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Ichnology of a Pennsylvanian Equatorial Tidal Flat

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Systematic Ichnology

Taxonomic Philosophy

The Waverly ichnofauna represents a real challenge to ichnotaxonomists. The biogenic structures are extremely abundant and the morphologic variability of the trace fossils is striking. In modern ichnology, contrasting philosophical perspectives have been adopted by different authors. As in the case of body-fossil taxonomists, the lumpers and the splitters represent two opposing ways of weighting trace-fossil morphology (Pickerill, 1994). In a simplified characterization, lumpers will tend to cluster all existing forms in a few essential ichnogenera, and splitters will find visible morphologic differences significant enough to create a plethora of new forms. From a philosophical perspective, lumpers can be characterized as more inferential, splitters as more empirical. Lumpers tend to favor behavior over morphology, trying to define the basic behavior that relates a group of structures, while splitters remain reluctant about the invisible links among morphologically dissimilar forms. This confrontation is a revisitation of the old debate about the roles of observation and theory in science.

Most ichnologists will agree with Bromley (1996, p. 166) that "in the final analysis, it is the morphology of the trace as an expression of animal behavior that is the basis of the name." To decipher the behavior of the tracemaker, however, may be quite a difficult task. Very frequently morphology in itself is considered sufficient to define new ichnotaxa, although its ethologic meaning is hardly understood. Some morphologic features can be objective in the sense of being observable and easily recognizable, and they may still not deserve any consideration at any ichnotaxonomic level. A drastic difference in morphology may actually provide evidence of extrinsic controls rather than behavioral determinants (see section on "Substrate"). Only morphologic characters that are known to reflect significant behavioral traits should be considered (i.e., ichnotaxobases; Bromley, 1996).

While morphology is observed, behavior must be inferred. The degree of behavioral inference varies with each particular case. For example, in the simplest case, there is almost a continuum from the morphologic observation of clearly preserved ventral anatomic features to the interpretation of a trace as a resting structure. However, analysis of most traces requires a larger inferential jump, involving knowledge of a complex array of biological, taphonomic, and environmental determinants.

Advantages of a dual nomenclature (i.e., two separate names for biotaxa and ichnotaxa) as well as the risks involved in the biotaxonomic identification of the tracemaker have been stressed by Bromley (1996). In some instances, inclusion of the actual taxonomic identification of the trace maker as an essential component of naming trace fossils (e.g., Seilacher, 1985; Hasiotis and Bown, 1992) has led to circular reasoning. Introduction of a biologically (or environmentally) based ichnotaxonomy will lead to a dual nomenclature for marine and continental trace fossils. Buatois, Jalfin, et al. (1997) noted that if biologic or sedimentologic criteria are applied to ichnotaxonomy, it will be virtually impossible to escape from circular reasoning when using trace fossils as an aid to interpret ancient depositional environments. In these situations, such a taxonomic system undercuts the information potential of trace fossils in sedimentology, stratigraphy, and paleoecology (Buatois, Jalfin, et al., 1997).

The importance of the biology of the tracemakers in understanding the ethologic significance of biogenic structures, however, is not always straightforward. Constructional possibilities are determined by intrinsic biologic factors, and therefore should be helpful in evaluating the relative significance of behavioral traits as reflected by trace-fossil morphology. In this sense, the biology of the tracemaker plays a role, albeit indirect, in trace-fossil taxonomy. Ethologic interpretation of a morphologic feature requires this broad biological framework. Similarly, an accurate understanding of the environmental conditions under which a trace fossil is created should enlighten our understanding of the structure, even if those factors are not formally considered in the nomenclature of trace fossils (cf. Goldring et al., 1997). Both paleobiological and environmental analyses provide significant clues that help decipher behavior and propose more robust taxonomic schemes.

In our efforts to characterize the Waverly ichnofauna, we have been conservative to avoid creating names with little ethological significance. At the same time, we have tried to provide an appropriate biological and environmental framework to maximize the information potential of the Waverly trace fossils. We have erected only one new ichnospecies, Protovirgularia bidirectionalis, whose distinctive morphology undoubtedly reflects a unique behavioral pattern. When possible, we have used a formal name to characterize a structure, which will lead to direct comparisons with other ichnofaunas. However, where morphology was indistinct or the available specimens were insufficient to recognize significant morphological and behavioral traits, we have used open nomenclature.

General Comments

In this section, trace fossils are described and discussed in terms of ichnotaxonomy, environmental and stratigraphic range, and possible tracemakers. Ichnotaxa are listed alphabetically and ichnofossils in open nomenclature are described at the end of the section. Associated ichnofauna refers to the other traces most commonly recorded on the same bedding plane. Descriptions are based on the analysis of more than one hundred collected rock slabs and additional specimens examined in the field. Specimens are housed at the Museum of Invertebrate Paleontology at the University of Kansas.

Ichnogenus Arenicolites Salter, 1857

Discussion--The ichnogenus Arenicolites includes vertical U-shaped burrows lacking spreiten, thus differing from the spreiten U-burrow Diplocraterion (Fürsich, 1974a; Hakes, 1976). A related ichnotaxon, Solemyatuba Seilacher, 1990a, is distinguished from Arenicolites by its elliptical cross section, indicative of a bivalve trace maker.

Arenicolites is interpreted as dwelling structures (domichnia) commonly produced by polychaetes (Goldring, 1962; Fürsich, 1974b), crustaceans (Goldring, 1962), and holothurians (Seilacher, 1990a; Bromley, 1996). Bromley (1996) discussed Arenicolites-like structures formed in modern environments by the polychaetes Chaetopterus variopedatus, Lanice conchilega, Amphitrite ornata, and Arenicola marina, the echiuran worms Urechis caupo and Echiurus echiurus, the holothurian Leptosynapta tenuis, and the enteropneust Balanoglossus clavigerus (see also Howard and Dörjes, 1972 for a discussion of the latter genus). U-shaped burrows also are produced by the polychaete Glycera alba (Ockelman and Vahl, 1970). Bromley (1996) cautioned against assuming that Arenicolites indicates a suspension-feeder, because in modern environments such burrows are produced by both suspension-feeders (e.g., Urechis caupo) and deposit-feeders (e.g., Echiurus echiurus). Ronan (1977) also criticized the assumption that U-shaped burrows are invariably produced by filter-feeders, providing several examples of Arenicolites-like burrows constructed by omnivorous polychaetes. Bajard (1966) also figured U-shaped burrows of Arenicola marina.

Although more typical of high-energy shallow-marine facies (e.g., Carey, 1978; Heinberg and Birkelund, 1984; Dam, 1990a; Pemberton, MacEachern, et al., 1992), Arenicolites also has been recorded in deep-marine (e.g., Crimes et al., 1981; Buatois and Mángano, 1992), marginal-marine (e.g., Hakes, 1976; Chaplin, 1982; Eagar et al., 1985), and continental (e.g., Bromley and Asgaard, 1979, 1991; Mángano et al., 1994) facies. Examples of this ichnogenus in tidal-flat facies have been mentioned by Hakes (1976), Ireland et al. (1978), Chamberlain (1980), Pollard (1981), and Narbonne (1984), among others. Arenicolites ranges in age from Cambrian to Holocene (e.g., Crimes, 1994; Bromley, 1996).

Arenicolites isp.

Fig. 24A-B

Specimens--Four specimens on slabs KUMIP 288500, KUMIP 288531, KUMIP 288532, and KUMIP 288552.

Description--Simple, U-shaped, vertical burrows without spreiten. Walls are smooth with a thin lining. Burrow fill is identical to host rock. Burrow depth is 17.2- 40.0 mm; arm width is 4.0-11.4 mm; spacing between is arms 71.4-201.7 mm. Preserved as full relief.

Figure 24--Arenicolites isp. Cross sectional view. A. Note associated Rosselia socialis (center). KUMIP 288552. x 0.5. B. Burrow-fill partially preserved, showing thin wall lining. KUMIP 288531. x 0.6.

Associated Ichnofauna--Nereites missouriensis, Protovirgularia bidirectionalis, P. rugosa, Curvolithus simplex, Conichnus conicus, Rosselia isp., Lockeia siliquaria, Psammichnites implexus, Halopoa isp., Palaeophycus tubularis, Cruziana problematica, and small cylindrical burrows.

