Benthic Macroinvertebrate Associations on a Carbonate-Clastic Ramp in Segments of the Eany Jurassic Back-Arc Basin of Northem Chile (26-290 S)

Benthic macroinvertebrate associations on a carbonate-clastic ramp in segments of the Eany Jurassic back-arc basin of northem Chile (26-290 S)

Martin Aberhan Inslllut IOr PaJiontologle dar UnlverslUlt, PlelcherwaJl 1, 0-97070 W:lrzburg, Germany

ABSTRACT

The distriblllion of facies and benthic fauna in segments of the marine Eariy Jurassic Andean Basin c·f northem Chile has been reconstructed based on sedimentological, taphonomic, and paleoecologicalevidence. In the areas studied, that is between Salar de Pedernales (26°S) and El Tránsito (29°S), the depositional system is interpreted as a h:>moclinal ramp, on which fourerwironmentaJ subdivisions can be distinguished: shallow siliciclastic ramp, mixed siliciclastic-carbonate ramp, midd/e carbona:e ramp, and deep carbonate rampo In an offshore direction, they reflect a general decrease in terrigenous sediment input, anergy level, grain-size, and oxygen supply, and a relative increase in carbonate production. More than 200 bulk collections of benthic macroinvertebrates, dominated by bivalves and brachiopods, were grouped into 27 associations (1-27) and 3 assemblages (E,F,G) by means of a O-mode cluster-analysis. Most associations are restricted to a single environmental subdivision and therefore proved to be useful indicators of generalenvironmental conditions and bathymetry. This is also true for guild-assemblages. Higher numbers of dominant guilds and a relatively broad guild-spectrum within highly-diverse associations are found in the more onshore settings. Due to a stable and consolidated substraie, on!he middle carbonate ramp only pedunculate brachiopods and epifaunal bivalves could gain high abundances in associations of moderate diversity. An even higher dominance of only a few guilds is found on the oxygen-controlled deep rampoTypical representatives of low-diversity associations were deposit-feeding nuculid bivalves in the upper dysaerobi:: biofacies zone and small, flat-'valved pectinacean bivalves in the lower dysaerobic zone.

Key words: Juras3lc, Macrobenthos, Assoclat/on, Pa/eoec%gy, Gulld, Oxygen, Facies, Northem Chile.

RESUMEN

Asociaciones de macroinvertebrados bentónicos en una rampa carbonatada-elástica en seg mentos de la cuenca de trasarco jurásica inferior, norte de Chile (26-29°S). Sobre la base de evidencias sedimentológicas, tafonómicas y paleoecológicas se reconstruye la distribución de las facies y la fauna bentónica en segmentos del Jurásico Inferior marino de la cuenca andina del norte de Chile. En las áreas estudiadas, entre Salar de Pedernales (26'S) y El Tránsito (29°S), el sistema de deposición se interpreta como una rampa homoclinal en la cual se distinguen cuate subdivisiones ambientales: rampa somera silicic/ástica, rampa mixta siliciclástica-carbonatada, rampa media carbonatada y rampa profunda carbonatada. En dirección costa afuera, se manifiesta un decrecimiento general en el aporte de sedimentos terrrgenos, del nivel energético, del tamaño de grano, del suministro de oxrgeno, y un incremento relativo de la pPOducción de carbonato. Más de 200 muestras, estadCsticamente representativas de macroinverlebrados

Rellfs/a Geológica d, Chile, Vol. 20, No. 2, p. 105·136. 13 Flgs., 1 table, December 1993. 106 BENTHIC MACROINVERTEBRATE ASSOCIATlONS ON A CARBONATE-CLASTlC RAMP IN SEGMENTS OF THE EARLY JURASSIC ...

bentónicos, dominadas por bivalvos y braquiópodos, han sido agrupadas en 27 asociaciones (1-27) y 3 asambleas (E,F,G) por medio del 'Q-mode cluster-analysis'. La mayoría de las asociaciones están restringidas a una subdivisión ambiental única y, por lo tanto, demuestran ser indicadores útiles de las condiciones ambientales generales y de la batimetrfa. Esto también resulta serválido para las asociaciones de gremios. En las asociaciones de mayor diversificación, correspondientes a las regiones mañnas más marginales, se encuentra un mayor número de gremios dominantes y un espectro de gremios relativamente amplio. En la rampa media carbonatada, debido a la presencia de un substrato estable y consolidado, sólo son abundantes braquiópodos pedunculados y bivalvos epifaunales, en asociaciones de diversidad moderada. Un predominio aún me.yorde sólo unos pocos gremios se encuentra en la rampa profunda, controlada por el tenorde oxrgeno. Representantes trpicos de las asociaciones de baja diversidad son los nucúlidos depositrvoros de la zona disaeróbica superior y pequeños pectrnidos, de valvas aplanadas, en la zona disaeróbica inferior.

Palabras claves: Juras/co, Macrobentos, Asociaciones, Pa/e06cologla, Gremios, OxIgeno, Facies, Norte de ChUe.

INTRODUCTION

The marine Early Jurassic sequence of the Andean Manceñido (1981,1991) and Prinz (1991), respec back-arc basin ot Chile and Argentina is well known tively. for its rich and diverse benthic fauna. Bivalves form In contrast to taxonomic analyses, paleo the numerically most important group and have been synecological work on the Jurassic macrobenthos of repeatedly iIIustrated in the literature. In particular, South America is rare. Damborenea et al. (1975) major parts of the class have been revised recently by studied the biofacies of the Argentine Liassic. They Damborenea (1&87a;1987b), Brachiopods, the sec already used cluster-analysis and a 'biotype' approach ond conspicuous benthic group, and corals have on the basis of presence and absence of genera. been recently documented monographically by Detailed information on distribution patterns of benthic

Tran' .... ncl de ..HuonCObamba ~ . '*"'1 ".~". ""l;.. 0 •• ~ Triasssic-Lower Cretaceous ". :~:::.

D sedlmentary facies

f!lJ volcanic facies

Precambrlan·P aleozolc

metamorphlc and ~ IgnecU$ rocks N I 4

o IOOOkm L---..J FIG. 1. Mesozolc. sedimenlary baslns 01 lhe Cordillera de los Andes soulh 01 lhe 'Transversal de Huancabarrba' (alter Dalziel. 1986). M. Aberhan 107

associations from the Liassic of northern Chile is and on the environmental interpretation of the known by the work of Aberhan (1992). Otherwise, no recognized benthic associations. The results may paleoecological studies have been carried out. serve as a useful basis for the analysis of benthic The purpose of this paper is to summarize the distribution patterns through space and time in the information given by Aberhan (1992) with special Andean Basin and hopefully allow comparison with emphasis on the spatial and temporal distribution of other Mesozoic back·arc basins in tne future. benthic mac'oinvertebrates within the Andean Basin

GEOLOGICAL SETIING

The M es noic paleogeography of the Andes (Fig. paleogeographic maps for various time slices have 1) consisted of several elongated, north·south trending been provided by Riccardi (1983) ar.d more recently basins whicr where borde red by Paleozoic basement by Riccardi et al. (1993). ridges (e.g. Riccardi, 1983; Dalziel, 1986; Riccardi et The UpperTriassicto middle Cretaceous evolution al., 1993). The north Chilean-Argentine back-arc of the Mesozoic marginal sea of Argentina and Chile basin, the se-called Andean Basin, was bordered by can be subdivided into several transgressive a large continental block in the east. To the west full regressive cycles of different orders (e.g. Riccardi, water exchange with the Paleo-Pacific was restricted 1983; Riccardi et al., 1993). In tt-e studied area by an active volcanic arc (e.g. Hervé et al., 1987; between 26° and 29°S the sea transgressed in an Riccardi et e.!. 1993). However, narrow connections eastern direction fromthe Sinemurian to the Toarcian. to the open ocean obviously existed during most of The subsequent regression started during the late the Jurassic (von Hillebrandt et al., 1986; Quinzio, Toarcian and continued untillate Baj.:>cian times (von 1987; Groschke et al., 1989). More detailed Hillebrandt, 1971; 1973; Riccardi et al., 1993).

STUOY AREAS ANO MAIN SECTIONS

Sedimertologic and ecological data have been near the Pliensbachianrroarcian boundary. Note, obtained by a bed by bed analysis of six sections that about 80 m aboye the base a gently dipping throligh characteristic Liassic deposits of northern thrust fault causes the repetition ot about 35 m of Chile (Fig. 2). The three main sections under study, upper Pliensbachian sediments. Sectíon 5 and section comprising between 220 and 280 m of upper 6 from Quebrada Pinte, located about 5 km apart Sinemurian te Toarcian sediments, are schematically from each other, hava baen described and iIIustrated illustrated in figure 3. Section 2 from the Lower by von Hillebrandt (1973). They can be easily Quebrada E Asiento, forming part of the Montandón correlated using both litho- and biostratigraphic Formation (Harrington, 1961), has been described criteria. previously by von Hillebrandt and Schmidt-Effing The remaining sections 1, 3, and 4 are not (1981) and Pérez (1982) who also clarified the reproduced here, since they either cover relatively stratigraphy. In addition, Pérez (1982) provided a short stratigraphical ranges (sections 3,4) or are only detailed description of litho- and biofacies and in represented by single bulk samples of particularly terpreted the depositional environments. According fossiliferous horizons (section 1). The interested to Pérez (1 982), the inferred bathymetry ofthe Liassic reader is referred to Aberhan (1992, p. 9, Fig. 4, and environmen:s ranged from a faw maters at tha Appendix 1). lowermost ~par Pliensbachian to more than 100 m 108 BENTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ...

30" '" ... "

...-

3.'

