Taphonomy and preservation of burrowing thalassinidean shrimps Author(s) :Gale A. Bishop and Austin B. Williams Source: Proceedings of the Biological Society of Washington, 118(1):218-236. 2005. Published By: Biological Society of Washington DOI: http://dx.doi.org/10.2988/0006-324X(2005)118[218:TAPOBT]2.0.CO;2 URL: http://www.bioone.org/doi/full/10.2988/0006-324X%282005%29118%5B218%3ATAPOBT %5D2.0.CO%3B2 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 118(1):218–236. 2005. Taphonomy and preservation of burrowing thalassinidean shrimps Gale A. Bishop and Austin B. Williams (GAB) Museum of Geology and Paleontology, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, South Dakota 57701 U.S.A. and Department of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460 U.S.A. e-mail: Gale.Bishop@sdsmt.edu (ABW) National Marine Fisheries Laboratory (Systematics), National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560 U.S.A. deceased 27 October 1999 Abstract.—Thalassinidean shrimp constitute six or seven families of highly specialized burrowing decapod crustaceans. Adaptations to a deep infaunal mode of life, exemplified by the ghost shrimps Callichirus and Callianassa, include an elongate subcylindrical body differentially sclerotized and divided into units functional for life in tubular burrows (i.e., burrowing, locomotion, flexion, pumping, and sealing). Death or molting within burrows may lead to positive biases of preservation of the more heavily sclerotized parts consisting of burrow buttons preserving chelipeds, anterior cephalothoracic regions, posterior abdomen, and walking legs. Thalassinidean chelipeds, particularly the fingers, are among the most common of decapod crustacean body fossils. Illustrations and examples are given to document the full range of fossil remains, including body fossils and trace fossils. An exceptional whole-body thalassinid, Axiopsis eximia, was described by Kensley and Williams (1990) from the Middle Eocene of South Carolina. The specimen is a silicified body fossil preserving virtually the entire animal except distal appendages. This specimen remains the only described silicified thalassinid. This paper is presented as a tribute to Brian Kensley and as closure for the Bishop-Williams collaborations spanning 25 years. Shrimp of the superfamily Thalassinoidea comprise several groups of decapod crustaceans with a significant fossil records that indicate ancient adaptation to a fossorial existence within tubular burrows in various marine substrates. In the context of this study, the classification of these burrowing shrimp is much less important that their functional role within ancient and modern ecosystems. Specializations in development, behavior, physiology, and morphology are well exemplified by species of the ghost shrimp genera Callichirus and Callianassa, which are abundant in many modern marine habitats (Rodrigues et al. 1986) from mid-tide levels (Bishop & Bishop 1992) to the outer continental shelf (Manning & Felder 1986). The potential for fossilization may be investigated in modern species of these genera as well as documenting the resulting fossils, for though the integument is often weakly calcified, even soft to the touch, it is functionally strengthened where necessary, sometimes strongly so in the chelipeds, anterior carapace, and posterior abdominal somites. A robust but scattered literature discusses the fossilization (e.g., Rodrigues 1983; Rodrigues & Suguio 1984) of this geologically old group, and our purpose here is to selectively consolidate and synthesize this information. We describe the general length of the VOLUME 118, NUMBER 1 geological record, the kinds of evidence constituting it, and infer the conditions under which fossilization occurs by deduction from parallels observed in the natural history of selected living species. The group as recognized here is composed of six or seven families, the lobster shrimps or Axiidae, Axianassidae of questionable status, ghost shrimps or Callianassidae, Callianideidae, Laomediidae, mud lobsters or Thalassinidae, and mud shrimps or Upogebiidae. Terms and Definitions The following concepts are used in this paper: Burrow button—A disk or button-like portion of sediment that formed around, and is preserved because of, the presence of portions of a fossil decapod crustacean. Disassociation unit—A natural aggregation of exoskeleton elements that are commonly preserved together, often chelipeds, abdomens, or cephalothoraxes. Whole body fossil—A fossil representing essentially a complete preserved decapod crustacean; however, some parts may be missing because of their thinness or fragility. Trace fossil—Markings left in the sediment by activities of an ancient organism, such as burrows, borings, or resting marks. Methods The morphology of living and fossil thalassinoids has been studied and documented in the field, by examining collections of preserved recent materials in the collections of the Department of Crustacea at the National Museum of Natural History, and by examination of fossil materials in many paleontologic collections, especially the Gale A. Bishop Decapod Collection now reposited in the Museum of Geology at South Dakota School of Mines and Technology. Documentation from the authors’ notes, sketches, and