Ordovician Nautiloids of Argentina

Florencio G. ACEÑOLAZA1 and Matilde S. BERESI2

Abstract: ORDOVICIAN NAUTILOIDS OF ARGENTINA. An up to date synthesis of the knowledge of Ordovician nautiloids from Argentina with its biostratigraphical framework is herein presented. Abundant ellesmerocerids within Tremadocian-Llanvirn strata are found in northwestern Argentina (Cordillera Oriental and Sierras Subandinas).

Arenig-Llanvirn material is described from the carbonates of Precordillera where mid to high diversity fauna comprises ellesmeroceratids, orthoceratids and tarphiceratids. The most diverse association is found in Tremadocian beds, with a remarkable depression in Llanvirn and Asghill strata from northwestern Argentina.

Rare Hirnantian elements have been found in Precordillera. Argentine material display, as expected, a widespread distribution.

Resumen: NAUTILOIDEOS ORDOVÍCICOS DE ARGENTINA. Esta contribución constituye una síntesis sobre el conocimiento de los nautiloideos ordovícicos de Argentina, reseñando los principales taxones determinados hasta el presente.

Asi también se presenta el marco estratigráfico de los hallazgos y la región en la que se encuentran. Se comprueba una notable abundancia de ellesmeroceratidos en capas del Tremadociano-Llanvirniano del noroeste de Argentina.

En los estratos Arenigianos-Llanvirnianos de la plataforma carbonática de Precordillera la fauna de nautiloideos es más frecuente y muestra una mayor diversidad siendo comunes los ellesmeroceratidos, orthocerátidos y tarphiceratidos. Se destaca que el registro de estos elementos se inicia a partir del Tremadociano, marcando una progresiva disminución partir del Llanvirniano. Por último se menciona su presencia en menor proporción en capas asghillianas del NOA y en niveles hirnantianos de la Precordillera. Los taxones presentes tienen una amplia distribución global.

Key words: Ordovician. Nautiloids. Argentina.

Palabras clave: Nautiloideos. Ordovícico. Argentina

Introduction

Nautiloids are known from the Ordovician beds of Argentina since the end of the 19th Century.

Kayser (1876) is the first author to describe and illustrate specimens of Lituites? sp. and Orthoceras sp. within fossils that were collected by Alfred Stelzner from limestones of Talacasto, Precordillera of San Juan.

The earliest material has been recognized in rocks of the Cambrian-Ordovician transition at Jachal, San Juan Province (Keller 1999, Cañas 1999). Even though they were not identified, they were localized in the basal and upper sector of La Silla Formation cropping out at La Laja, San Roque, Los Berros, Los Potrerillos, and La Silla localities (Missisquoia zone, sensu Cañas 1999).

Borello and Gareca (1961) also identified some Cyrtoceras sp. within Ordovician outcrops at Los Azules. The same year, Borello (1961) mentions the existence of a cyrtoconic nautiloid (“Westonoceras” sp.) in the Cerro Agua Negra. In addition, a varied fauna was described by Aceñolaza et al. (1977) from the limestones of Huaco, San Juan. The same year, Aceñolaza and Toselli (1977) described the presence of Arenig nautiloids in the volcanoclastic sequence cropping out in the vicinity of Chaschuil (Famatina System, Catamarca province). Kobayashi (1987), summarizes the nautiloid fauna from South American, with special mention to those from the Andean region of Argentina.

Tremadocian nautiloids were also collected in the Cordillera Oriental of northwest Argentina, Jujuy Province and determined as Endoceras by Harrington (1938). The most important collection due to the quantity and variety of specimens was collected and studied by Cecioni (1953, 1965) from Tremadocian and Arenig sequences of Jujuy province. The early mentioned collection opened up the scenario of nautiloid fossils in the Ordovician beds of northwest Argentina.

This paper is a summary with an up to date vision of Ordovician nautiloids from Argentina. We must point out, that descriptions offered herein, are transcriptions from the cited literature, and does not validate the taxonomic categories assigned by early authors. In some cases, invalid names were updated following the criteria of the Treatise on Invertebrate Paleontology (Teichert et al. 1964) and Flower (1964).

