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Ameghiniana

versión impresa ISSN 0002-7014

Ameghiniana vol.46 no.3 Buenos Aires jul./set. 2009

 

ARTÍCULOS ORIGINALES

The hexactinellid sponge Cyathophycus from the Upper Ordovician of the Argentine Precordillera

Marcelo G. Carrera1 and Gladys Ortega2

1CIPAL-CICTERRA-CONICET, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba. mcarrera@efn.uncor.edu
2Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba.

Abstract. A nearly complete specimen of a hexactinellid sponge has been recovered from Lower Ordovician siliciclastic rocks of the Argentine Precordillera. The material was collected in the upper member 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 family Dictyospongiidae is discussed; Cyathophycus has been considered as a transitional form between protospongiids with simple regular skeletons and dictyospongiids with more complex skeletons. Previous studies have stated that hexactinellid evolution through the Cambrian-Ordovician interval, like other sponge groups at this time, appears to have been based on strengthening the skeleton, and the occupation of 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 double- layered, with bundled spicule rays, and not as rigid as the typical dictyosponge skeletons. Lower and Middle Ordovician records are from black shale deposits, while younger occurrences show slightly different environmental settings (mudstone, shaly mudstone and dolomites), which indicate shallower but still quiet water deposits.

Resumen. La esponja hexactinélida Cyathophycus en el Ordovícico Superior de la Precordillera Argentina. Se describe e ilustra un ejemplar casi completo de una esponja hexactinélida proveniente de rocassiliciclásticas ordovícicas de la Precordillera Argentina. El material fue colectado en el miembro superior de la Formación Los Azules (Sandbiano). Se asigna el material a una nueva especie del género Cyathophycus 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 forma transicional entre las protosponjas con esqueletos simples y las dictiosponjas con esqueletos cuadriculados 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 de esta etapa transicional en la que los esqueletos son simples pero con dos paredes, y aún no lo suficientemente rígidos como los esqueletos de las dictiosponjas. Los registros del Ordovícico Inferior y Medio provienen de depósitos de pelitas negras mientras que registros algo más jóvenes se encuentran en calizas fangosas y dolomitas, más someros, pero aún de aguas tranquilas.

Key words. Porifera; Hexactinellida; Ordovician; Argentina.

Palabras clave. Porífera; Hexactinélida; Ordovícico; Argentina.

Introduction

The Ordovician sponges from the Argentine Precordillera have been described in several contributions (see Carrera, 2003; 2007; Carrera and Rigby, 1999 for a review). Most of these publications focused on taxonomy, paleoecology and paleogeography of the sponge fauna collected from Lower and Middle Ordovician limestones. Orchoclad demosponges are the main components of these sponge associations, which inhabited a former carbonate platform.
The sponge described in this contribution came from a Middle-Upper Ordovician siliciclastic unit that represents a deep basin depositional setting. This unit overlies a thick, Cambrian and Lower Ordovician carbonate succession.
Sponges recovered from the siliciclastic units in the Ordovician of Argentina are relatively scarce, with only a few published descriptions. Carrera (1998) described the hexactinellid sponge Larispongia from a Lower Ordovician unit of the Puna region. In the Lower Ordovician of the Famatina region, Esteban and Rigby (1998) described the cosmopolitan hexactinellid genus Protospongia and Beresi et al. (2006) described the hexactinellid sponge Diagoniella from Cambrian - Ordovician rocks of the Cordillera Oriental and the Subandean Ranges of Salta Province, Northwestern Argentina.
The records of isolated hexactinellid spicules are also limited, but they have been mentioned from the Cambrian limestones of the Precordillera (Beresi and Rigby, 1994), and from Ordovician limestones and shales 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 the Middle to Upper Ordovician shales of the Precordillera Basin, include a fairly complete sponge body to complement isolated hexactinellid spicules from the same location (Carrera, 2001).
The aim of this contribution is the taxonomic description of the new material and a discussion of the evolutionary significance of the geographic and environmental distribution of Cyathophycus.

