Copyright � 2006, SEPM (Society for Sedimentary Geology) 0883-1351/06/0021-0548/$3.00

PALAIOS, 2006, v. 21, p. 548–556

Research Report

DOI: 10.2110/palo.2005.p05-086r

A WOOD-FALL ASSOCIATION FROM LATE EOCENE DEEP-WATER SEDIMENTS OFWASHINGTON STATE, USA

STEFFEN KIEL1* and JAMES L. GOEDERT2

1Earth Sciences, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK and Department of Paleobiology, Smithsonian Museum of NaturalHistory, Washington, DC 20013-7012, USA; 2Burke Museum, University of Washington, Seattle, Washington 98195, USA

e-mail: steffen.kiel@gmx.de

ABSTRACT

Fossil wood fragments and an associated species-rich invertebrateassemblage, analogous to those found on wood falls in the deep seatoday, were found in late Eocene deep-water sediments of the LincolnCreek Formation in Washington State, United States. This assem-blage is the earliest known complex deep-sea biologic communitybased on decaying wood as its primary source of nutrients. The 495recovered fossils (exclusive of foraminiferans) belong to 21 species; 7species relied directly on the wood, either by ingesting it or by feedingon xylophagous microbes; these species are also the most abundant.Seven species were predators or scavengers that were most likelyattracted by the wood-dependent species. The remaining seven spe-cies represent predators, detritus feeders, and suspension feeders thatmay or may not have had a relation to the wood fall or its fauna.All species had a benthic mode of life, and pseudoplanktonic taxa areabsent, indicating that the colonization of the wood began only onceit had arrived on the deep-sea floor. The wood-dependent speciesbelong to taxa that fill the same ecologic niche in the deep sea today,indicating that the modern wood-fall ecosystem had evolved at leastby late Eocene time. There is no uniformity or specialization of dis-persal strategies among the recovered taxa; they rather reflect thoseof the phylogenetic group to which they belong. The wood-fall assem-blage described here shares several families with fossil whale fallsand cold seeps but very few species, a condition that can also beobserved at modern examples of these ecosystems.

INTRODUCTION

Wood falls are pieces of water-logged wood that have sunk to the deep-sea floor. Once they have arrived at the sea floor, they are commonlycolonized by distinctive faunal communities (Turner, 1978; Wolff, 1979;Marshall, 1983, 1985, 1988; Dell, 1987). These faunal communities differin their taxonomic composition, ecology, and life histories from the ben-thic soft-bottom fauna of the surrounding deep-sea floor, but they showsimilarities to ecosystems around sunken whale carcasses (whale falls),hydrocarbon seeps (cold seeps), and hydrothermal vents (Van Dover,2000; Smith and Baco, 2003). To a lesser extent they also show similar-ities to communities associated with other accumulations of plant materialin the deep sea (Wolff, 1979; Smith and Baco, 2003). Earlier studies ofwood-fall faunas focused on food chains and how the nutrients stored inthe wood are transformed and distributed across the sea floor (Turner,1978). In recent years, wood falls have attracted scientific attention be-cause they were considered to have served as an evolutionary steppingstone for taxa that now live at hydrothermal vents and cold seeps (Distelet al., 2000). This hypothesis requires that the taxa in question appeargeologically earlier at wood falls and whale falls than at cold seeps andhot vents.

Except for teredinid-bored wood, fossil wood-fall associations are rare.

*Corresponding author.

A few occurrences of docoglossate and cocculiniform limpets associatedwith wood or organic-rich sediments were described from Miocene andPliocene strata in New Zealand (Marshall, 1985, 1986), and the doco-glossate Pectinodonta xylodia was found with wood in early Oligocenestrata in Washington State, United States (Lindberg and Hedegaard,1996). The purpose of the present study is to document a highly localizedand species-rich wood-fall association from late Eocene deep-water sed-iments in Washington State, to reconstruct the ecology of this association,and to discuss the evolutionary implications. The newly discovered wood-fall assemblage provides an excellent opportunity to evaluate the woodstepping-stone hypothesis in light of these new data.

MATERIAL AND GEOLOGICAL SETTING

The studied wood fall is from fossil-poor deep-water sediments in thelower part of the Lincoln Creek Formation (LCF) in western WashingtonState, United States (Fig. 1). The LCF in the study area is an approxi-mately 3000-m-thick sequence of monotonous, thick-bedded siltstonesand mudstones of late Eocene to early Miocene age (Rau, 1966; Protheroand Armentrout, 1985). Stratigraphically the wood fall was located in thelate Eocene part of the sequence (Rau, 1966), approximately 275 m abovethe contact with the underlying basalts. It was found on the west bankof the Middle Fork of the Satsop River immediately south of where asmall stream enters from the east, approximately 200 m south and 190m west of the northeastern corner of Section 20, T. 21 N., R. 6 W., DryBed Lakes, USGS 7.5 minute quadrangle, Provisional Edition, 1990, Ma-son County, Washington State. Based on Foraminifera, Rau (1966) esti-mated that the sediments in which the wood fall was found were depos-ited at a depth between 500 and 800 m.

