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Large, complex burrow systems from freshwater deposits of the Monongahela Group (Virgilian), Southeast Ohio, USA
Palaeogeography, Palaeoclimatology, Palaeoecology 300 (2011) 128–137
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Large, complex burrow systems from freshwater deposits of the Monongahela Group (Virgilian), Southeast Ohio, USA Daniel I. Hembree ⁎, Gregory C. Nadon, Michael R. King Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH, 45701, USA
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Article history: Received 9 March 2010 Received in revised form 14 December 2010 Accepted 16 December 2010 Available online 22 December 2010 Keywords: Ichnology Ichnofossil Crustacean Continental
a b s t r a c t A large burrow complex is described from the Monongahela Group (Upper Pennsylvanian) in southeastern Ohio. The burrow complex consists of a branching network of elliptical to circular, horizontal tunnels, 1–18 cm in diameter, occurring on a single horizontal plane. The tunnels branch to form X-, T-, and Y-junctions with branch angles between 60 and 130°. Branching produces multiple polygonal galleries up to 60 cm wide connected by small horizontal tunnels. Vertical shafts connecting the galleries to the surface are not preserved. Tunnel surfaces are irregular with circular to elliptical nodes and depressions, elongate ridges and grooves, and 1.5–4.0 cm wide nodular patches typically located at branch points. The occurrence of these burrow complexes in the Monongahela Group represents one of the largest examples of a branching burrow complex older than the Cretaceous. The Monongahela burrow complexes are preserved within a succession of floodplain and palustrine sediments suggesting a freshwater habitat for the tracemaking organisms. Potential tracemakers of the Monongahela burrow complexes include a variety of Pennsylvanian crustaceans and vertebrates. The presence of these and similar Paleozoic ichnofossils indicate a long and complex evolutionary history of the trace-making behaviors associated with the construction of large, open burrow networks. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Geological setting
Large, interconnected burrow complexes, termed ophiomorphids by Seilacher (2007), are among the most recognized trace fossils and tend to be abundant in shallow water environments from the Mesozoic to the recent. This paper describes one of the oldest large, branching burrow complexes which occurs in the Upper Pennsylvanian (Virgilian) Monongahela Group of southeastern Ohio. The burrow complexes described in this paper are organized into multiple polygonal galleries of large diameter (N10 cm) tunnels connected by smaller diameter (1–5 cm) tunnels, with Y-, T-, and X-shaped branches, and irregular tunnel surface ornamentation. These ichnofossils likely represent dwelling burrows, possibly produced by crustaceans or vertebrates in a freshwater environment. The burrow complexes described here have some architectural similarities to other ichnogenera including Thalassinoides, Ophiomorpha, and Spongeliomorpha, but they also possess unique morphological properties not found in these ichnogenera. The significance of these burrows to the paleoenvironmental interpretation of the Monongahela Group is discussed as well as the paleoecological significance of large burrow complexes in the fossil record.
The Upper Pennsylvanian Monongahela Group of Ohio crops out in a northeast to southwest belt across the southeastern corner of the state lying between the Upper Pennsylvanian Conemaugh Group and Lower Permian (?) Dunkard Group (Sturgeon, 1958). At the time of deposition of the Monongahela Group, Ohio was located at a paleolatitude of 10° S at 320 Ma, and 7° S at 300 Ma (Opdyke and DiVenere, 1994). The Monongahela Group is composed mostly of thin beds of sandstone interbedded with light-gray to green laminated shale and blocky red to purple mudstone, as well as fine-grained massive, laminated, and brecciated limestone beds (Fig. 1) (Sturgeon, 1958; Nadon et al., 1998; Hembree, 2008; King, 2008). The fine-grained siliciclastic units are highly variable across exposures of the Monongahela, whereas the carbonate units are more laterally continuous (King, 2008). The strata of the Monongahela Group are interpreted as the deposits of meandering and anastomosed fluvial systems (Nadon et al., 1998; Nadon and Hembree, 2007; Hembree, 2008; King, 2008). The lenticular sandstone bodies include the deposits of both fluvial channels and levees. The associated thin siltstone and sandstone beds and laminae interbedded with mudstones represent deposits of crevasse splays and overbank flood deposits. Adjacent blocky mudstones with red, purple, green, and gray coloration are interpreted as paleosols as indicated by the presence of rhizoliths, horizonation, blocky textures, clay skins, and slickensides (Hembree, 2008; King, 2008). Carbonate units are interpreted as lacustrine and palustrine deposits forming in the floodplain system (King, 2008).
⁎ Corresponding author. Tel.: + 1 740 597 1495. E-mail address: [email protected] (D.I. Hembree). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.12.016
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Fig. 1. Middle to Late Pennsylvanian strata of southeastern Ohio are divided into the Allegheny, Conemaugh, and Monongahela groups. The Virgilian Monongahela Group consists primarily of fine-grained siliciclastics deposited on floodplains, shallow lakes, and marshes. Freshwater limestone and coal are also present.
The large burrow complexes described in this paper are present in the lower Fishpot Limestone, a succession of interbedded carbonate, shale, and mudstone beds that lie approximately 12 m above the base of the Monongahela (Fig. 2). The Fishpot Limestone consists of alternating layers of limestone, 10–50 cm in thickness, and fine siliciclastic layers millimeters to decimeters in thickness (Nadon et al., 1998). The lower Fishpot Limestone includes layers of laminated limestone defined by alternating mudstone and packstone. The packstone layers include grains of amorphous limestone, disarticulated bivalves, and ostracodes. The laminated limestone is overlain by an intraclastic grainstone consisting of grain-supported clasts N2 mm in diameter. The edges of the carbonate clasts can be matched between adjacent clasts and the composition of the clasts is a similar massive mudstone. The upper Fishpot Limestone consists of massive mudstone, wackestone, and packstone. Individual beds are typically composed of N40% allochems between 0.3 and 2.0 mm in diameter. The surfaces of most beds contain a polygonal pattern of infilled fractures. Body fossils from the carbonate facies include fish teeth and scales as well as ostracodes (Nadon et al., 1998; King, 2008). The carbonate facies of the lower Fishpot Limestone alternate with laminated siliciclastic gray to red mudstone. The mudstone has high clay content with relatively horizontal, parallel to subparallel laminae. Laminae are defined by increased concentration of silt-sized grains
and ostracodes. The units directly below and above the Fishpot Limestone consist of blocky to platy, red to mottled red-gray mudstones containing rhizoliths, clay skins, slickensides, and plant debris. The carbonate and siliciclastic facies of the Fishpot Limestone are interpreted as having been deposited in freshwater ponds, small lakes, or marshes on a distal floodplain. The high concentration of ostracodes and bivalves and absence of any other body fossils is suggestive of a freshwater lake system (Park and Gierlowski-Kordesch, 2007). The infilled polygonal structures on bedding planes are interpreted as mudcracks produced during periodic drying events. Such events would have also produced the brecciated pattern observed in the intraclastic grainstone. The carbonate accumulated in these water bodies through a variety of processes including fluvial transport of carbonate grains eroded from older, exposed carbonate deposits and transported as bed load and suspended load as well as quiet water accumulation of micrites from dissolved load (Gierlowski-Kordesch, 2010). Laminated mudstone interbedded with the carbonate facies are also interpreted as deposits of floodplain lakes likely the result of increased surface runoff and increased rates of sedimentation or a temporary switch in the sediment source from carbonate-dominated to siliciclastic-dominated areas. The mudstone units below and above the Fishpot Limestone are interpreted to be paleosols that developed on seasonally well-drained floodplains as
