Taphonomic losses become taphonomic gains: an experimental approach using the rocky shore gastropod, Tegula funebralis

t E~teGY ELSEVIER

Palaeogeography,Palaeoclimatology,Palaeoecology114 (1995) 197-217

Taphonomic losses become taphonomic gains: an experimental approach using the rocky shore gastropod, Tegulafunebralis Sally E. Walker a, James T. Carlton b a Department of Geology, The University of Georgia, Athens, GA 30602, USA b Maritime Studies Program, Williams College at Mystic Seaport, Mystic, CN 06355, USA Received 30 March 1994; revised and accepted 27 September 1994

Abstract

The gaps in the fossil record are not liabilities for paleontologists but rather assets for evolutionary and paleoecological studies. The fossil record of rocky shore invertebrates is deemed poor, resulting from the bias of a high energy (physical) environment. The poor fossil record of the Pleistocene rocky shore gastropod Tegulafunebralis appears to be no exception to this rule. However, field studies, including experimentally deployed shells in two habitats, reveal five important taphonomic processes that affect the resultant fossil resource for these gastropods: (1) the predilection for Tegula shells by the intertidal hermit crab, Pagurus samuelis, affects the longevity of the shells, (2) hermit crab-occupied shells of Tegula have a distinct array of bionts distinguishable from the living snail and empty, experimentally tethered shells, (3) biont types on tethered shells differ between two habitats (mudflat and rocky intertidal), and are thus useful for paleoenvironmental determinations, (4) physical processes greatly affected intact shell longevity of experimental shells at the rocky intertidal site, whereas at the mudflat site, mistaken predation by durophagous crabs was the most important agent of shell destruction, and (5) despite these taphonomic losses, the Pleistocene fossil record of Tegula retains a good record of the biological factors that affect its preservation, that of pagurid crustaceans and their gastropod shell-associated bionts. Taphonomic losses when viewed from a hierarchy of shell users, then, are gains in biological information that indicate the level of complexity within the shell-using community, and this record is not completely lost in high energy regimes.

1. Introduction

"... the fossil record can yield a great deal of biological information if the nature of the biases is understood" Valentine, 1989

The gaps in the fossil record are not, in every instance, liabilities for paleontologists but rather, assets for understanding the nature of evolution and community structure through time once the biases are understood. However, biases in molluscan paleontology have been limited to physical processes as the principal factor in controlling 0031-0182/95/$9.50© 1995ElsevierScienceB.V. All rights reserved SSDI 0031-0182 (94) 00094-8

assemblages (after Kidwell and Bosence, 1991). Current sorting, abrasion, dissolution, tectonics, and diagenesis are frequently invoked to explain missing information in the fossil record (Driscoll, 1967, 1970; Hallam, 1967; Driscoll and Weltin, 1973; Valentine, 1989). Predation, secondary shell inhabitants, and destructive boring and encrusting organisms may also affect the resultant fossil record of molluscs by destroying, transporting or reorienting shells, and modifying the taphonomic overprint (e.g., Teichert and Serventy, 1947; Smith, 1952; Trewin and Welsh, 1972; Shimoyama, 1985; Cadfe, 1968, 1989, 1990; Walker, 1989). Biological processes

198

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197~217

are archived on fossils, such as predator-prey associations represented by drill holes on invertebrate skeletons (reviewed by Kabat, 1990; Kowalewski, 1993) or symbiotic relationships in the fossil record (Brett and Cottrell, 1982; Morris et al., 1991; Walker, 1992). Although these predatory or symbiotic associations may result in a taphonomic loss of information (i.e., shell destruction), the shells that do survive provide a taphonomic gain of behavioral, ecological and evolutionary information on organisms which may not otherwise have a good fossil record (e.g., octopus, fish, crabs). Only in using a holistic approach, combining biological and physical factors, will gaps and biases be recognized as taphonomic gains, rather than taphonomic losses. For example, the excellent original record of marine benthic molluscs from the Pleistocene of California (Valentine, 1989) may also record an excellent original record of behavioral, ecological and evolutionary information recorded on the shells of those molluscs in the form of epi- and endobionts. Biological factors lend themselves to laboratory and field manipulations although field experiments testing the effects of predators on the potential invertebrate fossil record are rare (Plotnick, 1986; LaBarbera, 1981; Walker, 1988; Walker and Yamada, 1993). Additionally, experimental studies on molluscs are usually limited to pelecypods, which are easily manipulated in field and laboratory experiments (Boucot et al., 1958; Emery 1968; Lever and Thijssen, 1968; Clifton, 1971; Dent and Uhen, 1993). Rarely have gastropods figured in paleoecological experimentation except in shell hydrodynamic studies conducted in the laboratory (Nagle, 1967; Brenchley and Newall, 1970). Because of the fossil importance of gastropods, it is necessary to understand the paleoecological and taphonomic components affecting their representation in the fossil record. Taphonomic field studies are lacking in this regard and we wish to rectify this problem in order to refine taphonomic and paleoecologic work using gastropod shells. We tackled a subset of the problems outlined above by combining field experiments with descriptive field work using the rocky shore gastropod, Tegula funebralis. Tegula was used to address five

taphonomic questions, some of which are exclusive to gastropods and will affect their representation in the fossil record: (1) How do hermit crabs modify the postmortem taphonomic information (i.e., encrusting and boring organisms) on shells of rocky shore gastropods? (2) Are empty, experimentally tethered gastropods shells subject to similar taphonomic processes as hermit crab-occupied shells? (3) How long does a tethered gastropod shell last in a high-energy environment? (4) Are high energy environments detrimental to preservation of epi-and endobionts? (5) Is there fidelity between Recent epi-and endobionts and those preserved on Pleistocene Tegula?

2. Modern and fossil Tegula funebralis, an overview Rocky shore faunas, in general, are thought to be rare in the fossil record (Johnson, 1988; Bambach, 1986). However, new accounts of ancient rocky shore faunas are available, most notably from the Cretaceous (Lescinsky et al., 1991 and references therein; Johnson, 1988), Miocene (Baluk and Radwanski, 1991) and PlioPleistocene (Valentine, 1961, 1989; Kennedy, 1973; Marincovitch, 1976; Meldahl, 1993). Thus, it may eventually be possible to compare the taphonomic biases acting on rocky shore assemblages in a stratigraphic and geographic context. The black turban snail, Tegula funebralis, is an abundant rocky shore gastropod whose empty shells are frequently inhabited by the epibenthic hermit crab, Pagurus samuelis, on the north coast of California (Morris et al., 1980; Bollay, 1964). Both T. funebralis and P. samuelis co-occur in the mid-intertidal zone from Vancouver Island, British Columbia, to Central Baja California, Mexico (McLean, 1978; Morris et al., 1980). Despite its abundance in Recent habitats, Tegula funebralis (henceforth, referred to as Tegula) is relatively rare in the Pleistocene of California (Valentine, 1961 ). The rarity of Tegula is thought to be a factor of its rocky shore habitat and the weak nature of its shell microstructure (Valentine, 1961). We propose that rocky shore hermit crabs may also affect the abundance of Tegula through various taphonomic pathways. Experiments by Reese (1962, 1969) demonstrated that Tegula was

