PakoBios 18(4): 12-20, December 15, 1998 © 1998 University of California Museum of Paleontology Secondary tiering among Silurian epibionts in the Waldron Shale, Indiana, USA RODNEY WATKINS1 and PAULA E. McGEE Geology Department, Milwaukee Public Museum, 800 West Wells St., Milwaukee, WI 53233, USA; e-mail: rw@mpm.edu. Epibionts in the Silurian (Wenlockian) Waldron Shale, Indiana, lived as secondary tierers on crinoids, brachiopods and platyceratid gastropods. Bryozoans were the dominant epibionts, followed by cornulitids, small crinoids, corals, spirorbids, and brachiopods. Crinoid holdfasts exposed to encrustation from 0 to 5 mm above the sediment surface have fewer cpibiont species than taller brachiopods (to 2 cm height) and gastropods (to 5 cm height). Upright crinoid columns elevated higher than 5 cm above the bottom hosted encircling coral and bryozoan colonies, as well as other crinoids attached by distal coils of their columns. The platyceratid Naticonema niagarense lived, at least in part, on crinoid crowns at heights of up to 1 m above the bottom and hosted its own encrusting fauna dominated by bryozoans and cornulitids. The bryozoan-cornulitid assemblage on N. niagarense is the same as that on Strophostylus cyclostomatus, a platyceratid which has not been found on Waldron crinoids and was probably a bottom-dweller. The same association of encrusting epibionts occurred through all primary tier levels in the Waldron fauna, and host selectivity among epibiont groups was low. INTRODUCTION Tiering was an important part of the ecologic structure of shallow marine, epifaunal communities of the Paleozoic (Bottjer and Ausich, 1986). During the Silurian, primary tiers in epifaunal communities included a 0 to 2 cm tier dominated by brachiopods, a middle tier of corals, bryozo- ans and crinoids from 2 cm to approximately 10 cm, and an upper tier of crinoids that extended to 1 m above the bottom (Watkins and Hurst, 1977; Watkins, 1991; Taylor and Brett, 1996). This paper examines secondary tiering in this type of community in the Silurian Waldron Shale of Indiana, USA, where epibionts include corals, bryozoans, brachiopods, gastropods, spirorbids, cornulitids, and crinoids (Hall, 1879; Hallcck, 1973; Richards, 1974; Liddell and Brett, 1982; Brett, 1991; Peters and Bork, 1998). GEOLOGICAL SETTING The late Wenlockian Waldron Shale, a mixed carbonate- clastic unit up to 5 m thick (Drostc and Shaver, 1983; Archer and Feldmann, 1986), was deposited on a shallow cratonic shelf between the Michigan and Illinois basins (Fig. 1). Interbedded dolomitic shale and skeletal tempestites in the lower part of the Waldron, which includes the fauna studied here, were deposited between normal and storm wave base (Feldman, 1989). Skeletal material in both shale and tempestites consists of more than 70% crinoids and bryozo- ans, with fewer corals, brachiopods, molluscs and trilobites (Watkins, 1996). MATERIAL AND METHODS The material used in this study is part of the Thomas A. Greene Collection (G) at the University of Wisconsin- Milwaukee and was collected in the 1880s near the town of Waldron, Shelby County, Indiana (Siemann-Gartmann, 1 author for correspondence 1983). More detailed locality data are not available. Lithol- ogy and preservation of specimens are typical of Waldron Shale units A and B of Feldman (1989). A few other specimens in the Milwaukee Public Museum (MPM) were collected from the "Old Blue Ridge Quarry" south of Waldron (locality 10 of Feldman, 1989). Skeletal material examined for epibionts (Table 1) in- cluded crinoid holdfasts (a substrate for encrustation from 0 to 5 mm above the bottom), brachiopods (a substrate from 0 to 2 cm above the bottom), and platyceratid gastropods (a substrate from 0 to 5 cm, extending to decimeters