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indicated by the presence of deep penetrating root traces, welldeveloped blocky textures, clay cutans, and slickensides. The entire succession of facies containing the large burrow networks, therefore, is interpreted as a continental in origin and the burrows themselves are contained entirely within aquatic, lacustrine deposits. 3. Monongahela burrow complexes The ichnofossils described in this study were collected from an 11 m thick succession of the Monongahela Group, 16 km east of Athens, Ohio along U.S. Route 50 (N 39° 17.7′ W 81° 55.8′) (Fig. 3). The base of the section begins above the base of the Monongahela, which is not exposed at this locality, and starts with a 1-m-thick fine sandstone bed (Fig. 2). The horizon containing the burrows occurs 8.5 m above the base of the section and approximately 32 cm above the base of the Fishpot Limestone within an interval of interbedded laminated to massive mudstone and limestone (Figs. 2 and 4). The burrowed interval is capped by an 80 cm thick bed of massive, finegrained limestone. All of the burrows are present within the mudstone beds and are preserved as three-dimensional casts composed of a calcareous mudstone identical to the composition of the overlying limestone. All material described in this paper is held in the Ohio University Zoological Collections (OUZC), Athens, Ohio. The burrows are organized into multiple polygonal galleries of large diameter tunnels connected by smaller diameter tunnels (Fig. 5). Individual tunnels vary from 1–18 cm in width to 2–11 cm in height (Figs. 5 and 6). Width to height ratios for tunnels greater than 5 cm in width are typically N1 and ≤ 1 for tunnels less than 5 cm in width. The tunnels are straight or curved and up to 35 cm in length (Fig. 5). Large diameter tunnels branch to form Y-, T-, and X-junctions (Fig. 7). The angles between the branches of large diameter tunnels are between 60 and 130°. The branching large tunnels are connected into 50–60 cm wide polygonal galleries that are composed of up to six individual tunnel segments. The galleries also contain short (3–5 cm), small diameter (1–5 cm) side branches that either connect to larger tunnels or terminate as blind tunnels (Fig. 6C and D). The angles between the branches of small diameter tunnels are between 60 and 90°. All tunnels occur along a single horizontal plane and no vertical shafts connecting the tunnel galleries to the paleosurface are preserved. The surficial morphology of the tunnels consists of irregular circular to elliptical nodes and depressions as well as elongate ridges and grooves that cover the upper, lower, and side walls (Fig. 8A). Some portions of the burrow wall are also covered in a 1.5–4.0 cm wide, nodular surface texture (Fig. 8B). Individual nodules vary in size from 4.5 to 7.2 mm in diameter. These nodular patches mostly occur near branch points. The burrow fill of the specimens is composed of massive calcareous mudstone and calcareous breccia similar in lithology to the overlying limestone (Fig. 9). There is a sharp boundary between the burrow fill and the laminated mudstone of the surrounding rock matrix which can be seen in vertically cut sections (Fig. 9A). These sections also clearly show that the burrow fill extends to the outer irregular surface of the tunnel walls (Fig. 9B). 4. Discussion 4.1. Preservation and morphology Biogenic structures are the product of both organism behavior and the environment in which they were produced (Bromley, 1996). The final morphology of a trace fossil, however, is also dependent on taphonomic processes including compaction, diagenesis, and erosion. Fig. 2. Stratigraphic section of the Virgilian Monongahela Group exposed in the road cut containing the large burrow complexes. The vertical scale is in meters.
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Fig. 3. Map of the location of the study area in southeast Ohio. The locality is a roadcut on the north side of U.S. 50 approximately 17 km east of Athens.
Compaction can alter the cross-sectional morphology and height to width ratios of tunnels and chambers (Fürsich, 1973a; Bromley, 1996; Savrda, 2007). It can also cause portions of unfilled burrows to collapse making them difficult to recognize (Bromley, 1996; Savrda, 2007). Diagenetic alteration can change the size of burrows through the accretion of mineral layers or substantially change the surficial morphology (Fürsich, 1973a; Bromley, 1996; Savrda, 2007). The erosion of portions of burrows can alter the overall architecture of a burrow system. In subsurface galleries such as Thalassinoides, for example, the vertical shafts that connect the complex to the surface are typically eroded and not preserved (Fürsich, 1973a). Despite these changes, different properties of a trace fossil may be analyzed to interpret the relative roles of these processes on the final morphology and to determine the original morphology of the biogenic structure. The Monongahela burrows were passively filled by overlying sediment that entered through surface openings by gravitational and physical sedimentary processes. The passive nature of the burrow fill is clearly indicated by the vertical sections cut through the burrow casts. The burrow fill is distinct from the surrounding mudstone and identical to the lithology of the unit directly above the burrow complexes. The passive infill of the Monongahela burrow complexes suggests that the tunnels were kept open during their occupation
(Bromley, 1996). The open burrows were then filled passively by sediment infiltrating from above. The massive nature of the fill suggests that this occurred rapidly, possibly while the burrow complexes were still in use. The sediment completely filled the tunnels to produce the three-dimensional casts collected from the mudstone. The size of a trace fossil can be significantly altered by diagenesis; in some examples Thalassinoides diameter has been increased by 50– 75% (Fürsich, 1973a; Myrow, 1995). The key to recognizing this alteration of burrow size is the analysis of cross sectional views of individual tunnels and the burrow fill. The cross sections of the Monongahela burrow complexes show that the burrow fill extends to the edge of the exposed burrow casts. The fill is the same lithology of the unit above the burrowed horizon and has not been replaced by secondary mineral growth. This indicates that the burrows did not become enlarged by diagenetic alteration and instead the size of the burrow cast is the size of the original open tunnels. The recognition of the passive infill, therefore, aids in the interpretation of the original burrow morphology. The Monongahela burrow complexes have no preserved remains or indications of vertical shafts connecting the horizontally oriented tunnel galleries to the sediment surface. Passive infill from overlying
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and Frey, 1974; Curran and Martin, 2003) and exceptionally preserved examples of Thalassinoides (Kennedy, 1967). The irregular surficial morphology of the burrow casts are the result of both tracemaker activity and diagenetic overprinting. The irregular, elongate ridges and grooves were likely produced on the interior wall of the tunnels during excavation. It is important to note that the surficial features on the walls of the burrow casts are the negative images of the original features. Therefore, the positive relief features on the outer surface of the cast represent impressions on the interior of the original tunnel wall and the negative relief features on the cast represent protrusions. These surficial features indicate that the interior tunnel walls were not smooth and were not lined with mud, fecal material, or pellets. The nodular textures are likely diagenetic in origin and represent post-burial mineral growth possibly in zones of concentrated organic material. Their irregular shapes and sizes as well as their preservation on the outer surface of the burrow casts indicate that they are not pellets produced by the tracemaker. 4.2. Ichnotaxonomy
Fig. 4. The horizon containing the large burrow complexes (at arrow) occurs below the Fishpot Limestone (A). Large horizontal tunnels are visible in cross-section eroding out of the outcrop (B). The burrows are preserved as natural casts composed of calcareous mudstone similar in composition to the overlying strata.
unit suggests, however, that a connection to the surface environment did exist while the tunnels were open. Without the presence of these vertical shafts, passive infill of the subsurface tunnels from above would not be possible. Despite their lack of preservation, therefore, the nature of the burrow fill does indicate that the tracemakers maintained a connection to the surface through vertical shafts as observed in similar modern burrow networks (Shinn, 1968; Bromley
Ichnotaxonomy is based upon the morphology of ichnofossils without the consideration of possible tracemakers, depositional environments, or substrate conditions (Bromley, 1996; Pemberton et al., 2001; Bertling et al., 2006). Bertling et al. (2006) recommended that four ichnotaxobases related to burrow morphology be used when classifying trace fossils: overall shape, orientation, ornamentation, and internal structure. Considering these ichnotaxobases, the Monongahela burrow complex is similar to established ichnogenera of branching burrow networks such as Thalassinoides, Ophiomorpha, and Spongeliomorpha. Like these ichnogenera, the Monongahela burrow complexes consist of circular to elliptical tunnels interconnected along a single horizontal plane (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975). These ichnogenera also include branching tunnels, typically forming Y-shaped or T-shaped junctions (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975; Myrow, 1995). In contrast, the burrow complexes from the Monongahela possess Y-, T-, and Xshaped junctions within a single gallery, a suite of architectures not observed in these other ichnogenera. The tunnels at the junction
Fig. 5. A) A single slab (OUZC 5503) approximately 100 cm long and 50 cm wide containing two branching galleries (1 and 2) of large diameter-tunnels connected by a smalldiameter tunnel (at arrow). B) Line drawing of the burrow complex.
