Tracemaking Activities of Crabs and Their Environmental Significance: the Ichnogenus Psilonichnus

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Tracemaking Activities of Crabs and Their Environmental Significance: The Ichnogenus Psilonichnus

Robert W. Frey University of Georgia

H. Allen Curran Smith College, [email protected]

S. George Pemberton Alberta Geological Survey

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Recommended Citation Frey, Robert W.; Curran, H. Allen; and Pemberton, S. George, "Tracemaking Activities of Crabs and Their Environmental Significance: The Ichnogenus Psilonichnus" (1984). Geosciences: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/geo_facpubs/53

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Tracemaking Activities of Crabs and Their Environmental Significance: The Ichnogenus Psilonichnus Author(s): Robert W. Frey, H. Allen Curran and S. George Pemberton Source: Journal of Paleontology, Vol. 58, No. 2, Trace Fossils and Paleoenvironments: Marine Carbonate, Marginal Marine Terrigenous and Continental Terrigenous Settings (Mar., 1984), pp. 333-350 Published by: SEPM Society for Sedimentary Geology Stable URL: http://www.jstor.org/stable/1304788 Accessed: 19-04-2016 18:17 UTC

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This content downloaded from 131.229.93.39 on Tue, 19 Apr 2016 18:17:02 UTC All use subject to http://about.jstor.org/terms JOURNAL OF PALEONTOLOGY, V. 58, NO. 2, P. 333-350, 9 FIGS., MARCH 1984

TRACEMAKING ACTIVITIES OF CRABS AND THEIR ENVIRONMENTAL SIGNIFICANCE: THE ICHNOGENUS PSILONICHNUS1

ROBERT W. FREY, H. ALLEN CURRAN AND S. GEORGE PEMBERTON Department of Geology, University of Georgia, Athens 30602; Department of Geology, Smith College, Northampton, Massachusetts 01063 and Alberta Geological Survey, Terrace Plaza, 4445 Calgary Trail South, Edmonton, Alberta T6H 5R7, Canada

ABSTRACT-Modem crabs are common inhabitants of shallow subtidal, intertidal, and supratidal environments, and many crabs are capable of producing traces that can be preserved in the rock record. The first crabs, Early Jurassic in age, probably were not fossorial. By Cretaceous time, however, diverse endobenthic lineages were established. Many representatives of these lineages undoubtedly produced domiciles that are preserved in shallow marine to quasimarine sediments and that should be useful in characterizing the depositional environment of the sediments. None- theless, most such dwelling structures have been studied little and remain essentially unnamed. The ichnogenus Psilonichnus Fiirsich is amenable to the taxonomic concept of several forms of crab burrows; presently recognized ichnospecies include P. tubiformis Fiirsich and P. upsilon (n. ichnosp.). Future work may reveal the need for further ichnospecific differentiation. The occurrence of Psilonichnus upsilon and related burrow forms should prove to be a useful criterion for the identification of marine-margin facies in the rock record. Certain crabs also produce domiciles referable to Thalassinoides, Gyrolithes, and Skolithos, and possibly Macanopsis and Spongeliomorpha. Except for Skolithos, such structures traditionally have been attributed to shrimp, lobsters, or stomatopods. Ethologic and taxonomic re-evaluation of these burrow forms is needed.

INTRODUCTION reviews of related ichnogenera, modern bur- SEVERAL ichnogenera have been established rowing crabs, and the known fossil record of for trace fossils of presumed crustacean ori- crab or crab-like burrows. gin: Ardelia Chamberlain and Baer (1973), In terms of species diversity and adaptive Chagrinichnites Feldmann et al. (1978), Gy- radiations among crustaceans (26,000 rolithes Saporta (1884), Macanopsis Macso- species), the crabs (4,500 species) are ap- tay (1967), Ophiomorpha Lundgren (1891), proached in importance only by copepods Pholeus Fiege (1944), Spongeliomorpha Sa- (4,500 species), isopods (4,000 species), am- porta (1887), and Thalassinoides Ehrenberg phipods (3,600 species), and non-crab deca- (1944). Most of these burrows have been pods collectively (4,000 species) (Warner, ascribed to the activities of shrimp, lobsters, 1977). Thus, crab body fossils and crab bur- or stomatopods (Pemberton, Frey, and Walk- rows surely must be more common in the er, personal observ.). Fossil burrows such rock record than the present literature indi- as those excavated by moder crabs, al- cates. though well-documented locally (Richards, In addition to their preserved domiciles, 1975; Jenkins, 1975), have rarely received crabs may be represented in the ichnologic formal ichnogenus and ichnospecies names. record by various bioturbate textures im- With taxonomic emendation, the ichno- parted to host sediments (Edwards and Frey, genus Psilonichnus Fiirsich (1981) is ame- 1977, p. 228-230; Katz, 1980); they also may nable to many crab trace fossils and, hence, be responsible for appreciable bioerosion of is the major subject of our report. Presently sediments (Letzsch and Frey, 1980a, p. 208- recognized ichnospecies include P. tubifor- 210). Crabs thus have considerable impor- mis Fiirsich (1981) and P. upsilon n. ich- tance as geologic agents. nosp., described herein. We also present brief NATURAL HISTORY OF CRABS

1 Contribution number 498, University of Decapods are represented by about 8,500 Georgia Marine Institute, Sapelo Island. species, of which more than half are crabs. Copyright ? 1984, The Society of Economic 333 0022-3360/84/0058-0333$03.00 Paleontologists and Mineralogists and The Paleontological Society

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FIGURE 1-Dense population of the sand fiddler crab, Uca pugilator, feeding on an estuarine beach. Blackbeard Creek, Sapelo Island, Georgia.

