Aptian-Albian rudist bivalves (Hippuritida) from the Chilean Central Andes: Their palaeoceanographic significance

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AptianeAlbian rudist bivalves (Hippuritida) from the Chilean Central Andes: Their palaeoceanographic signicanceJean-Pierre Masse a, *, Francisco-Amaro Mourgues b, Mukerrem Fenerci-Masse aa CEREGE, Aix-Marseille University, Centre Saint-Charles, 13331 Marseille Cedex 03, Franceb Terra Ignota, Heritage and Geosciences Consulting, Dr. Ca diz 726, N~ un~oa, Santiago, Chile

a r t i c l e i n f o

Article history:Received 30 September 2014Accepted in revised form 7 December 2014 Available online

Keywords:Rudist bivalves Early Cretaceous Taxonomy Palaeobiogeography South America

a b s t r a c t

The study of AptianeAlbian rudist faunas from the Chilean Central Andes documents the presence of two forms, ascribed to the Monopleuridae: Douvillelia skeltoni an early Aptian species known hitherto from Mexico, and Jerjesia chilensis, an endemic Albian species from Chile and Peru, with a complex taxonomic history. The regional stratigraphy of the Central Andes, combining ammonites and rudists, is consistent with the Caribbean stratigraphic distribution of Jerjesia and Douvillelia. Andean occurrences of the two genera broaden their distributional area on the Pacic side of Americas, and testify their biostratigraphic value. Oceanographic conditions of the Chilean Pacic margin during the AptianeAlbian, including moderate but effective cold oceanic current, upwellings, high productivity and thermal instability, may acknowledge the low taxonomic diversity of rudist assemblages, which look impoverished when compared to their low latitude homologues from the Caribbean regions which were functioning as a dispersal centre.

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1. Introduction

Cretaceous, rudist-bearing carbonate platforms, tend to be globally restricted to 30oe35o south and north latitudes, so they extend farther beyond the tropics than their modern counterparts,i.e. coral reefs (Coates, 1973; Masse, 1992a,b; Simo et al., 1993; Philip, 1998; Skelton et al., 2003). However Eastern Asia, i.e. the Japanese archipelago, and the Pacic side of North America, where rudist occurrences are found at higher palaeolatitudes, do not t this overall latitudinal distribution (Masse, 1992a,b; Sano, 2012; Sano et al., 2013; Skelton et al., 2013). Rudist occurrences of Japan at a palaeolatitude in the range of 35oe45o N are somewhat anomalic. Nevertheless some debate exists on the palaeoposition of the different parts of the archipelago deduced from palinspastic re- constructions, for instance Hokkaido is close to its present day location (44oN) for Hay and De Conto (1999), at 50oN for Eldridge et al. (2000), and 35e36oN for Takashima et al. (2007). Whatever its amplitude, this latitudinal anomaly hardly corresponds to a late AptianeAlbian p.p. warm peak, as suggested by Takashima et al. (2007), because this event cannot explain the presence of

* Corresponding author.E-mail address: masse@cerege.fr (J.-P. Masse).

TithonianeBerriasian forms recorded from the same region (Sano and Skelton, 2010). We favour the warming effect of a Cretaceous precursor of the modern Kuroshio current (Japan current) is sup- ported by simulations of Aptian oceanic circulation (e.g. Fluteau et al., 2007). This present day warm (24oc annual average SST) north owing current, sustaining coral reefs in southern Japan, corresponds to the western branch of the North Pacic, clockwise ocean gyre. Similarly, rudist occurrences of North California (Sano et al., 2013; Skelton et al., 2013) with palaeolatitudes in the range of 40oN, do not t the above mentioned latitudinal range and their palaeoceanographic and/or palaeogeographic context is still unclear.The southernmost rudist (27oe30o S) from America: Hippurites chilensis, was known from Chile since the rst half of XIX century after the pioneer works of d'Orbigny (1842) about the fossils and geology of South America. Subsequently this species was either ignored or misinterpreted.The objectives of the present paper are to revise the systematic position and to analyse the taxonomic avatars of Hippurites chi- lensis, the rst rudist to have been described in America. It com- plements and supports the results briey exposed earlier (Mourgues et al., 2010) placing this form in the genus Jerjesia. In addition we provide data on some other forms, including the genus Dou- villelia, and discuss the biostratigraphic and palaeobiogeographic

