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Trace-fossil Distribution
Relationships Between Trace Fossils and Sedimentary Facies
Trace fossils are distributed irregularly throughout the Stull Shale Member succession. Biogenic structures have been recorded in units A1 (parasequence A), B1 (parasequence B), and D3 (parasequence D), but they are most abundant in unit B1.
Trace fossils in unit A1 (mud-dominated heterolithic facies) are restricted mainly to the sandstone interbeds. Sandstone soles contain low-diversity, but locally dense, assemblages of locomotion and grazing traces. Cruziana problematica is the dominant form, but Protovirgularia rugosa and Psammichnites grumula also are present. Monospecific assemblages or low-diversity assemblages are the rule, with traces often concentrated in small patches on certain bedding planes. Specimens of Palaeophycus tubularis are present in the mudstones, and they are filled with sand from overlying unit B1 and record a different bioturbation event (see section on "Ichnology of key stratal surfaces").
Unit B1 (sand-dominated heterolithic facies) displays an extraordinary abundance and diversity of trace fossils. This ichnofauna is dominated by bivalve resting (Lockeia ornata, L. siliquaria), escape (Protovirgularia rugosa) and feeding traces (Protovirgularia bidirectionalis), ophiuroid resting structures (Asteriacites lumbricalis), and arthropod locomotion traces (Cruziana problematica). Associated traces represent a wide variety of ethologic categories (domichnia, cubichnia, fodinichnia, pascichnia, and repichnia), and includes the following ichnotaxa: Arenicolites isp., Asterosoma? isp., Chondrites? isp., Conichnus conicus, Cruziana isp., Curvolithus simplex, Curvolithus multiplex, Diplocraterion isp. A, Diplocraterion isp. B, Nereites cambrensis, Nereites imbricata, Nereites jacksoni, Nereites missouriensis, Palaeophycus tubularis, Parahaentzschelinia ardelia, Pentichnus pratti, Phycodes palmatus, Phycodes isp., Phycosiphon incertum, Planolites beverleyensis, Psammichnites implexus, Psammichnites plummeri, Psammichnites? isp., Rhizocorallium irregulare, Rosselia socialis, Rusophycus isp., Skolithos isp., Teichichnus rectus, and Trichophycus isp. Biogenic structures left in open nomenclature include chip-shaped burrows, pelletoidal chains, small horizontal cylindrical burrows, small vertical burrows, and undetermined trackways. Most traces are preserved on bedding planes, and the degree of bioturbation is low, so the primary sedimentary fabric was not destroyed.
Unit D3 (trough cross stratified sandstones) contains a mono specific suite of Diplichnites cuithensis, a large trackway that may have been produced by the giant myriapod Arthropleura (Briggs et al., 1979). This ichnotaxon is present in the upper part of the sandstone.
Environmental Implications of Trace-fossil Vertical Distribution
Vertical distribution of trace fossils was controlled by substantial changes in environmental parameters, which in turn reflect the depositional evolution at the Waverly site.
Unit A1 is interpreted as having been deposited in a mixed- to mud-flat environment. This interpretation is supported by lenticular- and wavy-bedded deposits, wrinkle marks, relict troughs, and the near absence of body fossils. Scarcity of trace fossils may reflect severe living conditions and/or unfavorable preservational conditions. A low diversity of biogenic structures in mixed- to mud-flat facies is not surprising, because very few animals are able to inhabit the uppermost zone of tidal flats in tropical environments (Terwindt, 1988). Extremely high temperatures and desiccation usually prevent the establishment of diverse benthic communities in soft-sediment upper-intertidal zones (Newell, 1979; Reise, 1985). Taphonomic factors, however, also may have played a role. Preservation of trace fossils in tidal-flat deposits is favored by the presence of sandstone/mudstone interfaces. Lack of sandstone interbeds would inhibit preservation and visibility of biogenic structures, which is consistent with the concentration of trace fossils on the soles of interbedded thin sandstone beds. However, it is interesting to note that even at these interbeds, diversity is particularly low in comparison with the sandstone interbeds of Unit B.
Unit B1 reflects deposition in an intertidal-sand flat. Abundance and diversity of biogenic sedimentary structures in unit B1 record the activity of a diverse benthic community. Preservation of trace fossils on bedding planes was enhanced by the cyclic alternation of sand layers and mud partings. Some crowded bedding surfaces document time averaging, which records the activity of several communities (see section on "Evidence of time-averaged surfaces"). Although the envisaged sedimentary environment is rigorous and unstable, organisms were able to deal with these stressful conditions, probably as a result of the predictability of such an environment (see section on "Spatial heterogeneity").
Unit D3 represents the fill of an abandoned fluvial channel. The producer of the trackway D. cuithensis was most likely a terrestrial arthropod. Diplichnites cuithensis has been recorded exclusively from late Paleozoic subaerial deposits, commonly exposed fluvial bars, silted channels, and desiccated sheetfloods (Briggs et al., 1979; Briggs et al., 1984; Ryan, 1986).
Diversity and abundance of trace fossils abruptly decrease upward in the Waverly section, which reflects an overall shallowing-upward trend from lower-intertidal to fluvial facies. The highest ichnodiversity is recorded in the tidal-flat deposits, specifically in the sand-flat facies. Towards the upper part of the section, terrestrial deposition is indicated by the arthropod locomotion traces.