Some of the illustrated material is housed in the collections of the Facultad de Ciencias Naturales e Instituto Miguel Lillo under the prefix PIL (INSUGEO-Tucumán University), and the Instituto de Geología de la Facultad de Ciencias Exactas, Físicas y Naturales under PI-UNSJ 626-632 (San Juan University).

Geological and stratigraphical position of fossils

Ordovician nautiloids from different regions of Argentina are listed below, organized in the following way:

1.- Northwest Argentina (Jujuy and Salta provinces)

1.1.- Tremadocian

a) Purmamarca Region – Jujuy. Harrington (1957) named as Chañarcito Limestone a series of Tremadocian strata nearby Puerta Chañarcito, south of Purmamarca train station. Asaphellus catamarcensis is the most distinctive trilobite within the strata. Cecioni (1965) collected and described from limestones of Purmamarca region the following new species, whose taxonomic validity must be revisited:

Purmamarcoceras kobayashii Cecioni: Endogastric cyrtocone shell slightly curved, depressed, soft surface. Short chambers that define transversals sutures. Siphuncle circular reaching ventral wall. It has short septal necks, orthochoaniticals and thick rings.

Clarkoceras argentinum (Cecioni): Small orthoceracone laterally compressed. Marginal siphuncle, elliptic and laterally compressed. The septal necks have a length 2/3 of the height of the chamber.

Connecting rings display double stripes.

Protocyptendocera fuenzalidae Cecioni: Orthocone depressed in dorsoventral section. Straight sutures, circular siphuncle in a ventral position. Long septal necks, ¾ the length of the chamber and thick connecting rings in double stripes.

Protocyptendoceras corvalani Cecioni: Orthocone depressed with a marked elliptical section. Circular siphuncle in contact with ventral wall. It is different to P. fuenzalidae due to greater height of chambers.

Figure 1.- Localities with ordovician nautiloids

Robsonoceras compressum Cecioni: Orthocone of elliptical section, laterally compressed with straight transversal sutures. Marginal siphuncle with elliptical section.

Cyptendoceras (?) floweri Cecioni: Orthocone of subcircular section with variable diameter, slightly dorsoventral depressed. Suture surrounds ventral lobe, displaying short chambers and wide siphuncle.

Cyclostomiceras depressius Cecioni: Brevicone orthoconic, with elliptical section, dorsoventral depressed. Straight sutures with soft ventral saddles. Ventral siphon in contact with wall; short septal necks.

b) Tilcara – Jujuy. Specimen collected from a limestones of Tremadocian age near Tilcara (Cecioni 1965).

Protocyptendocera teicherti Cecioni: Orthocone deppressed with elliptical section. Wide siphuncle, depressed, of elliptical section measuring ¾ height of chamber. It is different by a larger number of chambers, and well defined ventral saddle.

1.2- Arenig- Llanvirn

a) Rio de las Capillas. Zapla Range – Jujuy. Cecioni (1953) described material from the outcrops on the western slope of Zapla Range. Nautiloids come from dark limestones with Hoekaspis schlagintweiti (Trilobita- Arenig/Llanvirn).

Cochlioceras paucistriatum (Cecioni): Orthocone whose shell has lirae figures inclined with an apical form. Elliptic section with a dorsal and ventral depression. Marginal large sized siphuncle, sub elliptical and depressed in outline. Short necks bended on a straight angle. Thick connecting rings with an increasing ratio of 11/100.

Cochlioceras paucistriatum (Cecioni): Orthocone with a elliptic to sub-triangular depressed section.

The shell surface has lirae. Wide marginal siphuncle with thick rings. Increasing ratio of 5/100.

Cochlioceras multistriatum (Cecioni): Ornamented Orthocone with slightly inclined lirae. Circular siphuncle in contact with shell. It differs from the previous ones on the increasing ratio (less than 2- 3/100).

Protocycloceras harringtoni Cecioni: Orthocone with elliptic section and a dorsoventral depression.