Geological setting and stratigraphy

The Argentinean Precordillera consists of a thick sedimentary succession of Cambro-Ordovician rocks showing a transition from a nearshore carbonate bank through mixed carbonate-siliciclastic slope deposits to basinal clastics.
The Lower Ordovician rocks consist of Tremadocian to Darriwilian platform carbonates (La Silla and San Juan Formations; Keller et al., 1994; Cañas, 1999). Carbonate sedimentation mostly ceased during the Darriwilian due to regional drowning, and the deposition of graptolitic black shales and mudstones (Los Azules and Gualcamayo Formations).
The Middle-Upper Ordovician Los Azules Formation (Harrington in Harrington and Leanza, 1957), a dominantly shaly unit, paraconformably overlies the San Juan Limestone (Lower-Middle Ordovician), and is unconformably overlain by the Carboniferous Guandacol Formation. The type locality for the study of this formation is the Los Azules creek, which is situated on the western flank of the Cerro Viejo of Huaco, Central Precordillera, San Juan Province (latitude 30º 11'-30º 15' S, longitude 68º 34'- 68º 35' W) (Borrello and Gareca, 1951; Cuerda and Furque, 1975; Furque, 1979). The formation is divided into three members with variable thickness in different sections (Ortega, 1987). A composite stratigraphic column of this unit is ca. 318 m in maximum thickness. 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 Undulograptus dentatus and Holmograptus lentus zones, and associated conodonts of the Lenodus variabilis Zone, indicates an early Darriwilian age for this member (Brussa et al., 2003; Ortega and Rickards, 2003; Ortega et al., 2007). The middle member (220 m thick) comprises grey siltstones with subordinate sandstones at its base, and yields graptolites of the Pterograptus elegans and Hustedograptus teretiusculus zones (Ortega, 1987; 1995) and conodonts of the Eoplacognathus suecicus and Pygodus serra zones (Hünicken and Ortega, 1987; Ottone et al., 1999), indicating a late Darriwilian age. The upper member (88 m thick) is composed by gray calcareous shales with mudstones at the base and contains coquinas in its upper part. Graptolites of the Climacograptus bicornis Zone and conodonts of the Amorphognathus tvaerensis Zone (Baltoniodus variabilis Subzone) (Ortega, 1987; Ortega and Albanesi, 1998; Ottone et al., 1999) allow an assignment of these levels to the upper Sandbian. The presence of a stratigraphic gap between the middle and upper members, which suppressed the upper part of the H. teretiusculus Zone and the entire Nemagraptus gracilis Zone, was noted by Ortega (1987).
The sponge described here was collected in the upper member of the Los Azules Formation at the Amarilla Creek (figure 1). The material is associated with graptolites, the trilobites Guandacolitus furquei Harrington and Leanza and Triarthrus jachalensis (Harrington and Leanza), and a palynological assemblage referable to LAU (Ottone et al., 1999; 2001). The graptolite fauna of the Climacograptus bicornis Zone is composed 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).


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

Systematic paleontology

Class HEXACTINELLIDA Schmidt, 1870
Order RETICULOSA Reid, 1958
Family DICTYOSPONGIIDAE ? Hall, 1884

Remarks. Cyathophycus was initially placed in the Dictyospongiidae by Hall (1884), and later in the Protospongiidae by De Laubenfels (1955). Finks (1983) placed the genus in the Hintzespongiidae based on the presence of two spicule layers in the body wall, an interpretation followed by Rigby (1995) and Rigby and Mehl (1994). However, Botting (2004) demonstrated that the nature of the second internal body wall in Cyathophycus is variable and some species have been described as having a monaxonbased layer. A layer of this type is not homologous to the hexactine-based layer of the Hintzespongiidae, so he placed again the genus in the Dictyospongiidae.
The Argentinean material possesses a gastral layer composed of irregularly oriented hexactine-based spicules (mostly of third order size) and monaxons, some disposed around parietal gaps and others in a continuous mesh. The nature of this second gastral layer partially supports Botting's argument for the inclusion of Cyathophycus in the Dictyospongiidae. Hintzespongia's basal layer is composed of the same hexactines seen the external layer, disposed around parietal gaps, but no monaxons are present. The constructional features of the second layer seen in Hintzespongia (Rigby and Gutschick, 1976) are different from those observed in the Argentinean species and those seen in the others species of Cyathophycus.