Remarks--The ichnotaxonomy of the different ichnospecies of Arenicolites is unclear and this ichnogenus is a candidate for taxonomic revision. Arenicolites isp. differs from A. carbonarius Binney, 1852, in the absence of successive subdivisions in one of the arms, from A. stather Bather, 1925, in being very thinly lined and having curved arms, and from A. variabilis Fürsich, 1974b, in having curved arms. In the absence of any consistent ichnotaxobases for Arenicolites, and because no bedding-surface expression of the U-burrow is available, we prefer to leave the designation of this form at the ichnogeneric level.

Our specimens are similar to A. curvatus figured by Goldring (1962, fig. 11). However, this ichnospecies has been included in Solemyatuba by Seilacher (1990a), who considered A. curvatus a junior synonym of Solemyatuba (Arenicolites) subcompressa. Solemyatuba includes two ichnospecies, S. subcompressa (Eichwald, 1860) and S. ypsilon Seilacher, 1990a. Solemyatuba subcompressa lacks a lower extension tube and has been recorded only from the Paleozoic, while S. ypsilon has a lower extension tube and is typically found in Mesozoic strata (Seilacher, 1990a). An ichnotaxon similar to S. subcompressa is the smaller Arenicolites graptolithoformis Hundt, 1831, from the Silurian of Germany. Nonetheless, re-examination of the type specimens is necessary to establish its affinity with Solemyatuba. Specimens from Waverly have smooth walls and longer diameters in the plane of the U, which are features present in Solemyatuba. However, we prefer to include our specimens in Arenicolites, because elliptical cross sections cannot be demonstrated because of partial preservation.

Ichnogenus Asteriacites von Schlotheim, 1820

Discussion--The first comprehensive study of stellate trace fossils was accomplished by Seilacher (1953). His study dealt with both experimental neoichnology and Mesozoic specimens, and it provided a detailed account of ophiuroid and asteroid burrowing techniques. Asteriacites differs from the asterozoan burrow Pentichnus Maerz, Kaesler, and Hakes, 1976, in the subcylindrical to subconical morphology of the latter, which represents permanent to semi-permanent domiciles rather than resting traces (Mángano et al., 1999).

Asteriacites commonly is interpreted as resting traces (cubichnia) of asterozoans, including ophiuroid and asteroid traces (Hantzschel, 1975). Aquarium experiments with Ophiura texturata and Astropecten aurantiacus led Seilacher (1953) to interpret A. lumbricalis as an ophiuroid resting trace and A. quinquefolis as an asteroid resting structure. In addition, Seilacher (1953, fig. 3) analyzed an occurrence of A. lumbricalis that ethologically corresponds to fugichnia (cf. Seilacher, 1953, figs. 3a and b). Ophiuroids are known to escape successfully from storm sedimentation events (Schäfer, 1972). Mángano et al. (1999) recently analyzed several occurrences of Asteriacites from Pennsylvanian units in Kansas, and they interpreted them either as escape (vertical repetition) or as hunting structures (horizontal repetition) of omnivorous epifaunal ophiuroids.

Occurrences of Asteriacites traditionally have been attributed to asterozoans without further distinction (e.g., Santos and Campanha, 1970; Brito, 1977; Muniz, 1979). More recently, some authors have related explicitly Asteriacites to ophiuroid tracemakers (e.g., Hakes, 1976; Mikuláš, 1990; West and Ward, 1990; Twitchett and Wignall, 1996; Wilson and Rigby, 2000). In two of these cases, Asteriacites is associated directly with ophiuroid body fossils (Mikuláš, 1990; West and Ward, 1990). Asteriacites commonly is produced by deposit feeding or omnivorous ophiuroids (Mángano et al., 1999).

Asteriacites has been reported from shallow-marine (e.g., Dam, 1990a) to deep-marine facies (e.g., Crimes and Crossley, 1991). Although some authors (e.g., Seilacher, 1983) have considered this ichnogenus as an indicator of normal-salinity conditions, West and Ward (1990) and Mángano et al. (1999) have shown that Asteriacites may be present in brackish-water, marginal-marine environments. Asteriacites is a common component of tidal-flat ichnofaunas (e.g., Hakes, 1976; Howard and Singh, 1985; Miller and Knox, 1985; West and Ward, 1990; Mángano et al., 1999), and ranges in age from Cambrian to Holocene (Mikuláš, 1992).

Asteriacites lumbricalis von Schlotheim, 1820

Figs. 25A-F, 26A-C

Specimens--Eighty-seven specimens on 23 slabs (KUMIP 288500, KUMIP 288503, KUMIP 288510, KUMIP 288511, KUMIP 288517, KUMIP 288519, KUMIP 288520, KUMIP 288521, KUMIP 288522, KUMIP 288523, KUMIP 288527, KUMIP 288528, KUMIP 288530, KUMIP 288535, KUMIP 288538, KUMIP 288542, KUMIP 288544, KUMIP 288546, KUMIP 288553, KUMIP 288556, KUMIP 288568, KUMIP 288570, KUMIP 288571), many incomplete specimens (i.e., partial disc impressions with less than five arms), and several other specimens examined in the field.

Description--Star-like convex hyporeliefs (fig. 25AF) or, less commonly, concave epireliefs (fig. 26A-C) of small to moderate size. The central disc commonly is poorly defined. Disc diameter is 5.3-8.5 mm, but typically 5.3-6.0 mm. A central depression is observed in a few specimens. Arms are moderately long (6.7-16.8 mm). Arm width (1.0-5.3 mm.) depends on arm shape. Arms display a wide variety of morphologies, and an individual specimen can show significant differences between arms. Arms may expand proximally and taper toward the tip, resulting in a lanceolate shape (cf. Seilacher, 1953, fig. 2a). In such cases, delicate longitudinal striations may cover the arms; transverse striations also are present. Other specimens show arms divided into two subparallel ridges covered by a regular transverse sculpture. In general, the distal part of the arm may bifurcate into twos or, more rarely, threes; may widen ending in a funnel-like shape; or may end in a single tip of a progressively tapering arm. Free arm length is typically two to three times the disc diameter, irrespective of size and arm morphology. Vertical repetition (sensu Seilacher, 1953) is recorded by successive impressions at different depths on the soles of some current-rippled sandstones. In most cases, repetition is partial, involving part of the central disc and arms. No clear "horizontal repetition" (sensu Seilacher, 1953) has been detected.

Figure 25--Asteriacites lumbricalis. Hypichnial preservation. A. Specimen with deep central disc and the arms extending upward. KUMIP 288519. x 1.4. B. Cluster of several specimens associated with Cruziana problematica. Note transverse striae in some of the specimens and branched arms. KUMIP 288519. x 1.3. C. Specimen with central depression and branched arms. KUMIP 288522. x 1.9. D. Note lateral repetitions and associated Cruziana problematica. KUMIP 28850. x 1.5. E. Specimen with central depression associated with Cruziana problematica. KUMIP 288500. x 1.2. F. Cluster of several specimens showing lateral repetition. KUMIP 288520. x 0.9.

Figure 26--Asteriacites lumbricalis. Epichnial preservation. A. Very shallow impression with straight arms and poorly preserved morphological details. KUMIP 288528. x 2.3. B. Note lateral repetition. KUMIP 288528. x 3.14. C. Specimen of A. lumbricalis crosscut by Curvolithus simplex (arrow). KUMIP 288527. x 1.2.

Associated Ichnofauna--Asteriacites lumbricalis commonly forms a dense shallow-tiered assemblage with Cruziana problematica. Other traces commonly associated are Lockeia siliquaria, L. ornata, Curvolithus simplex, Protovirgularia rugosa, P. bidirectionalis, small cylindrical burrows, and small vertical burrows.

Remarks--Analysis of mainly Mesozoic specimens of Asteriacites led Seilacher (1953) to recognize two ichnospecies, A. lumbricalis von Schlotheim, 1820, and A. quinquefolis Quenstedt, 1876, both being distinct in convex hyporelief. However, almost all re-examined specimens (Seilacher, 1953, Plate 10, Figs. 1 and 2 excepted) were placed in A. lumbricalis. Asteriacites lumbricalis includes relatively small star-like traces with a distinctive or poorly defined central disc, relatively narrow arms that may display proximal expansions, or bifurcated rays. Seilacher (1953) made an innovative characterization of the different preservational variations of A. lumbricalis, relating the proximal expansion and the regular sculpture of longitudinal and transversal striations to the circular movements of the disc and the digging activity of the tube feet, respectively (see Seilacher, 1953, Fig. 2, for further explanation). On the other hand, A. quinquefolis is less well defined in terms of its distinctive features. It is somewhat larger, with a very regular stellate outline, typically with a more inflated shape and shorter arms than A. lumbricalis. In convex hyporelief, A. quinquefolis typically shows an irregular raggy aspect that contrasts with the regularity of A. lumbricalis (cf. Seilacher, 1953, Table 10, Fig. 1a).