Not only are crabs among the most successful 1965), and appear in terrestrial environments of all decapods, more is known about their as well as in marine, brackish, and fresh biology than of any other single group of crus- waters. Most crabs that inhabit brackish or taceans (Kaestner, 1970; Warner, 1977; fresh waters must return to salt waters to Barnes, 1980). Members of the genus Uca, breed, e.g., Rhithropanopeus; strictly fresh- prodigious burrowers, perhaps have been water crabs include the Potamidae and their studied most intensively (Crane, 1975). allies, or "river crabs" (Chace and Hobbs, Crabs are divided into two major taxo- 1969; Banes, 1980). All land crabs occupy nomic groups: the Anomura-hermit crabs burrows or conceal themselves beneath pro- and their kin, and the Brachyura-or true tective cover, e.g., Cardisoma (Herreid, 1963; crabs. Here, we are concerned mainly with Henning, 1975); members of the genus Ge- brachyurans. They range in size from the tiny carcinus (e.g., Bliss et al., 1978) are best male oyster crab Pinnotheres ostreum and adapted for terrestrial life (Kaestner, 1970, sand dollar crab Dissodactylus mellitae, only p. 356). Terrestrial crabs may obtain respi- 2 to 4 mm in width, to the giant Australian ratory water by burrowing down to the water xanthid crab Pseudocarcinus gigas, which table. The same is true of the so-called am- may have a carapace width of 43 cm, a chela phibious crabs such as Uca (Frey and Mayou, of about the same length, and a body weight 1971). of about 14 kg (Waner, 1977). Some brachy- In addition to the various habitat adapta- urans, such as the soldier crab Mictyris tions mentioned above, as well as formal (Schmitt, 1965) and the fiddler crab Uca (Fig- taxonomic ranks, brachyurans may be divid- ure 1), may be extremely abundant locally. ed into five general, nonexclusive categories Many species leave conspicuous records of of life styles (Warner, 1977, p. 68-84). Each their forays (Figure 2). category is characterized by a particular be- Adaptations.- Brachyurans exhibit a broad havioral pattern and, in some cases, by spe- spectrum of habitat adaptations (Williams, cific morphological adaptations:

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FIGURE 2-Trackway of the shore crab Pachygrapsus crassipes crossing a ripple-marked sand flat. Associated burrows made by the ghost shrimp Callianassa californiensis. Lagoon near Torrey Pines, California. (Photo courtesy of J. E. Warme.)

1) Locomotion by means of walking, run- Locomotion, burrowing, and feeding ac- ning, or climbing. Some species progress very tivities ofbenthic crabs are most apt to leave slowly, whereas individuals of Ocypode, the an ichnologic record. Locomotion typically fastest of all running crabs, may attain speeds results in a trackway representing a single ex- up to 1.6 m/sec (Barnes, 1980). Many species cursion (Figures 2, 3A); in some instances, in this category also burrow. however, the path is used repeatedly. Feeding 2) Swimming, especially among the traces are especially common on substrate Portunidae. Posterior appendages typically surfaces exploited by amphibious crabs such are flattened to function as oars, e.g., Calli- as Uca (Figures 3B-C, 4A); similar surficial nectes. Many such crabs are adept at inter- traces-and dwelling structures-are pro- cepting prey animals in the water column; yet duced by the sand-bubbler crab Scopimera the crabs may seek temporary refuge, or may (Schmitt, 1965, figs. 53, 54). Underlying sed- conceal themselves from their prey, by bur- iment laminae may be disrupted, on a small rowing shallowly into the substrate. scale, where penetrated by tips of the dactyli 3) Burrowing, discussed below. (Schafer, 1972, fig. 140; Frey, 1973, fig. 5). 4) Incorporation of camouflage or incon- Such "undertracks" have some potential for spicuousness as a major adaptive feature. preservation in the rock record (cf. Goldring Cryptic coloration (Daldorfia), tufted hairs and Seilacher, 1971). (Pilumnus), and encrustations by sessile epi- Among burrowing crabs, two distinct be- bionts (Pisa) are typical means of conceal- havioral patterns are discernible: back-bur- ment. The ghost crab Ocypode is a master at rowers and side-burrowers (Warner, 1977, p. blending into its beach-sand background. 75-78). Back-burrowers tilt the body back- 5) Adaptation to commensal, symbiotic, or ward and dig with the walking legs. Loosened parasitic relationships with other species. sediment becomes a semifluid; the crab, uti- Members of the Pinnotheridae are well- lizing various morphologic adaptations, or- known associates of diverse shelled or bur- dinarily works itself downward until only the rowing organisms (Warner, 1977, p. 81-84), eyes or antennae remain visible (Savazzi, and the anemone-hermit crab association is 1982). In the portunid crab Macropipus equally well documented (Ross, 1974). On (Kaestner, 1970, p. 346), the flattened fifth the other hand, diverse invertebrates and pereiopods are pushed horizontally into the some fish may be common commensals in- sediment while the chelipeds are forced for- side crab burrows (Powell and Gunter, 1968). ward, thereby moving the body backward into

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This content downloaded from 131.229.93.39 on Tue, 19 Apr 2016 18:17:02 UTC All use subject to http://about.jstor.org/terms TRACEMAKING ACTIVITIES OF CRABS 337 the substrate; the buried crab then nestles its row, and deposited near or scattered around chelipeds along the anterior curvature of its the aperture (Figures 3C, 4B) (Chakrabarti, carapace, enclosing a sediment-free space 1972); this sediment reworking may alter the filled with respiratory water. The teeth of the grain-size distribution of particles (Chakra- anterior margin of the carapace extend over barti, 1980). In some species, the pelleted the water chamber to the chelae, straining out sediment may be used to construct a chimney detrital sand. The soldier crab Mictyris that effectively extends the burrow aperture (Schmitt, 1965, p. 135, fig. 55) buries itself vertically (Uca) or horizontally (Sesarma) extremely rapidly; its actions produce a cork- (Basan and Frey, 1977). Most side-burrowers screw spiral because it digs with the legs of are terrestrial or amphibious, yet a few sub- one side of its body while rotating its position littoral crabs-such as Goneplax-construct in the substrate by means of the legs on the their burrows underwater (Rice and Chap- other side. man, 1971). Back-burrowers thus are temporarily Although the structures excavated by side- embedded within the substrate. Resting traces burrowers generally serve as protective dom- so created would appear in the rock record iciles, more specific functions may be dis- as a pocket-like disruption of sediment lam- cerned. For example, males of some species inae (Schafer, 1972, figs. 226, 227), although of the ghost crab Ocypode dig spiraled bur- these structures remain poorly documented rows thought to be used in copulation in current geologic literature (cf. Hannibal (Hughes, 1973). Similarly, the location and and Feldmann, 1983). quality of burrow construction is intimately Side-burrowing, on the other hand, results related to reproductive patterns in the fiddler in the construction of a regular domicile (Fig- crab Uca pugilator (Christy, 1983). ure 5), which may provide shelter for ex- Side-burrowers are the chief subject of our tended periods of time. Various heterogene- report, because their traces are more likely to ities in habitat may influence the selection of be represented in the rock record. Several an actual burrow site. Juvenile through adult examples of their burrow construction are burrows of the ghost crab Ocypode are typi- cited in Table 1, and their fossil counterparts cally zoned according to beach slope and tide are discussed subsequently. levels (Frey and Mayou, 1971). In salt marsh- Geologic history.--The most primitive es, variations in the density of grass root mats crabs, the Dromiacea, first appeared in the may influence the density and distribution of Early Jurassic (Warner, 1977, p. 164-172, burrows of the fiddler crab Uca (Ringold, tables 4, 5, figs. 41A, 43). They probably 1979). Territoriality also may be involved; stemmed from the Glypheoidea, a macruran among freshwater crabs of the Pseudothel- lineage related to the spiny lobsters (Pem- phusidae, which burrow in rocky streams, the berton, Frey, and Walker, personal ob- most favorable sites are preempted by the serv.). Early dromiaceans lack the charac- larger, more dominant individuals (A. E. teristic brachyuran morphology, however, Smalley, 1983, personal commun.). and are not likely to have constructed dis- Side-burrowers employ their legs, the dac- tinct, readily preservable dwelling structures. tyli of which may be slightly flattened dor- Families of moder crabs containing fosso- soventrally, to excavate sediment. Usually, rial members seem to have become well es- only the legs on one side of the body are tablished by Cretaceous time, and their bur- engaged. Excavated sediment may be formed rows should be correspondingly prominent into a spherical pellet, carried out of the bur- in post-Jurassic rocks. Some possibly occur

