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Ameghiniana

Print version ISSN 0002-7014

Ameghiniana vol.47 no.3 Buenos Aires Sept. 2010

 

ARTÍCULOS ORIGINALES

Darriwilian bryozoans from the San Juan Formation (Ordovician), Argentine Precordillera

 

Marcelo G. Carrera1 and Andrej Ernst2

1CIPAL-CICTERRA-CONICET, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Velez Sarsfield 299, 5000 Córdoba, Argentina. mcarrera@com.uncor.edu
2Institute for Geosciences, Christian-Albrechts-University of Kiel, Ludewig-Meyn-Str. 10, D-24118, Germany. ae@gpi.uni-kiel.de

 


Abstract. A detailed study is carried out on the taxonomy of the bryozoan fauna from the upper levels of the San Juan Formation (Darriwilian) in the Talacasto and Cerro Viejo sections, Argentinean Precordillera. Three bryozoan genera are described and two new species are erected: Lamottopora multispinosa sp. nov., Aostipora sanjuanensis sp. nov., and Phylloporinidae sp. indet. The paleoecological significance of the bryozoan fauna is discussed. The ramose bryozoans Lamottopora and Aostipora dominate in the Talacasto section. Some colonies probably lived attached directly to the substrate, while others occur attached to the surface of the abundant sponge fauna in the section. The majority of bryozoans at the Cerro Viejo section occur as epibionts, with bases attached to the surface of sponges. Bryozoans collected in the matrix are scarce compared to the Talacasto section, and comprise a few fragments of ramose bryozoans and two small fragments of the reticulate Phylloporinidae sp. indet were recorded exclusively in this locality. The presence of this delicate form in Cerro Viejo is further evidence of the quiet water conditions suggested for these levels. Only the laminar form Nicholsonella occurs in slightly older levels (Floian) of the San Juan Formation. The low diversity recorded here shows a slight decoupling with the global pattern. The first local radiation (three genera) is minor compared with the important radiation that the phylum experienced elsewhere.

Resumen. Briozoos darriwilianos de la Formación San Juan (Ordovícico) de la precordillera argentina. Se ha realizado un estudio taxonómico detallado de la fauna de briozoos de los niveles superiores (Darriwiliano) de la Formación San Juan en las secciones de Talacasto y Cerro Viejo, Precordillera Argentina. Se describen tres géneros y se definen dos nuevas especies: Lamottopora multispinosa sp. nov., Aostipora sanjuanensis sp. nov. y Phylloporinidae sp. indet. Se discute el significado paleoecológico de la fauna de briozoos. Los briozoos ramosos como Lamottopora y Aoistopora dominan en la sección de Talacasto. Algunas colonias vivieron fijadas directamente sobre el sustrato, aunque también aparecen como epibiontes sobre la superficie de esponjas, que son muy abundantes en este intervalo. En la sección de Cerro Viejo los briozoos se encuentran mayoritariamente adheridos a la superficie de las esponjas, mientras que algunos fragmentos de briozoos ramosos, incluyendo dos colonias del briozoo reticulado Phylloporinidae sp. indet, aparecen en el sedimento. La presencia de estas formas delicadas en el Cerro Viejo es una nueva evidencia de la baja energía del agua como fue sugerido para estos niveles. En las calizas de la Formación San Juan sólo el género laminar Nicholsonella aparece en niveles algo más antiguos (Floiano). Esta baja diversidad muestra un desajuste con las curvas globales de diversidad del grupo. La primera radiación local (géneros), descripta en esta contribución, es también significativamente menor y muestra un desacople con la diversificación que experimenta el grupo para este mismo intervalo temporal en otras regiones.

Key words. Bryozoans; Ordovician; Argentina; Precordillera.

Palabras clave. Briozoos; Ordovícico; Precordillera; Argentina.