The sampled interval that contained the wood fall and the associatedinvertebrates was about 5 cm thick and extended laterally for less than 1m. This highly localized area stood out because of its high concentrationof molluscan fossils, which is in strong contrast to the almost fossil-freesurrounding sediments. It also yielded fragmentary wood and a few con-cretionary lumps. The great majority of the macrofossils and microfossilswere recovered from the unconsolidated sediments; the concretions con-tained additional fragmented wood, loose and in situ calcitic linings ofXyloredo borings, and small patches of pyrite. The concretions are com-posed of the same siltstone that forms the surrounding sediment but witha gray-green color and a micritic matrix. A detailed analysis of the dia-genesis of the concretions is beyond the scope of this study.

The wood-fall assemblage yielded 17 species of Foraminifera, most ofthem with agglutinated tests: Rhizammina sp., Dendrophrya sp., Kala-mopsis grzybowskii (Dylazanka), Placentammina placenta (Grzybowski),Psammosphaera fusca Schultze, Reophax splendidus Grzybowski, Reo-phax sp. 1, Reophax sp. 2, Cribrostomoides trinitatensis Cushman, (?)Labrospira pacifica Krasheninnikov, Cyclammina amplectens Grzybows-ki, Cyclammina rotundidorsata (Hantken), Chilostomelodes sp., Lenti-culina rancocasensis Olsson, Dentalina nana Reuss, Lagena sulcatifor-mis Pozarzska and Urbaneck, and Oridorsalis sp. The simultaneous oc-currence of C. amplectens and C. rotundidorsata further supports the late

PALAIOS 549LATE EOCENE WOOD FALL

FIGURE 1—Location of the LCF wood fall.

Eocene age estimate for the assemblage (D. Georgescu, personal com-mununication, 2005).

METHODS

Sample Processing and Documentation

The sampled fossil-bearing sediments were processed with a 5% to10% H2O2 solution and washed through sieves of various mesh sizes,and the residue was screened for fossils; 495 specimens (excludingForaminifera) were recovered. Specimens were photographed using PhilipsXL30 ESEM and Leo 1455VP scanning electron microscopes and Olym-pus Camedia C-3040 Zoom and Kodak Professional DCS Pro14n digitalcameras. Macrofossils were coated with ammonium chloride before beingphotographed. The fossils are deposited in the Cenozoic mollusk collec-tion of the U.S. National Museum of Natural History, Washington, D.C.(USNM). Specimens were measured with a Fowler ultra-cal II digitalcaliper with an accuracy of 0.01 mm. Shell length was measured for thelimpets; maximum diameter was measured for the coiled specimens. Theonly specimens that were measured are those in which the preservationof the apertural margin suggested that no significant portion of the shellwas lost.

Relationships between Wood and the Fauna

The highly localized occurrence of this wood-fall association withinfossil-poor deep-water sediments and the abundance of taxa that alsoinhabit wood falls today suggest that this assemblage represents an insitu wood-related faunal community. However, the possibility cannot beexcluded that some members of the assemblage have been washed in,either before, during, or after the wood served as food, substrate, orshelter. Some species might also have been living at the site prior to thearrival of the wood. The following approaches were used to assess thedegree to which the encountered species might be wood related

1. An Actualistic Taxonomic Comparison to Extant Wood-Fall Com-munities.—In this approach only those taxa that also occur at extant woodfalls are considered to be related to the wood. The disadvantage of thismethod is that it does not recognize extinct wood-related taxa and thuswill not reveal any evolutionary changes in the faunal compositionthrough time, because putatively extinct wood-related taxa are a priorineglected.

2. The Abundance of Individuals of a Particular Species.—Whereas

species are rare on the general deep-sea floor, they are often very abun-dant on wood falls (Marshall, 1988). Thus the assumption is made thatthe more abundant a species, the more likely it is to be an actual memberof the wood-fall community. This method has the potential to overcomethe problem of the a priori exclusion of extinct wood-fall taxa of theprevious approach. Drawing the line between species that are abundantenough to be considered wood-fall related and those that are not, however,is subjective. Another problem is that species from different trophic levelsoften differ markedly in abundance (e.g., an abundant primary consumerversus a rare predator).

3. Ecologic Relationships.—Here it is reasoned that a species whosemain food source was evidently available in the assemblage is more likelyto have been part of the wood-fall community than a species whose foodsource was not or can only be inferred. This approach has the potentialto overcome the problem of approach 2, i.e., that species from differenttrophic levels have different abundances. However, a problem similar tothat of the first approach arises, inasmuch as the feeding strategies ofmost fossil species are derived from those of their modern relatives. Thus,in the long run this approach likewise fails to detect ecologic changesthrough time, at least above the taxonomic level for which ecologic uni-formity can be assumed.