Spatial Heterogeneity
One of the most remarkable features of the Waverly tidal-flat deposits is the heterogeneous distribution of biogenic structures. Any casual observer inspecting the outcrop can find substantial differences in the trace-fossil content of different bedding planes. Furthermore, a detailed inspection of sandstone rippled tops or soles reveals patchiness or small-scale spatial heterogeneity. Although spatial heterogeneity is a major feature of modern tidal flats, it has not been recorded in the fossil record in previous ichnologic studies.
Zonation and patchiness of benthic communities and of the biogenic structures recording their behavior is a common characteristic of modern coastal environments (e.g., Schäfer, 1972; Anderson and Meadows, 1978; Newell, 1979; Reise, 1985; Tufail et al., 1989). This is particularly true for the intertidal zone, where the tidal cycles, tidal currents, river input, and wind processes result in a wide variety of salinity changes and hydrodynamic regimes (Meadows et al., 1998). In addition, the complex biogenic interactions of the intertidal zone produce further complexity in spatial and temporal heterogeneity (Reise, 1985; Bertness, 1999; Little, 2000). Heterogeneity occurs primarily at two scales.
At the larger scale, zonational distribution is expressed along the entire tidal range. This is shown by different animal communities living in different areas within the tidal flat, where substrate, exposure, temperature, and other environmental parameters differ substantially (i.e., sand, mixed, and mud flat). Only very few species are able to inhabit the entire tidal range (Reise, 1985). In general, biologic diversity and biomass decrease toward the level of high tide (Newell, 1979; Reise, 1985). Heterogeneity and predictability of the middle- to lower-intertidal environments result in high-species diversity containing species that are particularly adapted to utilize the resources of specific microhabitats (Sanders, 1968, 1969; Slobodkin and Sanders, 1969). In contrast, in the high-intertidal area conditions not only are more extreme, but they are characterized by high-temporal instability and unpredictability resulting in a decrease in species diversity. In this setting, physical factors, such as heating, frost, and water loss, playa crucial role on benthic macrofaunal communities. Although primary production by benthic microalgae increases in a landward direction, benthic consumers do not show a corresponding increase. This is related to difficulty for marine organisms to adapt to prolonged low-tide emersion (Reise, 1985). Arid climates cause more extreme upper intertidal conditions and corresponding impoverished communities than humid climates. In general, benthic organisms tend to be less specialized in their diet, often being trophic generalists.
Several authors have recognized tidal zonation based on biogenic sedimentary structures. For example, Frey, Howard, et al. (1987) distinguished three different ichnofaunas (brachyuran, molluscan, and holothurian assemblages) in low-energy, extensive macrotidal flats of the Yellow River, in South Korea, from the shore to more than 3.9 km seaward. Swinbanks and Murray (1981) recognized five sedimentological/floral zones in tidal flats of British Columbia, each characterized by different associations of animal structures. Different assemblages of biogenic structures also have been recognized in tidal flats from the North Sea by Gerdes et al. (1985), and the Bay of Mont-Saint-Michel in France by Larsonneur (1994). Aitken et al. (1988) discussed an interesting example of tidal-flat assemblages in a subarctic, ice-modeled coast. In fjord settings, coastal physiography plays a major role in organism distribution.
This scale of heterogeneity is reflected at Waverly by the broad vertical pattern in trace-fossil distribution previously discussed, where the mixed- and mud-flat assemblages are contrasted with the abundance and diversity of the sand-flat ichnofauna. Similar trends in trace-fossil distribution have been documented in a Silurian tidal flat by Narbonne (1984).
On a smaller scale, within each environment (e.g., on the sand flat), spatial segregation of species may reflect distinct microhabitats and partitioning of energy resources. Particular spatial array of organisms permits maximum utilization of available food resources (Newell, 1979). An example is the spatial separation of barnacles and limpets within the lower-intertidal zone of rocky shores (Lewis, 1961). Spatial partitioning patterns of intertidal organisms may be regarded as a mechanism by which organisms exploit particular food resources within the limits of their tolerance to environmental conditions, and at the same time minimize interspecific competition (Newell, 1979). In the middle and, particularly, in the lower-intertidal area, food resources are abundant and varied, but equally the organisms are bound by other species whose requirements may overlap with their own. Niche specialization, commonly reflected by patchiness, may effectively reduce interspecific competition. Selection commonly favors those behavioral responses that tend to restrict organisms to particular niches at which they convert energy more efficiently than their neighbors (Wolcott, 1973). Such adaptations are reflected by patterns of behavior, metabolism, or developmental changes (Newell, 1979).
Hogue and Miller (1981) recorded the existence of small patches of nematodes in ripple troughs, which they explained in terms of preferential accumulation of organic detritus (see section on "Substrate"). Reise (1985) documented segregation of assemblages of the amphipod Corophium volutator, the polychaete Arenicola marina, and the prosobranch Hydrobia in the sandy-tidal flat at Koningshafen. Mounds are stabilized and inhabited by the tube-dwelling amphipods, the prosobranch tends to concentrate at the fringe of the mounds, and the polychaete causes high sediment turnover in the surrounding areas. This distribution pattern records trophic partitioning, ensuring maximum utilization of the available food resources. Interestingly, Reise (1985) explained this distribution pattern in terms of sediment stabilizers and destabilizers (see section on "Substrate"). Another example of small-scale spatial heterogeneity is recorded by the distribution of Scolecolepsis squamata and Paraonis fulgens on a foreshore profile (Roder, 1971; Bromley, 1996). Scolecolepsis constructs vertical shafts on slight topographic rises; meanwhile the spiral traps of Paraonis occur in nearby depressions. Bromley (1996, p. 126) noticed that subtle modifications in the environment could result in the two communities alternating in a stratigraphic sequence. To quantify spatial heterogeneity in the modern intertidal zone, Meadows et al. (1998) established three 50-m transects in the lower intertidal zone of a bay in the Clyde Estuary, Scotland. The first transect was at a right angle with the sand waves (peak/trough transect), the second and third crossed the peak (peak transect) and trough (trough transect), respectively. Correlation, cluster, and principal component analysis highlighted patterns of spatial patchiness in the sedimentary environment (microhabitats) and macrobenthic community.
