Introduction

The Ordovician sponges from the ArgentinePrecordillera have been described in several contri-butions (see Carrera, 2003; 2007; Carrera and Rigby,1999 for a review). Most of these publications fo-cused on taxonomy, paleoecology and paleogeogra-phy of the sponge fauna collected from Lower and

Middle Ordovician limestones. Orchoclad demo-sponges are the main components of these spongeassociations, which inhabited a former carbonateplatform.

The sponge described in this contribution camefrom a Middle-Upper Ordovician siliciclastic unitthat represents a deep basin depositional setting.This unit overlies a thick, Cambrian and Lower Or-dovician carbonate succession.

Sponges recovered from the siliciclastic units inthe Ordovician of Argentina are relatively scarce,with only a few published descriptions. Carrera(1998) described the hexactinellid sponge Larispongiafrom a Lower Ordovician unit of the Puna region. In

AMEGHINIANA (Rev. Asoc. Paleontol. Argent.) - 46 (3): 449-459. Buenos Aires, 30-09-2009 ISSN 0002-7014

©Asociación Paleontológica Argentina AMGHB2-0002-7014/09$00.00+.50

1CIPAL-CICTERRA-CONICET, Facultad de Ciencias Exactas,Físicas y Naturales, Universidad Nacional de Córdoba. mcarrera@efn.uncor.edu2Museo de Paleontología, Facultad de Ciencias Exactas, Físicas yNaturales, Universidad Nacional de Córdoba.

The hexactinellid sponge Cyathophycus from the UpperOrdovician of the Argentine Precordillera

Marcelo G. CARRERA1 and Gladys ORTEGA2

Key words. Porifera. Hexactinellida. Ordovician. Argentina.

Palabras clave. Porífera. Hexactinélida. Ordovícico, Argentina.

Abstract. A nearly complete specimen of a hexactinellid sponge has been recovered from LowerOrdovician siliciclastic rocks of the Argentine Precordillera. The material was collected in the upper mem-ber of the Los Azules Formation (Sandbian). A new species of the genus Cyathophycus Walcott, C. extendis,is erected. The taxonomic position of the genus is still controversial. Their recent assignment to the fami-ly Dictyospongiidae is discussed; Cyathophycus has been considered as a transitional form between proto-spongiids with simple regular skeletons and dictyospongiids with more complex skeletons. Previousstudies have stated that hexactinellid evolution through the Cambrian-Ordovician interval, like othersponge groups at this time, appears to have been based on strengthening the skeleton, and the occupationof more turbulent, shallow-water environments, while remaining dominant in deep-water settings.Cyathophycus is a typical representative of this transitional stage in which the skeleton is simple, but dou-ble-layered, with bundled spicule rays, and not as rigid as the typical dictyosponge skeletons. Lower andMiddle Ordovician records are from black shale deposits, while younger occurrences show slightly dif-ferent environmental settings (mudstone, shaly mudstone and dolomites), which indicate shallower butstill quiet water deposits.Resumen. LA ESPONJA HEXACTINÉLIDA CYATHOPHYCUS EN EL ORDOVÍCICO SUPERIOR DE LA PRECORDILLERA AR-GENTINA. Se describe e ilustra un ejemplar casi completo de una esponja hexactinélida proveniente de ro-cas siliciclásticas ordovícicas de la Precordillera Argentina. El material fue colectado en el miembro supe-rior de la Formación Los Azules (Sandbiano). Se asigna el material a una nueva especie del géneroCyathophycus Walcott, C. extendis. La posición taxonómica del género Cyathophycus es aún controvertida,su asignación a la Familia Dictyospongiidae es discutida. Cyathophycus es considerado como una formatransicional entre las protosponjas con esqueletos simples y las dictiosponjas con esqueletos cuadricula-dos más complejos. Estudios previos muestran que las hexactinélidas en el intervalo Cambro - Ordovícico,como otros grupos de esponjas, refuerzan su estructura espicular para ocupar ambientes más turbulentos,aunque se mantienen dominantes en sectores más profundos. Cyathophycus es un típico representante deesta etapa transicional en la que los esqueletos son simples pero con dos paredes, y aún no lo suficiente-mente rígidos como los esqueletos de las dictiosponjas. Los registros del Ordovícico Inferior y Medioprovienen de depósitos de pelitas negras mientras que registros algo más jóvenes se encuentran en calizasfangosas y dolomitas, más someros, pero aún de aguas tranquilas.

