Trace fossils from the Mahantango formation (upper middle Devonian) Pike County, Pennsylvania.
KEY WORDS: marine trace fossils, Middle Devonian, eastern Pennsylvania, Mahantango Formation
Willard (1935) proposed the name Mahantango Formation for strata exposed along the north branch of Mahantango Creek in Snyder and Juniata Counties, Pennsylvania. The Middle Devonian Mahantango Formation is up to 400 meters thick and makes up the bulk of the Hamilton Group present in Pennsylvania and neighboring southeastern New York (Faill, 1985; Prave et al., 1996). It consists of predominantly fossiliferous mudstones and sandstones representing shallow marine deposits arranged in coarsening-upward cycles (Prave et al., 1996). The formation delineates prograding clastic wedges that were associated with the Devonian Acadian orogeny (e.g., Dennison and Hasson, 1976; Faill, 1985). As noted by Prave et al. (1996), numerous workers who have investigated the Mahantango Formation have offered contrasting explanations regarding the depositional cyclicity. Within the Hamilton Group, an overall cycle of initial deepening followed by shallowing to the Tully Limestone is apparently the signature of loading and relaxation of the crust by the mountain belt during Acadian Tectophase II of Ettensohn (1985). As such, these include a prograding shoreline with accompanying storms (Goldring and Bridges, 1973), a fluvial-dominated delta (e.g., Willard, 1935; Kaiser, 1972; Hoskins, 1978), and storm-generated sand ridges (Sarwar and Smoot, 1983; Sarwar, 1984). In contrast, Prave et al. (1996) concluded that cyclicity could be explained due to frequent progradation and retreat of what was basically a straight, tide-influenced shoreline onto a storm-dominated marine shelf.
Thick deposits of the Mahantango Formation are present in a series of very steep, cliff exposures along Route 209 between Bushkill and Milford, Pennsylvania. Extensive fragmentation along bedding and cleavage intersections as well as weathering along vertical joint faces has resulted in an expansive apron of shale-chip rubble. As such, prospecting for trace fossils is typically limited to scattered small clasts, most ranging from 2.5-15 cm, and consisting of an assortment of upper, middle, and lower portions of the steep exposures. However, during the spring of 2008 the author, while riding along Route 209 in Pennsylvania (5.6 km south of the Milford Bridge, latitude 41[degrees]16' 19", longitude 74[degrees]50' 35", Fig. 1), came upon large clasts that had recently slumped after an extensive rainstorm. Closer inspection revealed that most of the clasts, ranging from 30-45 cm or more in length, came from an isolated lowermost exposure of the Mahantango Formation not subject to mixing from above. Due to the distinctive freshness of the sheered face, the clasts were estimated to represent an approximate thickness of 6 m. The rocks are dominantly dark-gray mudstones. Weathering varies from brownish-gray, olive-gray, pale-brown, and grayish red-purple. Detailed field investigation revealed the presence of the trace fossils Cruziana isp., Dactylophycus quadripartitum, Diplichnites isp., Helminthoidichnites tenuis, Helminthopsis hieroglyphica, Planolites annularis, Planolites beverleyensis, Protovirgularia dichotoma, ?Psammichnites isp., Treptichnus bifurcus, and Treptichnus pollardi. Associated fossils include cephalopods and bivalvia. In addition, there is evidence of fracture cleavage.
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Illustrated specimens are housed in the collection of the New Jersey State Museum, Trenton, New Jersey, with repository numbers NJSM 21880-21892 inclusive, with the remaining material in collections of the Department of Geology and Meteorology, Kean University, Union, New Jersey.
Ichnogenus Cruziana d'Orbigny 1842 (Figs. 2, 3)
Material: Ten specimens. Figured specimens NJSM 21880-21881.
Description: Straight to slightly curved specimens preserved in convex hyporelief. Trace width 3-4 mm, and up to approximately 60 mm long, composed of two lowrelief symmetrical lobes, each 0.75-1 mm wide, separated by a shallow median furrow. Transverse scratch marks are somewhat faint though discernible.