It has one ring every two chambers and straight and transversal sutures.

Protocycloceras bonarellii Cecioni: Orthocone of circular section with rings slightly adoral inclined.

Smooth suture on dorsal lobes. Circular siphon with short septal necks and thick connecting rings.

Its main difference is the presence of clearly defined dorsal lobes.

Protocycloceras spp. Three specimen are mentioned, but they were not assigned scientific names due to bad preservation.

Paracyclostomiceras floweri Cecioni: Laterally compressed orthocone. Length about 32 mm and diameter 14 mm. Smooth shell defining weak dorsal and ventral lobes. Small ventral siphon and short septal necks.

b) Arroyo Garrapatal. Zapla Range – Jujuy. In this locality Cecioni (1953, 1965) described the following species from limestones cropping out on the eastern slope of Zapla Range. Hoekaspis schlagintweiti (Trilobita) is the most remarkable associated fossil.

Belloceras milleri Cecioni: Orthocone bearing a strong elliptic section withseveral transversal ridges.

Large sized siphuncle, marginally placed and in contact with the ventral wall. Orthochoanitic septal necks are developed. Septa are inclined, with thick connecting rings and an increasing ratio of 12/100.

Paracyclostomiceras depressum Cecioni: Dorsoventral depressed orthocone. Sutures weakly developed with dorsal and ventral lobes. Siphon circular and very small, in contact with the ventral wall.

Protocycloceras stefaninii Cecioni: Orthocone rings with a depressed elliptical transversal section.

The rings are regularly separated, adapical inclined, one every three chambers. Straight transversal suture cutting the rings in ventral position. The siphon large sized, circular and with thick septal necks.

c) Arroyo San Lorenzo. Ledesma – Jujuy. The following specie was found in a limestone cropping out at San Lorenzo river, near Ledesma city, Jujuy province.

Desioceras floweri Cecioni: Thin orthocone with circular section. Shell covered by lirae. Rings are weekly defined and regularly spaced. Sutures show rounded ventral saddles. Siphon marginal and with a sub circular outline. Aneuchoanitic lacking structure in connecting rings.

d) Espinazo del Diablo – Jujuy. Aceñolaza (1966), described in the Sepulturas Formation outcropping by the western slope of El Espinazo del Diablo, near Cajas, a sequence of sandstones and shales with abundant nautiloids similar to the material previously described by Cecioni (1953).

Recognized taxa: Protocycloceras stefaninii, Protocycloceras sp., Paracyclostomiceras floweri Cecioni and Cyclostomiceras depresius, Cochlioceras sp. (Lám.1 fig.A ).

e) Tafna – Jujuy. Loss (1948) pointed out the existence of nautiloid shells within Arenig strata near Tafna (west of La Quiaca, Jujuy) that he assigned to the Endoceratide. Arenig graptolites are accompanying fauna.

f) Quebrada de Incamayo – Salta. Different fragments of orthoconics nautiloids were found and assigned to Cameroceras sp.( Pl. 1, fig. B )

1.3.- Caradoc/Asghill

a) Zapla / Puesto Viejo Ranges – Jujuy. Cecioni (1953) mentioned abundant undescribed nautiloids from outcrops of Arroyo Moralito, eastern flank of Zapla Range, Jujuy. The presence of ? Dawsonoceras sp ( Pl.1, fig. F) has been recognized in the eastern slope Sierra de Puesto Viejo by one of the authors (FGA). This material is a orthocone with circular section and conspicuous rings and lirae ornamentation on the suface of shell.

2.- Catamarca Province.

a) Chaschuil: Aceñolaza and Toselli (1977) recognized nautiloids within fossils of the Suri Formation at Chaschuil (Catamarca Province, Famatina System). An interesting environment has been interpreted for this unit with vulcanism associated to sedimentation (Arenig-Llanvirn). These authors pointed out the scarce presence of orthoconical forms.