~ N \ o 200 km '-----' FIG.2. Locallonotworklngareasand 10"'" L--...... L_....J' studied secUons (1-6). 70"20'

SEOIMENTARY FACIES ANO OEPOSITIONAl ENVIRONMENTS

In the studi3d sections of the Early Jurassic of water depth with progressive distance from the shore. northern Chile, twelve facies types have be en Four facies zones can be distinguished: recognized which range from pure siliciclastics through a- an onshore facies belt, including facies A-C, was mixed carbonate/siliciclastics to pure carbonates. characterized by a high input of siliciclastic mate The facies types are sUlT,lmarized in tabl~ 1 and their rial which prevailed over the rate of carbonate stratigraphic distribution can be drawn from figure 3. production. Unfossiliferous, large-scale cross They have been interpreted in terms of physical bedded, coarse-grained sandstones reflect a high processes and can be assignedto distinct depositional energy level and episodically high sedimentation environments. Below, a brief overview of the main rates in an environment aboye fair-weather wave facies and an interpretation ofthe spatial arrangement base. With increasing distance from the shore, of their corresponding environments is presented. grain size decreased, . thus giving rise to fine The dominating facies types, that is, types A-G of grained sandstones and siltstones bearing a rich table 1, are characterized by uniform, relatively thick benthic fauna. They can be attributed to envi sedimentary packets with gradual transitions between ronments of intermediate water energy and various facies. They are interpreted to represent sedimentation rates; different environments on a more or less gradually b- a secand facies belt, which is situated more sloping sea flocr, exhibiting a continuous increase in offshore than 80, comprises the spectrumof micritic M.Aberhan 109

Quebrada El Asiento Quebrada Pinte ~ section 2 section 5 section 6 K ---

)~~ e F .C:;'" lo. e o .~ 1.\ ~'" u lo. l.' '"o ~ .&

1.\ e '" :cu .Q'" '"e F .S! s:: E

l.' l.' e E .~ @limestonc oC u '" D lA\. ~marlsl.Onc .Q'" :; I.~ '"e p c=J siltslone .S! O sandstone s:: F 1,\ O grutile JI' ~ volcanic rack nOI ). ....na...... '"" shcll bcd exposed E I.~ hardground

ooids II'" I.~ e 1: O patch red D 1.\ )A\. ='" 0 corals ... v v S 1.\ v 1", Thalassinoitks A V V ), I~ t? ~ Planoliles D 1.\ J.\ Á Chondriles e si s --. Tcich ichnus e le l.\.. ~ Diplocralt!fion ~ Ophiomorpha trough ), ~ cross-bcdding planar 10m ~ cross-bc.dding 1 ., small-scaJe e si s g cross-bcdding ... - p;uallcl -- lamination ~ fauh e si s g majar unconfonnity

FIG. 3. Maln studled seclions through the Uasslc 01 Quebrada El Asiento (secUon 2) and Quebrada Pinte (sec\lons 5, 6). A to L denote lacles types as deflned In table 1. Graln slze: c =clay; si = sin; s = sand; 9 = grave l. Carbonate IIthology: M = mudstone; W = wackeStone; P =packstone; G =gralnstone. TABlE 1. MAIN FEATURES OF THE 12 FACIES TYPES RECOGNIZEO IN THE EARlY JURASSIC OF NORTHERN CHilE ANO INTERPRETATION OF CORRESPONOING DEPOSITIONAL ENVIRONMENTS. o

Facies type. ComposlUon Physlc:al sedlmentllry BloIurtla1lon Trace fo•• n. Body f08s11. Seclmentation Energy level Interpretatlon ot strudure. ",te dllposlllonal envlronment

A unloooUKorouo, modorately quartz¡ ,a,ely lold""a, eroolve basol abo"nt vary rara marina hlgh hlgh "",rglnftl "",rln.. Iftn ""Hft; CD m sorted, coarse1l,lIined sandstones and blodasts; matrix: trough eross-beddlng bivalvas strcng Inlluence 01 eurrants/ Z lo IIne conglomeratas; eobbles IIIty mlerRe to sparlte waves ...:r comrnon up to 9 cm In langth ñ

B • unlossllHerous, we.-sorted, quartz; malrlx: sparlta eroslve base; oceu' Th./assInoldes absent high Ngh rrigr.tlng shaltow subtidal medlum-gralned sandstones planar cross-bedding sand "-,, strcng Inlluenee ofl l>~ O eurrents/waves Z m< c· weU-sor1ed, line-gralned quartz and blodasts; pa,allellaminatlon abundant Th./as5Jnoldes blvalves Intermediate Intermediate shalow siliclclas!le ,amp; I ~ sandstones to slltstones mstrlx: sUty marl to Planolltes gastropods below wave base, Inlluenee . m rrierlte Chondr~es plantdebris ot c:urrents al reichlchnus ~ D • mlerltle sandstones to sandy quatU: and blodasts; generally-ra,e; common Oph/omorpha blvalves Intermedlate Intermediate rrixed slllelelastle-carbonate > wackestones ,arely leidspar, mlea, eroslve base, Planolltes brachlopods ramp; Intermlttent '"O granMle rock Iragments; trough and planar Tha/as5Jnoldes gast,opods Inlluenee of eurrent. '"O mat,be: mler"e eross-beddlng Dlp/ocraterlon corals ~ O E • wacka- to packstones blodast., peloids; abundant Tha/as5Jnoldes brachlopods Intermedlate Intermediate rriddle carbonate ,amp; z rarely quatU: Chondr~es blvalves tolow tolow wlnnowing eommon '" crlnoids Oz > F • b,ownlsh mudo to wackestones rarely bloclast., quartz, rare PlanolHes ammonoid. Iow Iow deep offsho,e carbonate o > and leldspar Chondr'$S nueuUd blvalves ramp l> Tha/as5Jnold$S al ~ G • bHurrinou. mudo to wackestones; ,arely bloclasts, quartz, absent ammonoids Iow Iow deepest part of ramp; ~ m Inte,ealatlon 01 winnowed lags and leldspa, Posldonotis oxygen-controled O benthle launa > H • _II-sorted grainstones peloids, agg'egate trough eross-beddlng oceu, brachlopods hlgh turbulent envlronment on ... Tha/asslnoldes hlgh ñ'" g,alns, blodasts Oph/omorpha blvalves a shallow carbonate ramp, crlnoid. most likely shoals lar Irom ~ any elastle Inlluenee ~ " I • coral-stromatoporold blost,ome matrbe: mler"e rare Tha/asslnold$S corals 10101 Iow 10101 energy envlronment on z blvalve OO,lngs stromatoporoids a shallow carbonate ramp, '"m In organlsms blvalves la, I,om elastlc Inlluence ~ ~ brachlopoá. m Z J • moderately dlverse patch-reel. blvalve OOrlngs co,als Iowto Intermediate shalow neal$hore ramp ül O In organlsms stromatoporoids Intermedlate blvalves " :r... brachlopod. m 1,!1 l> K • hardg,ounds le'Nginous crusls Tha/as5Jnold$S eneNsting oysters ext,emely Iowto deep offshore ramp r common -< blvalve OOrlngs and serpulids Iow; Intermediate < In substrate U/hophaga omission e l> > '" L • Fe-oolHes Fe-ooids, bioelasls eommon blvalves Iow Iow rrixed slllclelastlc·carbonate ñ'" quartz; matrix: micrite arnmonoid. ramp M.Aberhan 111

sandstones to sandy or silty wackestones. These facies types. mixed siliciclastic-carbonate deposits are The organization of facies be:ts more or less summarized in facies type D. They reflect roughly parallel to the coastline and continuously grading into corresponding rates of terrigenous sediment each othersuggests depositional en'lironments which accumulation and autochthonous carbonate . spatially were passing into each and which temporally production. In this respect they act as a link were replacing each other. A.::cordingly, the between the siliciclastic-dominated facies zone depositional system has been interpreted as a ramp a- and the carbonate-dominated facies zone C-. rather than a rimmed shelf or an isolated platform. Sedimentation rate and energy level are Ramps are gently sloping platforms on which shallow, interpreted to have been intermediate; high-energy facies onshore pass downslope into c- a third facies ,belt is defined by the widely spread deeper-water,low-energyfacies offshore (Ahr, 1973). facies type E and replaced zone b- in a still farther Accordingly, facies belts a- to d- from aboye can be offshore direction.lt is characterized by a reduced assigned to a shallow siliciclastic ramp, a mixed input of terrigenous material and a high rate of siliciclastic-carbonate ramp, a middle carbonate ramp, carbonate production. Whilst coarse-grained ma and a deep carbonate ramp, respectively. terial was deposited in onshore settings, a consi It has been distinguished between homoclinal derable amount of clay- and silt-sized particles ramps and distally steepened rafTlls (Read, 1982; was bypassedto environments stillfarther offshore 1985). Homoclinal ramps generally lack significant towards the basin centers. Consequently, the sediment gravity flow deposits as 'Nell as slumping dominant lithofacies are wacke- and packstones structures in the deep-water facies. Due to the which are rich in bioclasts. They are interpreted to apparent absence of a slope break and associated have been deposited at times of intermediate to mas s flow deposits, the Early Jurassic ramp of low, but rather continuous sedimentation rates, northern Chile is interpreted as a homoclinal rampo under the influence of weak currents; Due to the lack of protecting reef belts and barrier d- an offshore facies belt, formed by facies types F systems, facies organization and stratification types and G, occupied a position still further offshore on ramps are usually highly influenced by storm than zone C-, in the deepest parts of the basin. activity (e.g. Aigner, 1985; Wright. 1986; Lee and Low rates of sedimentation of fine-grained Kim, 1992; Holland, 1993). However, typical features terrigenous clastics and autochthonous lime mud of a storm-dominated sedimemation, such as under low energy conditions led to the sandstone-mudstone couplets, sharp bases of beds, sedimentation of monotonous series of mud- to graded bedding, hummockycross-t:edding, andshell wackestones and silty marls. In part, bituminous beds with a high percentage of articulated shells, sediments point towards a restricted water were of subordinate importance in the .studied circulation and oxygen-deficient conditions on sections. The obvious scarcity of hurricanes and the sea floor. winter storms can be explained by the The relative spatial position of the less common paleogeographic position ofthe Andean Basin on the facies types H-L can be drawn from table 1 and figure eastern margin of an ancient ocean behind a volcanic 3. Most commonly, they represent spatially and arc (see paleogeographic maps of Barron et al., temporaliy limited environments which were 1981; Smith et al., 1982; Ziegler et a.'., 1982; Scotese, associated with one of the aboye mentioned,main 1991 ).

BENTHIC ASSOCIATIONS

Using relative abundances of individual taxa, 202 into 27 associations by means of a O-mode cluster bulk collections of benthic macroinvertebrates from analysis (Aberhan, 1992). Bulk samples have been Early Jurassic sections of northern Chile, comprising taken from single layers of uniform lithology. Obviously more than 40,000 specimens, have been grouped transported, mixed, or diagenetically biased samples 112 BENTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTlC RAMP IN SEGMENTS OF THE EARLY JURASSIC ...

lOO 600 700 IlOO 900 1000 1100

n,p ~_---20 60

_---17 lO

10 FIG.4. Rarelactlon curves 01 assoclatlons "ind n., =numberspecles; n... =number 01 u lS lO 100 200 JOO '00 lOO 600 700 !lOO 900 1000 1100 Indlvlduals. For key see Appendlx 1.