Subsequent work on new collections led to the recognition of three additional ichnospecies, A. stelliformis Osgood, 1970, A. gugelhupf Seilacher, 1983, and A. aberensis Crimes and Crossley, 1991. Asteriacites stelliformis is characterized by arms showing a distinctive chevron-like sculpturing (Osgood, 1970). Asteriacites gugelhupf is characterized by its conical shape and represents deep domiciles (Seilacher, 1983). As noted by Mikuláš (1990), however, permanent or semi-permanent deep burrows representing dwelling traces should not be included within Asteriacites. Accordingly, A. gugelhupf is regarded more appropriately as Pentichnus gugelhupf (Mángano et al., 1999). Asteriacites aberensis is represented by small five-rayed impressions with the diameter of the central area large compared to the length of the rays (Crimes and Crossley, 1991).

Taking into account Seilacher's (1953) characterization, the Waverly specimens must be included in A. lumbricalis, which display multiple preservational variations related to the digging technique. Specimens of Asteriacites at Waverly were probably produced by an ophiuroid. Depth of excavation, digging technique, and typical behavior of an ophiuroid tracemaker can explain morphologic differences between and within specimens. When digging into the sediment, the central disc and proximal arms twist back and forth, resulting in the disappearance of the disc contour and enlargement of the proximal arms that misleadingly looks asteroid-like in outline (Seilacher, 1953, Fig. 2.2). Very fine longitudinal striations are evidence of sideways arm movements, whereas transverse striations resulted from the digging activity of tube feet. Arms with funnel-like open tips indicate that, although the disc and proximal arms were completely hidden in the sand, the arm tips extended above the substrate, sweeping sediment sideways. This behavior has been recorded in recent ophiuroids inhabiting shallow waters (Thorson, 1957). General configuration and depth of the structure suggest that epifaunal ophiuroids, rather than infaunal burrowers, were responsible for Asteriacites lumbricalis.

Ichnogenus Chondrites von Sternberg, 1833

Discussion--The taxonomy of Chondrites has been reviewed by Fu (1991). A somewhat similar form is Phymatoderma Brongniart, 1849. However, Phymatoderma is distinguished from Chondrites by a more complex branching pattern that includes secondary tunnels (Miller, 1998).

Historically considered a feeding trace (fodinichnion), recent work suggests that Chondrites may represent specialized feeding behavior that involves chemosymbiosis, being interpreted as a sulfide pump (Fu, 1990; Seilacher, 1990; Bromley, 1996). Seilacher (1990a) suggested lucinoid bivalves, such as Thyasira, as modern analogues of the Chondrites trace maker.

Chondrites is a facies-crossing form, recorded in marginal-marine (e.g., Archer and Maples, 1984), shallow-marine (e.g., Frey, 1990), and deep-marine facies (e.g., Buatois and Mángano, 1992; Orr, 1995). It has been regarded that the Chondrites animal developed adaptations to cope with oxygen-depleted conditions (Bromley and Ekdale, 1984; Savrda, 1992). Chondrites has also been reported in tidal-flat facies by Chamberlain (1980). Chondrites ranges in age from Cambrian to Holocene (Crimes, 1987; Ekdale, 1977).

Chondrites? isp.

Fig. 27

Figure 27--Chondrites? isp. preserved at the top of a sandstone bed with flat-topped ripples. KUMIP 288563. x 1.3.

Specimens--Five slabs (KUMIP 288500, KUMIP 288527, KUMIP 288537, KUMIP 288542, KUMIP 288563) containing numerous specimens, the actual number of which is not possible to assess.

Description--Regularly branching system forming a dendritic network. Tunnel fill different from host rock. Tunnel width is 0.6-1.4 mm. Preserved as positive epireliefs or, more rarely, negative epireliefs in ripple troughs.

Associated Ichnofauna--Chondrites? isp. commonly is associated with Curvolithus simplex and Protovirgularia bidirectionalis, but other ichnotaxa may also be present (e.g., Asteriacites lumbricalis, Nereites missouriensis).

Remarks--The Waverly specimens are only tentatively assigned to Chondrites because the typical branching pattern of this ichnotaxon is not completely evident. The specimens analyzed are always concentrated in ripple troughs. An identical preferential distribution was observed in Carboniferous tidal-flat deposits in roadcuts exposures along Kansas Highway 166 in Chautauqua County, southeastern Kansas. Differentiation of Chondrites? isp. from the inorganic branched and reticular structures described previously is extremely difficult in some cases. It also is possible that some of the inorganic structures were initiated by cracking along the traces themselves, a situation commonly observed in modern tidal flats (e.g., Bajard, 1966; Baldwin, 1974).

Ichnogenus Conichnus Männil, 1966

Discussion--The taxonomy of Conichnus and related plug-shaped burrows has been discussed by Pemberton et al. (1988). Conichnus has been considered a senior synonym of Amphorichnus Männil, 1966, by Frey and Howard (1981) and Pemberton et al. (1988). However, subsequent re-examination of the type specimen of Amphorichnus suggests that this is a valid ichnogenus (A. Ekdale, written communication, 2000). Conichnus is characterized by its conical to subcylindrical geometry, rounded base, and lack of ornamentation.

Conichnus commonly is interpreted as a dwelling structure (domichnion) or a resting trace (cubichnion) of anemones or anemone-like organisms (Pemberton et al., 1988). Presence of a thin lining is suggestive of a more or less permanent domicile (domichnia).

Conichnus typically has been reported in shallowmarine environments (e.g., Frey and Howard, 1981; Vossler and Pemberton, 1988; Nielsen et al., 1996; Curran and White, 1997), although examples have been recorded in intertidal facies (Hiscott et al., 1984; Weissbrod and Barthel, 1998). Conichnus ranges in age from Cambrian to Holocene (Hiscott et al., 1984; Curran and Frey, 1977).

Conichnus conicus Männil, 1966

Fig. 28A-B

Figure 28-Conichnus conicus. Both are basal bedding-plane views. A. Basal view of a specimen showing unornamented apical disc. KUMIP 288509. x 1.4. B. Oblique view of a specimen with lateral corrugations. KUMIP 288568. x 1.26.

Specimens--Two slabs (KUMIP 288509, KUMIP 288568) containing two specimens and one slab (KUMIP 288552) having two possible other specimens.

Description--Vertical, conical, very thinly lined burrows circular in cross section. Burrow-fill apparently is identical to host rock, although more resistant to weathering. Burrow wall displays corrugations that may suggest a crude funnel-like layering. Where the base of the burrow is visible, a small, planar, apical disc without ornamentation is observed. Height is up to 45.5 mm. Burrow diameter is 25.0-32.2 mm. Apical disc diameter is 10.2-16.6 mm. Preserved as full relief protruding from base and top of sandstone beds.

Associated Ichnofauna--Lockeia siliquaria, L. ornata, Cruziana problematica, Asteriacites lumbricalis, and Protovirgularia rugosa.

Remarks--Conichnus conicus is distinguished from C. papillatus (Männil, 1966) by the lack of an apical protuberance, and from C. conosinus Nielsen et al., 1996, by the absence of an upper dish-shaped depression.

Ichnogenus Cruziana d'Orbigny, 1842

Discussion--Cruziana is distinguished from Didymaulichnus Young, 1972, by the presence of transverse striations representing scratch marks (Aceñolaza and Buatois, 1993; Keighley and Pickerill, 1996). Poor preservation and the generally smooth lobes of the Waverly specimens cloud their distinction from Didymaulichnus, particularly D. lyelli. However, careful examination of the specimens shows the presence of transverse striations locally, placing these traces in Cruziana.