4. Comparison to the Background Fauna.—Species that are knownfrom the background fauna of the Lincoln Creek Formation are morelikely to be present regardless of the wood than species that do not occurelsewhere in the formation. However, the utility of this approach is clearlylimited because species that are higher up in the food chain are, in fact,likely to recruit from the background fauna.

RESULTS AND DISCUSSION

Fauna

All species were determined to the lowest taxonomic level possible.Those species indicated as new will be described in detail in a forthcom-ing taxonomic paper. The paleoecology of each species inferred fromextant relatives is summarized in Table 1. This table also provides furtherinformation on their taxonomic position. Most specimens have their orig-inal shell mineralogy and microstructures preserved and have a thin coatof silt adhering to them.

Bivalvia.—Specimens of the wood-boring Xyloredo sp. (Fig. 2A) reach5 mm in height. Characteristic calcitic tube linings of this genus werefrequently found in the concretions and loose in the surrounding sedi-ments. The mytilid Idas n. sp. (Fig. 2B) is the oldest record of this genus.Some specimens have the prodissoconch (larval shell) preserved, whichcan be distinguished from the brownish adult shell by its whitish color.This difference in color is most probably due to a difference in miner-alogy; while the prodissoconch is composed entirely of aragonite, theouter layer of the adult shell is made of calcite (Carter, 1990). The prod-issoconch is 560 �m long and high, indicating a long free-swimminglarval stage.

Gastropoda.—With 15 species and more than 400 specimens, the Gas-tropoda are by far the most species-rich, abundant, and ecologically di-verse group in the assemblage. Dominant is a nondescript skeneiformspecies (Fig. 2H) up to 4.5 mm in diameter. Morphologically similarspecies occur on extant wood falls (Marshall, 1988), but unfortunatelythe protoconch (larval shell) of the LCF species is not preserved, whichprevents a more precise classification. Neither is its crossed lamellar shellmicrostructure conclusive for taxonomic purposes. The same shell struc-ture and lack of protoconch also prevents further identification of thecocculiniform limpet (Figs. 2D–E) that grew to about 4 mm in length.The finding of Leptogyra n. sp. is interesting (Figs. 2F–G) in that itrepresents the first fossil record of this vent- and wood-fall-inhabitinggenus.

Provanna antiqua (Fig. 2C) is reported here for the first time from awood fall. Squires (1995) documented the highly variable sculpture ofthis species, including all transitions between smooth, spiral, axial, and

550 PALAIOSKIEL AND GOEDERT

TABLE 1—Ecology and feeding strategies of the recovered specimens inferred from their extant relatives.

Taxon Ecology inferred from extant relatives Source

Bathybembix sp. (Trochidae: Gastropoda) Herbivore, detritus feeder; mainly deep water; knownfrom kelp falls

Miller et al. (2000)

Buccinids (Neogastropoda) General predators and scavengers; shallow and deepwater

Taylor et al. (1980)

Cadulus sp. (Scaphopoda) Predator, mainly Foraminifera, but also other micro-prey; shallow and deep water

Shimek (1990); Gudmundsson et al. (2003)

Cocculinid (Gastropoda) Lives on various biogenic substrates in the deep sea;presumably bacteria grazer

Haszprunar (1998); Smith and Baco (2003)

Dentalium laneensis (Scaphopoda) Predator, mainly Foraminifera, but also other micro-prey; shallow and deep water

Shimek (1990); Gudmundsson et al. (2003)

Exilia sp. (Fasciolariidae: Gastropoda) Predator on amphipods; mainly deep water Harasewych (1987); Kantor et al. (2001)Foraminifera Use wood as shelter and substrate; detritus feeder,

predators?Wolff (1979)

Gemmula sp. (Turridae: Gastropoda) Predator on polychaetes; egg cases of this familyfound attached to wood; shallow and deep water

Turner (1978); Taylor et al. (1980)

Idas n. sp. (Mytilidae: Bivalvia) At seeps, whale falls and wood falls; mainly deepwater; chemosymbiotic or suspension feeding

Distel et al. (2000); Smith and Baco (2003);Olu et al. (2004)

Isocrinid (Echinoderm) Suspension feeder; wood as substrate? Baumiller (1997); Meyer (1997)Leptogyra n. sp. (Neomphalidae: Gastropoda) Occurs at vents and wood falls; presumably bacteria

grazerMarshall (1987); Waren and Bouchet (1993)

Macrocyprinid (Ostracoda) Sediment ingester, detritus feeder Maddocks (1990)Muricid (Neogastropoda) Predator on mollusks and other invertebrates; shallow

and deep waterPonder (1998)

Natica? weaveri (Naticidae?: Gastropoda) Predator on mollusks? Grazer? Shallow and deep wa-ter

Bandel (1999); Kase and Ishakawa (2003)

Parasyrinx n. sp. (Turridae: Gastropoda) Predator on polychaetes; egg cases of this familyfound attached to wood; mainly deep water

Turner (1978); Taylor et al. (1980)