were excluded from the analysis. Therefore, the Fürsich, 1991). On the whole, information 1055 appears associations as well as three assemblages, which to be predictable within certain limits and the bias is are statistically less significant, are regarded to Iikely to be constant within distinctive time-environ represent the autochthonous to parautochthonous mental units. relics of ancient communities. In orderto avoidthe repetition of long descriptions As a rule, associations of the geological past here, only the trophic nucleus of each association suffered 1055 of soft-bodied.elements. Consequently, and assemblage (that is, the numerically most taxonamic andtrophic composition as well as diversity important elements that make up 80% of an values are more or less biased if compared to former association) is Iisted in appendix 1 (the identification living communities. Furthermore, fossil associations of rhynchonellid brachiopods has been carried out represent time-averaged community relics of non according to the preliminary results of a Ph.D. thesis contemporaneous ancient communities (Walker and (V.Lüdemann; Berlin University) and an unpublished Bambach, 1971; Fürsich and Aberhan, 1990). As manuscript on Liassic rhynchonellids from Cal ama, such they cannot record short-term fluctuations but North Chile (X. Shi; China University of Geosciences, rather reflect the general environmental conditions Beijing). The publication of new species from the which persisted over longer time periods. Liassic of Argentina, based on detailed morphometric Despite these restrictions, community analysis and thin sections, makes a revision of the paleoecology has proved in numerous case studies, Chilean material necessary. Judging from the figured to be a sensitive tool in reconstructing Phanerozoic specimens in Manceñido (1991) some species which marine paleoenvironments, if used in combination prevail in assemblage zones from Argentina also with sedimentoiogical and taphonomic data (e.g. dominate in associations from northern Chile of the Ziegler et al., 1968; Rhoads et al., 1972; Scott, 1974; same age. In order to avoid the use of nomina nuda, 1990; Duff, 1975; Fürsich, 1977; 1981; 1984a; 1984b; an open nomenclature has therefore been adopted Fürsich and Werner, 1986; Hurst and Watkins, 1981; for some rhynchonellids in this study). Oschmann, 1988; Wignall, 1990a; Aberhan and Also included in appendix 1 is information on the M.Aberhan 113

shallow siliciclastic ramp

deep carbonate ramp 20mL- Skm

I I I F .19" 25 ~ 24 G olO I I I 23 27 ... 1 26 I I I I I "lO 19 :e 20 21 18 .! 21 I 1 I 22 i!! E .! ¡¡; 16 17 ;:; I I I 15 I " I I 14 j 13 12 ~ I I II I ~ I 10 ..i I I FIG. 5. Envlronnenl-tlme dlagram. Assocla- 9 .. 8 1I0ns (1-27) and assemblages (E-G) ·C" I I 7 I ~ 6 E 4 5 are dravrn according lo Iheir age and .. 3 1 spatlal pJslllon on a homoclinal rampo ti;" 2 I I I ;:; For key see appendlx 1. ~ I I

occurrence of associations with regard to facies epibyssate suspension-feeders, free-Iying epifaunal types, autecological data on life habit .and feeding suspension-feeders, cemented epifaunal suspension mode of the dominant faunal elements, and the feeders, cemented microcarnivores, pedunculate diversity valJes of each association. Diversity has suspension-feeders, and epifaunal herbivores and/ been measured as species richness (N), which is the or detritus-feeders. number of species present, and evenness (D), Based on the ramp model outlined aboye, the expressed by the formula D =1/L PI 2, whereby PI is distribution of dominant species, guilds, and asso the relative frequency of the íth species (MacArthur, ciations and their relation to environmental factors 1972, p. 197). The evenness reflects the frequency such as rate of sedimentation, grain-size, consistency distribution of species within an association. To allow and mobility of substrates, energy level, and oxygen the compar son of associations, which are often supply shall now be reconstructed in more detail. A based on differing numbers of individuals, species summary diagram, depicting the spatial and tempo diversity is also expressed as rarefaction curves ral distribution of associations and assemblages is (Sanders, 1968; Tipper, 1979; Fig. 4). presented in figure 5. The spatial distribution of guilds Those species which exhibit the same mode of is reconstructed in figure 13. life and feeding type are grouped together in a guild. To assign species to guilds is performed without SHALLOW SILlCICLASTIC RAMP BIOFACIES regard to theirtaxonomic position and points out, that those species belonging to a particular guild overlap significantly in their niche requirements (e.g. Root, Description 1967; Bambach, 1983). The most common guilds that have be~n distinguished in this study are shallow Whilst the relatively coarse-grahed sandstones infaunal sLspension-feeders, shallow infaunal of facies type A and B are unfossiliferous, the fine deposit-feeders, deep infaunal suspension-feeders, grained sandstones and si~stones hold a rich benthic endobyssate (semi-infaunal) suspension-feeders, fauna.lt is represented bythe Sinemurian associations 114 BENTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURAS SIC ...

1,2, and 6 (Fig. 5; Appendix 1). In order to convey an Interpretation impression of the benthic colonization of the Sinemurian sea floor, the reconstruction of the The good preservation of the fauna and the Paralle/odon hirsonensis association is given as an presence of articulated bivalves indicate tha! lateral example in figure 6. The dominant faunal elements of transport was insignificant in the formation of these the shallow siliciclastic ramp are the semi-infaunal associations. The preservation of aragonitic forms as filter-faeding bivalvas Gervillella araucana, steinkerns shows that a taphonomic bias of the fauna Paralle/odon hirsonensis, and Bakevel/ia waltoni, the through selective solution of aragonitic shells can be pedunculata, suspension-feeding brachiopod ruled out. Therefore, the associations are regarded Lobothyris cf. ovatissima, and the shallow infaunal, as the parautochthonous relics of the preservable filtar-feeding bivalves Protocardia sp. A and par! of former communities. Anisocardia sp. A. Consequantly, the dominating Lithofacies and taphonomic data point to an guilds are serri-infaunal, shallow infaunal and intermedia!e energy level and the influence of pedunculate suspension-feeders. A less common moderately strong currents. According to the high guild is repras:mted by epifaunal, presumably percentage of suspension-feeders waterenergy must herbivorous gastropods, which are essentially have been sufficient enough to keep nutrients in restricted to this environment (Fig. 13). suspension. The relatively common herbivorous In the various associations one-fourth to one-fifth gastropods let presume, that the sea boltom was of fhe bivalves is preserved with both valves. The colonized by benthic algae, thus indicating an degree of abras on and breakage is generally low. environment well within the photic zone. The high Encrustation of shells by serpulids occurs very rarely percentages of epifaunal and semi-infaunal forms and no traces of bioerosion have been found on suggest a fairly stable substrate. At the same time shells. Most forms with aragonitic shell mineralogy substrate consistency must have been soft enough to are preserved as steinkerns. allow the colonization by a burrowing infauna. A

FIG.6. Blolope reco;lSlruclion 01 lhe Paral/e/odon hirsonensis associallon. lIIuslrated are Ihose species whlch lorm lhe Irophic nucieus. 1- Terebra/u/a cf. oval/ss/ma; 2- Para//e/odon h/rsonensis; 3- Gervil/e/Ia araucana; 4- Bakevell/a wa//oni; 5- Modio/us gigan/eus; 6- S/riae/aeon/na /ransal/anliea; 7- hlgh-spired gastropod; 8- Lithotroehus? andinus; 9- Cardinia andium; 10- P/anolites sp.; 11- Telchiehnus sp.; 12- Chondrites sp.; benlhlc algae hypolhellcal. M.Aberhan 115 relatively stable low-stress environment is also carbonate ramp are association 4 in the upper corroborated by the intermediate to high diversity Sinemurian, associations 16, 17, 20 and 21 in the values (Fig. 4; Appendix 1). The nearly complete upper Pliensbachian, and associat.ons 23, 24, 25, absence of encrusters and borers points out, that and 26 as well as assemblage Fin Toarcian strata shells did net remain exposed on the sea bottom for (Fig. 5; Appendix 1). The upper Pliersbachian Weyla a long time and that the rate of sedimentation, alata/Entolium corneolum association, which is therefore, was rathercontinuous. However, to enable reconstructed in figure 7, is regarded as a the settlement of fixo-sessile brachiopods, sedi characteristic representative. As is typical for most mentation rates cannot have been particularly high. associations of this environment, it exhibits a In summary, the associations characterize a comparatively broad spectrum of lite habits, a high relatively or·shore depositional environment on a diversity (Fig. 4; Appendix 1), and its constituent shallow siliciclastic ramp below fair-weather wave samples possess a high faunal densi:y. The dominant base with in:ermediate rates of sedimentation. organisms of the mixed siliciclastic-carbonate ramp are bivalves, which are represented by different guilds and morphotypes. In particular, these are the M1XED 31L1CICLASTIC-CARBONATE RAMP BIOFACIES deep infaunal suspension-feeders Pholadomya hemi cardia, Pholadomya fidicula, Pleuromya uniformis, Description Pachymya rotundocaudata, and Gresslyaperegrina, the shallow infaunal suspension-feeder Protocardia Common associations of the mixed siliciclastic- striatula, the free-Iying epifaunal suspension-feeders

FIG.7. Biolo¡:e reconslruetion 01 Ihe Weyla alata/Entolium corneolum assoeiation. IlIuslraled are lhe species Irom Ihe Irophie nueleus 1- Weyl¡: alata; 2- Entolium corneolum; 3- Pleuromya uniformis; 4- Montlivaltia sp.; 5- Pholadomya corrugata; 6- Pinna el. radiata; 7- Grypraea d. darwini; B- Arlieidae gen. el sp. nov.; 9- 'Terebratula' sp. A; 10- Mesomiltha bellona; 11- Quadcatirhynchla sp. B; 12- Thala3sinoides sp.; 13- Diplocraterion sp.; 14- Ophiomorpha sp. 116 BENTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURAS SIC oo .

Entolium cornaolum, Wayla alata, and Gryphaaa MIDDLE CARBONATE RAMP BIOFACIES darwini, the cemented epifaunal suspension-feeder Actinostraon solitarium. and finally the lucinids Description Mesomiltha balbna and Mesomiltha? huayquimilli (see also figure 13). The latter two are classified as Benthic associations of the middle carbonate infaunalsurface deposit- and/or suspension-feeders ramp are the Sinemurian associations 3, 5, 7, 8, and via mucus tubes :for a discussion offeeding strategies 9, the lower Pliensbachian associations 10, 11, 12, in Lucinacaa see a.g. Liljadahl, 1991). , 3, 14, and 15, and the upper Pliensbachian As a rule, more than ha~ and up to 85% of the association 18 and assemblage E (Fig. 5; Appendix bivalves are preserved with both valves. Many of 1). They are dominated by epifaunal suspension them, especially deep infaunal forms, occur in life feeders, in particular pedunculate brachiopods position. Tha level of abrasion and breakage of shells (Lobothyris cf. ovatissima, Lobothyris subpunctata, is very low. Encrusting serpulids, small oysters, and Ouadratirhynchiacrassimedia, Ouadratirhynchía sp. bryozoans as well as boring bivalvas and algae occur A, Gibbírhynchia curvíceps, Spirifarina chilensis), ocassionally, bu: in general the degree of bioerosion free-Iying bivalves (Weyla alata, Weyla titan, Entolium and encrustation of shells is low. The aragonitic corneolum), and crinoids ('Pentacrinites' sp.) (Fig. fauna is representad as steinkerns and in some 13). More rarely, the cemented oyster Exogyra sp. A samples also carries original shell material. occurs; representatives of the infauna are very rareo On the other hand, the degree of bioturbation is fairly Interpretation high. Tracefossils, however, basically Thalassinoídes, exhibit a very low diversity. Some of the dominant As for the fauna of the shallow siliciclastic ramp, species of the middle carbonate ramp are iIIustrated the taphonomic data show that we clearly deal with in figure 8, the reconstruction ofthe Spiríferína chilansis the time-averaged in situ remnants of former association. Standing also for other associations of communities wh:>se soft body fauna has been remo the middle carbonate ramp, it rellectsthe conspicuous ved. The good p-eservation quality of fossils and the lack of infauna apart from trace fossils, a high faunal dominanca of filter-feeders indicate an intermediate density of epifauna, and a relatively low diversity, if energy level. The well-balanced percentage 01 faunal compared to associations fromthe mixed siliciclastic elements living upon and within the sediment is carbonate ramp (se e al so figure 4; Appendix 1). suggestive 01 a substrate, which was sufficiently Almost all brachiopods are articulated and often stable for a sessile epifauna and on the other hand occur as small clusters on bedding surfaces. Bivalves soft enough to enable a rich endobenthic life. High are commonly found single-valved, more rarely faunal densities, ,igh species richness and evenness, bivalved and in lite position. The degree of breakage and an equable cistribution of guilds within the various is moderate to low. In some samples the percentage associations, i" dicate a stable and predictable of shell-encrusting serpulids, oysters, and bryozoans environment which, for the most part, was free of as well as the number of bivalve and brachiopod physical stress. Most niches were occupied and a borings, as indicated by Gastrochaenolites and clear structure according to the partitioning 01 spatial Podíchnus reach intermediate levels. Shells are resources has ceen developed. Such biologically preferentially oriented in a concave-up position within controlled environments, which are close to their the sediment. Calcitic brachiopod and bivalva shells carrying capacity are typically dominated by K are preserved. In contrast, aragonitic forms are rare strategists. K-strategists are characterized by a and, primarily they exist as steinkerns only. In some relatively long life span, a relatively low reproduction cases aragonitic gastropods and the solitary coral rate and a high lellel 01 specialization (MacArthur and Montlivaltia are preserved with recrystallized shell Wilson, 1967). In this case, lor example, Weyla alata material. seems to have lollowed up this strategy. M.Aberhan 117