[FIGURE 2 OMITTED]
Remarks: Trilobites are commonly designated as the marine producers of Cruziana (Seilacher, 1970). In contrast, for continental deposits, possible producers include notostracan crustaceans (Bromley and Asgaard, 1979; Pollard, 1985). Though Cruziana is typically a shallow marine form (e.g., Crimes et al., 1977; Mangano et al., 1996), it has also been reported from marginal marine (e.g., Buatois and Mangano, 1997), deep marine (e.g., Pickerill, 1995), and continental strata (e.g., Bromley and Asgaard, 1979).
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Ichnogenus Dactylophycus Miller and Dyer 1878 (Fig. 4)
Dactylophycus quadripartitum Miller and Dyer 1878
Material: Two specimens. Figured specimen NJSM 21882.
Description: Small, branching burrows which radiate outward in one direction in a flabellate manner. Four branches can be distinguished, most of which terminate in a pointlike configuration. Individual branches have a diameter 1.5-2 mm, with maximum length up to 8 mm. Preserved in convex hyporelief. On one specimen, one of the branches exhibits delicate annulations on the lateral ridges which are transverse to a narrow median furrow, imparting a bilobate appearance, while on a second branch these features are very faint and poorly preserved. The lack of annulations on the other branches is due to recent erosion. A second specimen possesses a flat median groove on two of the branches taking up more than half of the width, while a third branch exhibits faint annulations.
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Remarks: Dactylophycus is a relatively rare trace fossil having been reported from Upper Ordovician shallow-water marine deposits in the Cincinnati area, Ohio (Miller and Dyer, 1878; Osgood, 1970), and Upper Cambrian? to Lower Ordovician lagoonal strata from Newfoundland, Canada (Fillion and Pickerill, 1990). Thus, to the author's knowledge, this represents the youngest occurrence of this trace fossil. Osgood (1970), in an excellent restudy of the type materials noted that Miller and Dyer (1878), designated two ichnospecies, D. tridigitatum and D. quadripartitum, with the former being the type ichnospecies. The difference between the two ichnospecies was based on the number of branches, which Osgood (1970) noted was not a critical factor due to the variability of branches from specimen to specimen. Interestingly, however, he indicated that while only specimens of D. quadripartitum could be located in the Botanical Museum at Harvard University, both ichnospecies should be retained until D. tridigitatum is located. Suzanne Constanza (Harvard University, Botanical Museum) was kind enough to send this author close-up photographs of the three slabs exhibiting D. quadripartitum (all numbered HBM 3174). Overall, as noted by Osgood (1970), the diameter of the branches (2-4 mm) as well as the maximum length (up to 15 mm) is greater than the present specimen. The same is true for specimens (2-2.3 mm wide, up to 10 mm long) reported by Fillion and Pickerill (1990). Though Osgood (1970) considered Dactylophycus a deposit feeder, both he and Fillion and Pickerill (1990) noted the absence of a main shaft. Interestingly, though one of the present specimens shows a hint of what could have been a portion of a shaft, there is no evidence of a master tunnel (cf. Osgood, 1970).
Ichnogenus Diplichnites Dawson 1873 (Fig. 5)
Material: Several specimens on a single slab. Figured specimen NJSM 21883.
Description: Main trackway consists of two parallel rows of transverse ridges, lacking a median groove. Individual ridges are spindle-shaped, partially smooth but also exhibit striations which are parallel to the trackway axis. Series comprises 5 distinct, three additional poorly preserved imprints. Ridges are 4-7 mm long, 0.75-2 mm wide. Trackway length of series of 5 ridges is 15 mm, preserved width 7-15 mm, and they are spaced 2-2.5 mm apart. Other portions of the slab exhibit sporadic spindle-shaped ridges, but not occurring in a series.