In marine deposits, Cruziana is interpreted as produced by trilobites (e.g., Seilacher, 1970). Long ploughs of Cruziana are related either with locomotion (repichnia) or grazing (pascichnia) activities within the sediments (Seilacher, 1970). In continental environments other arthropods, such as notostracan crustaceans, also produce Cruziana (Bromley and Asgaard, 1972, 1979; Pollard, 1985). Pollard (1985) noted that Cruziana (and its cubichnion companion Rusophycus) also occurs in Devonian and Carboniferous continental strata that predate the first occurrence of notostracan body fossils, and therefore their biologic affinities are unknown. In recent tidal flats, some detritus-feeding amphipods are able to produce bilobated structure comparable to Cruziana.

Although more typical of shallow-marine facies (e.g., Crimes et al., 1977; Fillion and Pickerill, 1990; Mángano et al., 1996), Cruziana also has been recorded from deep-marine (e.g., Pickerill, 1995), marginal-marine (e.g., Buatois and Mángano, 1997), and continental deposits (e.g., Bromley and Asgaard, 1972, 1979). Cruziana is a common element in Paleozoic tidal-flat environments (e.g., Baldwin, 1977; Narbonne, 1984; Durand, 1985; Legg, 1985; Mángano et al., 1996; Mángano et al., 2001; Stanley and Feldmann, 1998; Mángano and Buatois, 2000; Mángano and Astini, 2000; Astini et al., 2000) and ranges in age from Cambrian to Cretaceous (e.g., Crimes et al., 1977; Fregenal-Martinez et al., 1995; Buatois et al., 2000).

Cruziana problematica (Schindewolf, 1921)

Fig. 29A-F

Figure 29--Cruziana problematica and Cruziana isp. All are basal bedding-plane views. A. General view of a monospecific assemblage of Cruziana problematica. Note specimens oriented parallel to ripple crests and troughs, whose morphology is casted from the underlying bed. KUMIP 288508. x 0.26. B. Close-up view. Note overcrossing among different specimens of Cruziana problematica. KUMIP 288510. x 0.38. C. Detailed view of some specimens of Cruziana problematica showing poorly developed scratch marks. KUMIP 288511. x 0.79. D. Specimens of Cruziana problematica preserved as strings showing only locally the bilobate structure. KUMIP 288513. x 0.52. E. Specimen of Cruziana problematica displaying self-overcrossing in a Gordia-like fashion. KUMIP 288540. x 0.46. F. Multiple overcrossing among different specimens of Cruziana problematica at different depths. KUMIP 288515. x 0.4. G. Larger specimen of Cruziana isp. associated with several poorly preserved individuals of Cruziana problematica. KUMIP 288510. x 0.86.

Specimens--Forty-seven slabs (KUMIP 288500, KUMIP 288501, KUMIP 288502, KUMIP 288503, KUMIP 288505, KUMIP 288506, KUMIP 288507, KUMIP 288505, KUMIP 288509, KUMIP 288510, KUMIP 288511, KUMIP 288512, KUMIP 288513, KUMIP 288514, KUMIP 288515, KUMIP 288516, KUMIP 288517, KUMIP 288518, KUMIP 288519, KUMIP 288521, KUMIP 288522, KUMIP 288523, KUMIP 288527, KUMIP 288531, KUMIP 288533, KUMIP 288534, KUMIP 288535, KUMIP 288536, KUMIP 288538, KUMIP 288540, KUMIP 288541, KUMIP 288542, KUMIP 288543, KUMIP 288545, KUMIP 288546, KUMIP 288552, KUMIP 288554, KUMIP 288556, KUMIP 288559, KUMIP 288563, KUMIP 288565, KUMIP 288567, KUMIP 288568, KUMIP 288569, KUMIP 288570, KUMIP 288571, KUMIP 288572) containing approximately 727 specimens and several others examined in the field.

Description--Straight to gently sinuous bilobate traces. Lobes are mostly smooth and symmetrical, with lateral margins slightly curved. Median longitudinal furrow is narrow and shallow. Poorly preserved transverse striations are visible in some specimens. Trace width is 1.4-6.6 mm, but commonly 2.5-5.8 mm. Length is 27.0-230.0 mm, but typically 80.0-150.0 mm. Axial terminations commonly are lacking. A few specimens display partial preservation as negative hyporeliefs, revealing the unilobate upper surface of the trace. Depth is variable, but most specimens are very shallow structures. Preserved mostly as positive hyporeliefs.

Some bed soles exhibit almost a monospecific assemblage of C. problematica in multiple preservational variants. Some specimens are partially or completely represented by an irregular unilobated string of rock (fig. 29D). In many cases, these irregular strings become more regular unilobated traces or poorly bilobated structures. In these crowded surfaces, specimens commonly crosscut each other. Locally, they may display different patterns of distribution, such as several subparallel specimens following ripple topography (fig. 29A).

Cruziana problematica commonly is not connected to well-developed resting structures. Some specimens, however, show incipient resting features, commonly associated with a shift in the vertical direction of movement. These structures are sLightly wider than the connected locomotion trace and are consistently 4.6-8.6 mm long. They are comparable with Rusophycus carbonarius, but they do not display the typical coffee-bean shape of that ichnospecies.

Associated Ichnofauna--Cruziana problematica either forms monospecific assemblages or is associated with Asteriacites lumbricalis in dense assemblages, and commonly is crosscut by Lockeia siliquaria, Nereites imbricata, Protovirgularia bidirectionalis, P. rugosa, Curvolithus simplex, Conichnus conicus, and Psammichnites grumula, among many other ichnotaxa.

Remarks--The studied specimens are similar to C. problematica described by Fillion and Pickerill (1990) and to the type specimens described by Schindewolf (1921) as Ichnium problematicum. Isopodichnus osbornei recorded by Glaessner (1957) displays a wider median furrow and is commonly preserved as shallow furrows in epirelief, resembling Diplopodichnus biformis (Buatois et al., 1998c). Jensen (1997) analyzed the type specimens of Fraena tenella Linnarsson, 1871, and concluded that this ichnospecies is identical to Cruziana problematica. He therefore regarded Cruziana problematica as a junior synonym of Cruziana tenella. However, because Cruziana tenella is a poorly known ichnospecies, we prefer to retain the widely used Cruziana problematica to promote nomenclatorial stability.

Some specimens of C. problematica, such as those at the base of gutter casts, most likely represent simple locomotion structures. However, crowded occurrences of C. problematica may well record grazing within the sediment (pascichnia).

Cruziana isp.


Specimens--One slab (KUMIP 288510) containing a single specimen.

Description--Straight horizontal trace consisting of two lobes separated by a relatively shallow, but well-defined median furrow. Lobes are relatively flat. Faint, thin, discontinuous transverse scratch marks cover the lobes. External ridges are absent. Length is 29.7 mm; width is 12.4-14.4 mm. Width changes slightly along the specimen. Preserved in positive hyporelief.

Associated Ichnofauna--Cruziana problematica, Asteriacites lumbricalis, and Lockeia siliquaria.

Remarks--This single specimen of Cruziana isp. clearly differs in morphology and size range from C. problematica. Although size commonly is not a good ichnotaxobase, C. problematica has a consistently smaller size range and a distinctive mode of occurrence (i.e., long, straight to sinuous, commonly overlapping bilobated ridges). Discontinuous transverse endopodal scratches are not distinctive enough to allow a detailed ichnotaxonomic evaluation.