Provanna antiqua (Provannidae: Gastropoda) Family occurs at vents, seeps, whale falls, and woodfalls; bacteria grazer

Waren and Bouchet (2001); Smith and Baco(2003)

Skeneid (Trochoidea: Gastropoda) Several unrelated groups known from wood falls;presumably bacteria grazer; mainly deep water

Marshall (1988)

Xylocythere sp. (Ostracoda) Commensal or parasitic? Detritus? Shallow and deepwater

Maddocks and Steineck (1987)

Xyloredo sp. (Pholadidae: Bivalvia) Ingests wood, has xylophagous endosymbionts; onlydeep water

Turner (1973); Distel and Roberts (1997)

reticulate sculpture. About half of the LCF wood-fall specimens aresmooth; the other half show transitions to spiral or weakly reticulatesculpture. Several predatory neogastropod species are present; six of themcan be assigned to genera or families, including the Buccinidae (Fig. 2M),Fasciolariidae (Exilia sp.; Fig. 2I), Muricidae, and Turridae (Gemmula sp.and Parasyrinx n. sp.; Figs. 2K–L). The largest of them is a muricid (Fig.2J) at 12 mm in height. The presence of three specimens of Natica?weaveri (Fig. 2Q) may present a paleoecological problem. If this speciesindeed belongs to the shell-boring, carnivorous gastropod family Natici-dae, then the absence of the typical drill holes from the molluscan shellsin the assemblage may indicate that the naticids were not feeding on theabundant mollusks at this site. Because this is rather unlikely, the absenceof drill holes could suggest that the naticids were present as dead shellsprior to the arrival of the wood. However, the absence of naticid drillholes could also indicate that the naticid specimens in fact belong to thefamily Ampullinidae, a grazing group that builds convergent shells (Ban-del, 1999; Kase and Ishakawa, 2003) and not the Naticidae. The largestspecimen in the assemblage is the trochid Bathybembix sp. (Fig. 2P),which is 20 mm high. Bathybembix is a typical deep-water genus, andone extant member, B. bairdii, was found associated with kelp falls (Mill-er et al., 2000).

Scaphopoda.—Two species are present: Dentalium laneensis (Fig. 2N)reaches up to 13 mm in length, whereas the largest Cadulus sp. (Fig. 2O)is only 2.3 mm long. Cadulus sp. may be mistaken for elongate pteropods(planktonic gastropods). The specimens in the LCF wood-fall assemblagewere clearly identified by their crossed lamellar shell microstructure,whereas the shells of pteropods are composed of a peculiar so-calledscrew layer (Bandel, 1977).

Ostracoda.—The most interesting ostracod in the assemblage is Xylo-

cythere sp. (Figs. 2S–T), with 6 specimens between 570 and 590 �m inlength. Extant members of this genus have so far only been found athydrothermal vents and wood falls. Rare fossil species as old as Oligo-cene were reported from DSDP cores but without any connection to wood(Maddocks and Steineck, 1987; Steineck et al., 1990; van Harten, 1992).Five specimens of a macrocyprinidid are present (Fig. 2U) and are be-tween 1.1 mm and 1.57 mm long. In the northeastern Pacific Ocean today,this genus has never been reported from farther north than southern Cal-ifornia (Maddocks, 1990). One 0.76-mm-long specimen of the deep-seagenus Krithe (Fig. 2R) was found.

Other.—Two isolated stem plates of an isocrinid (Echinodermata) anda fragment of a single middle valve of an unidentified polyplacophoranwere found.

Colonization

All recovered species had a benthic mode of life. Although several ofthe species evidently or possibly had a planktonic larval stage, theselarvae are not known to settle on floating wood but settle only on the seafloor. Teredinid bivalves are known to infest floating wood, but xylopha-gains do not (Turner, 1955, 1966). No barnacles like balanids or scalpel-lids (goose-neck barnacles) were found in the assemblage. These seden-tary animals often colonize drifting objects like wood; their absence fromthe LCF wood fall indicates that the wood had not drifted for an extendedperiod of time. Thus the colonization of the wood by the recovered in-vertebrates began once the wood had arrived on the deep-sea floor. Thecourse of this colonization, however, is difficult to reconstruct based onthe present evidence.

PALAIOS 551LATE EOCENE WOOD FALL

FIGURE 2—The fauna recovered from the concretions and the surrounding sediments. (A) Xyloredo sp. (USNM 528918). (B), Idas n. sp. (USNM 528919). (C) Provannaantiqua Squires (USNM 528920). (D, E) Cocculiniform limpet (USNM 528921). (F) Protoconch of Leptogyra n. sp. showing reticulate sculpture (USNM 528922). (G)Adult shell of Leptogyra n. sp. (USNM 528923). (H) Skeneiform gastropod (USNM 528924). (I) Exilia sp. (USNM 528925). (J) Muricid (USNM 528926). (K) Parasyrinxn. sp. (USNM 528927). (L) Gemmula? sp. (USNM 528928). (M) Buccinid (USNM 528929). (N) Dentalium laneensis Hickman (USNM 528930). (O) Cadulus sp. (USNM528931). (P) Bathybembix sp. (USNM 528933). (Q) Natica? weaveri Tegland (USNM 528932). (R) Krithe sp. (USNM 529333). (S) Xylocythere sp. (USNM 529334). (T)close-up of Xylocythere sp. (USNM 529335). Arrows � clusters of micropores in two pits on shell’s surface. (U) Macrocyprinid (USNM 529336).