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signicance of the rudist assemblages, focussing on comparisons with Caribbean faunas. This study is based on the analysis of historic collections and specimens collected by one of us (F.A.M.), their dating is founded on an up-to-date biostratigraphy of the Cha-n~arcillo Basin (Mourgues, 2007). The specimens are housed in thePaleontological Collection of the Geological and Mining Survey of Chile, Santiago (SNGM).A key problem we will address is that South America rudist occurrences which match the overall latitudinal distribution of the group, are somewhat anomalic if we consider the sea surface oceanic currents. By reference to the modern regional oceanog- raphy characterized by the existence of the Peru (or Humbolt) current, a cold current owing northward off western South America, and forming the eastern branch of the South Pacic gyre, is assumed to have existed during the Early Cretaceous (Hay, 1995).

2. Regional geological setting

Rudist bearing beds are present in the Pabello n and Arqueros Formations of the Chan~arcillo basin, one of the retroarc Andean ba- sins lining South America. The inll of the Chan~arcillo Basin(Atacama basin) (Fig. 1), comprises more than 2000 m of marine carbonate rocks and minor volcaniclastics at the base and top of the succession, known as the Chan~arcillo Group in the sense ofSegerstrom and Ruiz (1962), which ranges in age from late Berriasian to early Albian (Mourgues, 2007). This Lower Cretaceous succession crops out in the southern Atacama region as a NNE-trending belt

from La Serena to Copiapo , parallel to the current plate margin (Fig. 2). Southward of this belt, there are only isolated outcrops. During late Aptianeearly Albian times the Pacic Gondwana active margin was characterized by a geodynamic setting with a closing back arc basin which suffered a strong tectonic and volcanic activity (Are valo et al., 2005). This activity was reected in the record of synsedimentary tectonic features, volcaniclastics ows and lava events into the regressive platform carbonate sediments of the upper Pabello n Formation (Mourgues, 2007), that is the uppermost lith-ostratigraphic unit of the Chan~arcillo Group. This group is covereddisconformably eastwards by the Cerrillos Formation (Segerstrom and Parker, 1959), a thick subaerial volcaniclastic sequence ascribed to the early Albianeearly Maastrichtian (Maksaev et al., 2009).

3. Biostratigraphy of rudist bearing formations

The Pabellon Formation was originally (Tavera, 1956; Corvalan, 1974) assumed to be upper Barremian, based on the presence of Agria blumenbachi (formerly Hippurites chilensis Jerjesia chi- lensis). The publication of an ammonite fragment attributed to Parahoplites gr. nuteldiensis (J. Sowerby), subsequently raised the age of formation to the early late Aptian (Pe rez et al., 1990). The recent revision of ammonite specimens collected by Pe rez et al. (1990) show the presence of Neodeshayesites, this result combined with the discovery of Hypacanthoplites-bearing beds at the Que- brada El Molle, lead to assign the top of the Pabello n Formation to the lower Albian (Mourgues, 2007).

Fig. 1. Location map and stratigraphic succession of the Chan~arcillo Group (modied from Mourgues, 2007; numerical ages from Cohen et al., 2013).

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*formationsD Totoralillo Formation (Kit)D Member 2 (Kia2): carbonate platform Fossil localityD Nantoco Formation (Kin)Member 1 (Kia1): volcanicFig. 2. Location map and geological context of the study stratigraphic sections.