Remarks: Keighley and Pickerill (1998) discussed the problems associated with both the initial introduction of the Diplichnites by Dawson (1873), as well as the subsequent adaptation for varied morphologies (see also Fillion and Pickerill, 1990). Although the specimen closely resembles Merostomichnites strandi (Stormer, 1934) as illustrated in Hantzschel (1975) and Neef (2004), Keighley and Pickerill (1998) detail why the use of that ichnotaxon for trackways is contentious. Diplichnites has been recorded in marine (e.g., Seilacher, 1955) as well as nonmarine (e.g., Lucas et al., 2004) deposits. Due to the problems associated with the varied trackway morphologies linked to Diplichnites, assignment is best left in open ichnospecific nomenclature.
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Ichnogenus Helminthoidichnites Fitch 1850 (Fig. 6)
Helminthoidichnites tenuis Fitch 1850
Material: Ten specimens. Figured specimen NJSM 21884.
Description: Small, simple, smooth, unbranched, straight to gently winding burrows preserved in hyporelief and epirelief. Burrow fill is structureless and similar to surrounding sediments. Width ranges from 0.5-1.5 mm, and is constant within individual specimens; maximum observed length 80 mm. Crossovers between different specimens are present.
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Remarks: Although Hantzschel (1975) placed Helminthoidichnites Fitch 1850 in with Gordia Emmons 1844, Hofmann and Patel (1989) did not agree, citing the distinctive self-overcrossing of Gordia compared to the secondary amount of looping of Helminthoidichnites. In addition, Hofmann and Patel (1989) indicated that Helminthoidichnites does not exhibit the type of meander habit and is less sinuous than Helminthopsis. A number of researchers agree with this assessment (e.g., Narbonne and Aitken, 1990; Jensen, 1997; Buatois et al., 1997), and consider Helminthoidichnites a valid ichnogenus. This trace is very similar to Unisulcus minutus reported by Hitchcock (1858). Interesting, Jensen (1997) noted that theoretically, the distinction between Helminthoidichnites and Planolites is the lack of difference between the fill and surrounding sediment, a difficult assessment in burrows of very small size.
This ichnogenus ranges in age from the Late Precambrian to the Cretaceous (Narbonne and Aitken, 1990; Fregenal-Martinez et al., 1995). It has been reported from marine (e.g., Jensen, 1997) and nonmarine (e.g., Buatois et al., 1997; Uchman et al., 2004) deposits.
Ichnogenus Helminthopsis Heer 1877 (Fig. 7)
Helminthopsis hieroglyphica Heer in Maillard 1887
[FIGURE 7 OMITTED]
Material: Three specimens. Figured specimen NJSM 21885.
Description: Horizontal, smooth, unbranched, unlined, irregularly meandering trails or burrows. Diameter varies from 1.0-1.5 mm, though is constant within each specimen; maximum preserved length of 65 mm. Preserved in negative epirelief and positive hyporelief. Specimens display straight segment separated by irregular meanders.
Remarks: The greater sinuosity and irregular meandering differentiates Helminthopsis from Helminthoidichnites (Hofmann and Patel, 1989). Helminthopsis is distinguished from Gordia by its lack of self-overcrossing and tendency to meander (Pickerill, 1981). Recent in-depth reviews by Han and Pickerill (1995) and Wetzel and Bromley (1996) suggested retaining three of the previous established ichnospecies (up to 22) of this ichnogenus. However, while agreeing on two of the ichnospecies (H. hieroglyphica, H. abeli), they differed on the third, namely Han and Pickerill (1995) designating H. granulata, and Wetzel and Bromley (1996) choosing H. tenuis (see also Pickerill et al. (1998) and Wetzel et al. (1998) for further details). There is general agreement for H. hieroglyphica, specifically, the distinctive display of straight segments between irregular meanders.
Helminthopsis is interpreted to represent a grazing trail produced by a deposit-feeding organism (Buatois et al., 1997). In marine environments, proposed trace-makers include worm or worm-like forms (Chamberlain, 1971; Dam, 1990) and priapulans and polychaetes (Ksi kiewiez, 1977). Nonmarine producers may include nematodes and arthropods (Mangano et al., 1996; Metz, 1996).