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

preferred over other gastropod species by Pagurus samuelis. Preference for Tegula by hermit crabs may seriously affect the preservation of those shells. That is, gastropod shells used by hermit crabs may facilitate postmortem destruction of the shell by predators (e.g., Vermeij, 1977, 1987; LaBarbera and Merz, 1992) or bioeroders (Smyth, 1990). Living Tegula shells provide a substrate for many bionts (Alleman, 1968; Bollay, 1964; Peppard, 1964; Brewer, 1975), but these bionts differ in composition or location on hermit crabinhabited shells (this paper). The potential for using bionts to recognize hermit crab-inhabited shells from ancient rocky shore deposits fossil was first proposed by Carlton (1972) for Tegula. Carlton's observations on Tegula have remained untested until this paper. 3. Terminology/abbreviations "Hermitted" will be used here in place of "hermit crab-occupied shell" (after Vermeij, 1977), and "pagurized" will be used to describe shells with bionts uniquely associated with hermitted shells (after Seilacher, 1969). Not all hermitted shells are pagurized either because of the behavior of the crab (e.g., Isoeheles pilosis: Walker, 1988), or because the shells represent a new input into the hermit crab guild from the gastropod death assemblage, and are thus free of pagurid bionts. Fossil specimens were examined from the following museums denoted by abbreviations in the text: Los Angeles County Museum of Natural History, Department of Invertebrate Paleontology (LACMIP), University of California Museum of Paleontology (UCMIP), and California Academy of Sciences (CAS). 4. Taphonomic signatures of epibenthic hermit crabs and living snails from a high energy environment: a field study Shells of living and hermitted Tegula were examined for the presence of epi- and endobionts to determine the extent of taphonomic modifications produced by hermit crabs and to develop taphonomic criteria for distinguishing between the two

199

types of occupants in the fossil record of temperate, rocky intertidal zones.

4.1. Methods An exposed rocky intertidal site (Horseshoe Cove) at Bodega Bay, Sonoma County, California (38°18 N lat., 123°03 W long.) was chosen for this study (Fig. 1). Fifty Tegula and fifty hermitted shells were collected from the mid- to high intertidal zone once every month for 21 months, from January 1985 until October 1986. One month, an additional 24 shells from each species were collected (for another study) and were added to the final sample size. A total of 985 Pagurus samuelis and 1074 Tegula were used (89 Pagurus granosimanus were also collected as "hermitted shells," but were not used in this study). All shells collected during the 21 months were measured for the following: (1) Shell height, (2) last whorl thickness and, (3) two basal diameters called "aperture A" and "aperture B" (Fig. 2). Shell height is often a misleading indicator of true height for Tegula, as apices were frequently eroded. A simple linear regression was performed on three measurements ("aperture A," "aperture B," and last whorl thickness) using living Tegula snails with intact apices. On the basis of this analysis, "aperture A" was taken as the best estimate of shell height (n = 50, r 2 = 0.95). Epi- and endobionts (bionts) on living Tegula and hermitted shells were determined by examining the apertural and exterior shell regions. The apertural region of Tegula (Fig. 3a) was examined for encrusting and boring bionts. The difference in biont species composition and location on the shell was noted for 21 months of the study. Statistical significance of these associations were tested with Mann-Whitney U-test (Sokal and Rohlf, 1981). Biont associations in regard to size class were tested with a Kruskal-Wallis Test (Sokal and Rohlf, 1981). The shell exterior was divided into four quadrats on a subsample of Tegula (Fig. 3b, c) to determine if (1) bionts had a preference for a particular part of the shell and if (2) this distributional pattern was different between snails (n=50) and hermit crabs (n = 50). The statistical significance of these associations was tested with a T-test (Sokal and Rohlf, 1981).

200

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

"

Bo I

~ H ; ~ e s h o e Cove ~

Pacific Ocean ~ ~. !

,

("[".i

.:"" "' . "

~

0 I

N~

. "................... " " ... .

1 km 1

2 f

Fig. 1. Study area at Bodega Bay, SonomaCounty,California.

Fig. 2. Shell measurements used: H=height; LW=last whorl height; B=aperture B; A =aperture A. Aperture A was found to be the best indicator of shell height.

4.2. Results~discussion Bionts inhabiting the apertural area on Tegula Living snails and hermitted shells had potentially preservable bionts (i.e., those that have a fossil record) within or near the shell aperture (Table 1).

The majority of living snails (84%) had clean apertural areas in contrast to 39% of hermitted shells. Living snails and hermitted shells had small, round (< 0.25 ram), shallow pits made by the green alga, Gomotia, on the base of the shell. A boring bryozoan (Penetrantia ?concharum; Osburn, 1953) also made small (< 0.25 ram) pits but had connecting stolons on the base of shells. Penetrantia occurred in low frequencies that were not significantly different between living snails and hermitted shells (Table 1). Therefore, this type of Penetrantia cannot be used as a criterion to distinguish between shells inhabited by snails and hermit crabs. Overall, 61% of the hermitted shells were encrusted with suspension-feeding or filter-feeding bionts (Table 1). Spirorbis spp., the most frequent epibiont, occurred on 54% of the shells producing

S.E. Walker, J. T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 19~ 217

.mbilired'eft.re cu,ar base (basal)

A

201

Table 1 Frequency of biont occurrence (number of bionts/total sample size) on apertural areas of Tegulafunebralis shells inhabited by the living snail (n=1074) and Pagurus samuelis (n=985) at Horseshoe Cove, Bodega Bay, California for 21 months. Key: + =present; - absent; numbers in parentheses are numbers of shells with biont; No biont =clean shells; pagurized shells = minimum number of bionts that indicate a hermit crab occupant. Mann-Whitney U-test (two-tailed probability): *= statistically significant P < 0.05; .... = not statistically significant; 1=Unidentified boring bryozoan=0.42 probability (not significant); 2 No biont=0.0000 probability, statistically significant, P<0.05 Biont

Tegula funebralis

Pagurus samuelis

(Living) Gomontia sp.

+

+

-

0.54 (533) 0.001 (1)

-

0.01 (16)

-

0.30 (291)

0.16 (168) 0.84 (906) 2'* -

0.12 (119) 1..... 0.39 (389) 2.* 0.61 (596)

(boring green alga) Polyehaeta

Fig. 3. A. Shell locations associated with the aperture. B, C.