FIGURE 3--Crawling and grazing traces of the sand fiddler crab, Uca pugilator. Washover sands within a salt marsh, St. Catherines Island, Georgia. A, crossing trackways, with distinctive dactylus imprints. Bird track at lower left corner. B, C, grazing traces, those in C superimposed on raindrop imprints. Cheliped scrapings in the substrate surface reflect deposit-feeding activities (Miller, 1961); individual scrapes are sharper in moist sediment (C) than in wet sediment (B). Small sediment pellets (3-4 mm) discarded during feeding; large pellets (> 10 mm) discarded during burrow excavation.

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r .. rL r FIGURE 5-Domicile of the ghost crab Ocypode quadrata exposed by storm-wave erosion of a sand dune. Cabretta Island, Georgia. Length and configuration of burrow arms controlled partly by topography of substrate surface (cf. Frey and Mayou, 1971). Only uppermost part of basal shaft (below bifurcation) is exposed. Cheliped sculptings common locally within burrow.

'A in Upper Jurassic rocks (Fiirsich, 1974, fig. --Y r? r, '; .'L c; Ir 7). ' ?? :C_ *?s?; ?1.*?' ji?c. .-?? v?.?' MODERN AND ANCIENT CRAB BURROWS r '' iP.*'- ... Modem benthic crabs and their burrows are abundant in numerous sedimentary environments, from terrestrial clastics to shoal- ing marine carbonates. Representative brachyuran families (Table 1) include: 1) por- tunids-swimming crabs, 2) xanthids-mud crabs, 3) goneplacids-xanthid-like, bottom- dwelling crabs, 4) ocypodids- amphibious crabs, 5) grapsids-crabs of diverse habits and habitats, and 6) gecarcinids-land crabs. Cer- tain anomuran sand crabs, such as Albunea (Farrow, 1971, fig. 10), also may construct distinctive burrows; most, however, produce FIGURE 4-Surficial lebensspuren made by ocyp- odid crabs. Locality as in Figure 3. A, radially only bioturbate textures in the sediment (Frey disposed grazing traces of the fiddler crab Uca and Howard, 1972, p. 173, fig. 2). pugilator, wet sediment. B, entrance to large Fossorial members of the above groups, as burrow of the ghost crab Ocypode quadrata sur- a whole, are most characteristic of high-in- rounded by profuse sediment pellets discarded tertidal to supratidal environments. Terres- by Uca pugilator, dry sediment. C, a similar trial burrowing crabs also are significant lo- surface crossed by the white-tailed deer Odo- coileus virginianus. cally (Bliss et al., 1978). Of the latter, Cardisoma (Table 1) is perhaps best known