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At Quebrada Carrizalillo (Fig. 3A) Jerjesia biostromes are part of a transgressive succession that unconformably overlies porphyry andesites. At Quebrada El Molle (Fig. 3B) Jerjesia e bearing beds are observed as biostrome block components in a calcareous breccia olistostrome. This conspicuous unit is capped by clastic e carbonate marine sequences containing a mideearly Albian Neodeshayesites horizon and is supported by Hypacanthoplites bearing marine de- posits of latest Aptian age (Aguirre-Urreta et al., 2007). The age for Jerjesia chilensis beds is therefore early Albian.The type locality of Jerjesia chilensis correspond to the Arqueros ancient silver mine (29o490 S). In this locality the Lower Cretaceous is represented by a volcanic succession with two well dened marine carbonate intercalations (Fig. 3C). Aguirre and Egert (1962) dened de Arqueros Formation as a volcano-sedimentary succes- sion made of ve members, and recognized precisely the strati- graphic situation of rudists in the upper member of the formation. Recently detailed stratigraphic sections were performed at the type locality and two rudists biostromes were identied in the middle and top parts of the Member 4 (Kia4) of the Arqueros Formation. The rst member of the Arqueros Formation (Kia1) is composed by a thick porphyry andesite lava ow with locally pillow structures. The overlying unit is a massive bed of shoreface limestone and marlyesherty (silty) hemipelagic limestone with gastropods, spa- tangoid echinids and pectinids, with interbedded thin grainstones containing trigoniids and other bivalves (Kia2). A mid homoge- neous volcanic member (Kia3) is formed by porphyry andesite and is covered unconformably by a new marine succession (Kia4) made of a thinly rhythmic bedded mudstone and coarse sandstone, with two massive rudist biostromes having a muddy matrix. At the upper part of the section a calcareous breccia is present. This car- bonate interval is covered by a thin manganese-rich volcaniclastic red sandstone which passes upward to a thick andesitic lava suc- cession (Kia5).Douvillelia beds are present in the uppermost marine strata of the Pabellon Formation at Cerro Pajonales (Fig. 3D). Stratigraphic indices, i.e. ammonites, were identied below and mark the upper Barremian Antarcticoceras domeykanus zone and the lower Barre- mian Emericiceras and Shasticrioceras zones (Mourgues, 2007). Douvillelia beds are observed just below the regional unconformity underlying the Cerrillos Formation. In this locality the upper part of the Pabello n Formation appears to be missing by erosion, this interpretation is supported by the absence of porphyry andesite (see above) or any volcanic episode recognized regionally, e.g. Quebrada Carrizalillo, Quebrada El Molle and Llano de Arqueros, dated as late Aptian, i.e.-115 Ma (Morata et al., 2001; Morata and Aguirre, 2003). The foregoing suggests that the Douvillelia- bearing beds are early Aptian in age. The identication of Douvillelia skeltoni Alencaster and Pantoja-Alor corroborates this dating (see discussion below).

4. Systematic palaeontology

The classication used herein refers to Skelton (2013a,b).Abbreviations used for systematic descriptions. LV, left valve; RV, right valve; Ant, anterior side; Post, posterior side; Vent, ventral; Dors, dorsal; t, tooth; at, anterior tooth; pt, posterior tooth; ct, central tooth; am, anterior myophore; pm, posterior myophore; lg, ligament groove; bc, body cavity; ol, outer shell layer; il, inner shell layer; AB, anterior band; PB, posterior band; IB, interband; Dap, antero-posterior diameter; Ddv, dorso-ventral diameter; L, length.Order Hippuritida Newell, 1965Suborder Hippuritidina Newell, 1965, in Skelton, 2013b Superfamily Radiolitoidea d'Orbigny, 1847, in Skelton, 2013a Family Monopleuridae Munier-Chalmas 1873Genus Jerjesia Alencaster 1986