 

Introduction

Middle and Upper Ordovician bryozoans from the Argentine Precordillera are conspicuous elements in mixed calcareous and siliciclastic units. In contrast, very few occurrences of bryozoans have been reported from the Lower Ordovician limestones (Carrera, 1995).
Keller et al. (1993) and Carrera (2003a) recorded several genera of trepostome and cryptostome bryozoans from different Middle and Upper Ordovician units, although no complete taxonomic descriptions were given.
This contribution is a new report of Argentinean bryozoans and is part of a major project that involves a taxonomic survey of Ordovician bryozoans from Argentina. Previous records include Prasopora argentina Carrera, 2003b, from the Middle Ordovician units,
and two new species-Moyerella spinosa Ernst and Carrera, 2008, and Phylloporina sassitoensis Ernst and Carrera, 2008-based on material collected in the Sassito Formation (Upper Ordovician).
Early Ordovician limestones of the Argentine Precordillera are represented by the fossiliferous San Juan Formation. The outstanding feature of these limestones is the abundance of benthic fauna, mainly brachiopods, gastropods, sponges and echinoderms. Bryozoans are very scarce; only the genus Nicholsonella Ulrich, 1890, occurs in the middle (Floian) part of the formation (Carrera, 1995), and some trepostomes (Carrera, 2003a) in the uppermost (Darriwilian) beds. Numerous specimens were collected from these upper levels, revealing a higher diversity than previously recognized.
The aim of this contribution is to provide a detailed taxonomic study of the bryozoan fauna from the upper levels of the San Juan Formation and to analyze the paleoecological significance of the bryozoans and their relationship with the associated fauna.

Geological setting and stratigraphy

The Lower Paleozoic of the Argentine Precordillera consists of a sequence of sedimentary rocks including a 2,500 m thick sequence of Cambrian and early Ordovician limestones. The younger carbonate unit is represented by the San Juan Formation. This unit is a muddy fossil-bearing carbonate sequence developed over a Cambrian to Lower Ordovician carbonate platform succession (Cañas, 1999).
Earliest conodonts recorded from the formation demonstrate that the age of the lower levels is latest Tremadocian (Paltodus deltifer Zone, Keller et al., 1994; Albanesi et al., 1998). The top of the San Juan Formation has been dated at several localities. In the northern sections, conodonts and graptolites rendered a lower Middle Ordovician age (Isograptus victoriae maximus) for the transitional beds between the San Juan Formation and the black shales of the overlying Gualcamayo Formation (Ortega et al., 1985), whereas elsewhere conodonts have proved a Darriwilian age for the upper limestone beds (Lehnert, 1995; Albanesi et al., 1998).
The bryozoan fauna is concentrated in the upper-most twenty meters of the San Juan Formation (Darriwilian). These horizons are included in the brachiopod Ahtiella Biozone (Herrera and Benedetto, 1991) that partly corresponds to the Lenodus variabilis conodont Biozone (Albanesi and Ortega, 2002). The bryozoan material was mainly collected at two typical sections of the Argentine Precordillera where superb outcrops of the San Juan Limestone lie exposed: the Talacasto and Cerro Viejo sections (figure 1). Co
nodonts recorded at the two localities belong to the P. horridus Subzone of the L. variabilis Biozone (Albanesi and Ortega, 2002; Albanesi et al., 2006).


Figure 1. Geological map showing location of the sampled sections and a composite stratigraphic column of the San Juan Formation /
mapa geológico indicando las secciones muestreadas y la columna estratigráfica compuesta de la Formación San Juan. 1, Cerro Viejo. 2, Talacasto.

Taphonomic and paleoecologic studies (Sánchez et al., 1993, 1996; Carrera, 2000) reveal the autochthonous nature of the fossil assemblages.
General sedimentological features of the bryozoans-bearing limestones indicate a shallow subtidal environment in middle to distal ramp settings (Cañas, 1999). A detailed biofacies study of the San Juan Formation was carried out by Carrera et al. (1999) and Carrera (2001).