photographs of the preserved portions of thalassinoids has been supple- 219 mented by observations recorded in the extensive literature on the group. Live specimens of many ghost shrimp were observed in the field (Bishop & Bishop 1992) and documented in notebooks and photographically as opportunity allowed (Fig. 1). Daily activities of Callichirus major Say, 1818 were documented in an aquarium (GAB) by recording observations on videotape and film (Fig. 1h). Extensive photographic documentation made to document the systematics of thalassinoids described from the Bishop Collection has been reinterpreted to function as a primary record of the preservational process involved in the taphonomy of the thalassinoids. The burrows excavated by thalassinoids into the substrate (Fig. 1b, e, f, g) function as structural components of their skeletal support, provide a unique microenvironment, and act as their shelter, but also strongly bias their preservation potential and their known fossil record. This unique mode of life necessitates the enhancement of earlier concepts of decapod taphonomy (Bishop 1986, Bishop & Williams 1986). Functional Morphology The body plan of Callichirus major, functionally representative of many thalassinoids, can be subdivded into burrowing, locomotion, flexion, pumping, and sealing units (Fig. 1a). This breakdown is admittedly artificial because many functions are shared among units (e.g., burrowing is shared by chelipeds, walking legs, and mouthparts, which also perform other functions such as food gathering, eating, grooming, etc.). The units are differentially sclerotized. Hardened parts are restricted to the cephalothoracic region anterodorsal to the cervical groove; to abdominal somites 3, 4, and 5, bearing pleopods greatly broadened as flap-like diaphragm-shaped pumping appendages that can also move sand during burrowing (Pohl 1946:75); to abdominal segment 5 with its uropods, and the telson, bearing thick fringes of setae that act as 220 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 1. The body and sedimentary structures of Recent ghost shrimp (Callichirus major) on St. Catherines Island, Georgia. a, Male C. major removed from its burrow by a yabby bait pump; b, Male C. major found dead on the beach apparently after leaving its burrow (arrow); c, Claw variation seen within C. major, female minor (left) and major claws (right) top, male minor claw (second row) and left and right major claws (bottom); d, Cheliped disassociation units and cheliped fragments of C. major found in the swash-line on the beach; e, Burrow of C. major with expelled rod-shaped fecal pellets; f, Washed in burrow mouths of burrow systems of VOLUME 118, NUMBER 1 seals; to appendages such as eyestalks, mandibles and exposed elements of mouthparts, large chelipeds (Fig. 1d); and to the thoracic sternum with its walking legs. Gill covers of the cephalothorax are exceedingly flexible and easily deformed, even to the point of rolling back on themselves during capture, and the first two abdominal somites are soft to the touch, allowing the elongate body to turn around ‘‘. . . head over heels resembling a tractor tread in motion. . . .’’ (Pohl 1946; GAB, pers. observ.) within tubular burrows (Fig. 1h). Offsetting the differential scleritization of callianassids, however, is the much more firmly sclerotized exoskeleton of the Axiidae which resembles that of freshwater crayfishes or clawed lobsters. Although thalassinoids exhibit considerable diversity in this respect, the chelipeds of all species are well mineralized and can be used for defense. A pinch from the major claw of a male C. major taken from its burrow can draw blood from fingers of the collector (GAB). In many fossorial thalassinoids, the burrow functionally becomes part of the ‘‘skeletal’’ structure of the organism (Fig. 1g), allowing the integument to be reduced because the burrow walls replace many of its functions. Thalassinoids and Their Substrates Thalassinoid shrimps have far more profound influences on benthic environments than their cryptic infaunal existence might imply, circumstances that in turn have bearing on their fossilization. Pohl (1946) reported population densities of the Carolinian ghost shrimps, Callichirus [5Callianassa] major, of 0.44/m2 or 1780 animals/acre. Pryor (1975) reported that 100 ghost shrimps were flushed out of an area with 10 burrow mouths. Frey et al. (1978:217) re- 221 ported washing more than 100 animals to the surface from an area with 8 exposed burrow mouths and that burrow density increases seaward (see their fig. 11). Individual density thus seems to be correlated with burrow density, which in turn varies along the strand and especially across the beach. Pohl (1946:73) found the number of burrows to be relatively constant across the beach except from ½ to ¾ tide levels. Pryor (1975) reported 2–4 burrow openings/m2 in wave-dominated foreshores, 20/m2in 10 m of water, and a maximum of 450/m2 in protected tidal pools and shore zones of Mississippi Sound (these may have been burrows of Biffarius biformis (Biffar 1971) which is also present in great densities on protected tidal sand flats of the Georgia Coast). Bishop & Bishop (1992) investigated the distribution of ghost shrimp burrows across North Beach, St. Catherines