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Fig. 6. Variations in tunnel diameter within the burrow complexes. A) Slab (OUZC 5503) containing two different tunnel styles: a large-diameter (11 cm) elliptical tunnel (1) and a small diameter (4 cm) circular tunnel (2). B) A small, 1 cm diameter, tunnel (at arrow) branching off of a larger, 7.5 cm diameter, tunnel (OUCZ 5504). C) Examples of small-diameter tunnels within a burrow complex (OUZC 5503): (1) a poorly preserved, small-diameter tunnel connecting two galleries; and (2) short, blind tunnel. D) A curved, small diameter tunnel (at arrow) connecting two tunnels within the same gallery (OUZC 5503).
points are typically enlarged in Thalassinoides, often up to three times the width of the adjoining tunnels (Kennedy, 1967; Häntzschel, 1975). The Monongahela burrows do not possess enlarged tunnels of this scale at the junctions or elsewhere in the burrow complex. The overall architecture of the Monongahela burrow complexes is also unique in their organization into 50–60 cm polygonal galleries of large (8–18 cm) diameter tunnels connected to each other by small (1–5 cm) diameter tunnels. The surficial morphologies of Thalassinoides, Spongeliomorpha, and Ophiomorpha are characterized by smooth-walled tunnels, tunnels with walls ornamented with regular, repeating elongate ridges, and tunnels with mammillate textures covering the entire outer wall respectively (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975; Frey et al., 1978). Tunnels of the Monongahela burrow complexes possess a mixture of elliptical nodes and depressions as well as irregular elongate ridges and grooves, an ornamentation pattern not observed in these other ichnogenera. Based on morphological similarity, the Monongahela burrow complexes may be assigned to the ichnogenus Thalassinoides. The difference in tunnel ornamentation between the Monongahela burrow complexes and both Spongeliomorpha and Ophiomorpha prevent their assignment to either of these ichnogenera. The Monongahela burrow complexes may represent a new ichnospecies of Thalassinoides, but morphological variation due to substrate and preservation cannot be discounted. 4.3. Paleoecological significance The geologic range of branching burrow networks such as Thalassinoides, Ophiomorpha, and Spongeliomorpha extends from the Cambrian to the recent (Häntzschel, 1964, 1975; Fürsich, 1973b; Bromley and Frey, 1974; Miller and Byers, 1984; Carmona et al., 2004). Thalassinoides first becomes abundant in the Ordovician, but these burrows are generally very small in diameter, ranging from 4 to 40 mm (Sheehan and Schiefelbein, 1984; Myrow, 1995; Ekdale and Bromley, 2003). Spongeliomorpha first appears in the Early Permian
while definitive specimens of Ophiomorpha first appear in the Triassic although some potential Late Carboniferous specimens have been described (Bromley and Frey, 1974; Häntzschel, 1975; Frey et al., 1978; Carmona et al., 2004). These two ichnogenera do not become common until the Early Mesozoic, and the Paleozoic examples are much smaller than younger examples (Häntzschel, 1975; Carmona et al., 2004). Unnamed branching burrow networks from continental settings include Lower Triassic burrows from well-drained paleosols of the Driekoppen Formation of South Africa and the Fremouw Formation of Antarctica (Groenewald et al., 2001; Hasiotis et al., 2004). The tunnels of these burrow complexes are 100–150 mm in diameter and are attributed to fossorial tetrapods (Groenewald et al., 2001; Hasiotis et al., 2004). Monongahela burrow complexes, therefore, represent one of the oldest large diameter (N100 mm) branching burrow complexes reported from the continental and marine fossil record. As a reference, Thalassinoides of this size have not been previously reported from strata older than the Cretaceous and include specimens with tunnels diameters of 100–120 mm (Pemberton et al., 1984; Carmona et al., 2004; Neto De Carvalho et al., 2007). Only much smaller diameter (7–11 mm) Thalassinoides have been reported from the Lower Pennsylvanian Fentress Formation in central Tennessee (Miller and Knox, 1985). The specific tracemaker of the Monongahela burrow complexes is unknown since no body fossils were found within the burrow casts or in the surrounding matrix. The architectural and surficial morphology of the burrows, however, can be used to interpret likely tracemakers. The presence and morphology of tunnel linings, branching patterns, the shape and relative dimensions of tunnels and chambers, and markings or bioglyphs on tunnel walls produced by tracemaker appendages, body, or head may all be used to help identify tracemakers. The absolute size of tunnels and chambers is not as useful in tracemaker identification. This is because many small animals may produce burrows that are much larger than their body, especially when the animals are gregarious or colonial. As a result, attempts to equate burrow diameter with organism diameter can
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Fig. 8. Surficial morphology of the burrow casts. A) Burrow surfaces are covered in elongate ridges (R) and grooves (G), and irregular circular to elliptical nodes (N) and depressions (D) (OUZC 5504). B) Portions of the tunnel walls are covered in a mammillate texture. Individual mounds are 4.5–7.2 mm in diameter (OUZC 5503).
Fig. 7. Three different branching patterns observed in the burrow complexes. A) Y-shaped junction (OUZC 5503). B) T-shaped junction (OUZC 5503). C) X-shaped junction (OUZC 5505).
easily lead to incorrect interpretations. The interpreted environment of deposition indicated by sedimentology and body fossils, in this case a freshwater lacustrine environment, may also be used to refine the potential list of tracemakers. For the Monongahela burrow complexes, marine organisms as well as terrestrial continental organisms may be excluded from consideration. Crustaceans (Cl. Malacostraca) are typically associated with subaqueous, branching burrow networks with the same morphology as Thalassinoides in modern shallow marine and freshwater environments as revealed by studies these burrow systems in the field and
laboratory (Shinn, 1968; Sellwood, 1971; Bromley and Frey, 1974; Curran and Frey, 1977; Hasiotis and Mitchell, 1993; Myrow, 1995; Bromley, 1996). While rare, the association of crustacean body fossils and branching burrow complexes also occurs in the fossil record. Body fossils of the decapod crustacean Callianassa have been found associated with Thalassinoides from the Early Miocene (Hayward, 1976) and Eocene (Glaessner, 1947; Frey et al., 1984). Neto de Carvalho et al. (2007) described the occurrence of the decapod crustacean Mecochirus within Early Cretaceous Thalassinoides from Portugal. Bromley and Asgaard (1972) described Early Jurassic (Toarcian) Thalassinoides containing fossils of the decapod crustacean Glyphea. This association of the Thalassinoides morphology with various crustacean taxa in the modern and ancient, therefore, suggests that the Monongahela burrow complexes may have also been produced by crustaceans. There are a number of crustacean taxa present in the Pennsylvanian whose extant members produce branching burrows (Brooks et al., 1969; Glaessner, 1969; Schram et al., 1978; Schram, 1981; Schram and Mapes, 1984; Wills, 1998). Crustaceans in general, including various branchiopods and malacostracans, have been present in shallow marine environments since the Cambrian (Brooks et al., 1969; Schram, 1981; Wills, 1998) and freshwater environments since the Devonian (Fayers and Trewin, 2003). A variety of crustacean taxa interpreted as rapacious carnivores, low-level carnivores, filter feeders, and detritus feeders have been collected from Pennsylvanian strata such as those of the Upper Pennsylvanian Mazon Creek area of Illinois (Schram, 1981). These crustacean fossils occur in units interpreted as the deposits of both nearshore marine and brackish water environments (Schram, 1981). Malacostracans are preserved in a range of depositional environments with highly variable salinities by the Carboniferous suggesting that these taxa had developed advanced osmoregulatory mechanisms (Briggs and
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Fig. 9. Cross sections of tunnel segments from the burrow complexes showing the calcareous mudstone and breccia burrow fill. A) A small, 5 cm diameter, burrow showing a sharp boundary between the burrow fill and the laminated mudstone of the surrounding unit. B) A large, 16 cm diameter, burrow showing that the fill extends to the outer surface of the tunnel cast.