Ichnogenus Curvolithus Fritsch, 1908

Discussion--Curvolithus is distinguished from similar forms (e.g., Gyrochorte, Psammichnites) by its trilobate upper surface. However, traces with variable morphology usually have been included in Curvolithus, resulting in a rather complex ichnotaxonomic situation (cf. Fillion and Pickerill, 1990). Although Curvolithus has a trilobate upper surface, its lower surface has been regarded either as quadralobate (e.g., Maples and Suttner, 1990), trilobate (e.g., Webby, 1970; Hakes, 1976, 1977), bilobate (e.g., Heinberg, 1970; Fürsich and Heinberg, 1983; Heinberg and Birkelund, 1984; Lockley et al., 1987), and even unilobate (e.g., Chamberlain, 1971; Badve and Ghare, 1978). Confusion resulted in part because, other than Mikuláš (1992) and Rindsberg (1994), little attention has been paid to the original specimens described by Fritsch (1908). In comparing his Carboniferous specimens with the types, Rindsberg (1994) noted some differences between the type specimens of Curvolithus and traces subsequently assigned to this ichnogenus by later authors. In an attempt to resolve these problems, Buatois, Mángano, Mikuláš, et al. (1998) redescribed the type specimen of Curvolithus and reviewed the taxonomy of this ichnogenus. These authors mentioned six ichnospecies recognized in the stratigraphic record: C. multiplex Fritsch 1908, C. gregarius Fritsch 1908, C. davidis Webby 1970, C. annulatus Badve and Ghare 1978, C. aequus Walter et al. 1989, and C. manitouensis Maples and Suttner 1990. Additionally, they defined another ichnospecies, Curvolithus simplex. Buatois, Mángano, Mikuláš, et al. (1998) retained C. multiplex for specimens with a smooth trilobate upper surface and a quadralobate lower surface and removed C. gregarius from Curvolithus. Buatois, Mángano, Mikuláš, et al. (1998) also regarded C. davidis and C. annulatus as nomina dubia, and they considered C. manitouensis as a junior synonym of C. multiplex. Curvolithus aequus has a bilobate lower surface and was interpreted as washed-out specimens of Didymaulichnus.

Curvolithus is regarded as a locomotion trace (repichnion) of carnivores, most likely gastropods, flatworms, or nemerteans (Lockley et al., 1987; Buatois, Mángano, Mikuláš, et al., 1998). The internal structure and production of this ichnogenus was analyzed by Heinberg (1973), who demonstrated that sediment excavated by the Curvolithus-animal was transported along its sides and packed in pads behind it.

Curvolithus commonly is associated with shallow-marine facies, both normal salinity and brackish, and it typically occurs in the Cruziana ichnofacies (Buatois, Mángano, Mikuláš, et al., 1998). Lockley et al. (1987) defined the Curvolithus ichnofacies as a subset of the Cruziana ichnofacies (actually Curvolithus association; see Bromley, 1990, 1996) that indicated delta-influenced nearshore environments. Occurrences of Curvolithus in tidal-flat deposits were reported by Hakes (1976, 1977, 1985) and Martino (1989, 1996). We are unaware of deep-marine occurrences of Curvolithus. Badve and Ghare (1978) noted that C. annulatus from the Jurassic Gajansar Beds of India occurred in the Zoophycos and Nereites ichnofacies. However, a critical analysis of the ichnotaxa present in the association (e.g., Arenicolites, Monocraterion, Scolicia, Planolites, Nereites, Thalassinoides) and the overall aspect of the assemblage suggests the Cruziana ichnofacies. Chamberlain (1971) and Hantzschel (1975) also regarded grooved tubes described by Keij (1965) from Miocene brackish-water deposits of Borneo as Curvolithus. Specimens from the Cambrian of Poland were questionably included in Curvolithus by Fedonkin (1977); they are bilobate and probably belong to another ichnogenus. Curvolithus ranges in age from Precambrian to Miocene (Webby, 1970; Keij, 1965).

Curvolithus multiplex Fritsch, 1908

Fig. 30

Figure 30--Curvolithus multiplex. Basal bedding-plane view. KUMIP 288500. x 1.6.

Specimens--A single specimen on slab KUMIP 288500.

Description--Horizontal, straight to curved trace. Lower surface consists of four flat smooth lobes. Lobes are 1.9-2.6 mm wide. Total trace width is 7.0-7.5 mm. Maximum observed length of the trace is 56.3 mm. Lobes are separated by three narrow angular furrows. Laterally, inner lobes gradually merge to form a single central lobe. Trace-fill is identical to the host rock. Upper surface cannot be observed. Preserved as positive hyporelief.

Associated Ichnofauna--Asteriacites lumbricalis, Diplocraterion isp. A., Protovirgularia bidirectionalis, Curvolithus simplex, and Cruziana problematica.

Remarks--Curvolithus multiplex is distinguished from the other Curvolithus ichnospecies by having a smooth trilobate upper surface and a quadralobate lower surface (Buatois, Mángano, Mikuláš, et al., 1998).

Curvolithus simplex Buatois, Mángano, Mikuláš and Maples, 1998

Fig. 31A-F

Figure 31--Curvolithus simplex. A. Several specimens preserved on the top of rippled sandstone. Specimens in the upper right with outer lobes tapering toward the center and enveloping the central lobe. KUMIP 288550. x 0.3. B. Close-up of top of rippled sandstone showing specimens preserved on both crests and troughs. KUMIP 288549. x 0.5. C. Large specimen of C. simplex preserved on the base of sandstone bed. KUMIP 288531. x 0.37. D. Large specimen of C. simplex overprinting a background association of Cruziana problematica and crosscut by a deeper structure, possibly Protovirgularia bidirectionalis (arrow). Basal bedding-plane view. KUMIP 288500. x 0.62. E. Specimen of C. simplex, preserved on the base of sandstone bed, crosscut by a bilobate structure (probably Protovirgularia rugosa). KUMIP 288500. x 1.22. F. Top of bed view showing specimens with the outer lobes enveloping the inner lobe. KUMIP 288549. x 0.62.

Specimens--Twenty-six slabs (KUMIP 288500, KUMIP 288514, KUMIP 288516, KUMIP 288519, KUMIP 288522, KUMIP 288527, KUMIP 288528, KUMIP 288531, KUMIP 288533, KUMIP 288534, KUMIP 288541, KUMIP 288542, KUMIP 288543, KUMIP 288544, KUMIP 288548, KUMIP 288549, KUMIP 288550, KUMIP 288551, KUMIP 288552, KUMIP 288554, KUMIP 288555, KUMIP 288558, KUMIP 288559, KUMIP 288561, KUMIP 288569, KUMIP 288571) containing approximately 202 specimens and several others recorded in the field.

Description--Horizontal to oblique or, more rarely subvertical, straight to curved to sinuous, endostratal trace consisting of three smooth lobes on lower and upper surface. The central lobe ranges from 2.0 to 7.9 mm in width. Outer lobes are narrower and flatter than the central one, ranging from 1.4 to 4.0 mm in width. Total trace width is 2.7-14.5 mm. Maximum observed length of the trace is 77.6 mm. Each outer lobe is separated from the central lobe by a narrow angular furrow. In certain specimens, outer lobes gradually taper toward the center, enveloping the central lobe and giving the appearance of a narrower bilobate structure. Trace-fill is identical to the host rock. Segments preserved on ripple tops are usually very short, while those preserved on sandstone soles are long. Preserved as full relief, as well as positive hyporeliefs and epireliefs.

Associated Ichnofauna--Asteriacites lumbricalis, Diplocraterion isp. A, Protovirgularia bidirectionalis, Curvolithus multiplex, and Cruziana problematica, as well as other ichnotaxa.

Remarks--Buatois, Mángano, Mikuláš, et al. (1998) noted that there was no available ichnospecific name to cover the most common specimens of Curvolithus (i.e. with a trilobate smooth upper surface and a trilobate to unilobate smooth lower surface), which either were assigned erroneously to C. multiplex or classified as Curvolithus isp. Therefore, they proposed the ichnospecies C. simplex for such traces. Curvolithus simplex includes specimens with both concave and convex lower surfaces.

Ichnogenus Diplichnites Dawson, 1873

Discussion--Considerable confusion persists regarding the use of the ichnogenus Diplichnites. It was erected by Dawson (1873) to name trackways reported from deltaic Carboniferous deposits of Nova Scotia, which were believed to be produced by crustaceans, annelids, or myriapods. Subsequently, Seilacher (1955) applied this name to trilobite trackways from the Cambrian of Pakistan. However, Briggs et al. (1979, 1984) suggested restricting Diplichnites to nontrilobite trackways. The name Diplichnites is applied herein regardless of the tracemaker identity and based strictly on trackway morphology. However, it should be stated that the ichnotaxonomy of arthropod trackways, and of the ichnogenus Diplichnites in particular, is in need of revision.

Diplichnites cuithensis Briggs, Rolfe, and Brannan, 1979

Fig. 32A-C

Figure 32--Diplichnites cuithensis. A. Specimen preserved as positive hyporelief. KUMIP 288578. x 0.27. B. Superposition of imprints due to coalescing of adjacent footfalls in specimen preserved as positive hyporelief. KUMIP 288576. x 0.36. C. Specimen preserved as negative epirelief. KUMIP 288577. x 0.36.