Materials and methods

The present investigation is based on ten samples, from which 17 thin sections were produced. All specimens are deposited in the Centro de Investigaciones Paleobiológicas CIPAL, Universidad Nacional de Córdoba (CEGH-UNC).
Bryozoans were observed in thin sections using a transmitted light microscope. The following morphological characters were measured and used for statistics: Autozooecial Aperture Width, Autozooecial Aperture Spacing, Acanthostyle Diameter, Wall Thickness in Exozone, Autozooecial Diaphragm Spacing, Mesozooecia Width, Mesozooecial Diaphragm Spacing. The spacing of structures is measured as a distance between their centers. Additional quantitative characters are the Number of Mesozooecia and Acanthostyles surrounding each autozooecial aperture. Statistics were summarized using arithmetic mean (X), sample standard deviation (SD), coefficient of variation (CV), and minimum (MIN) and maximum (MAX) value.

Systematic palaeontology

Phylum Bryozoa Ehrenberg, 1831
Class Stenolaemata Borg, 1926
Order Trepostomida Ulrich, 1882
Suborder Amplexoporina Astrova, 1965
Family Aisenvergiidae Dunaeva, 1964

Genus Lamottopora Ross, 1963

1963. Lamottopora Ross, p. 58-59; Brown, 1965: 999-1000.
1978. Trematopora Astrova, p. 80 (pars).

Type species. Lamottopora duncanae Ross, 1963. Ordovician; USA (Vermont).

Emended diagnosis. Ramose and encrusting colonies. Autozooecia curving gently from the endozone into the exozone. Autozooecial apertures oval to rounded with well developed cingulum. Diaphragms absent to rare in autozooecia. Abundant mesozooecia with abundant diaphragms, thick-walled, restricted to exozone. Mesozooecial apertures completely sealed by laminated skeleton. Acanthostyles abundant, often arranged around autozooecial cingulum or in mesozooecial walls. Walls thin in endozone, thickened in exozone, displaying obliquely laminated microstructure with dark zooecial boundaries. Maculae and monticulae may be present.
Comparison. Lamottopora Ross, 1963, differs from Eridotrypa Ulrich, 1893, by having abundant mesozooecia, well developed acanthostyles and rare to absent autozooecial diaphragms.
Occurrence and age. Two species were previously reported: Lamottopora duncanae Ross, 1963, and Lamottopora pauca Brown, 1965, from the Middle Ordovician of Vermont and Kentucky, respectively. The present material is described from the upper levels of the San Juan Formation (Darriwilian), San Juan Province, Argentina.

Lamottopora multispinosa sp. nov. Figures 2.1-8; 4.1; Table 1


Figure 2. 1-8, Lamottopora multispinosa sp. nov. 1, paratype (CEGH-UNC 9339), oblique branch section, scale bar = 1 mm / paratipo, section transversal de una rama, escala = 1 mm. 2, paratype CEGH-UNC 23940, longitudinal branch section, scale bar = 1 mm / paratipo, sección longitudinal de una rama, escala = 1 mm. 3-4, paratype CEGH-UNC 23941, longitudinal sections of encrusting overgrowths, showing autozooecia and mesozooecia, scale bars 1 mm for 3 and 0.2 mm for 4 / secciones longitudinales de sobrecrecimientos incrustantes, mostrando autozoecios y mesozoecios, escala 1 mm en 3 y 0,2 mm en 4. 5, holotype CEGH-UNC 23939, longitudinal section showing mesozooecial diaphragms and laminated skeleton of the exozone, scale bar = 0.2 mm / holotipo, sección longitudinal mostrando los diafragmas de los mesozoecios y el esqueleto laminado en la exozona, escala= 0,2 mm. 6, paratype CEGH-UNC 23940, longitudinal section showing mesozooecia and laminated skeleton of the exozone, scale bar = 0.2 mm / paratipo, sección longitudinal mostrando los mesozoecios y el esqueleto laminado en la exozona, escala= 0,2 mm. 7, paratype CEGH-UNC 23941, tangential section showing mesozooecial and autozooecial apertures, scale bar = 0.5 mm / paratipo, sección tangencial mostrando las aberturas de los mesozoecios y autozoecios, escala= 0,5 mm. 8, holotype, tangential section showing autozooecial apertures and acanthostyles, scale bar = 0.2 mm / holotipo, sección tangencial mostrando las aberturas de los autozoecios y los acantoporos, escala= 0,2 mm. 9-11, Aostipora sanjuanensis sp. nov., holotype CEGH-UNC 23937. 9, branch longitudinal section, scale bar = 1 mm / holotipo, sección longitudinal de una rama, escala = 1 mm. 10, branch longitudinal section, arrow indicates vesicles, scale bar = 1 mm / sección longitudinal de una rama, la flecha indica las vesiculas, escala = 1 mm. 11, branch cross section, scale bar = 1 mm / sección tangencial de una rama, escala = 1 mm.