Type species. Jerjesia encina Alencaster 1986Emended diagnosis. Thick-shelled cylindro-conical RV, LV low conical. Radial bands inconspicuous. Myocardinal apparatus strong, myophores on both valves located on transverse thickenings of the shell margin, parallel or slightly oblique to the commissural plan, am larger than pm with an antero-ventral elongation. Small body cavity with a rounded transverse outline in RV, small conical body cavity with a limited cardinal platform in the dorsal side of LV. Commissural plan strongly oblique to the shell axis. Ligament ridge marked externally by a ligament groove.Discussion. In her original description Alencaster (1986) mentioned: 1 e an accessory cavity anked by a transverse plate in the LV (see g. 3, plate 1 and g. 6, plate 3), 2 e central tooth with a curved peaked termination engaged in a small canal at the base of the opposite anterior tooth (see g. 2, p. 55). Figures provided by the author show that:1 e the so-called transverse plate and accessory cavity represent the section of the cardinal platform and adjacent body cavity of the LV; this controversial interpretation rules out the placement of Jerjesia in the Caprotinidae Gray emend. Skelton (2013a); it also precludes, as suggested by Skelton (2013a), the assignment of the genus to the Polyconitidae;2 e the peculiar relationships of the central and anterior teeth and sockets, illustrated on g. 2 (p. 55), hardly correspond to the photographs of plates 1e3; this point is important because it was considered as one of the key attributes of the genus (Alencaster, p. 49).The emended diagnosis and the foregoing remarks suggest that Jerjesia must belong to the Monopleuridae. The myophoral orga- nization of Jerjesia departs from those of the petlalodontid mono- pleurids including: Petalodontia Pocta, Debrunia Masse and Fenerci- Masse, Pseudopetalodontia Masse et al., Araeopleura Cox and Mathesia Mainelli, having all one or two myophoral plates on the LV, protruding into the opposite valve. It also differs from Mono- pleura Matheron by the wide transverse, myophoral, anterior and posterior thickenings of the RV, and from Bicornucopina Hofmann by its low domal LV and teeth-sockets morphology. The genus Glossomyophorus Masse et al. has a myophoral plate on the LV, protruding in a cavity of the opposite valve; a myophoral organi- zation much alike that of the Caprotinidae than the Monopleuridae (Masse et al., 1994). Arnaudia Fischer, placed in the Monopleuridae by Skelton (2013a), but assigned to the Radiolitidae by Khn (1944), has a depressed LV and lacks a ligament ridge. Gyropleura Douville has a posterior myophoral plate in the RV. The foregoing shows that Jerjesia is a valid taxonomic entity.Jerjesia chilensis (d'Orbigny) Fig. 41842 e Hippurites chilensis d'Orbigny, p. 107, pl. 22, g. 161929 e Agria blumenbachi Studer, Steinmann, p. 116e117, g. 128 1967 e Agriopleura sp., Herm, p. 662, pl.1, g. 1; pl. 2, gs. 2 and 3 1977b e Agriopleura, Jurgan, p. 422e423, g. 241999 e Agriopleura sp. aff. A. blumenbachi (Studer), Mourgues, p.156. Pl. 5, g. 7, Figs. text 33 and 34Type locality. Llanos de Arqueros, northeast of La Serena, Chile (Fig. 2).Type level. Member 4 of the Arqueros Formation (Kia4), lower Albian.Study material and localities. 4 isolated bivalve specimens, (SNGM-1991 to 1994), from Llano de Arqueros, 1 isolated specimen (SNGM 1185-1), two blocks cut in slabs (SNGM 1185-2 and 4), and two clusters of silicied specimens (SNGM 1185-3 and 4), from Quebrada El Molle.Taxonomic avatars of Hippurites chilensis d'Orbigny.D'Orbigny (1842) described Hippurites chilensis from fossils samples collected by Ignacio Domeyko near the Arqueros ancient

Fig. 3. Stratigraphic sections of the upper part of the Pabello n Formation at (A) Quebrada Carrizalillo, and (B) Quebrada el Molle, and the upper part of the Arqueros Formation at (C) Llanos de Arqueros, with position of Jerjesia chilensis bearing beds. The Cerro Pajonales (D) section shows the lower part of the Pabellon Formation and the position of the Douvillelia skeltoni bearing beds.

