Early Myriapod-Like Body Fossil and Associated Walking
Trace from a Silurian Lagerstätte, Ohio
Cary Easterday
(Department of Geological Sciences, Ohio State University, Columbus, Ohio, USA)
(now at Department of Earth Science,
Northeastern Illinois University, Chicago, Illinois, USA)
19 October 1999
The
Tymochtee Formation (pronounced “Tymock”) (Upper Silurian) of south-central
Ohio has produced a fossil specimen preserved in Silurian plattenkalk.
This specimen was originally described as a eurypterid. It is actually
one of the earliest, or is the earliest, terrestrial myriapod or myriapod-like
animal. The fossil adds to the sparse record of Paleozoic myriapods, and
allows a comparison of a trace and its trace-maker.
The
fossil is from the Latham Limestone Quarry in Mifflin Township of Pike County,
Ohio, USA. This quarry was known as the Ralph Rogers Quarry when the
fossil was collected many years ago.
The
Silurian of Ohio is generally moderately fossiliferous and the fossils are
poorly preserved. The Silurian is generally not too well exposed in Ohio,
except in quarries. The stratigraphic section below is what is exposed in
the Latham Limestone Quarry.
The
specimen is a 2.5 cm long (incompletely preserved) body fossil and walking
trace.
Other
Tymochtee Dolomite fossils in Ohio include abundant ostracods, rare molluscs,
rare brachiopods, uncommon eurypterids, and locally abundant
stromatoporoids. It is a generally low diversity biota. There are
also small vertical tubes and horizontal traces in the Silurian Tymochtee
Dolomite of Ohio.
Sedimentary
structures in the Tymochtee Dolomite include thinly laminated beds, mudcracks,
laminar to laterally linked stromatolites, oscillation ripples, carbonate
“flaser bedding”, some mottled beds, teepee structures, “puckered” surfaces,
and flat-pebble conglomerates. Other sedimentological features include
some aeolian quartz grains, gypsum/anhydrite, and salt & gypsum crystal
casts.
The
Tymochtee Dolomite is usually interpreted as a peritidal carbonate environment
(shallow subtidal, intertidal, supratidal, evaporitive (sabkha)).
The
fossil has an incomplete head shield with a missing posterior, plus 10
distinctive, ring-like body segments. Thin appendages do appear; the
“eye” on the head shield may or may not be an eye; an antennae pair or another
appednage pair is folded under the head shield; the body is parallel-sided,
like all modern myriapods. The eurypterids have non-parallel sided bodies
(even in juveniles), and have robust appendages (not seen here). This
fossil is therefore not a eurypterid as originally interpreted.
The
trace fossil associated with the body fossil is dextrally curved (to the
right), which is also seen in other body-trace fossil associations, and has 2
parallel rows of closely spaced, small triangular impressions (coxal
impressions). The trace has many thin, backwardly curved impressions
(walking appendage impressions). Modern myriapods have backwardly curved
walking appendages.
Taphonomy: the
appendages are inwardly curved (possible dessication). Arcing is seen in
the leg traces from the movement of head and legs. No evidence of
scavenging.
Other early myriapod/myriapod-like fossils: 1) marine Wheeler Shale (Middle Cambrian); 2)
terrestrial traces in the Ordovician Borrowdale Group; 3) terrestrial traces in
the Upper Ordovician Juniata Formation; 4) terrestrial Lower to Upper Silurian
of Shropshire, England and other localities; 5) terrestrial Lower to Upper
Devonian of Rhynie, Scotland and Gilboa, New York and other localities.
Other known body fossil-trace fossil associations: Mesolimulus
and final walking traces from the Jurassic Solnhofen Limestone; Flexicalymene
and Rusophycus
from the Cincinnatian Series (Upper Ordovician).
Conclusions:
1)
a myriapod-like body fossil and associated trace fossil add to a sparse
published record of early Paleozoic myriapodan arthropods
2)
the fossil tends to confirm that some tracheates (ateloceratan or “uniramian”
arthropods) were air-breathing and terrestrial by the Late Silurian
3)
the fossil tends to confirm that some walking traces from marginal-marine
environments of the early Paleozoic have been correctly attributed to
tracheates.