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the Lower Ordovician of the Famatina region,Esteban and Rigby (1998) described the cosmopolitanhexactinellid genus Protospongia and Beresi et al.(2006) described the hexactinellid sponge Diagoniellafrom Cambrian - Ordovician rocks of the CordilleraOriental and the Subandean Ranges of Salta Pro-vince, Northwestern Argentina.

The records of isolated hexactinellid spicules arealso limited, but they have been mentioned from theCambrian limestones of the Precordillera (Beresi andRigby, 1994), and from Ordovician limestones andshales from the same basin (Gnoli and Serpagli, 1980;Mehl and Lehnert 1997; Beresi and Heredia, 2000;and Carrera, 2001).

The specimens collected by the coauthor in theMiddle to Upper Ordovician shales of thePrecordillera Basin, include a fairly complete spongebody to complement isolated hexactinellid spiculesfrom the same location (Carrera, 2001).

The aim of this contribution is the taxonomic de-scription of the new material and a discussion of theevolutionary significance of the geographic and envi-ronmental distribution of Cyathophycus.

Geological setting and stratigraphy

The Argentinean Precordillera consists of a thicksedimentary succession of Cambro-Ordovician rocksshowing a transition from a nearshore carbonatebank through mixed carbonate-siliciclastic slope de-posits to basinal clastics.

The Lower Ordovician rocks consist ofTremadocian to Darriwilian platform carbonates (LaSilla and San Juan Formations; Keller et al., 1994;Cañas, 1999). Carbonate sedimentation mostlyceased during the Darriwilian due to regionaldrowning, and the deposition of graptolitic blackshales and mudstones (Los Azules and GualcamayoFormations).

The Middle-Upper Ordovician Los AzulesFormation (Harrington in Harrington and Leanza,1957), a dominantly shaly unit, paraconformablyoverlies the San Juan Limestone (Lower-MiddleOrdovician), and is unconformably overlain by theCarboniferous Guandacol Formation. The type local-ity for the study of this formation is the Los Azulescreek, which is situated on the western flank of theCerro Viejo of Huaco, Central Precordillera, San JuanProvince (latitude 30º 11'-30º 15' S, longitude 68º 34'-68º 35' W) (Borrello and Gareca, 1951; Cuerda andFurque, 1975; Furque, 1979). The formation is divid-ed into three members with variable thickness in dif-ferent sections (Ortega, 1987). A composite strati-graphic column of this unit is ca. 318 m in maximumthickness. The lower member (5-10 m thick) is made

up of dark argillites, siltstones and k-bentonite strata.The presence of graptolites that represent the Undu-lograptus dentatus and Holmograptus lentus zones, andassociated conodonts of the Lenodus variabilis Zone,indicates an early Darriwilian age for this member(Brussa et al., 2003; Ortega and Rickards, 2003; Ortegaet al., 2007). The middle member (220 m thick) com-prises grey siltstones with subordinate sandstones atits base, and yields graptolites of the Pterograptus ele-gans and Hustedograptus teretiusculus zones (Ortega,1987; 1995) and conodonts of the Eoplacognathus sue-cicus and Pygodus serra zones (Hünicken and Ortega,1987; Ottone et al., 1999), indicating a late Darriwilianage. The upper member (88 m thick) is composed bygray calcareous shales with mudstones at the baseand contains coquinas in its upper part. Graptolitesof the Climacograptus bicornis Zone and conodonts ofthe Amorphognathus tvaerensis Zone (Baltoniodus vari-abilis Subzone) (Ortega, 1987; Ortega and Albanesi,1998; Ottone et al., 1999) allow an assignment of theselevels to the upper Sandbian. The presence of a strati-graphic gap between the middle and upper mem-bers, which suppressed the upper part of the H.teretiusculus Zone and the entire Nemagraptus gracilisZone, was noted by Ortega (1987).