Quadrants used in the external shell biont study, QI= Quadrant 1, Q2= Quadrant 2, and so on.

Spirorbis sp. Polydora sp. Cirrepedia Chthamalusfissus Cheilostomata

Hippothoa hyalina Ctenostomata

small ( < 2 m m ) calcareous tubes a r o u n d the aperture, often leaving a space indicating hermit crab m o v e m e n t (i.e., abrasion by the carapace) (Plate I,a,b). Encrusting Hippothoa hyalina (Plate I,a), the next m o s t frequent biont, etches the outer shell leaving small oval-shaped pits (Plate I,c). T h e acorn barnacle, Chthamalusfissus, and the boring spionid, Polydora sp. (Plate I,d, Pleistocene example), rarely occurred on the shells. The occurrence o f all apertural-inhabiting bionts was not significantly different between size classes of Tegula except for Penetrantia, which occurred on larger shells (Table 2). Bionts occurred on all size classes o f hermitted shell, indicating no spatial restriction for these bionts and that all shell sizes of Tegula used by the hermit crab, Pagurus samuelis, can be pagurized. T h e two m o s t i m p o r t a n t bionts useful for distinguishing between living snails and hermit crabs in high energy environments were: spirorbids and Hippothoa hyalina (Fig. 4). Hippothoa is an excellent indicator o f hermitted shells in the fossil record (Walker, 1988, 1992). A similar etching b r y o z o a n occurs on the e u r o p e a n coast (Electra sp.) and has

Penetrantia ?concharum No bionts Pagurized shells

been used by Boekschoten (1967) to infer hermit crab-inhabitation o f shells. To a lesser extent, b a m a c l e s were also indicative of hermitted shells but not the boring bryozoan.

Bionts on the exterior shell of Tegula Spirorbid polychaetes and barnacles occurred on the shell surfaces of b o t h living snails and hermitted shells (Table 3). Spirorbids were rare and because the frequency of occurrence was low, statistical tests were not performed. Chthamalus fissus occurred on significantly m o r e living snails shells than on hermitted shells (T-test, pooled, T = 5 . 1 1 8 , P < 0 . 0 0 1 ) . Chthamalus location over the aperture (Quad 1) in b o t h snails and hermit crabs was significant (snails: one-way A N O V A F = 7.74, P = 0.006, P < 0.05; hermitted shells: one-way A N O V A F = 6 . 5 6 1 , P = 0 . 0 1 1 , P < 0.05). Thus, external bionts can not be used as reliable criteria

202

S.E. Walker, J. 72 Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197 -217

PLATE I

A.Pagurized Tegulafunebral& with spirorbids and Hippothoa in umbilicus and notch region, Recent, Horseshoe Cove, Bodega Bay, California, shell height 20.1 mm. B. Pagurized T. funebralis with spirorbids and Hippothoa in aperture illustrating hermit movement area (arrow), Pleistocene, Pacific Beach, San Diego, California, CAS 53780.02, shell height 16.7 rnm. C. Pagurized 7". funebralis with encrusting Hippothoa and Hippothoa etchings (arrow), Pleistocene, San Pedro, California, CAS 1478.01, shell height 16.3 mm. D. Typical spionid occurrence in umbilicus (arrow) and borings from a bryozoanon T. Ji*nebralis, Pleistocene, San Pedro, California, CAS 1478.01, shell height, 16.3 mm.

S. E~ Walker, J. T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

203

Table 2 Kruskal-Wallis (non-parametric ANOVA) to test whether apertural-inhabiting bionts are independent of size class groups for T. funebralis shells inhabited by the living snails or hermit crab, Pagurus samuelis. Key: * = statistically significant (p < 0.05); ns= not significantly different between the shell size classes; df= degrees of freedom; P = probability Bionts

Size class

Chi-square/df

P

1:9.0-14.9 (mm)

II:15.0-18.9 (mm)

11I:19.0-22.9 (mm)

IV:23.0-28.9 (mm)

38

17 257

90 479

61 132

52.68/3 55.98/3

0.00" 0.00"

20 83 151 109

61 138 274 169

37 56 90 53

33.14/3 4.99/3 12.0/3 10.6/3

0.00" 0.66 ns 0.10"s 0.16"~

Living snails

Penetrantia No Bionts Hermitted shells

Penetrantia Hippothoa hyalina Spirorbis sp.

1 14 18 58

No biont

to distinguish between hermits and living snails in the fossil record.

5. Experiment: shell longevity and taphonomic signatures on empty shells ENCRUSTING BRYOZOAN

SPIRORRIDS

B O R I N G BR Y O Z O A N

BA RNA Cl. ES

Fig. 4. Typical biont locations on T. funebralis shells. Table 3 Spirorbids and barnacles on the exterior shell of Tegula funebralis (*= significantly different) Encruster

Quad 1

Quad 2

Quad 3

Quad 4

Spirorbids

Spirorbis Snails Hermits Barnacles

2 3

2 1

2 2

2 1

23* 10"

17 8

17 5

12 3

Chthamalus fissus Snails Hermits

Shells were tethered at a mud flat (low energy site) and rocky intertidal (high energy site) to compare taphonomic processes in the two habitats: (1) Shell longevity (i.e., intact shells) and (2) differences in epi- and endobiont settlement. Bionts that settled on empty, experimentally tethered shells were then compared to bionts associated with living snails and hermitted shells, to refine the criteria by which to recognize pre- and postmortem use of shells in the fossil record.

5.1. Methods Empty Tegula were tethered in the mid-intertidal zone at Horseshoe Cove (quartz diorite substrate) and in the mid-intertidal mudflat at the University of California Biological Reserve (fine sand and silt substrate)(Fig. 1). Initially, twenty unplugged Tegula were emplaced in December 1985 at the mudflat site. After two weeks, hermit crabs inhabited 10% of the shells. Subsequently, all additional tethered shells were plugged with paraffin to prevent hermit crab occupancy. In January, March and July of 1986, thirty additional plugged shells

204

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

were emplaced at the mudflat to study seasonal differences in biont settlement. A total of 110 tethered shells were emplaced at the mudflat site. Each shell array consisted of ten shells which were tethered to plastic rope (1.8 m in length) by monofilament line (5 cm). The shells were spaced approximately 5 cm apart on the ropes, which were then tied between two iron posts and anchored in the mud so that the shells rested on the substrate. In the rocky intertidal, five shells plugged with paraffin were tethered to plastic rope (30cm in length) by monofilament line (5cm length). Three sets of tethered lines (of 5 plugged shells each) were epoxied with Z-spar Marine Epoxy to the quartz diorite substrate. Each line was placed in different tide pools which overlapped with the occurrence of Tegula and P. samuelis. A total of 45 shells were emplaced at the rocky intertidal site. Shells were monitored every month during low tide for a total of 11 months (December 1985-October 1986) for the mudflat site, and 10 months for the rocky intertidal site. Each shell was examined for biont occurrence, especially boring or filamentous algae and calcareous bionts (spirorbids, bryozoans or barnacles). Shell loss was also noted.