Table 1. Descriptive statistics of Lamottopora multispinosa sp. nov. See text for abbreviations. / estadística descriptiva de Lamottopora multispinosa sp. nov. Ver texto para las abreviaturas.

2003a. Hallopora sp. 1; Carrera, p. 162, pl. 8, figs. 4-7.

Etymology. The species name refers to presence of abundant acanthostyles.
Holotype. Ramose zoarium CEGH-UNC 23939.
Paratypes.
CEGH-UNC 9339 and CEGH-UNC 23940-23944.
Type horizon. Upper levels of the San Juan Formation (Darriwilian).
Type locality. Quebrada Ancha Section, Talacasto, Argentina.

Diagnosis. Encrusting colonies with branched ramose extensions; long autozooecia, having rounded apertures with distinct peristomes; 4-7 rounded to polygonal mesozooecia surrounding each autozooecial aperture, sealed by calcitic skeleton on colony surface; 7-9 large acanthostyles surrounding each autozooecial aperture, locally absent maculae indistinct, consisting of irregularly arranged macrozooecia and clusters of mesozooecia.
Description. Encrusting colonies with branched ramose extensions. Encrusting sheets 1.32-2.34 mm thick. Branched colonies 3.1-3.9 mm in diameter, with 1.8-2.5 mm wide endozones and 0.65-0.70 mm wide exozones. Autozooecia long, growing at first parallel to the branch axis in endozone; in early exozone bending towards the colony surface at angles of 21º-37° and intersecting the colony surface at angles of 72º-90°. Autozooecial apertures rounded, with well developed cingulum. Cingulum 0.030-0.055 mm thick. Autozooecial diaphragms absent or rare. Mesozooecia abundant, rounded to polygonal in cross section, 4-7 surrounding each autozooecial aperture, restricted to the outer exozone, bearing abundant diaphragms, often beaded in places of diaphragm attachment, sealed by calcitic skeleton on colony surface. Autozooecial walls granular, 0.005-0.010 mm thick in endozone; coarsely laminated, weakly to strongly thickened in exozone, 0.030-0.055 mm thick in exozone. Acanthostyles locally abundant, large, having indistinct cores, originating in the outermost exozone, 7-9 surrounding each autozooecial aperture; locally lacking. Maculae indistinct, consisting of irregularly arranged macrozooecia and clusters of mesozooecia.
Comparison. Lamottopora multispinosa sp. nov. differs from Lamottopora duncanae Ross, 1963 in having smaller branch diameters (3.1-3.9 mm vs. 4.0-6.0 mm in L. duncanae), and more abundant acanthostyles (7-9 vs. 2-3 per autozooecial apertures in L. duncanae). The new species differs also from Lamottopora pauca Brown, 1965 by having smaller branch diameters (3.1-3.9 mm vs. 5.0-8.0 mm in L. pauca) and abundant mesozooecia.