Island, Georgia, and determined that the burrow openings of the Carolinian Ghost Shrimp (C. major) form a continuous burrow strand paralleling the mid-tide line with variable burrow mouth densities ranging from 0 to 15 burrows per square meter. Subsequent data taken on South Beach, St. Catherines Island show a similar pattern but with higher maximum burrow densities of 40 burrows per square meter. Burrows of the Georgian Ghost Shrimp B. biformis occupy a similar position of the beach but are restricted to sand tidal flats on sound margins, oceanic inlets, and in ephemeral runnel pools along the beach. The density of burrows of this smaller ghost shrimp tends to be much higher, reaching a maximum density of 483 burrows per square meter on the south margin of St. Catherines Sound (Bishop & Bishop 1992:14). As thalassinoids burrow, they turn over the sediment. MacGinitie (1934), for ex- ← C. major; g, Excavated transverse cross sections of filled (left) and open (right) burrows of C. major showing burrow lining of fecal mud at 20 cm below beach surface; h, Burrowing C. major in aquarium showing carapace and abdomen of animal in a burrow turn-around. Scales as imaged. 222 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON ample, determined that thalassinoid bioturbation turned over a beach in California to a depth of 75 cm in 240 days, with most activity concentrated in the upper 45 to 50 cm. Such activity must destroy older sedimentary structures while forming new ones or form overprinted burrow systems as was observed by Howard and Scott (1983). Suchanek (1983) found that Callianassa species in shallow lagoonal sea grass communities at St. Croix, W.I., funnel massive quantities of sediment (up to 2.59 kg/m2/ day) into subsurface galleries, gleaned for organic material and sorted. Colin et al. (1986) discussed the dominant role of ghost shrimps in bioturbation and water pumping at Eniwetok, Marshall Islands, along with a review of such recorded activities. Branch and Day (1984) and Mukia and Koike (1984) reviewed this subject for estuarine areas of South Africa and Japan, respectively. One of the most characteristic indicators of the occupancy of a burrow by a callianassid is the recurrent expulsion and surface accumulation of fecal pellets (Fig. 1e). Several estimates of fecal pellet production have been made; Pohl (1946) counted 10 to 100 pellets per expulsion. Pryor (1975) calculated that an average of 2480 pellets/burrow/day are produced by callianassids. Each pellet weighed approximately 1.4 mg (dry), consisting of 90–98% clay and siltsized quartz and up to 3.68% organic carbon. Frankenberg et al. (1967) calculated that 456 6 118 pellets/burrow/day could be produced by C. major. The coherent pellets are easily moved by the swash of breaking waves and flood tides, accumulating as crescent-shaped strings marking the shoreward reach of each breaking wave. The pellets represent concentrations of mud removed from suspension in the water column by the ghost shrimp and changed into the hydrodynamic equivalent of sand-sized grains, allowing mud-sized sediment to accumulate in sandsized hydrologic regimes along foreshore and near-shore areas as mud lenses trapped in ripples, runnels, or discrete mud layers of up to 30 cm thickness in an environment normally too high energy for the sedimentation of mud. Calculations of Pryor (1975) show that a density of 10 burrows/m on Sapelo Island, Georgia, would produce 12.3 metric tons of fecal mud/year, enough to cover that area to a depth of 4.5 mm in that time. In the Mississippi Sound the density is much greater, 500 burrows/m2, and the fecal mud production much greater, reaching 618 metric tons/km2/year, or enough to cover that area with 141 mm of mud/year. Passage of clay through the digestive system of Callianassa or Callichirus modifies its mineralogy, destroying chlorite, partly destroying mixed-layer clays, and partly disordering kaolinite and illite. Upon settling, the fecal pellets slowly lose their coherent shape and become indistinguishable as pellets. The high organic content of these muds produces a micro-reducing environment that may result in the formation of glauconite (Pryor 1975:1244). Habitats occupied by thalassinoids are, however, highly variable. Lobster shrimps such as Axiopsis serratifrons (A. Milne Edwards, 1873) and Coralaxius abelei Kensley and Gore, 1980 occur in coral and coral rubble. Callianassids tend to occupy sandier habitats, as on the beaches of the Atlantic coasts where they are one of the most abundant beach dwellers (Rodrigues & Shimizu 1986). The mud shrimp, Upogebia, tends to occupy muddy substrates in marshes and lower salinity areas (Williams 1986), although some species of this group burrow in stony corals (Kleemann 1984) or sponges (Williams 1987). The Australian mud lobster, Thalassina anomala (Herbst, 1804), occupies muddy lithotopes in and adjacent to mangrove swamps. Rabalais et al. (1981) correlated several genera and species with sediment types varying from coarse to fine in a variety of depths to a maximum of 134 m in the northern Gulf of Mexico. Salinities across this array of environments range from those of hypo- or VOLUME 118, NUMBER 1 223 Fig. 2. Summary of taphonomy of thalassinoid fossils from living animal