Ichnogenus Planolites Nicholson 1873 (Fig. 8)
Planolites annularis (Walcott 1890)
Material: One specimen. Illustrated specimen NJSM 21886.
Description: Straight, horizontal, unbranched, ellipsoidal burrow exhibiting regularly spaced annulations. Burrow diameter 4 mm and constant, preserved length 25 mm. A burrow lining is not present, and the burrow fill is lighter in color and is of coarser grain size than that of the host sediment. Preserved in convex hyporelief.
Remarks: Planolites is distinguished from Palaeophycus by the lack of a wall lining and having burrow fill that differs from the host strata (Pemberton and Frey, 1982). Keighley and Pickerill (1995) considered the absence of a wall lining as a distinctive feature of Planolites. It has been reported from marine (e.g., Heinberg and Birkelund, 1984; MacNaughton and Pickerill, 1995; Bromley and Uchman, 2003) as well as nonmarine (e.g., Gierlowski-Kordesch, 1991; Pickerill, 1992; Buatois and Mangano, 1993) strata. The specimen of P. annularis is most comparable to a similar ichnotaxon in Osgood (1970, Plate 77, figure 3), as well as weathered specimens from the same location (cf. Osgood, 1970) collected by the author (Metz, 1996). Planolites has been interpreted to represent active backfilling of a temporary burrow by a mobile deposit-feeding organism, such as polychaetes (Pemberton and Frey, 1982).
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Ichnogenus Planolites Nicholson 1873 (Fig. 9)
Planolites beverleyensis (Billings 1862)
Material: Two specimens. Illustrated specimen NJSM 21887.
Description: Simple, horizontal, smooth-walled, circular to ellipsoidal unlined burrows. Diameter 3 mm, fairly constant within each specimen; maximum length observed is 50 mm. Burrow fill is coarser and lighter in color than the surrounding sediment. Preserved in convex hyporelief.
Remarks: Planolites beverleyensis differs from P. montanus in being a larger size and more gently curved. A lack of annulations and/or ornamentation distinguishes it from other ichnospecies of Planolites.
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Ichnogenus Protovirgularia M'Coy 1850 (Fig. 10)
Protovirgularia dichotoma M'Coy 1850
Material: One specimen. Illustrated specimen NJSM 21888.
Description: Straight, unbranched, horizontal trace, 30 mm in length, 3 mm in diameter, a portion of which exhibits a median ridge with paired, lateral, wedge-shaped, closely- spaced appendages. Lateral appendages are normal to median ridge, with length of 1 mm. Preserved in convex hyporelief.
Remarks: Recently, a number of researchers have provided excellent, in-depth reviews of Protovirgularia (e.g., Han and Pickerill, 1994; Seilacher and Seilacher, 1994; Uchman, 1998; Mangano et al., 2002). Han and Pickerill (1994), in citing morphologic variation displayed by Paleozoic specimens from eastern Canada, noted that only P. dichotoma is distinctive, and that other subsequently named ichnospecies be regarded as synonyms. In contrast, Seilacher and Seilacher (1994) recognized four additional ichnospecies in addition to P. dichotoma, namely P. rugosa (Miller and Dyer, 1878), P. tuberculata (Williamson, 1887), P longespicata (deStefani, 1885), and P trianularis (Macsotay, 1967). In turn, Uchman (1998) added three additional ichnospecies citing the collection of Ksi kiewicz (1977), namely P vagans, P. obliterata, and P. dzulynskii. Finally, Mangano et al. (2002) added P. bidirectionalis. Based on revisions by Han and Pickerill (1994), citing morphologic variation exhibited by specimens of Protovirgularia, and comments by other authors cited above, the present specimen is assigned to P. dichotoma. Interestingly, P. rugosa is a typically short trace typically connected to a smooth, Lockeia-like burrow, though evidence of this is lacking on the bedding surface.