Family Trematoporidae Miller, 1889

Genus Aostipora Vinassa de Regny, 1921

1911. Trematopora Bassler, p. 267-268 (pars).
1921. Aostipora Vinassa de Regny, p. 230.

Type species. Trematopora cystata Bassler, 1911. Middle Ordovician; Estonia.

Emended diagnosis. Ramose branched colonies, with non-uniform endozones. Autozooecia long, growing parallel to the branch axis in endozones, curving more or less gently in exozones. Autozooecial cross sections polygonal in endozones, becoming rounded-polygonal to rounded in exozones. Autozooecial diaphragms absent in endozone, few to absent in exozones. Autozooecial walls thin in endozones, thickened in exozones, showing distinct fine lamination. Mesozooecia common to abundant, partitioned by both diaphragms and cysts in the same mesozooecium. Massive extrazooidal vesicles occurring sporadically or in aggregations. Acanthostyles few to abundant.
Comparison. Aostipora Vinassa de Regny, 1921, is similar to Trematopora Hall, 1852, but differs by the presence of vesicles. Aostipora is also similar to Batostoma Ulrich, 1882, but differs from it by presence of vesicles and by having rare diaphragms.
Occurrence and geological age.
This genus includes Aostipora cystata (Bassler, 1911) and Aostipora kuckersiana (Bassler, 1911), both from the Middle Ordovician of Estonia (Kuckers Shale, C2).

Aostipora sanjuanensis sp. nov. Figures 2.9-11; 3.1-5; Table 2

Table 2. Descriptive statistics of Aostipora sanjuanensis sp. nov. See text for abbreviations / estadística descriptiva de Aostipora sanjuanensis sp. nov. Ver texto para las abreviaturas.

Etymology. The species name refers to San Juan Province, Argentina.
Holotype. Branched colony CEGH-UNC 23937.
Paratype. CEGH-UNC 23938.
Type horizon. Upper levels of the San Juan Formation (Darriwilian).
Type locality.
Quebrada Ancha Section, Talacasto, Argentina.

Diagnosis. Branched colonies with narrow exozones; long autozooecia with rounded-polygonal apertures; autozooecial diaphragms absent in endozones, rare in exozones; mesozooecia rare to common; 6-7 moderately large acanthostyles surrounding each autozooecial aperture; maculae not observed; vesicles with rounded roofs common in exozones, covered by laminated skeleton, occurring in mesozooecia, locally building massive accumulations.
Description. Branched colonies 1.9-2.5 mm in diameter, with narrow exozones. Autozooecia long, growing at first parallel to the branch axis in endozone; in early exozone bending to the colony surface at angles of 22º-32° and intersecting the colony surface at angles of 42º-50°. Autozooecial apertures rounded-polygonal. Autozooecial diaphragms absent in endozones, rare in exozones. Mesozooecia rare to common, rounded to polygonal in cross section, restricted to the outer exozone, bearing abundant diaphragms and locally vesicles, often beaded in places of diaphragm attachment, sealed by calcitic skeleton on colony surface. Autozooecial walls granular, 0.005-0.010 mm thick in endozone; coarsely laminated, strongly thickened in exozone, with distinct serrated autozooecial boundaries, 0.055-0.100 mm thick in exozone. Acanthostyles abundant, moderately large, having distinct cores and narrow laminated sheaths, originating in the outermost exozone, 6-7 surrounding each autozooecial aperture. Maculae not observed. Vesicles with rounded roofs common in exozones, covered by laminated skeleton, occurring in mesozooecia, locally building massive congregations.
Comparison. Aostipora sanjuanensis sp. nov. is similar to A. cystata (Bassler, 1911) from the Middle Ordovician of Estonia, but differs in having fewer mesozooecia and larger autozooecial apertures (0.10-0.16 mm vs. 0.08-0.10 mm in A. cystata, measured from Bassler, 1911: fig. 160d).