The sponge described here was collected in theupper member of the Los Azules Formation at theAmarilla Creek (figure 1). The material is associatedwith graptolites, the trilobites Guandacolitus furqueiHarrington and Leanza and Triarthrus jachalensis(Harrington and Leanza), and a palynological assem-blage referable to LAU (Ottone et al., 1999; 2001). Thegraptolite fauna of the Climacograptus bicornis Zone iscomposed of Cryptograptus tricornis (Carruthers),Glossograptus ciliatus Emmons, Dicranograptus cf. D.spinifer Lapworth, Dicellograptus divaricatus (Hall),Dicellograptus spp., Nemagraptus gracilis (Hall),Climacograptus bicornis (Hall) and Orthograptus sp.(Ortega and Albanesi, 1998; Kaufmann, 2008).

Systematic paleontology

Class HEXACTINELLIDA Schmidt, 1870Order RETICULOSA Reid, 1958

Family DICTYOSPONGIIDAE ? Hall, 1884

Remarks. Cyathophycus was initially placed in theDictyospongiidae by Hall (1884), and later in theProtospongiidae by De Laubenfels (1955). Finks(1983) placed the genus in the Hintzespongiidaebased on the presence of two spicule layers in thebody wall, an interpretation followed by Rigby(1995) and Rigby and Mehl (1994). However, Botting(2004) demonstrated that the nature of the second in-ternal body wall in Cyathophycus is variable and some

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Hexactinellid sponge from the Ordovician of Argentina 451

species have been described as having a monaxon-based layer. A layer of this type is not homologous tothe hexactine-based layer of the Hintzespongiidae, sohe placed again the genus in the Dictyospongiidae.

The Argentinean material possesses a gastral lay-er composed of irregularly oriented hexactine-basedspicules (mostly of third order size) and monaxons,some disposed around parietal gaps and others in acontinuous mesh. The nature of this second gastrallayer partially supports Botting's argument for theinclusion of Cyathophycus in the Dictyospongiidae.Hintzespongia's basal layer is composed of the samehexactines seen the external layer, disposed aroundparietal gaps, but no monaxons are present. The con-structional features of the second layer seen inHintzespongia (Rigby and Gutschick, 1976) are differ-ent from those observed in the Argentinean speciesand those seen in the others species of Cyathophycus.

The view of Cyathophycus as a primitive memberof the family Dictyospongiidae is consistent, but thecharacters found in the species currently included inCyathophycus could also justify the erection of a newfamily. The genus requires a complete revision andthis point is beyond the scope of this paper.

Genus Cyathophycus Walcott, 1879

Type species. Cyathophycus reticulatus Walcott, 1879

Cyathophycus extendis n. sp.Figures 3 - 5

Etymology. Extendis (Lt.): wide, extensive, long; re-ferring to the horizontally longer quadrules.

Material and occurrence. A nearly complete sponge, HolotypeCORD- PZ 14217 A, and a small fragment paratype CORD- PZ

14217 B. Upper member of the Los Azules Formation, upper Sand-bian (Upper Ordovician), Amarilla creek, San Juan Province, Ar-gentine Precordillera. The material is housed at the PaleontologyMuseum of Córdoba University.