above the bottom for platyceratids that lived on crinoid crowns). Encrusted specimens of crinoids, brachio- pods and gastropods are termed "hosts" in this paper, regardless of whether encrustation is considered to have occurred on living or dead substrates. Counts of the number of cpibiont individuals and colo- nies were made on most, but not all, of the host specimens examined (Table 2). These data tend to exaggerate the importance of small solitary taxa, such as cornulitids, and minimize the importance of larger bryozoan colonies; this paper does not use them in the following discussions of relative abundance of epibionts. Instead, relative abun- dances of epibiont taxa, following Lescinsky (1997), arc given as the percent of encrusted host specimens on which an epibiont taxon occurred (Table 3). Percent of host covered by encrusting epibionts was estimated (Table 1), but no attempt was made to measure areas covered by separate epibiont taxa. ENCRUSTING EPIBIONTS Bryozoans arc the most common encrusters in the Waldron and are represented by at least 16 species (Liddell and Brett, 1982). In this study, bryozoans were identified as five morphologic groups. Sheet trepostomes consist of very thin zooaria composed of a single layer of abutted polygonal WATKINS & McGEE-TlERING OF SILURIAN EPIBIONTS 13 North 90° W 85" W Fig. 1. Location of the Waldron Shale at Waldron, Indiana, USA. Wenlockian paleogeography modified from Droste and Shaver (1983). zooecia that reach 0.5 mm in diameter. Most of these colonies are Palcscbara maculata Hall. Other sheet bryozo- ans include several species and arc dominated by Eistnlipora McCoy and Bcrcnicca Lamouroux. Zoaria of sheet bryozo- ans range from 2 mm to over 5 cm in diameter. Less common discoidal bryozoans from 0.5 mm to 4 mm diameter include several species of ceramoporids (Hall, 1879; Liddcll and Brett, 1982). Attachment bases of ramose bryozoans are another epibiont type that reach 7 mm in diameter. These may have one or more domal to broken cyclindrical projec- tions. Rare ptilodictyid holdfasts are included in this cat- egory. Stoloniferous bryozoans, which may represent ctenostomes, consist of radial clusters of tubular zooecia that are connected by networks of stolons. Colonics extend over linear distances that reach 4 cm. Locally common encrusters include cornulitids and crinoid holdfasts. Individuals of Cornulitcs proprius Hall range from 0.8 mm to 20 mm in length. Encrusting crinoids are represented by two holdfast categories of Brett (1981). Simple discoidal holdfasts range from 0.8 mm to 3.7 mm in diameter, and dendritic radicular holdfasts range from 0.3 mm to 12 mm in diameter. Dendritic holdfasts with a pcntalobate lumen are referable to Encalyptocrinitcs Goldfuss, but most dendritic holdfasts are smaller than 5 mm and are taxonomicallv indeterminate. Other encrusters include a single species of spirorbid that ranges from 0.7 mm to 2.5 mm in diameter. Encrust- ing favositids from 1.5 mm to 11 mm in diameter represent colonics that died as juveniles; larger favositids in the Waldron overgrew their hosts and rested directly on the sediment (Fig. 2A). Rare craniaccan brachiopods and soli- tary rugose corals are less than 10 mm in diameter. ENCRUSTERS ON CRINOID HOLDFASTS, BRACHIOPODS, AND GASTROPODS Crinoid holdfasts Crinoid holdfasts in growth position within the Waldron were described by Hall (1879) and Siemann-Gartmann (1983). The holdfasts occur in dolomitic shale and on the tops of thin, normally-graded beds of storm deposited wackestone and packstone. Most of the holdfasts studied were anchored entirely within soft sediment, but a few show an initial attachment to a skeletal particle, with radicular cirri expanding laterally into the matrix. The holdfasts represent a