relevant to biostratigraphic and paleoenviromental interpretations are a thin coquina of
hyolithid molds (A), and trace fossils such as Protichnites (B, C), Rusophycus (D), and
Climactichnites (E). Whereas the majority of the trace fossil producers and hyolithids likely
inhabited marine settings, it has been suggested that the organisms that produced Climactichnites
and Protichnites may have also ranged into settings that were subaerially exposed (Yochelson and
Fedonkin, 1993; MacNaughton et al., 1999). Evidence for subaerial exposure includes desiccation
features such as sand-loaded polygonal mudcracks (F). Width of field of view in A is 11 cm, B is
2
30 cm, C is 50 cm, D is 26 cm, E is 110 cm, F is 65 cm.
Appendix 4: Medusae impressions exhibit a wide range of morphologies, and are preserved
in convex and concave epirelief on bed surfaces (A-F,J,K), and bed soles (G-I). Although most
beds overlying medusae were examined in-situ, some part-counterpart specimens were extracted
intact, whereby overlying layers could be separated from the initial stranding surface. For example,
A-C illustrate a typical rippled surface characterized by three medusae impressions; in these images
the overlying layer is successively removed (including the sediment ring which forms around the
carcass) revealing a small mound of sediment presumably representing the margins of the
medusoid carcass or its gastrovascular cavity. This sediment mound is not rippled like the
remainder of the inferred stranding surface (C), and in many cases exhibits weak radial or tripartate
or quadripartate cracks in the center (upper specimen in C, I). Many specimens are elongated
parallel to inferred current directions (E,F,H,K), suggesting post-stranding orientation of
specimens, perhaps concomitant with organism escape behaviors (e.g., far right specimen in H).
Some specimens exhibit rills on sediment rings deposited around carcass margins (K), and
multiple generations of ripples in adjacent and overlying sediments (G), suggesting carcass
exposure through multiple tidal cycles and possible reworking of sediments within marginal rings
after carcass decay. Rare foam marks (D) on thin sediment layers overlying specimens are
consistent with deposition in an intertidal setting. Width of field of view in A-C is 65 cm, D is 85
cm, E is 90 cm, F is 40 cm, G is 75 cm, H is 85 cm, I is 43 cm, foreground in J is 95 cm, K is 29
cm.
Appendix 2
(for GSA Data Repository)
convex margin central sediment mound convex margin
7
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