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ichnologically. Other species of land crabs TABLE I-Selected examples of studies on moder crab generally remain poorly studied from this burrows. (The albuneids are anomuran crabs; all others are brachyurans.) standpoint. Freshwater crabs (e.g., Chace and Hobbs, 1969) are comparatively rare, and ALBUNEIDAE none is known to be an extensive burrower. Albunea (Farrow, 1971). PORTUNIDAE However, the habit is widespread among Ovalipes (Caine, 1974). members of the Pseudothelphusidae (A. E. XANTHIDAE Smalley, 1983, personal commun.); their typ- Menippe (Powell and Gunter, 1968). Panopeus (Basan and Frey, 1977; Pemberton and ical environment is small streams, where they Frey, personal observ.). burrow under and between rocks. The saber Eurytium (Basan and Frey, 1977). crab Platychirograpsus typicus, in contrast, GONEPLACIDAE Goneplax (Rice and Chapman, 1971). burrows into the clay banks of rivers (Mar- OCYPODIDAE chand, 1946, p. 94). Uca (Frey and Howard, 1969; Frey, 1970; Frey and Fossil burrows attributed to crabs have been Mayou, 1971; Farrow, 1971; Braithwaite and Talbot, 1972; Allen and Curran, 1974; Basan and reported from the Upper Cretaceous of Brit- Frey, 1977; Garrett, 1977; Ferreira, 1980). ish Columbia (Richards, 1975), the Miocene Ocypode (Hayasaka, 1935; Utashiro and Horii, 1965; Frey and Mayou, 1971; Farrow, 1971; or Pliocene of Japan (Nomura and Hatai, Braithwaite and Talbot, 1972; Hill and Hunter, 1936), the ?Miocene of Taiwan (Hayasaka, 1973, 1976; Allen and Curran, 1974; Ferreira, 1935), the Miocene of Poland (Radwaniski, 1980; Chakrabarti, 1981). Macrophthalmus (Farrow, 1971; Braithwaite and 1977a, 1977b), the Miocene and Pliocene of Talbot, 1972). Australia (Jenkins, 1975), the Pleistocene of Ilyoplax (Hayasaka, 1935). GRAPSIDAE the southeastern United States (Frey and Sesarma (Braithwaite and Talbot, 1972; Allen and Mayou, 1971; Curran and Frey, 1977) and Curran, 1974; Basan and Frey, 1977; Garrett, the Bahamas (Figure 6), and possibly the Oli- 1977). Pachygrapsus (Warme, 1971). gocene of Egypt (Bown, 1982, p. 281). GECARCINIDAE Subfossil crab burrows also occur in relict Cardisoma (Shinn, 1968; Braithwaite and Talbot, Holocene salt-marsh deposits of Georgia 1972). (Frey and Basan, 1981; Pemberton and Frey, personal observ.) and South Carolina (personal observ.). Evidence for crabs as the tracemaker is cir- Because prominent crab burrows generally cumstantial in most ancient settings; the are most abundant in intertidal, supratidal, strongest conclusions obviously are drawn and terrestrial environments-facies having when the fossil burrows closely resemble relatively low potential for preservation- modem crab burrows. Nevertheless, body re- fossil crab burrows are inherently less abun- mains were found in association with the bur- dant in the rock record than other crustacean rows reported by Richards (1975), Jenkins burrows made in low intertidal and subtidal (1975), and Frey and Basan (1981). They also environments (e.g., Frey, Howard, and Pryor, were observed in the relict Holocene burrows 1978; Dworschak, 1983). Even where the ap- from South Carolina, mentioned above. In- propriate facies are preserved, the crab bur- dividuals of many species of crustaceans or- rows may be truncated by scour horizons dinarily desert their burrows before death (Radwanski, 1977a, fig. 1) or otherwise mod- (Schafer, 1972), probably accounting for the ified or poorly preserved, as by concretionary dearth of body parts preserved inside bur- overgrowth; diagenetic mineralization may rows. Whether the same is true for most crabs impart thick "burrow linings" where none is not known, however. existed during the lifetime of the crab (Frey Aside from the obvious limitations of cir- and Basan, 1981, P1. 5, figs. 22-23). cumstantial evidence, certain fossil burrows In general, therefore, crab burrows are less resembling modem crab burrows are older abundant, less well preserved, and less readi- than the oldest known, unequivocally endo- ly identifiable in the rock record than such benthic crab remains. "Crustacean burrows" structures as Ophiomorpha (Frey, Howard, (Fiirsich, 1974, fig. 7) from the Upper Juras- and Pryor, 1978). However, where ancient sic of England are possible examples. crab burrows are preserved, they may have

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This content downloaded from 131.229.93.39 on Tue, 19 Apr 2016 18:17:02 UTC All use subject to http://about.jstor.org/terms TRACEMAKING ACTIVITIES OF CRABS 341 considerable significance in the interpreta- tion of associated depositional environments (Frey and Mayou, 1971), and they warrant formal trace fossil names. Finally, interpretations of many such fossil burrows are hampered by the relatively small amount of information available for numer- A ous kinds of modem crab burrows. For example, in the Pleistocene of North Carolina we observed large concretionary burrow systems (Curran and Frey, 1977, P1. 5d) possibly made by the stone crab Menippe mercenaria; yet a paucity of information on modem burrows of M. mercenaria (Powell and Gunter, 1968, p. 286-287) precludes rigorous unifor- mitarian comparison. Comparable burrows have been cast, by means of polyester resin, in Georgia estuarine environments (Figure 7). No other local decapods (Williams, 1965) B or stomatopods (Manning, 1969) are of a size consistent with this tracemaker; however, even though individuals of the crab were seen in close proximity, none was observed within the large burrows sampled. Additional work 20cm on burrows of M. mercenaria from the Geor- gia coast therefore is being undertaken (Walker and Frey, in progress). FIGURE 7-Polyester cast of probable stone crab burrow, Menippe mercenaria. Muddy sand of TAXONOMY OF BRACHYURAN AND tidal stream point bar within a salt marsh, be- OTHER CRUSTACEAN BURROWS tween Sapelo and Blackbeard islands, Georgia. The ichnogenus Psilonichnus, as emended Cheliped sculptings common on burrow walls. herein, is well suited for the accommodation A, steep oblique view. B, shallow oblique view (cf. Powell and Gunter, 1968). (Sketches by G. of certain kinds of fossil crab burrows. Yet, Maddock.) somewhat similar burrows are constructed by various shrimp or shrimp-like animals (Dworschak, 1983)-as well as by some cray- fish (Hobbs, 1981)-and certain crab bur- fossil burrows were reported by Nomura and rows are referable to ichnogenera other than Hatai (1936), but no body remains of crabs Psilonichnus (Table 2). For example, body were observed. fossils of the crabs Ommatocarcinus (Jen- Similarly, individuals of the same species kins, 1975) and Longusorbis (Richards, 1975) of crab may construct burrows referable to are associated with Thalassinoides. The same different ichnospecies or even different is true of burrow systems excavated by the ichnogenera. Juveniles of the modem ghost moder crab Panopeus herbsti in coherent crab Ocypode quadrata (Frey and Mayou, muds (Letzsch and Frey, 1980a, fig. 5; Pem- 1971, P1. 2, fig. 3) sometimes excavate simple berton and Frey, personal observ.); similar vertical shafts in foreshore sediments that, in

FIGURE 6-Psilonichnus upsilon in its type area. Pleistocene calcarenites, San Salvador Island, Bahamas. A, inclined Y-shaped burrow; right arm slightly smaller in diameter (2.5-3 cm) than left arm (4-4.5 cm); length is 1.2 m. B, enlargement of bifurcation in A. C, upper part of bifurcated burrow, 2.5-3 cm in diameter; position of right arm, behind plane of rock exposure, indicated by arrow. D, segment of apparently unbranched shaft, ca. 5 cm in diameter and 1.35 m in length.