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

The view of Cyathophycus as a primitive member of the family Dictyospongiidae is consistent, but the characters found in the species currently included in Cyathophycus could also justify the erection of a new family. The genus requires a complete revision and this 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


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 the specimen showing main quadrules and first- and second-order spicules, x 5.5. 3, Close-up of the lower part of the sponge showing main quadrules and the second layer with small gaps, x 8. 4, Main quadrule with bundled hexactine rays and a hexactine spicule in the center, x 16. 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 mostrando 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 capa 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 las espículas en el borde de la esponja incluyendo posibles espículas de orden cero, x 3. 6, Detalle del la figura previa 5, x 12,5.

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

Material and occurrence. A nearly complete sponge, Holotype CORD- PZ 14217 A, and a small fragment paratype CORD- PZ 14217 B. Upper member of the Los Azules Formation, upper Sandbian (Upper Ordovician), Amarilla creek, San Juan Province, Argentine Precordillera. The material is housed at the Paleontology Museum of Córdoba University.

Diagnosis. Probably vasiform, medium size sponge. Skeleton of regularly oriented hexactine-based spicules in a reticulate pattern forming slightly regular quadrules in a main, external layer. Quadrules formed by tracts of superposed overlapping rays of 1-3 ranks of spicules. First-order quadrules 2.3-2.5 mm wide, 2.1-2.3 high at maximum observed diameter. 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 rays 1.3-1.4 mm, horizontal tangential rays 1.3-1.5 mm. Third-order spicules with vertical tangential rays 0.6 mm, horizontal tangential rays 0.5 mm. Rare fourthorder and large zero-order spicules occur. A second layer, composed of small, irregularly oriented hexactine- based spicules of approximately third-order size and small monaxons, is disposed beneath the first quadruled layer.
Description. Probably vasiform sponge 45 mm high and 28 mm width at the top, 8 mm at the preserved base. Diameter expands upward to 20 mm at intermediate length. It is difficult to stablished the right form of the specimen. The holotype has ragged margins on all sides except the osculum. The sponge is preserved as a white stain, showing a laterally compressed body, in a calcareous shale. It is possible to distinguish the reticulation formed by well-preserved hexactine spicules with the naked eye.
Skeleton composed of regularly oriented hexactine- based spicules in a reticulate pattern, forming moderately regular quadrules in the main, external layer. These quadrules are formed by tracts of superposed overlapping rays of 1-3 ranks of spicules.
At least three ranks of quadrules are present. The first order quadrules range 2.3- 2.5 mm wide, 2.1-2.3 high at maximum observed diameter. Second order quadrules 1.2-1.5 mm wide, 1.4-1.5 mm high. Third order 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 and horizontal tangential rays 2-2.5 mm. Spicules with prominent proximal and distal rays, preserved as molds, or as small nodes or short cylindrical rods where the skeletal structure is best preserved.
Second-order spicules with vertical tangential rays 1.3-1.4 mm, horizontal tangential rays 1.3-1.5 mm. Third-order spicules with vertical tangential rays 0.6 mm, horizontal tangential rays 0.5 mm. Exceptional fourth-order spicules with vertical rays 0.25 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.
A second, gastral layer, composed of small, irregularly- oriented hexactine-based spicules of approximately third-order size and small monaxons, is disposed beneath the first quadruled layer (figures 3.3, 4.1, 4.4). Small parietal gaps are present in some well preserved 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 irregularly oriented. It is difficult to distinguish true monaxons from broken hexactine rays; we only ascribed as monaxon type those spicules with diactine terminations.
The oscular rim is composed of small hexactines, irregularly distributed, and possible small monaxons. The white-spotted preservation of the second layer and the oscular rim obscure detailed spicule features and arrangement.
A small area of the skeletal net, possesses long slender diactines in vertical bundles, protruding 10 mm from the oscular fringe (figures 4.5, 4.6). These diactins are cross-connected by second order-size hexactines forming a quadruling. Most diactins range from 4-6 mm long and are 0.05 mm wide. At a first glance is difficult to distinguish the real significance of this small fragment. It is clear that they develop from the oscular rim, but apparently they are not a part of the main skeleton. This separated spicule mesh could be the overlain fragment of another sponge or the attachment of a juvenile specimen.