Heterogeneity related to local microtopography, typically bedforms or small positive areas within an isochronous horizon, is well represented at Waverly. Examples of patchiness are the mounds characterized by dense aggregations of U-shaped tubes (Protovirgularia bidirectionalis) and small vertical burrows (fig. 64A-D). These structures may be compared with Corophium volutator mounds (Reise, 1985) and tube-building polychaete worms (Jones and Jago, 1993), which increase sediment stability. U-shaped, mucus-lined bivalve structures can be interpreted as stabilizers, which trapped the tide-transported sediment resulting in small positive elements on the tidal-flat surface. The reason why the infaunal burrowers chose this particular spot of the substrate is more difficult to assess, but it must have been related to some particularly attractive feature of the sediment, as site selection is rarely random. Larvae preference for settlement in particular sites has been associated with specific features of the substratum, including physical properties (e.g., grain roundness) and biological components (e.g., organic film induced by bacteria, type of interstitial organisms, presence/absence of seagrass) (Newell, 1979; Reise, 1985). Wilson (1954, 1955) showed that the most important factor in the settlement of Ophelia bicornis is the presence of a film of microorganisms on the surface of sand grains. Meadow and Anderson (1968) made a survey of microorganisms attached to grains of intertidal sand and found an uneven distribution, with microorganisms tending to concentrate in small pits and grooves within the surface. Microbial stabilization also plays a major role in creating topographic irregularities, such as erosive remnants, mounds, domal upheavals, and projecting bedding planes in tidal flats (Gerdes et al., 1994; Gerdes, Klenke, et al., 2000; Gerdes, Krumbein, et al., 2000; Noffke et al., 1996; Noffke, 1999).
Figure 64--Sediment mounds. A. Upper view of an intensely bioturbated mound. KUMIP 288542. x 0.45. B. Lateral view of mound shown in A. Note shafts of Protovirgularia bidirectionalis and small cylindrical burrows. KUMIP 288542. x 0.45. C. Upper view of an irregular-shaped mound with smooth surface. KUMIP 288574. x 0.28. D. Basal view of mound shown in C. Note presence of Palaeophycus tubularis and abundant shell fragments. KUMIP 288574. x 0.28.
Another example of patchiness is the preferential presence of Psammichnites implexus in ripple troughs. Small-scale spatial heterogeneity probably documents the effects of bedform topography on the partitioning of food resources (see section on "Substrate"). For example, specimens of Psammichnites implexus commonly are concentrated in ripple troughs, where they display almost a guided meandering pattern. Absence of self-overcrossing suggests phobotaxis. Specimens are isolated in separate troughs and do not overlap. Presence of guided meanders records a highly specialized feeding strategy comparable with that of Helminthorhaphe in deep-marine settings (Seilacher, 1977a; Uchman, 1995). This distribution may reflect food searching in ponded areas established in ripple troughs during the low tide. The searching pattern indicates that troughs acted as sites of accumulation of organic detritus, being organic-rich at the sediment-water interface and within the uppermost millimeters of the sediment.
Heterogeneity also is recorded by nuculoid bivalve structures (fig. 65). The paucispecific assemblage of Lockeia ornata and Protovirgularia rugosa on localized stratigraphic levels may be interpreted as recording tidal flat heterogeneity across the tidal range. Although the sand-dominated interval of the Waverly section represents a sand-flat environment, the mixed-flat transition is probably present. Stratigraphic changes in trace-fossil assemblages may record subtle shifts between adjacent zones of the intertidal area. Alternatively, physical or biological disturbance may result in temporal modifications of benthic community structure (see section on "Tiering structure and ichnoguilds").
Figure 65--Heterogeneous distribution of Lockeia ornata preserved on the base of a sandstone bed. KUMIP 288552. x 0.23.
Trace-fossil Paleoecology
Environmental Controls
Many factors define the niche and survival range of animal species. Within a particular ecosystem, some of these factors are particularly relevant, becoming limiting factors (Brenchley and Harper, 1998). In the tidal zone, environmental controls acquire particular ecological significance. The tidal-flat zone represents a harsh environment where marine organisms often approach the survival boundaries of their tolerance range to environmental extremes. Although tidal flats are primarily a marine habitat, they are subject to the extremes of terrestrial climate, heating, frost, desiccation, and rain (Reise, 1985). Temperature, time of exposure to subaerial conditions, salinity, hydrodynamic energy, and substrate are effective limiting factors.