Potential trace-makers proposed for Protovirgularia include crabs (Gumbel, 1879), annelids or arthropods (Richter, 1941; Volk, 1961; Claus, 1965; Greiner, 1972), scaphopods (Seilacher and Seilacher, 1994), and bivalves (e.g., Hakes, 1977; Han and Pickerill, 1994; Seilacher and Seilacher, 1994, Mangano et al., 1998). Since bivalves are present in the Mahantango Formation, they are the likely trace-maker responsible for Protovirgularia dichotoma.
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Ichnogenus Psammichnites Torell 1870 (Fig. 11)
Material: One specimen. Illustrated specimen NJSM 21889.
Description: Straight, bilobed trail, diameter 7 mm, maximum length 60 mm, preserved in epirelief. Surface of each lobe is elevated, somewhat rounded and smooth with sporadic, moderately well-preserved transverse striae forming segments 1-2 mm wide. Median groove shallow, 2 mm wide.
Remarks: Overall, the somewhat poor preservation and lack of a clear ridge only allows for tentative ichnogeneric assignment. Several investigators (e.g., Hofmann and Patel, 1989; McIloy and Heys, 1997; Mangano et al., 2002; Seilacher, 2007) have helped to clarify the internal structure and taxonomy of the complex morphology of this trace fossil. Nevertheless, partly due to morphologic variability, there has been confusion as to the taxonomy of Psammichnites and other related ichnogenera (e.g., Olivellites, Crossopodia, Plagiomus; see Mangano et al. 2002, for details). Seilacher (2007) provided an excellent review of the numerous preservation variants of Psammichnites. Psammichnites is a common trace fossil found in Cambrian rocks (e.g., Hofmann and Patel, 1989; Seilacher, 1997), reappears in the Silurian, Carboniferous, and the Permian (see Mangano et al., 2002 for further details). Proposed Cambrian trace-makers include worms (e.g., Matthew, 1890), crustaceans (Torell, 1870), gastropods (Hantzschel, 1975), mollusks (e.g., Glaessner, 1969), annelids (McIlroy and Heys, 1997), and echiurans (Runnegar, 1982). Recently, Seilacher-Drexler and Seilacher (1999) suggested halkieriids as a possible producer, while Mangano et al. (2002) noted the possibility of a molluscan trace-maker for Psammichnites.
[FIGURE 11 OMITTED]
Ichnogenus Treptichnus Miller 1889 (Fig. 12)
Treptichnus bifurcus Miller 1889
Material: Eight specimens. Illustrated specimen NJSM 21890.
[FIGURE 12 OMITTED]
Description: Straight to curved horizontal burrows, 1-2 mm in diameter, individual segments 4-7 mm in length, with short projections 1-1.5 mm extending from junctures between longer segments. Several of the projections curve upward, others are swollen compared to the main burrow. Burrow fill is similar to the host rock. Preserved in convex hyporelief.
Remarks: Maples and Archer (1987) emended the original description of Treptichnus, while Buatois and Mangano (1993) and Buatois et al. (1998) provided an in-depth reevaluation of this ichnotaxon. Where the burrow segments of T. bifurcus are straight, projections occur on alternate sides, where curved, the projections occur on the outside of the curved portion. The projections represent bedding plane indications of oblique shafts (Maples anal Archer, 1987; Buatois and Mangano, 1993). Burrow fill is structureless and similar to host strata. Several examples of T. bifurcus appear to grade into T. pollardi, and vice-versa. Proposed trace-makers for Treptichnus include vermiform animals and insect larvae (Buatois and Mangano, 1998). Most recordings of Treptichnus have been from nonmarine (e.g., Uchman et al., 2004) environments, though marginal marine (e.g., Archer et al., 1995) to deep marine (e.g., Crimes et al., 1981) examples have also been noted.
Ichnogenus Treptichnus Miller 1889 (Figs. 13, 14)
Treptichnuspollardi Buatois and Mangano 1993
Material: At least forty-five specimens. Illustrated specimens NJSM 21891-21892.