Clarkson, 1989). Eocarids, for example, are well-known from coal swamp deposits of the Pennsylvanian suggesting that they inhabited freshwater environments (Schram, 1981). Syncarids are also abundant in the Pennsylvanian (Schram, 1981). Modern syncarids of the genus Anaspides, Paranaspides, and Micraspides inhabit freshwater lakes and rivers in Tasmania suggesting the possibility of similar environmental preferences for older syncarid taxa (Williams, 1965; Swain and Reid, 1983). While these crustaceans have not been associated with burrows, the presence of these animals in environments similar to the Monongahela lake systems does present the possibility that there may have been some freshwater burrowing malacostracan species in the Pennsylvanian. Vertebrates also construct complex burrow systems (Hasiotis et al., 2007). While terrestrial tetrapods produce complex burrow systems, the sediment in which the Monongahela burrows were produced are lacustrine in origin and show no sign of pedogenic modification. While some fish also produce complex burrows in marine environments (Rice and Johnstone, 1972), burrowing fish in modern freshwater environments such as lungfish tend to produce simple vertical or horizontal structures (Atkinson and Taylor, 1991). In addition, those fish that do produce complex burrow systems in marine environments often tend to produce these structures in concert with other organisms such as crustaceans (Farrow, 1971; Atkinson, 1974; Karplus et al., 1974). In these situations, the burrows possess two or more distinct architectural morphologies. Based on the comparison of their architectural morphology with modern crustacean burrow systems and taking the environment of deposition into consideration, the Monongahela burrow complexes are interpreted as the product of the burrowing activity of freshwater crustaceans. Specifically, the regular and repeating branching pattern of the Monongahela burrows strongly resembles those produced by crustaceans in both the modern and ancient (Weimer and Hoyt, 1964; Bromley, 1967, 1996; Shinn, 1968; Frey and Mayou, 1971; Bromley
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and Frey, 1974; Ott et al., 1976; Swinbanks and Luternauer, 1987; Pemberton et al., 2001; Carmona et al., 2004; Neto de Carvalho et al., 2007; Seilacher, 2007). The two-dimensional nature of the burrow complexes and the arrangement of the branching tunnels into a repeating polygonal pattern are also similar to previously studied crustacean burrow networks (Carmona et al., 2004). The Monongahela burrows do not resemble burrows produced by freshwater fish or tetrapods, the only other likely tracemakers of appropriate size and behavioral complexity to inhabit in the environment of deposition. The Monongahela burrow complexes are interpreted as dwelling structures due to complex, branching nature of the burrow network. In addition, the passive nature of the burrow fill that is similar in all of the tunnels indicates that the entire burrow complex was kept open, another characteristic of dwelling structures. Dwelling burrows are often the product of several interrelated behaviors and portions of the burrow complex likely served as sites of feeding and reproduction as well. The variation in the diameter of the component tunnels suggests that the burrow complexes were likely the dwellings of multiple individuals of different sizes. There are many examples in modern aquatic environments where several burrowing organisms will produce interconnected complexes including crustaceans (Atkinson, 1974; Atkinson et al., 1982). In these examples of commensalism, however, the morphologies of the individual elements of the burrow complex are distinct. In the Monongahela burrow complex, the large and small tunnels are identical with the exception of size. There are no distinct differences in the morphology or surficial features to interpret them as the burrows of different organisms. It is possible that they were produced by smaller and potentially younger individuals. They may also simply represent tunnels excavated for locomotion purposes only. The larger tunnels may have been expanded to serve as active dwelling chambers for multiple individuals. 4.4. Paleoenvironmental significance The succession of sedimentary strata in the Monongahela Group is indicative of a continental setting. The Fishpot Limestone has previously been interpreted as a freshwater to brackish palustrine limestone while the underlying and overlying strata include deposits of meandering and anastomosed fluvial systems as well as paleosols (Nadon et al., 1998; Hembree, 2008; King, 2008). As such, there is no direct evidence of marine influence in the stratigraphic interval containing the burrow complexes. Differences in the architectural and surficial morphology of trace fossils are typically due to variations in either the behavior of the tracemaker or the nature of the substrate (Bromley, 1996; MacEachern et al., 2007). Variations in substrate properties such as composition and compaction can result in morphological differences within a single burrow network. For example, linings can be present in one portion of a Thalassinoides complex and absent in another. Such linings are typically indicative of less cohesive substrates which require additional structural support to maintain open burrows (Bromley, 1996; Pemberton et al., 2001). This is supported by studies of modern burrow systems where tunnels cross both loose and cohesive substrates (Bromley and Frey, 1974; Bromley, 1996; Pemberton et al., 2001). The tunnels of the Monongahela burrow complexes lack a conspicuous lining and were passively filled by sediment from the overlying Fishpot Limestone. This suggests that the sediment was firm enough to allow open tunnels to be maintained without extra support. Surficial ornamentation can also provide information about the substrate. Scratch-marks preserved on tunnel walls are suggestive firmer substrates (Bromley, 1967, 1996; Pemberton et al., 2001). Clay rich sediments, for example, can preserve the scratches and scrapes produced by arthropod appendages while excavating a tunnel (Bromley, 1996). Softer substrates lack the cohesion to preserve these types of excavation marks. The irregular, elongate ridges and grooves on the walls of the Monongahela burrow complexes
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were likely produced during burrow construction. These may represent irregular surface indentations and scratch marks created by the tracemaking organisms either as the tunnels were initially excavated or expanded over time. The combination of sedimentological and ichnological evidence suggests that the burrow complexes were produced in firm mud along the margin of a shallow body of fresh water. The presence of large burrow complexes within the shoreline mud suggests that oxygen and nutrient availability in the depositional environment were relatively high. These high levels would be required to sustain a population of organisms capable of producing the extensive tunnel complexes. The energy expenditure required to construct the 50–60 cm wide galleries would need to be justified by the availability of resources in the immediate surroundings. The burrows were likely filled with sediment as water levels rose and the shoreline migrated inland. The in situ brecciation of the burrow fill suggests that some post-fill desiccation of the mud occurred soon after the flooding event.
5. Conclusions This study documents large burrow complexes from the Upper Pennsylvanian (Virgilian) Monongahela Group of southeastern Ohio. The ichnofossils consist of interconnected, branching mazes of elliptical to circular tunnels with irregular surface textures. These complexes differ from previously described branching burrow networks in their organization into multiple polygonal galleries of large diameter tunnels connected by smaller diameter tunnels, the presence of Y-, T-, and X-shaped branches, and tunnel surface ornamentation consisting of irregular circular to elliptical nodes and depressions as well as elongate ridges and grooves. While the absence of body fossils in the burrow casts and the surrounding matrix makes the assignment of a tracemaker difficult, the architectural morphology of the burrow complexes is similar to burrow complexes constructed by modern crustaceans. The presence of the large burrow complexes provide important paleoecological and paleoenvironmental information about the Monongahela Group. The burrow complexes indicate the presence of a large population of animals in the lacustrine paleoenvironments of the Monongahela Group despite the absence of abundant body fossils. In order to sustain the population of organisms necessary to construct the burrow networks, high levels of oxygen and nutrients must have been present in the subsurface environment. The nature of the passive infilling and the surficial morphology of the tunnels are suggestive of a firm substrate. The occurrence of these ichnofossils in the Upper Pennsylvanian Monongahela Group represents one of the largest diameter complex burrow networks from the Paleozoic. The average diameter of the tunnels is significantly larger than previously described specimens of Paleozoic Thalassinoides. Specimens of Thalassinoides from the Ordovician and Pennsylvanian vary from only 3 to 11 mm in average diameter whereas the tunnels in the burrows described here average 12 cm in diameter. In addition, these burrow complexes are preserved within a sequence of floodplain and palustrine sediments suggesting a freshwater habitat for the tracemaking organisms. This is unusual for large Paleozoic burrow complexes that are typically associated with marginal marine to marine depositional environments. The presence of these trace fossils in addition to other Paleozoic forms of Thalassinoides, Ophiomorpha and Spongeliomorpha indicate a long and complex evolutionary history of the trace-making behavior associated with the construction of large, open burrow networks.
Acknowledgements We would like to thank F. Surlyk and J.P. Zonneveld for their reviews and comments that helped improve this manuscript.
