The Valanginian Olcostephaninae Haug , 1910 ( Ammonoidea ) from the Andean Lower Cretaceous Chañarcillo Basin , Northern Chile *

Ammonites of the genus Santafecites Etayo-Serna and subgenus Olcostephanus (Viluceras) AguirreUrreta and Rawson are described for the first time from Chile. The succession of Olcostephaninae from the Chañarcillo Basin of northern Chile is described in the light of new collections and revision of historical material. The occurrence of mixed Andean and Mediterranean faunas supports the correlations proposed with the ammonite scales of the Neuquén Basin and the Mediterranean Province.


Introduction
Olcostephanids are an important component of the Valanginian ammonite faunas of the Chañarcillo Basin (northern Chile), where they occur at two discrete intervals in the lower part of succession.The first one is characterized by the conspicuous occurrence of the cosmopolitan Olcostephanus (Olcostephanus) atherstoni (Sharpe, 1856) whereas the other is a level with Olcostephanus (Viluceras) permolestus (Leanza, 1957).
The first Olcostephaninae species mentioned in the Chilean literature is Olcostephanus (O.) curacoensis (Weaver, 1931) but the ammonites were not described and/or illustrated (Tavera, 1956;Corvalán, 1974;Corvalán in Segerstrom et al., 1963).These authors assigned the Argentinean species to different genera, all of which are now regarded as junior objective synonyms of Olcostephanus Neumayr, 1875.It was reported from several localities of the Punta del Cobre and Abundancia formations south and north of Copiapó.O. (O.) curacoensis is herein considered, in agreement with Riccardi et al. (1971) and Aguirre-Urreta and Rawson (1997), a synonym of O. (O.) atherstoni.
The second level is characterized by a very distinctive fauna of evolute olcostephanids.This was recorded by Corvalán in Segerstrom et al. (1963) and Corvalán (1974) with a new species, 'Holcostephanus copiapoensis nov.sp.', but neither a description nor an illustration was given; this is therefore a nomen nudum according to the International Code of Zoological Nomenclature.Examination of the specimen labeled 'Holcostephanus copiapoensis nov.sp.' in the Corvalán collection by one of us (FAM), allows us to regard this taxon as an invalid synonym of Olcostephanus (Viluceras) permolestus.
The insights issued from this paleontological study permits to discuss and precise the former ammonite biostratigraphic long distance correlations between the European and South Central Andean regions.In this way, the stratigraphic record of the Andean Chañarcillo Basin, represent a link between the Neuquén Basin and Tethys ammonite biostratigraphic record.

Stratigraphy and fossil localities
The oldest sequences recorded until now from the marine Chañarcillo Basin are the Punta del Cobre and Abundancia formations, which are respectively assigned to an Early and Late Valanginian age (Mourgues, 2004;Aguirre-Urreta et al., 2007).These outcrops are located southeast of the city of Copiapó, in the northernmost part of the Chañarcillo Basin (Fig. 1).Nevertheless, (?) Middle to Upper Berriasian ammonites (Malbosiceras gr.malbosi in Aguirre-Urreta et al., 2007, fig. 6A) were collected 80 km north of Copiapó and are currently under study.
The Abundancia Formation ('Capas de Abundancia', Biese in Hoffstetter et al., 1957) is the oldest lithological unit of the Chañarcillo Group (Segerstrom and Parker, 1959).It is characterized by well bedded grey mudstones interbedded with arkoses, which pass vertically and laterally into the Nantoco Formation limestones, and lies on the Punta del Cobre Formation (Segerstrom and Ruiz, 1962;Marschik and Fontboté, 2001, p. 413).This latter unit is composed of a pile of andesitic to basaltic andesitic lava flows, conglomerates, sandstones, tuffs and dacitic dome complexes, including sedimentary layers in its upper levels.The contact between this unit and the overlying Abundancia Formation is transitional, and has been defined by the first occurrence of a continuous bed of massive limestone or its metamorphosed equivalent.The following localities have been investigated.

Cerro La Vinchuca
Discontinuous outcrops of the Chañarcillo Group were observed southwest of Inca del Oro, which were previously investigated by Moraga (1977).Two different levels have yielded ammonites, where two distinctive faunas were identified.From bottom to top these are: Lissonia Gerth, 1925(to which Raimondiceras Spath, 1924 is likely to be a synonym) and O. (O.) atherstoni.The historical collection of SERNAGEOMIN contains a single specimen herein identified as Santafecites santafecinus (d'Orbigny, 1842) from the same locality (Chong collection, SNGM 07) and described below.