In this section, we discuss the role of environmental parameters as controlling factors of the Waverly ichnofauna. Although environmental factors are considered separately, it is important to understand that the limits of tolerance of intertidal invertebrates are defined in terms of multi-variable responses, rather than in terms of isolated factors (Newell, 1979). Salinity, temperature, and exposure to subaerial conditions are intimately linked and are strongly dependent on latitudinal position and climate. On the other hand, hydrodynamic energy and substrate conditions also are interconnected and dependent on coastal topography and physiography. The resultant ichnofauna therefore is shaped by the interplay of key environmental parameters overprinted by taphonomic factors.
Salinity
Coastal environments experience large fluctuations in salinity. Periodic emersions and submersions of the intertidal zone are matched by periodic fluctuations in salinity. Additionally, seasonal rains and drainage from the continent significantly affect the salinity and position of the water table. Salinity shifts, together with exposure and temperature, are typically drastic in the upper-intertidal area and diminish towards the lower-intertidal zone (Newell, 1979; Reise, 1985). In general, salinity tolerance controls the zonational distribution of intertidal animals, with the more euryhaline species occurring more abundantly in the upper-intertidal zones (Newell, 1979). Complex hydrologic conditions of the tidal flat promote particular behavioral strategies for protection, such as infaunalization. Inhabiting a burrow or temporary refuge in the sediment is an effective strategy in avoiding salinity variations (Howard, 1968; Pemberton and Wightman, 1992). In low-energy settings, close to the low-water mark, surface salinity changes have little effect on the salinity of interstitial water below a depth of about 2 cm (Sanders et al., 1965; Johnson, 1967). Many tidal-flat inhabitants have developed biological rhythms (e.g., circa-tidal, circa-semilunar rhythms) of vertical or horizontal migration controlled by tide cyclicity (Palmer, 1995). Many species hide in their burrows during low tide and are active during high tide. For example, the modern crab Sesarma reticulatum hides in its burrow during low tides and roams the surroundings during high tides (Palmer, 1967, 1995; Seiple, 1981). Horizontal migration is a well-documented strategy to minimize the dramatic salinity shifts in the upper-intertidal zone. An example of tide-associated migration is the activity pattern of the modern predaceous isopod Eurydice pulchra, which lives buried in the sand flat during emersion, but rises into the water column with flood tide to swim at the water's edge and feed on epifauna, infauna, and debris. It retreats seaward with ebb tide and reburies itself for protection (Warman et al., 1991).
Marine invertebrate surface activity on the tidal flat is typically more intense during high tide (Pienkowski, 1983; Vader, 1964). In contrast, many semi-terrestrial and terrestrial animals (e.g., terrestrial crabs, the modern intertidal beetle Thalassotrechus barbarae) may display a peak of activity during low-tide emersions (Palmer, 1995). Other adaptations to stressful salinity conditions involve protection by organic substances (e.g., mucus), and osmoregulation (Kinne, 1964). Some animals combine several strategies for better protection. For example, the modern Corophium is a good osmoregulator and a wellknown burrower that can tolerate salinities between 2‰ and 47‰.
In addition to the sedimentologic evidence, the diversity and complexity of forms recorded at Waverly indicate a sand-flat environment, probably very close to low-water level, where stress conditions were ameliorated by short-time exposure (cf. Swinbanks and Murray, 1981). From a biological perspective, lower intertidal animal communities resemble contiguous sub-wave base assemblages (Schäfer, 1972; Reise, 1985). The scarcity of biogenic structures in upper-intertidal deposits suggests that physico-chemical conditions were extreme, preventing the development of an abundant resident fauna. Cruziana problematica commonly is associated with other ichnotaxa in sand-flat deposits of the Stull Shale Member. Paucispecific occurrences of C. problematica in the mud- and mixed-flat deposits may record either a wide environmental range of its producer or short-term incursions into this zone. High-density assemblages of these arthropod traces may not record upper-intertidal inhabitants, but landward migrations from the lower-intertidal zone. As in the case of many recent crustaceans, these migrations regulated by tidal cyclicity may have been connected to the search for food.
Temperature
Tidal flats commonly exhibit rapid changes in temperature related to periodic subaerial exposure. Several studies of modern environments have documented the relationships between temperature and animal-sediment interactions (e.g., Green and Hobson, 1970; Yeo and Risk, 1981; Aitken et al., 1988). However, application of these concepts to the study of fossil cases is still in its infancy. Commonly, the high-intertidal zone in tropical environments is an extremely inhospitable habitat for marine organisms due to very high temperatures, long time of exposure, and abnormal salinities. As a consequence, the highest density of biogenic structures in tropical tidal flats is in the lower-intertidal zone (Terwindt, 1988). On the other hand, tidal flats in colder areas may exhibit a high density of biogenic structures in the upper-intertidal zone (e.g., Yeo and Risk, 1981). Therefore, bathymetric displacement of certain species along latitudinal gradients is common (Reise, 1985). For example, the bivalve Gemma gemma lives in intertidal areas in northern North America and in subtidal areas in the south to avoid the hazards of high temperatures on tidal flats (Green and Hobson, 1970). Aitken et al. (1988) documented biogenic structures in modern subarctic tidal flats and noted a dominance of vertical domiciles of bivalves and polychaetes. These authors compared subarctic and temperate tidal flats in terms of biogenic structures and noted that some forms, such as Corophium volutator, were abundant in temperate tidal flats but absent from subarctic intertidal areas.
