Attachment

A wide range of habitats was exploited by Early Cambrian echinoderms, including deep slope (Poleta Formation, California) to shallow shelf detrital facies, and less common shallow carbonate bioherms and associated facies. Based upon functional morphology and taphonomy, we believe, contrary to Smith (1988), that most Early Cambrian echinoderms were attached to firm or hard substrates in life and that the limited availability of these substrates (mostly skeletal fragments) limited the distribution of the echinoderms. The fossils commonly occur in siliciclastic-dominated sequences such as fine-grained siltstones and shales that presumably formed soft substrates. Assuming that the echinoderms were not usually transported into these settings, the only available attachment sites appear to have been skeletal debris, such as trilobite molts and rare brachiopod or hyolith shells.

Specimens associated with attachment sites are rare, and the attachment mechanism in other cases is uncertain, although functional morphology and taphonomy provide important clues. No known Early to Middle Cambrian echinoderms were skeletally attached. The edrioasteroids Stromatocystites and Camptostroma had basal disks that are interpreted to have enabled clinging by suction. There is a system of radiating ridges and plate rings in the loosely plated aboral surface that was capable of being withdrawn upward, forming a partial vacuum (Smith and Jell 1990). Presumably the animals released from attachment sites following death (Guensburg and Sprinkle 1994b). Blastozoans are considered to be the sister group to edrioasteroids (Derstler 1985), and Early Cambrian examples Kinzercystis and Lepidocystis apparently retained attachment disks. Specimens of Lepidocystis are rarely attached to trilobite exoskeletons (Sprinkle 1973: plate 3, figures 1-4). The paleoecology of helicoplac-oids is more problematic. These spindle-shaped echinoderms are most often preserved flattened parallel to bedding, but a few specimens are vertical, with a thecal pole buried in shale. Attachment sites have not been identified.

Attachment structures of Middle Cambrian edrioasteroids and eocrinoids are often modified versions of the basal disk described above (figure 19.1). For the most part, these fossils occur in fine-grained siliciclastic and mixed siliciclastic to carbonate (mi-critic) sequences of the outer detrital belt (Sprinkle 1976). The diverse and widespread eocrinoid Gogia and close relatives were the most common echinoderms during this time. Specimens occasionally occur attached to skeletal fragments (Sprinkle 1973: plate 23, figures 1-6) by a small multiplated button-shaped holdfast at the end

Figure 19.1 Reconstruction of soft-substrate echinoderm community from the Middle Cambrian Burgess Shale (British Columbia, Canada). Community is reconstructed at the base of a carbonate bank in about 150 m of water and includes short- and long-stalked eocrinoids (Gogia, left, and G. radiata, left center), the cri-noid Echmatocrinus (right center), the edrioas-teroid Walcottidiscus (right rear), and tiny mo bile Ctenocystis (left front). Echinoderms, which make up less than 5 percent of the fauna, are shown with other components of the fauna, including trilobites, sponges, Marrella, a hyolith, and the priapulid Ottoia. Front width of block diagram about 0.5 m. Source: Modified from Sprinkle and Guensburg (1997) by James Sprinkle and Jennifer Logothetti.

Figure 19.1 Reconstruction of soft-substrate echinoderm community from the Middle Cambrian Burgess Shale (British Columbia, Canada). Community is reconstructed at the base of a carbonate bank in about 150 m of water and includes short- and long-stalked eocrinoids (Gogia, left, and G. radiata, left center), the cri-noid Echmatocrinus (right center), the edrioas-teroid Walcottidiscus (right rear), and tiny mo bile Ctenocystis (left front). Echinoderms, which make up less than 5 percent of the fauna, are shown with other components of the fauna, including trilobites, sponges, Marrella, a hyolith, and the priapulid Ottoia. Front width of block diagram about 0.5 m. Source: Modified from Sprinkle and Guensburg (1997) by James Sprinkle and Jennifer Logothetti.