Figure 4. Cyathophycus extendis n. sp. CORD- PZ 14217A and B. 1, Lower part of the sponge specimen showing quadrules of the primary spicule net and the second layer with small parietal gaps. Note at the upper left margin an isolated quadrule slightly elevated over the second layer, 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 CORDPZ 14217B, x 14. 4, Detail of the spicule net showing first quadrule layer and, underneath, the second layer with small hexactines, possible monaxons, and faintly marked parietal gaps, x 12. 5, Upper part of the sponge showing the oscular rim and the possible related juvenile or fragmented sponge (right margin), x 4. 6, Detailed of the same area with bundles of long monaxons and hexactine spicules forming quadrules, x 12. / 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 segunda 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 CORDPZ 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 monaxonas, y poros murales pobremente marcados, x 12. 5, Parte superior de la esponja mostrando el anillo del ósculo y la posible esponja juvenil o fragmento adosado (margen derecho), x 4. 6, Detalle de la misma área con manojos de largas monaxonas y hexactinas formando cuadros., x 12.

Some white dots of apparently protruding spicules from the oscular rim occur suggesting the presence of a marginalia.
The margins of the sponge, root tuft or other attachment structure are not preserved.
Discussion. The Argentinean material fits well with the diagnosis of the genus Cyathophycus Walcot, 1879, as emended by Rigby and Mehl (1994) and modified by Botting (2004). The species assigned to Cyathophycus include a variety of morphologies and structures, but some consistent features appear clearly in the specimen described here. The partial bundling of spicule rays in the regular orthogonal quadruling of the primary layer is characteristic. This is a character noted in the more expanded description of C. reticulatus made by Rauff (1894). He noted that the tracts are moderately bundled, with 2-6 rays in a bundle. He also noted that some tracts of probable long diactines occur as rods in the skeleton, a feature rarely found in others species included in Cyathophycus (see Rigby, 1995). In C. extendis sp. nov., the bundles of the rest of the skeleton, as in the other species, are formed by the juxtaposition of hexactine rays.
The form of the species included in Cyathophycus is variable, from conical- cylindrical and vasiform to subspherical. The size is also variable from small spherical forms 17 mm in diameter to others up to 120 mm tall and 35 mm wide in the cylindrical or conical forms. Our specimen falls in the mid-range of sizes included in the genus. Although the form of each species included in the genus is apparently constant, diverse grades of preservation (some of the species described are fragments) prevent a clear taxonomic differentiation.
The second layer is either present or absent, and if it is present consists of small, irregularly oriented hexactines and monaxons, or only monaxons (Botting, 2004). The first description of the genus (Walcott, 1879; 1881) makes no mention of the inner layer of the type species C. reticulatus and the spicules are referred as thread- like, possibly implying monaxons. Dawson and Hinde (1889) also found no inner layer. Rigby and Mehl (1994) stated that the inner layer consists of irregularly arranged hexactines or their derivates, surrounding parietal gaps. Rigby and Chatterton (1994) found a possible dermal layer with parietal gaps in the species C. mackenziensis, but spicules could not be distinguished. Rigby (1995) described specimens of C. reticulatus and C. pseudoreticulatus Rigby, 1995 and found no trace of the inner spicule layer in either species. As we stated in the discussion of the family, the second layer of our specimen can be clearly accommodated by these varieties of the secondary layer as currently included in the diagnosis of the genus.
Differentiation among species was mainly based of spicule and quadruling sizes. However, Boting (2003) demonstrated that this features are variable depending on the size of the specimen. We include here the measurements of spicule and quadruling sizes of several species of the genus in order to illustrate the different relation of these characters in the Argentinean sponge. The quadruling of C. extendis n. sp. is slightly wider than high, and the horizontal rays of hexactines are longer than the vertical ones. This feature separates it from the rest of the species described 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 relation observed in the Argentinean sponge.
