Late Triassic), Near Milford, New Jersey

Cent. Eur. J. Geosci. • 4(1) • 2012 • 3-8 DOI: 10.2478/s13533-011-0058-7 Central European Journal of Geosciences First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey Communication Robert Metz1∗, 1 Department of Geology and Meteorology, Kean University Union, 07083 New Jersey, USA Received 29 November 2011; accepted 3 February 2012 Abstract: Mudstones of the Triassic Passaic Formation, near Milford, New Jersey, have yielded the first evidence of an arthropod impression in that formation. Associated trace fossils include Helminthoidichnites, Lockeia, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia ichnofacies. Associ- ated sedimentary structures include desiccation cracks and raindrop impressions. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure. Keywords: fossil arthropod impression, Passaic Formation, Triassic, New Jersey, mudstones © Versita sp. z o.o. 1. Introduction come an integral part of geologic research. As noted in many summaries, e.g., [5] ichnology comprises an impor- tant tool in identifying many physico-chemical parameters Near Milford, New Jersey, a slab representing fine- (e.g., fluvial-sediment input, wave energy, subaerial versus grained lacustrine deposits of the Late Triassic Passaic subaqueous exposure, oxygenation), especially when inte- Formation has yielded the impression of an arthropod. grated with stratigraphic and sedimentological analysis. Previous studies at this site have addressed the sedimen- As such, trace fossils are unique in that they represent tology, structure, and noted the presence of burrowing paleontologic entities as well as biogenic sedimentary [1–3], while nearby [4] detailed several trace fossils from structures [5], and contrary to body fossils trace fossils al- the Passaic Formation. ways represent in situ record of an ancient biota. Though marine studies still dominate, non-marine ichnologic re- The intent of this paper is to document, to the author’s search has greatly increased during the past decades, as knowledge, the first record of an arthropod impression from a diversity of ichnofaunas has been recorded from a va- the Late Triassic Passaic Formation of New Jersey, and riety of paleoenvironments e.g., [6–15]. However, with all to comment on the paleoenvironmental and paleoecolog- the many publications (particularly non-marine) documen- ical conditions when the arthropod was emplaced in the tation of the organism or organisms responsible for the substrate. traces is often rare or nonexistent. As such, many factors are involved including insufficient hard parts, burial envi- Ichnologic investigations of sedimentary rocks have be- ronment, reworking of the sediment, lack of quick burial, presence of scavengers, and the degree of sediment sat- ∗E-mail: [email protected] uration, among others. Besides, many of the nonmarine 3 First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey Figure 1. Location map, showing Passaic Formation. Elevation in feet. traces are attributed to soft-bodied organisms (e.g., an- sures of the Passaic Formation located along Spring Glen nelids, nematodes), thus offering little chance for even Road (Route 627) approximately 2 km north of Milford partial preservation. in Hunterdon County, New Jersey (Fig. 1, 40◦57’22”N, 75◦10’41”W). The arthropod-bearing slab was collected from a small talus pile, and exhibited sedimentary fea- 2. Geologic setting tures similar to an exposure that is located several meters upslope which did not yield additional arthropod fossils. The Newark basin formed during the early stage fragmen- Exposures at this location are represented by cyclical tation of the supercontinent Pangea and represents one mudstones below members L and M of the Late Trias- of the largest exposed rift basins in eastern North Amer- sic Passaic Formation [16, 22]. [1] noted that the expo- ica. The basin fill, collectively referred to as the Newark sure exhibits alternating massive, ledge-forming deposits Supergroup, is represented by more than 6 km of non- of poorly-sorted siltstone and less resistant fine-grained marine sedimentary and igneous rocks deposited over an units. Desiccation cracks are common, raindrop impres- interval of approximately 30 million years [16]. Extensive sions are also present, and most of the strata are biotur- coring by the Newark Basin Coring Project established a bated. detailed orbitally tuned geomagnetic polarity time scale and stratigraphy for the Late Triassic [16, 17]. Research on Newark basin lacustrine strata suggest 3. The specimen that the deposits exhibit evidence of being controlled by Milankovitch climate cycles, as expressed by repet- The slab on which the specimen is impressed (Fig. 2) is itive transgressive-regressive lake-level successions [16, composed of reddish-brown siltstone overlain and under- 18, 19]. The Passaic Formation represents the most lain by claystone laminae exhibiting desiccation cracks widespread and thickest (>5000 m) of these deposits, and on their upper surfaces. The arthropod (New Jersey State is made up of marginal fluvio-lacustrine and cyclical la- Museum [NJSM 23195]), preserved as an impression, is custrine reddish brown, sporadically gray, black, and tan oval in shape (Fig. 2(b)), 3.5 cm in length, and 2 cm in mudstones, sandstones and conglomerates [20]. Sedimen- maximum width, and occurs in epirelief on the reddish- tary structures include desiccation cracks, ripple marks, brown claystone. No traces of body tissue have been raindrop impressions, cross-bedding, and tool marks. Ver- found. Uneven segmentation is seen preserved across tebrate footprints, plants, fish, and conchostracans, are most of the specimen, with two of the segments observed also present [21]. The deposits of the Passaic Formation being approximately 2 mm in length. Their lack of im- reflect climate-controlled precipitation-evaporation cycles pressions of the appendages may possibly be due to the that are characteristic of smaller Van Houten cycles, which final positioning after death of the organism, or subse- in turn are modulated by the short McLaughlin cycles, and quent detachment and removal prior to burial. Along one long modulating cycles [16]. side of the specimen there is a very narrow ridge of up- The slab described in this report was collected from expo- lifted thin clay likely resulting from the arthropod being 4 Robert Metz, (a) (b) Figure 2. Fossil arthropod from the Passaic Formation, Milford, New Jersey. A. Desiccation cracks on slab (yellow arrows), as well as interpreted locations where organisms rested or burrowed (white arrows), thereby influencing crack formation upon drying. Scale in mm. B. Closeup showing arthropod framed in Fig. 2(a). Note (arrow) uplifted thin clay due to pushing down of arthropod into underlying sediment. Scale in mm. covered and gently squeezed into the underlying sedi- print Rhynchosauroides as well as desiccation cracks, tool ment (Fig. 2(b)). The slab is 1 cm thick, and approxi- marks, cross-bedding, and raindrop impressions. mately 11 X 14 cm wide. The surface of the slab does not exhibit walking or crawling traces associated with movement of the arthropod. In addition to the arthropod, the trace fossils Helminthoidichnites and Spongeliomor- 4. Discussion pha are preserved in epirelief. Upon closer inspection, the shape of the desiccation cracks exhibit patterns dif- After viewing the slab, Conrad Labandeira, David Grimaldi ferent to illustrated classic examples [23, 24], in lacking (e.g., noted that the ovoid tagma shows evidence of uneven distinctive evidence of orthogonal or nonorthogonal frac- segmentation), and Alexandr Rasnitsyn agreed that it con- ture plans. Indeed, most of them intercept locations where tains an arthropod. Interestingly, Alexander Ponomarenko organisms likely moved along, rested, or burrowed (Fig. and Kirill Eskove stated the possibility of it being a whip 2). As such, I suggest that initially these were traces ini- scorpion (personal communication, 2011), though Jason tiated by organisms which upon drying of the mud pref- Dunlop noted that the lack of a prosoma and associated erentially influenced the directions of the resultant des- appendages likely precludes such a designation (personal iccation cracks [25]. On the underside, several examples communications, 2011). Thus, unfortunately, the arthro- of the trace fossil Helminthoidichnites are present. Asso- pod cannot be identified to a lower taxonomic rank and ciated slabs exhibit the trace fossils Helminthoidichnites, is unidentifiable. Previous studies of the Passaic strata Lockeia, Scoyenia, Spongeliomorpha, and the reptile foot- making up most of this exposure [2, 26] describe it as com- 5 First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey posed of lacustrine, fluvial, and floodplain deposits. In a variable trace [30, 40]. Interestingly, for the fossil arthro- study of Passaic trace fossils nearby the present location, pod from the Passaic Formation, no apparent evidence of [4] assigned them to the Scoyenia ichnofacies, which is such is exhibited. Thus, I suggest that in the present ex- typically affiliated with lowstand desiccated substrates in ample, the arthropod could have been carried by wind or underfilled lakes [27]. A similar assignment can be made water onto the sediment surface after death, or have been for this investigation. The

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