Lindberg and Hedegaard (1996) suggested that colonization of woodby the limpet Pectinodonta is a singular event caused by a cloud oflarvae, followed by intense competition for the limited space availableon the sunken log. This conclusion was based on the striking uniformityof the size-class structure of these limpets found on recent and fossilwood and on a growth pattern of initially rapid lateral growth followedby a switch to more vertical growth (Lindberg and Hedegaard, 1996). A

size-class analysis of the two most abundant species at the LCF woodfall (Fig. 3), a coiled gastropod and a limpet, add an interesting perspec-tive to the colonization of wood in the deep sea. Whereas the size-classstructure of the limpet could be interpreted as a singular colonizationevent, in the case of the coiled skeneid it suggests two colonizationevents. The limited space on a sunken log should be a limiting factor forall species colonizing it, not only for limpets. However, limpets may be

552 PALAIOSKIEL AND GOEDERT

FIGURE 3—Size class structure of the skeneid and the cocculinid gastropods, usingten intervals. Thin line represents the superimposed normal distribution.

FIGURE 4—Ecologic relationships. (A) Generalized food web of a wood-basedecosystem in the deep sea. (B) Inferred relationships between predator and preyamong those taxa in the LCF assemblage that are not directly related to the wood.Stippled arrows indicate relationships where either the food preferences of the pred-ators are only hypothesized (indet. neogastropods) or where there is no direct evi-dence for the prey in the LCF assemblage but its presence can be inferred frommodern analogs (amphipods, polychaetes).

better prepared to defend their space than are coiled gastropods becausethey can attach themselves firmly to the substrate. For the same reason,limpets may also be less vulnerable to shell-crushing and especiallyaperture-peeling predators.

There are indicators that the LCF wood-fall community represents atime-averaged assemblage. Turner (1978) indicated that the diversity andthe number of predatory species at experimentally submerged wood fallsincreased over time. The very fragmentary wood of the deposit shows thatthe assemblage was preserved at a very late stage of decay of the wood.Thus the high diversity and the large number of predatory species at theLCF wood-fall site could have accumulated over an extended period oftime and possibly even encompass successional stages of colonization.

Because wood falls form ephemeral and widely scattered, so-calledislands in the deep sea, it was hypothesized that their colonization re-quires a motile larval phase (Turner, 1978). Among species living at ventsand seeps, which represent equally scattered habitats, however, no specialadaptation in dispersal strategies was found; rather, they reflected thedispersal strategy of the phylogenetic group to which they belong (Lutzet al., 1986; Kiel, 2006). The species found at the late Eocene wood-fallcommunity show a wide range of dispersal strategies, as inferred frompreserved larval shells and by analogy to their living relatives. For ex-ample, Idas n. sp. has a free-swimming, plankton-feeding larva, Lepto-gyra sp. shows a free-swimming but nonfeeding larva, and Provannaantiqua has direct development (Kiel, 2006). The ostracods have no free-swimming larval stage, but they may have been transported on fish asundigested eggs (Maddocks and Steineck, 1987). Thus, the wood-fallspecies described here also show neither uniformity nor a special adap-tation among their dispersal strategies but reflect those of the phylogeneticgroup to which they belong, analogous to the mollusks at vents and seeps.

Ecology

Many species could be identified only to the level of family or genus,but a reconstruction of their feeding strategies and ecologic needs is stillpossible, because these taxa show ecologic uniformity at the taxonomiclevel to which they have been identified. Nine of the recovered specieshave extant relatives living at wood falls. These are the cocculinid, Idasn. sp., Leptogyra n. sp., Provanna antiqua, the skeneid, the two turrids,Xylocythere sp., and Xyloredo sp. Among them, only Xyloredo is endemicto wood falls; the additional habitats of the other groups are outlined inTable 1. Eleven species have no close relatives reported from modernwood falls (Bathybembix sp., Cadulus sp., Dentalium laneensis, Exiliasp., Krithe sp., Natica? weaveri, the two buccinids, the isocrinid, themacrocyprinid, and the muricid).