5.2. Results~discussion Rocky intertidal shell loss Surprisingly, despite the occurrence of heavy winter storms of 1986 in Horseshoe Cove, loss of experimental tethered shells was minor (Fig. 5a). One shell was missing in April, possibly torn from the lines. In October 1986, 43 shells were still present. No predatory damage was observed on any of the shells. Loss of external shell matrix exposing the nacreous layers beneath, and pitting of the shells started within two months of emplacement of all tethered lines. At the end of 10 months, all shells were abraded with exposed nacreous layers and perforated shell apices. Thus, it appears that shell longevity of experimental shells tethered in the middle intertidal is affected mostly by physical factors which abrade the shells, and not biological factors, such as predators.

-~

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ROCKY INTERTIDAL 1--:-...... 3--,-Fig. 5. ( t o p ) P e r c e n t o f

p l u g g e d n = 15

plugged n = 15 pluggedn= 15

T. funebralis shells r e m a i n i n g ( n u m b e r

of shells remaining/numberof initial shells x 100) per month after emplacement at Horsehoe Cove (rocky intertidal site). (bottom) Percent of tethered shells with algae (boring or filamentous) per month (number of shells with algae/number of shells present each month × 100).

Rocky intertidal biont settlement Within a month of emplacement, boring green algae was present within tethered shell apertures and external parts of shells (Fig. 5b). Few spirorbids settled on the tethered shells during the ten months of the study (7 of the 45 shells had spirorbids). Most spirorbid settlement occurred in the summer for all tethered lines. Spirorbids settled within the umbilicus, apertural notch and next to the paraffin plug of shells with apertures facing into the substrate. Spirorbids settled on the external shell surfaces only where the shell was resting on the substrate (i.e., in a dark area). Despite ubiquitous barnacle settlement on the adjacent rocks and within the tidepools, no barnacle settlement was observed on the experimental shells. Mud flat site shell loss Within four months of emplacement, 70% of unplugged Tegula shells at the mudflat site were lost (Fig. 6a). Three unplugged Tegula (15% of original shells) remained at the end of the study in October. A total of 73 plugged Tegula shells were left after 11 months (66% of the original shells) with most shell loss in the spring. Often,

S.E. Walker,J. T. Carlton/Palaeogeography, Palaeoclimatology,Palaeoecology114 (1995) 197-217 -

100.

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MUDFLAT 1 --o-unplugged n = 30 2 - - - * - - - - plugged n = 30 3--,-plugged n = 30 4 .......... plugged n = 30

Fig. 6. (top) Percent of T. funebralis shells remaining (number of shells remaining/number of intial shells× 100) per month after emplacement at the Biological Reserve (mud flat site). (bottom) Percent of tethered shells with algae (boring or filamentous) per month (number of shells with algae/number of shells present each month × 100). the shells were crushed or peeled, leaving body whorl fragments and pieces of the outer lip lying next to the tethered lines. These shell breakage patterns were produced by Cancer productus "predation" on the empty shells, which were common in the area during M a r c h - M a y 1986 (refer to Walker, 1988). Walker and Yamada (1993) showed that mistaken predation on empty shells by crabs is quite common and need not be limited to living snails or hermitted shells. The occurrence of durophagous crabs at the mudflat site affected shell survivorship (i.e., intact shells) more than physical factors associated with the rocky intertidal site.

Mud flat site shell bionts Algae (unidentified Chaetophorales algae, Ulva sp., Enteromorpha sp.) and diatoms were the dominant bionts on Tegula shells throughout the study period (Fig. 6b). However, algae became sparse after June. The decrease in algal turf encrusting the shells coincided with a large algal (Ulva sp.) bloom on the mudflat, which smothered the shells

205

for the latter part of the summer. An anoxic layer (rich in hydrogen sulfide) under the canopy of algae developed, precluding biont settlement and growth. Black sulfide precipitates were not as evident on Tegula as they were on tethered Olivella shells ("oxides" of Walker, 1988). One barnacle (Balanus crenatus) was observed on the apex of a tethered Tegula shell. It remained throughout the duration of the study, even after the shell was crushed by a crab and the fragment it was on was left tethered. A large settlement of barnacles occurred in April 1986 covering the iron posts which anchored the tethered shells, but no shells became infested with the barnacles. Scouring and intermittent burial of the experimental shells most likely prohibited barnacle settlement (see discussion on Olivella, Walker, 1988: pp. 56-57).

6. Is the taphonomic signature from the living shell-inhabiting community retained in a recent death assemblage? Taphonomic loss of information was studied from a beachdrift collection of Tegula shells representing 12 collecting periods. A total of 200 Tegula whole or fragmented shells were recovered from walking the Horseshoe Cove strandline for one hour each time. The shells and fragments were examined for biont traces and then compared to the record of recent shells from Horseshoe Cove.

6.1. Results~discussion The beachdrift shells retained little of the biont taphonomic information that was evident within the living assemblage of snails and hermit crabs (Table4). Only a small percentage (11%) had pagurized taphonomic signatures. Spirorbid tubes and traces of tubes were present in suture areas or within apertural regions. Etch patterns of Hippothoa were also present in apertural areas only. Bionts were preserved in "taphonomic refugia", such as sutures, umbilicular or apertural regions of the shell. These areas are least likely to be abraded by the coarse substrate of Horseshoe Cove. The low occurrence of pagurized shells may

206

S.E. Walker, J. 7~ Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197~17

Table 4 Taphonomic composition of T. funebralis beach drift. A "time-averaged" sample from 12 collections (June-July 1983 1986; onehour search time along 100 ft. strandline). No Biont: indicates that the shells were clean; Any: is the minimum amount of biont(s) necessary to indicate a hermit crab occupied the shell. Size class (mm)

Half shell

Last whorl

Whole shell

Pitting

Spirorbids

Hippothoa

No biont

Any

0.0-9.9 n=47 (0.21)

1 (0.02)

4 (0.94)

2 (0.04)

3 (0.061

6 (0.13)

2 (0.04)

38 (0.81)

7 (0.15)

10.0 14.9 n = 116

6 (0.05)

79 (0.68)

31 (0.27)

9 (0.08)

7 (0.06)

6 (0.05)

99 (0.85)

12 (0.10)

15.0-19.9 n=36 (0.17)

3 (0.08)

2 (0.05)

30 (0.83)