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TABLE 2-Ichnogenera most likely to include crabs as The degree to which spiraled burrowing tracemakers. Pholeus is omitted because of the un- patterns exhibited by certain modern crabs certainty of its taxonomic status. (Schmitt, 1965, p. 135, fig. 55; Farrow, 1971, A-Taxonomic Key p. 465, fig. 7) may approach typical speci- Unbranched burrows. mens of the fossil burrow Gyrolithes (Table 1. Predominantly vertical; essentially uniform in 2) remains to be seen. Ratios of coil height diam eter ...... Skolithos to coil diameter are very different; yet, the 2. Erect, arcuate; with basal chamber .... Macanopsis mere presence of a distinct helix satisfies the Sparsely branched burrows: chieftaxonomic criterion (Bromley and Frey, 1. Erect spirals ...... Gyrolithes 1974, p. 320-321). Similarly, some forms of 2. Externally striated components of variable orientation ...... Spongeliomorpha the striated burrow Spongeliomorpha- which 3. J-, Y-, or U-shaped, erect components ...... is fully intergradational with Thalassi- ...... Psilonichnus Well-integrated burrow systems ...... Thalassinoides noides-need ethologic reevaluation from the viewpoint of brachyuran tracemakers; the B - Diagnoses same is true of Macanopsis. Skolithos-Cylindrical to subcylindrical, straight to gently Thalassinoides systems produced by crabs curved, distinctly walled, vertical to steeply inclined (Richards, 1975, p. 1856) are more apt to burrows (Alpert, 1974). Macanopsis- Slightly to highly curved J-shaped burrows exhibit cheliped sculptings along burrow walls terminating in a basal chamber; upper part of burrow than those excavated by shrimp (Figure 8), essentially vertical (Macsotay, 1967). lobsters (Pemberton, Frey, and Walker, per- Gyrolithes- Rarely branched, spiraled burrows; helix essentially vertical, consisting of dextral, sinistral, or re- sonal observ.), or stomatopods. We spec- versing coils (Bromley and Frey, 1974). ulate that Spongeliomorpha specimens ex- Spongeliomorpha- Sparsely developed burrow systems; components vertical to horizontal, characterized by cavated by crabs would bear much shorter, sets of longitudinal or oblique, fine, elongate striations more stumpy or bulbous sculptings than those on exterior of burrow casts (Fiirsich, Kennedy, and produced by shrimp or shrimp-like animals Palmer, 1981). Psilonichnus- Predominantly vertical J-, Y-, or U-shaped (Hantzschel, 1975, fig. 67.2a-b; Fiirsich, structures of variable diameter; lateral branches, if Kennedy, and Palmer, 1981, P1. 3, figs. 3-6). present, form singular or bifurcated culs-de-sac and Furthermore, swollen bifurcations and nodal tend to emanate from vertical shafts (this report). Thalassinoides- Three-dimensional burrow systems enlargements along burrow components, seen consisting predominantly of smooth-walled, essen- in various specimens of Thalassinoides, tially cylindrical components of variable diameter; branches Y- to T-shaped, enlarged at points of bifur- Spongeliomorpha, and Gyrolithes (Kennedy, cation (Howard and Frey, 1984). 1967; Bromley and Frey, 1974; Fiirsich, Ken- nedy, and Palmer, 1981), are more characteristic of shrimp or shrimp-like aninals than of crabs. Local burrow enlargements, called the fossil record, could only be designated as "turn arounds," are necessary for reversals Skolithos; adults of the same crab produce in direction of movement by animals having structures typically referable to Psilonichnus a circumference similar to the diameter of upsilon. In modern salt marshes of Georgia, the burrow; but these swellings are of no ad- the fiddler crab Uca pugnax may construct vantage in the bidirectional movements of Skolithos-like shafts where population den- crabs. sity is low (Basan and Frey, 1977, PI. 4f) or Among Thalassinoides systems attribut- integrated burrow systems referable to Tha- able to crabs, those reported by Jenkins (1975) lassinoides paradoxicus where population seem to be most typical of the ichnogenus in densities are high (Frey, Basan, and Scott, terms of overall burrow configuration. Those 1973, fig. 1D; Letzsch and Frey, 1980b, fig. reported by Nomura and Hatai (1936) and 2B). Even more problematical, taxonomi- Richards (1975) are less typical in that they cally, is the occasional interpenetration of seem to consist of vertical or inclined shafts burrows of Uca pugnax, Sesarma reticula- stemming from a horizontal basal tunnel; tum, and Eurytium limosum in these marsh- however, comparable configurations have es (Basan and Frey, 1977, P1. 4a, c). Some- been observed in Thalassinoides systems at- what comparable ethologies and burrow tributable to shrimp (Figure 8). morphologies probably are to be expected in As presently defined, the main distinction the fossil record. between unbranched, J-shaped burrows of

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FIGURE 8- Reconstruction of smooth-walled burrows referable to Thalassinoides paradoxicus. Impure limestones at transitional contact between Clinchfield Sandstone and overlying Ocala Limestone (Eocene), near Perry, Georgia. Associated cheliped fragments, some inside burrows, indicate that Callianassa sp. was the tracemaker. A, unusual burrow systems in which horizontal bifurcations are comparatively rare; burrow fills more resistant to outcrop weathering than host stratum. B, burrow systems in which horizontal and vertical bifurcations are almost equally prominent; the structures therefore are somewhat more typical of T. paradoxicus.

modem crabs (Figure 9) (see Frey and How- on fossil burrows attributable to crabs, fur- ard, 1969, P1. 3, fig. 4) and the fossil burrow ther differentiation of the ichnogenus Psilon- Macanopsis (Table 2) is the presence of a ichnus may prove necessary. prominent basal chamber or cell in the latter.

Although slight enlargements have been ob- SYSTEMATIC ICHNOLOGY served at the base of certain crab burrows, we know of none that closely duplicate Ma- Ichnogenus PSILONICHNUS Fiirsich, 1981 canopsis. Sandstone casts or rods HAYASAKA, 1935, p. 99- Thus, the majority of fossil and recent crab 100, P1. 1, fig. 5, P1. 2, fig. 2. burrows observed to date exhibit a charac- Fossil burrows STEPHENSON, 1965, p. 850-851, fig. 1. teristic, definitive range of morphologic vari- Burrows of Callianassa sp. RADWANSKI, 1969, p. ations amenable to ichnological taxonomy. 93, fig. 33.3. Certain intergradations present taxonomic Callianassid burrows RADWANSKI, 1970, p. 386- problems and require subjective judgments; 388, fig. 4a, PI. 6, figs. a-c. but these problems, conceptually, are no dif- Ghost crab burrows FREY AND MAYOU, 1971, p. ferent from those of any other trace fossil 67-68, P1. 4, fig. 3a-b. ?Crustacean burrows FORSICH, 1974, p. 10-11, fig. group (Pemberton and Frey, 1982). 7a-c. Most fossil crab burrows with which we J-shaped burrows CURRAN AND FREY, 1977, p. are familiar, other than the exceptions out- 158-159, P1. 5, fig. e; BELT, FREY, AND WELCH, lined above, fall within a distinctive but here- 1983, p. 250-252. tofore unnamed trace fossil group. For this Burrows attributable to ghost crabs RADWANSKI, group we propose the name Psilonichnus up- 1977a, p. 219-221, fig. la, P1. 2, figs. 1-2a, b, silon. P. upsilon corresponds rather closely to P1. 3, figs. 1-4; RADWANSKI, 1977b, p. 233-235, P1. 3, fig. bl-4. moder burrows of the ghost crab Ocypode Psilonichnus FURSICH, 1981, p. 157. and certain other members of the Ocypodi- ?Ichnofossil type 4 BowN, 1982, p. 280-281, fig. dae. As additional information accumulates 11F.