Diagnosis. Probably vasiform, medium size sponge.Skeleton of regularly oriented hexactine-basedspicules in a reticulate pattern forming slightly regu-lar quadrules in a main, external layer. Quadrulesformed by tracts of superposed overlapping rays of1-3 ranks of spicules. First-order quadrules 2.3-2.5mm wide, 2.1-2.3 high at maximum observed diame-ter. Vertical tangential rays of first-order spicules 1.8-2.2 mm long and horizontal tangential rays 2-2.5 mm.Second-order spicules with vertical tangential rays1.3-1.4 mm, horizontal tangential rays 1.3-1.5 mm.Third-order spicules with vertical tangential rays 0.6mm, horizontal tangential rays 0.5 mm. Rare fourth-order and large zero-order spicules occur. A secondlayer, composed of small, irregularly oriented hexac-tine-based spicules of approximately third-order sizeand small monaxons, is disposed beneath the firstquadruled layer.Description. Probably vasiform sponge 45 mm highand 28 mm width at the top, 8 mm at the preservedbase. Diameter expands upward to 20 mm at inter-mediate length. It is difficult to stablished the rightform of the specimen. The holotype has ragged mar-gins on all sides except the osculum. The sponge ispreserved as a white stain, showing a laterally com-pressed body, in a calcareous shale. It is possible todistinguish the reticulation formed by well-pre-served hexactine spicules with the naked eye.

Skeleton composed of regularly oriented hexac-tine-based spicules in a reticulate pattern, formingmoderately regular quadrules in the main, externallayer. These quadrules are formed by tracts of super-posed overlapping rays of 1-3 ranks of spicules.

At least three ranks of quadrules are present. Thefirst order quadrules range 2.3- 2.5 mm wide, 2.1-2.3high at maximum observed diameter. Second orderquadrules 1.2-1.5 mm wide, 1.4-1.5 mm high. Thirdorder quadrules 0.6-0.7 mm wide, 0.5-0.6 mm high.

First-order spicules with basal ray diameters 0.09-0.11 mm, vertical tangential rays 1.8-2.2 mm long andhorizontal tangential rays 2-2.5 mm. Spicules withprominent proximal and distal rays, preserved asmolds, or as small nodes or short cylindrical rodswhere the skeletal structure is best preserved.

Second-order spicules with vertical tangentialrays 1.3-1.4 mm, horizontal tangential rays 1.3-1.5mm. Third-order spicules with vertical tangentialrays 0.6 mm, horizontal tangential rays 0.5 mm.Exceptional fourth-order spicules with vertical rays0.25 mm long, and horizontal rays 0.3 mm long.Zero-order spicules are present but very scarce, 3.0-3.3 mm in ray length and not regularly oriented.

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Figure 1. Geologic map of the fossiliferous locality, Cerro Viejo, SanJuan Precordillera (modified from Furque, 1979 and Ortega, 1987) /mapa geológico de la localidad fosilífera, cerro Viejo, Precordillera de SanJuan (modificado de Furque, 1979 y Ortega, 1987).

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A second, gastral layer, composed of small, irreg-ularly-oriented hexactine-based spicules of approxi-mately third-order size and small monaxons, is dis-posed beneath the first quadruled layer (figures 3.3,4.1, 4.4). Small parietal gaps are present in some wellpreserved areas, these are 0.6-0.8 mm in diameter,and are found at a density of 5 per square millimeter.Small monaxons in the gastral layer are irregularlyoriented. It is difficult to distinguish true monaxonsfrom broken hexactine rays; we only ascribed asmonaxon type those spicules with diactine termina-tions.

The oscular rim is composed of small hexactines,irregularly distributed, and possible small monax-ons. The white-spotted preservation of the secondlayer and the oscular rim obscure detailed spiculefeatures and arrangement.

A small area of the skeletal net, possesses longslender diactines in vertical bundles, protruding 10mm from the oscular fringe (figures 4.5, 4.6). Thesediactins are cross-connected by second order-sizehexactines forming a quadruling. Most diactinsrange from 4-6 mm long and are 0.05 mm wide. At afirst glance is difficult to distinguish the real signifi-cance of this small fragment. It is clear that they de-velop from the oscular rim, but apparently they arenot a part of the main skeleton. This separatedspicule mesh could be the overlain fragment of an-other sponge or the attachment of a juvenile speci-men.