Puquios area
The Puquios area is characterized by complex tectonics that affects the whole Mesozoic sequence.It is known in the literature as the 'Caos de Puquios' and was studied by Sepúlveda and Naranjo (1982) and Mpodozis and Allmendinger (1992).These authors describe a series of 200 m of bioclastic and oolithic limestones that yielded ammonites of Late Valanginian to Barremian age.The material is kept in the collection of SERNAGEOMIN and contains a single specimen of Santafecites santafecinus (Sepúlveda collection, SNGM 1536) described below.

Quebrada Meléndez
The Quebrada Meléndez is one of the reference sections for the Lower Cretaceous of the Chañarcillo Basin.The succession crops out in a tributary creek, east of the Copiapó valley (Fig. 2).It shows the most complete series of the Upper Valanginian to Lower Aptian strata.From this locality O. (O.) curacoensis and Lissonia riveroi (Lissón, 1907) were reported (Corvalán, 1974).We have observed that near the top of the Abundancia Formation, an 'Ammonitico rosso' bed contains abundant O. (Viluceras) permolestus associated with numerous neocomitids, such as juvenile N. (Neocomites) peregrinus (Rawson and Kemper, 1978), Neocomites (Sabbaiceras) beaumugnensis (Sayn, 1907) and Rodighieroites cardulus Company, 1987.These neocomitids are under study with associated material from others localities.
The upper fossiliferous level with O. (O.) atherstoni was also identified at Quebrada Nantoco.We collected three specimens from that locality (SNGM 1021 [7-9]), which are associated with a specimen of Bochianites sensu lato, several meters above a bed with Lissonia.

Systematic Paleontology
The material described herein is stored in the Paleontological Collections of the Servicio Nacioal de Geología y Minería (SERNAGEOMIN) Museum, Santiago, with the prefix SNGM.Most of the material belongs to the collection of one of us (F.A.M.).Specimens from the Corvalán, Chong and Sepúlveda collections have also been included in this study.
Dimensions of specimens are indicated as follow: d: diameter; wh: whorl height; wt: whorl thickness; wu: width of umbilicus.M: macroconch; m: microconch.In the synonymy list, v indicates that we have seen the specimen(s).
Diagnosis: Compressed to strongly inflated cadicones, with strongly arched to well rounded venter.Primary ribs usually present on umbilical wall, commonly terminating in tubercles at umbilical shoulder, from which arise straight or slightly curved secondary ribs, usually in fasciculate bundles.Commonly 3-4 secondaries per bundle, although there may be as many as 6-9 or as few as 2. Secondary ribs may bifurcate on the flanks, while intercalated ribs between bundles are the rule.Ribbing always passes uninterrupted across venter; also it may weaken in some species.Parabolae or constrictions may or may not be present, but are absent on the outer whorls of macroconchs.The genus is dimorphic; microconchs are small and bear lappets on the aperture; macroconchs are larger, with a simple peristome (modified after Cooper, 1981, p. 161).Occurrence: Olcostephanus (Olcostephanus) is widely distributed in the Mediterran-Caucasian and Indo-Pacific subrealms of the Tethyan Realm and adjacent areas of the Boreal Realm sensu Westermann (2000).In the western Tethys, the subgenus is known to range from the highest Upper Berriasian (F.boissieri Zone, T. otopeta Subzone) to the high Lower Hauterivian, L. nodosoplicatum Zone, O. (O.) variegatus biohorizon (Bulot, 1990a;Bulot, 1992;Bulot et al., 1993;Bulot and Thieuloy, 1995).In South America, Olcostephanus (Olcostephanus) is known from Colombia, Perú, Chile and Argentina.The oldest occurrence was reported from the early Valanginian of the Neuquén Basin (Aguirre-Urreta and Rawson, 1999) and possibly Colombia (Etayo Serna, written communication 1999).'Middle' Valanginian Olcostephanus of the atherstoni group occur in all countries listed above except Perú (Riedel, 1938;Haas, 1960;Riccardi et al., 1971;this paper).Late early Hauterivian Olcostephanus of the sayni-variegatus (Paquier, 1900) plexus are known from all those countries except Chile (Riedel, 1938;Haas, 1960;Robert et al., 1998;Aguirre-Urreta and Rawson, 2001).