Cyathophycus quebensensis Dawson, 1889 (in Dawson and Hinde, 1889), has quadrules that are vertically elongated and nearly 2 -3 times as high as wide, a clearly different arrangement that the horinzontally elongated quadrules observed in C. extendis n. sp. A long marginalia is seen in C. quebensensis as figured in the restoration made by Dawson.
C. mackenziensis Rigby and Chatterton, 1994, is a small steeply obconical sponge with first order quadrules normally up to 1 mm high and wide, although some fragments reach to 2.4 high and 1.4 mm wide.
C. pseudoreticulatus Rigby, 1995, is a conico-cylindrical sponge with first order quadrules 1.4-1.6 mm high and 1.00-1.30 mm wide in the lower parts, to 2.0 mm high and 1.6-1.8 mm wide in upper part of the skeleton. These are smaller quadrules than those of our material, and again higher than wide.
C. reticulatus? described by Rigby (1995) is a small sponge, 20 mm wide and 15 mm high with quadrules 2.5 mm and 1.5 mm wide, different from the quadrate dimensions of the Argentinean sponge.
C. loydelli Botting, 2004, is a small sub-spherical sponge with spicule maximum ray length of 1.2 mm. The subspherical shape is characteristic of this species.
C. simpsonensis Rigby and Mehl, 1994, differs from our material in lacking a prominent reticulation that forms a pronounced quadruling with tracts of several spicule rays. Besides, the quadrules in C. simpsonensis are larger (3 to 5 mm) than those of the Argentinean species.

Evolutionary significance of Cyathophycus' s environmental distribution

The genus Cyathophycus is one of the typical Lower Paleozoic forms. The records of the genus have a long temporal range from the Lower Ordovician, possibly Cambrian, to the Middle Devonian. It was considered as a transitional form between protospongiids with simple regular generally unbundled skeletons and dictyospongiids with quadruled, thicker more complex skeletons (Rigby, 1986), or near the base of the dictyospongiid clade (Botting, 2004). However, more important is their consideration as a possible basal taxon in demosponge origins (see Botting, 2003).
In recent studies of the evolutionary history of the lower Paleozoic spiculate sponges, Carrera and Rigby (2004) and Carrera and Botting (2008) emphasized the differences shown by Cambrian and post- Cambrian sponges. Cambrian spiculate sponges, represented mainly by monaxonids and hexactinellids, are structurally simple, extremely thin-walled forms in which the spicules form essentially a single layer. These characteristics can be considered as an adaptation of filter feeders in low-energy environments. In complex skeletons, the passage of water through more complicated channels and cavities minimizes the destructive effects of strong currents in agitated waters. This is a primary condition for being efficient filter feeders in shallow-water environments. Early Ordovician, shallow-water sponges typically show these characteristics, with orchoclad "lithistid" demosponges as the dominant forms.
Carrera and Botting (2008) also stated that the hexactinellid evolutionary trends through the Cambrian-Ordovician interval, like other sponge groups at this time, appear to have been on strengthening the skeleton, and occupation of more turbulent, shallow-water environments while also remaining dominant in deep-water settings. Rather than becoming hypercalcified, or developing lithistid-like or fused skeletons, hexactinellids developed thick but unconsolidated walls of large spicules.
Large hexactinellid spicules, including monaxons, are abundant in many Middle Ordovician shallowwater deposits, although complete sponges are rarely preserved (Botting, 2005). A great variety of hexactinellid sponges remained as primary constituents of deep water settings. The earliest known true dictyosponges with relatively rigid, thin walls also occurred at this time, and should have a better record in suitable environments than is currently known. Hexactinellids in deeper water facies developed a range of strategies for strengthening the body wall, including the evolution of multiple spicule layers, strands of hexactines, and parietal gaps.
In this context, Cyathophycus is a typical representative of this transitional or intermediate stage in which the skeleton is simple, but double-layered, with bundled spicule rays to increase strength, but not to the extent seen in dictyosponges.