At Waverly, the highest density and diversity of ichnofossils is, by far, concentrated in lower-intertidal deposits, close to low tide. High temperatures and increased desiccation risk in the upper-intertidal zone were probably major limiting factors. This interpretation is consistent with paleogeographic reconstructions that suggest an equatorial position for Kansas during the Pennsylvanian (Scotese and McKerrow, 1990; West et al., 1997).
Substrate
Substrate control on trace fossil morphology of the Waverly ichnofauna is striking. Almost all forms were affected by the substrate and serve as useful tools for measuring substrate properties (fig. 66A-E). In general, it is possible to identify ichnotaxa irrespective of substrate effects (fig. 66A-D). In a few cases, however, deformation is so severe that accurate identification cannot be achieved (fig. 66E). Vertical and horizontal differences in substrate conditions influence the diversity, abundance, and distribution of intertidal organisms (Newell, 1979, Reise, 1985).
Figure 66--Substrate fluidity and morphology of trace fossils. All photos are base of bed views. A. Fluted base with deformed specimens of Protovirgularia rugosa connected to escape structures. KUMIP 288525. x 0.79. B. Load-casted surface with inflated, starfish-shaped Asteriacites lumbricalis. KUMIP 288523. x 1.66. C. Firm sandstone base with brittlestar-shaped Asteriacites lumbricalis (arrow). KUMIP 288530. x 0.79. D. Soft substrate with high pore-water content having poorly preserved sinuous trace (Psammichnites? isp. [arrow]) and Cruziana problematica. Note the gradation between bilobed forms and chains of knobs related to soft-sediment deformation. KUMIP 288515. x 0.55. E. Highly deformed sandstone base with large undetermined traces. KUMIP 288563. x 0.55.
Morphologic variability of trace fossils between and along bedding planes reflects controls operating at different scales. Because tidal flats are regularly exposed and submerged by the tides, the concomitant pore-fluid content within the sediment will vary during a tidal cycle. On the other hand, the low-tide landscape commonly is characterized by the presence of tide pools within a generally emerged area, resulting in a range of substrate conditions along an isochronous surface.
The effects of microtopography on sediment grain size, sorting, and organic richness have been investigated. Thurn and Griffiths (1977) analyzed the hydraulic circulation of water through the pore system of ripple marks in sand. They found that water entered through the troughs and exited through the crests along a pressure gradient. This circulation pattern results in a re-sorting of sediment with small grains and organic debris being drawn into the troughs. Organic matter is trapped in the sediment to a depth equal to the height of the ripple crest. The localized distribution of organics accounts for the aggregation of meiofauna and invertebrate grazers in troughs (Jansson, 1967; Harrison, 1977, Newell, 1979; see also section on "Spatial Heterogeneity").
Whereas the anatomy of body fossils is controlled by inherited genetic factors, the morphology of trace fossils is strongly controlled by external factors (Goldring et al., 1997). Substrate type and consistency are important extrinsic factors that determine both burrowing technique and infaunal community composition (Bromley, 1996). Substrate consistency embraces the intricate interplay of multiple factors (e.g., grain size, sorting, water content, organic matter content, shear strength, and mucus binding) that define the mechanical properties of the sediment (cf. Bromley, 1996). Sediment composition directly influences substrate consistency. Carbonate substrates may be subjected to progressive dewatering stages (soupground, softground, firmground, and hardground) associated with increasing compaction and cementation (Ekdale et al., 1984; Ekdale, 1985; Lewis and Ekdale, 1992). Goldring (1995) recently introduced the term "looseground" for soft sand and gravel as distinct from soft mud and silt (softground).
Most studies concerning trace fossils and substrate control have focused on the evolution of carbonate substrates and how this affects community composition (e.g., Bromley, 1975; Goldring and Kazmierczac, 1974; Mángano and Buatois, 1991; Lewis and Ekdale, 1992; Bromley and Allouc, 1992), or on the erosional exhumation of firm siliciclastic sediments (e.g., MacEachern et al., 1992; MacEachern and Pemberton, 1997). Recent work in siliciclastic ichnology, however, emphasizes that the process of dewatering and the concomitant changes in substrate properties is a continuum rather than a series of compartmentalized stages (cf. Buatois, Jalfin, et al., 1997). Therefore, a scale of morphologic variation of ichnofossils depicting substrate evolution can be constructed.
Maples and West (1990) suggested that morphologic variability of bivalve traces at Waverly was controlled by substrate fluidity. Protobranch bivalve traces at Waverly exhibit a complex array of relationships controlled by bivalve behavior and substrate character (Mángano et al., 1998). In defining a range of substrate conditions, presence of impregnated walls, sharpness of delicate morphologic details, and degree of deformation are important observations (Goldring, 1991). The sharp chevrons of Protovirgularia bidirectionalis suggest penetration in relatively firm, dewatered substrates. Structures with sharp, closely spaced chevrons represent what can be characterized as the firmer end of the softground range ("non-fluid sediments" of Maples and West, 1990).