Description: Microconch [m]
: the microconch consists of immature examples of 25-30 mm diameter, with at least some part of the body chamber preserved.Septation ceases at diameters of about 20-25 mm.The shell is slightly involute, cadicone with moderately depressed to semicircular whorls.The umbilical slope is moderately steep with a rounded margin.The umbilicus is moderately deep and narrow where its diameter (wu) is approximately 28% of the shell diameter (d).The venter and flanks are rounded.Maximum width of whorl at umbilical bullae.Rursiradiate primary ribs arise near the umbilical seam, progressively becoming prominent on the umbilical slope, and forming bullae-like tubercles on the upper part of the umbilical margin.There are 9-12 primary ribs per half-whorl.Bundles of three to four (exceptionally 5) secondary ribs associated with each bulla, but an additional one or two may be intercalated between two bundles.There are 40-45 secondary ribs per half-whorl.No constrictions were observed our examples.

Macroconch [M]
: the shell of the macroconch is moderately small with diameters between 70-85 mm.Some specimens are septate to at least 40-50 mm.The body chamber occupies at least ¾ of the last whorl.These forms are more involute than the microconchs, with the umbilicus (wu) reaching approximately a fifth of the shell diameter (d).The umbilical slope is steep with a rounded margin.Ribbing is finer and denser than in microconchs, and slightly rursiradiate.There are 13-15 primary ribs per half-whorl.Bundles of four to six secondary ribs are associated with each bulla, but an additional rib may be intercalated between two bundles.There are 55-70 secondary ribs per half-whorl.Neither constrictions nor parabolae was observed in our material.
On the other hand, some of the South African microconchs figured by Cooper (1981, p. 265, Fig. 115 Bulot (1990aBulot ( , 1992)), first occur in the middle part of the B. campylotoxus Subzone (B.subcampylotoxus biohorizon sensu Bulot and Thieuloy, 1995).The acme of the species ranges from the upper part of the B. campylotoxus Subzone to the uppermost part of the K. biassalense Subzone (B.campylotoxus biohorizon to K. inostranzewi Zone sensu Bulot and Thieuloy, 1995).These forms, always cadicones, lose at the adult stage the sphaerocone morphology of the precursors.It is among those populations that the morphology is closer to the typical O. (O.) atherstoni from South Africa.They also include large numbers of typical O. (O.) guebhardi.
At the top of the K. biassalense Subzone, the populations are affected by a new morphological change that affects mainly the density of ribbing, while the general form of the shell remains unchanged.Bulot (1990aBulot ( , 1992) ) introduced the name querolensis for this morphotype that shows its acme in the S. verrucosum Subzone and disappears at permolestus (Leanza, 1957).
the top of the K. pronecostatum Subzone.Most of the Chilean macroconchs described herein clearly belong to this morphotype.Similarly, Ettachfini (1991) showed that the morphotype succession identified in south-east of France is also represented in the Valanginian of the Essaouira basin (Morocco).In South-eastern Spain, O. (O.) atherstoni is represented only by the morphotype querolensis, which shows the same distribution as in France.The German morphotype hollwedensis Bulot (='Proastieria' sensu Stolley, 1937) is known only from the lower part of the P. hollwedensis Zone of the Lower Saxony and Yorkshire basins of Northern Europe (Kemper et al., 1981).Similar forms also occur in the 'Marnes à Astieria' of the Jura platform in Switzerland (Bulot, 1992).This morphology is by far the closest to the typical O. (O.) atherstoni from South Africa.The time span of the hollwedensis morphotypes merely represents an equivalent of the uppermost B. campylotoxus and lower K. biassalense Subzones of the Mediterranean standard ammonite scale.
Diagnosis: An evolute to serpenticone subgenus of Olcostephanus characteristically with welldeveloped constrictions and flares, particularly in the adult growth stage.Discussion: The relationships between Viluceras, Simbirskites, Lemurostephanus and Olcostephanus have been discussed at length by Aguirre-Urreta and Rawson (1999, p. 343 and 351-352) and we fully agree with those authors on the systematic position of Viluceras as a subgenus of Olcostephanus.As already mentioned above and for the reasons given by Aguirre-Urreta and Rawson (1999, p. 351-352), we also regard Lemurostephanus as a junior subjective synonym of Olcostephanus s. str.
Occurrence: Until the recognition of its occurrence in Chile (Mourgues, 2004), the subgenus was known only from the middle part of the Upper Valanginian of the Neuquén Basin in Argentina (Aguirre-Urreta and Rawson, 1999).The reported occurrence of O. (V.) permolestus from south-east France (Autran, 1993) is erroneous and will be discussed below.Holotype: By original designation, the specimen figured by Leanza (1957, p. 16, Pl. 3, Fig. 1), from El Durazno hill, Central West Argentina.Universidad de Buenos Aires, CPBA 7018.Refigured by Aguirre-Urreta (1993, Pl. 2, Fig. 4) and Aguirre-Urreta and Rawson (1995, Pl. 1, Fig. g;1999, p. 350, Fig. 5a).Material: 19 specimens (table 4 and 5 margin.From each bulla arise associated bundles of two to three (frequently two) secondary ribs, gently rursiradiate on the upper third of the whorl.An additional one may be intercalated between two bundles.Constrictions are frequent.Remarks: Both our macroconchs and microconchs fall in the range of intraspecific variation accepted by Aguirre-Urreta and Rawson (1999) for this species (compare our Figs.8B, E and K with their Figs.5h, i  and n).As in the Argentinean material, microconchs show a significant variation in rib density, degree of inflation, as well as in strength and frequency of constrictions and flares.Outside southern South America, a single specimen was reported from south-east France by Autran (1993) as O. (Lemurotephanus) permolestus.As already noted by Bulot (1990a) and Aguirre-Urreta and Rawson (1999), this record is based on the misidentification of macroconch forms of O. (O.) nicklesi.