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Large, complex burrow systems from freshwater deposits of the Monongahela Group (Virgilian), Southeast Ohio, USA Daniel I. Hembree ⁎, Gregory C. Nadon, Michael R. King Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH, 45701, USA
a r t i c l e
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Article history: Received 9 March 2010 Received in revised form 14 December 2010 Accepted 16 December 2010 Available online 22 December 2010 Keywords: Ichnology Ichnofossil Crustacean Continental
a b s t r a c t A large burrow complex is described from the Monongahela Group (Upper Pennsylvanian) in southeastern Ohio. The burrow complex consists of a branching network of elliptical to circular, horizontal tunnels, 1–18 cm in diameter, occurring on a single horizontal plane. The tunnels branch to form X-, T-, and Y-junctions with branch angles between 60 and 130°. Branching produces multiple polygonal galleries up to 60 cm wide connected by small horizontal tunnels. Vertical shafts connecting the galleries to the surface are not preserved. Tunnel surfaces are irregular with circular to elliptical nodes and depressions, elongate ridges and grooves, and 1.5–4.0 cm wide nodular patches typically located at branch points. The occurrence of these burrow complexes in the Monongahela Group represents one of the largest examples of a branching burrow complex older than the Cretaceous. The Monongahela burrow complexes are preserved within a succession of floodplain and palustrine sediments suggesting a freshwater habitat for the tracemaking organisms. Potential tracemakers of the Monongahela burrow complexes include a variety of Pennsylvanian crustaceans and vertebrates. The presence of these and similar Paleozoic ichnofossils indicate a long and complex evolutionary history of the trace-making behaviors associated with the construction of large, open burrow networks. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Geological setting
Large, interconnected burrow complexes, termed ophiomorphids by Seilacher (2007), are among the most recognized trace fossils and tend to be abundant in shallow water environments from the Mesozoic to the recent. This paper describes one of the oldest large, branching burrow complexes which occurs in the Upper Pennsylvanian (Virgilian) Monongahela Group of southeastern Ohio. The burrow complexes described in this paper are organized into multiple polygonal galleries of large diameter (N10 cm) tunnels connected by smaller diameter (1–5 cm) tunnels, with Y-, T-, and X-shaped branches, and irregular tunnel surface ornamentation. These ichnofossils likely represent dwelling burrows, possibly produced by crustaceans or vertebrates in a freshwater environment. The burrow complexes described here have some architectural similarities to other ichnogenera including Thalassinoides, Ophiomorpha, and Spongeliomorpha, but they also possess unique morphological properties not found in these ichnogenera. The significance of these burrows to the paleoenvironmental interpretation of the Monongahela Group is discussed as well as the paleoecological significance of large burrow complexes in the fossil record.
The Upper Pennsylvanian Monongahela Group of Ohio crops out in a northeast to southwest belt across the southeastern corner of the state lying between the Upper Pennsylvanian Conemaugh Group and Lower Permian (?) Dunkard Group (Sturgeon, 1958). At the time of deposition of the Monongahela Group, Ohio was located at a paleolatitude of 10° S at 320 Ma, and 7° S at 300 Ma (Opdyke and DiVenere, 1994). The Monongahela Group is composed mostly of thin beds of sandstone interbedded with light-gray to green laminated shale and blocky red to purple mudstone, as well as fine-grained massive, laminated, and brecciated limestone beds (Fig. 1) (Sturgeon, 1958; Nadon et al., 1998; Hembree, 2008; King, 2008). The fine-grained siliciclastic units are highly variable across exposures of the Monongahela, whereas the carbonate units are more laterally continuous (King, 2008). The strata of the Monongahela Group are interpreted as the deposits of meandering and anastomosed fluvial systems (Nadon et al., 1998; Nadon and Hembree, 2007; Hembree, 2008; King, 2008). The lenticular sandstone bodies include the deposits of both fluvial channels and levees. The associated thin siltstone and sandstone beds and laminae interbedded with mudstones represent deposits of crevasse splays and overbank flood deposits. Adjacent blocky mudstones with red, purple, green, and gray coloration are interpreted as paleosols as indicated by the presence of rhizoliths, horizonation, blocky textures, clay skins, and slickensides (Hembree, 2008; King, 2008). Carbonate units are interpreted as lacustrine and palustrine deposits forming in the floodplain system (King, 2008).
⁎ Corresponding author. Tel.: + 1 740 597 1495. E-mail address: [email protected] (D.I. Hembree). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.12.016
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Fig. 1. Middle to Late Pennsylvanian strata of southeastern Ohio are divided into the Allegheny, Conemaugh, and Monongahela groups. The Virgilian Monongahela Group consists primarily of fine-grained siliciclastics deposited on floodplains, shallow lakes, and marshes. Freshwater limestone and coal are also present.
The large burrow complexes described in this paper are present in the lower Fishpot Limestone, a succession of interbedded carbonate, shale, and mudstone beds that lie approximately 12 m above the base of the Monongahela (Fig. 2). The Fishpot Limestone consists of alternating layers of limestone, 10–50 cm in thickness, and fine siliciclastic layers millimeters to decimeters in thickness (Nadon et al., 1998). The lower Fishpot Limestone includes layers of laminated limestone defined by alternating mudstone and packstone. The packstone layers include grains of amorphous limestone, disarticulated bivalves, and ostracodes. The laminated limestone is overlain by an intraclastic grainstone consisting of grain-supported clasts N2 mm in diameter. The edges of the carbonate clasts can be matched between adjacent clasts and the composition of the clasts is a similar massive mudstone. The upper Fishpot Limestone consists of massive mudstone, wackestone, and packstone. Individual beds are typically composed of N40% allochems between 0.3 and 2.0 mm in diameter. The surfaces of most beds contain a polygonal pattern of infilled fractures. Body fossils from the carbonate facies include fish teeth and scales as well as ostracodes (Nadon et al., 1998; King, 2008). The carbonate facies of the lower Fishpot Limestone alternate with laminated siliciclastic gray to red mudstone. The mudstone has high clay content with relatively horizontal, parallel to subparallel laminae. Laminae are defined by increased concentration of silt-sized grains
and ostracodes. The units directly below and above the Fishpot Limestone consist of blocky to platy, red to mottled red-gray mudstones containing rhizoliths, clay skins, slickensides, and plant debris. The carbonate and siliciclastic facies of the Fishpot Limestone are interpreted as having been deposited in freshwater ponds, small lakes, or marshes on a distal floodplain. The high concentration of ostracodes and bivalves and absence of any other body fossils is suggestive of a freshwater lake system (Park and Gierlowski-Kordesch, 2007). The infilled polygonal structures on bedding planes are interpreted as mudcracks produced during periodic drying events. Such events would have also produced the brecciated pattern observed in the intraclastic grainstone. The carbonate accumulated in these water bodies through a variety of processes including fluvial transport of carbonate grains eroded from older, exposed carbonate deposits and transported as bed load and suspended load as well as quiet water accumulation of micrites from dissolved load (Gierlowski-Kordesch, 2010). Laminated mudstone interbedded with the carbonate facies are also interpreted as deposits of floodplain lakes likely the result of increased surface runoff and increased rates of sedimentation or a temporary switch in the sediment source from carbonate-dominated to siliciclastic-dominated areas. The mudstone units below and above the Fishpot Limestone are interpreted to be paleosols that developed on seasonally well-drained floodplains as