4 (0.11

4

I

27

5

(0.11)

(0.03)

(0.75)

(0.14)

20.0-24.9 n=16

3 (0.19)

0

13 (0.81J

4 (0.25)

0

0

12

0

13 (0.06)

125 (0.58)

76 (0,35)

20 (0.09)

17 (0.07)

(0.54)

(O.75)

(0.07) Total

n=200

reflect the type and amount of predation occurring within the intertidal zone at Horseshoe Cove. That is, freshly killed gastropods are best represented in this beach drift example from Horseshoe Cove. The lack of hermitted shells in the beach drift of Horseshoe Cove may be a factor of seasonal variation in wave regime (storms that recycle buried shells or redistribute shells), type and availability of predators, hermit crab recruitment or shell strength. Hermitted shells are weaker, and thus more vulnerable to crushing than are the living gastropods (LaBarbera and Merz, 1992). Including predation by crabs, it follows that hermitted shells may also be more vulnerable to destruction in high energy regimes, and are less likely to survive in the "potential" fossil record of rocky beach drift associations. The distribution of living T. funebralis in the intertidal is most likely controlled by sea stars (Paine, 1969; Menge, 1972; Doering and Phillips, 1983). Sea stars, such as Pycnopodia helianthoides, are voracious in their proclivity for gastropods but do not damage the shell during predation events, nor are epibionts removed from the shells during predation (Walker, 1990). Many of these shells

9 (0.04)

176 (0.82)

24 (0.11)

could become beach drift before hermit crabs had a chance to occupy them. Crabs are also a major predator of Tegula (Riedman et al., 1981 ). Cancer crabs were not seen in the middle to high intertidal of Horseshoe Cove, nor were experimental shells attacked, but crabs could intercept shells in the lower zone, where Cancer appeared to be common (see Walker, 1990). Many shells in the beach drift were peeled and crushed, presumably as a result of crabs.

7. Retention of the taphonomic signature in the Pleistocene of California Given the restrictions posed by the Recent death assemblage study, how well were epi- and endobionts represented on Tegula shells from the Pleistocene of California? Were the taphonomic signatures eliminated by the high energy regime or were they remarkably preserved? Were the Pleistocene shells of Tegula represented by onceliving snails, or were they hermit crabs in snail's clothing?

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoelimatology, Palaeoecology 114 (1995) 197-217

Z1. M e ~ o ~ Museum specimens of Pleistocene Tegula funebralis from the Pacific coast of California and Baja, Mexico, were used to test the applicability of the biont criteria. Because additional bionts were found in the fossil samples that were not present in the recent northern California study, an additional sample of Recent shells (n = 30) from the southern range of T. funebralis were examined from the Palos Verdes Peninsula, California, to determine if modern bionts from that region were similar to those present in the Pleistocene collections from southern California.

7.2. Results~discussion Tegula fossils were rare at most localities in California, except for San Diego County, which yielded many bulk samples. Many fragments, presumably from Cancer crab predation, and biont fossils and/or trace fossils occurred in these collections. Thirty-nine localities were examined, for a total of 713 fossil Tegula, of which, between 0% and 66% were pagurized depending on locality (Table 5). Pagurized shells represent a total of 29% of all Tegula fossils, with apertural regions occupied by spirorbids, encrusting/etching bryozoa (Hippothoa), boring bryozoa (?Penetrantia) and rarely, the slipper shell, Crepidula. No encrusting barnacles were present on the fossils. Spirorbids were the most frequently encountered body fossil per total sample size (23%) with Hippothoa traces the second most common fossil (7%; Table5). Apertural-inhabiting Crepidula ?perforans and boring spionid polychaetes were present in lower frequencies. The slipper shell, Crepidula was not present in Recent habitats from northern California but it was commonly associated with hermitted shells from the Monterey region south to San Diego County (Carlton and Roth, 1975; Kennedy, 1973). A Recent sample of hermitted Tegula shells from southern California had the same complement of epi- and endobionts on all size classes of shells as the fossils (Fig. 7a-c). Etching traces and body fossils of Hippothoa were present, but rare, on larger shell size classes in Recent and fossil Tegula

207

as were spionid borings (Plate II,a). These biont fossils and/or trace fossils commonly occurred in the umbilicus and apertural notch area of hermitted shells. If shell layers were missing, steinkerns (internal sediment casts) of fossil Tegula also contained impressions of spionid borings (Plate II,b). Despite C. ?perforans occurrence in Recent Palos Verdes collections (Fig. 7a, Plate II,c) it was not present on fossils from Palos Verdes. One Pleistocene C. perforans was found within the aperture of a fossil Tegula from Pacific Beach, San Diego County (Plate II,d). The delicate and friable nature of this slipper shell and the adverse affects of curation (which removes material from the aperture) most likely contributed to its lack of representation. As shown in the Recent study, not all hermitted shells are pagurized. It is thus likely that many of the Tegula shells were hermitted in the California Pleistocene. In some localities, Tegula was poorly preserved or was damaged by crabs (S.E. Walker, pers. obs.). We strongly suggest that the preference of P. samuelis for Tegula shells has, in part, contributed to this biased sample. Most importantly, the occurrence of pagurids and their use of shells can be demonstrated by the shell-associated fossil bionts. Even in high energy regimes, the biont taphonomic signature remains, despite the perils of preservation.

8. Conclusions

While the Pleistocene fossil record of molluscs in California is excellent in itself (Valentine, 1989), the rocky intertidal record also records an excellent shell-dwelling microhabitat, that of epi-and endobionts on shells of fossil gastropods. In turn, these bionts also provide information on the actual inhabitant of the shell, whether it was the living snail or a hermit crab. Thus, the gastropod fossil record from the California Pleistocene is not only good it provides a record for the diversity of bionts and secondary inhabitants that also use empty shells from gastropods. Epi- and endobionts on and within fossil gastropod apertures have historically been attributed to postmortem settlement on empty shells (Arua,