Trueman et al. (1966) investigated the effects of substrate, particularly grain size, on the rate of burrowing. Paradoxically, it seems that the easier the penetration, the worse the anchorage, and vice versa. A dilatant medium becomes firm and more resistant to shear as increased force is applied, whereas a thixotropic system shows reduced resistance to increased rates of shear. As a consequence, anchorage requires a substance with dilatant qualities, whereas motion is facilitated by a thixotropic system (Trueman and Ansell, 1969). For example, in soft estuarine silts penetration is easy, but firm anchorage is difficult. Conversely, secure anchorage is attained in dilatant beach sands, but penetration is harder because of the increased resistance of the substrate. Factors involved in the penetration and protraction phase tend to compensate one another so that the difference in the rate of burrowing may not be determined solely by grain size. Trueman et al. (1966) noticed that compacted sediment is stiffer, which results in a decreased burrowing rate. Accordingly, the frequency of the digging cycle and depth of penetration in each sequential movement decreases as burrowing into deeper levels proceeds (Ansell, 1962). Although quantitative experimental work has not focused specifically on water content and how it affects the mechanical properties of the sediment, within any given grain-size range a more compacted sediment will be less fluid and stiffer, offering increased resistance to penetration.
In chevron locomotion traces, such as Protovirgularia, the distance between two chevrons represents each sequential set of movements, and each chevron indicates the site of anchoring of the foot flaps within the sediment. Sharp, closely spaced chevrons account for short steps, with the animal struggling to advance in a stiff, resistant sediment. Longer distances between chevrons, like those observed in some delicate Protovirgularia rugosa, may reflect relatively coherent, but less resistant substrates, resulting in lower shell friction and allowing smoother and easier movement during the protraction phase. Sediment that is too fluid may result in irregular and highly deformed trace morphologies, recording the difficulties of the foot in obtaining a secure anchorage. As the amount of pore water increases, neither the fine morphologic details of L. ornata nor its pedal flap anchor are cast by soft, fluid sand. This may explain the mostly smooth locomotion traces resembling Palaeophycus tubularis (Planolites of Maples and West, 1990) that are connected to poorly defined L. ornata structures. Intermediate, poorly cohesive, relatively fluid substrates lead to intermediate morphologies, such as Imbrichnus (Hallam, 1970) and other forms that exhibit evidence of a bifurcated foot but display considerable deformation.
In short, dissimilar morphotypes respond to the same style of motion controlled by different substrate properties (Mángano et al., 1998). In other words, once intrinsic biologic factors were established (i.e., a bifurcated muscular foot), substrate consistency played the main role in the morphologic variability of trace fossils. Thus, Protovirgularia, Walcottia, Uchirites, Imbrichnus, and Chevronichnus do not represent major behavioral differences; they mainly record differences in the degree of dewatering and other related properties of the sediment. At a different scale, biologic diversity also could contribute to trace-fossil variability, as more than one bivalve species may have been involved in the production of biogenic structures. Species-level constraints, however, do not appear to have controlled the basic pattern of burrowing behavior.
The position of the trace maker within the substrate, particularly in relation to the sand-mud casting interface, is also a major influence in trace fossil morphology (Goldring et al., 1997). Subtle up-and-down movements that result in different morphologic expressions of an ichnotaxon can sometimes explain gradational changes along locomotion structures. For example, some typical chevron specimens of Protovirgularia grade into bilobate Didymaulichnus-like structures that, in turn, grade into smooth-walled Palaeophycus-like burrows. Elsewhere, some chevron traces grade laterally into smooth-walled forms, or into bilobate structures that locally show some chevrons. These differences in morphology within a single trace are toponomic rather than morphoethologic, and accordingly only one name, the one that better reflects the ethology of the animal, should be applied to the whole structure. In deeper variants, chevrons disappear, resulting in bilobate or even apparently smooth simple burrows (cf. Seilacher and Seilacher, 1994, pl. 1b).
The other important point about substrate is that animals are not passive to the physical properties of the sediment, but actually can substantially modify substrate attributes (Bromley, 1996). Woodin and Jackson (1979) and Woodin (1983) classified organisms into functional groups according to the effects, both direct and indirect, on the properties of the surrounding sediment, and the manner in which they make the environment more or less suitable for colonization by other organisms. Reise (1985) identified stabilization as promoting biologic interactions, whereas no benthic species will benefit directly from sediment destabilization. On this basis, two main functional groups can be distinguished: sediment stabilizers and sediment destabilizers. Mobile, mostly detritus-feeder infauna and epifauna, but also some sedentary organisms, whose feeding and defecation activities may provide abundant particles in suspension, destabilize the substrate (Rhoads and Young, 1970; Rhoads, 1974). In contrast, sedentary organisms that build mucus-lined tubes within the sediment reduce resuspension and erosion, and represent sediment stabilizers. For example, the tubebuilding polychaete worm Lanice conchilega acts like the steel reinforcing rods in concrete and increase the rigidity and stability of the sand (Jones and Jago, 1993). Rhoads and Young (1970) proposed that one feeding group may affect negatively another trophic group to the point of making life impossible for the affected group. The expected result of trophic amensalism is that where deposit feeders are abundant, development of suspension feeders is limited. In this framework, Reise (1985) explained the incompatibility of dense assemblages of organisms in the sand flats of Konigshafen in the North Sea.
At Waverly, bivalves that constructed mucus-lined, U-shaped burrows (Protovirgularia bidirectionalis), together with worms that produced lined vertical domiciles, most likely acted as sediment stabilizers. Evidence for this hypothesis comes from preferential concentration of these structures in small mounds (fig. 64A-B), resulting in the peculiar microtopography of some bedding surfaces. In contrast, dense concentrations of mobile detritus-feeding nuculanid bivalves (tracemakers of Protovirgularia rugosa and Lockeia ornata) may have acted as sediment destabilizers. Additionally, dense assemblages of Nereites may have changed significantly the nature of the substrate, encapsulating within the sediment significant amounts of defecation products. The intruding up-and-down movements of the Curvolithus tracemaker may have played a destabilizing role, particularly in some dense assemblages. Especially in the mud- and mixed-flat zones, microbial binding may have contributed significantly to stabilize the sediment because microbial mats shelter the substrate against erosion (see "Interpretation" of unit A1).