in its burrow (left) into fossils consisting of burrows, fecal pellets, and body fossils of the organism as disassociation units preserved in ‘‘burrow buttons,’’ resulting in the diverse outlined modes of preservation (right). hypersaline estuaries to that of normal seawater. Taphonomy of Thalassinoid Decapod Crustaceans Taphonomic processes operative in the preservation of decapod crustaceans have been investigated by Bishop (1986, 1987) and summarized by Bishop (1986) and Bishop and Williams (1986). For most marine decapods, these processes involve conditions of the remains at time of burial, decomposition, breakup into disassociated units (including carapace, sternum, abdomen, and appendages), and disintegration of these units into isolated elements or fragments (Fig. 2). The taphonomic model envisioned for most marine decapods clearly has limited application to fossorial, deep infaunal decapods such as thalassinideans because the latter are not immediately exposed to the ravages of physical and bio- logical forces that affect remains of decapods that live in the water column or the interface between it and the substrate. Because many thalassinoids spend virtually their entire lives within burrows, molting and death are thought to occur there, thus limiting physical forces that might lead to their disintegration while at the same time enhancing their chances for preservation. Recently molted C. major, for example, have commonly been collected from burrows on the Georgia coast (GAB), whereas collecting over an interval of four years has yielded only one observation of a live C. major outside its burrow (Fig. 1b) and only eight observations of body parts (all male major chelipeds) outside the animals’ burrows (Fig. 1c). Burial conditions.—Thalassinoid decapods might enter the preservation process as corpses, molts, or disassociation units. Living animals, particularly those occupy- 224 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON ing solitary burrows, might occasionally be buried beneath shifting sediment, but most mortality can also be envisioned as resulting from drastic causes such as voluminous shifting sediments (i.e., storm beds), poisoning by red tides, or severe and prolonged changes in oxygen levels, salinity, etc. Because of the cryptic life of these animals, nothing is known about their disposal of exuviae or, in the case of communal burrows, of corpses. Such remains may be recycled through ingestion by congeners or commensals within the burrow system, and there is the possibility that some species may clean their burrow systems by systematically carrying debris such as exoskeletal fragments and corpses into disposal chambers that subsequently may be closed off. Decomposition.–Plotnick (1986) experimentally investigated decomposition of a caridean shrimp, Pandalus danae Stimpson, l857, finding that fresh corpses shallowly buried in situ are rapidly destroyed by scavengers, bacteriological decay, and infaunal bioturbation, seldom lasting more than one to two weeks. Specimens in oxic and anoxic jar experiments decomposed rapidly within two weeks, and any physical disturbance disentegrated them rapidly after one week. Plotnick concluded that ‘‘. . . physical disturbance of a decaying organism [caridean shrimp] is as or possibly more important than the decay process itself in destroying buried remains of shrimp’’. The rapidity of decomposition of these lightly sclerotized remains in an experimental setting probably is similar to that undergone by thalassinoids, and should be repeated with representatives of the latter, but the results indicate that preservational processes probably occur very rapidly if a fossil shrimp is to be produced. Disassociation.–As thalassinoid shrimps decompose, they can be expected to disintegrate into disassociation units comprised of the more heavily sclerotized parts of the exoskeleton, especially the large cheliped, but also the other larger legs, anterior cephalothoracic region, and posterior part of the abdomen (Fig. 2). Heterocheley and sexual dimorphism of some species might favor preservation of the major chelipeds of males. Few observations of disassociated thalassinoid units have been reported in the literature. However, one of us (GAB) observed major chelipeds of male C. major collected by Eric Bishop in the swash zone of Sapelo Island, Georgia, at low tide 12 April 1987. Both of us (GAB and ECB) observed major chelipeds of male C. major in the swash on St. Catherine’s Island at low tide on 12 May 1989. These were not molts, for none had the ischial molting suture open. These are the only evidences of Recent thalassinoids seen by GAB outside their burrows and confirm that chelipeds do form a coherent disassociation unit. Diagenesis and lithification.