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First record of a horseshoe crab trackway from the Bloomsburg Red Beds (Silurian: Ludlow Series) of New Jersey.

ABSTRACT: A trackway (Kouphichnium Nopcsa 1923) attributed to a horseshoe crab is described for the first time, from the Bloomsburg Red Beds (Silurian: Ludlow Series), of the Delaware Water Gap National Recreation Area, New Jersey. The Bloomsburg Red Beds have generally been regarded as a meandering stream-floodplain deposit subject to temporary marine transgressions. The associated monospecific, sporadic, simple burrowing, infaunal trace fossil Skolithos Haldeman 1840, as well as the presence of the putative horseshoe crab trackway on the same bedding surface as a previously documented ancestral horseshoe crab, strongly suggests a marginal marine-brackish water origin for the strata.

KEY WORDS: horseshoe crab trackway, Silurian, Bloomsburg Red Beds, New Jersey

INTRODUCTION

Ancestral horseshoe crabs (Class Merostomata, Order Xiphosurida, Suborder Synziphosurina), often found associated with eurypterids in Silurian and Devonian strata, are sporadically represented in the geologic column (Stormer, 1955). Horseshoe crabs range in age from the Ordovician to the Holocene, and though found in marine deposits, most Silurian samples seem to be in marginal-marine, brackish or perhaps freshwater sediments (Stormer, 1955). Ancestral horseshoe crabs of Silurian or Devonian age are known from multiple continents (e.g., Stormer, 1955). Examples from eastern North America include ones reported from New York State (Clarke, 1902; Ruedemann, 1916; Babcock et al., 1995), Pennsylvania (Babcock et al., 1995), and New Jersey (Metz, 2000). In addition, there are reports of trackways, burrows, resting traces, or locomotion trails of these forms (e.g., Wright and Benton, 1987; Babcock et al., 1995; Romano and Whyte, 2003).

This paper describes an ancestral horseshoe crab trackway from the Bloomsburg Red Beds (Silurian: Ludlow Series) of northwestern New Jersey. Found in the Delaware Water Gap National Recreation Area, this is the first trace fossil evidence of horseshoe crabs in this deposit.

GEOLOGIC SETTING AND PREVIOUS WORK

The Bloomsburg Red Beds (Silurian: Ludlow) are approximately 460 meters thick, and consist of red, green, and gray siltstones, shales, sandstones and conglomeratic sandstones in upward-fining sequences (Epstein, 2001). Sedimentary structures include cross-bedding, desiccation cracks, small-scale ripples and cut-and-fill (Epstein, 1971). Fossil evidence includes local occurrences of fish scales (Beerbower and Hait, 1959), an ancestral horseshoe crab (Metz, 2000), as well as the trace fossils Rusophycus Hall 1852, Cruziana d'Orbigny 1842, and Skolithos (Martino and Zapecza, 1978; Metz, 2000). The Bloomsburg Red Beds have been interpreted to have formed in a meandering stream-floodplain environment subject to temporary marine transgressions (e.g., Smith, 1967; Epstein, 1971; Martino and Zapecza, 1978).

A trackway of a horseshoe crab was found in the Bloomsburg Red Beds at Buttermilk Falls in the Delaware Water Gap National Recreation Area of northwestern New Jersey (41[degrees]08'05" latitude, 74[degrees]53'11" longitude, Fig. 1). The trackway (Fig. 2), occurs beneath an overhang, and is uncollectable due to the presence of extensive fractures. The trackway occurs on the sole of an overlying 38 cm thick, light-gray, mostly medium-grained, lithic arenite (N 70[degrees] E, 50[degrees]NW, Fig. 3). The overlying 38 cm section of lithic arenite consists of 5 cm of parallel laminations that exhibit small-scale ripples 0.3 m updip of the trackway; followed by 10 cm of small-scale, cross strata; 7 cm of parallel laminations, 8 cm of small-scale, cross strata, and 8 cm of parallel laminations, in ascending order. Interestingly, vertical burrows of the trace fossil Skolithos linearis Haldeman 1840 penetrate portions of the entire section, though the degree of bioturbation is low. This unit is underlain by 50 cm of highly fractured, reddish brown siltstone. In turn, this siltstone is underlain by 80 cm of light-gray, lithic arenite exhibiting alternating horizontal laminations and cross-strata, as well as sporadic Skolithos burrows. Thus, overall, the entire section represents a portion of an upward-fining sequence. The trackway lies 13 m downdip along the same bedding surface as that of an ancestral horseshoe crab (Metz, 2000), and approximately 1 m stratigraphically higher in the section than the trace fossils Rusophycus and Cruziana described from the same location by Martino and Zapecza (1978).