Interpretation various samples (e.g. Fig. 9), which re11ects a rather low degree 01 sorting and by the predominantly The largely component-supported sediment 1abric concave-up orientation 01 shells within the sediment. and high concentrations 01 skeletal elements are Low sedimentation rates can be in1erred 1rom the interpreted ro have been caused by winnowing 01 predominance 01 epifauna, an intense bioturbation, 1ine-grained sediment by moderate currents. and occasionally higher levels 01 encrustation. However, general good preservation and the Consequently, the high1aunal densities are interpreted occurrence 01 brachiopods in small clusters, which as autochthonous to parautochtonous accumu-Iations are likely to reflect the original colonization pattern on 01 hard parts due to a combination 01 low rates 01 the ancient sea floor, indicate that destruction 01 sedimentation, high rates 01 bioproduction, and shells by CLrrents and extensive lateral transport winnowing 01 1ine-grained material. have not taken place. This is 1urther corroborated by These 1actors obviously resulted in a stable and the size 1requency distribution 01 brachiopods in consolidated, bioclast-rich substrate upon which

_. ___ ._ _· -~-~~r\.-:-:-... -~~;:--, ",_ ,..... ':.:; ... ..:/ :: :': :":;'/ .~. :': ,~0 -' < 0 :,: .

',,", ., ' ,'-, ' .', ¡~,';" . : .' , ' O' :'~ "-.::'~" ' ;r.:" ' . " .,'. ., , ....; ....

FIG. 8. Slolope reconslruellon 01 lhe Splriferina chlJensls assodallon wMh specles lrom lhe Irophlc nucleus 1- S¡:/tiferlna chllensls; 2· Lobot/lyr/s el. ovatlssima; 3- Weyla a/ata; 4· 'Penlacrinltes $p.; S- Tllalasslnoldes sp.; 6- shell podo 118 BEN-HIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ... extensive braehiopod- and epifaunal bivalve DEEP CARBONATE RAMP BIOFACIES assoeiations Wele able to thrive, covering large parts of the ancient sea floor. At least some of the Description braehiopods were growing upon eaeh other as indicated by abundant traces of Podichnus on the shells of Lobotr.yris subpunctata. The associations represented on the deep As aragonit c shells of the infauna occur only carbonate ramp are associations 19 and 22 from the sporadically in various samples, some preservational upper Pliensbachian and association 27 as well as bias, that is most of the aragonitic shells dissolved assemblage G from the T oarcian (Fig. 5; Appendix during diagenesis, eannot be ruled out. However, 1). As the four associations differ markedly from each since aragonitic gastropods and scleractinean corals other, they are described and discussed separately. are sometimes preserved with recrystallized shells, The Nucu/ana ovum association (22) is char the absence of the burrowing forms appears to reflect acterized by the high dominance of a mobile, shallow an original featu·e of these communities. As indicated infaunal, deposit-feeding nuculid bivalve, Nucu/ana by an abundant, but poorly-diverse ichnofauna, some ovum, which occurs in moderately bioturbated dark specialized organisms - most likely crustaceans coloured mud- to wackestones. The diversity of the must have been able to penetrate the substrate while association and the number of guilds are very low it apparently was unsuitable for burrowing bivalves. (Fig. 4; Appendix 1). Deep infaunal forms are com pletely lacking. Nearly half of the bivalves are articulated without evidences of encrustation or FIG. 9. Slze fraque"lCy dlstrlbutlon 01 Lobolhyrls d . ovallsslma In bioerosion; aragonitic fauna is preserved with shell. Ilve SaI11lles oHIle Lobolhyrlsd. ovallsslma assoclatlon. N: The Gryphaea sp. A association (19) is dominated number 01 studled Indtvlduals. by the small, free-Iying oyster Gryphaea sp. A (Appendix 1). Further common elements are brachiopods. Epifaunal diversity is intermediate (Fig. Lobothyris er. tlvatissima (ass. 3) • 4), infauna occurs only sporadically. Nearly all organisms are epifaunal filter-feeders. A relatively "~ low percentage of bivalves has been found articulated. ': ~ The degree of breakage is low; signs of encrustation and bioerosion are absent. Aragonitic fauna is preserved as steinkerns. The association occurs in moderately bioturbated wackestones. The Posidonotis cancel/ata association (27) has an extremely low diversity (Fig. 4; Appendix 1) and is essentially formed by a single species, the epibyssate and/or free-Iying, filter-feeding bivalve Posidonotis cancellata. It occurs in non-bioturbated, fine-bedded, black mud- and wackestones. The individuals are mainly single-valved and often coneentrated as shell pavements. They are mainly preserved as internal and external molds. ' The Propeamussiumpumilumassemblage (G) is characterized by a moderately diverse epifauna, dominated by the free-Iying, suspension-feeding pectinaceans Entolium corneo/um and Prope amussium pumi/um. On the other hand, infaunallife has been restricted to a single bivalve, the lucinid Mesolinga sp., which might have lived in symbiosis with sulfide-oxydizing bacteria. Apart from a relatively lowdegree of articulation in bivalves, the preservation of fossils is fair. Thefaunal samples are from brownish M.Aberhan 119 mud- to wackestones which are bioturbated by infaunaJ diversity, which is essentially restricted to Chondrites. They alternate w~h black marls which the Jucinid Mesolinga Recent representatives of this are devoid o" fossils. bivalve family are known to Jive in symbiosis with chemo-autotrophic bacteria. These oxidize HzS to Interpretation sulphate or thiosulphate and thus protect their hosts against poisonous effects (e.g. Felbeck et al., 1983; Taphono-nic data, monotonous Itthofacies, and Reid and Brand, 1986; Vetter et al., 1991). In orderto faunal cOll1>osition point to a quiet offshore simultaneously meet the metabolism of bivalves and environment distinctly below wave-base for the four bacteria, chemosymbiosis requires bng-term stable associations They are all interpreted to have been conditions with a baJanced juxtaposition of both 0z structured by more or less lowered levels of free and HzS (Felbeck et al., 1983; Oschmann, 1993). oxygen, as snall be demonstrated below. These demands are realized close to the 0/HzS The Nuculana ovum association is the only interface. In addition, the frequently occurring trace association where infaunal deposit-feeders, in this fossil Chondrites is regarded as an indicator of lowered case nuculid bivalves, are significant. According to 0z-conditions (Bromley and Ekdale, i 984; Seilacher, the facies in Nhich they occur in the Chilean Liassic, 1990). nuculids preferred a fine-grained substrate rich in In summary, composition and structure of the organic matter, indicative of low energy conditions. benthic fauna suggest an oxygen-controlled This bivalve group is known as a dominant component environment with a position of the 0/HzS interface of oxygen-co,trolled environments fromthe Paleozoic fairly close to the surface of the substrate (Fig. 10). tothe Recen~ (e.g. Bader, 1954; Kammer etal., 1986; Recent propeamusiids are most commonly found on Wignall, 1990b). Jn particular, in an analysis of the the continental slope and in the deep sea and are rare Lower Jurassic of England, Nuculana ovum has in environments shallower than 60 m (Waller, 1972). been onlyfound in high abundance in an environment This may also serve as an upper bathymetric value where oxygen-deficiency was evident (Morris, 1979). fortheformer habitat ofthe Propeamussiumpumilum Low diversit'j, the environmental tolerances of the assemblage. dominant species, and the structure of the association In the Gryphaea sp. A associati·::>n divers~y and which completely lacks deep infaunal guilds indicate dominant elements of the epifauna are - apart from a lowered oxygen-supply of the benthic fauna. Propeamussíum - essentially the same as for the Accordingly, the O/HzS interface has been Propeamussíum pumilum assemblage. This points reconstructed at a position a few centimeters below to similar living conditions for the inhabitants of the the surface of the substrate (Fig. 10). Below the 01 surface of the substrate in both associations. The HzS interface, the presence of toxic HzS prevented nearly complete lack of infauna indicates high stress settlement ot any infauna. Above the interfacethe 0z conditions within the sediment, which are interpreted level increased giving rise to an impoverished fauna to have been caused by a shortage of free oxygen consisting basically of nuculids. and raised concentrations of toxic HzS. Accordingly, The low percentage of epifauna, however, is not the 0/HzS interface has been placed close to the interpreted as a consequence of low-oxygen surface of the substrate in a similar pos~ion to that of conditions attheseafloor. Rather, biogenic reworking the Propeamussium pumilum assemblage (Fig. 10). ofthe muddysubstrate by nuculids most likely resulted In the Posídonotís cancel/ata association, the in a water-rich, thixotropic sediment surface. This extreme dominance of Posidonotís cancel/ata which excluded m::>st filter-feeding as well as epifaunaJ occurs in high numbers points to an opportunistic life sessile organisms, a case of faunaJ interaction which strategy of that species (see also Hallam, 1977). is known fromthe Recent as trophic group amensalism Opportunists are characterized by rapid growth rates (e.g. Rhoads and Young, 1970; Bloom et al., 1972; and high fertil~y rates (Levinton, 1970). This enables Aller and Dcdge, 1974). them to occupy free niches or ha:litats with -high Propeamussiumpumilum, being characteristic of densities within a short period of time. Possibly, the assemblage of the same name, has been Posídonotís cancel/ata al so lived in symbiosis with interpreted as a benthic opportunistic bivalve which chemo-autotrophic bacteria, a mode of feeding that preferred Jow-energy conditions and was able to has been considered for the morphologically and toJerate Jowered oxygen conditions (Johnson, 1984). ecologically related Bositra from the Posidonien Low oxygen supply is further indicated by the very low schiefer of south-west Germany (Savrda and Bottjer, 120 BE~HIC IoIACROINVERTEBRATE ASSOCIATlONS ON A CARBONATE-CLASnC RAMP IN SEGMENTS OF THE EARLY JURASSIC •••

1987; Seilacher, 1990). Here, extremely low substrate. Even on the sea floor oxygen-supply never diversities, the dominance of an opportunistic species, has been sufficient to give rise to a more diverse epi and the absenc9 of a shelly ¡nfauna and trace fossils benthic life. For these reasons, the O¡HaS interface are indicators of anoxic conditions within the substrate has been reconstructed in apositíon at, to immediately and of poor 0a-concentrations on the surface of the above, the sediment/water interface (Fig. lO).

