fossil from the Silurian Tuscarora Fm., Ridge and Valley Province,
alleghaniensis (scale: long bar 4 in)
an Early Silurian trace fossil, covers the base of
a sandstone bed from the Tuscarora
The critter making these burrows lived some 435
million years ago.
A closer view illustrates the branching nature of the
burrows, many of which display strongly defined ridges
oriented at right angles to the long axis of the burrow.
(scale: long bar 4 in)
the upper, yellowish-gray unit, Arthrophycus
burrows extend below the base of the bed and cut into the
darker, lower unit.
approximates bed contact.
the lower, gray shaly unit, horizontal burrows appear as
lighter ovals and masses (circled).
view of Arthrophycus extending into the lower shaly
trace fossils occur in a Tuscarora Fm. exposure in the
Lewistown Narrows in the recently cut north bench along PA
is the name paleontologists assigned to a group of
trace fossils that range from single to compound elongate
burrows displaying transversely annulated (ringed or banded)
applies to those compound, fan-like
assemblages of annulated burrows appearing on the bases of
sandstone beds. This fossil was first described in
1831 observed on an ornamental slab of rock in front of a
tavern at the base of Shade Mountain, Mifflin Co., PA (Rindsberg
and Martin, 2003, p. 202-203).
What are trace
ichnofossils (Greek ikhnos for "track" or "trace")
by geologists, result from the activity of an individual
of organisms who
modify the environment in which they dwell. In this
case, both single and fan-like
typically marked by transverse ridges (articulations), were generated by
animals tunneling through a muddy and sandy substrate within
a shallow marine environment. (The
illustration for this
web page shows a shallow marine substrate covered with
sand-size material somewhat comparable to that Silurian setting
Over time, the mud turned to
mudstone (shale), and the silt and sand became siltstone and sandstone,
a process geologists call lithification.
As the mudstone weathers away, the trace fossils are exposed on the bottoms
of the siltstone and sandstone layers.
What can we say
about the rocks containing
During the Early Silurian
Period (in lower Paleozoic Era), our part of the northern
Appalachian basin was actually located somewhere between 20° to 25° S latitude (that's below
the Equator). Siliciclastic (quartz-rich) sediment, being eroded
from (the Taconic) highlands to our east (equivalent to off the coast of
New Jersey), was carried by rivers westward across southeastern and central PA toward the shore of a large
(epeiric) sea that covered much of the interior of the US to the west of
Throughout the Ridge and Valley of central PA the Tuscarora Formation comprises interbedded,
quartz-rich sandstone (quartz arenite), siltstone, and mudstone beds. These bedrock
layers were deposited in a variety of shallow, marginal-marine
environments, including tidal channels, barrier bars, tidal flats, and
Here, in this part of
northcentral PA, the Tuscarora Formation underlies the northwestern
ridge of Bald Eagle Mountain and many of the ridges to the south.
addition to Arthrophycus,
types of trace fossils are reported in the Tuscarora Fm.; specifically
Rusophycus (Cotter, 1983).
Trace fossils such as these aid geologists in synthesizing
paleoenvironmental interpretations for geologic units,
coastal configuration, water
type of current or wave activity.
get its name?
Arthrophycus alleghaniensis is the ichnogenus
and ichnospecies designation for this specific trace fossil.
ichnogenus name, is used to refer to this general category
of traces, especially if the ichnospecies is unable to be
keeping with the formal classification system (taxonomy)
used by biologists and paleobiologists (paleontologists) for
identifying and naming organisms over the last 300 hundred
years, this and all trace fossils belong to a special category of
fossils formally called ichnofossils.
Rindsberg (2001) helps us clarify the taxonomic history
for Arthrophycus alleghaniensis.
This trace fossil was originally named Fucoides
alleghaniensis in 1831 by R. Harlan.
described it as an extinct species of fossil vegetable.
Recognize that at this time the scientific understanding of fossils,
especially trace fossils, was in its infancy.
Notable, however, is the fact that this trace fossil was the
first to be named in North America. Later, in 1852,
noted geologist James Hall supplied the generic ichnogenus name
Arthrophycus, and all subsequent references to this fossil
continue to use this name.
from the Greek; specifically, "arthro" means joint or
pertaining to the joints, and "phycus" (phŷkos) refers to
seaweed or algae. These traces were originally
interpreted as the remains of plant matter.
unique to northcentral PA?
in Ordovician-Silurian rocks throughout the Appalachian
Mountains of eastern North America (Ontario-Alabama).
Arthrophycus brongniartii, the second to be named in
North America, has recently been documented from Alabama.
It has, however, also been reported from localities around
the world as well as in both younger and older geologic
sequences. For example,
isp. have been reported from Lower to Middle Ordovician
Portugal; a photo of
Libya is included above; and
Arthrophycus minimus, has been described from Upper
Cambrian rocks in Argentina.
What can we say
about the critter who made
Published work in
suggests that the
animals that made Arthrophycus are
thought to be invertebrate arthropods (perhaps
In 2003, Rindsberg and Martin proposed that a
Raphiophorid trilobite could have been the critter
responsible for these burrows; and they suggest that
2) is candidate for the tracemaker of
Arthrophycus (p. 201).
However, no direct
evidence, specifically, no
body fossils have been documented from these rock units to date.
Therefore, anyone who finds such evidence would be making a significant
contribution to the understanding of these specific trace fossils as
well as to the science of paleobiology (paleontology).
1983, Shelf, paralic, and fluvial environments and eustatic
sea-level fluctuations in the origin of the Tuscarora Formation
(Lower Silurian) of central Pennsylvania: Journal of Sedimentary
Petrology, v. 53, no. 1, p. 25–49.
Rindsberg, A. K.,
November 13, 2001, Arthrophycus or Harlania?:
Posting # 3, Skolithos archives list serve, RedIRIS - Spanish
Academic Network (a forum on trace fossils).
Rindsberg, A. K.,
and Martin, A. J., 2003, Arthrophycus in the Silurian
of Alabama (USA) and the problem of compound trace fossils:
Palaeogeography, Palaeoclimatology, Palaeoecology, v. 192, p.
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