soft-sediment form of the "dendritic radicular" category of Brett (1981). Radicular cirri diverge from the base of the column, branch repeatedly, and taper in diameter from several millimeters to less than 1 mm at their distal ends. Cirri penetrated the sediment to a depth of 1.5 cm to 3.0 cm. Table I. Summary of host taxa examined for encrusting epibionts. Size range of holdfasts and gastropods is diameter; size range of brachiopods is height of articulated shells. N=number of host specimens examined; Ne=number of host specimens with encrusting epibionts. HOSTS Size (mm) N Ne encrusted % cover (ave.) No. of epibiont taxa CRINOIDS: Encalyptocrinitcs holdfasts PLATYCERATID GASTROPODS: Naticonctna niajjarense Strophostylus cyclostomatus BRACHIOPODS: Mcristina maria Eospirifer radiants Sultatina sulcata 38-154 101 61 63 15 20-57 _6 76 100 25 20-51 29 29 100 3(1 13-23 ¦11 41 100 25 16-27 29 27 93 15 13-24 38 38 100 2(1 12 10 12 10 14 PALEOBIOS, VOL. 18, NUMBER 4, 1998 Table 2. Numbers of encrusting epibiont individuals and colonies counted on the following hosts: Eh—Eucctlyptocrinites holdfasts; Nn=Naticotiema nia/iarensr, Sc=Stropbostyhts cyclostomatur, Mm=Mcristina maria; EtrEospirifer radiants; Ss=Sitlcatina sulcata. HOSTTAXA Eh Nn Sc Mm Ho Ss EPIBIONT TAXA: favositid 5 4 - 3 4 9 solitary rugose corals - 1 - 1 - 1 sheet trepostomes 12 89 29 54 9 16 other sheet bryozoans 88 94 47 134 87 177 discoidal bryozoans 2 30 6 23 21 39 ramose bryozoans 7 5 3 10 29 34 stoloniferoiis bryozoans 9 9 3 12 4 (. craniacean brachiopod - - - 1 - - spirorbid 3 10 5 2 3 11 42 cornulitid - 250 117 40 5 28 discoidal crinoid holdfasts 1 7 8 2 4 6 dendritic radicular crinoid holdfasts 1" 10 7 8 IS 10 TOTAL KPIBIONT INDIVIDUALS & CO I.ONI KS NUMBKR OF ENCRUSTED HOST SPECIMENS 144 64 509 54 225 25 311 34 189 25 368 36 Complete holdfasts range from 3.8 cm to 15.4 cm in diameter, and the largest partial holdfasts suggest a maxi- mum diameter of approximately 20.0 cm. Hall (1879) attributed these holdfasts to Eucctlyptocrinites, based on one complete individual of Eucctlyptocrinites crassus (Hall) (Feldman 1989, tig. 4). Encrusting cpibionts were observed on 63% of holdfasts, where epibiont cover ranges from less than 1% to 95% (Table 1). As many as four epibiont groups and nine individuals and colonies occur on single holdfasts, but 56% of encrusted holdfasts include only one epibiont group. Epibionts occur mainly on the proximal parts of radicular cirri and the basal column but are sporadically present on the distal limits of cirri (Fig. 2B). Fistuliporid bryozoans are dominant, ranging from small colonies restricted to indi- vidual cirri to colonies 7.6 cm in diameter that cover nearly the entire radicular cirri and intervening sediment. Fistuliporid sheets also extend onto the basal part of the column, and on ten holdfasts, they grew over the tops of truncated columns (Fig. 2D). Sheet trepostomes to 2.3 cm in diameter are less abundant. Small encrusting crinoid holdfasts of dendritic radicular form occur on individual cirri (Fig. 2E) or on the base of the column. Other epibionts, which occur on 13% or fewer holdfasts, include favositids, ramose brvozoans, discoidal bryozoans, stolonif- eroiis bryozoans, spirorbids, and discoidal crinoid holdfasts (Tabic 3). There is no evidence that epibionts encrusted live hold- fasts. The spatial occurrence of encrusters indicates that as much as 5 mm of the holdfasts were exposed above the sediment surface. Exposure of holdfasts may have been related to storm scour and removal of most of the crinoid column and crown. Column remnants on the holdfasts are truncated at heights of 3 mm or less above the radicular cirri, and encrusting bryozoans and microcrinoids extend over the truncated column in 17% of encrusted