208

S.E. Walker, J.Z Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

Table 5 Pleistocene localities with biont fossils and/or trace fossils present on Tegula funebralis shell apertures from California and Baja California. Key: S a m p l e = s a m p l e size; A n y = n u m b e r of shells with m i n i m u m n u m b e r o f fossil bionts that indicate a hermit crab once-inhabited the shell; N o biont = no encrusting or boring organisms on shell; - = not present; numbers in parentheses indicate the frequency of shells (number of shells/per sample size) that has a particular biont; * = three of the T. funebralis contained encrusting coralline algae within the aperture, which is also characteristic of hermit crab-inhabited shells; ** =encrusting bryozoan, Antropora tincta represented instead of Hippothoa. Localities

n

Spirorbids

Near Rosarito Beach, Punta Baja, Mexico U C L A loc. 2717 48 26 U C L A loc. 2718 33 0 U C L A loc. 3162 II 2 Punta Baja, Mexico U C M P A-9592 5 0 Northwestern Baja, Mexico U C M P B-3095 11 2 Punta Cabras,Baja, Mexico U C M P A-9585 2 0 Punta China, Baja, Mexico U C M P A-9595 4 0 U C M P A-9596 9 1 U C M P A-9002 12 0 Pt. Loma, San Diego Co., California U C L A loc. 3605 64 35 L A C M I P loc.23605 216 61 L A C M I P loc. 11701 28 7 Pacific Beach, San Diego Co. CAS 53980.02 6 0 San Juan Capistrano, Oranage Co., California L A C M I P loc. 58 21 0 San Clemente Island, Los Angeles Co., L A C M I P loc. 12577 22 3 San Pedro, Los Angeles Co., California, CAS 1478.01 3 1 D e a d m a n Island, Los Angeles Co. CAS 61647.01 2 0 N o b Hill, Los Angeles Co. CAS 1189.01 2 1 CAS 61645.02 5 0 Pt. Ferman, Los Angeles Co. L A C M I P loc. 345 2 0 L A C M I P loc. 2673 2 1 San Pedro Sand, Los Angeles Co. L A C M I P loc. 332 3 0 L A C M I P loc. 5194 1 0 L A C M I P loc. 300 71 0 L A C M I P loc. 430 6 0 U C L A loc. 2314 5 3 U C L A loc. 4242 l0 6 Palos Verdes Sand, San Pedro, Los Angeles Co. CAS 97.01 9 2 CAS 61649.01 2 0 Palos Verdes, Los Angeles Co. U C M P A-6734 2 0 U C L A loc. 1309 1 0 U C L A loc. 1310 3 0

Hippothoa Spionids

Crepidula Penetrantia

Any

No Biont

1 0 1

2 1 3

0 0 0

0 1 3

28 (0.58) 1 (0.03) 6 (0.54)

20 32 2

0

I

0

0

1 (0.20)

4

0

0

0

1

2(0.18)

8

0

1

0

0

1 (O.5O)

1

0 0 0

0 0 1

0 0 0

0 2 1

0 1 (0.11) 1 (0.08)

4 6 1o

1 3 0

13 14 0

0 0 0

23 46 1

40 (0.61) 58 (0.27) 7 (0.25)

19 158 21

1

3

1

3

4 (0.66)

2

1"*

0

1

0

2 (0.09)

19

0

4

0

9

7 (0.32)

15

1

0

0

2

2 (0.66)

1

0

0

0

0

0

2

1 I

0 1

0 0

0 0

1 (0.50) 1 (0.20)

1 1

0 0

0 0

0 0

0 0

0 1 (0.50)

2 1

0 1 4 1 0 0

0 0 2 0 0 1

0 0 0 0 0 0

0 0 0 0 1 0

0 1 (1.0) 5 (0.07) 1 (0.16) 3 (0.60) 6 (0.60)

0 0 67 5 2 4

I 1

1 0

0 0

1 0

4 (0.44) 1 (0.50)

4 1

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

2 l 3

S.E. Walker, J. T. Carlton/Palaeogeography, Palaeoclirnatology, Palaeoecology 114 (1995) 197 217

209

Table 5 (continued) Localities

n

Spirorbids

Hippothoa

Pt. Vincente, Los Angeles Co. (Palos Verdes: Terrace 3) LACMIP loc. 5100 25 9 0 Rancho Palos Verdes, Los Angeles Co., Terrace 2 LACMIP loc. 6154 4 1 0 Terrace 4 LACMIP loc. 6153 4 2 0 ?Terrace 4 LACMIP loc. 2057 33 2 0 Palos Verdes Hills, Los Angeles Co., Terrace 5 LACMIP 1310 4 0 0 Santa Barbara Qd., Santa Barbara Co., California UCMP A-4965 3 0 1 Cayucos, San Luis, Obispo Co. UCLA loc. 3393 10 0 0 Total Frequency

713

166 0.23

22 0.03

1982; Boekschoten, 1967; Sch/ffer, 1972; Baluk and Radwanski, 1985). But biont settlement on gastropod apertures in soft-sedimentary environments is also indicative of hermit crab-occupancy (Walker, 1988, 1992). Here, we have shown that rocky intertidal shells also contain taphonomic signatures unique to hermitted shells: spirorbid polychaetes, the slipper limpet Crepidula from southern California shells, the encrusting bryozoan, Hippothoa, and to a minor extent barnacles (Chthamalus) and spionid polychaetes (Polydora sp.) which occupy the apertural areas. In all, 61% of the Recent hermitted shells were pagurized, while the remaining 39% of shells had no indication of hermit crab-occupancy. This is startling, as it indicates that not all hermitted shells will be recognized in the fossil record and that their occurrence will be underrepresented. We speculate that if 50% of gastropod shells are pagurized from ancient rocky shore deposits, then all of the shells were most likely inhabited by hermit crabs before burial. The taphonomic problems associated with hermit crabs using gastropod shells have been discussed by Walker (1989). Environmental determinations may also be deduced, using habitat-restricted bionts, such as the spirorbids we found in this study. Spirorbids were very common on rocky shore hermitted shells, but did not occur on experimental shells in the

Spionids

Crepidula

Penetrantia

Any

No Biont

0

0

6

a12 (0.48)

13

0

0

0

1 (0.25)

3

0

0

2

2 (0.50)

2

1

0

3

3 (0.09)

30

0

0

0

0

4

0

0

0

1 (0.33)

2

0

0

50 0.07

2 0.002

0

0

10

107 0.15

207 0.29

483 0.68

mud flat site (this study) or rarely occurred on hermitted shells encountered at the mud flat site (Walker, 1988: Table 1). The occurrence of these mud flat spirorbids most likely indicates that the hermit crab had transported a rocky shore gastropod into the mud flat area. The rocky shore gastropod leads two different lives from a fossil point of view, that of the living snail, and one as a hermitted shell (Fig. 8). We have already discussed how living gastropods can be recognized from hermitted shells, but the crucial question is whether these taphonomic signatures remain after the shells have tumbled in the high energy beach zone. We examined a modern death assemblage of Tegula and found that the taphonomic signatures were retained on the shells from a coarse, quartz diorite beach substrate. Surprisingly, most of the death assemblage contained whole shells. Few pagurized shells and fragments were encountered. The bionts were preserved preferentially in what we call "taphonomic refugia" or areas which are more resistant to scouring forces in these high energy environments. Taphonomic refugia on Tegula were the indented suture areas between adjacent whorls, the apertural notch, umbilicus, and internal aperture regions. We surmise that the beach drift assemblage represented mostly predation on living snails, by crabs and sea stars. Pagurized Tegula shells were rare,

S.E Walker,J. T. Carlton/Palaeogeographv Palaeoclimatology,Palaeoecology114 (1995) 197 217

210

(16.1

1.00

[s] .80,

re)

k : : :l~=J I~IIIII[II

.40..