–The generally fragile exoskeleton of thalassinoids suggests that they seldom might be preserved as fossils, a conclusion that is contradicted by the robustness of their fossil record. The protective substrate apparently works as a strong promoter of fossilization is a positive taphonomic feedback system. Fossil Thalassinoids The geologic record of thalassinoid shrimps consists of body fossils and trace fossils (Fig. 1). Glaessner (1969:R477) cited a body fossil record back to the Early Jurassic of Germany, from which Magila was described, and recorded Etallonia, Magila, Protaxius, and Upogebia from the Late Jurassic. Förster (1977) described Magila dura bicrista from the Upper Jurassic (Oxfordian) of Poland. Trace fossil burrows, usually said to be constructed by thalassinoids, named Thalassinoides and Ophiomorpha, are known from rocks as old as Jurassic and perhaps even the Upper Ordovician (Sheehan & Schiefelbein 1984), but those earliest records are not directly attributable to thalassinoids (Frey & Mayow 1971) and may represent burrows of glypheoid lobsters (Förster 1977), or unknown organisms in the earliest case. VOLUME 118, NUMBER 1 The spectrum of expected remains of thalassinoids is presented (Fig. 2) as a guide to the types of fossils observed and to be expected in the fossil record. The types of occurrences of living ghost shrimp (Fig. 2) and recently dead, virtually whole mud lobsters (Fig. 3) are then summarized and documented as more typical trace fossils (Fig. 4) and body fossils (Fig. 5). Thalassinoids are among the most common of fossil decapods. Body fossils, particularly in fine-grained clastic and carbonate lithosomes, are frequently abundant. Trace fossils attributable to thalassinoids normally include both abundant burrows and currently understudied fecal pellets. Collections from many fossil localities contain abundant thalassinoids. For example, in the Early Cretaceous Glen Rose Formation of Texas, thalassinoids constitute 18% of the decapod taxa but 91% of the specimens (Bishop 1983b). Thalassinoid remains are often predominantly claws (Bishop 1981), but some partial-body and wholebody fossils are known. Vernon (1951) stated that ‘‘Some portions of the Inglis member (Upper Eocene, Florida) are characterized by abundant fragments of mud shrimp, Callianassa sp. [C. inglisestris Roberts, 1952], claws of which are so common and prominent that this particular facies was called ‘the shrimp claw limestone’ in the process of mapping.’’ Partial body fossils are known in the North American Cretaceous deposits such as the Coon Creek Formation of northern Mississippi, where claws and partialbody fossils constitute 9% of known taxa but 27% of the specimens (Bishop 1983a; Bishop, in press) and the bulk of the fauna in the Gammon Ferruginous Member of the Pierre Shale at the Heart Tail Ranch Locality (Bishop 1985). Whole-body thalassinoid fossils are occasionally found in fine clastic rocks at the surface (Kensley & Williams 1990) or in well borings (e.g., Rathbun 1935, pl. 7, figs. 1–5) as compressed fossils. Well known Australian localities have been commercially exploited sources of fossil whole-body mud lobsters, T. anomala, for many years (Eth- 225 eridge & McCulloch 1916, Murray & Hanley 1986). Thalassinoid burrows are complex tubular structures, extending as much as one to five meters into the substrate, often with branches and turn-around chambers. Forms of Recent callianassid burrows have been determined by filling them with fiberglass resin before excavation and study of their shapes (Shinn 1968), as have those of Thalassina (Förster & Barthel 1978). Weimer & Hoyt (1964) demonstrated the analogy of Ophiomorpha to recent Callianassa burrows in their often cited Georgia work, identifying Ophiomorpha as a marine sand beach ‘‘index’’ trace fossil. Frey et al. (1978) established that Callianassa construct burrows of different morphologies in different substrates after recruitment to the bottom as larvae (Frey & Howard 1975) and are analogous to trace fossil ichnogenera such as Ardelia, Gyrolithes, Ophiomorpha, Teichichnus, and Thalassinoides; see also Ekdale et al. (1984) for a general explanation of trace fossils. Wall morphology of fossil thalassinoid burrows is dependent upon physical characteristics of substrates, i.e., mammallation is common in sandstones, whereas burrows are smooth in mudstones (Ager & Wallace 1970, Kennedy & Sellwood 1970, Kern & Warme 1974, Frey et al. 1978). The burrows of many species are lined with mucus or fecal pellets pushed into the substrate. The direct occurrence of fossil thalassinoids within trace fossil burrows is unusual but occasionally has been reported or figured (Mertin 1941, Shinn 1968, Waage 1968, Beikirch & Feldmann 1980). Murray and Hanley (1986) documented finely preserved T. anomala from northern Australia are molts of mud lobsters preserved in Satltersche Position in molting chambers within the their burrows, lithified within the last 5000 years. The feces of callianassids are brownish to slate gray, rod-shaped striated cylinders that range from about 0.75–1 mm diameter, 2–5 mm long to 3–4 mm diameter, and 10 mm long (Shinn 1968, Pryor 1975). Rod- 226 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON shaped, striated fecal pellets attributable to thalassinoids are known from the fossil record as old as the Cretaceous (Bishop 1977: Figs. 6C, 7L; Bishop 1987: Fig. 4I). Thalassinoid burrow trace fossils.