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p. W79, fig. 51.1), which consists of an unbranched, slightly to highly curved J-shaped burrow terminating in a basal, subhemis- pherical chamber; the upper part of the burrow is vertical, and the lower part becomes increasingly oblique with depth; in rare cases the basal chamber is horizontal. Pholeus remains poorly known; the ichnogenus was established on the basis of specimens found in a museum, and has yet to be described in detail from a field locality. It may well prove to be a form of Thalassinoides (cf. Figure 8). Macanopsis, on the other hand, may be more closely related to Skolithos, even though partially overlapping the morphology of simple J-shaped specimens of Psilonichnus (cf. Figure 9). As presently defined, the chief distinguishing feature of Macanopsis is the basal chamber; however, that trait may prove to be most significant as a taxonomic char- acter within the overall group ofunbranched, shaft-like burrows typified by Skolithos (the FIGURE 9-J-shaped burrows of the ghost crab ichnogenus Skolithos and related forms are Ocypode quadrata. Polyester casts; lower back- presently undergoing extensive monographic shore, Sapelo Island, Georgia. Ghost crab bur- reevaluation; Pemberton and Frey, in prep- rows in uppermost foreshore are simple, in- aration). clined shafts; in upper backshore and dunes, burrows typically are branched (cf. Frey and Mayou, 1971). PSILONICHNUS TUBIFORMIS Fiirsich, 1981 Psilonichnus tubiformis FORSICH, 1981, p. 157- 158, PI. 1, figs. 1-2, PI. 4, fig. 5. Emended diagnosis.- Predominantly ver- Diagnosis. - Psilonichnians consisting typ- tical, cylindrical, unlined burrows ranging ically of upper, Y- or U-shaped components, from irregular shafts to crudely J-, Y-, or grading downward to predominantly verti- U-shaped structures; lateral branches, not cal, straight to slightly curved or twisted shafts necessarily the same diameter as parent bearing irregularly spaced, horizontal or trunks, may be present and tend to form sin- oblique dead-end branches, some of which gular or bifurcated culs-de-sac. bifurcate. Discussion. -Burrows are preserved as end- Discussion. -The ichnospecies is distin- ichnia and are interpreted as dwelling struc- guished primarily on the basis of the lower, tures of fossil decapod crustaceans, including vertical shafts and irregular lateral branches. brachyuran crabs. Diameters of most speci- P. upsilon n. ichnosp. consists mainly of in- mens fall within the 1 to 10 cm range, de- clined shafts bearing relatively few, or no, pending partly on the identity and ontoge- branches; where present, the branches either netic stage of the tracemaker; lengths may be comprise, or tend to be congruent with, the as much as 1.5 to 2 m, although many spec- overall Y-shaped configuration of the bur- imens are considerably shorter. rows (cf. Frey and Mayou, 1971, P1. 2, fig. 4; More or less similar ichnogenera include Chakrabarti, 1981, text-figs. 11-12). Pholeus (Fiege, 1944, p. 401-404, 415-416, In its type area, P. tubiformis occurs in 426, figs. a, 1; Hintzschel, 1975, p. W93, fig. marginal marine to terrestrial deposits, pos- 59.1), which consists of two or more vertical sibly a low-lying coastal plain inundated sub- or inclined shafts stemming from a horizon- sequently by marine waters (Fiirsich, 1981, tal basal tunnel, and Macanopsis (Macsotay, p. 158). We speculate that these burrows may 1967, p. 32, figs. 44,46, 60; Hantzschel, 1975, have been excavated by a thalassinidean