.

[8)

2

0

iJ Itllli

~

m

,00 SIZE CLASS

OF SHELL

[2]

1.00.

o .80,

[5)

(sJ

U~ ~0 . 6 0 ,

I~

.40°

~

.20

~

.OC

[I) (I) (ij r~j~

m JIIItlN

f*tN'II[III :'<" 3

II

|

1

2

"

J

•

. . . . . .

II| ....

I

4

SIZE CLASS OF SHELL

1.oo1 0

i-1

~ .60,

>.~

~

[2)

(2)

(2)

..° .o. ••"" •• iii

[I73

i!i

i!ii

~ (181 !

D

[~

.40,

Spirorbids

~

No

Encrusting bryozoan

[=~

Barnacles

biont

Spionid

I

/

Crenidula

Boring bryozoan

~

Pagurid shell bionts

.20.

0

~

.00

i il 2

i .:.

11111 3

4

SIZE CLASS OF SHELL Fig. 7. Frequency of bionts (number of bionts/shell size class) on Recent T. funebralis shells and frequency of biont fossils and/or trace fossils (number of bionts/shell size class) on Pleistocene Tegulafunebralis. Numbers above histograms represent the number of shells with that particular biont. "Pagurid indicating bionts" represents the minimum number of bionts that determines whether a shell has been occupied by a hermit crab. (top) Palos Verdes, Los Angeles Co., Recent, (n = 30). (middle) Palos Verdes, Los Angeles Co., Pleistocene (n = 28), pooled Pleistocene samples from these localities: LACMIP 1310; LACMIP 1309; CAS 97.01; CAS 1198.01; CAS 61647.01; CAS 1478.01; CAS 61649.01; CAS 61645.01. (bottom) Pt. Loma, San Diego Co., Pleistocene (n = 30), UCLA Loc. 3605.

S.E. Walker, J. T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995) 197-217

211

P L A T E II

A. Pagurized T. funebralis with spionid bore hole (short arrow) and Hippothoa etching (long arrow), Pleistocene, San Pedro (Nob Hill), California, CAS 61645.02, shell height 20.0 mm. B. Pleistocene steinkern of T.funebralis with spionid trace fossil imprints on outer whorl (arrow points to a spionid tunnel). Fungal pits are evident in apex of steinkern; Pleistocene, CAS 52980, shell height 14.7 mm. C. Pagurized Tegula funebralis with spirorbids in notch and umbilicus, Crepidula ?perforans in aperture (arrow), Recent, Palos Verdes, California, shell height 12.7 mm, Kuris collection. D. Pagurized Tegula funebralis with Crepidula ?perforans (arrow) in aperture and boring bryozoan on base, Pleistocene, Pacific Beach, San Diego Co., California, CAS 53780.02, shell height 16.7 mm.

212

S.E Walker, J. 72 Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197-217

I LivingSnailI DeathRelatedFactors

I

1. Natural Death

2. Sea Star Predation

1. Bird predation

I

Crab Predation

I

2. Fish Predation

3. Storms

I

i

V

t

ImmediateTaphonomicResults 1. Epibionts on external shell surfaces only: barnacles and spirorbids 2, Shell intact for the most part •

~

I shens I crushed I

I I II II

/

e

/ /

1 Shells I crushed I ,, 2. Shells peeled 3. Epibionts retained on fragments

~,~

~

| | I |

MarineHermitCrabs ~1

a.) Epibionts: spirorbids, barnacles on mid-intertidal shells within apertures b.) Predators: birds, scientists, in mid-intertidal 2. Between-Habitat Effects (including low intertidal):

THEPOTENTIAFOSSI L LRECORD ofRockyShoreGastropods 1. Snail Taphonomic Factors: Pristine apertural areas; barnacles (common)and spirorbids on external shell surfaces. In high-energy regimes, epibionts will be preserved within "taphonomic refugia" (sutures and rugosities of the shell). Shells are subject to "predation" while empty or inhabited by a living snail or hermit crab.

a.) Epibionts: spirorbids, barnacles, bryozoans, spionids, Creoidula within apertures. b.) Predators: crabs, sea stars, birds, fish.

2. Hermit Crab Taphonomic Factors: Apertural areas colonized by a diversity of epibionts; In high energy regimes, these bionts will be represented by traces of etching bryozoa (Hinoothoa sp.) or body fossils within taphonomic refugia (apertural notch, aperture, umbilicus, columella, outedip). External bionts cannot be distinguished between hermits and living snails. 3. Fragments of shells produced by predators. Crab predation recognized by deeply peeled apertures, last whorl fragments, or apical fragments with sheared edges.

Fig. 8. Taphonomic gauntlet for rocky shore gastropods from temperate regions, based on Tegula.

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 197~17

we surmised, because the shells were recycled by the hermit population to a point that they were rarely "preserved" in the modern beach assemblage. In contrast, the Pleistocene Tegula retained between 0% and 66% of the biont information indicating pagurized shells depending on locality (refer to Table 5). Overall, 29% of the fossil Tegula from rocky shore deposits totaling 39 localities, retained the taphonomic signature of pagurization. This overall percentage is half the finding for pagurized shells from the living population of hermitted Tegula funebralis. As in the modern beach drift example, pagurized shells are not well represented in modern or fossil death assemblages from rocky shore localities. We suggest this difference may come from the preference of hermit crabs for Tegula, further enhancing its destruction in high energy environments. The taphonomic gauntlet for gastropods is not restricted to just the level of living snails and hermitted shells. Experiments with tethered shells in two different habitats (mud fiat and rocky intertidal) showed that biotic and physical factors were very different between the two habitats (Fig. 9). Physical factors, such as abrasion, were more prevalent at the rocky shore site than at the mud flat site. While this may be intuitively obvious, what is surprising is how long the shells lasted at the rocky site. Despite 10 months tethered in a high energy system on quartz diorite substrate, only one shell was missing, and abrasive damage was only evident within 10 months of emplacement of the experimental lines. In contrast, at the mud flat site, biotic factors were the primary determinant of shell longevity. Cancer crab predation contributed to shell loss of Tegula, with unplugged shells (which occasionally housed hermit crabs), sustaining the greatest amount of predation with only 30% of the shells surviving at the end of the study. Of the plugged Tegula (to eliminate the hermit crab problem), 66% of the shells survived. Although Cancer crabs were present in the lower intertidal of the rocky shore site, they apparently did not migrate up shore with the high tides as has been reported to occur (Robles, et al., 1989). Peeled and crushed fragments of Tegula brunnea were frequently encountered around dens of