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

The distribution of Cyathophycus is consistent with this pattern and most of Lower and Middle Ordovician records came from black shale or siltstone deposits: Cyathophycus reticulatus Walcott, 1879, Utica shale, New York (Lower Ordovician); Cyathophycus quebensensis Dawson 1889, Quebec (Lower Ordovician?); C. pseudoreticulatus Rigby, 1995, Vinini Formation, Nevada (Lower Ordovician); Cyathophycus sp. Mergl, 2008, Prague Basin, Czech Republic (Lower Ordovician). Younger occurrences show slightly different environmental settings (mudstone, shaly mudstone and dolomites), shallower but also quiet water deposits: C. loydelli Botting, 2004, Central Wales (Upper Ordovician); ?C. reticulatus Rigby, 1971, Lake Winnipeg, Manitoba Canada (Upper Ordovician); C. mackenziensis 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 the Argentine Precordillera occured in two different depositional and partially coeval settings (figure 6): the hexactinellid spicule occurrences in the San Juan Formation (Floian and Darriwilian intervals), recorded by Gnoli and Serpagli (1980), Mehl and Lehnert (1997), Carrera (2001) and the hexactinellid isolated spicules in the black shale deposits of the Gualcamayo- Los Azules formations (Dapingian- Darriwilian to Sandbian) mentioned by Carrera (2001) and the Las Aguaditas Formation (Mehl and Lehnert, 1997). C. extendis n. sp. came from the deep basin deposits that postdate the San Juan carbonate sequence. They are coeval to a mixed carbonate- siliciclastic unit (Las Aguaditas Formation) which recorded isolated hexactin and pentactin spicules (Mehl and Lehnert, 1997) and is interpreted as a former deep platform to upper slope environment.


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

The occurrence of hexactinellid spicules in the carbonate platform appear to represent a particular event, coincident with both transgresive episodes: the Floian transgression and the middle to late Darriwilian sea level rise. These environmental changes are clearly shown by the sponge associations recognized in the Ordovician limestones of the Argentine Precordillera (Carrera, 1997) and the environmental influence is also clearly detected in the associated fauna (Carrera, 2001; Cech and Carrera, 2002). These punctuated occurrences in the carbonate deposits contrast with several records of hexactinellid spicules and now a complete sponge from the siltstone units of the Precordillera.
C. extendis n. sp. belongs to a deep-water association consisting mainly of graptolites, trilobites and inarticulate brachiopods (figure 7). In spite of limited favorable physical conditions for a more diverse association, the sponge was evidently able to tolerate the siliciclastic input and moderately low oxygen content of the environment. Evidence of turbidity or a soft substrate is difficult to evaluate. Root-tuft attachments are found in others species of the genus (Botting, 2004) suggesting a soft substrate for anchoring. Unfortunately, the preservation of our specimen prevents a definite conclusion about the attachment structure.


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

Establishing the water depth of fossiliferous siltstones is difficult; however, the association can be ascribed to an intermediate situation between the demosponge- dominated communities that inhabited the carbonate platform (Carrera, 2000; 2001; 2003) and the graptolite-dominated black shales deposits that underlie the C. extendis levels.

Acknowledgements

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

References

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