holdfasts. Truncated holdfasts overgrown by bryozoans also served as the nucleus for local growth of "microbioherms" in the Waldron (Archer and Feldman, 1986). Brachiopods Articulated shells examined for epibionts include coarsely- ribbed Sitlcatina sulcata (Cooper), finch-ribbed Eospirifer radiatus (Sowerby), and smooth Mcristina maria Hall (Table 1; Fig. 3A-C). Mcristina Hall and Sitlcatina Schmidt lacked a pedicle opening. In Eospirifer Schuchert, the delthyrium is partly blocked by the beaks, which probably limited pedicle attachment. All three taxa represent unat- tached or weakly attached recliners. These brachiopods, which range from 2.2 cm to 4.0 cm in shell width, offered epibionts a substrate that ranged from 0 cm to approximately 2 cm above the substrate (Fig. 3A C). Epibiont cover on the brachiopods ranges from 1% to 75% and is dominated by sheet bryozoans. Other groups include ramose, discoidal, and stoloniferoiis bryozoans, as well as corals, craniaceans, spirorbids, cornulitids, and small crinoid holdfasts (Tables 2, 3). As many as eight epibiont groups and 29 individuals and colonics occur on single WATKINS & McGEE-TIERING OF SILURIAN EPIBIONTS 15 Tabic 3. Relative abundance of encrusting epibionts, measured as percent of encrusted host specimens on which epibiont taxon occurs. Sec Table I for sample size of host specimens. El/=Eucalyptocrinitesholii(asta; N>i= Nuticoticma niajjarcnst- Sc=StrophostyIus cyclmtomatus\ Mm-Mcristina maria; Eif Eospirifer radiatur, Ss~ ¦¦Sulcatum sulcata. HOSTTAXA Eh (%) Nn (%) Sc (%) Mm (%) Eo (%) Ss (%) Kl'IBIONTTAXA: favositid 6 8 7 12 11 21 solitary rugose corals - 1 - 2 - 3 sheet trepostomes 17 92 93 76 22 37 other sheet bryozoans '2 76 86 93 100 97 discoidal bryozoans 3 -11 24 44 4-4 47 ramose bryozoans 1 1 14 14 24 48 58 stoloniferous bryozoans 13 17 14 24 15 16 craniacean brachiopod - 5 - 2 - - spirorbid 5 18 17 37 30 S3 cornulitid - 8-1 97 61 22 47 discoidal crinoid holdfasts 2 9 21 5 15 11 dendritic radicular crinoid holdfasts 21 13 L7 15 26 21 Fig 2. Epibionts on Eucalyptocrinites holdfasts. (A) Favositid attached to radicular cirrus and extending onto surrounding sediment, G17749, x3. (B) Sheet bryozoan enveloping radicular cirrus, G17746, x4. (C) Coiled column of commensal crinoid, cemented to radicular cirrus of host, MPM28505, x3. (D) Sheet fistuliporid covering central part of holdfast and truncated column, G49351, x4. (E) Small crinoid holdfast on radicular cirrus, G49277, x7. 1() PALEOBIOS, VOL 18, NUMBER 4, 1998 Fig 3. Examples oi cpibiont distribution on Fossil brachiopods. Note cpibiont growth stops at the brachiopod commissure in all cases suggesting hosts were alive at time of encrustation. (A) Sulcatum sulcata, G54942). (B) Eospirifcr radiatus, Ci 18527). (C) Mcristina maria, G55047). shells, and 75% of brachiopod shells have three or more cpibiont groups. Epibiont occurrence on the three brachio- pod species is similar, although sheet trepostomes are more common on the smooth shells of M. maria (Table 3; Fig. 3C). Available evidence suggests growth of epibionts on both living and dead brachiopod shells. On many shells, epibionts occur on the brachial valve and the anterior part of the pedicle valve, as would be expected on a live individual reclining in soft sediment. In these cases, growth of bryo- zoan colonies stops abruptly at the brachiopod commis- sure, which also suggests colonization of a living host (Fagerstrom, 1996; Ixscinsky, 1997). On 35% of observed shells, bryozoan and favositid colonies grew across the brachiopod commissure. This indicates encrustation of a dead host and provides a minimum estimate for post- mortem colonization. Platyceratid gastropods StrophostyluscyclostomatusHaW—Strophostyhts cyclostomatus has a smooth circular aperture and