213

Cancer crabs in the lower intertidal of the rocky shore site (Walker, 1990). Tegula funebralis fragments in the beach drift at the rocky site also indicated crab predation. Perhaps if shells were tethered in the lowest intertidal at the rocky shore site, these shells would have experienced heavy predation similar to the mud flat site. A question may be raised concerning the fate of gastropod shells that are not experimentally tethered. This question can not be simply answered, as numerous factors could affect the taphonomic fate of gastropod shells--from other competing shell-inhabiting species (McLean, 1983) to physical factors. Vance (1972) added over 12,000 empty shells to a rocky intertidal reef in the San Juan Islands, Washington, and showed that hermit crab density was greatly increased by the increased availability of shells. However, compared to the amount of shells released, few shells were reencountered after a year (Vance, 1972: table 1). He attributed some of this loss to currents sweeping the shells away. He did not mention the eventual fate of those shells nor the fact that the hermit crab he studied (P. hirsutiusculus) has a large bathymetric distribution (intertidal to 110 m; Morris et al., 1980) and could take the shells out of the original environment. Alternately, another experiment released 1410 hermitted Tegula gallina shells in a rocky intertidal habitat near San Luis Obispo, California, to determine resource partitioning and quality of the shell resource over time (Kuris et al., unpubl, ms.). Shell attrition was greatest in the first two months of the study, with increasing loss due to wear (abrasion) after the first six months of the study (averaging about 500 recaptured shells during this time). After nine months within the hermit crab guild, most of the shells were too damaged to be used and were abandoned by the crabs. Kuris et al. (unpubl.) also found that complete encrustation (e.g., Hippothoa, barnacles, spirorbids) occurred within 120 days of the expermentally released shells. Walker (unpub.) conducted a similar experiment by releasing 500 pristine but empty shells of Olivella biplicata into the rocky intertidal zone of Horseshoe Cove. Within one month, only 31% of the shells were recaptured, with 85% of the shells

214

S.E. Walker,J.T. Carlton/Palaeogeography,Palaeoclimatology,Palaeoecology114 (1995) 197-217

[ Experimental Shells

Experimental Shells I Rocky Intertidal (Teaulal

Mudflat Intertidal (Tegula)

V

V

TAPHONOMIC PROCESSES AND/OR SIGNATURES ON EXPERIMENTAL, EMPTY SHELLS Freauencv of Occurrence

Type of Process/ Signature

Frequency of Occurrence

Rare

SHELL LOSS

Extremely Common

Common

PHYSICAL ABRASION

None

None

SHELL PREDATORS

Cancer crabs: common

Common

EPIBIONTS

Rare

Rare

ENDOBIONTS

Rare

None

SULFIDE RESIDUE

Common

FOSSIL RECORD

FOSSIL RECORD

1. Physical abrasive factors present: worn apices, holes in shells, abraded shell surfaces.

1. Physical breakdown minimal in 11 months.

2. Mistaken predation dependent on habitat/tidal level; movement of predators or hermits crabs.

Mistaken Predation by Cancer crabs is high; peeled and crushed shells are common. 3. Preservable Epibionts: none 4. Endobionts: algal pittings

3. Preservable Epibionts: spirorbids, mid- to high intertidal. 4. Endobionts: boring green algae.

Fig. 9. Taphonomicdifferencesin shellpreservationbetweenrockyintertidaland mud flat sitesbased on experimentallytetheredshells. occupied by Pagurus granosimanus, 3% occupied by P. samuelis and 14% were empty. Only bluegreen algae were encountered on the shells within the first month of release. Within four months, 32% of the shells were recaptured with 20% of the shells encrusted with spirorbids. Six months later, when the experiment was terminated, 26% of the Olivella shells were recaptured, all occupied by Pagurus granosimanus; 27% had spirorbid encrustation and 1% of the shells had Hippothoa. Thus, it appears that at least 30-35% of experi-

mentally released gastropod shells (hermitted or empty) are retained within the intertidal guild for at least 6 months, before significant shell wear and encrustation occurs. Shell abrasion was extensive within ten months on experimentally tethered shells (this study) and within nine months on released hermitted shells (Kuris et al., unpubl.). It is not known what happens to the rocky intertidal shells when they are swept away on these precipice shores nor how periodic burial affects them. In burial experiments conducted in the mudflat at the

S.E. Walker, J.T. Carlton/Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995) 197-217

Bodega Bay Reserve, Walker (1988: p. 57) showed that some pagurized bionts will not survive burial on a short-term basis (within 90 days). Barnacles were preserved least, with encrusting bryozoans surviving intact after 90 days of burial. It is surmised, based on what was preserved in the beach drift at horseshoe cove, that the taphonomic loss of bionts during burial and exhumation would be much greater at the rocky shore site than in the quieter, mudflat environment. The experimental shells in the rocky intertidal were restricted to the same tidal height and tide pools as the hermit crab, P. samuelis, but did not develop the diversity of bionts evident on the pagurized shells. Encrusting bryozoans and barnacles were not present on experimentally tethered shells, but were present on P. samuelis at the same tidal height. This discrepancy may be due to the hermit crab's ability to transport the shells across different habitats, tidal heights, subtidal depths and their propensity to switch shells with other species (Hazlett, 1981; Walker, 1989). In the Pleistocene of California, the gastropod fossil record is compromised by the predilection of hermit crabs for gastropod shells and constrained by the destructive influence of predatory crabs. These two biological agents are important taphonomic modifiers of the gastropod fossil record. Physical factors may be important in the formation of fossil assemblages, but biological factors appear to be, in the environments examined here, an overriding causal factor influencing the quality of the fossil resource in these assemblages.

Acknowledgements We thank A. Boucot, G. Cadre, E. Dilworth, K. Flessa, A. Johnson, M.A.R. Koehl, W. Miller III, J. Valentine, and an anonymous reviewer for their critique of this manuscript. A. Kuris and C. Hickman provided invaluable advice. A. Kuris kindly provided the Recent shells from Palos Verdes Peninsula and access to his unpublished work on hermit crab guilds. Research at Bodega Marine Reserve was made possible by C. Hand, J. Clegg, and P. Siri. Access to Museum collections was kindly provided by G. Kennedy (LACMNH),

215

D. Fautin and D. Chivers (CAS), and D. Lindberg (UCMP). J. West (UCB) kindly identified the green algae. This research was supported, in part, by Sigma Xi, the Lerner-Gray Fund, Committee on Grants and Research (UCB), and a Department of Paleontology Friends of Fossil Summer Research Grant to SEW. Revisions were supported by NSF grant EAR-9196158 to SEW.

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