Order Cryptostomida Vine, 1884
Suborder Phylloporinina Lavrentjeva, 1979
Family Phylloporinidae Ulrich, 1887
Phylloporinidae sp. indet Figures 3.6-8; 4.2


Figure 3. 1-5, Aostipora sanjuanensis sp. nov. 1-2, paratype CEGH-UNC 23938, branch longitudinal sections showing autozooecia with diaphragms in exozone and vesicles (arrow), scale bars 0.2 mm / paratipo, secciones longitudinales mostrando los autozoecios con diafragmas en la exozona y vesiculas (flecha), escala = 0,2mm. 3, holotype CEGH-UNC 23957, branch longitudinal section showing vesicles, scale bar = 0.5 mm / holotipo, sección longitudinal mostrando las vesículas, escala = 0,5 mm. 4-5, holotype CEGH-UNC 23937, tangential section showing autozooecia and mesozooecial apertures and acanthostyles, scale bars 0.5 mm for 4 and 0.2 mm for 5 / holotipo, sección tangencial mostrando las aberturas de los autozoecios, los mesozoecios y los acantoporos, escalas 0,5 mm en 4 y 0,2 mm en 5. 6-8, Phylloporinidae sp. indet, CEGH-UNC 23945, tangential section of the colony showing arrangement of autozooecia and autozooecial diaphragms, scale bars 1 mm for 6 and 7 and 0.5 mm for 8 / sección tangencial de la colonia mostrando la disposición de los diafragmas en los autozoecios y los mesozoecios, escala 1 mm en 6 y 7 y 0,5 mm en 8.


Figure 4. Different colony forms and sizes of the San Juan Formation bryozoans / diferentes formas y tamaño de las colonias de briozoos de la Fromación San Juan. 1, A highly ramose colony of Lamottopora multispinosa sp. nov. (CEGH-UNC 0291) from the Talacasto locality / colonia fuertemente ramosa de Lamottopora multispinosa sp. nov. de la localidad de Talacasto. 2, A reticulate colony of Phylloporinidae sp. indet (CEGH-UNC 23946) from the Cerro Viejo section / colonia reticulada de Phylloporinidae sp. indet de la sección de Cerro Viejo.

Material. Single tangential thin section CEGH-UNC 23945 and a zoarium CEGH-UNC 23946.

Description. Colonies of unknown shape consisting of slender, apparently unlinked branches. Branch bifurcation frequent. Branches 0.44-0.60 mm wide. Autozooecia tubular; arranged in 3-4 alternating rows on branches; growing parallel to the reverse branch wall for a long distance, bending sharply in exozones. Autozooecial apertures oval, 0.07-0.10 mm wide, spaced 0.31-0.41 mm from centre to centre longitudinally. Autozooecial diaphragms abundant, curved distally. Acanthostyles and heterozooecia absent.
Comparison. The present material is similar to the genus Phylloporina Ulrich, 1887, in its internal morphology. However, Phylloporina has a reticulated colony with anastomosed branches. Branch linkage could not be observed in the present material. Another similar genus, Pseudohornera Roemer, 1876, has a colony consisting of unlinked branches. However, the present species differs in budding of autozooecia along the reverse side, whereas autozooecia in Pseudohornera bud along the median lamina. The taxonomic position of this species remains uncertain until sufficient material is studied.
Occurrence and geological age. Upper levels of the San Juan Formation (Darriwilian); Cerro Viejo Section, San Juan, Argentina.