Posidollotis cancellata ass.

FIG. 10. ReeonslruelJon 01 oxygen-conlrolled assocJatlons and arrangemenl aJong an oxygen gradlenl. Nucu/ana ovumass.: 1: Nucu/ana ovum; 2: Plano/ltes sp.; 3: Chondrltes sp.; 4: shell pod; Propeamuss/um pumllum ass.: 1: Enlo//um corneo/um; 2: Propeamuss/um pumHum; 3: Mesol/nga sp.; 4: Gryphaea sp. A; s: Lobolhyris el. subpunctala; 6 : Chondrltes sp.; Gryphaea sp. A ass.: 1: Gryphaea sp. A; 2: Lobolhylfs subpunctata; 3: Rud/rhynch/a aIf. rud/s; 4: EnloHum comeo/um; S: Ael/noslreon so/italfum; 6: sheU pod; Pos/donols cance/lata ass.: 1: Pos/donot/s canee/lata. Note lhe poslllon ollhe O¡H.S Interface. M.Aberhan 121

OXYGEN GRADIENTS ON A DEEP CARBONATE RAMP

As it has become clear from the foregoing ancient shelf and epeiric basin anoxia are more interpretati01 the fauna of the deep carbonate ramp dynamic. Accordingly, a modilied oxygen zonation was controlled by a restricted water circulation which concept has been delined for these environments resulted in a reduced supply of oxygen of the benthic (see Tyson and Pearson, 1991:. In particular, fauna. According to their taxonomic composition and Oschmann (1990; 1991 a; b)proposed apoikiloaerobic ecological structurethe associations can be arranged enviroment to accommodate environments with along an oxygen gradiént (Fig. 10). This is expressed seasonally fluctuating oxygen conditions. It is by a successively higher position of the 0jH2S inter characterized by micro-Iaminated sediments, the face within t1e sediment. The interface ranges from dominance of opportunistic species which are able to a few centimeters below the surface of the substrate exlend their planktonic larval stag3s (in particular to a position where it coincides with the surface of the shallow inlaunal, filter-feeding bivalves), and a substrate or even lies slightly aboye it. stochastically-controlled launal non-equilibrium. Traditionally, oxygen-controlled environments Faunal composition and dynamics 01 oxygen have been subdivided into three biofacies zones, controlled environme!1ts from the Cllilean Liassic can mainly depending on water depth (Rhoads and Morse, neither be interpreted with the classical oxygen 1971; Byers, 1977): a- The aerobic biolacies (> 1 mil zonation concept 01 the dysaerobic biolacies nor with I 02) is characterized by a diverse shelly and soft the poikiloaerobic model. While n both models bodied berthic fauna and bioturbation; b- the endobenthic launa dominates, in the Chilean Liassic dysaerobic biofacies (0.1-1 mili 02) consists of a epibenthic bivalves prevail. (Note, that the low small, low diversity fauna dominated by infauna with percentage 01 epilauna in the Nuculana ovum little or no elements with hardparts and moderate association does not directly result from a low oxygen bioturbation: and c- the anaerobic biofacies «0.1 mil supply, but presumably was caused by trophic group 1°2) lacks a macrobenthicfauna and bioturbation and amensalism). the corresponding environment typically exhibits The absence 01 micro-Iaminated sediments and laminated sediments. the long-term stability in faunal co~osition indicate Further studies revealed a gradational transition oxygen-delicient conditions which - in contrast to the between the aerobic and dysaerobic biolacies. Several poikiloaerobic environment - exis:ed over longer authors described dysaerobic, low-diversity periods. Forexample, inthe uppermcst Pliensbachian associations which were composed 01 shelly organism to lowermost Toarcian beds 01 Quebrada El Asiento, 01 different taxonomic composition in the various the Posidonotis cancel/ata association is the only case studies (e.g. Savrda et al., 1984; Thompson et benthic fauna that occurs for some tens of meters of al., 1985; K3mmer et al., 1986). Consequently, the sediment. These are interpreted to correspond to a delinition ol:he dysaerobic biolacies has been exten period 01 several hundred thousand years. The lack ded. of short-term fluctuations in oxygen supply may be A further subdivision, the exaerobic biofacies, explained by the paleogeographic position of the has been acded by Savrda and Bottjer (1 987; 1991). Andean Basin (see aboye): water exchange with the It is characterized by the unusual occurrence of open ocean most likely was restricted and short-term shel/y epibenthic bivalves in laminated, organic-rich events such as storms were rareo sediments. The bivalves are interpreted to have lived Due to their temporal stability, the Chilean in symbiosis with sulphur-oxidizing bacteria at the associations are assigned to the modified version of anaerobic-cysaerobictransition. Here, the exaerobic the dysaerobic biofacies and furtherdemonstrate the biolacies is treated as a special development 01 the variability in faunal composition of t1is environment. dysoxic environment, being characterized by the Based on the faunal distribution in thestudied sections, occurrence of sulphide-oxidizing bacterial mats. a faunal zonation along an oxygen gradient can be This Cl3ssical oxygen zonation has been reconstructed (Fig. 11). It may serve as an example elaborated on the continental slope and basin 01 faunal distribution in oxygen-.:ontrolled ramp environments, which are characterized by long-term environments which are characterized by a long stable conditions. In contrast, many modern and term relative environrnental stability. Four biofacies 122 BENTHIC MACROINVERTE8RATE ASSOCIATtONS ON A CARSONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ... zones can be distinguished, with emphasis on an longer-term stable environments with the position upper and a lower dysaerobic biofacies (Fig. 11). 01 the O/H2S interface only a few centimeters a- the aerobic zone is characterized by a highly below the surface of the substrate; diverse fauna which Is dominated by K-strategists. c- the lower dysaerobic zone harbours low-diversity It is composed of variable percentages 01 infaunal benthic assoc!ations, which almost axclusively and eplfaunalfiiter-feeders and exhibits high faunal conslst of epifaunal filter-feeders. The fauna densitíes. Important environmental factors which becomes increasingly dominated by r-strategists, inlluenced composition and structure of which belong to the group of small, flat-valved associations are rate of sedimentatlon. rate 01 pectinacaans. Also organisms which Incorporate biogenic reINorking, substrate consistency and chemo-autotrophic bacteria as symbionts gain in stability, and energy level; importance. b- an upper d¡saerobic zone is formad by low Besides the Posídonotis cancel/ata association diversity ben1hic associations which are dominated from the Liassic of Chile, similarly structured by nuculids. As deposit-feeders they are regarded associations are known from Europe which occur as K-strategists (see Levinton, 1972). Deep in sediments 01 the same age. These are ¡nfaunal forms are absent which is a.lso true forthe Kauffman's' Posidonía' paleocommunity from the following two zones. Faunal densities are Posidonienschiefer (Kauffman, 1981), an considerably tuned down in comparlson with the analogous benthic fauna Irom the lower aerobic zone. Opalinuston (Aalenian) 01 Switzerland (Etter, Nuculid bivalves formed an integral part 01 oxygen 1990), and the fauna of the bituminous shale controlled communities since the Late Paleozoic facies of northem England (Morris, 1979; 1980); and can be traced as a dominant component of d- an anaerobic zone is characterized by the com this envlronment until mid Jurassic times. This plete absence of benthic organisms and a rieh evolutionaryconservative bivalve group (Levinton, ammonite fauna. 1974) was oDviously able to thrive successfully in

lower dj'saerobic %One l.anaerobic

epifaufta infaunl

nucullcls

r-.s'raltaísa

""omosy"'biosll oIiv

ENVIRONMENTAL SIGNIFICANCE OF GUILD-ASSEMBLAGES

From the spatial distribution of associations (Fig. exhibiting subtidal and near-intertidal sand bars, 5) it becomes evident that, as a rule, assoclations and shoals, shallow lagoons and an offshore shelf assemblages can be clearly assigned to one out of environment. In addition, variations in depth of these four environrr.ental subdivisions of the rampo In other shallow water habitats have been on y moderate. In words, benthic associations are fairly useful indicators contrast, the ramp model proposed here for the Early of bathymetry. This appears, to be in contrast to other Jurassic of northern Chile is characterized by a studies (6.g. Fürsich, 1976), whic~ showed that morphologically rather uniform, gently dipping sea different associations were occupying similar floor and by a water depth ranging from a few meters environments and similar associations lived in different to at least several tens of meters. Consequently, environments. Accordingly, the macroinvertebrate these differences in the suitability of associations for fauna by its own is not regarded as a useful tool bathymetric interpretations can be explained by two interpreting bathymetry (Fürsich, 1976). differing models of platform topography. On a ramp, The relatively low bathymetric significance of environmental factors are distributed more linear associations B.g. from the Coralllan of England and than on a morphologically strongly subdivided shelf Normandy (Fürsich, 1976) is interpreted to be due to and correspondingly this is reflected in the faunal a morphologically more differentiated sea floor, distribution.

distanee

IS FIG.12.

Dendrogram of a Q.mode clus ter·analysls (hlerarchlcal ag 10 glomeratlve mEthod alter Ward, program SPSSIPC+) and gulld-compo31t1on 01 re suning gulld-asSEmblages (1- VI). Associatlons (1·27) and assemblages (E-G) are cases; guilds are varlablas: a = cemented ep 'Iaunal sus l6fl.! 4 16 17 lO 1111 611 n 3 7 1 , 10 11 1117 I~ IS E 5 13" G 2~ penslon-Ieeder 11 UI IV v VI b =epibyssate suspenslon· leeder c = Iree-Iying eplaunal sus a ..-...... penslon-Ieecler i ~ ~ I O El d =shallow Inlaunal sus b O ~ ~ ~ IJ n ~ penslon-Ieeder e '-"'- I;:;:;:;:::;:;:;:;:j 8 = ¡nlaunal surface deposlt IJ GIl O... G G andIor suspeDSion-leeder dU- 81 D rm.. ~ I I f = deep infaunal3uspension feeder ·0 [] a I I I 9 = seml-Infaunal suspenslon ro I:;:;:;:;:;:;:;:;:;:;j []I].... [J I I ~ feeder 1;:;:;:;:;:;:;:;:;:;1 h =epifaunalhert:ivoreandlor r.-tL I O D I ~ detritus-Ieeder h O- I I IJ I I I = pedunculate 3uspenslon I I':':':':':':':'j...... 1] I:;:;:;:;:;:::::::;:::;:;:;:;:j [] feeder. -ª- ~ O.. 124 BENTIi'C MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC AAMP IN SEGMENTS OF THE EARL~ JUAASSIC •••