–Fossil burrows of thalassinoids exhibit a broad range in structure which depends upon such factors as morphology and behavior of the burrow maker, characteristics of the substrate being burrowed, and diagenetic history (Ekdale et al. 1984). There is no clear way to link ancient thalassinoid taxa with specific burrow forms unless the animal remains are preserved within. Burrows of thalassinoids have morphologies that overlap and have been assigned to various ichnogenera including Ardelia, Gyrolithes, Teichichnus, and Ophiomorpha (Frey et al. 1978). Bromley and Frey (1974) discussed the interrelationships and morphologic variation of some of these ichnogenera, and Spongeliomorpha, concluding that burrows of these types can be constructed by many crustaceans (including thalassinoids, brachyurans, and stomatopods) and that the morphology of a single burrow may cross ichnotaxonomic boundaries. Body fossils of thalassinoids are sometimes obviously preserved within burrows or burrow segments, and the name ‘‘burrow button’’ is herein coined to indicate body fossils preserved as parts of burrows in the form of button-like disks (see Ehrenberg 1938; Mertin 1941; Glaessner 1947, 1969: fig. 243.1; Waage 1968, pl. 8c; Shinn 1968, pl. 111, fig. 3; Murray & Hanley 1986). The close association of burrows with thalassinoid body fossils has been cited as evidence for fossil burrows, Ophiomorpha, being produced by Callianassa (Pickett et al. 1971). The preservation of major and minor thalassinoid chelipeds in proximity (Rasmussen 1971, figs. 11, 14, 17; Labadie & Palmer 1996) may be taken as evidence for the remains being burrow fossils, because the chelipeds would almost certainly have been disintegrated by all but the most passive external physical conditions if not protected in a burrow. It follows from this reasoning that many thalassinoid fossils actually may be burrow fossils because of their burrowing habits. Abundant burrow fossils have been collected from the Cretaceous of the Northern Atlantic Coastal Plain in the Merchantville Formation of the C&D Canal, and the Cretaceous of Sounding Creek, Alberta, that preserve cheliped pairs preserved in close proximity and parallel to one another as observed by GAB. These paired chelipeds consist of major and minor chelipeds with virtually all segments preserved. These burrow fossils are preserved as small, phosphatic concretions associated with larger sandstone concretions preserving burrow cross sections and soft-sediment deformation structures. Articulated paired chelipeds of Protocallianassa are found in abundance in the Merchantville Formation of Delaware preserved in siderite concretions. The most elegant of the known wholebody thalassinoids occur in the Pleistocene or Holocene deposits of northern Australia (Etheridge & McCulloch 1916, Bennett, 1968, Murray & Hanley 1986) and the Miocene of southwestern Japan (Karasawa & Nishikawa 1991). Deposits containing these exquisite fossils, preserved as calcareous concretions formed about T. anomala buried in mud, occur in many localities. Specimens are found washing out of mangrove swamps, often in proximity to living populations of this species in nearby muddy substrates. Murray and Hanley (1986) were convinced that some if not all of these fossils formed during the last 7000 years and → Fig. 3. Traces and trace fossils of thalassinoids. a, Hypothetical burrow systems showing habitat partitioning between C. major (deep burrows), Biffarius biformis (shallow burrows), and alphaeids (bifurcating burrows in mud) (total vertical scale 5 m). Fossils of thalassinoids include: b, Striated, channeled, rod-shaped fecal pellets VOLUME 118, NUMBER 1 227 preserved as part of a phosphatic concretion, Cretaceous Piuerre Shale, Corson County, SD (GAB 4-2010); c, Claw of Protocallianassa sp. preserved in a sand-filled burrow, Eocene, Twiggs Claystone, Houston Co., GA (GAB 34-18); d and i, Ophiomorpha sp., smooth form, from the Coon Creek Formation, Cretaceous, Union Co., MS; e–f, Fecal pellet chamber fills of Upogebia racheochir; f associated with the crab Notopocorystes dichrous, GAB 7308), Eagle Ford Formation, Garza-Little Elm Reservoir, TX (GAB 5051); g–h, Smooth burrow (Serpula wallacensis or Indian Bead) attributable to Protocallianassa russelli (GAB 4-0386), Pierre Shale, Mobridge, SD. (b–c 32.0; d–i 31.0.) 228 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 4. Body fossils of thalassinoids are represented by cepahlic disks, chelipeds, and claws. Whole-body fossil thalassinoids are preserved three dimensionally in nodules (see Fig. 5), three dimensionally in claystones (see Fig. 6), and as compressions in shale. a, Burrow button preserving Protocallianossa mortoni (GAB 37260), Coon Creek Formation, Union Co., MS; b–c, Burrow button preserving (b) the cephalothorax and abdomen (TX1QD-Inv-5) and (c) abdomen of Upogebia rhacheochir (GAB 9108151), Eagle Ford Formation, Denton Co., TX; d, Burrow button preserving chelae and cephalothorax of Protocallianassa mortoni, Coon Creek Formation, Union Co., MS; e–h, Burrow buttons preserving cheliped pairs of Protocallianassa mortoni, Coon Creek Formation, Union Co., MS (GAB 37-208); i, j, and l, Cheliped disassociation units of (i and l) Proto- VOLUME 118, NUMBER 1 that their formation may be an ongoing process. The consistent mode of preservation, with the central part of the carapace pushed inward and sometimes flipped over, the claws spread anteriorly, the branchiostegites spread laterally, and the abdomen tightly tucked beneath the sternum, was interpreted as evidence that these remains represent exuviae fossilized in burrows after