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indicated by the presence of deep penetrating root traces, welldeveloped blocky textures, clay cutans, and slickensides. The entire succession of facies containing the large burrow networks, therefore, is interpreted as a continental in origin and the burrows themselves are contained entirely within aquatic, lacustrine deposits. 3. Monongahela burrow complexes The ichnofossils described in this study were collected from an 11 m thick succession of the Monongahela Group, 16 km east of Athens, Ohio along U.S. Route 50 (N 39° 17.7′ W 81° 55.8′) (Fig. 3). The base of the section begins above the base of the Monongahela, which is not exposed at this locality, and starts with a 1-m-thick fine sandstone bed (Fig. 2). The horizon containing the burrows occurs 8.5 m above the base of the section and approximately 32 cm above the base of the Fishpot Limestone within an interval of interbedded laminated to massive mudstone and limestone (Figs. 2 and 4). The burrowed interval is capped by an 80 cm thick bed of massive, finegrained limestone. All of the burrows are present within the mudstone beds and are preserved as three-dimensional casts composed of a calcareous mudstone identical to the composition of the overlying limestone. All material described in this paper is held in the Ohio University Zoological Collections (OUZC), Athens, Ohio. The burrows are organized into multiple polygonal galleries of large diameter tunnels connected by smaller diameter tunnels (Fig. 5). Individual tunnels vary from 1–18 cm in width to 2–11 cm in height (Figs. 5 and 6). Width to height ratios for tunnels greater than 5 cm in width are typically N1 and ≤ 1 for tunnels less than 5 cm in width. The tunnels are straight or curved and up to 35 cm in length (Fig. 5). Large diameter tunnels branch to form Y-, T-, and X-junctions (Fig. 7). The angles between the branches of large diameter tunnels are between 60 and 130°. The branching large tunnels are connected into 50–60 cm wide polygonal galleries that are composed of up to six individual tunnel segments. The galleries also contain short (3–5 cm), small diameter (1–5 cm) side branches that either connect to larger tunnels or terminate as blind tunnels (Fig. 6C and D). The angles between the branches of small diameter tunnels are between 60 and 90°. All tunnels occur along a single horizontal plane and no vertical shafts connecting the tunnel galleries to the paleosurface are preserved. The surficial morphology of the tunnels consists of irregular circular to elliptical nodes and depressions as well as elongate ridges and grooves that cover the upper, lower, and side walls (Fig. 8A). Some portions of the burrow wall are also covered in a 1.5–4.0 cm wide, nodular surface texture (Fig. 8B). Individual nodules vary in size from 4.5 to 7.2 mm in diameter. These nodular patches mostly occur near branch points. The burrow fill of the specimens is composed of massive calcareous mudstone and calcareous breccia similar in lithology to the overlying limestone (Fig. 9). There is a sharp boundary between the burrow fill and the laminated mudstone of the surrounding rock matrix which can be seen in vertically cut sections (Fig. 9A). These sections also clearly show that the burrow fill extends to the outer irregular surface of the tunnel walls (Fig. 9B). 4. Discussion 4.1. Preservation and morphology Biogenic structures are the product of both organism behavior and the environment in which they were produced (Bromley, 1996). The final morphology of a trace fossil, however, is also dependent on taphonomic processes including compaction, diagenesis, and erosion. Fig. 2. Stratigraphic section of the Virgilian Monongahela Group exposed in the road cut containing the large burrow complexes. The vertical scale is in meters.
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Fig. 3. Map of the location of the study area in southeast Ohio. The locality is a roadcut on the north side of U.S. 50 approximately 17 km east of Athens.
Compaction can alter the cross-sectional morphology and height to width ratios of tunnels and chambers (Fürsich, 1973a; Bromley, 1996; Savrda, 2007). It can also cause portions of unfilled burrows to collapse making them difficult to recognize (Bromley, 1996; Savrda, 2007). Diagenetic alteration can change the size of burrows through the accretion of mineral layers or substantially change the surficial morphology (Fürsich, 1973a; Bromley, 1996; Savrda, 2007). The erosion of portions of burrows can alter the overall architecture of a burrow system. In subsurface galleries such as Thalassinoides, for example, the vertical shafts that connect the complex to the surface are typically eroded and not preserved (Fürsich, 1973a). Despite these changes, different properties of a trace fossil may be analyzed to interpret the relative roles of these processes on the final morphology and to determine the original morphology of the biogenic structure. The Monongahela burrows were passively filled by overlying sediment that entered through surface openings by gravitational and physical sedimentary processes. The passive nature of the burrow fill is clearly indicated by the vertical sections cut through the burrow casts. The burrow fill is distinct from the surrounding mudstone and identical to the lithology of the unit directly above the burrow complexes. The passive infill of the Monongahela burrow complexes suggests that the tunnels were kept open during their occupation
(Bromley, 1996). The open burrows were then filled passively by sediment infiltrating from above. The massive nature of the fill suggests that this occurred rapidly, possibly while the burrow complexes were still in use. The sediment completely filled the tunnels to produce the three-dimensional casts collected from the mudstone. The size of a trace fossil can be significantly altered by diagenesis; in some examples Thalassinoides diameter has been increased by 50– 75% (Fürsich, 1973a; Myrow, 1995). The key to recognizing this alteration of burrow size is the analysis of cross sectional views of individual tunnels and the burrow fill. The cross sections of the Monongahela burrow complexes show that the burrow fill extends to the edge of the exposed burrow casts. The fill is the same lithology of the unit above the burrowed horizon and has not been replaced by secondary mineral growth. This indicates that the burrows did not become enlarged by diagenetic alteration and instead the size of the burrow cast is the size of the original open tunnels. The recognition of the passive infill, therefore, aids in the interpretation of the original burrow morphology. The Monongahela burrow complexes have no preserved remains or indications of vertical shafts connecting the horizontally oriented tunnel galleries to the sediment surface. Passive infill from overlying
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and Frey, 1974; Curran and Martin, 2003) and exceptionally preserved examples of Thalassinoides (Kennedy, 1967). The irregular surficial morphology of the burrow casts are the result of both tracemaker activity and diagenetic overprinting. The irregular, elongate ridges and grooves were likely produced on the interior wall of the tunnels during excavation. It is important to note that the surficial features on the walls of the burrow casts are the negative images of the original features. Therefore, the positive relief features on the outer surface of the cast represent impressions on the interior of the original tunnel wall and the negative relief features on the cast represent protrusions. These surficial features indicate that the interior tunnel walls were not smooth and were not lined with mud, fecal material, or pellets. The nodular textures are likely diagenetic in origin and represent post-burial mineral growth possibly in zones of concentrated organic material. Their irregular shapes and sizes as well as their preservation on the outer surface of the burrow casts indicate that they are not pellets produced by the tracemaker. 4.2. Ichnotaxonomy
Fig. 4. The horizon containing the large burrow complexes (at arrow) occurs below the Fishpot Limestone (A). Large horizontal tunnels are visible in cross-section eroding out of the outcrop (B). The burrows are preserved as natural casts composed of calcareous mudstone similar in composition to the overlying strata.
unit suggests, however, that a connection to the surface environment did exist while the tunnels were open. Without the presence of these vertical shafts, passive infill of the subsurface tunnels from above would not be possible. Despite their lack of preservation, therefore, the nature of the burrow fill does indicate that the tracemakers maintained a connection to the surface through vertical shafts as observed in similar modern burrow networks (Shinn, 1968; Bromley
Ichnotaxonomy is based upon the morphology of ichnofossils without the consideration of possible tracemakers, depositional environments, or substrate conditions (Bromley, 1996; Pemberton et al., 2001; Bertling et al., 2006). Bertling et al. (2006) recommended that four ichnotaxobases related to burrow morphology be used when classifying trace fossils: overall shape, orientation, ornamentation, and internal structure. Considering these ichnotaxobases, the Monongahela burrow complex is similar to established ichnogenera of branching burrow networks such as Thalassinoides, Ophiomorpha, and Spongeliomorpha. Like these ichnogenera, the Monongahela burrow complexes consist of circular to elliptical tunnels interconnected along a single horizontal plane (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975). These ichnogenera also include branching tunnels, typically forming Y-shaped or T-shaped junctions (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975; Myrow, 1995). In contrast, the burrow complexes from the Monongahela possess Y-, T-, and Xshaped junctions within a single gallery, a suite of architectures not observed in these other ichnogenera. The tunnels at the junction
Fig. 5. A) A single slab (OUZC 5503) approximately 100 cm long and 50 cm wide containing two branching galleries (1 and 2) of large diameter-tunnels connected by a smalldiameter tunnel (at arrow). B) Line drawing of the burrow complex.