GUILD-ASSEMBLAGES IN THE EARLY JURAS SIC OF blages stress the ecological structure of 1he benthic CHILE fauna. The dendrogram of figure 12 reveals, that only To explore the relationship between benthic fau two associations, the Nuculana ovum association na and environments still further, it will be examined and the Posidonotis cancel/ata association, cannot whether the associations of a particular be assignedto any ofthe identified guild-assemblages. subenvironment can be characterized by a specific AII other associ,ations exhibit a high degree of similarity guild-structure and whetherthe complexity of adaptive to, at least, one other association. This beco mes strategies varies between the four subenvironments. most evident in guild-assemblage V, in which seven For this purpose, the relative percentage of guilds associations - all 01 them strongly doninated by has been calculated for each association and pedunculate, suspension-feeding brachiopods - are assemblage. Subsequently, the associations have grouped together. The relative abundance of guilds been grouped into guild-assemblages by means of a in other guild-assemblages can be de~icted 1rom O-mode cluster-analysis with the relative abundances figure 12. of guilds serving as variables (Fig. 12). Each of the Next, the spatial distribution of associations from resulting six guild-assemblages groups together a single guild-assemblage shall be examined. In the related associations with a very similar percentage of majority of cases the associations formhg a guild the various guilds. In contrast to associations which assemblage can be assigned to the same enviro n are defined on the basis of relative abundances of mental subdivision (Fig. 5). However, all en·,ironments taxa only, this c1assi1ication adds a 1urther dimension harboured faunas from more than a shgle guild to environmental interpretrttion since guild-assem- assemblage. The shallow siliciclastic ra:-np carried

• l ñ 4"'u "

i ~, j

DISTRIBUTION 0 1' GUlLDS

~lr~j.;li~~I,~;:¡¡:,,;:· =-~----- ~

FIG. 13. Onshore·olfshore changes 01 Envlronmental laClo/S and seml-quanllallve CilslrlbuUon 01 gullds on an Early Jurasslc ré11T9 01 northern Chlle. For key 01 gullds see Il gu~ 12. Second gulld Irom above; shalow Inlaunal deposl leeder. M. Ab9rhan 125 representatives of guild-assemblage 111 and IV, which gradients exist, e.g. with regard to bathymetry and are characterized by the dominance of pedunculate distance from a sediment source. filter-feeders and semi-infaunal filter-feeders In general, the number of dominant guilds is respectively. TIle mixed siliciclastic-carbonate ramp higher and the guild-spectrum within associations is was the preferred habitat of guild-assemblages I and broader in the more onshore habitats of the shallow 11, which exhitlit high abundances of deep infaunal siliciclastic and mixed ramp as corr.pared to the filter-feeders and epifaunal free-Iying filter-feeders. offshore settings of the middle and deep carbonate The middle carbonate ramp harboured guild ramp (Fig. 13). This can be explained by a relatively assemblages V and the majority of associations of VI, high degree of environmental heterogeneity in more which are dominated by pedunculate and epifaunal, onshore settings. There, important environmental free-Iying sus¡::ension-feeders respectively. Finally, factors such as rate of sedimentation, consistency the deep carbonate ramp was inhabitated by a minor and stability of the substrate, water energy, salinity, part of guild-assemblage VI and the associations 22 temperature, and supply with nutrients are subject to and 27, which could not be assigned to any of the stronger temporal and spatial fluctuations than in established guid-assemblages and are characterized more offshore habitats. Accordingly, the specific by the extrerre dominance of a single guild. The composition of a given association is thought to distribution of guilds along an onshore-offshore retlect a distinct combination of environmental gradient and their relative importance are iIIustrated conditions within a multi-dimensional continuum of in figure 13. environmental factors. In contrast, the lowvariability in guild-composition on the middle ramp corresponds to a spatially and DISCUSSION temporally more homogeneous environment. Due to aconsolidated substrate, onlythe niches on top ofthe The good correlation of guild-assemblages and substrate could be occupied in high densities. Finally, facies zones proves, that they can be used as envi in the deep ramp, the high dominance 01 a single guild ron mental incficators. This is particularly true of within associations indicates a high-stress depositional settings with a fairly linear distribution of environment. Under these rigorous condilions environmental factors. This is the case on platforms ecospace has been utilized considerabl~'less intensely with a ramp topography, as discussed in this study.lt than in the more hospitable environ:nents further also might be expected on the continental slope or in onshore. deep sea basins, if rather linear environmental

SPATIAL DISTRIBUTION OF BIVALVES VERSUS BRACHIOPODS

Faunal sarrples fromthe Early Jurassic of northern 1991). On the other hand, brachiopods occur in high Chile mostcommonly are either dominated by bivalves abundances in eastern Europe (e.g.Tch.jumatchenco, or by articulate brachiopods. However, while bivalve 1972; Dzik, 1979; Heliasz and Racki, 1980), in the dominated associations ocurred in all four environ submediterranean provine e (Fürsich and Sykes, mental subdivisions 01 the ramp, brachiopod 1977), and in so me other parts of Eurcpe (see Ager, dominated associations are restricted to the middle 1965). In contrast to the more eurytopic bivalves (e.g. carbonate rarr.p (Fig. 13). In order to achieve a more Rudwick, 1970; Steele-Petrovic, 1979) \he occurrence thorough understanding of this distribution pattern, of brachiopods in high abundances appears to be the biological adaptations and requirements of both bound to a specific combination of environmental groups shall be brietly discussed below. parameters. In most quantitative paleoecologlcal studies of Above all, a stable and firm substrate is regarded Jurassic bent,ic macroinvertebrates, brachiopod as an important prerequisite for a successful survival domlnated associations are rare or absent (e.g. Duff, of brachiopods (see also Aberhan, 1992, p. 71). 1975; Fürsich, 1977; 1984b; Fürsich and Werner, Articulate brachiopods further require a relatively 1986; Oschmann, 1988; Wignall, 1990a; Heinze, stable environment. They are largely excJuded from 126 BEWTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ••• environments with fluctuating or extreme conditions characterized by a reduced input of terrigenous ma concerning oxygen supply or salinity. Due to their terial and the accumulation of skeletal hardparts fixo-sessile IHe habitat they are restricted to caused by a high rate of carbonate production, environments with low to moderate rates of moderate winnowing of fine-grained sediment and sedimentation. In particular, they avoid habitats in low rates of sedimentation. This resulted in a stable which mud and silt are actively deposited (Rudwick, and consolidated substrate. Relative distance from 1970). Jurassb articulate brachiopods presumably the shore, bathymetric position, and a fairly constant, had a non planktotrophic larval development moderate to low energy level guaranteed a relatively (Valentine and Jablonski, 1983) and therefore a high stability and predictability of living conditions. lower dispersal potential than pléinktotrophic bivalves. Competition with epifaunal bivalves for space In contrast to bivalves, brachiopods failed to develop may well have occurred and possibly is reflected in mechanisms to cope with advanced predators such the fluctuating dominance pattern of both groups. as thick shells, spines, and swimming ability. In However, also other biological factors such as the summary, the adaptable superiority of eurytopic activities of predators or stochastic influences in the bivalves over more stenotopic brachiopods appears recruitment of larvae may have played an important to provide a plausible explanation for the observed role. Furthermore, variations in relative abundance of distribution pattern of both groups (see also Stanley, taxa can also be due to slight fluctuations of physical 1974; Gould and Calloway, 1980; Thayer, 1985; environmental parameters such astemperature,light, Rhoads and Thayer, 1991). water energy, or primary production (e.g. Rees et al., Having these factors in mind, the striking 1977; Buchanan et al., 1978; Gray, 1977; 1984), abundance of brachiopods on the Early Jurassic which normally remain undetected in the fossil re middle carbonate ramp of northern Chile becomes cord. more plausible. This fully marine environment was

CONCLUSIONS

• The Early Jurassicof northern Chile is represented material and low rates of sedimentation by shallowto deep subtidal marine environments. characterized the middle carbonate rampo It was The depositional system has been reconstructed colonized by associations dominated by epifaunal as a homoclinal rampo Reflecting the distribution free-Iying bivalves or artículate brachiopods. As a of environmental factors (water energy, input of stenotopic group which depended on a relatívely terrigenous material, carbonate production, firm and stable substrate, stable and normal oxygen supply, and others), four depositional marine oxygen and salinity conditions, and low to facies have been recognized: shallow siliciclastic moderate sedimentation rates, articulate ramp, mixed siliciclastic-carbonate ramp, middle brachiopods were able to thrive on the carbonate ramp, and deep carbonate rampo consolidated substrates of this subenvironment. • Benthic associations can clearly be assigned to • In the offsho"re habitats of the deep carbonate distinct subenvironments on a ral'l1'. Furthermore, ramp, a low-diversity benthic fauna has been guíld-asse/":1blages proved to be of high environ controlled by low oxygen concentrations. Three mental significance in environments with a associations and one assemblage can be relatively "near distríbution of environmental assigned to a long-term stable dysoxic factors, as s the case on a rampo environment. According to the position of the O; In the Earl~" Jurassic of Chile the more onshore H2S interface, they can be arranged qualitatively shallow siliciclastic and míxed silicíclastic• along an oxygen gradient. An upper dysaerobic carbonate ramp harboured highly-diverse biofacies zone has been recognized, which is associations which indicate hospitable,low-stress dominated by deposit-feeding nuculid bivalves. conditions. Theywere dominated by K-strategists In the lower dysaerobic zone thin-shelled, flat which bel01g to semi-infaunal, deep infaunal, valved fílter-feeding pectinacean bivalves pre and epifaunal, free-Iying filter-feeders. vailed exhibiting an opportunistic larval strategy. • Further offshore, reduced input of terrigenous M.Aberhan 127

ACKNOWLEDGEMENTS

The author would like to thank F.T. Fürsich and of P. Sprechmann (Universidad de la República, W. Oschmann (University Würzburg) tor numerous Montevideo) who translated the Engfish abstract and discussions and F.T. Fürsich and several anonymous of S. Sch6dlbauer (Sayrisches Geologisches reterees for their comments on the manuscript. The Landesamt, Munich) for drawing the biotope author also thanks A. von Hillebrandt (Technical reconstructions, is particularly acknowledged. University, Serlin), E. Pérez (Servicio Nacional de Photographic work has been carried out by H. Schirm. Geología y Minería, Santiago), and L.A. Quinzio The study was financially supported :Jy a grant of the (Universidad Católica del Norte, Antofagasta, Chile) Deutsche Forschungsgemeinschaft (Fu 131/9-1), for their support in carrying out lield work. The work which is gratefully acknowledged.