has not been observed on crinoid tegmens; it is interpreted as a bottom-dweller. Specimens range in diameter from 2.0 cm to 5.1 cm, and they offered a substrate for encrustation that extended up to 5 cm above the bottom. Epibiont cover ranges from 1% to 90%, and individual gastropods contain up to live epibiont groups and 25 individuals and colonies; 76% of shells host three or more epibiont groups. Sheet trepostomes are the most common epibiont in both frequency of occurrence and percent cover, and single colonies can extend from the umbilical region to the apex. Other sheet bryozoans are the second most common epibiont, and discoidal, ramose and stoloniferous bryozo- ans are also common. Cornulitids are subordinate to bryo- zoans in amount of cover but are equally abundant in frequency of occurrence, being present on 97% of the specimens. Single gastropods host as main as 23 cornulitids, which occur over all parts of the shell, with variably ori- ented apertures. Ixss common epibionts include corals, spirorbids, and small crinoid holdfasts (Table 3). Growth relationships indicate colonization ol live ,S'. cyclostomatus by bryozoans and cornulitids. On 34% of shells, sheet trepostomes are overgrown by the gastropod inductura (Fig. 4A), indicating occupation of a live host. Many other sheet trepostomes extend to the edge of the gastropod aperture without extending inside. This also suggests encrustation of a live host. Cornulitids are oxer- lapped by the inductura on one specimen of S. cyclostomatus (Fig. 4A), which also indicates a life relation between epibiont and host. Naticonema niagarense (Hall)— Naticonema niagarense is known to have a life position on the tegmen of the crinoid PeriechocrinusMorris(Kluessendorf, 1983; Boucot, 1990). The vast majority of N. niagarense specimens in the Waldron occur as loose shells in the sediment matrix, but their irregular apertures suggest growth against the surface of crinoid tegmens. The Waldron specimens are 2.0-5.7 cm in diameter. As bottom dwellers or dead shells, N. niagarense offered a substrate for encrustation extending up to 5 cm above the bottom, while as a crinoid epibiont, it lived several decimeters above the bottom. Epibiont cover on N. niagarense ranges from 1% to 80%, and individual gastropods contain up to seven epibiont groups and 31 individuals and colonies; 57% of shells host three or more epibiont groups. Sheet trepostomes are the most common epibiont in both frequency of occurrence and percent cover, and single colonies can extend from the umbilical region to the apex. Other sheet bryozoans are the second most common epibiont, and discoidal, ramose and stoloniferous bryozo ans are also common. Cornulitids are subordinate to bryo- zoans in amount of cover but are nearly as abundant in frequency of occurrence, being present on 84% of N. niagarense specimens. They occur singly and in groups of up to eight intergrown individuals, and single gastropods host as many as 26 cornulitids. Cornulitids occur over all parts of the shell, and their apertures are variably oriented. Less common epibionts include corals, spirorbids, craniaceans, and small crinoid holdfasts (Table 3). Growth relationships indicate colonization of live N. niagarenscby bryozoans and cornulitids. Sheet trepostomes are overgrown by the gastropod inductura on 11% of N. WATKINS & McGEE-TIERING OF SILURIAN EPIBIONTS 17 Fig4. Epibionts on Strophostylus cyclnstomatns(A) and Natkimcma uiajjarcnsc(B-F). (A) Sheet trepostome and cornulitids overgrown by gastropod inductura, G49949, x2. (I?) Sheet trepostome overgrown by gastropod inductura, G49284, x2. (('.) Upper part of whorls with cornulitids, sheet trepostomes and, in center right, stoloniferous bryozoan, G49960, x3. (D) Detail of irregular aperture showing