This content downloaded from 131.229.93.39 on Tue, 19 Apr 2016 18:17:02 UTC All use subject to http://about.jstor.org/terms TRACEMAKING ACTIVITIES OF CRABS 345 decapod somewhat like Upogebia (cf. Dwor- bedded Pleistocene calcarenites exposed on schak, 1983) in a low-energy, brackish water sea cliffs along the northeast (Atlantic-facing) setting. Alternatively, the burrows may have coast of San Salvador Island, Bahamas. Based been excavated by shrimp or shrimp-like an- on physical sedimentary characteristics, as- imals in an extremely shallow, possibly in- sociated fossils, and stratigraphic position, the tertidal environment near the shoreline (Fir- deposits are interpreted to have accumulated sich, 1981, p. 165). in an uppermost foreshore to backshore environment. Fine-grained eolian calcarenites PSILONICHNUS UPSILON n. ichnosp. immediately overlie the beds bearing P. up- Figure 6A-D silon. Further information on the geologic Burrows of Callianassa sp. RADWANSKI, 1969, p. setting of San Salvador is given by Curran 93, fig. 33.3. (1984). Callianassid burrows RADWANSKI, 1970, p. 386- Segments of P. upsilon shafts may be rather 388, fig. 4a, P1. 6, figs. a-c. common at this locality; holes of the proper Ghost crab burrows FREY AND MAYOU, 1971, p. diameter can be located easily in the friable 67-68, P1. 4, fig. 3a-b. calcarenites of the sea-cliff exposures. How- J-shaped burrows CURRAN AND FREY, 1977, p. ever, these beds also contain rhizocretions. 158-159, PI. 5, fig. e; BELT, FREY, AND WELCH, 1983, p. 250-252. The hard calcrete cores of large-diameter rhi- Burrows attributable to ghost crabs RADWANSKI, zocretion shafts frequently break out of the 1977a, p. 219-221, fig. la, PI. 2, figs. 1-2a, b, rock, leaving unlined, relatively straight shafts P1. 3, figs. 1-4; RADWANSKI, 1977b, p. 233-235, that can be difficult to distinguish from seg- P1. 3, fig. bl-4. ments of P. upsilon. The Y-shaped branching Y-shaped crab burrows CURRAN, 1984, p. 312- pattern thus is the key to positive identifi- 321. Fig. 4A. cation of P. upsilon; but the juncture in many Diagnosis. - Psilonichnians consisting typ- cases is not preserved. We therefore recom- ically of gently inclined, sparsely branched to mend a conservative approach to the iden- unbranched, J- or Y-shaped burrows; in- tification of P. upsilon specimens here; after clined shafts straight to slightly arcuate; further study and increased familiarity with branches slightly to markedly curved, not various preservations of the form, however, horizontal. it may yet prove to be a common trace fossil. Origin of name. -Derived from the Greek Comparison with modern analogs.-Bur- yupsilon, meaning bare, mere, or simple Y; rows of the ghost crab Ocypode quadrata are refers to the typically Y-shaped burrow form. common on the narrow, upper foreshore and Repository. -Type specimens, from the backshore zones of the carbonate-sand Pleistocene of San Salvador Island, Bahamas, beaches of San Salvador Island. These bur- have been placed in the Paleontological Col- rows are unlined; most have diameters of 1 lections of the Department of Geology, Uni- to 3 cm; and many have two entrances that versity of Georgia. Specimen numbers: ho- join below in a Y-shaped pattern. However, lotype, TF-Q-001; paratype, TF-Q-002. single-entry burrows are far more common Description of specimens in the type area. - on these beaches than are double-entry forms. Unbranched to Y-shaped, unlined burrows; Specimens of P. upsilon preserved in ad- shafts, steeply inclined to bedding, typically jacent Pleistocene beds are comparable with 2.5 to 4.5 cm in diameter and up to 1.2 m the modem burrows of Ocypode quadrata and or more in length. Where branched, the angle undoubtedly were formed by that animal. Di- of bifurcation is 65? to 80?; typically, one ameters of Pleistocene shafts are somewhat branch is slightly smaller in diameter than larger than typical shafts of moder burrows the other. Openings of branches at the ancient on San Salvador Island, but they are within substrate surface are somewhat enlarged. the range of diameters reported for 0. qua- Terminus of burrows not preserved; there- drata burrows along the Georgia coast (Frey fore, measured shaft lengths are less than the and Mayou, 1971, table 1). Thus, specimens original lengths. of P. upsilon here have a direct modern an- Occurrence and geologic setting.--Well- alog in the burrows of Ocypode quadrata- preserved specimens of P. upsilon occur rare- excavated by a tracemaker of narrow envi- ly in medium- to fine-grained, planar cross- ronmental preference and well-known habits

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(Frey and Mayou, 1971; Allen and Curran, vironmental reconstructions. For example, 1974; Hill and Hunter, 1973, 1976)-andcan the somewhat aberrant specimens of Thalas- be used as a reliable indicator of ancient up- sinoides paradoxicus mentioned previously per foreshore and backshore environments, (Figure 8) occur in clayey, somewhat fissile as represented by rocks on San Salvador Is- limestone; these beds are underlain by rela- land and in other, similar settings. tively clean sandstones characterized by ver- Other occurrences.--The friable, poorly tical shafts of Ophiomorpha nodosa and are consolidated Pleistocene backshore sedi- overlain by limestones characterized by hor- ments of Georgia and northeastern Florida izontal tunnels of Thalassinoides suevicus are easily scraped or excavated; serial sec- (Frey and Seilacher, 1980, fig. 4). The T. par- tions of these deposits reveal P. upsilon mor- adoxicus animal thus seems to have exploited phologies that are virtually identical with a peculiar environment transitional between burrows of the moder ghost crab Ocypode the nearshore sands and offshore carbonates. quadrata (Frey and Mayou, 1971, Pls. 2-3), For similar reasons, Psilonichnus upsilon including the common association with am- might occur in facies other than the shoreline phipod cryptobioturbation (P1. 4, fig. 3a). sequence mentioned above; one possibility is North Carolina specimens of P. upsilon (Cur- point-bar deposits within the seaward part of ran and Frey, 1977, PI. 5e) are less well pre- an estuarine sequence (Frey and Howard, served but are clearly the same as the Geor- 1980; Howard and Frey, 1980a). gia-Florida forms. Similarly, during beach progradation, long In the Miocene of Poland, upper parts of shafts of P. upsilon (e.g., Figure 6D), although burrows have been removed by erosion (Rad- originating at a substrate surface in the back- wanski, 1977a, p. 221, fig. 1); the remainder shore, might penetrate well into underlying of the structure is quite like equivalent com- foreshore deposits replete with Ophiomorpha ponents of P. upsilon mentioned above. Bur- nodosa. In such instances, the spatial range rows described by Hayasaka (1935, P1. 1, fig. of the two ichnospecies (cf. Frey and Mayou, 5, P1. 2, fig. 2) and Stephenson (1965, fig. 1) 1971, fig. 3) would overlap both laterally and probably can be ascribed to P. upsilon, al- vertically. Somewhat comparable trace fossil though additional evaluation is required. overprintings have been observed in related settings (Howard and Scott, 1983, p. 178- ENVIRONMENTAL SIGNIFICANCE 182). Deep penetrations of this sort possibly Ancient depositional environments in also account for the difficulty experienced by which populations of Psilonichnus upsilon Radwanfski (1977a, p. 222-224) in interpret- might abound constitute a mixture of mar- ing the occurrence of P. upsilon shafts in marginal-marine and quasimarine facies. Typical ly sands containing scour horizons, whether moder environments include the upper- part of a shoreline, embayment, or estuarine most foreshore and backshore of beaches, sequence. dunes, washover fans, and supratidal flats. Nevertheless, Pleistocene shoreline se- Marine conditions characterize the back- quences of Georgia and northeastern Florida shore during spring and storm tides, and also (Howard and Scott, 1983) are virtually iden- characterize washover fans and supratidal tical with their Holocene counterparts (How- flats during storm surges. In contrast, mari- ard and Frey, 1980b): well-laminated fore- time eolian conditions characterize dunes, the shore deposits characterized by vertical backshore during neap tides, and washover Ophiomorpha nodosa are overlain by cross- fans and supratidal flats during non-storm laminated backshore deposits characterized periods. Relatively few invertebrate trace- by Psilonichnus upsilon. The only exceptions makers are tolerant of such stressful condi- are local isolated lenses of cross-laminated tions (Dorjes and Hertweck, 1975); among sand containing P. upsilon that seem to rep- marine or quasimarine species, only the am- resent the backshore- or dune-like part of phibious crabs have succeeded in exploiting nearshore bars or small spits; but these fea- this highly variable coastal zone. tures, occupied by Ocypode quadrata, also In the rock record, vertical sequences and are known from the moder coast. associated physical sedimentary structures Finally, unlike most other marine or mar- also are important in ichnological and en- ginal-marine facies, these deposits may con-