Specimens--Three specimens on three slabs (KUMIP 288576, KUMIP 288577, KUMIP 288578) and three additional specimens studied in the field.

Description--Straight trackways consisting of two parallel rows of similar tracks preserved as positive hyporelief or negative epirelief. Individual trackways traced up to 440 mm long. Width of trackway is 233.0-302.4 mm. Space between rows is 101.6-177.7 mm. Imprints represented by elongated and sigmoidal ridges oriented normal to the axis of the trackway. In most cases, details of individual imprints not preserved due to superposition of imprints, soft-sediment deformation, or both. In well-preserved forms, each imprint tends to be sharply defined, shallow, and tapers toward axis. Imprints 49.1-96.7 mm long; width 6.5-22.4 mm. Imprints closely spaced between 9.5-37.1 mm. Superposition due to coalescing of adjacent footfalls is common. In one specimen, individual imprints cannot be identified because they coalesce into a single ridge that forms the row (fig. 32B).

Associated Ichnofauna--No other traces are associated with D. cuithensis.

Remarks--Diplichnites cuithensis is interpreted as a locomotion trace (repichnion) produced by the giant myriapod Arthropleura (Briggs et al., 1979). A detailed analysis of the morphology of this ichnospecies can be found in Briggs et al. (1979). These authors discussed several potential arthropod tracemakers, including myriapods, eurypterids, and scorpions, concluding that D. cuithensis was produced by the former. Assuming the estimation of Ryan (1986) that the body length of Arthropleura is 3.75 times the width, the Waverly arthropleurids would be at least 1.13 m long. This estimation falls within the Arthropleura range suggested by other authors.

Diplichnites cuithensis has been recorded from Namurian deltaic channel-fill deposits of Arran, Scotland (Briggs et al., 1979); Westphalian alluvial deposits of New Brunswick (Briggs et al., 1984); and Westphalian to Early Permian channel-bar facies of Nova Scotia (Ryan, 1986). The depositional environment of Diplichnites cuithensis is typically subaerial, commonly exposed fluvial bars, silted channels, and desiccated sheet-flood deposits. Diplichnites cuithensis ranges in age from Namurian to Early Permian (Briggs et al., 1979; Ryan, 1986).

Ichnogenus Diplocraterion Torell, 1870

Discussion--The presence of spreiten connecting the arms distinguishes Diplocraterion from the related U-burrow Arenicolites (Fürsich, 1974a). Rhizocorallium, another U-shaped trace, also has spreiten, but it differs from Diplocraterion in its horizontal to subhorizontal orientation. Corophioides Smith, 1893 and Polyupsilon Howell, 1957a are considered junior synonyms of Diplocraterion (Goldring, 1962; Frey and Chowns, 1972; Fürsich, 1974a; Fillion and Pickerill, 1990).

Functional analysis of the spreiten provides a key to the ethologic significance of Diplocraterion. Fürsich (1974a) concluded that the spreiten may result either from growth of the inhabitant or vertical movement of the structure by the inhabitant to maintain an optimum distance from the sediment-water interface. Diplocraterion may be regarded as a dwelling structure or domichnion (Cornish, 1986; Ekdale and Lewis, 1991), or as an equilibrium structure or equilibrichnia (Bromley, 1996). Although Diplocraterion usually has been considered as the work of suspension feeders (Cornish, 1986; Mason and Christie, 1986; Dam, 1990b; Ekdale and Lewis, 1991; Jensen, 1997), Bromley (1996) has shown that this is not always the case and that the origin of vertical U-spreiten traces may be considerably more complex. Bromley (1996) contrasted the activities of the amphipod Corophium volutator, a detritus feeder that constructs Diplocraterion-like burrows in muddy sediments, with the suspension feeder Corophium arenarium that produces similar, but mucus-lined, structures in sand. Polychaete annelids also have been suggested as tracemakers of Diplocraterion (Arkell, 1939).

Diplocraterion is especially common in high-energy, shallow-water environments. However, it is a facies-crossing ichnotaxon that ranges from deep-marine (e.g., Crimes, 1977) to shallow-marine (e.g., Bromley and Hanken, 1991; Chaplin, 1996; Paczesna, 1996; Orlowski and Zylinska, 1986) and marginal-marine environments (e.g., Chaplin, 1985; Mángano and Buatois, 1997). Although most occurrences are restricted to marine settings, Diplocraterion recently has been recorded in continental deposits (Kim and Paik, 1997; Zhang et al., 1998). Diplocraterion is a common component of tidal-flat environments, being particularly abundant in high-energy, lower-intertidal sand flats (e.g., Ireland et al., 1978; Chamberlain, 1980; Pollard, 1981; Narbonne, 1984; Cornish, 1986; Mason and Christie, 1986; Weissbrod and Barthel, 1998). Diplocraterion ranges in age from Cambrian to Holocene (Jensen, 1997; Bromley, 1996).

Diplocraterion isp. A

Fig. 33A

Specimen--One specimen on a single slab (KUMIP 288500).

Description--U-shaped burrow observed as dumb-bell semirelief. Spreite is protrusive. Arms are very thinly lined and are filled with the same lithology as the host rock. Burrow surface displays corrugations. Arm terminations are separated by a zone of reworking representing bedding-plane expression of spreiten. Width is 19.8 mm. Arm thickness is 6.6-8.5 mm. Preserved as positive hyporelief.

Associated Ichnofauna--Cruziana problematica, Protovirgularia bidirectionalis, Curvolithus simplex, C. multiplex, and Asteriacites lumbricalis.

Remarks--Terminology follows that proposed by Fürsich (1974a). The Waverly specimen compares favorably with other occurrences of dumb-bell semireliefs (e.g., Fillion and Pickerill, 1990, fig. 7.5; Zhang et al., 1998, fig. 11B). Preservation is restricted to the bedding plane, which precludes ichnospecific assessment. Hypichnial preservation and larger size distinguishes this ichnospecies from Diplocraterion isp. B.

Diplocraterion isp. B

Fig. 33B-D

Figure 33--Ichnospecies of Diplocraterion. A. Diplocraterion isp. A. U-shaped burrow preserved as dumb-bell hyporelief. KUMIP 288550. x 0.99. B. Diplocraterion isp. B. Cluster of several specimens preserved at the top of a rippled sandstone. KUMIP 288529. x 0.75. C. Diplocraterion isp. B. Small specimens preserved as negative epirelief. KUMIP 288514. x 1.03. D. Diplocraterion isp. B. Poorly preserved specimens in a well-weathered rippled sandstone top. KUMIP 288554. x 0.52.

Specimens--Seven slabs (KUMIP 288514, KUMIP 288516, KUMIP 288519, KUMIP 288529, KUMIP 288539, KUMIP 288541, KUMIP 288554) with approximately 84 specimens and many others examined in the field.

Description--U-shaped burrow observed as dumbbell depressions on bedding planes. Arms are very thinly lined. Burrow surface is smooth and lacks ornamentation. Arm terminations typically are separated by a furrow reflecting reworked sediment that records the bedding-plane expression of the protrusive spreiten. In some cases, both arms appear isolated as paired small circles with no apparent spreiten. Unequal development of limbs is common. Width is 5.8-13.0 mm. Arm thickness is 1.1-3.8 mm. Preserved as negative epireliefs.

Associated Ichnofauna--Commonly associated with Curvolithus simplex and Protovirgularia bidirectionalis, and, more rarely, with Halopoa isp., and arthropod tracks.

Remarks--Specimens of Diplocraterion isp. B described here are similar to those figured by Arkell (1939, pl. VII, figs. A-B), Mason and Christie (1986, fig. 2), and Bromley and Hanken (1991, figs. 10 and 11) preserved on bedding planes. Our specimens probably represent the bases of U-shaped burrows. Preservation restricted to bedding planes precludes ichnospecific assessment. In contrast to Diplocraterion isp. A, Diplocraterion isp. B is smaller, preserved as negative epireliefs, and typically occurs in high densities.

Ichnogenus Halopoa Torell, 1870

Discussion--The ichnogenus Halopoa recently was reviewed by Uchman (1998). It includes predominantly horizontal traces covered with longitudinal ridges or wrinkles and composed of overlapping cylindrical probes. Asterosoma von Otto, 1854, also has longitudinal wrinkles, but it is characterized by a radial morphology (Seilacher, 1969; Hantzschel, 1975; Pemberton, MacEarchen, et al., 1992). The ichnogenus Asterophycus Lesquereux, 1876, also is star-shaped and has longitudinal wrinkles, and is most likely a junior synonym of Asterosoma (Chamberlain, 1971; Schlirf, 2000).