Proterocameroceras tolai ( Pl. 1, fig. E ) : Orthoconical shell of circular to sub-circular section, slightly flattened in a dorsal-ventral way. Marginal siphuncle covering 1/3 of shell diameter. Septum loxochoanitic. Sutures simples, defining regularly spaced chambers. Juvenile spacimens display epi and hiposeptales chamber deposit.

Proterocameroceras sp.: Fragments of a orthoconical shell compressed and with oval section. The increasing ratio of the shell from the apex to the opening is approximately 1.5 mm every 10 mm.

Slight ornamented and inclined shell can be recognized. No details of internal structures could be observed.

3.- San Juan Province.

During the Cambrian and Early Ordovician the Precordillera of western Argentina was a carbonate shelf deeping eastwards with a clastic basin westwards. Thick limestone sequences characterise early Ordovician strata in the region (Arenig - Lower Llanvirn). Limestones of San Juan Formation display remarkable outcrops interpreted to be deposited on a warm platform developed in the Eastern and Central belts of the Cuyo Precordillera (sensu González Bonorino and González Bonorino 1991).

Description of Ordovician nautiloids from the Precordillera are scarce, although there are many references to the presence of these fossils in the literature.

The older nautiloids had been recorded in limestones of La Silla Formation (Tremadocian) cropping out in diverse localities as Jáchal, San Juan Province (Gamboa 1985, Keller 1999).

The San Juan Formation (Arenig-Lower Llanvirn) has yielded a large collection of nautiloids from diverse localities of central and eastern Precordillera. Nautiloids are abundant in the fossiliferous limestones of the middle part and the top of San Juan Formation. In terms of abundance, the fauna of San Juan Formation is dominated by numerous specimens of the ellesmeroceratids, endoceratids and orthocerids. These records significantly increase the diversity of the group during the Lower and Middle Ordovician in the Precordillera.

Nautiloids that were not described and figured are presented herein, from diverse localities of the central and eastern Precordillera of San Juan Province: Quebrada Las Lajas, Sierra Chica de Zonda (Bordonaro et al. 1987), Loma de los Piojos (Jáchal), Niquivil, Tambolar, Gualilán and Talacasto (Beresi 1986), Quebrada Gustavo, Sierra de Villicúm (Peralta and Beresi 1999), Las Aguaditas section, Sierra de La Trampa (Cabaleri 1990). Nautiloids are mentioned in Sierra Las Higueras, Salagasta area, northern sector of the Precordillera of Mendoza (Beresi et al. 1998) and Ponón Trehué Formation in San Rafael block, south Mendoza province (Keller 1999, Heredia verbal communication).

Some nautiloids have also been found above the units overlying San Juan Formation, in the black shales of Gualcamayo and Los Azules formations.

Lituites sp. and orthoceratids have been also mentioned from the Lower Llanvirn facies described by Baldis and Beresi (1981) in the Sierra de Villicum (Baldis et al. 1984) Las Aguaditas section and Sierra La Trampa (Cabaleri 1985). Several orthoconic longicones are also present within black shales of Gualcamayo and Los Azules formations. Above the latter, in the clastic successions of Upper Ordovician age (Las Plantas and Trapiche formations) nautiloid fauna display a significant decrease.

A clear diversity decrease is recorded during the uppermost Ashgill (Hirnantian), interpreted to be caused by the known glacial event. Few rare and scarce orthoconic longicones are associated to the Hirnantia fauna within the strata outcropping in the Villicúm Range (Sánchez et al. 1991).

a) Huaco

The sequence of San Juan Formation at Cuesta de Huaco, is a classical section with nautiloids as follows (Aceñolaza et al.1977):

Barnesoceras stelzneri (Pl 1, Fig. P, Pl. 2. , Fig. A ): Endogastric cyirtocone, subcircular in transverse section to depressed with strongly adoral expansion. Dorsal Siphuncle, marginal, normallly 1/3 of shell diameter. Septal necks loxochoanítiques and short suture with moderated lateral lobes.

cf. Oelandoceras sp.: ( Pl.1, figs. J,M,O) Cyrtocone longiconique, endogastric, elliptic-depressed in transversal section. Suture with a ventral saddle and smoth lateral lobe. Maximun width of chambers is 8 mm.