Fig. 4. Jerjesia chilensis (d'Orbigny) from Quebrada El Molle (SNGM 1185-1). A, lateral view of a bivalve specimen in a bouquet (SNGM 1185-1a). B, Dorsal view of an isolated specimen, showing the ligament groove (lg) of the RV and the attened LV (SNGM-1b). CeD, Longitudinal section of B specimen, showing the internal characters, and their interpretation (E, F). G, Longitudinal section of a well preserved bivalve specimen showing the myocardinal elements and their interpretation. H, close up of the cardinal organization gured in (G) (Llanos de Arqueros, SNGM-1991). I, Transverse section of the RV of a well preserved specimen of Llanos de Arqueros (SNGM-1992). J, K, Antero- posterior section of an isolated LV, partly silicied, showing the myophores. L, M, Ibid., dorsal section showing the teeth (slabs from Quebrada el Molle SNGM 1185-2 and 4). Scale bar 1 cm.

mine, northeast of La Serena city. Darwin (1846) assigned to the d'Orbigny species some specimens coming from the same locality. During the XX century the d'Orbigny's taxon was either ignored or re-interpreted. So Hippurites chilensis was ignored by Fritzsche (1923) who ascribed the forms of Arqueros to Agria blu- menbachi (Studer), he identied the same taxon at Potrero Seco in the Copiapo valley; and in the Lower Cretaceous of Per, namely at Huallanca and Huaraz. Steinmann's work (1929) tends to bear out the former identications for both Chile and Peru specimens. Subsequently Herm (1967) studied the shell microstructure of the bivalves collected from the Arqueros and Lo Prado formations and similarly, identied Agriopleura sp.. The ensuing works made in Chile mentioned the same taxon with different nomenclatural

combinations all revolving around Agriopleura (Tavera, 1956; Corvala n, 1974; Jurgan, 1977a,b; Mourgues, 1999, 2000).The following deals with the assignment of Hippurites chilensis and the so-called Agriopleura blumenbachi from Chile and Peru, to a single taxon Jerjesia chilensis (d'Orbigny).Description (Fig. 4). RV cylindrical or cylindro-conical, length up to 8e9 cm, with a rounded elliptical transverse outline (Dap > Ddv). Average dimensions Dap 2.6 cm, Ddv 2.3 cm. Outer shell smooth, ligament groove well dened, marked by downward deected growth lines; radial bands inconspicuous. Flattened domal LV. Myophores of LV corresponding with slightly oblique transverse thickenings, small conical body cavity restricted to the dorsal side, teeth relatively short and subequal. Myophores of RV

oblique (inward inclination) transverse thickenings. Body cavity of RV relatively small and rounded. Compared to the outer shell layer, originally calcitic and frequently silicied, the inner shell layer is relatively thick except on the ventral side where the two layers are nearly of same thickness.Remarks. Well preserved (i.e. non-silicied) specimens from Lla- nos de Arqueros document the teeth and sockets architecture whereas myophores exhibit a peculiar colour. Serial longitudinal sections of a specimen (Fig. 4G, H) show: teeth with a convex lami- nated microstructure, whereas sockets are lled by a concave upward (towards the commissure) laminated microstructure, and myo- phores on both valves are in black. Growth banding is also recorded in the shell of the Mexican species Jerjesia encina and the black coloured myophores (bandas oscuras) (Alencaster, 1986) as well. We assume that these features merely reect a diagenetic context favouring their preservation, i.e. possibly the preservation of aragonite. Usually they are not preserved, and the inner shell layer has a homogeneous sparry structure (Fig. 4C, D), see also Herm (1967).Discussion. The above descriptions show that Hippurites chilensis d'Orbigny lack the generic attributes of this genus: mainly the shell infoldings of the RV and the canaliculated and porous structure of the LV (Skelton, 2013a). Similarly it does not match the characters of Agriopleura: mainly the protruding myophores of the LV, usually depressed, and the salient bands, depressed interband and incon- spicuous ligament groove of the RV (Masse and Fenerci-Masse, 2014).Jerjesia was hitherto represented by a single species J. encina found in lower Albian beds from the Pihuamo region (Jalisco) of the southwestern Pacic side of Mexico (Alencaster, 1986). The overall morphology of J. chilensis is similar to J. encina the dimensions of