Halopoa is interpreted as a feeding structure (fodinichnion) produced by deposit-feeding crustaceans (Nathorst, 1881). Ksiazkiewicz (1977) suggested priapulid worms as tracemakers, but his proposal was rejected by Uchman (1998). Uchman (1998, 1999) noted that the origin of the longitudinal striation and wrinkles may be diverse, including microfaulting due to tension caused by the producer (cf. Osgood, 1970; Seilacher, 1990).

Halopoa is present in open marine environments, both shallow (Torell, 1870; Jensen, 1997) and deep (Ksiazkiewicz , 1977; Buatois et al., 2001) water. Halopoa ranges in age from Cambrian to Miocene (Seilacher, 1955; Crimes and McCall, 1995).

Halopoa isp.

Fig. 34A-F

Figure 34--Halopoa isp. All photos are of base of beds with the exception of C and E, which show preservation at the top of sandstone beds. A. Specimen showing two branches oriented oblique to the bedding plane. KUMIP 288554. x 0.8. B. Large trace segment with laterally persistent striations. KUMIP 288555. x 0.8. C. Trace segments at the top of sandstone bed. Note associated Nereites missouriensis (upper right). KUMIP 288531. x 0.7. D. Paired striated trace segments. KUMIP 288531. x 0.5. E. Trace segment at the top of a sandstone bed with interference ripples. KUMIP 288544. x 0.3. F. Trace segment showing concentrical fill. KUMIP 288575. x 0.5.

Specimens--Thirteen slabs (KUMIP 288519, KUMIP 288531, KUMIP 288532, KUMIP 288534, KUMIP 288535, KUMIP 288538, KUMIP 288544, KUMIP 288550, KUMIP 288551, KUMIP 288554, KUMIP 288555, KUMIP 288572, KUMIP 288575) containing twenty-five specimens.

Description--Horizontal to rarely oblique traces characterized by longitudinal wrinkles or striae. Trace segments have an inflated shape, pinching out laterally. Striae are commonly laterally continuous, but they may anastomose or merge in some specimens. Trace segments occur alone or branch to form pairs. Tunnels are thickly lined and have a concentric fill. Trace segments are 6.0-27.7 mm wide and up to 160.9 mm long. Striae are 0.3-2.6 mm wide and up to 79.9 mm long. Wall lining is up to 3.2 mm thick. Preserved in full relief on both tops and bases of sandstone beds.

Associated Ichnofauna--Asteriacites lumbricalis, Cruziana problematica, Protovirgularia bidirectionalis, Curvolithus simplex, and Trichophycus isp.

Remarks--Two ichnospecies of Halopoa were recognized by Uchman (1998): H. imbricata Torell, 1870, and H. annulata (Ksiazkiewicz, 1977). Halopoa imbricata is unbranched and has relatively long and continuous furrows and wrinkles, while H. annulata is branched and has perpendicular constrictions. The Waverly specimens are classified at the ichnogeneric level because the overall morphology of the trace cannot be detected. The specimens studied differ from single concentrically filled traces, such as Rosselia or Cylindrichnus, in their horizontal orientation. Concentric fill, inflated trace segments, and continuity of striation distinguish the Waverly specimens from Palaeophycus striatus.

Ichnogenus Lockeia James, 1879

Discussion--Although Pelecypodichnus Seilacher, 1953, still is used as an ichnogenus by some authors (e.g., Eagar and Li, 1993), it should be abandoned, because it is a junior synonym of Lockeia. Lockeia James, 1879, was once considered as a nomen oblitum rather than the senior synonym of Pelecypodichnus (Hakes, 1977; Bromley and Asgaard, 1979; Wright and Benton, 1987). The status of Lockeia, however, was revised by Maples and West (1989). Based on the Principle of Priority (ICZN, 1985), they considered Lockeia to be the senior synonym of Pelecypodichnus. Umbonichnus Karaszewski 1975 is a poorly known junior synonym of Lockeia (Rindsberg, 1994). Lockeia is distinguished from Sagittichnus Seilacher, 1953, by the arrowhead shape of the latter (Gluszek, 1995).

Several ichnospecies of Lockeia have been proposed: L. siliquaria James, 1879, L. amygdaloides (Seilacher, 1953), L. ornata (Bandel, 1967a), L. czarnockii (Karaszewski, 1975), L. elongata Yang, 1984, L. avalonensis Fillion and Pickerill, 1990, L. triangulichnus Kim, 1994, L. cordata Rindsberg, 1994, and L. hunanensis Zhang and Wang, 1996. Lockeia siliquaria, the most widespread ichnospecies, is oval to almond-shaped, typically tapering only at one end with the other end somewhat rounded. However, L. siliquaria may display very irregular outlines, corrugated sides, or a peripheral rim (Mángano et al. 1998). All these features are related to the paleobiologic affinity of the bioturbator or substrate fluidity, and do not involve modifications of behavior. Lockeia siliquaria is considered the senior synonym of L. amygdaloides (Seilacher and Seilacher, 1994). Lockeia czarnockii is indistinguishable from almond-shaped L. siliquaria. Fillion and Pickerill (1990) suggested that L. ornata was a questionable form that probably should be included in Walcottia. Our observations of Bandel's specimens, including the types, indicate that L. ornata is a distinct ichnotaxon, and its diagnostic feature is the presence of a concentrically ornamented surface. Many specimens of L. ornata are often in physical continuity with Protovirgularia rugosa, forming a compound trace fossil. Lockeia cordata probably is a preservational variant of L. siliquaria, and is not distinctive enough in terms of ethology to warrant recognition as a separate ichnospecies. Lockeia avalonensis is spheroid to sub-ovate in form with steep margins and, rarely, a shallow carinal crest (Fillion and Pickerill, 1990). Observations of the Waverly specimens suggest that distinguishing among spheroid, subovate, and almond-like Lockeia can be quite difficult. Data on L. siliquaria from Waverly clearly show that small specimens of L. siliquaria tend to be more spherical, and they resemble the "stuffed burrows" of Pollard (1981). Accordingly, more spherical specimens may record structures of juvenile forms or a different biologic species rather than real behavioral variants. The question of whether or not this subtle change in shape deserves a different ichnospecies name remains problematic.

Lockeia triangulichnus is most likely an inorganic structure. In any case, the solely subtriangular outline would not be of behavioral significance, but would be a feature more related to the paleobiology of the tracemaker (e.g., its foot morphology, shell form). The taxonomic validity of L. hunanensis is difficult to evaluate, because the quality of the illustrations of the type specimens is very poor.

Lockeia typically occurs as convex hyporeliefs on the soles of sandstones, occasionally displaying an overlying shaft (cf. Seilacher, 1953). Lockeia historically has been interpreted as a resting structure (cubichnion) (Seilacher, 1953; Osgood, 1970, Fillion and Pickerill, 1990; Rindsberg, 1994). However, some specimens of Lockeia, particularly those representing the lower end of relatively deep structures, may document semi-permanent domiciles (i.e., domichnia). Bivalves are the typical tracemakers of Lockeia (Seilacher and Seilacher, 1994). However, conchostracans also may produce similar traces in continental settings (Pollard and Hardy, 1991).

Lockeia has been reported from shallow-marine (e.g., Seilacher, 1953; Osgood, 1970; Fillion and Pickerill, 1990; Kim, 1994), marginal-marine (e.g., Bandel, 1967; Hakes, 1977; Wright and Benton, 1987; Rindsberg, 1994; Mángano and Buatois, 1997), and deep-marine facies (e.g., Crimes et al. 1981; Yang et al., 1982), in addition to continental environments (e.g., Bromley and Asgaard, 1979; Gluszek, 1995). Hakes (1976, 1977, 1985), Rindsberg (1994), and Mángano et al. (1998), among others, recognized Lockeia in tidal-flat facies. Lockeia ranges in age from Late Cambrian/Early Ordovician to Pleistocene (Fillion and Pickerill, 1990; Pemberton and Jones, 1988). Specimens reported from the Vendian (Late Precambrian) as Lockeia isp. by McMenamin (1996) do not display the characteristic morphology of this ichnogenus.

