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