Robsonoceras sp.: (Pl. 1,fig. C) Orthocone with a moderated adoral expansion. In transverse

Plate 1.- Some Ordovician nautiloids of Argentina A.-Cochlioceras sp. Espinazo del Diablo, Jujuy (x0,4). B.-Cameroceras sp. x : 0,5 Incamayo. Salta. C.- Robsonoceras sp. x 0.3 Espinazo del Diablo, Jujuy; D.- Protocycloceras sp.. x 0.5 Huaco, San Juan; E.- Protocameroceras tolai. x 0.7 Chaschuil, Catamarca.; Sierra de Famatina ; F. Dawsonoceras sp , x 0.5. Sierra Puesto Viejo, Jujuy.; G: Angelinoceras sp. x 0,25 Huaco. San Juan.. H.- Curtoceras kayseri. x 0,25 Huaco, San Juan.; I.- Clinoceratidae. Indet. x 2. Sierra de Villicúm. PI-UNSJ 631; J,M, O: Oelandoceras sp. x 0,3 Huaco, San Juan.; K: Piloceras sp. x 0,5 Huaco, San Juan.. L.- Orthoceras sp. x: 0,3 Huaco, San Juan; N.and P. Barnesoceras stelzneri. X 0,5 Huaco, San Juan..

section subcircular. Submarginal siphuncle with 1/3 ratio of fragmacone. Ortocuanitic septal neck.

Surface with thin transverse costae ornamentation.

Protocycloceras sp.: ( Pl.1 , fig. D): Annulate orthocone of transverse circular to subelliptic section.

Maximun diameter is 24 mm in oral position. Shell with 3 mm rings and thick chambers up to 5-7 mm.

Cyptendoceras sp: Circular to subcircular depressed orthocone. Hemichoanítics septal neck with circular to subcircular siphuncle, Diameter is 4 mm. Chambers of variable diameter between 2 to 3 mm.

Piloceras sp.: ( Pl. 1, fig. K) Transversally subelliptical to circular sectioned endogastric cyrtocone.

Shell is ornamented with transversal costae. Oral separation is about 7 mm getting smaller adapicallyto reach 4 mm. cf. Anaspyroceras sp.: Transverse circular orthocone. Ventral siphuncle with chambers that delimitate a straigth suture of a thickness up to 1mm.

Curtoceras kayseri Aceñolaza et al. ( Pl. 1. fig H; Pl.2 fig. B ) Gradually expanded tarphyceracone, whorl section subcircular to sligtly depessed. Dorsal siphuncle with orthochoanitic septal necks.

Suture simple with a ventral and a dorsal saddle with wide lateral lobe. Fragmocone reaches a width of 110 mm.

cf. Angelinoceras sp.( Pl. 1 fig. G ): Gyroceracone strongly expanded to oral position; subcircular in transverse section. Suture simple. Possibly juvenil fragment of genus.

b) Talacasto

This is a classical area were Kayser (1876) described few nautiloids as Lituites sp. and two forms of Orthoceras sp. In addition, Borello (1961) described from the western slope of Sierra de Agua Negra (eg. Westonoceras sp., exogastric cyrtocone, compressed, with ventral siphuncle strongly recumbent septal necks. Simple suture, transversally striated shell. In same locality Beresi (1986) identified Barnesoceras stelzneri (possibly Lituites sensu Kayser), Curtoceras kayseri and Angelinoceras sp.