Some white dots of apparently protrudingspicules from the oscular rim occur suggesting thepresence of a marginalia.

The margins of the sponge, root tuft or other at-tachment structure are not preserved.Discussion. The Argentinean material fits well withthe diagnosis of the genus Cyathophycus Walcot,1879, as emended by Rigby and Mehl (1994) andmodified by Botting (2004). The species assigned toCyathophycus include a variety of morphologies andstructures, but some consistent features appear clear-ly in the specimen described here. The partial bun-dling of spicule rays in the regular orthogonalquadruling of the primary layer is characteristic.This is a character noted in the more expanded de-scription of C. reticulatus made by Rauff (1894). Henoted that the tracts are moderately bundled, with 2-6rays in a bundle. He also noted that some tracts ofprobable long diactines occur as rods in the skeleton,a feature rarely found in others species included inCyathophycus (see Rigby, 1995). In C. extendis sp. nov.,the bundles of the rest of the skeleton, as in the oth-er species, are formed by the juxtaposition of hexac-tine rays.

The form of the species included in Cyathophycusis variable, from conical- cylindrical and vasiform tosubspherical. The size is also variable from smallspherical forms 17 mm in diameter to others up to120 mm tall and 35 mm wide in the cylindrical orconical forms. Our specimen falls in the mid-range ofsizes included in the genus. Although the form ofeach species included in the genus is apparently con-stant, diverse grades of preservation (some of thespecies described are fragments) prevent a clear tax-onomic differentiation.

The second layer is either present or absent, and ifit is present consists of small, irregularly orientedhexactines and monaxons, or only monaxons (Bot-

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Figure 2. Stratigraphic column of the upper member of the LosAzules Formation in the Amarilla creek, Cerro Viejo area / colum-na estratigráfica del miembro superior de la Formación Los Azules en laquebrada Amarilla, área del Cerro Viejo.

Hexactinellid sponge from the Ordovician of Argentina 453

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Figure 3. Cyathophycus extendis n. sp. CORD- PZ 14217A. 1, General view of the complete specimen, x 1.5. 2, Detail of the lower part of thespecimen showing main quadrules and first- and second-order spicules, x 5.5. 3, Close-up of the lower part of the sponge showing mainquadrules and the second layer with small gaps, x 8. 4, Main quadrule with bundled hexactine rays and a hexactine spicule in the center, x16. 5, Spicule arrangement in the border of the sponge including possible zero order spicules, x 3. 6, Close up of the previous figure 5, x 12.5./ Cyathophycus extendis n. sp. CORD- PZ 14217A. 1, Vista general del espécimen completo, x 1.5. 2, Detalle de la parte basal del espécimen mostran-do los cuadros principales y las espículas de primer y segundo orden, x 5.5. 3, Detalle de la parte basal mostrando los cuadros principales y la segunda ca-pa con pequeños poros, x 8. 4, Cuadro principal con radios de hexactinas coalescentes (en atado) y una hexactina en el centro, x 16. 5, Distribución de lasespículas en el borde de la esponja incluyendo posibles espículas de orden cero, x 3. 6, Detalle del la figura previa 5, x 12,5.