This content downloaded from 131.229.93.39 on Tue, 19 Apr 2016 18:17:02 UTC All use subject to http://about.jstor.org/terms TRACEMAKING ACTIVITIES OF CRABS 347 tain a mixture of invertebrate and vertebrate BRAITHWAITE, C. J. R. and M. R. TALBOT. 1972. lebensspuren (Figures 3A, 4C) (Howard and Crustacean burrows in the Seychelles, Indian Ocean. Palaeogeography, Palaeoclimatology, Frey, 1980b). Presence of bird and tetrapod Palaeoecology, 11:265-285. tracks is unequivocal evidence for the prox- BROMLEY, R. G. and R. W. FREY. 1974. Rede- imity of a terrestrial environment, and their scription of the trace fossil Gyrolithes and taxo- overlap with Psilonichnus upsilon is equally nomic evaluation of Thalassinoides, Ophio- strong evidence for a high-intertidal or su- morpha and Spongeliomorpha. Geological pratidal environment. Society of Denmark, Bulletin, 23:311-335. CAINE, E. A. 1974. Feeding of Ovalipes gua- ACKNOWLEDGMENTS dulpensis (Saussure) (Decapoda: Brachyura: Portunidae), and morphological adaptations to For their valuable reviews of the prelimi- a burrowing existence. Biological Bulletin, 147: nary manuscript, we thank Horton H. Hobbs, 550-559. James D. Howard, and Austin B. Williams. CHACE, F. A., JR. and H . H. HOBBS, JR. 1969. Equally valuable reviews of the final manu- The freshwater and terrestrial decapod crusta- script were given by Gale A. Bishop, Rodney ceans of the West Indies, with special reference to Dominica. United States National Museum, M. Feldmann, and Franz T. Fiirsich. Bulletin 292, 258 p. We also are grateful to the College Center CHAKRABARTI, A. 1972. Beach structures pro- of the Finger Lakes, Bahamian Field Station, duced by crab pellets. Sedimentology, 18:129- and its staff on San Salvador Island, for full 134. logistical support of the part of this study .1980. Influence ofbiogenic activity of ghost conducted in the Bahamas (by Curran); thanks crabs on the size parameters of beach sediments. also are due Robert C. Titus, Hartwick Col- Senckenbergiana Maritima, 12:183-199. 1981. Burrow patterns of Ocypode cera- lege, for directions and an introduction to the tophthalma (Pallas) and their environmental beds bearing fossil crab burrows on San Sal- significance. Journal of Paleontology, 55:431- vador Island. 441. CHAMBERLAIN, C. K. and J. L. BAER. 1973. REFERENCES Ophiomorpha and a new thalassinid burrow from the Permian of Utah. Brigham Young Univer- ALLEN, E. A. and H. A. CURRAN. 1974. Biogenic sity, Geology Studies, 20:79-94. sedimentary structures produced by crabs in la- CHRISTY, J. H. 1983. Female choice in the re- goon margin and salt marsh environments near Beaufort, North Carolina. Journal of Sedimen- source-defense mating system of the sand fiddler tary Petrology, 44:538-548. crab, Uca pugilator. Behavioral Ecology and So- ALPERT, S. P. 1974. Systematic review of the ciobiology, 12:169-180. genus Skolithos. Journal of Paleontology, 48: CRANE, J. 1975. Fiddler Crabs of the World. 661-669. Ocypodidae: Genus Uca. Princeton University BARNES, R. D. 1980. Invertebrate Zoology. W. Press, Princeton, 736 p. B. Saunders, Philadelphia, 1089 p. CURRAN, H. A. 1984. Ichnology of Pleistocene BASAN, P. B. and R. W. FREY. 1977. Actual- carbonates on San Salvador, Bahamas. Journal palaeontology and neoichnology of salt marshes of Paleontology, 58:312-321. near Sapelo Island, Georgia, p. 41-70. In T. P. and R. W. FREY. 1977. Pleistocene trace Crimes and J. C. Harper (eds.), Trace Fossils 2. fossils from North Carolina (U.S.A.), and their Geological Journal, Special Issue 9. Holocene analogues, p. 139-162. In T. P. Crimes BELT, E. S., R. W. FREY and J. S. WELCH. 1983. and J. C. Harper (eds.), Trace Fossils 2. Geo- Pleistocene coastal marine and estuarine se- logical Journal, Special Issue 9. quences, Lee Creek Mine, p. 229-263. In C. E. DORJES, J. and G. HERTWECK. 1975. Recent bio- Ray (ed.), Geology and Paleontology of the Lee coenoses and ichnocoenoses in shallow-water Creek Mine, North Carolina, I. Smithsonian marine environments, p. 459-491. In R. W. Contributions to Paleobiology, 53. Frey (ed.), The Study of Trace Fossils. Springer- BLISS, D. E. and others. 1978. Behavior and Verlag, New York. growth of the land crab Gecarcinus lateralis DWORSCHAK, P. C. 1983. The biology of Upo- (Freminville) in southern Florida. Bulletin of gebia pusilla (Petagna) (Decapoda, Thalassini- the American Museum of Natural History, dea). I. The burrows. Marine Ecology, 4:19-43. 160(2): 114-151. EDWARDS, J. M. and R. W. FREY. 1977. Substrate BOWN, T. M. 1982. Ichnofossils and rhizoliths characteristics within a Holocene salt marsh, of the nearshore fluvial Jebel Qatrani Formation Sapelo Island, Georgia. Senckenbergiana Ma- (Oligocene), Fayum Province, Egypt. Palaeo- ritima, 9:215-259. geography, Palaeoclimatology, Palaeoecology, EHRENBERG, K. 1944. Erganzende Bemerkungen 40:255-309. zu den seinerzeit aus dem Miozan von Burg-

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