Figure 4A shows a generalized food web resulting from a wood fallas outlined by Turner (1978). When the species from the LCF wood-fallassemblage and their ecological needs are superimposed on this model,three groups of species can be distinguished: those with ecologic rela-tionships, those with only hypothetical ecologic relationships, and thosewithout apparent relationship to the wood fall. Ecologic relationships canbe inferred for the wood ingesters (Xyloredo sp. and the polyplacophoran)

and the microbe grazers (the cocculinid, the skeneid, Leptogyra n. sp.,and Provanna antiqua), and for those predators and scavengers shown inFigure 4B whose preferred prey is present in the assemblage (i.e., thebuccinids, muricid, Cadulus sp., and Dentalium laneensis). Only hypo-thetical relationships can be constructed for Exilia sp., Bathybembix sp.,the turrids, and the macrocyprinid, as their preferred food has not beenfound at the fossil site. In modern wood falls (Table 1), the preferred foodfor Exilia sp. is amphipods, and turrids prey on polychaetes. The grazingtrochid Bathybembix sp. might have been attracted to detritus from thewood or from its inhabitants. Macrocyprinidid ostracods are solitary,sediment-ingesting bottom crawlers (Maddocks, 1990), and the presence offive specimens in the LCF wood-fall assemblage could indicate that theywere attracted by the abundance of nutrients at this site. There are nowapparent ecologic relationships between the wood fall and the ostracodKrithe, the doubtful naticid Natica? weaveri, and the isocrinid.

The source of nutrients for Idas at the wood falls is not clear, althoughIdas is present at many modern wood falls. It may either be a suspensionfeeder like most other mytilids or may rely on chemoautotrophic (sul-phophilic) bacteria like most bathymodiolines, or both. Turner (1978)showed that the sea floor surrounding experimentally submerged woodfalls was covered by yellow-brown fecal pellets and other metabolic by-products of the animals in and on the wood. This cover of excrement is

PALAIOS 553LATE EOCENE WOOD FALL

TABLE 2—Fossil species abundance in relation to presumed niche, including known relatives at modern wood-fall deposits. Internal ranking is by the number of specimens.See text for further details about rankings.

Taxon Modern relatives No. of Specimens Ecologic relationship In background fauna? Rank

Skeneid yes 341 wood related no 1Cocculinid yes 39 wood related no 1Leptogyra n. sp. yes 39 wood related no 1Idas n. sp. yes 17 wood related no 1Provanna antiqua yes 10 wood related occurs at seeps 1Xyloredo sp. yes 8 wood related no 1Xylocythere sp. yes 6 wood related ? 1Muricid no 5 potential prey present no 2Buccinid 1 no 4 prey available no 2Buccinid 2 no 4 prey available no 2Cadulus sp. no 10 prey available yes 3Dentalium laneensis no 2 prey available yes 3Parasyrinx n. sp. yes 2 food only inferred yes 4Gemmula? sp. yes 1 food only inferred yes 4Macrocyprinid no 5 food only inferred ? 5Bathybembix sp. no 1 food only inferred yes 5Exilia sp. no 1 food only inferred yes 5Indet. neogastropods ? 5 prey available? ? 6Natica? weaveri no 3 ? yes 6Isocrinid no 2 Substrate? yes 6Krithe sp. no 1 ? ? 6

likely to cause anoxic conditions in the sediment and to sequester sulfides,which, after being released into the water, may be used as nutrients forthe putative endosymbionts of Idas.

The species abundance distribution shows that those species that mostlikely depended directly on the wood or on the microbes degrading it arealso the most abundant (Table 2). The only exception is Cadulus sp., apredatory scaphopod that preys on foraminiferans. The distinction be-tween species that are known from the background fauna of the LCF(Armentrout, 1973) and those that are not provides additional evidenceregarding their ecologic relation to the wood only in the cases of thebuccinids, scaphopods, and the muricid. The preferred prey is present forall of these species, but whereas the scaphopods occur throughout thesurrounding sediments and may thus have been at the site prior to thearrival of the wood or may have been washed in, the buccinids and themuricid are not known from the background fauna.

Based on the above assessment, the species are ranked into six cate-gories (Table 2), listed below.

1. Species that have relatives at modern wood falls, are dependent onthe wood as food source, and are not known from the background fauna.They include all wood ingesters, microbe grazers, Xylocythere sp., andperhaps also Idas n. sp. These are also the most abundant species.

2. Predatory species whose prey is found in the wood-fall assemblage.These predators (the buccinid and muricid gastropods) have not beenfound in the background fauna but are absent from modern wood falls.

3. Same characteristics as in category 2, but also occurring in thebackground fauna, e.g., the two scaphopods Dentalium laneensis andCadulus sp.

4. Species with relatives at modern wood falls, but the presence oftheir food is only inferred, and they are also known from the backgroundfauna; these are the two turrid gastropods.

5. Same characteristics as in category 4, but without relatives at mod-ern wood falls; these are the detritivores Bathybembix sp. and the ma-crocyprinidid, and the predatory gastropod Exilia sp.

6. Species with no apparent connection to the wood: Krithe sp.,Natica? weaveri, the isocrinid, and undetermined predatory gastropods.Figure 5 shows a reconstruction of the wood fall including most speciesof the categories 1 to 3 and the polyplacophoran. Also the taxa of cate-gories 4 and 5 are considered here as belonging to the wood-fall com-munity, although the evidence for their relation to the wood is less directthan for those of the first three categories.