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Figure 4. Cyathophycus extendis n. sp. CORD- PZ 14217A and B. 1, Lower part of the sponge specimen showing quadrules of the primaryspicule net and the second layer with small parietal gaps. Note at the upper left margin an isolated quadrule slightly elevated over the secondlayer, x 4.8. 2, Detail of the second layer and parietal gaps, x 15. 3, Detail of irregularly-oriented second-order spicules of the paratype CORD-PZ 14217B, x 14. 4, Detail of the spicule net showing first quadrule layer and, underneath, the second layer with small hexactines, possible mon-axons, and faintly marked parietal gaps, x 12. 5, Upper part of the sponge showing the oscular rim and the possible related juvenile or frag-mented sponge (right margin), x 4. 6, Detailed of the same area with bundles of long monaxons and hexactine spicules forming quadrules, x12. / Cyathophycus extendis n. sp. CORD- PZ 14217A and B. 1, Parte basal de la esponja mostrando cuadros de la red de espículas principal y la se-gunda capa con pequeños poros parietales. En el margen superior izquierdo se observa un cuadro aislado ligeramente elevado sobre la capa inferior, x 4.8. 2,Detalle de la capa basal y los poros murales, x 15. 3, Detalle de espículas de segundo orden irregularmente orientadas correspondientes al paratipo CORD-PZ 14217B, x 14. 4, Detalle de la red espicular mostrando la capa de cuadro primaria y por debajo la segunda capa con pequeñas hexactinas, posibles mon-axonas, y poros murales pobremente marcados, x 12. 5, Parte superior de la esponja mostrando el anillo del ósculo y la posible esponja juvenil o fragmentoadosado (margen derecho), x 4. 6, Detalle de la misma área con manojos de largas monaxonas y hexactinas formando cuadros., x 12.

Hexactinellid sponge from the Ordovician of Argentina 455

ting, 2004). The first description of the genus (Wal-cott, 1879; 1881) makes no mention of the inner layerof the type species C. reticulatus and the spicules arereferred as thread- like, possibly implying monax-ons. Dawson and Hinde (1889) also found no innerlayer. Rigby and Mehl (1994) stated that the innerlayer consists of irregularly arranged hexactines ortheir derivates, surrounding parietal gaps. Rigby andChatterton (1994) found a possible dermal layer withparietal gaps in the species C. mackenziensis, butspicules could not be distinguished. Rigby (1995) de-scribed specimens of C. reticulatus and C. pseudoretic-ulatus Rigby, 1995 and found no trace of the innerspicule layer in either species. As we stated in thediscussion of the family, the second layer of our spec-imen can be clearly accommodated by these varietiesof the secondary layer as currently included in the di-agnosis of the genus.

Differentiation among species was mainly basedof spicule and quadruling sizes. However, Boting(2003) demonstrated that this features are variable

depending on the size of the specimen. We includehere the measurements of spicule and quadrulingsizes of several species of the genus in order to illus-trate the different relation of these characters in theArgentinean sponge. The quadruling of C. extendis n.sp. is slightly wider than high, and the horizontalrays of hexactines are longer than the vertical ones.This feature separates it from the rest of the speciesdescribed so far in Cyathophycus.

Rauff (1894) measured first order quadrules in C.reticulatus as generally 3 mm high and 2 mm wide,which are different from the quadruling size relationobserved in the Argentinean sponge.

Cyathophycus quebensensis Dawson, 1889 (in Daw-son and Hinde, 1889), has quadrules that are verti-cally elongated and nearly 2 -3 times as high as wide,a clearly different arrangement that the horinzontal-ly elongated quadrules observed in C. extendis n. sp.A long marginalia is seen in C. quebensensis as figuredin the restoration made by Dawson.

C. mackenziensis Rigby and Chatterton, 1994, is asmall steeply obconical sponge with first orderquadrules normally up to 1 mm high and wide, al-though some fragments reach to 2.4 high and 1.4 mmwide.

C. pseudoreticulatus Rigby, 1995, is a conico-cylin-drical sponge with first order quadrules 1.4-1.6 mmhigh and 1.00-1.30 mm wide in the lower parts, to 2.0mm high and 1.6-1.8 mm wide in upper part of theskeleton. These are smaller quadrules than those ofour material, and again higher than wide.

C. reticulatus? described by Rigby (1995) is a smallsponge, 20 mm wide and 15 mm high with quadrules2.5 mm and 1.5 mm wide, different from thequadrate dimensions of the Argentinean sponge.

C. loydelli Botting, 2004, is a small sub-sphericalsponge with spicule maximum ray length of 1.2 mm.The subspherical shape is characteristic of thisspecies.