[FIGURE 1 OMITTED]

THE HORSEHOE CRAB TRACKWAY

The trackway, preserved in positive hyporelief, consists of well preserved impressions on the left side, and less distinct impressions on the right side (Fig. 2). A median groove is lacking. The total length of the trackway is 3 cm, and the maximum external width is approximately 4 cm. The scratches on the left side of the trackway display greater curvature than those on the right side, which are mostly linear in form. Overall, the shape of the tracks varies from gently curvilinear, through slightly convex outwards, straight, to bifid. The angle of the individual scratches to the midline is greater on the right than on the left. The individual scratches range in length from 0.2-0.7 cm.

Though the pattern is somewhat limited in extent, the general form and disposition of the trackway is most similar to the ichnotaxon Kouphichnium Nopcsa 1923. Kouphichnium is commonly ascribed to the activity of xiphosaurans. This is a convincing interpretation for this trace, as it occurs in close proximity and along the same bedding surface as a previously recorded ancestral horseshoe crab (Metz, 2000). This trackway records the first evidence of such in the Bloomsburg Red Beds.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The Bloomsburg Red Beds exhibit well-defined, fining-upward sequences, and comprise fine to coarse grained sandstone grading upward into siltstone and shale (Smith, 1967). In turn, the existence of these deposits suggests a meandering stream-floodplain interpretation (e.g., Smith, 1967; Epstein, 1971). However, the presence of fish scales and the trace fossils Rusophycus, Cruziana, and Skolithos indicate shallow marine to brackish conditions, at least intermittently (Beerbower and Hait, 1959; Martino and Zapecza, 1978; Metz, 2000). In contrast, within the horseshoe crab-bearing trackway strata, alternation of cross-bedded sand overlain in turn by parallel-laminated sand, is similar to that observed in modern crevasse splays (O'Brien and Wells, 1986). Furthermore, the presence of lamination points to a lack of active shallow-tier burrowers. As such, the ichnofabric, characteristic of a higher energy setting, is consistent with a crevasse splay interpretation. Interestingly, however, horseshoe crab remains are often found in inferred brackish and freshwater deposits of the Silurian and Devonian; they are less commonly known from shallow marine environments (Stormer, 1955; Eldredge, 1974). Martino and Zapecza (1978) indicated that trilobites were the tracemakers usually responsible for Rusophycus and Cruziana, and concluded that marine waters were responsible for deposition of portions of the Bloomsburg Red Beds. However, the trace fossil Rusophycus does not always indicate a trilobite burrow (e.g., Seilacher, 1960) and even nonmarine occurrences of this ichnotaxon have been recorded (e.g., Bromley and Asgaard, 1979). In addition, Skolithos is commonly interpreted as opportunistic, vertical dwelling burrows formed by annelids or phoronids (Osgood, 1970; Alpert, 1974). Though most distinctive of shallow marine environments (e.g., Bottjer and Droser, 1992) characterized by high current energy or wave sites, it is a wide-ranging form, having been found in deep-marine (Crimes, 1977) to nonmarine deposits (e.g., Woolfe, 1990). Of particular relevance to interpretation of the Bloomsburg succession are the works of Ranger and Pemberton (1992), Pemberton and Wrightman (1992), Gingras et al. (1999), and MacEachern and Gingras (2007), who suggested that these conditions characterize brackish-water trace fossil assemblages hy: 1) a low diversity; 2) domination by monospecific suites; 3) traces typically present in marine rather than freshwater environments; 4) a trend toward simple burrowing structures formed by opportunistic organisms; (5) sporadically distributed ichnogenera; and 6) infaunal traces predominating over epifaunal ones. Thus, the ichnological character of the Bloomsburg Red Beds suggests a brackish water origin (near a marine shoreline) for the trackway-bearing strata.

ACKNOWLEDGMENTS

I thank the two anonymous reviewers for critically reviewing and making very useful suggestions for improvement of the original manuscript. I thank Pat Lynch, Chief, Resource and Research Planning Division, Delaware Water Gap National Recreation Area, for access to collect and study specimens. I thank Rodrigo Pellegrini, Registrar, New Jersey State Museum, and Jeffrey Shreiner, Delaware Water Gap National Recreation Area, for their continued support. Special thanks are given to Simon Braddy, University of Manchester, who provided valuable comments on the trackway.

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DEPARTMENT OF GEOLOGY AND METEOROLOGY, KEAN UNIVERSITY, UNION, NEW JERSEY, 07083, RMETZ@KEAN.EDU
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Author:Metz, Robert
Publication:Bulletin of the New Jersey Academy of Science
Article Type:Report
Geographic Code:1USA
Date:Sep 22, 2009
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