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Fig. 6. Variations in tunnel diameter within the burrow complexes. A) Slab (OUZC 5503) containing two different tunnel styles: a large-diameter (11 cm) elliptical tunnel (1) and a small diameter (4 cm) circular tunnel (2). B) A small, 1 cm diameter, tunnel (at arrow) branching off of a larger, 7.5 cm diameter, tunnel (OUCZ 5504). C) Examples of small-diameter tunnels within a burrow complex (OUZC 5503): (1) a poorly preserved, small-diameter tunnel connecting two galleries; and (2) short, blind tunnel. D) A curved, small diameter tunnel (at arrow) connecting two tunnels within the same gallery (OUZC 5503).
points are typically enlarged in Thalassinoides, often up to three times the width of the adjoining tunnels (Kennedy, 1967; Häntzschel, 1975). The Monongahela burrows do not possess enlarged tunnels of this scale at the junctions or elsewhere in the burrow complex. The overall architecture of the Monongahela burrow complexes is also unique in their organization into 50–60 cm polygonal galleries of large (8–18 cm) diameter tunnels connected to each other by small (1–5 cm) diameter tunnels. The surficial morphologies of Thalassinoides, Spongeliomorpha, and Ophiomorpha are characterized by smooth-walled tunnels, tunnels with walls ornamented with regular, repeating elongate ridges, and tunnels with mammillate textures covering the entire outer wall respectively (Kennedy, 1967; Fürsich, 1973b; Häntzschel, 1975; Frey et al., 1978). Tunnels of the Monongahela burrow complexes possess a mixture of elliptical nodes and depressions as well as irregular elongate ridges and grooves, an ornamentation pattern not observed in these other ichnogenera. Based on morphological similarity, the Monongahela burrow complexes may be assigned to the ichnogenus Thalassinoides. The difference in tunnel ornamentation between the Monongahela burrow complexes and both Spongeliomorpha and Ophiomorpha prevent their assignment to either of these ichnogenera. The Monongahela burrow complexes may represent a new ichnospecies of Thalassinoides, but morphological variation due to substrate and preservation cannot be discounted. 4.3. Paleoecological significance The geologic range of branching burrow networks such as Thalassinoides, Ophiomorpha, and Spongeliomorpha extends from the Cambrian to the recent (Häntzschel, 1964, 1975; Fürsich, 1973b; Bromley and Frey, 1974; Miller and Byers, 1984; Carmona et al., 2004). Thalassinoides first becomes abundant in the Ordovician, but these burrows are generally very small in diameter, ranging from 4 to 40 mm (Sheehan and Schiefelbein, 1984; Myrow, 1995; Ekdale and Bromley, 2003). Spongeliomorpha first appears in the Early Permian
while definitive specimens of Ophiomorpha first appear in the Triassic although some potential Late Carboniferous specimens have been described (Bromley and Frey, 1974; Häntzschel, 1975; Frey et al., 1978; Carmona et al., 2004). These two ichnogenera do not become common until the Early Mesozoic, and the Paleozoic examples are much smaller than younger examples (Häntzschel, 1975; Carmona et al., 2004). Unnamed branching burrow networks from continental settings include Lower Triassic burrows from well-drained paleosols of the Driekoppen Formation of South Africa and the Fremouw Formation of Antarctica (Groenewald et al., 2001; Hasiotis et al., 2004). The tunnels of these burrow complexes are 100–150 mm in diameter and are attributed to fossorial tetrapods (Groenewald et al., 2001; Hasiotis et al., 2004). Monongahela burrow complexes, therefore, represent one of the oldest large diameter (N100 mm) branching burrow complexes reported from the continental and marine fossil record. As a reference, Thalassinoides of this size have not been previously reported from strata older than the Cretaceous and include specimens with tunnels diameters of 100–120 mm (Pemberton et al., 1984; Carmona et al., 2004; Neto De Carvalho et al., 2007). Only much smaller diameter (7–11 mm) Thalassinoides have been reported from the Lower Pennsylvanian Fentress Formation in central Tennessee (Miller and Knox, 1985). The specific tracemaker of the Monongahela burrow complexes is unknown since no body fossils were found within the burrow casts or in the surrounding matrix. The architectural and surficial morphology of the burrows, however, can be used to interpret likely tracemakers. The presence and morphology of tunnel linings, branching patterns, the shape and relative dimensions of tunnels and chambers, and markings or bioglyphs on tunnel walls produced by tracemaker appendages, body, or head may all be used to help identify tracemakers. The absolute size of tunnels and chambers is not as useful in tracemaker identification. This is because many small animals may produce burrows that are much larger than their body, especially when the animals are gregarious or colonial. As a result, attempts to equate burrow diameter with organism diameter can
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Fig. 8. Surficial morphology of the burrow casts. A) Burrow surfaces are covered in elongate ridges (R) and grooves (G), and irregular circular to elliptical nodes (N) and depressions (D) (OUZC 5504). B) Portions of the tunnel walls are covered in a mammillate texture. Individual mounds are 4.5–7.2 mm in diameter (OUZC 5503).
Fig. 7. Three different branching patterns observed in the burrow complexes. A) Y-shaped junction (OUZC 5503). B) T-shaped junction (OUZC 5503). C) X-shaped junction (OUZC 5505).
easily lead to incorrect interpretations. The interpreted environment of deposition indicated by sedimentology and body fossils, in this case a freshwater lacustrine environment, may also be used to refine the potential list of tracemakers. For the Monongahela burrow complexes, marine organisms as well as terrestrial continental organisms may be excluded from consideration. Crustaceans (Cl. Malacostraca) are typically associated with subaqueous, branching burrow networks with the same morphology as Thalassinoides in modern shallow marine and freshwater environments as revealed by studies these burrow systems in the field and
laboratory (Shinn, 1968; Sellwood, 1971; Bromley and Frey, 1974; Curran and Frey, 1977; Hasiotis and Mitchell, 1993; Myrow, 1995; Bromley, 1996). While rare, the association of crustacean body fossils and branching burrow complexes also occurs in the fossil record. Body fossils of the decapod crustacean Callianassa have been found associated with Thalassinoides from the Early Miocene (Hayward, 1976) and Eocene (Glaessner, 1947; Frey et al., 1984). Neto de Carvalho et al. (2007) described the occurrence of the decapod crustacean Mecochirus within Early Cretaceous Thalassinoides from Portugal. Bromley and Asgaard (1972) described Early Jurassic (Toarcian) Thalassinoides containing fossils of the decapod crustacean Glyphea. This association of the Thalassinoides morphology with various crustacean taxa in the modern and ancient, therefore, suggests that the Monongahela burrow complexes may have also been produced by crustaceans. There are a number of crustacean taxa present in the Pennsylvanian whose extant members produce branching burrows (Brooks et al., 1969; Glaessner, 1969; Schram et al., 1978; Schram, 1981; Schram and Mapes, 1984; Wills, 1998). Crustaceans in general, including various branchiopods and malacostracans, have been present in shallow marine environments since the Cambrian (Brooks et al., 1969; Schram, 1981; Wills, 1998) and freshwater environments since the Devonian (Fayers and Trewin, 2003). A variety of crustacean taxa interpreted as rapacious carnivores, low-level carnivores, filter feeders, and detritus feeders have been collected from Pennsylvanian strata such as those of the Upper Pennsylvanian Mazon Creek area of Illinois (Schram, 1981). These crustacean fossils occur in units interpreted as the deposits of both nearshore marine and brackish water environments (Schram, 1981). Malacostracans are preserved in a range of depositional environments with highly variable salinities by the Carboniferous suggesting that these taxa had developed advanced osmoregulatory mechanisms (Briggs and
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Fig. 9. Cross sections of tunnel segments from the burrow complexes showing the calcareous mudstone and breccia burrow fill. A) A small, 5 cm diameter, burrow showing a sharp boundary between the burrow fill and the laminated mudstone of the surrounding unit. B) A large, 16 cm diameter, burrow showing that the fill extends to the outer surface of the tunnel cast.