Palaeoecology

The bryozoan bearing limestones have been the subject of sedimentological (Cañas, 1995, 1999; Carrera and Astini, 1998) and paleoecological (Sánchez et al., 1993, 1996; Carrera, 1997, 2001; Sorrentino et al., 2009) studies. In general, the upper interval of the San Juan Formation is a muddy, fossil-bearing, shelf-ramp sequence dominated by skeletal-litho-clastic wackestone-packstones and nodular wackestones and mudstones. All these facies were deposited in shallow to deep subtidal conditions, low-energy settings rarely affected by storms (thin layers of biolithoclastic grainstones). A detailed study of biofacies of the San Juan Formation was carried out by Carrera et al. (1999) and Carrera (2001). The bryozoan interval considered here was placed in the demosponge biofacies. Differences among coeval stratigraphic sections of this biofacies allowed recognition of geographic differences in the distribution of the fauna (Sánchez et al. 1996; Carrera, 1997, 2000, 2001).
Taxonomic diversity of the demosponge biofacies is the highest of all the intervals analyzed in the San Juan Formation, and communities are dominated either by sponges (24 species) or articulate
brachiopods (23 species). Trilobites, bryozoans, mollusks, and pelmatozoans occur in subordinate abundance. The high diversity values, and the high number of trophic types and guilds, indicate a significant increase in ecospace utilization (Sánchez et al., 1993; Waisfeld and Sánchez, 1996; Waisfeld et al., 2003). Bryozoans represent a small portion of this highly diverse assemblage. However, they are important components, together with crinoids, in the epizoan association mainly attached to sponges (Carrera, 2000).
The top of the San Juan Formation in the Talacasto section is represented by highly fossiliferous nodular wackestones and packstones where a diverse fauna occurs, mainly including brachiopods and sponges. Huge palmate to saucer shaped sponges (Carrera, 1997, 2000) are the most distinctive feature. A significant number of ramose bryozoans (Lamottopora, Aostipora) were collected here. Complete colonies and fragments of these bryozoans appear embedded in matrix, while small bryozoan encrustations occur on sponges within the same levels (Carrera, 2000). At this locality Lamottopora shows simple to highly ramose forms three to six centimeters long (figure 4.1), while Aostipora is a stick-like bryozoan rarely bifurcating and its colonies are smaller than those of Lamottopora.
The Cerro Viejo assemblage is represented by a high percentage of elongated sponges indicative of low energy conditions; palmate to dish-shaped sponges are very rare (Carrera, 1997). This locality is interpreted as
a middle to distal ramp setting rarely affected by storm waves. Cañas (1995) described these upper levels in the Cerro Viejo section as nodular wackestones and marlstones deposited in more distal conditions.
Bryozoans in the Cerro Viejo assemblage occur in high percentage as epibionts, with bases attached to the surface of sponges (Carrera, 2000). The encrusting bryozoans usually show a laminar base and an incipient erect zoarium. Some of these specimens resemble the immature part of the ramose trepostomes recorded in the matrix. Bryozoans collected in the matrix are scarce compared with those recorded in the Talacasto section. Only a few fragments of the ramose bryozoans occur here, and two small fragments of Phylloporinidae sp. indet were recorded exclusively at this locality. The erect, unifoliate form of this zoarium resembles the reticulate morphologies. Fragmentary preservation precludes observation of linked branches; they are highly branched colonies but apparently unlinked.
Although reticulate or unlinked unifoliate colonies are generally assumed to inhabit quiet water environments, some examples show that many can also inhabit areas of moderate to high energy conditions (McKinney and Gault, 1980; McKinney and Jackson, 1989; Taylor, 1999). Anastomoses between branches undoubtedly increase the rigidity of erect bryozoans, but apparently are a poor adaptation to strong currents (Taylor, 1999). Instead, there is a good correlation between branch diameter and habitat depth (Schopf et al., 1980), with a clear bias toward more ro
bust branched morphotypes in shallower environments. Consequently, the slender and delicate form of our specimen (figure 4.2) suggests quiet water conditions, a feature that complements the evidence provided by other paleoecological and sedimentological analyses.
Bryozoans afforded elevated sites on sponges that could have offered advantageous locations in the water column. Sponges provided the primary hard substrates for attachment; no other groups bear encrusters. The large size and elevation above the substrate offered by sponges probably made them more suitable substrates than brachiopods, gastropods, crinoids or other skeletal alternatives.
Bryozoans very seldom settle directly onto sediment, unless the grains are pebble-sized or larger. Settlement often occurs on elevated substrates (Taylor, 1999; Hageman et al., 2000). Paleozoic carbonate environments, in which processes such as early diagenesis and microbial production of sediment are common features, can produce indurated sediments for attachment. However, by far the most abundant hard substrates in modern and ancient seas are those provided by dead shells and other bioclastic debris and, in some cases, by the skeletons of the living biota as well (Wilson and Palmer, 1992). In both sections sponges are colonized by epizoans, but the highly fossiliferous packstone facies of the Talacasto section also shows firm-ground surfaces that could have served as substrates for some of the large colonies found there (figure 5). The absence of such colonies in the Cerro Viejo section may be due to a more unstable sediment-water interface (nodular mudstones and wackestones) where turbidity could have been slightly greater. The absence of huge palmate to laminar sponges, commonly found with attached organisms in the Talacasto section, cannot be ruled out as another possible cause of the difference.


