CHANGE IN EDENIAN TRIARTHRUS SPECIES ALONG A DEPTH GRADIENT IN THE TACONIC FORELAND BASIN, PENNSYLVANIA

A study was conducted to identify and interpret the Triarthrus trilobites found in the Upper Reedsville Formation at Swatara Gap, PA and the Antes Shale at Bellefonte, PA. Both units are of Early Eden age. They also contain Triarthrus species resembling both beckii and eatoni, which have been much described by many past workers. Although some workers have regarded the two species as synonymous, we agree with those who regard them as discrete species.

Triarthrus beckii has a short palpebral lobe that terminates at a line that intersects the distal ends of the S2 furrows plus a maximum of 16 thoracic segments. Lines drawn from the posterolateral angles of the fixigena/cephalon, through the abaxial ends of the S1 lateral glabellar furrows, intersect (sag.) on the anterior glabellar furrow/border of the cephalon. T. eatoni has a long palpebral lobe that terminates at a line that intersects the distal ends of the S1 furrow plus a maximum of 14 thoracic segments. Lines drawn from the posterolateral angles of the fixigena/cephalon, through the abaxial ends of the S1 lateral glabellar furrows, intersect (sag.) on the frontal lobe of the glabella. Shape of the glabella and posterior end of the facial suture were also used to distinguish the species but are less reliable in flattened/sheared specimens.

The Antes Shale at Bellefonte, PA is a deep ramp (basin axis) dark-gray to black calcareous shale containing only T. beckii. Like other reported occurrences of beckii, it was the dominant member of a low diversity fauna that lived under what other workers have described as dysaerobic/dysoxic. The Reedsville Formation (top) at Swatara Gap, PA contains both eatoni and beckii. This is the first record of both species occurring in the same location, lithology (olive gray, micaceous, silty mudstone that is interbedded with cross-laminated siltstone and very fine-grained sandstone) and community (high-diversity fauna living under oxic/aerobic conditions). Triarthrus eatoni and beckii lived together as uncommon members of a diverse community in the oxic/aerobic lower shoreface environment of eastern Pennsylvania while only beckii (larger and more abundant) lived 70 miles west as the dominant member of a low diversity fauna living in a deeper (below wave base) dysaerobic/dysoxic basin-axis environment.