molting. Otherwise, whole-body remains of thalassinoids are rare. Rathbun (1926: pl. 30, figs. 1–2) described and figured compressed whole-body Upogebia eocenica Rathbun, 1926 from the Eocene of Lewis County, Washington, and (Rathbun 1935: pl. 7, figs. 1–5) Callianassa aquilae Rathbun, 1935, from the Late Cretaceous Eagle Ford formation of Texas from both surface outcrops and well cores. Stenzel (1945) described and figured a whole-body Upogebia rhacheochir Stenzel, 1945 from the Late Cretaceous Britton Formation of the Eagle Ford Group near Dallas Texas. Glaessner (1956, 1969) reproduced figures of several wholebody thalassinoids (Upper Jurassic Protaxius, and Upper Cretaceous-Eocene Protocallianassa) and reconstructions of wholebody remains. Karasawa (1989) figured several thalassinoids preserved as whole body fossils (see Karasawa 1989: pl. 1, figs. 9–10). An exceptional whole-body thalassinoid, Axiopsis eximia Kensley & Williams, 1990, was described from the middle Eocene of South Carolina. The unique specimen (Fig. 6) is preserved as a silicified body fossil that preserves virtually the entire animal except distal segments of the appendages. This remarkable specimen is the only silicified thalassinoid known to this author that has been described. Its mode of preservation indicates that future collecting tech- 229 niques should include acid digestion of appropriate partially silicified limestones to recover decapods preserved in this manner. Other localities have yielded less complete whole-body fossils. The Coon Creek Formation (Ripley Formation) contains abundant body parts preserved in articulated positions (Bishop in press) characteristically somewhat squashed by compaction in sandy mudstones (Fig. 3). The Britton Formation of the Eagle Ford Group is known to produce whole-body thalassinoids from several localities preserved as compressions, including Rathbun’s (1935: pl. 7, figs. 1–2) Grayson County specimen and several specimens found in drill cores (Rathbun 1935: pl. 7, figs. 3–5), or in concretions, from Stenzel’s California Crossing locality (1945: pl. 42, figs. 1–3), the Texcrete rock quarry, Hill locality, and GarzaLittle-Elm reservoir (all in Fig. 3); see Bishop et al. (1992). Several parts of whole-body thalassinoids were described from chalk of the Pflugerville Formation near Austin, Texas (Belkirch and Feldmann 1980: fig. 8a–cc), and Roberts (1962: pl. 84) figured a whole-body thalassinoid from New Jersey, but the quality of the illustration masks nearly all detail. Exceedingly abundant disassociated chelipeds (see Fig. 4) have led several authors to conclude that ‘‘crab’’ claws constitute the majority of the decapod crustacean fossil record. Disassociated chelipeds (Fig. 4) are sometimes preserved whole (Rathbun 1926: pl. 81, fig. 8, Via Boada 1969: pl. 11, fig. 16, pl. 2, fig. 1; Karasawa 1989: pl. 2, fig. 6) but more commonly are preserved as partial disassociation units because the long thin ischium and merus are easily broken from the stouter spatulate carpus and propodus during fossilization or collection, or ← callianassa mortoni (GAB 37-162), Union Co., MS and j, Protocallianassa russelli (GAB 36-360), Heart Tail Ranch Locality, Butte Co., SD; k, m, and n, Propodal disassociation units consisting of right major propodi of (k) Protocallianassa inglisestris, Twiggs Clay, Houston Co,, GA (GAB 34-17), (m) P. russelli (GAB 14-33), Pierre Shale, Pennington Co., SD, and (n) P. russelli (GAB 36-137), Heart Tail Ranch Locality, Butte Co., SD. (a, c–h, l 31.0; i, m, n 31.5; b, j, k 32.0.) 230 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 5. Thalassina anomala (Herbst, 1804), from Australia, a whole-body fossil thalassinoid preserved in a calcareous nodule or concretion. a, Dorsal aspect showing splayed thoracic walls and push-in cephalic disk; b, Ventral aspect showing enrolled abdomen, and c, Left lateral aspect showing alignment of chelipeds, thorax, and enrolled abdomen. From Holocene Quaternary mangrove swamp mud, shore of Gulf of Carpentaria, northern Australia (GAB specimen). Scale 5 1.03. VOLUME 118, NUMBER 1 231 Fig. 6. Photograph of a, dorsal, b, ventral, and c, left lateral aspect of silicified specimen of Axiopsis exima Kensley and Williams, 1990 (USNM 219431) described from the Middle Eocene lower Warley Hill Marl, Sumter County, South Carolina. Scales are 23 and 13. 232 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON may be hidden and not obvious unless prepared out of the matrix (Roberts 1962: pl. 83, figs. 11–12; Stenzel 1945: pl. 42, figs. 7–10, Föster 1979: figs. 1–2; Bishop 1983b: fig. 3d, 1985: fig. 6.3; Karasawa 1989: pl. 1, fig. 7). Dissociated abdomen units are scarcer than cheliped units (Rathbun 1935: pl. 16, figs. 1–2, Stenzel 1945: pl. 42, figs. 4–6; Bishop et al. 1992: fig. 7B). The record may be meager because the loosely articulated abdomen fragments are more easily scattered than are the more firmly articulated chelipeds. The lack of diagnostic characters on the abdomen leads to difficulty in identification and a reluctance by most paleontologists to describe and name such fragments leading to a condition of under-reporting in the literature. One of the few records of a disassociated thalassinoid anterior cephalothoracic region is Robert’s (1962: p. 173, pl. 84, figs. 