sheet trepostomes extending to margin, G49971, x2.5. (K) Detail of irregular aperture showing sheet trepostomes extending to margin, G49623, x2.5. (F) Discoidal bryozoan, G49607, x3.5. (G) Spirorbid, G49666, x3.5. niagarense specimens (Fig. 4B), indicating occupation of a live host. Main' other sheet trepostomes extend to the edge of the gastropod aperture without extending inside (Fig. 41"), E), which also suggests encrustation of live hosts. On two specimens of N. niaijarense, cornulitids that were overlapped by the gastropod whorl continued growing above the suture, which indicates a living host-epibiont association. EPIBIONTS ON UPRIGHT CRINOID COLUMNS Robust columns ofcrinoids provided substrates that elevated epibionts decimeters above the bottom. Uninterrupted growth of an epibiont around the entire circumference of the column indicates a life relationship with an upright crinoid (Boucot, 1990). In the Waldron, this condition has been reported for fevositids (Hall, 1879; Halleck, 1973) and also occurs in sheet trepostomes and stoloniferous bryozoans (Fig. 5D, F.). Peters and Bork (1998) described epibionts on three well-preserved specimens of Ecalyptocrinites in which complete crowns are articulated with partial columns up to 8 cm in length. These specimens were preserved by rapid, storm-related burial, and epibionts that occupied living, upright columns include clusters of rhynchonellid brachio- pods that were pedically attached, as well as encrusting favositids, bryozoans, spirorbids, and cornulitids (Peters and Bork, 1998). Some Paleozoic crinoids lived as epibionts with the distal part of their column coiled around the column of a host crinoid (Brett, 1981; Guensburg, 1992). Evidence from both holdfasts and pluricolumnals indicates this type of relation among Waldron crinoids, but the specific identifi- cation of the encrusting crinoid with this attachment struc- ture is unknown. Fragments of a slender crinoid column, up to 1 mm in diameter and 1.5 cm long, are coiled around the base of the column on nine Eucalyptocrinites holdfasts. In two occurrences, stereom of the holdfast partially oxer- grew the coiled column, and in one occurrence, the coiled column is partly cemented to a radicular cirrus (Fig. 2C). A bryozoan encrusting the holdfast also extends over the coiled column in one specimen, and in another, cirri of an encrusting microcrinoid holdfast extends from the coiled column onto the host holdfast. These occurrences indicate a primary relationship between the coiled columns and holdfasts rather than a chance dcpositional relationship. Seven pluricolumnals in the collections contain a spiral groove defined by unequal growth of the stereom (Fig. 5 A, B). These grooves range from 1 mm to 2 mm in width; their maximum length is unknown, and they spiral around 18 PALEOBIOS, VOL 18, NUMBER 4, 1998 D Fig. 5. Kpibionts on crinoid pluricolumnals. (A) Spiral groove, G50058, x2. (1?) Spiral groove, MPM 28506, x3. (C) Spiral groove retaining a fragment of coiled column of commensal crinoid, MPM28507, x3. (D) Sheet trepostome associated with swollen area of column, G50120, x3. (E) Stoloniferous bryozoan, G50073, x3. the host column for lengths of 1.0 cm to 2.5 cm. In one specimen, the groove is occupied by a fragment of slender crinoid column like those associated with the holdfasts (Fig. 5C). The coiled columns on holdfasts and the spiral grooves in pluricolumnals represent the same crinoid epibiont. This same crinoid-crinoid epibiontic relationship has been figured and discussed by Peters and Bork (1998). DISCUSSION Irrespective of whether hosts were dead or alive, near-surface tier divisions among encrusting epibionts is indicated by data for the Waldron Eucalyptocrinitcsho\dfasts, brachiopods and platyceratid