Plate 2. Sections demonstratives of internal characteristic of some Ordovician nautiloids of San Juan Formation, in San Juan Province (Precordillera): A.- Curtoceras kayseri. Section showing cameras and siphuncle position. x 0,5 Huaco. B.- Barnesoceras stelzneri Longitudinal section showing dorsal position of the siphuncle (crystalized calcite) x 0,5. Huaco. C.-Subfamily Orthoceratinae gen. et sp. indet. Section of phragmacone showing position of siphuncle. Internal mold of phragmacone,x 2. Sierra de Villicúm PI-UNSJ 626 .D.-Subfamily Pseudorthoceratinae gen et sp indet. Tangential section through adapical portion of phragmacone cameral deposits, x 2. Sierra de Villicum . PI-UNSJ 627 .-E Poligramoceras sp. Dorsal and apical view .of specimen Vi. 42, exposing siphuncle segments and adapical development of cameral deposits, x 2., Sierra de Villicum . PI-UNSJ 628. F.G: Subfamily Pseudorthoceratinae gen et sp indet . F. Ventral section of more adapical portion of a phragmacone, X 1. G. Magnified view illustring siphuncle segments, x 4. Villicum Range. PI-UNSJ 629 H: Subfamily Michelinoceratinae gen et sp indet. Ventral section of adapical portion showing siphuncle segments, X 2., Sierra de Villicum. PIUNSJ 630. I: Family Orthoceridae gen et sp indet. Ventral section of a fragmentary specimen exposing cameral deposits. Adapical portion showing cameral deposits, x 2, Sierra de Villicum PI-UNSJ 632. J: Family Endoceratidae gen et sp indet Ventral view exposing siphuncle segments and cameral deposits, x 3, Sierra de Villicum Range. PI-UNSJ 638. K.-Family Endoceratidae gen et sp indet. Dorsal-ventral view exposing siphuncle segments and cameral deposits, x 1, Sierra de Villicum. PI-UNSJ 640.- L: Cochlioceras sp. Dorsal ventral sections through a nearly complete phragmacone, x 1.5. Sierra de Villicum. PI-UNSJ 636. M, N: Vaginoceras sp. M. Dorsal-ventral sections through a nearly complete phragmacone exposing siphuncle, venter to right, x 1. N. Magnified view showing siphuncle segments, x 2., Sierra de Villicum. PI-UNSJ 634. O: Family Baltoceratidae gen et sp indet Naturally

weathered section of a fragmentary specimen exposing siphuncular rod, x 2.5. Sierra de Villicúm PI-UNSJ 639. P: Family Endoceratidae gen et sp indet. Dorsal-ventral view exposing wide siphuncle venter to right, x 2., Sierra de Villicum PI-UNSJ 635. Q: Family Orthoceridae gen et sp indet. Dorsal-ventral section exposing siphuncle and cameral deposits, venter to left, x 4. San Juan Formation, Sierra de Villicum PI-UNSJ 633. R: Proterovaginoceras sp. Dorsal-ventral section revealing siphuncle and cameral deposits, x 1.5., Sierra de Villicum . PI-UNSJ 637. S: Family Proterocameroceratidae gen et sp indet. Ventral view exposing siphuncle segments (holochoanitic septal neck), x 1.5. Sierra de Villicum. PI-UNSJ 641.

c) Loma de los Piojos.

Small sized ortoconids orthocerid and ellesmerocerid were recognized in the outcrops of the Loma de los Piojos, SW Jáchal, San Juan. Specimens belong to Cyptendoceras sp. and Curtoceras kayseri. (Beresi, 1986).

d) Los Azules.

Borello and Gareca (1951) recognized in the Central Precordillera of San Juan the presence of the genus Cyptendoceras sp.

e) Sierra deVillicum

In the eastern slope of Sierra de Villicum the Lower Ordovician (Arenig to Lower Llanvirn) thick carbonate sequence bears several forms as follows ( Beresi, 1996; Peralta and Beresi 1999):

Association I: Lower-Middle Member of San Juan Formation. Fauna occurs in a thick fine-grained dense micrite being characterized by a low-diversity of large endocerid (Vaginoceras sp.) and few smooth orthoconic ellesmerocerids. Accompanying fauna is dominated by gastropods and bryozoos.

Vaginoceras sp.: Orthoconic longicone, gradually expanded (apical angle 3-4 degrees), circular in cross section (maximum observed shell diameter: 24,8 x 23 mm) with straight transverse sutures.

Ventral siphuncle with endosiphuncular tube; septal neck holochoanitic with connecting rings thick. Cameral length: 5-7 mm, approximately 1/4 diameter of shell.