which are, however, signicantly higher (D > 5 cm, L up to 10 cm), moreover in the Mexican species the inner shell layer is relatively much thicker and the LV more oblique to the shell axis. We ascribe to Jerjesia chilensis dense tubular congregations (Fig. 5C, D) found by M. Floquet (personal communication, 2013) in the Huayhuash cordillera (Chiquian region, Peru). The Jerjesia beds are overlain by ammonite bearing marls with Prolyelliceras ulrichi, an index of the middle Albian (L. Bulot, personal communication, 2013). We as- sume that the monopleurid rudists from Peru, mentioned by Fritzsche (1923) as Agria, and especially those gured by Steinmann (1929) as Agria blumenbachi, actually represent Jerje- sia chilensis. This species is morphologically comparable to Mono- pleura marcida White (1884) from the middle Albian of Texas and Mexico (Garcia-Barrera, 2006). Internal characters of the LV are also somewhat similar but the myophores of the RV of M. marcida are represented by ledges (see Skelton, 1978). Monopleura sp. (p. 518, g. 5) gured by Skelton et al. (2013) from the dredged material (assumed to be Albian) of the Darwin Guyot, in the Mid-Pacic Mountains, has a low capuloid LV and is characterized by the commarginal elongation of the posterior tooth and nearly sym- metrical myophores, both features depart from Monopleura s.s., this form has therefore some afnity with Jerjesia.The Chilean record of the Jerjesia chilensis matches the age of the Mexican J. encina, and broadens the geographic distribution of the genus, represented by two vicariant species, which extends on the Pacic side of both central and south America, over about 48o in latitude (18oN to about 30oS).Jerjesia chilensis, J. encina, Pseudopetalodontia felixi (Douville ), Tepeyacia gregaria (Palmer) (see Masse et al., 2007) and Monopleura marcida (Perkins, 1974; Scott, 1981) are sub-cylindrical elevators

Fig. 5. Jerjesia chilensis assemblages from Chile (Llanos de Arqueros) (A, B), and Peru (Chiquian region) (by courtesy of M. Floquet, Aix-Marseille University) (C, D). A, Packed assemblage with upright oriented individuals. B, Silicied bouquet (SNGM 1185-4). C, D, Plan view of irregularly packed assemblages in growth position (eld views).