C. simpsonensis Rigby and Mehl, 1994, differs fromour material in lacking a prominent reticulation thatforms a pronounced quadruling with tracts of sever-al spicule rays. Besides, the quadrules in C. simpso-nensis are larger (3 to 5 mm) than those of theArgentinean species.

Evolutionary significance of Cyathophycus' senvironmental distribution

The genus Cyathophycus is one of the typical Lo-wer Paleozoic forms. The records of the genus have along temporal range from the Lower Ordovician,possibly Cambrian, to the Middle Devonian. It wasconsidered as a transitional form between proto-spongiids with simple regular generally unbundled

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Figure 5. Schematic cut-away reconstruction of Cyathophycus ex-tendis n. sp. (scale bar is 10 mm in overall view and 1 mm in de-tail view) / reconstrucción esquemática en corte de Cyathophycus ex-tendis n. sp. (la escala representa 10 mm en la vista del ejemplar com-pleto y 1 mm para el detalle de las espículas).

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skeletons and dictyospongiids with quadruled,thicker more complex skeletons (Rigby, 1986), ornear the base of the dictyospongiid clade (Botting,2004). However, more important is their considera-tion as a possible basal taxon in demosponge origins(see Botting, 2003).

In recent studies of the evolutionary history of thelower Paleozoic spiculate sponges, Carrera and

Rigby (2004) and Carrera and Botting (2008) empha-sized the differences shown by Cambrian and post-Cambrian sponges. Cambrian spiculate sponges, rep-resented mainly by monaxonids and hexactinellids,are structurally simple, extremely thin-walled formsin which the spicules form essentially a single layer.These characteristics can be considered as an adapta-tion of filter feeders in low-energy environments. In

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Figure 6. Stratigraphic distribution of Ordovician hexactinellid sponges in the Argentine Precordillera / distribución estratigráfica de lasesponjas hexactinélidas en la Precordillera Argentina.

Hexactinellid sponge from the Ordovician of Argentina 457

complex skeletons, the passage of water throughmore complicated channels and cavities minimizesthe destructive effects of strong currents in agitatedwaters. This is a primary condition for being efficientfilter feeders in shallow-water environments. EarlyOrdovician, shallow-water sponges typically showthese characteristics, with orchoclad "lithistid" demo-sponges as the dominant forms.

Carrera and Botting (2008) also stated that thehexactinellid evolutionary trends through theCambrian-Ordovician interval, like other spongegroups at this time, appear to have been on strength-ening the skeleton, and occupation of more turbu-lent, shallow-water environments while also remain-ing dominant in deep-water settings. Rather than be-coming hypercalcified, or developing lithistid-like orfused skeletons, hexactinellids developed thick butunconsolidated walls of large spicules.

Large hexactinellid spicules, including monaxons,are abundant in many Middle Ordovician shallow-water deposits, although complete sponges arerarely preserved (Botting, 2005). A great variety ofhexactinellid sponges remained as primary con-stituents of deep water settings. The earliest known

true dictyosponges with relatively rigid, thin wallsalso occurred at this time, and should have a betterrecord in suitable environments than is currentlyknown. Hexactinellids in deeper water facies devel-oped a range of strategies for strengthening the bodywall, including the evolution of multiple spicule lay-ers, strands of hexactines, and parietal gaps.

In this context, Cyathophycus is a typical represen-tative of this transitional or intermediate stage inwhich the skeleton is simple, but double-layered,with bundled spicule rays to increase strength, butnot to the extent seen in dictyosponges.