Lockeia ornata (Bandel, 1967a)

Fig. 35A-D

Figure 35--Lockeia ornata. All photos are of base of KUMIP 288552. A. Specimen of L. ornata connected with chevron locomotion traces. Chevron orientation indicates that the animal exited the resting structure. x 0.88. B. Close-up of specimen in A showing concentric ornamentation. Delicate, fine longitudinal ridges present on both sides of Lockeia siliquaria. x 1.9. C. Rosary structures resulting from the alignment of several specimens of Lockeia ornata and radial or fan-like patterns. x 0.8. D. Localized high density of L. ornata superimposed on a background assemblage of Cruziana problematica. x 0.89.

Specimens--Eight slabs (KUMIP 288521, KUMIP 288543, KUMIP 288549, KUMIP 288552, KUMIP 288557, KUMIP 288558, KUMIP 288562, KUMIP 288568) with approximately 217 specimens.

Description--Elongate, relatively small almond-shaped structures preserved as positive hyporeliefs. Delicate, sharp, concentric ridges resembling growth interruptions in a bivalve shell are diagnostic (fig. 35B). Length is 12.0-26.8 mm; width is 6.5-13.4 mm. In some specimens, a longitudinal median ridge (carina) occurs. Chevroned, smooth or roughly bilobated locomotion traces commonly are connected to L. ornata (fig. 35A). This form typically exhibits a gregarious mode of occurrence, with local patches of high density. Looping, radial, and rosary patterns formed by serial alignment of Lockeia commonly are observed (fig. 35C-D).

Associated Ichnofauna--Lockeia ornata typically is associated with Protovirgularia rugosa, P. bidirectionalis, Cruziana problematica, Asteriacites lumbricalis, and Palaeophycus tubularis.

Remarks--Bandel (1967a) found specimens of Lockeia exhibiting similar ornamentation, and he proposed the ichnospecies Pelecypodichnus ornatus (= Lockeia ornata). It can be argued, however, that specific substrate conditions are required for preservation of Lockeia ornata. Although the presence of concentric ornamentation was the diagnostic feature selected by Bandel (1967a), L. ornata exhibits a unique mode of occurrence, which suggests a pattern of behavior that differs significantly from that depicted by L. siliquaria. Connection of Lockeia ornata with spicate locomotion traces indicates a high degree of mobility along horizontal planes. Rosary structures (i.e., individual Lockeia aligned one behind the other) have been noted by several authors (e.g., Linck, 1949; Osgood, 1970; Wright and Benton, 1987; Seilacher and Seilacher, 1994). Seilacher and Seilacher (1994) proposed a new ichnospecies, L. serialis, based on the serial alignment of structures. These authors also suggested that L. serialis, first documented from the German Keuper (Triassic), has environmental significance (i.e., continental environments). However, our observations of Pennsylvanian tidal-flat facies indicate that serial alignment is quite common in brackish and normal-marine settings. Radial arrangements and looping record a patterned feeding strategy with constant repositioning in search for food. Seilacher and Seilacher (1994, pl. 1, Figs. c-e) illustrated radial structures produced by the modern bivalve Macoma.

In its movement, the L. ornata tracemaker cut the sand-mud casting interface at different angles, resulting in a highly variable range of length-to-width ratios (L/W) (fig. 36; see also Mángano et al., 1998, Fig. 11). Lockeia ornata records ventral and antero-ventral areas of the bivalve trace maker. Elongate forms represent almost horizontal orientations, whereas less elongate forms suggest inclined to subvertical orientations. Other specimens of L. ornata are connected to relatively short chevroned structures (Protovirgularia rugosa) that reveal a bifurcate foot. These structures can be interpreted as escape structures related to tidal sedimentation (see "Remarks" of P. rugosa). In the late Paleozoic, this mode of life was exploited almost exclusively by nuculoid protobranch bivalves (Stanley, 1968). Mángano et al. (1998) suggested Phestia, a Pennsylvanian nuculanid with concentric ornamentation, as the most likely tracemaker of L. ornata in the Waverly tidal flat.

Figure 36--Width/length regression curves of Lockeia ornata (triangles) and Lockeia siliquaria (circles). Note high degree of variability in witdh/length ratio for L. ornata versus L. siliquaria. The wider range of L. ornata reflects the undulating movements of its deposit-feeding producer that crossed the sand-mud interface with different orientations.

Lockeia siliquaria (James, 1879)

Fig. 37A-F

Figure 37--Lockeia siliquaria. A. Several specimens preserved as positive hyporelief on the sole of a sandstone bed. KUMIP 288553. x 0.29. B. Several specimens of Lockeia siliquaria associated with Cruziana problematica and preserved as positive hyporelief. KUMIP 288572. x 0.3. C. Specimen preserved as protruding shaft on the top of a rippled sandstone bed. Field photo. D. Superposition of three specimens at the base of a sandstone bed. KUMIP 288553. x 1.1. E. Differential preservation of Lockeia siliquaria at the top of a sandstone bed. Small protruding shafts are preserved together with large depressions. KUMIP 288553. x 0.44. F. Several specimens of Lockeia siliquaria with associated bivalve shells on the base of a sandstone bed. Field photo. Length of hammer is 33.5 cm.

Specimens--Eighteen slabs (KUMIP 288508, KUMIP 288509, KUMIP 288510, KUMIP 288511, KUMIP 288514, KUMIP 288522, KUMIP 288528, KUMIP 288531, KUMIP 288540, KUMIP 288541, KUMIP 288551, KUMIP 288552, KUMIP 288553, KUMIP 288554, KUMIP 288556, KUMIP 288569, KUMIP 288571, KUMIP 288572) containing 91 specimens and several others examined in the field.

Description--Almond-like or oval-shaped traces. Typically, these forms taper toward one end with the other end more rounded. However, some specimens exhibit irregular shapes. Length is 12.3-45.2 mm and width is 8.9-22.7 mm. Large specimens display hypichnial ridges up to 17.9 mm deep, but typically the depth is about 10 mm. The surface usually is smooth, although some specimens show corrugated lateral sides. A marginal rim is observed in a few specimens. A longitudinal ridge, or carena, is present occasionally. Some large specimens are strongly tilted to one side. Large specimens may occur singly (particularly some large specimens) or in patches, forming groups of three or more. Overlap between specimens is quite common, particularly in densely covered sandstone soles (fig. 37D). No preferred long-axis orientation has been detected. Alignments of one form behind the other, forming chainlike structures similar to those described by Seilacher (1953) from the Triassic of Germany ("pseudo-preferred orientation" of Osgood, 1970), have not been observed. Cross sectional views of some specimens show two basic patterns 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. The burrow fill may be massive, suggesting a passive filling of the structure, or the burrow fill may show a poorly defined meniscus-like structure in the lower part of the shaft.

Associated Ichnofauna--Lockeia siliquaria is present throughout the sequence, and it is associated with such other forms as Curvolithus simplex, Asteriacites lumbricalis, and Cruziana problematica. A few stratigraphic horizons, however, are characterized almost exclusively by high densities of L. siliquaria.

Remarks--Morphologic variability and mode of occurrence of these traces are consistent with L. siliquaria. Lockeia siliquaria commonly occurs in connection with inclined or vertical shafts, suggesting that the bioturbator was able to move vertically. Lockeia siliquaria records either the anterior area of the trace maker or the foot compressing the sediment. Morphologic variability and corrugated sides suggest the second possibility as more plausible. Ethologically, L. siliquaria either represents dwelling structures (domichnia) of suspension feeders or fugichnial responses to changing environmental conditions, rather than short-lived resting traces (cubichnia). Absence of intergradation with locomotion structures (Protovirgularia) suggests a bivalve tracemaker with a wedge-shaped foot rather than one with a bifurcated foot. The abundance and multiple modes of preservation of Lockeia siliquaria in time-averaged surfaces also suggest relatively deep structures that survived, at least partially, the destructive effects of coastal erosion. In the late Paleozoic very few bivalves, probably only primitive lucinids and a few anomalodesmatids, were capable of burrowing to intermediate depths (Stanley, 1968). At Waverly, the pholadomid Wilkingia is the most likely tracemaker of L. siliquaria (Mángano et al., 1998).

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Kansas Geological Survey, Geology
Placed on web May 21, 2015; originally published 2002.
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