In sum, the LCF wood-fall assemblage consists of 21 species (exclud-ing foraminiferans), of which 7 species belong to taxa that have closerelatives at modern wood falls and a tight ecologic relationship to thewood. A further seven species are predators whose prey items are eitherpresent in the assemblage or can be inferred from extant wood falls. Thislarge number of predatory species is in accordance with theoretical con-siderations and observations at extant wood falls (Turner, 1978). Thereconstruction of the food chain at the LCF wood-fall site indicates twotrophic levels among the predators, because some predators were likelyto be prey themselves (Fig. 4B). The extremely low number of shelleddetritus feeders at the LCF wood-fall site indicates that this ecologicalniche may have been occupied by polychaetes, nematodes, and amphi-pods, analogous to extant wood falls.

A single site like the one described here is unlikely to reflect the entireregional pool of species that would be able to colonize this habitat (Has-egawa, 1997; Amano and Little, 2005). For example, the LCF wood-fallassemblage lacks Pectinodonta xylodia, a species that was described fromearly Oligocene wood falls in similar deep-water sediments of the Makahand Pysht formations, only 100 km northwest of the LFC wood-fall lo-cality. Whether this is the result of the slight age difference, a differencein paleobathymetries, a difference in the kind of wood between the twosites, or simply a matter of chance remains to be tested.

Origin of the Wood-Fall Fauna

The LCF wood-fall assemblage demonstrates that many of the taxathat utilize wood in the deep sea today filled this same ecologic niche bythe late Eocene. These taxa are Xyloredo, the mytilid bivalve Idas, thegastropods Leptogyra and Provanna, and the ostracode Xylocythere sp.Wood-boring bivalves of the Xylophagainae are the most important ele-ments in the food chain of extant wood falls, because they turn woodinto nutrients for other organisms, either in the form of fecal pellets oras prey or carrion (Turner, 1973, 1978). The Xylophagainae evolved inthe Late Cretaceous (Turner, 1969). Most other major members of modernwood-fall associations and the Eocene organisms described here (e.g.,mytilids, buccinid and turrid neogastropods, cocculiniform limpets, andskeneiform gastropods) were also present in the Late Cretaceous. Thus,wood-fall ecosystems today may have evolved in Late Cretaceous time,but fossil evidence is still scant.

Earlier wood-fall communities could have harbored the same microbegrazers as modern wood falls, but had different wood ingesters (for

554 PALAIOSKIEL AND GOEDERT

FIGURE 5—Sketch reconstruction of the wood-fall community (drawing by Mary Parrish).

TABLE 3—Taxonomic composition of the LCF wood fall compared to those of otherselected chemosynthetic habitats. LCF � LCF wood fall; WAs � Eocene seeps inWashington State (Goedert and Squires, 1990; Squires and Goedert, 1995; Kiel, 2006);WAw � Oligocene whale falls in Washington State (Goedert et al., 1995); JMw �Japanese Miocene whale falls (Amano and Little, 2005); Rw � Recent whale falls(Smith and Baco, 2003); Rv � recent vents (Kenk and Wilson, 1985; Waren andBouchet, 2001). Extant seeps share all shown taxa with the LCF wood-fall assemblageand were therefore not included in this table.

Taxon or habitat LCF WAs WAw JMw Rw Rv

Cocculinids X X X XNeomphalids X X XSkeneids X XProvannids X X X X XMytilids X X X X X XChitons X X X

example, chitons) that were less efficient than xylophagains. If the as-sumption that xylophagain fecal pellets result in high concentrations ofsulfides is correct, then the absence of xylophagains could also result inthe absence of certain chemosynthetic microbes in earlier wood falls.Woody plants evolved in the Late Devonian and were possible foodsources in the deep sea from then on. Among wood-ingesting chitons,Sirenko (2004) suggested a Jurassic origin for the Leptochitonidae and alate Mesozoic origin for the genus Nierstraszella. He even suggested aCarboniferous origin for the family Ferreiraellidae, on the basis of thegenus Glaphurochiton, although that genus was not found with wood butin black and dark shales, presumably a paleoenvironment rich in organicmaterial. Proposals of ancient origins for deep-sea taxa should be treatedcautiously. Paleozoic or early Mesozoic origins were assumed for hydro-thermal-vent faunas discovered in the late 1970s (McLean, 1981; New-man, 1985), many of which subsequently proved to be of more recentorigin (see Little and Vrijenhoek, 2003, for a review). One might spec-ulate that the delayed recovery of forest ecosystems immediately afterthe Permian-Triassic mass extinction (Looy et al., 1999) may have re-sulted in starvation among some deep-sea wood feeders. Driftwood wasusually heavily colonized by hemipelagic crinoids and oysters from theLate Devonian to the Jurassic (Seilacher and Hauff, 2004). The effect ofthis colonization on the amount, distribution, and accessibility of woodto deep-sea, xylophagous invertebrates is still unknown.