Biostratigraphic implications
As in the Neuquén Basin, the genus Olcostephanus occurs at two main distinct levels.The Even if the First Apparition Data (FAD) of Karakaschiceras sensu Bulot and Thieuloy (1995) and Neohoploceras Spath, 1939 occur at the base of the K. biassalense Subzone of the Mediterranean zonal scheme, we herein consider that the Argentinean K. attenuatum Subzone better correlates with the K. pronecostatum Subzone of the Mediterranean zonal scheme.This view is supported by the fact that Neohoploceras arnoldi sensu Aguirre-Urreta (1998) is closely allied, if not identical, to Neohoploceras gr.depereti/schardti sensu Bulot and Thieuloy (1995).
biostratigraphic long distance correlations, for the Valanginian ammonite biozones.
, Fig. 129B) under O. (O.) baini baini, considered herein as a junior subjective synonym of O. (O.) atherstoni, hardly differ from the microconchs of O. (O.) guebhardi.Similarly, the larger macroconchs of O. (O.) guebhardi (morphotypes querolensis and hollwedensis) fall within the adult size of the smaller South African and Argentinean O. (O.) atherstoni.As a consequence, we cannot see any reason to keep O. (O.) guebhardi separate from O. (O.) atherstoni.Because of priority rules, O. (O.) guebhardi is a junior subjective synonym of O. (O.) atherstoni.Nevertheless, it should be noted that in Southeastern France the oldest representatives of O. (O.) atherstoni, O. (O.) guebhardi 'forme primitive' in
lower one is characterized by a monospecific assemblage of O. (O.) atherstoni, while the upper one is marked by the co-occurrence of O. (V.) permolestus with various Neocomitidae such as N. (Neocomites) per-egrinus, Neocomites (Sabbaiceras) beaumugnensis and Rodighieroites cardulus.Both O. (O.) atherstoni and O. (V.) permolestus were retained by Aguirre-Urreta and Rawson (1997, 1999) as index species for zones and subzones of the Valanginian of the Neuquén Basin (Fig. 9).As already pointed out by those authors, the lower and middle part of the O. (O.) atherstoni Zone, O. (O.) atherstoni and Karakaschiceras attenuatum subzones, span the Lower/Upper Valanginian boundary on evidence of ammonites common to Argentina and the Mediterranean Tethys.Until now, O. (O.) atherstoni was considered by one of us (L.G.B.) as the southern hemisphere counterpart of the North Tethyan species O. (O.) guebhardi.As discussed above in the light of the Chilean material, we are now convinced that the two taxa should be considered as synonyms.For the reasons given in our discussion of the intraspecific variability of O. (O.) atherstoni, we therefore consider that the base of the Argentinean O. (O.) atherstoni Zone correlates with the middle part of the B. campylotoxus Zone of the Mediterranean zonal scheme (Fig. 9).