Another effect of infaunal burrowers on the chemical properties of the substrate is the increase in oxygen circulation through the sediment. The depth and abruptness of the redox potential discontinuity depends on the amount of oxidizable organic matter within the sediment and oxygen flow. Subsurface deposit feeders extend the oxic layer of sediment from close to the surface down to their depth of feeding in modern environments (e.g., Rhoads and Germano, 1982; Reise, 1985). According to sedimentologic and ichnologic evidence, both persistent wave action and burrowing activity within the sediment may have combined to provide good oxygen circulation in the sand flats at Waverly.
Hydrodynamic Energy
Hydrodynamic energy is a common limiting factor in coastal environments. High energy of tides, waves, and currents strongly controls patterns of trace-fossil distribution along tidal shorelines. High-energy zones of tidal flats are typically dominated by vertical burrows, commonly Diplocraterion or Skolithos (e.g., Cornish, 1986; Simpson, 1991). Overall features of the Waverly tidal flat, such as ichnotaxonomic composition and dominance of horizontal structures of deposit feeders and grazers, suggest a moderate- to low-energy coastal setting. Moderate- to low-energy conditions also were prevalent in the lower, sandy-intertidal zone. Deep gutter-cast structures, flute marks, truncation of vertical shafts, palimpsest surfaces, and presence of transported burrows (fig. 67A-B), however, suggest occasional events of high energy that were able to sculpt the tidal-flat surface and move a considerable amount of sediment. These erosive events were most likely related to storms, or possibly to allocyclic changes of sea level (see section on "Ichnology of key stratal surfaces").
Figure 67--Transported burrows at the base of a sandstone bed. Both photos from KUMIP 288556. A. General view. Note molds of bivalve shells and associated locomotion traces of bivalves. x 0.36. B. Close-up. x 0.9.
Evidence of Time-averaged Surfaces
Lockeia siliquaria is moderately abundant throughout the sand-flat deposits of unit B1. Some stratigraphic levels, however, exhibit a high density of structures with specimens crosscutting each other at the base and differentially preserved at the top, either as protruding shafts or as shallow epichnial depressions. Cross sectional views of some specimens show two basic modes of preservation: (1) hypichnial ridges connected to endichnial shafts that cut across thin sandstone beds, and (2) hypichnial ridges connected to short endichnial shafts that are truncated by physical sedimentary structures (e.g., low-angle tabular crossbedding). The burrow fill may be massive, suggesting a passive filling of the structure, or may show a poorly defined meniscus-like structure in the lower part of the shaft.
One distinctive stratigraphic horizon displaying segregation of bivalve traces is of particular interest. A sandstone lens composed of amalgamated thin sandstone layers and interbedded mudstone partings contains L. siliquaria associated with the ophiuroid resting trace Asteriacites lumbricalis. A slab collected from this bed provides a unique opportunity to analyze in detail the trophic type and behavioral response of the tracemaker of L. siliquaria to environmental dynamics. The bottom and top of the slab were mapped in detail, and the maps subsequently superimposed to outline crosscutting relationships between forms and vertical repetition (fig. 68A-E). Superposition of those maps shows that many sole structures correlate with structures on the upper surface (vertical repetition), commonly showing some component of lateral displacement. However, a closer look reveals that some crowded areas at the base are barely populated at the top; in other words, some Lockeia hypichnia are not connected to corresponding epichnia. Conversely, nearly all bivalve shafts whose openings are visible at the top reach the base. Crosscutting relationships between hypichnial ridges of Lockeia are very common at the base. Finally, Lockeia siliquaria on the upper surface can be preserved as large oval depressions (negative epirelief) or protruding shafts (positive epirelief).
Figure 68--Lockeia siliquaria palimpsestic horizon. A. Photo of the base of a large slab with abundant Lockeia siliquaria preserved as positive hyporelief. B. Upper surface of the slab with Lockeia siliquaria preserved as large oval depressions (negative epirelief) or protruding shafts (positive epirelief). C. Map of base of slab. D. Map of top of slab. E. Superimposed map of specimens preserved on base (stipple) and top (white) of slab illustrating truncation between individuals, correspondence between basal and upper structures (with common horizontal displacement), and lack of a counterpart in some forms. All x 0.07.
The amalgamated sandstone lens with preferential preservation of L. siliquaria can be interpreted as a palimpsest fabric (fig. 69). Osgood (1970) attempted to explain the crosscutting of traces on bedding soles by a sequence of colonization events. Scouring followed a first colonization event. Then, new sedimentation and recolonization resulted in a time-averaged surface that recorded the work of several communities of burrowers. At Waverly, Lockeia siliquaria preserved as hypichnial ridges and/or epichnial depressions or ridges further complicate the situation. Cross sectional views of polished slabs record at least two successive colonizations. A first generation of Lockeia, now preserved as hypichnial ridges, was eroded almost completely, and this episode was followed by traction deposition. Large specimens of Lockeia display negative epireliefs, suggesting shafts truncated by erosional events, with only their lower ends preserved. A second generation of Lockeia suggests a later colonization event. These are preserved as shafts protruding from the top of the bed, suggesting connection with a missing upper surface and the pumping up of sediment during upward movement. Palimpsest surfaces in intertidal environments also have been recognized on the basis of body-fossil analysis by West et al. (1990), who noted evidence of heterochronous community replacement in modern intertidal relict exposures of marsh surfaces and suggested analogous situations in the Carboniferous of the Appalachian basin and elsewhere.