of a short-to-long multiplated stalk (figure 19.1). The lower holdfast surfaces are not well known, so it is uncertain whether suction was still used for adherence or if there was actually skeletal cementation to the attachment surface. Lichenoides is a Gogia relative whose thickened plates of the lower theca as an adult possibly anchored the animal. Cymbionites is a problematic Middle Cambrian taxon known by a greatly thickened basal plate that must have enabled anchoring in a similar manner. Edrioas-teroids such as Totiglobus and Edriodiscus had basal disks functionally similar to those of earlier relatives (Guensburg and Sprinkle 1994b). A Totiglobus from southern Idaho is attached to a trilobite free-cheek. The earliest probable crinoid Echmatocrinus occurs attached to worm tubes (figure 19.1), hyoliths, and possible stalks of other Echmatocrinus specimens using a medium-length stalk tipped by a low conical holdfast that appears to have been cemented to the attachment surface (Sprinkle 1973; Sprinkle and Collins 1998).

Late Cambrian echinoderms are poorly known, but based upon skeletal debris, they were locally common in shallow shelf environments of cratonic seas, and echino-derms were among the first metazoans to attach to widespread hardgrounds devel-

Figure 19.2 Reconstruction of hardground and soft-substrate echinoderm communities from the Upper Cambrian Snowy Range Formation (Wyoming, USA). A flat-pebble conglomerate bed is slowly being covered by soft muddy substrate (right), but thicker parts of the bed (left) have become pitted and corroded during a long period of exposure on the shallow sea floor. Two genera of stemmed trache-locrinid eocrinoids (left center), along with many additional holdfasts, a biscuit-shaped edrioasteroid (upper left), a sponge (lower left), and several Billingsella calciate brachio-pods, are attached to this lithified surface, while two cornute stylophorans (right front), a solute homoiostelean (right rear), and a trilo-bite feed in the soft muddy sediment. Front width of block diagram about 0.5 m. Source: Modified from Sprinkle and Guensburg (1997) by James Sprinkle and Jennifer Logothetti.

Figure 19.2 Reconstruction of hardground and soft-substrate echinoderm communities from the Upper Cambrian Snowy Range Formation (Wyoming, USA). A flat-pebble conglomerate bed is slowly being covered by soft muddy substrate (right), but thicker parts of the bed (left) have become pitted and corroded during a long period of exposure on the shallow sea floor. Two genera of stemmed trache-locrinid eocrinoids (left center), along with many additional holdfasts, a biscuit-shaped edrioasteroid (upper left), a sponge (lower left), and several Billingsella calciate brachio-pods, are attached to this lithified surface, while two cornute stylophorans (right front), a solute homoiostelean (right rear), and a trilo-bite feed in the soft muddy sediment. Front width of block diagram about 0.5 m. Source: Modified from Sprinkle and Guensburg (1997) by James Sprinkle and Jennifer Logothetti.

oped on grainstones, intraformational limestones, and cryptalgal biohermal mounds. Some hardground surfaces are encrusted by numerous subconical massive cemented holdfasts (figure 19.2), which, based on association with disklike columnals having trilobate lumens and distinctive thecal plates, we assign to eocrinoids. No Late Cambrian crinoids are known. This is curious because they commonly encrusted Ordovician hardgrounds (Palmer and Palmer 1977; Brett and Brookfield 1984; Guensburg 1992; Guensburg and Sprinkle 1992; Sprinkle and Guensburg 1995). The eocrinoid Ridersia may represent a sister taxon to later rhombiferans (Jell et al. 1985) and has a strongly heteromorphic stem that may indicate a free-living adult life mode. Edrio-asteroids continued to attach with a basal disk, but there were modifications that presumably increased efficiency by adding a well-developed peripheral rim that sealed the thecal margin (Smith and Jell 1990). Undescribed edrioasteroids from the Late Cambrian of Missouri have long conical aboral surfaces that could have been inserted into firm but plastic siliciclastic substrates or attached to skeletal fragments. Early and Middle Ordovician attached echinoderms continued encrusting hardgrounds and other solid surfaces. Eocrinoids, paracrinoids, and crinoids all exploited these surfaces in great numbers. Rootlike and radicular holdfasts among crinoids first appeared during the Middle Ordovician, corresponding to the rapid ecologic radiation of this group.

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