[FIGURE 13 OMITTED]
[FIGURE 14 OMITTED]
Description: Simple, zigzag, curved, or straight burrow segments possessing small pits, indicating openings of vertical shafts, located either at junction between, or at some position within individual segments. Diameter of segments 1 mm, individual segments range in length from 4-8 mm. The maximum number of burrow segments is 7, most have less. Sporadic burrow segments have short projections at junctures, similar to T. bifurcus. There are several examples of small pits as well as zigzag burrows lacking pits reflecting upper layer and lower layer morphology, respectively. Preserved in convex hyporelief and concave epirelief.
Remarks: The presence of pits and absence of projections distinguishes Treptichnus pollardi from T. bifurcus. Interestingly, Maples and Archer (1987) contended that Plangtichnus erraticus though similar to Treptichnus bifurcus, differed in lacking the presence of short projections that extended from junctures between individual segments. In contrast, Buatois and Mangano (1993) noted, in general, that Plangtichnus be considered a synonym of Treptichnus, due to evidence that both ichnotaxa actually represent different portions of a three-dimensional structure (e.g., individual erosional levels). Buatois and Mangano (1993) also considered in a similar light T. pollardi. Thus, in the present material, the presence of pits lacking evidence of segments, as well as segments lacking pits compares quite favorably to that illustrated by Buatois and Mangano (1993, p. 219, figure 3B, D). As with Treptichnus bifurcus, most recordings of T. pollardi have come from nonmarine environments (e.g., Buatois and Mangano, 1998).
The most recent studies in Pennsylvania and adjacent New York suggest that cyclic deposition of the Mahantango Formation could be best explained due to progradation and retreat of what was a straight, tide-influenced shoreline onto a storm-dominated marine shelf (e.g., Slattery, 1993, Prave et al., 1996). Detailed measurements of sections by Slattery (1993) and Prave et al. (1996) resulted in definition of 18-20 facies which were subsequently grouped into seven facies associations. In contrast, the present research reflects a single location in Pennsylvania and limited stratigraphic thickness. As such, however, Slattery (1993) provided a detailed description of a measured section which lies approximately 0.7 km north of the present site. Comparison to that of Slattery (1993) indicates that the deposits at the present site are most similar to a portion of Facies Association 1, dominated by locally fossiliferous, medium-to dark-gray mudstone. Slattery (1993) and Prave et al. (1996) interpret the mudstones of Facies Association 1 as that of hemipelagic deposits formed in an open marine setting (= offshore of MacEachern et al., 2007) below fair-weather wave base, reflecting relatively low energy levels.
The mudstones of the Mahantango Formation along Route 209 (Figure 1) yielded a moderate diversity and abundance of trace fossils. The biogenic structures are dominantly horizontal traces of deposit-feeding organisms (e.g., Planolites, Helminthopsis, Treptichnus). As such, the overall diversity and characteristics of the dominantly horizontal, biogenic structures allow the trace fossil assemblage to be referred to the Cruziana ichnofacies (Frey and Seilacher, 1980). In addition, given the depositional conditions, it is likely that a variety of tracemaking organisms readily took advantage of the relatively organic-rich sediments. Interesting, Slattery (1993) noted the presence of the Cruziana ichnofacies in similar deposits, but did not provide further details relative to the specific ichnotaxa.
I thank Luis A. Buatois for critically reviewing a preliminary version of this manuscript. I also thank Pat Lynch, Chief, Resource and Research Planning Division, Delaware Water Gap National Recreation Area, for access to study and collect specimens. I thank Shirley Albright and Rodrigo Pellegrini, New Jersey Science Museum, where the illustrated specimens are housed, and Jeffrey Shreiner, Delaware Water Gap National Recreation Area, for their continued support. I thank the two anonymous reviewers who provided insightful reviews that significantly improved the manuscript.
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DEPARTMENT OF GEOLOGY AND METEOROLOGY, KEAN UNIVERSITY, UNION, NEW JERSEY, 07083, RMETZ@KEAN.EDU
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|Publication:||Bulletin of the New Jersey Academy of Science|
|Date:||Jun 22, 2009|
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