Figure 69--Sequence of events leading to a palimpsest fabric. A. First colonization event. B. Erosion event that scoured the structures leaving only their lower ends (typical Lockeia preservation). C. Renewed deposition and passive infill of structures. D. Second colonization event. E. New erosion episode resulting in a time-averaged horizon with palimpsest fabric. After Mángano et al. (1998).
Tiering Structure and Ichnoguilds
Tiering consists of the vertical stacking of organisms within a single habitat (Bromley and Ekdale, 1986). Marine infaunal communities typically are tiered as a consequence of vertical partitioning of the endobenthic habitat due to environmental and chemical gradients and also to the type and availability of food resources. Oxygen content, organic matter, and substrate consistency vary with respect to the sediment/water interface (Bromley and Ekdale, 1986; Bromley, 1990, 1996). Trace fossils can be grouped into ichnoguilds (Bromley, 1990, 1996). The ichnoguild concept involves three aspects, which result from adaptations of the guild concept in paleontology as proposed by Bambach (1983): (1) bauplan (whether a structure is stationary, semi-permanent, or transitory), (2) food source (trophic type), and (3) use of space (tier). The ichnoguild concept has been applied to the study of both marine (Ekdale and Bromley, 1991) and continental ichnofaunas (Buatois, Mángano, Genise, et al., 1998e).
Tiering and ichnoguild analysis has serious limitations in ancient tidal-flat settings due to temporal instability of community structure and time averaging of fossil faunas. Community structure in shoreline environments is shaped by the interaction of physical and biological forces. Significant changes may occur seasonally (e.g., as result of physical stress) or stochastically. For example, the introduction of a new predator may trigger a top-down effect modifying community composition (Bertness, 1999). Accordingly, what we reconstruct here as tiering structure is most likely an idealized time-averaged picture of the tidal-flat community.
Careful examination of crosscutting relationships, burrowing depth, and wall sharpness suggest a tiering structure for the benthic fauna of the Waverly tidal flat. This information has been integrated with trophic types and bauplan to define ichnoguilds. The tiering structure and ichnoguild model of the Waverly tidal flat (fig. 70) have been constructed using only information based on those traces that are abundant enough or that provide direct information about depth with respect to the sediment-water interface.
Figure 70--Tiering structure and ichnoguilds of the Waverly tidal-flat ichnofauna.
Horizontal sinuous to meandering trails of Nereites missouriensis, Phycosiphon incertum, and Psammichnites implexus represent the shallowest tier. This tier invariably is preserved at the tops of ripple-bedded sandstones and most likely reflects the work of animals moving at the mud-sand interface. The ophiuroid resting trace Asteriacites lumbricalis and arthropod horizontal locomotion burrows, Cruziana problematica, characterize a slightly deeper tier. This tier typically is preserved at the bases of sandstone beds and probably records burrowing immediately below the sand-mud interface. An intermediate tier is represented by horizontal to subhorizontal trails Curvolithus simplex, the bivalve resting/feeding trace Lockeia ornata, and its associated escape trace Protovirgularia rugosa. Curvolithus simplex commonly crosscuts elements of the shallower tiers (e.g., Asteriacites lumbricalis) and is preserved both at the tops and bases of sandstones. Lockeia ornata and Protovirgularia rugosa are preserved on the soles of sandstone beds and usually crosscut shallower traces, such as Cruziana problematica. Preservation of fine sculpture in some specimens of Lockeia ornata indicates a firmer substrate. Finally, the deepest tier is occupied by dwelling and feeding structures of bivalves assigned to Lockeia siliquaria and Protovirgularia bidirectionalis, and the sea anemone burrow Conichnus conicus.
Four ichnoguilds have been recognized. Ichnoguild I consists of subsuperficial, vagile, deposit-feeder structures of worms and mollusks, represented by Nereites missouriensis, Phycosiphon incertum, and Psammichnites implexus. Ichnoguild II includes very shallow, vagile to semi-permanent, deposit-feeding structures of ophiuroids (Asteriacites lumbricalis) and arthropods (Cruziana problematica). Ichnoguild III consists of shallow, vagile, deposit-feeding and predaceous structures, recorded by Curvolithus simplex, Protovirgularia rugosa, and Lockeia ornata. Ichnoguild IV includes relatively deep, permanent to semi-permanent traces of suspension feeders, deposit feeders, and predators, represented by Protovirgularia bidirectionalis, Lockeia siliquaria, and Conichnus conicus.
With the exception of the relatively deep ichnoguild IV, the Waverly ichnofauna is dominated by shallow tiers. Preservation of shallow-tier structures is less common in post-Paleozoic tidal-flat assemblages, which are typically dominated by deep, elite traces of crustaceans. Ichnoguild analysis of the Waverly trace fossil assemblage provides a snapshot of the rich benthic fauna inhabiting the first few centimeters below the sediment/water interface.
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Kansas Geological Survey, Geology
Placed on web May 21, 2015; originally published 2002.
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