Clarkson, 1989). Eocarids, for example, are well-known from coal swamp deposits of the Pennsylvanian suggesting that they inhabited freshwater environments (Schram, 1981). Syncarids are also abundant in the Pennsylvanian (Schram, 1981). Modern syncarids of the genus Anaspides, Paranaspides, and Micraspides inhabit freshwater lakes and rivers in Tasmania suggesting the possibility of similar environmental preferences for older syncarid taxa (Williams, 1965; Swain and Reid, 1983). While these crustaceans have not been associated with burrows, the presence of these animals in environments similar to the Monongahela lake systems does present the possibility that there may have been some freshwater burrowing malacostracan species in the Pennsylvanian. Vertebrates also construct complex burrow systems (Hasiotis et al., 2007). While terrestrial tetrapods produce complex burrow systems, the sediment in which the Monongahela burrows were produced are lacustrine in origin and show no sign of pedogenic modification. While some fish also produce complex burrows in marine environments (Rice and Johnstone, 1972), burrowing fish in modern freshwater environments such as lungfish tend to produce simple vertical or horizontal structures (Atkinson and Taylor, 1991). In addition, those fish that do produce complex burrow systems in marine environments often tend to produce these structures in concert with other organisms such as crustaceans (Farrow, 1971; Atkinson, 1974; Karplus et al., 1974). In these situations, the burrows possess two or more distinct architectural morphologies. Based on the comparison of their architectural morphology with modern crustacean burrow systems and taking the environment of deposition into consideration, the Monongahela burrow complexes are interpreted as the product of the burrowing activity of freshwater crustaceans. Specifically, the regular and repeating branching pattern of the Monongahela burrows strongly resembles those produced by crustaceans in both the modern and ancient (Weimer and Hoyt, 1964; Bromley, 1967, 1996; Shinn, 1968; Frey and Mayou, 1971; Bromley
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and Frey, 1974; Ott et al., 1976; Swinbanks and Luternauer, 1987; Pemberton et al., 2001; Carmona et al., 2004; Neto de Carvalho et al., 2007; Seilacher, 2007). The two-dimensional nature of the burrow complexes and the arrangement of the branching tunnels into a repeating polygonal pattern are also similar to previously studied crustacean burrow networks (Carmona et al., 2004). The Monongahela burrows do not resemble burrows produced by freshwater fish or tetrapods, the only other likely tracemakers of appropriate size and behavioral complexity to inhabit in the environment of deposition. The Monongahela burrow complexes are interpreted as dwelling structures due to complex, branching nature of the burrow network. In addition, the passive nature of the burrow fill that is similar in all of the tunnels indicates that the entire burrow complex was kept open, another characteristic of dwelling structures. Dwelling burrows are often the product of several interrelated behaviors and portions of the burrow complex likely served as sites of feeding and reproduction as well. The variation in the diameter of the component tunnels suggests that the burrow complexes were likely the dwellings of multiple individuals of different sizes. There are many examples in modern aquatic environments where several burrowing organisms will produce interconnected complexes including crustaceans (Atkinson, 1974; Atkinson et al., 1982). In these examples of commensalism, however, the morphologies of the individual elements of the burrow complex are distinct. In the Monongahela burrow complex, the large and small tunnels are identical with the exception of size. There are no distinct differences in the morphology or surficial features to interpret them as the burrows of different organisms. It is possible that they were produced by smaller and potentially younger individuals. They may also simply represent tunnels excavated for locomotion purposes only. The larger tunnels may have been expanded to serve as active dwelling chambers for multiple individuals. 4.4. Paleoenvironmental significance The succession of sedimentary strata in the Monongahela Group is indicative of a continental setting. The Fishpot Limestone has previously been interpreted as a freshwater to brackish palustrine limestone while the underlying and overlying strata include deposits of meandering and anastomosed fluvial systems as well as paleosols (Nadon et al., 1998; Hembree, 2008; King, 2008). As such, there is no direct evidence of marine influence in the stratigraphic interval containing the burrow complexes. Differences in the architectural and surficial morphology of trace fossils are typically due to variations in either the behavior of the tracemaker or the nature of the substrate (Bromley, 1996; MacEachern et al., 2007). Variations in substrate properties such as composition and compaction can result in morphological differences within a single burrow network. For example, linings can be present in one portion of a Thalassinoides complex and absent in another. Such linings are typically indicative of less cohesive substrates which require additional structural support to maintain open burrows (Bromley, 1996; Pemberton et al., 2001). This is supported by studies of modern burrow systems where tunnels cross both loose and cohesive substrates (Bromley and Frey, 1974; Bromley, 1996; Pemberton et al., 2001). The tunnels of the Monongahela burrow complexes lack a conspicuous lining and were passively filled by sediment from the overlying Fishpot Limestone. This suggests that the sediment was firm enough to allow open tunnels to be maintained without extra support. Surficial ornamentation can also provide information about the substrate. Scratch-marks preserved on tunnel walls are suggestive firmer substrates (Bromley, 1967, 1996; Pemberton et al., 2001). Clay rich sediments, for example, can preserve the scratches and scrapes produced by arthropod appendages while excavating a tunnel (Bromley, 1996). Softer substrates lack the cohesion to preserve these types of excavation marks. The irregular, elongate ridges and grooves on the walls of the Monongahela burrow complexes
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were likely produced during burrow construction. These may represent irregular surface indentations and scratch marks created by the tracemaking organisms either as the tunnels were initially excavated or expanded over time. The combination of sedimentological and ichnological evidence suggests that the burrow complexes were produced in firm mud along the margin of a shallow body of fresh water. The presence of large burrow complexes within the shoreline mud suggests that oxygen and nutrient availability in the depositional environment were relatively high. These high levels would be required to sustain a population of organisms capable of producing the extensive tunnel complexes. The energy expenditure required to construct the 50–60 cm wide galleries would need to be justified by the availability of resources in the immediate surroundings. The burrows were likely filled with sediment as water levels rose and the shoreline migrated inland. The in situ brecciation of the burrow fill suggests that some post-fill desiccation of the mud occurred soon after the flooding event.
5. Conclusions This study documents large burrow complexes from the Upper Pennsylvanian (Virgilian) Monongahela Group of southeastern Ohio. The ichnofossils consist of interconnected, branching mazes of elliptical to circular tunnels with irregular surface textures. These complexes differ from previously described branching burrow networks in their organization into multiple polygonal galleries of large diameter tunnels connected by smaller diameter tunnels, the presence of Y-, T-, and X-shaped branches, and tunnel surface ornamentation consisting of irregular circular to elliptical nodes and depressions as well as elongate ridges and grooves. While the absence of body fossils in the burrow casts and the surrounding matrix makes the assignment of a tracemaker difficult, the architectural morphology of the burrow complexes is similar to burrow complexes constructed by modern crustaceans. The presence of the large burrow complexes provide important paleoecological and paleoenvironmental information about the Monongahela Group. The burrow complexes indicate the presence of a large population of animals in the lacustrine paleoenvironments of the Monongahela Group despite the absence of abundant body fossils. In order to sustain the population of organisms necessary to construct the burrow networks, high levels of oxygen and nutrients must have been present in the subsurface environment. The nature of the passive infilling and the surficial morphology of the tunnels are suggestive of a firm substrate. The occurrence of these ichnofossils in the Upper Pennsylvanian Monongahela Group represents one of the largest diameter complex burrow networks from the Paleozoic. The average diameter of the tunnels is significantly larger than previously described specimens of Paleozoic Thalassinoides. Specimens of Thalassinoides from the Ordovician and Pennsylvanian vary from only 3 to 11 mm in average diameter whereas the tunnels in the burrows described here average 12 cm in diameter. In addition, these burrow complexes are preserved within a sequence of floodplain and palustrine sediments suggesting a freshwater habitat for the tracemaking organisms. This is unusual for large Paleozoic burrow complexes that are typically associated with marginal marine to marine depositional environments. The presence of these trace fossils in addition to other Paleozoic forms of Thalassinoides, Ophiomorpha and Spongeliomorpha indicate a long and complex evolutionary history of the trace-making behavior associated with the construction of large, open burrow networks.
Acknowledgements We would like to thank F. Surlyk and J.P. Zonneveld for their reviews and comments that helped improve this manuscript.
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