1–3) description of Protocallianassa cliffwoodensis Roberts, 1962. The poor record of preservation for this disassociation unit is understandable because such fragments would be set aside as unidentifiable by virtually all paleontologists. Other isolated thalassinoid units or fragments of them are virtually impossible to identify, except for elements of the chelipeds, especially the palms and fingers. The bulk of the paleontologic record for thalassinoids is represented by isolated claw parts (Rathbun 1926: pls. 25–28, 1935: pl. 15; Bishop 1985: fig. 6.2, 6.4; Via Boada l969: pls. 1–2) upon which most species level taxa are based. The practice of naming such fragments, although necessary in many cases, is fraught with error, for as Rathbun (1935, p. 29) pointed out for just one genus, ‘‘in identifying chelae of Callianassa it must be taken into consideration that the major and minor chela of the same specimen may differ not only in size but in form and ornamentation; that those of the female differ from those of the male and [those of] the old from the young. The wide distribution of a species also promotes di- versity of form. An extensive series of specimens is needed to determine the composition of a species in this genus. . . .’’ Claw fragments of Ctenocheles Kishinouye, 1926 with its elongate finely toothed fingers are extremely fragile and difficult to preserve intact. Such claws could be confused with those of Oncopareia Bosquet, 1854 (Glaessner 1969: p. R478) and may have been described under the name of Ischnodactylus Pelseneer, 1886. Summary Thasassinoid shrimp living in many Recent benthic marine habitats are important bioturbators during feeding and often deep infaunal burrowing activities. Detritus feeding species are responsible for sedimentation of normally suspended marine clay through its accretion in fecal pellets deposited in environments characterized by sandsized sediment. Abundance and the burrowing mode of life in many habitats have led to an impressive fossil record for the group. Body fossils are known from the Early? Jurassic, becoming diverse and abundant by the Cretaceous, and consequently comprise significant proportions of known Cretaceous and Tertiary marine benthic faunas. Trace fossils attributable to the thalassinoid shrimps and similar infaunal organisms include several ichnogenera of burrows that are first recorded from the Late Paleozoic or Early Mesozoic, become abundant by Early Cretaceous, and continue in abundance through Cenozoic marine sediments. Fecal pellets of thalassinoids are present in the fossil record but are not often assigned to the group. The accumulation of Recent blankets and lenses of fecal muds would indicate that fossil mud beds in sand lithosomes might have a similar origin. Fossil thalassinoid fecal pellets are known from the Late Cretaceous but can be expected in much older rocks. The record of thalassinoid body fossils consists of a few whole-body, some partialbody, and an abundance of disassociated re- VOLUME 118, NUMBER 1 mains. Differential mineralization of the thalassinoid body leads to disassociation of the chelipeds, walking legs, cephalothoracic disk, and abdomen. Each of these units may disintegrate into unrecognizable fragments, although parts of the claws often constitute the most significant portion of the record. Near confinement of the record to this disassociation unit leads to a widely recognized taxonomic difficulty stemming from interspecies similarities, within-species heterocheley, sexual dimorphism, and ontogenetic change. It will be necessary to document the range of these variations in many Recent thalassinoid taxa if the fossil record of these structures is ever to be understood. Fossils attributable to the thalassinoid shrimps and similar infaunal organisms include several ichnogenera of burrows which are first recorded from the Late Paleozoic or Early Mesozoic, become abundant by Early Cretaceous, and continue in ample measure through Cenozoic marine sediments. Acknowledgments This manuscript was written over a number of years by Bishop and Williams, until Williams’ death in 1999 and Bishop’s retirement in June 1999, when it was despondently put on the shelf as a project that would likely never be finished. The incentive to finish the paper has been provided by this memorial volume to Brian Kensley, another of Austin’s active collaborators, especially because it involved a uniquely preserved fossil thalassinoid, Axiopsis eximia. Because this specimen is seminal in finishing this paper it was rephotographed as Fig. 6 by Thomas R. Waller and Warren C. Blow, assisted by Jo Ann Sanner of the Department of Paleobiology, National Museum of Natural History, Smithsonian Inistitution, Washington, D.C. The plates were constructed by Neal A. Larson of Hill City, SD. The many land owners and land managers are acknowledged for their assistance in granting permission and permits to col- 233 lect so many of these fossorial denizens of beaches and sea bottoms. Literature Cited Ager, D. V., & P. Wallace. 1970. The distribution and significance of trace fossils in the uppermost Jurassic rocks of the Boulonnais, northern France. Pp. 1–18 in T. P. Crimes & J. C. Harper, eds., Trace fossils. 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