Association II: Upper- Middle Member of San Juan Formation. The fauna is found in skeletal wackestones characterized by an abundant and diverse small longicone baltoceratids (Cochlioceras sp.). Large endocerids (Dideroceras sp., Cyptendoceras sp.), a variety of small to medium gradually expanding longicone orthoceratids (Polygrammoceras sp. and ?Anaspyrocera sp.), geisonoceratid orthoceratids, some small fusiform specimens and a few coiled nautiloids. The nautiloid taxa represent about 65% of the entire fauna and are associated with articulated brachiopods, bathyurid trilobites, pelecipods, sponges and fragment of crinoids. Among mentioned species are: ?Anaspyroceras sp.: Annulate orthocone with compressed circular section (maximum observed shell diameter: 13,3 mm) and simple transverse sutures. Central siphuncle, septal necks short orthocoanitic. Connecting rings thin and cylindrical. Mural cameral deposit.

Cochlioceras sp: Gradually expanded orthoconic longicone (apical angle 4º) with compressed circular section (dorsoventral diameter 20 mm x 14 mm). Wide ventral siphuncle (1/3 diameter of the shell). Moderate cameral length (4.5mm).

Polygrammoceras sp.: Orthoconic longicone with straight transverse sutures and circular cross section (maximum observed shell diameter: 14.4 x 13 mm); with empty, subcentral orthochoanitic siphuncle and expanded segments within camerae. Cameral chamber short. Mural and episeptal cameral deposits developed adapically.

Proterovaginoceras (=Dideroceras) sp.: Orthoconic longicone with subcircular section, (maximum observed shell diameter: 19 x 17 mm). Straight sutures and expanded angle up to 4º. Ventral siphuncle (1/3 diameter of the shell) with endosiphuncular tube, septal necks macrochoanitic.

Association III: The Upper member of San Juan Formation. Limestone with conodonts of E.

suecicus Zone (Sarmiento 1985, 1991), representing an age slightly above the beginning of Llanvirn. The assemblage III is dominated by a diverse and abundant nautiloid fauna predominantly composed of longiconic orthocones (Endocerids, orthoceratids and ellesmerocerids) of middle and small shell length. Scarce breviconic orthocones and cyrtocones are also present.

Numerous K-bentonite layers occur in the upper part of the section, which is characterized by argillaceous mudstone interbeded with thin yellowish clays (K-bentonitic layers) associated to a decreasing of the fossil fauna. Marlstone and grainstones with iron oxide are also associated in the sequence, where grainstones occurs with common nautiloid accumulation (shell pavement) (Astini 1997). These shell beds are composed of large longiconic orthoconids with low diversified nautiloid fauna showing preferential alignments. This mechanical assemblage shows transportation and reorientation due to a stable hydrodynamic condition and can be classified as a sedimentological fossil concentration.

Nautiloid fauna is exclusively comprised by groups with concentrations of large endocerids (?Cameroceras sp.) with large-diameter tubular siphuncles, nearly complete shells are parallel to the bedding planes, with phragmacones of a maximum observed shell diameter of 9 cm. An estimated 60 cm length has been recorded also. The nautiloids are about 75 % of all fauna within this grainstone levels. Among the early mentioned material are:

? Cameroceras sp.: Orthoconic longicone with circular to slightly depressed cross section. Straight, transverse sutures. Cameral chambers short. Tubular large siphuncle, diameter up to 60 percent.

Association IV: Lower Member of Gualcamayo Formation. This member consists of an alternation of black calcareous layers (limestones with cephalopods) with dark non-calcareous pelites and shales.

Carbonate deposition (Paraglossograptus tentaculatus Zone) is characterized in fossiliferous wackestonespackstones that grades to fossiliferous mudstones and marlstones. The low diversity nautiloid fauna is characterized by small slender longiconic orthocerids (?Michelinoceras), which is accompanied by nileid trilobites, few brachiopods and scarce sponges.

? Michelinoceras sp.: Medium sized slender orthocone. Circular cross section and long camerae.

Tubular siphuncle. Septal necks straight with connecting rings cilindrical and homogeneous.

