Arthrophycus alleghaniensis

A trace fossil from the Silurian Tuscarora Fm., Ridge and Valley Province,
Central PA

  
Arthrophycus alleghaniensis (scale: long bar 4 in)

Arthrophycus alleghaniensis, an Early Silurian trace fossil, covers the base of a sandstone bed from the Tuscarora Formation. 

The critter making these burrows lived some 435 million years ago.

 

A closer view illustrates the branching nature of the Arthrophycus alleghaniensis burrows, many of which display strongly defined ridges oriented at right angles to the long axis of the burrow.


Arthrophycus alleghaniensis
(scale: long bar 4 in)

 

In the upper, yellowish-gray unit, Arthrophycus  burrows extend below the base of the bed and cut into the darker, lower unit.

Yellow-brown line approximates bed contact.

In the lower, gray shaly unit, horizontal burrows appear as lighter ovals and masses (circled).

 

Closer view of Arthrophycus extending into the lower shaly unit.

These trace fossils occur in a Tuscarora Fm. exposure in the Lewistown Narrows in the recently cut north bench along PA Route 22/322.

 

A photo of Arthrophycus from the Akakus Fm.  (Silurian), near Ghat, SW Libya.

North African Photo Gallery, (photo S. Luning)

 

 

 

Arthrophycus alleghaniensis FAQs

What is Arthrophycus alleghaniensis ?

Arthrophycus 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) markings. 

Arthrophycus alleghaniensis 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 fossils?

Trace fossils (1, 2, 3, 4), called ichnofossils (Greek ikhnos for "track" or "trace") by geologists, result from the activity of an individual or group of organisms who modify the environment in which they dwell.  In this case, both single and fan-like branching burrows, typically marked by transverse ridges (articulations), were generated by animals tunneling through a muddy and sandy substrate within a shallow marine environment.  (The background illustration for this web page shows a shallow marine substrate covered with sand-size material somewhat comparable to that Silurian setting discussed above.)

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 Arthrophycus alleghaniensis?

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 us.

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 lagoons.  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.

In addition to Arthrophycus, several other types of trace fossils are reported in the Tuscarora Fm.; specifically Cruziana, Monocraterion, Planolites, and Rusophycus (Cotter, 1983).  Trace fossils such as these aid geologists in synthesizing  paleoenvironmental interpretations for geologic units, including coastal configuration, water depth, and type of current or wave activity.

How did Arthrophycus alleghaniensis get its name?

The name Arthrophycus alleghaniensis is the ichnogenus and ichnospecies designation for this specific trace fossil.  (Arthrophycus, the ichnogenus name, is used to refer to this general category of traces, especially if the ichnospecies is unable to be identified.)  In 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.

A. K. Rindsberg (2001) helps us clarify the taxonomic history for Arthrophycus alleghaniensis.  This trace fossil was originally named Fucoides alleghaniensis in 1831 by R. Harlan.  Unfortunately, he incorrectly 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.

Arthrophycus derives 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.

Is Arthrophycus alleghaniensis unique to northcentral PA?

The ichnotaxon Arthrophycus (Arthrophycus isp.) occurs 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, Arthrophycus alleghaniensis and Arthrophycus isp. have been reported from Lower to Middle Ordovician sequences in Portugal; a photo of Arthrophycus from Libya is included above; and a new ichnospecies, Arthrophycus minimus, has been described from Upper Cambrian rocks in Argentina.

What can we say about the critter who made Arthrophycus alleghaniensis ?

Published work in Alabama suggests that the animals that made Arthrophycus are thought to be invertebrate arthropods (perhaps trilobites).  In 2003, Rindsberg and Martin proposed that a Raphiophorid trilobite could have been the critter responsible for these burrows; and they suggest that Cryptolithus (1, 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).

 

 

 

 

 

REFERENCES
Cotter, 1983 Cotter, E., 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. 2549.
Rindsberg, 2001 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 and Martin, 2003

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. 187-219.

 

 

 


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Last Update: 01/12/10