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(sensu Skelton and Gili, 2002) which tend to build dense aggre- gations, a mode of assemblage remarkably developed during the Albian in the New World (Fig. 5). As elevators they may represent fast growing, somewhat opportunistic species, able to rapidly colonize shallow water settings, a key adaptative strategy in un- stable environments with, possibly, high nutrient contents due to the oceanographic (i.e. coastal upwellings) and volcanic context, i.e. conditions assumed to increase trophic resources (Allmon and Martin, 2014).Genus Douvillelia Alencaster and Pantoja-Alor 1998Type species. Douvillelia skeltoni Alencaster and Pantoja-Alor 1998 (Fig. 6A)Emended diagnosis. Thick-shelled form with an exogyriform RV (apex twisted) having two longitudinal grooves on the ventral side bounding three rounded salient ridges with convex upward growth lamellae, marked by two swellings of the commissure; interband a furrow; moderately convex LV. Well developed body cavities, dorsally oblique and conical in both valves, larger in RV. Myo- cardinal apparatus aligned parallel to the dorsal margin. Myo- phores on both valves located on transverse thickenings of the shell margin, parallel (posterior side) or slightly oblique (anterior side) to the commissural plan, am transversally elongated and larger than pm. Central tooth with a crescentic transverse section, with ante- rior socket located in the concave anterior side; narrow apex of anterior tooth obliquely inserted in its socket. Ligament groove and inner crest poorly dened.Discussion. In their original description of the genus Alencaster and Pantoja-Alor (1998) noticed the overall similarity of Douville- lia with Horiopleura and Polyconites, notwithstanding the absence of pediculated myophores in the RV of the former genus. They also recognized some afnities of D. skeltoni with Monopleura salazari Palmer but rejected the placement of their taxon in Monopleura. As suggested by Skelton (2013a) the myophoral characters of Dou- villelia do not match those of the Polyconitidae, and the above emended diagnosis leads to assign this genus to the Monopleur- idae. In placing Douvillelia in the Caprotinidae, Alencaster and Pantoja-Alor referred to the denition of the family given by Dechaseaux et al. (1969), which included several genera which have been formerly or subsequently assigned to distinctive families, mainly the Polyconitidae MacGillavry (1937) and the Caprinidae d'Orbigny (Skelton and Masse, 1998; Skelton, 2013a). The family Caprotinidae Gray is presently restricted to two closely related genera Caprotina and Chaperia (Skelton, 2013a). For the denition of the genus we focus on the monopleurid myocardinal organiza- tion and the peculiar morphology of the RV with an exogyriform habit and a trilobate, lamellar, ventral side associated with commissural swellings. The salient radial bands and depressed interband depart from that of Monopleura. Noticed that the myo- cardinal traits of Douvillelia are much alike those of Jerjesia (see above).Description of the Chilean specimens of Douvillelia Douvillelia skeltoni Alencaster and Pantoja-Alor 1998 Fig. 6 (BeF)Study material and localities. 10 RV, most of them partly embedded in a ferrigeneous rocky matrix and more or less silicied, some partly preserved; all from Cerro Pajonales (numbers SNGM 1983e1990).Thick-shelled form with an exogyriform RV (apex twisted) having two longitudinal grooves on the ventral side, bounding three rounded or attened salient ridges with convex, festooned, upward growth lamellae, marked by two swellings of the commissure; interband a furrow. Antero-posterior elongation, dorsal side attened and smooth, ventral side convex and lamellar, angular junction between the dorsal and posterior sides. Well marked ligament groove, located close to the junction in question.

LV poorly preserved. Large body cavity in RV. Myocardinal apparatus badly preserved but aligned on the dorsal side of the RV.The placement of the Chilean specimens in Douvillelia is mainly based on the characters of the ventral side of the RV and its commissural festoons. The average commissural dimensions given below support the identity of the specimens from Mexico and Chile (Ddv 3.9 cm and Dap 6.0 cm for the Mexican specimens; Ddv 4.0 cm and Dap 5.0 cm for the Chilean specimens) which are therefore assigned to Douvillelia skeltoni.Discussion. Douvillelia skeltoni was hitherto known from a single area: the Pacic side of the Huetamo region of the Mexico (Michoacan), in beds of early Aptian age (Alencaster and Pantoja- Alor, 1998). Geological data provided by Abad (1976, 1980) on rudists collected from the lower part of the Pabellon Formation at Vallenar, and by Jurgan (1977a) from Cerro Pajonales, and all ascribed to Agria, suggest that the rudists in question actually belong to Douvillelia skeltoni. The Chilean record of the species matches the age of the Mexican one and broadens the geographic distribution of the species which extends on the Pacic side of both central and south America, over about 48o in latitude (18oN to about 30oS).