5.- Mendoza province

Orthoconic longicone nautiloids from the top of San Juan Formation (Arenig), have been mentioned from Sierra Las Higueras , Salagasta area in northern Mendoza (Beresi et al. 1997).

Moreover nautiloids without taxonomic identification, has been cited in the carbonate sequence of Ponón Trehué Formation (Llanvirn) San Rafael Block, southern Mendoza (Keller 1999).

Biostratigraphical and paleoenvironmental considerations

The nautiloids of Argentina are recorded since the Lower Ordovician. These are particularly abundant in the Arenig and Llanvirn, becoming scarce towards the upper part of the System.

Representatives of the ellesmerocerids have an important abundance in outcrops of northwestern Argentina, representing 95% of the specimens. Endocerids follow in order of importance, even if we do not consider that Flower (1964a, b) and Hook and Flower (1977) postulated that the latter are an integral part of the first mentioned group. Cyrtoconical or gyroconical forms have not been found yet in outcrops of Jujuy province. In the Precordillera there is a more heterogeneous content, with representatives from the orders which include orthocerids and tarphycerids.

The set of nautiloids from Argentina is related to genera of global distribution. Many of them have been specifically determined, even though its validity may deserve a deeper analysis. Up to the present, we do not have a definite opinion about it.

In a general sense, nautiloids can be identified with Canadian genera (Laurentia), although some material is similar to that of the Baltic area and Australia.

The nautiloid fauna of the Argentine Precordillera is recovered from three distinct environments:

a) open-shelf subtidal limestones biofacies with thin-to medium-bedded burrowed skeletal wackestones and packstones of an early Arenig age (Oepikodus evae conodont zone). b) transitional facies from carbonate sedimentation (platy-ribbon limestones) with graptolitic black shales overlying San Juan Formation associated to a widespread hardground of Lower Llanvirn age (Eoplacognathus suecicus conodont zone) and c) marlstones and black shales of a Lower Llanvirn age within the lower member of Gualcamayo Formation.

The ellesmeroceroids and endoceroids nautiloids with a variety of orthocean, strophomenid, clitambonitacean brachiopods, bathyurid trilobites, pelecypods, sponges and algae, flourished in the outer carbonate platform biofacies related to a warm-water paleonvironment.

The brevicone, coiled and orthocerid nautiloids with accompanying fauna of brachiopods and nileid trilobites are found in the transicional biofacies.

The nautiloid association of San Juan Formation is associated to a subtidal water carbonate facies deposited in a warm open platform. Sometime during or after the late Arenig, the flooding of platforms limited the carbonate sedimentation and contributed to the diminishing nautiloid faunae together with deposition of black shale facies during the early Llanvirn (Association IV.- Beresi and Heredia, 1996).

Fossiliferous association of the upper levels with grainstones could have been originated by several taphonomic factors related to a major transgression and regional volcanic events during the Upper Arenig-Lower Llanvirn (Carrera and Astini, 1998). This type of conditions generated stressed environments that had strong impact on faunal composition, diversity and paleocommunity structure as it happened in other regions of the world (eg. Huff et al., 1992) Nautiloid generic diversity coincide with the rise and fall of the sea level (Crick ,1993). According to this author, from the Arenig up to the Ludlow the ratio of endemic and non-endemic fauna is reasonably constant and follow a same general pattern with sea level. The pronounced decrease in diversity recorded in the Late Ordovician was apparently a response to the short Hirnantian glaciation with high atmospheric CO2 concentration (Poussart et al., 1999).

According to the early referred data Crick (1993), concluded that nautiloids were sensitive to three biogeographic barriers: 1) water depth related to implosion limits of shell 2) distance between adjacent shallow water shelves associated to reproductive style and 3) cooler waters outside subtropical and tropical latitudes modified by shallow water epeiric seas. The patterns strongly suggest that nautiloid environments were optimal in lower latitude, warm, shallow water environments as for the Precordillera.

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Recibido: 5 de Agosto de 2002

Aceptado: 7 de Octubre de 2002