gastropods (Fig. 6). Holdfasts, which provided a hard surface 0 to 5 mm above the bottom, include fewer epibionts than taller skeletal substrates, and sheet bryozoans greatly dominate the encrusting assemblage. Brachiopods, reaching 2 cm above the bottom, and dead or bottom- dwelling platyceratids, which extended to 5 cm, hosted a greater abundance and diversity ofencrusters than on hold- fasts. Dominance of sheet bryozoans on the brachiopods and gastropods is also less marked than on holdfasts. Sheet trepostomes and cornulitids become progressively more common from brachiopods to taller platyceratids. Live, upright columns of Waldron crinoids extended the vertical range of epibionts to decimeters above the bottom (Peters and Bork, 1998). These epibionts included favositids, bryozoans, brachiopods, and other crinoids. Or- dovician to Carboniferous examples of similar epibionts on crinoid columns have been described by Fckert (1984), Boucot (1990), Powers & Ausich (1990), Brett (1991), Guensburg (1992) and Sandy (1996). Individuals of the platyceratid Naticonema niagarense that lived on the crowns of the crinoid Periechocrinus may have been elevated as much as 1 m above the bottom, based on the column lengths of similar Periechocrinus in the Wcnlock Limestone of Britain (Watkins & Hurst, 1977). N. niagarense hosted an abundant and diverse epibiont assemblage, and at least sheet trepostomes and cornulitids, its two most common epibionts, occupied live gastropods. Richards (1974) and Brett (1991) suggested that Waldron cornulitids attached to N. niagarense represent secondary tierers on crinoids. This type of relationship was described in detail by Morris & Felton (1993), who figured cornulitid- encrusted platyceratids in life position on Ordovician crinoid crowns. The epibiont assemblage on N. niagarense does not represent a unique high-tier community, as a nearly identi cal assemblage of epibionts occurs on the bottom-dwelling platyceratid Strophostylus cyclostomatus (Tables 2, 3). Mor- ris & Felton (1993) reviewed many uncertainties about platyceratids as crinoid epibionts, including mode of feed- ing, whether a platyceratid individual spent all its life on the same host, or whether an individual divided its time be- tween the bottom environment and several crinoid hosts. CONCLUSIONS Waldron encrusters, dominated by bryozoans and including corals, brachiopods, spirorbids, cornulitids, and small crinoids, are typical of epibiont communities of Ordovician lo Car- boniferous age (Lescinsky 1996,1997). As secondary tierers, Waldron epibionts occupied all heights attained by primary tierers ranging from encrusters on skeletal substrates at the WATKINS &McGEE-TIERING OF SILURIAN EPIRIONTS 19 CO w ¦a i_ . ¦*- 03 Q. T «0 N o > < m M ° 2 UJ O u. LT O Q -J Z O LU I— Q CO CO CO LU CO UJ s| £EN LUO •cr Om CO < o N o >- a: m UJ CO O < CO CO 2 CO O > < < co UJ o <¦ > N LL O 0 Q 1- > -J co or O O m T Q CO III _i M _l cr h- < Q RBI < Q LU O o O O LT o III CO 0- CO X Q co Q co CO LT CO O LU CO 1 O z o < z> cr cr o 7 -^ PERCENT OF ENCRUSTED HOSTS ON WHICH EPIBIONT OCCURS co a LT o o cm i5 100 —1 Fig. 6. Near surface tiering ol encrusting epiobionts, based on data from crinoid holdfasts, brachiopods and platyceratid gastropods. sediment surface to Nnticoncma niagarense, a platyceratid gastropod encrusted by its own epifauna, that lived on crinoid crowns reaching heights of 1 m above the bottom. The selectivity shown by epibionts in this tiering appears to have been relatively low. Favositids, brv'ozoans other than sheet trepostomes, spirorbids and small crinoids are equally common on a variety of skeletal substrates and appear to have been non.selective in relation to tier level and host taxa. 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