The distribution of Cyathophycus is consistent withthis pattern and most of Lower and Middle Ordovicianrecords came from black shale or siltstone deposits:Cyathophycus reticulatus Walcott, 1879, Utica shale,New York (Lower Ordovician); Cyathophycus quebe-nsensis Dawson 1889, Quebec (Lower Ordovician?); C.pseudoreticulatus Rigby, 1995, Vinini Formation,Nevada (Lower Ordovician); Cyathophycus sp. Mergl,2008, Prague Basin, Czech Republic (Lower Ordo-vician). Younger occurrences show slightly differentenvironmental settings (mudstone, shaly mudstoneand dolomites), shallower but also quiet water de-

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Figure 7. Paleoenvironmental reconstruction of the Cyathophycus deep water association (Attachment structure of C. extendis n. sp. in-ferred from other species of the genus) / reconstrucción paleoambiental de la asociación de Cyathophycus en aguas profundas. (Estructura defijación de C. extendis n. sp. inferida a partir de otras especies del género).

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posits: C. loydelli Botting, 2004, Central Wales (UpperOrdovician); ?C. reticulatus Rigby, 1971, Lake Winni-peg, Manitoba Canada (Upper Ordovician); C. macken-ziensis Rigby and Chatterton, 1994, Mackenzie Mts.,Canada (Silurian) and C. simpsonensis Rigby and Mehl,1994, Park Range, Nevada (Devonian).

Records of isolated hexactinellid spicules in theArgentine Precordillera occured in two differentdepositional and partially coeval settings (figure6): the hexactinellid spicule occurrences in the SanJuan Formation (Floian and Darriwilian intervals),recorded by Gnoli and Serpagli (1980), Mehl andLehnert (1997), Carrera (2001) and the hexactinellidisolated spicules in the black shale deposits of theGualcamayo- Los Azules formations (Dapingian-Darriwilian to Sandbian) mentioned by Carrera(2001) and the Las Aguaditas Formation (Mehl andLehnert, 1997). C. extendis n. sp. came from thedeep basin deposits that postdate the San Juan car-bonate sequence. They are coeval to a mixed car-bonate-siliciclastic unit (Las Aguaditas Formation)which recorded isolated hexactin and pentactinspicules (Mehl and Lehnert, 1997) and is interpret-ed as a former deep platform to upper slope envi-ronment.

The occurrence of hexactinellid spicules in thecarbonate platform appear to represent a particularevent, coincident with both transgresive episodes:the Floian transgression and the middle to late Da-rriwilian sea level rise. These environmental changesare clearly shown by the sponge associations recog-nized in the Ordovician limestones of the ArgentinePrecordillera (Carrera, 1997) and the environmentalinfluence is also clearly detected in the associatedfauna (Carrera, 2001; Cech and Carrera, 2002). Thesepunctuated occurrences in the carbonate depositscontrast with several records of hexactinellidspicules and now a complete sponge from the silt-stone units of the Precordillera.

C. extendis n. sp. belongs to a deep-water associationconsisting mainly of graptolites, trilobites and inarticu-late brachiopods (figure 7). In spite of limited favorablephysical conditions for a more diverse association, thesponge was evidently able to tolerate the siliciclastic in-put and moderately low oxygen content of the environ-ment. Evidence of turbidity or a soft substrate is diffi-cult to evaluate. Root-tuft attachments are found in oth-ers species of the genus (Botting, 2004) suggesting a softsubstrate for anchoring. Unfortunately, the preserva-tion of our specimen prevents a definite conclusionabout the attachment structure.

Establishing the water depth of fossiliferous silt-stones is difficult; however, the association can be as-cribed to an intermediate situation between thedemosponge- dominated communities that inhabit-ed the carbonate platform (Carrera, 2000; 2001; 2003)

and the graptolite-dominated black shales depositsthat underlie the C. extendis levels.

Acknowledgements

The authors want to thanks to J.K. Rigby and J. Botting fortheirs helpful reviews and supportive comments. MGC acknowl-edge support from CONICET, PIP 5599 and FONCYT, PICT 2004-21857 (BID 1728 OC/AR). Appreciation is extended to S. Druettafor the artistic reconstructions.

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Recibido: 4 de septiembre de 2008.Aceptado: 31 de marzo de 2009.

AMEGHINIANA 46 (3), 2009