Figure 5. Palaeoecological reconstruction of the bryozoan-bearing interval in the Talacasto section. The association is dominated mainly by sponges and brachiopods; bryozoans occur attached directly to the substrate or onto the sponges as epizoans (Carrera, 2000) / reconstrucción paleoecológica del intervalo con briozoos en la sección de Talacasto. La asociación se encuentra principalmente dominada por esponjas y braquiópodos, los briozoos aparecen adheridos al sustrato o sobre las esponjas como epizoos (Carrera, 2000).

The composition of the Argentinean limestone assemblages corresponds to the typical tropical carbonate environment recognized worldwide for this period. Alberstad and Repetski (1989) included these carbonate environments in the sponge-algal facies belt. And more recently, Waisfeld et al. (2003) included this carbonate biotic association in the brachiopod-demosponge assemblage type. Bryozoans are rare components of these associations or are at least volumetrically negligible until the Middle Ordovician. The Argentinean bryozoans in this interval are completely subordinate to brachiopods and sponges and it seems to be the case for all Lower Ordovician occurrences in shallow carbonate environments. Unfortunately, there are very few occurrences of bryozoan-rich deposits specifying the relative abundance with respect to other associated fossils; as a consequence, quantification of Middle to Upper Ordovician associations remains uncertain. In the particular case of bryozoan carbonates, they were most common in tropical and subtropical areas in the Paleozoic (Taylor and Allison, 1998). The shallow carbonate location of the bryozoans described here is in agreement with this pattern.
Few Lower Ordovician bryozoan records appear in the literature (Taylor and Ernst, 2004) compared with the highly diverse bryozoan associations that occur later in the Ordovician and younger periods. The earliest records of bryozoans occur in the Tremadocian, and their first radiation occurs in the Floian (Taylor and Ernst, 2004).
In the Argentinean limestones, only the laminar encrusting form Nicholsonella occurs in slightly older levels (Floian) of the San Juan Formation (Carrera, 1995). The very low diversity recorded herein shows a slight decoupling with the global pattern. In the same way, the first local radiation (only three genera) attributed to the Darriwilian bryozoans described in this contribution, is minor compared with the important radiation that the phylum experienced at the same time elsewhere (Taylor and Ernst, 2004). Later, in younger units of the Precordillera (Sandbian-Katian, Carrera, 2003a, 2003b; Ernst and Carrera, 2008) bryozoans acquire levels of diversity comparable with those of the global pattern.

Acknowledgments

The authors want to thank P.D. Taylor (London) and A. Jiménez-Sánchez (Zaragoza) for their helpful reviews and comments. Appreciation is extended to F.K. McKinney (Boone) and C. Buttler (Cardiff), for comments to the bryozoan taxonomy. MGC acknowledges support from CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), PIP 2009 (112-200801-00861) and ANCYPT (Agencia Nacional de Promoción Científica y Tecnológica), PICT 2004-21857 (BID 1728 OC/AR). The authors want to thank S. Druetta for the artistic reconstruction of the bryozoan association.

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Recibido: 26 de mayo de 2009.
Aceptado:
11 de diciembre de 2009.