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Savrda, C.E.; Eottjer, O.J.; Gorsline, O.S. 1984. S, No. 4, p. 423-434. Oevelopments of an oxygen-deficient marine biofacles Tyson, R.V.; Pearson, T.H. 1991. Modem and anclent model: evidem::e from Santa Moniea, San Pedro, and continental shelf anoxia: an overview. In Modam and Santa Barbara basins, California Continental ancient shelf anoxia (Tyson, R.V.; Pearson, T.H.; Borderland. American Association of Petro/eum editors). Geological Society of America, Special Geologists, Bulletin, Vol. 68 p. 1178-1192. Publication, Vol. 58, p. 1-24. Scotese, C.A. 199";. Jurassic and Cretaceous pi ate tectonic Valentine, J.W.; Jablonski, 0.1983. Larval adaptationsand reconstructions. Pa/aeogeograpphy. Pa/aeoclimatology, pattems of brachiopod diversity in space and time. Palaeoeco/ogy, Vol. 87, p. 493-501. Evolution, Vol. 37, p. 1052-1061. Seott, R.W. 1974. 8ay and shoreface benthic communities Vetter, R.O; Powell, M.A.; Somero, G.N. 1991. Metazoan in the Lower Cretaceous. Lethaia, Vol. 7, p. 315-330. adaptations to hydrogene sulphide. In Metazoan life Seott, A.W. 1990. Modals and stratigraphyof Mid-Cretaceous without oxygen (Bl)'ant, C.; editor). Chapman and Ha/I, reef commurities, Gulf of Mexlco. Concepts in p. 109-128. London. Sedimentology and Pa/eontology, Vol. 2, 102 p. Walker, K,A.; Bambach, A.K. 1971. The significance of Seilacher, A 1990. Abberrationsin bivalve evolution related fossil assemblages from fine-grained sediments: time to photo- and chemosymbiosis. Historieal Biology, Vol. averaged communities. Geologica/ Society ofAmeriea, 3, No. 4, p. 283-311. Abstracts with Programs, Vol. 3, p. 783-784. Smith, A.G.; Hurley, A.M.; Briden, J.C. 1982. Waller, T.A.; 1972. The functional signifieance of some shell Paltiokontinentale Weltkarten des Phanerozoikums. microstructures in the Pectinacea (Mollusea: Bivalvia). Enke, 102 p. Stuttgart. Intemationa/ Geologica/ Congress, No. 24, Section 7, Stanley, S.M. 1974. What has happened to the articulate p. 48-56. Montreal. brachiopods? Geologieal Societyof Amerlea, Abstracts Wignall, P.B. 1990a. Benthic palaeoecology of the late with Programs, Vol. 6, p. 966-967. Jurassic Kimmeridge Clay of England. Special Papers Steele-Petrovic, M.H. 1979. The physiological differences in Palaeontology, Vol. 43, 74 p. between articulate brachiopods and filter-feeding Wignall, P.B. 1990b. Observations on the evolution and bivalves as a factor in the evolution of marine level classification of dysaerobic communities. In bottom communities. Palasontology, Vol. 22, p. 101- Paleocommunity temporal dynamics: the long-term 134. development of multispecies assemblies (Miller, W. 111; Tchoumatchenco, P. 1972. Thanatocoenoses and biotopes editor). Pa/eontologieal Society, Specia/ Publication, of Lower Jurassic brachiopods in central and westem Vol. 5, p. 99-111. Bulgaria. Pa1aeogeography, Palaeoclimatology, Wright, V.P. 1986. Facies sequenceson acaroonate ramp: Palaeoeco/ogy, Vol. 12, p. 227-242. the Carbonife rous Limestone of South Wales. Thayer, C.W. 1985. Brachiopods versus mussels: Sediment%gy, Vol. 33, p. 221-241. competition, p ~edation, and palatability. Science, Vol. Ziegler, AM.; Cocks, LA.M.; Bambach, A.K. 1968. The 228, p. 1527-1528. composition and structure of Lower Silurian marine Thompson, J.B.; t.lullins, H.T.; Newton, C.A.; Vercoutere, communities. Lethaia, Vol. 1, No. 1, p. 1-27. T.L 1985. Altemative biofacies model for dysaerobic Ziegler, AM.; Scotese, C.A.; Barrett, S.F. 1982. Mesozoic communities. L.ethaia, Vol. 18, p. 167-179. and Cenozoic paleogeographic maps. In Tidal friction Tipper, J.C. 1979. Rarefaction and rarefiction -the use and and the earth's rotation 11 (Brosche, P.; Sandermann, J.; abuse of a mellod in paleoecology. Pa/eobiology, Vol. editors). Sprlnger Verfag, p. 240-252. M.Aberhan 131

APPENDIX 1

TROPHIC NUCLEI OF THE BENTHIC ASSOCIATIONS ANO-ASSEMBLAGES OCCURRING IN THE EARL Y JURASSIC OF NORTHERN CHILE. Lite hablt: E = epilaunal; Is = shallow inlaunal; Id = deep inlaunai; SI = semi-inlaunai; b = byssate; bo = bori~; e = cemented; t = Iree-Iying; m = llloblle; p = peduncuiate. Trophic group; 0= deposH-leeder; H = grazing herbivore or detritus-Ieeding; MC = microcarnivore; S = suspension-Ieeder; SOS = surface deposit- andlor suspension-Ieeder. Oiversity values: N = ranga 01 species rictmess, mean values in brackets; O = evenness.

Species rel. abundanee presenee life habit trophie % % group

1 GeNillella amucana association (2 samples 2::9 individual s; N = 14-19 (16.5); 0= 3.9; lacies C) GeNillella aratJcana Damboronea, 1987b 46.0 100.0 SI S Paralle/odon hirsonensis (d'Arehiae, 1843) 18.8 100.0 SI S 8akevellia (8akevellia) waltoni (Lyeett, 1863) 6.7 100.0 SI S Modiolus (Mo:iiolus) giganteus Ouenstedt, 1857 6.2 100.0 Eb S Lithotrochus ? andinus (Morieke, 1894) 4.2 100.0 Em H

2 Paralle/odon hirsonensis assoeiation (see ligure 6) (3 samples; 371 individuals; N = 13-21 (16.6); 0= 10.6; lacies C) Lobothyris el. ovatissima Ouenstedt, 1858 22.1 100.0 Ep S Paralle/odon nirsonensis (el' Arehiae , 1843) 17.8 100.0 SI S GeNillella araucana Damborenea, 1987b 12.1 100.0 SI S 8akevellia (Eakevellia) waltoni (Lyeett, 1863) 10.8 100.0 SI S Modiolus (Modiolus) giganteus Ouenstedt, 1857 4.3 100.0 Eb S Striactaeonina transatlantica (Behrendsen, 1891) 4.3 66.7 Em H high-spired gastropod 3.5 33.3 ?Em ?H Lithotrochus? andinus (Morieke, 1894) 3.5 100.0 Em H Cardinia andÍJm (Giebel, 1861) 3.2 33.3 Is S

3 Lobothyris el. ovatissima assoeiation (35 samples; 4553 individuals; N = 5-15 (9.1); 0= 1.6; laeies O, E) Lobothyris el. ovatissima Ouenstedt, 1858 78.9 100.0 Ep S Quadratirhynchia crassimedia Buekman, 1918 3.4 57.1 Ep S

4 Pholadomya hemicardia assoeiation (2 samples; 236 individuals; N = 11-12 (11.5); 0= 3.4; lacies D) Pholadomya (Pholadomya) hemicardia Roemer, 1836 35.6 100.0 Id S Entolium (En'olium) comeolum (Young and Bird, 1828) 19.9 100.0 El S Lobothyris el. ovatissima Ouenstedt, 1858 14.4 100.0 Ep S Weyla (Weyla) a/ata (von Bueh, 1838) 14.0 100.0 El S Pachymya (Arcomya) sp. A 5.9 100.0 Id S

5 Weyla alata/Lobothyris el. ovatissima assoeiation (6 samples; 544 individual s; N = 8-17 (11.8); 0= 5; lacies E) Weyla (Wey:a) alata (von Bueh, 1838) 38.1 100.0 El S Lobothyris e". ovatissima Ouenstedt, 1858 17.3 100.0 Ep S Plagios toma laeviusculum J. Sowerby, 1822 11.0 83.3 Eb S Pholadomya (Pholadomya) hemicardia Roemer, 1836 6.1 66.7 Id S Chlamys (Chlamys) textoria (Sehlotheim, 1820) 5.3 66.7 Eb S Entolium (Entolium) comeolum (Young and Bird, 1828) 3.3 100.0 El S 132 BENT-iIC MACROINVERTEBRATE ASSOCIATlONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURAS SIC ...

Speeies reJ. abundance presence life habit trophic % % group

6 Protocardia sp. A association (8 samples; 975 individuals; N ..12-16 (13.0); O .. 5.1; facies e) Lobothyris cf. ovatissima Quenstedt, 1858 39.3 100.0 Ep S Protocardia (Protocardia) sp. A 13.3 75.0 Is S Anisocardia sp. A 10.5 75.0 Is S Modiolus (ModiolUs) giganteus Quenstedt,1857 8.6 100.0 Eb S Entolium (Entolium) comeolum (Young and Bird, 1828) 8.5 62.5 Ef S Pleuromya uniformis (J. Sowerby, 1813) 2.6 50.0 Id S

7 Quadratirhynchia crassimedia association (5 samples; 633 individual s; N .. 7-13 (10.2); 0.3.1; facies E) Quadratirhynchia crassimedia Buckman,1918 46.0 100.0 Ep S Lobothyris cf. ovatissima Quenstedt, 1858 31.9 100.0 Ep S Gibbirhynchia cUNiceps (Quenstedt, 1858) 9.0 80.0 Ep S

8 Gibbirhynchia curviceps association (3 samples; 607 individuals; N .. 8-12 (9.7); O .. 1.7; facies E) Gibbirhynchia curviceps (Quenstedt, 1858) 74.3 100.0 Ep S Quadratirhynchia crassimedia Buckman, 1918 13.8 100.0 Ep S

9 Spiriferina chilensis association (see figure 8) (15 samples; 1884 individuals; N ..7-14 (10.0); O .. 2.4; facies E) Spiriferina chilensis (Forbes, 1846) 63.2 100.0 Ep S Lobothyris cf. ovatissima Quenstedt, 1858 14.1 80.0 Ep S Weyla (Weyla) elata (von Buch, 1838) 3.6 93.3 Ef S

10 Lobothylis su!Jpunctata association (16 samples; 2102 individual s; N = 5·13 (7.5); 0= 1.7; facies E) Lobothylis subpl!nctata (Davidson.1851) 76.6 100.0 Ep S Weyla (Weyla) elata (von Buch, 1838) 8.0 93.6 Ef S

11 Quadratirhynd.lia sp. A association (16 samples; 1844 individuals; N.. 5-17 (11.3); 0= 3.4; facies E) Quadratirhynchia sp. A 51 .8 100.0 Ep S Lobothylis SUbpL.'nctata (Davidson, 1851) 12.6 93.8 Ep S Weyla (Weyla) elata (von Buch, 1838) 6.9 100.0 Ef S Weyla (Weyla) titan (Moricke, 1894) 4.0 68.8 El S Exogyra sp. A 3.6 43.8 Ec S Plagios toma lae~'iusculum J. Sowerby,1822 3.3 93.8 Eb S

12 Lobothylis su!Jpunctata/Quadratirhynchia sp. A association (10 samples; 132'3 individuals; N.. 7-14 (10.1); 0= 3.5; facies E) Lobothyris subpt.:nctata (Davidson,1851) 47.9 100.0 Ep S Quadratirhynchia sp. A 22.9 100.0 Ep S Montlivaltia sp. 4.5 70.0 Ec MG Weyla (Weyla) ~/ata (von Buch, 1838) 3.8 80.0 El S Spilifelina chileneis (Forbes, 1846) 2.9 20.0 Ep S M.Aberhan 133

Species rel. abundance presence lite habit trophic % % group

13 Exogyra sp. A association (10 samplas; 1246 individuals; N=12-19 (14.6); 0=8.1; facias E) Exogyra sp. A 23.5 100.0 Ec S Weyla (Weyla) alata (von Buch, 1838) 16.1 100.0 Ef S Weyla (Weyla) titan (Moricke,1894) 15.9 90.0 Ef S Plagiostoma iaeviusculum J. Sowarby,1822 7.0 100.0 Eb S Chlamys (Chiamys) textoria (Schlothaim, 1820) 6.3 90.0 Eb S Ctenostreon wrighti Bayla, 1878 5.4 90.0 Ef S Ouadratirhynchia sp. A 4.4 60.0 Ep S Lobothyris subpunctata (Oavidson, 1851) 3.1 60.0 Ep S