Wood Falls and Other Chemosynthetic Ecosystems

Modern vents, seeps, whale falls, and wood falls share numerous in-habitants on higher taxonomic levels, but only a few species (Tunnicliffe

and Fowler, 1996; Smith and Baco, 2003). The same appears to be thecase when the LCF wood-fall fauna is compared to other fossil chemo-synthetic ecosystems in Washington State of similar age. At the specieslevel the LCF wood fall shares only Idas n. sp. with whale falls (Kieland Goedert, 2006) and Provanna antiqua with cold seeps (Squires,1995). At the family level the LCF wood fall shares mytilids, provannids,turrids, cocculinids, and polyplacophorans with cold seeps (see Table 3for a summary of habitat overlaps).

Wood falls in the deep sea have been considered to be evolutionarystepping stones for invertebrates during their adaptation to hydrothermalvents and cold seeps. For example, based on a molecular phylogeny ofbathymodiolins from whale falls and wood falls, seeps, and vents, Distelet al. (2000) suggested that wood and bone associations preceded the ventand seep specialization in this group. The fossil record of vents, seeps,whale falls, and wood falls is still very patchy, and the first occurrencesof the modern bathymodiolins in their respective fossil habitats are strat-igraphically very close to each other. The oldest record of Bathymodiolusfrom a seep is from the middle Eocene (Kiel, 2006), the oldest record ofIdas and other modiolids on whale falls is from the early Oligocene(Goedert et al., 1995), and here Idas is reported from the late Eocene.Two additional groups with species shared among vents, seeps, whalefalls, and wood falls are the provannids and cocculiniform limpets. Pro-vanna appears simultaneously at seeps (Squires, 1995) and wood falls inthe late Eocene but at whale falls only in the Miocene (Amano and Little,2005). Earlier records of this group have yet to be confirmed: Campbell(1996) reported provannids from Early Cretaceous cold seeps in Califor-nia, Little (2002) suggested procerithiids from a Late Cretaceous ventcommunity on Cyprus could be provannids, and Bandel and Kiel (2000)described Desbruyeresia antiqua from Late Cretaceous shelf deposits inSpain, although the habitat of this species is unknown. Cocculiniformlimpets appear almost simultaneously at seeps and on bones of a leath-erback turtle in the middle Eocene (Goedert and Squires, 1990; Marshall,1994), and records from fossil whale falls are still lacking (Amano andLittle, 2005).

These three groups (mytilids, provannids, cocculinids), however, showone common pattern in their fossil history: whale falls were always thelast to be colonized. This finding suggests that whale falls were initiallycolonized by a subset of the seep and wood-fall fauna that was presentat the time whales evolved. This suggests that, rather than being an evo-lutionary stepping stone toward vents and seeps, whale falls may haverepresented new niches available to taxa that were already adapted toephemeral reducing environments.

PALAIOS 555LATE EOCENE WOOD FALL

CONCLUSIONS

All species recovered from the LCF wood fall were benthic marinespecies, indicating that their colonization of the wood commenced oncethe wood had arrived on the deep-sea floor and not while it was afloaton the ocean’s surface or still in a terrestrial environment. Seven of the21 recovered species relied directly on the wood or on the microbesdegrading it and encompass most wood-dependent groups known fromextant wood falls, indicating that the diversity and taxonomic compositionof present-day wood-fall communities was achieved at least by the lateEocene. The LCF wood fall provides little evidence to either support orreject the stepping stone hypothesis, inasmuch as mytilids, provannids,and cocculinids appear almost simultaneously at wood falls and coldseeps. Fossil whale-fall communities harbor a similar set of taxa but areconsistently younger than wood-fall and seep communities, suggestingthat they were initially colonized (in part) by a subset of the seep andwood-fall fauna. Thus, rather than being an evolutionary stepping stonetoward vents and seeps, whale falls instead may have represented newniches available to taxa that were already adapted to ephemeral and re-ducing or microbe-rich environments. Undoubtedly, these taxa subse-quently radiated and diversified within the whale-fall environment.

ACKNOWLEDGMENTS

We would like to thank Klaus Bandel, Forest Gahn, Gene Hunt, ConradLabandeira, Bruce Marshall, Jim McLean, and Craig Smith for their helpwith identifying taxa and for fruitful discussions; Cris Little, Jim McLean,Kathy Campbell, Martin Thiel, and an anonymous reviewer made valu-able suggestions on an earlier draft of the manuscript, which are gratefullyacknowledged. We are especially grateful to Mary Parrish for the fineillustration of the wood fall and to Dan Georgescu for identification ofthe foram assemblage and its stratigraphic implications. Simpson TimberCompany (Shelton, Washington, USA) allowed access to the locality.This study was supported by the Charles D. and Mary V. Walcott fellow-ship of the Department of Paleobiology at the Smithsonian Museum ofNatural History to SK.

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