5. Palaeoenvironmental, palaeogeographical and palaeoclimatic signicance of Chilean rudist assemblages

AptianeAlbian rudist from Chile are characterized by their very low taxonomic diversity, limited to two genera Jerjesia and Dou- villelia, belonging to a single family, the Monopleuridae; and the monospecic state of the corresponding assemblages, characters allowing to dene a palaeobiogeographic entity equivalent to the modern PerueChile Province sensu Fernandez et al. (2000). These features contrast with those of the Caribbean regions where rela- tively high diversity assemblages are documented for the early Aptian, including 4 or 5 families and 10 genera (Harris and Hodson, 1922; Alencaster and Pantoja-Alor,1996, 1998; Chartrousse and Masse, 2004; Mitchell, 2013a), and the Albian with 6 families and 22 genera (Palmer, 1928; Alencaster, 1986; Chartrousse, 1998; Scott,2002; Garcia-Barrera, 2006; Masse et al., 2007; Mitchell, 2013a,b, Skelton et al., 2013). However the two Chilean rudists: Jerjesia and Douvillelia are Caribbean genera, and one of them is a Caribbean species. This similarity suggests that the Caribbean domain was a dispersal centre and probably a centre of origin for both the northern and southern American Pacic margins, as it was for the main part of the Pacic domain (Skelton et al., 2013). The south- ward dispersion of Jerjesia and Douvillelia from the Caribbean to the southern Pacic American margin shows that sea surface circula- tion, conveying larvae, did not conform the trajectory imposed by oceanic gyres owing towards the equator; consequently coastal eddy currents counteracting the gyres, were probably the main agents for dispersal. This dispersal mode contrasts with the island hopping mode invoked for the colonization by rudists of the Pa- cic domain (Skelton et al., 2013).The stratigraphic distribution of rudists on the Pacic side of America conforms their Caribbean and Pacic mode (Skelton et al., 2013): rudists are absent in the late Aptian and disappeared in the late Albian. In Chile, the regional geology shows that a late Aptian active volcanism and a late Albian widespread continental regime, were the main causal factors for rudist demise.The sketch diagram of Fig. 7, modied from Ramos and Aleman (2000), illustrates the palaeogeographic location of rudist-bearing platform carbonates in the island and back-arc settings associated with the Andean subduction, it also acknowledges the peculiar oceanographic context which characterizes the Chilean margin during the mid-Cretaceous. As stated above, the palaeoceanographic

Fig. 6. Douvillelia skeltoni Alencaster and Pantoja-Alor. A, type gure of Douvillelia skeltoni from Huetamo (Mexico), reproduced from Alencaster and Pantoja-Alor (1998), g. 7 (1),p. 24, ventral view of RV showing the longitudinal salient lamellar ridges, i.e. ventral bands, bounded by grooves. Be F, Specimens from Cerro Pajonales (Chile). B, Ventral view of RV showing the ventral bands and interband. C, Transverse section of RV and position of ventral bands and interband (SNGM 1985). D, Close-up of the commissure of the specimen in B, showing the ventral, festooned morphology corresponding to the bands and interband (SNGM 1983). E, Antero-ventral view of RV showing the lamellar habit (SNGM 1986). F, Dorsal side (RV) with position of the ligament groove (SNGM 1984). Scale bar 1 cm.

situation of Chile during the Cretaceous suggests that the Humbolt current may have been present off the Chilean shelf (Hay, 1995), a model supported by climatic simulations (e.g. Fluteau et al., 2007). This model postulates SST lower than 18oc, potentially detrimental

for coastal rudist communities, as they are presently for zoox- antellate coral communities (Montaggioni, 2007). Nevertheless the following observations may contribute to explain the presence of rudist communities.

Fig. 7. Palaeogeographic sketch of the central Andean region showing the association of rudist bearing platform carbonates with volcanic arc settings and their structural and palaeoceanographic context.

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1 e Applying the zooxantellate coral model to rudist commu- nities is misleading. The Chilean shelf is not only characterized by a low SST (