14 Weyla aSSDciation (4 samplas; 5-2 individuals; N=4-13 (9.0); 0=2.7; facias E) Weyla (Weyla) alata (von Buch ,1838) 54.3 100.0 Ef S Weyla (Weyla) titan (Moricka, 1894) 12.5 75.0 Ef S Lobothyris subpunctata (Oavidson, 1851) 8.4 75.0 Ep S Ouadratirhynchia sp. A 4.7 50.0 Ep S Exogyra sp. A 4.3 75.0 Ec S

15 Entolium comeolum association (3 samplas; 740 individual s; N = 7-16 (10.9); 0= 3.2; facias E) Entolium (En:olium) comeolum (Young and Bird, 1828) 40.8 100.0 Ef S Weyla (Weyla) a/ata (von Buch,1838) 38.1 100.0 Ef S Montlivaltia sp. 2.8 51.7 Ec MC

16 Pachymya rotundocaudata association (3 samplas; 349 individuals; N = 21-29 (25.7); O = 14.4; facias O) Pleuromya u.,iformis (J. Sowarby,1813) 15.5 100.0 Id S Pachymya (Pachymya?) rotundocaudata (A. Laanza,1942) 13.8 100.0 Id S Cardinia andium (Giabal,1861) 8.3 100.0 Is S Weyla (Weyla) alata (von Buch, 1838) 7.4 100.0 Ef S Rudirhynchia aff. rudis Buckman, 1918 5.2 100.0 Ep S Gryphaea (GJyphaea) darwini (Forbas, 1846) 4.9 100.0 Ef S Tetrarhynchia tatrahedra (J. Sowarby, 1812) 4.9 66.7 Ep S Pseudotrapezium anca/ruzi (A. Laanza, 1942) 4.0 100.0 Is S Pinna (Pinna) cf. radiata Münster, 1837 3.7 100.0 SI S Pho/adomya (Pho/adomya) fidicula (J. Sowarby, 1821) 3.4 66.7 Id S Protocardia (Protocardia) stria/ula (J. da C. Sowarby) 3.2 66.7 Is S Chlamys (Ct:lamys) textoria (Schlothaim, 1820) 2.9 66.7 Eb S Pholadomya (Pholadomya) corrugata Koch and Ounker,1877 2.6 66.7 Id S Plagios toma laeviusculum J. Sowa rby, 1822 2.0 100.0 Eb S

17 Pleuromya uniformis association (8 samplas; 1068 individuals; N =9-34 (23.0); 0= 7.9; facias O) Pleuromya uniformis (J. Sowarby,1813) 28.3 100.0 Id S Weyla (Weyla) alata (von Buch,1838) 18.3 100.0 Ef S Protocardia 'Protocardia) striatula (J. da C. Sowarby) 6.5 87.5 Is S 134 BENTHIC MACROINVERTEBRATE ASSOCIATIONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ...

Species rel. abundance presence lite habit trophic % % group

Pho/adomya (Pho/adomya) corrugata Koch and Dunker,1877 4.3 62.5 Id S Lobothyris subpunctata (Davidson,1851) 3.9 87.5 Ep S Cardinia andium (Giebel, 1861) 3.5 62.5 Is S P/agiostoma /aeviuscu/um J. Sowerby,1822 2.8 87.5 Eb S Tetrarhynchia tet:'ahedra (J. Sowerby,1812) 2.4 62.5 Ep S Montlivaltia sp. 2.3 62.5 Ec MC Mesomiltha bello.1a (d'Orbigny,1850) 2.3 62.5 Id SDS Ctenostreon wri[jJti Bayle,1878 2.2 100.0 Ef S Pinna (Pinna) ef. radiata MOnsler,1837 2.2 100.0 SI S Rudirhynchia aft. rudis Buekman,1918 2.2 50.0 Ep S

18 Lobothyris subpunctatalRudirhynchia aft. rudis association (4 samples; 489 individuals; N = 14-18 (15.5); D = 5.0; facies E) Lobothyris subpúflctata (Davidson,1851) 39.5 100.0 Ep S Wey/a (Wey/a) alafa (von Buch,1838) 15.7 100.0 Ef S Rudirhynchia aft. rudis Buekman,1918 8.8 100.0 Ep S P/euromya unifofTTIis (J. Sowerby,1813) 8.6 100.0 Id S GeNillaria pallas (A. Leanza,1942) 4.7 50.0 SI S Gryphaea (Grypl-aea) darwini (Forbes,1846) 3.1 75.0 Ef S

19 Gryphaea sp. A assoeialion (see figure 10) (4 samples; 414 individual s; N = 5-15 (9.5); D = 4.0; facies F) Gryphaea sp. A 45.7 100.0 Ef S Lobothyris subpunctata (Davidson,1851) 15.5 75.0 Ep S Rudirhynchia aft. rudis Buekman,1918 10.9 75.0 Ep S Ento/ium (Ento/ium) corneo/um (Young and Bird,1828) 6.0 50.0 Ef S Actinostreon so/iIBrium (J. de C. Sowervy,1824) 5.1 75.0 Ee S

20 Wey/a alatal Ento/ium comeo/um assoeiation (see figure 7) (11 samples; 1254 individual s; N = 14-29 (19.0); D = 7.4; facies D) Wey/a (Weyla) alata (von Bueh,1838) 32.2 100.0 Ef S Ento/ium (Entolium) corneo/um (Young and Bird,1828) 12.2 100.0 Ef S P/euromya uniformis (J. Sowerby,1813) 6.1 90.9 Id S Mont/ivaltia sp. 5.8 81.8 Ee MC Pho/adomya (Pholadomya) corruga fa Koeh and Dunker, 1877 5.4 72.7 Id S Pinna (Pinna) cf. radiata MOnster,1837 4.5 100.0 SI S Gryphaea (Grypraea) cf. darwini (Forbes,1846) 3.8 72.7 El S Aretieidae gen. el sp. nov. 3.4 81.8 Is S 'Terebratula' sp. A 3.2 36.4 Ep S Mesomiltha bello.1a (d'Orbigny, 1850) 2.5 63.6 Id SDS Quadratirhynchia sp. B 2.5 45.5 Ep S

21 Mesomi/tha be/lona assoeiation (3 samples; 697 individual s; N = 9-40 (26.8); D = 12.9; facies D) Mesomiltha bello.1a (d'Orbigny,1850) 16.2 100.0 Id SDS Weyla (Wey/a) alata (von Bueh,1838) 13.1 100.0 Ef S Enfo/ium (Enfo/iuM) corneo/um (Young and Bird,1828) 11.5 100.0 Ef S Gryphaea (Gryphaea) ef. darwini (Forbes,1846) 9.2 66.7 Ef S M.Aberhan 135

Species rel. abundance presence lite habit trophic % % group

Pleuromya uniformis (J. Sowerby, 1813) 5.5 100.0 Id S Myophorella (Myophorella) araucana (A. Leanza, 1942) 5.0 66.7 Is S Pholadomya (Pholadomya) corrugata Koch and Dunker,18n 4.7 66.7 Id S 'Terebratula' sp. A 4.2 66.7 Ep S Montliva/tia sp. 3.6 66.7 Ec MC 'Terebratula' sp. B 3.0 66.7 Ep S Pholadomya (Pholadomya) fidicula (J. Sowerby,1821) 2.3 100.0 Id S Ouadratirhynchia sp. B 2.0 66.7 Ep S

22 Nuculana ovum association (see figure 10) (4 samples; 563 individual s; N = 3-7 (4.8); D .. 1.4; facies F) Nuculana (MJculana) ovum 85.3 100.0 1m D

23 Gryphaea darwini association (6 samples; 1079 individuals; N = 9-19 (15.3); D = 5.5; facies D, L) Gryphaea (Gryphaea) darwini (Forbes,1846) 37.3 100.0 Ef S Pleuromya l.Jfliformis (J. Sowerby,1813) 15.6 100.0 Id S Rhynchonelloidea sp. A 9.4 100.0 Ep S Actinostreon solitarium (J. de C. Sowerby,1824) 7.0 100.0 Ec S Uthophaga sp. 3.9 16.7 lbo S Lobothyris subpunctata (Oavidson,1851) 3.9 83.3 Ep S Vaugonia (Vaugonia) exotica (Moricke,1894) 3.4 66.7 Is S

24 Actinostr80n solitarium association (3 samples; 345 individual s; N = 7-13 (9.7); O .. 3.0; facies O) Actinostreon solitarium (J. de C. Sowe rby ,1824) 54.2 100.0 Ec S Lobothyris s.Jbpunctata (Davidson,1851) 11.0 33.3 Ep S Gryphaaa (Gryphaea) darwini (Forbes,1846) 10.4 100.0 El S Entolium (Entolium) comeolum (Young and Bird,1828) 9.3 100.0 El S

25 Mesomiltha ? huayquimili association (3 samples; &.35 individual s; N = 12-17 (15.0); 0= 6.9; facies O) Mesomiltha ? huayquimili (A. Leanza, 1942) 29.0 100.0 Id SOS Pleuromya u,iformis (J . Sowerby,1813) 20.9 100.0 Id S Modiolus (Modiolus) imbrica tus J. Sowerby,1818 6.0 100.0 Eb S Vaugonia (\-'augonia) exotica (Moricke,1894) 5.7 66.7 Is S Gresslya pe~egrina (Phillips,1829) 5.5 100.0 Id S Chlamys (Cnlamys) textoria (Schlotheim,1820) 3.9 66.7 Eb S Gryphaea (Gryphaea) darwini (Forbes,1846) 3.9 100.0 Ef S Lobothyris SlJbpunctata (Oavidson, 1851) 3.9 66.7 Ep S Mesomiltha bellona (d'Orbigny,1850) 3.4 33.3 Id SOS

26 Gresslya peregrina association (3 samples; 579 individuals; N = 15-26 (19); 0= 5.9; facies O) Gresslya p9fegrina (Phillips,1829) 28.2 100.0 Id S Pleuromya uniformis (J. Sowerby,181,3) 21 .2 100.0 Id S Pholadomya (Pholadomya) fidicula (J. Sowerby,1821) 20.0 100.0 Is S 136 BENTHIC MACROINVERTEBRATE ASSOCIATlONS ON A CARBONATE-CLASTIC RAMP IN SEGMENTS OF THE EARLY JURASSIC ...

Speciea rel. abundance preaence life habit trophic % % group

Cercomya (Cercomya) undu/ata (J. de C. Sowerby,1827) 3.6 100.0 Id S Mesomiltha bel/ona (d'Orbigny,1850) 3.5 100.0 Id SDS Modiolus (Modio/;Js) imbricatus J. Sowerby,1818 3.5 100.0 Eb S

27 Posidonotis cancel/ata association (ses figure 10)

(4 samples; 599 individuaJs; N lO 1·3 (2.0); D .. 1.0; facies G) Posidonotis cancel/ata (A. Leanza,1943) 98.2 100.0 Eb/f S

Manuscrlpt recelved: AprU 15. 1993; accepted: Juiy 9, 1993.