darriwilian bryozoans from the san juan formation (ordovician), argentine precordillera
TRANSCRIPT
Introduction
Middle and Upper Ordovician bryozoans fromthe Argentine Precordillera are conspicuous ele-ments in mixed calcareous and siliciclastic units. In
contrast, very few occurrences of bryozoans have be-en reported from the Lower Ordovician limestones(Carrera, 1995).
Keller et al. (1993) and Carrera (2003a) recordedseveral genera of trepostome and cryptostome bryo-zoans from different Middle and Upper Ordovicianunits, although no complete taxonomic descriptionswere given.
This contribution is a new report of Argentineanbryozoans and is part of a major project that involvesa taxonomic survey of Ordovician bryozoans fromArgentina. Previous records include Prasopora argenti-na Carrera, 2003b, from the Middle Ordovician units,
AMEGHINIANA (Rev. Asoc. Paleontol. Argent.) - 47 (3): 343-354. Buenos Aires, 30-09-2010 ISSN 0002-7014
©Asociación Paleontológica Argentina AMGHB2-0002-7014/10$00.00+.50
1CIPAL-CICTERRA-CONICET, Facultad de Ciencias Exactas,Físicas y Naturales, Universidad Nacional de Córdoba, Av. VelezSarsfield 299, 5000 Córdoba, Argentina. [email protected] for Geosciences, Christian-Albrechts-University of Kiel,Ludewig-Meyn-Str. 10, D-24118, Germany. [email protected]
Darriwilian bryozoans from the San Juan Formation(Ordovician), Argentine Precordillera
Marcelo G. CARRERA1 and Andrej ERNST2
Key words. Bryozoans. Ordovician. Argentina. Precordillera.
Palabras clave. Briozoos. Ordovícico. Precordillera. Argentina.
Abstract. A detailed study is carried out on the taxonomy of the bryozoan fauna from the upper levels of theSan Juan Formation (Darriwilian) in the Talacasto and Cerro Viejo sections, Argentinean Precordillera. Threebryozoan genera are described and two new species are erected: Lamottopora multispinosa sp. nov., Aostiporasanjuanensis sp. nov., and Phylloporinidae sp. indet. The paleoecological significance of the bryozoan faunais discussed. The ramose bryozoans Lamottopora and Aostipora dominate in the Talacasto section. Somecolonies probably lived attached directly to the substrate, while others occur attached to the surface of theabundant sponge fauna in the section. The majority of bryozoans at the Cerro Viejo section occur asepibionts, with bases attached to the surface of sponges. Bryozoans collected in the matrix are scarce com-pared to the Talacasto section, and comprise a few fragments of ramose bryozoans and two small fragmentsof the reticulate Phylloporinidae sp. indet were recorded exclusively in this locality. The presence of this del-icate form in Cerro Viejo is further evidence of the quiet water conditions suggested for these levels. Onlythe laminar form Nicholsonella occurs in slightly older levels (Floian) of the San Juan Formation. The low di-versity recorded here shows a slight decoupling with the global pattern. The first local radiation (three gen-era) is minor compared with the important radiation that the phylum experienced elsewhere. Resumen. BRIOZOOS DARRIWILIANOS DE LA FORMACIÓN SAN JUAN (ORDOVÍCICO) DE LA PRECORDILLERA ARGENTI-NA. 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 san-juanensis sp. nov. y Phylloporinidae sp. indet. Se discute el significado paleoecológico de la fauna de brio-zoos. Los briozoos ramosos como Lamottopora y Aoistopora dominan en la sección de Talacasto. Algunas colo-nias vivieron fijadas directamente sobre el sustrato, aunque también aparecen como epibiontes sobre la su-perficie de esponjas, que son muy abundantes en este intervalo. En la sección de Cerro Viejo los briozoos seencuentran mayoritariamente adheridos a la superficie de las esponjas, mientras que algunos fragmentos debriozoos ramosos, incluyendo dos colonias del briozoo reticulado Phylloporinidae sp. indet, aparecen en elsedimento. La presencia de estas formas delicadas en el Cerro Viejo es una nueva evidencia de la baja en-ergía del agua como fue sugerido para estos niveles. En las calizas de la Formación San Juan sólo el génerolaminar Nicholsonella aparece en niveles algo más antiguos (Floiano). Esta baja diversidad muestra un de-sajuste con las curvas globales de diversidad del grupo. La primera radiación local (géneros), descripta enesta contribución, es también significativamente menor y muestra un desacople con la diversificación queexperimenta el grupo para este mismo intervalo temporal en otras regiones.
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and two new species-Moyerella spinosa Ernst and Ca-rrera, 2008, and Phylloporina sassitoensis Ernst and Ca-rrera, 2008-based on material collected in the SassitoFormation (Upper Ordovician).
Early Ordovician limestones of the Argentine Pre-cordillera are represented by the fossiliferous SanJuan Formation. The outstanding feature of these li-mestones is the abundance of benthic fauna, mainlybrachiopods, gastropods, sponges and echinoderms.Bryozoans are very scarce; only the genus Nichol-sonella Ulrich, 1890, occurs in the middle (Floian) partof the formation (Carrera, 1995), and some treposto-mes (Carrera, 2003a) in the uppermost (Darriwilian)beds. Numerous specimens were collected from the-se upper levels, revealing a higher diversity than pre-viously recognized.
The aim of this contribution is to provide a detai-led taxonomic study of the bryozoan fauna from theupper levels of the San Juan Formation and to analy-ze the paleoecological significance of the bryozoansand their relationship with the associated fauna.
Geological setting and stratigraphy
The Lower Paleozoic of the Argentine Precordille-ra consists of a sequence of sedimentary rocks inclu-ding a 2,500 m thick sequence of Cambrian and earlyOrdovician limestones. The younger carbonate unitis represented by the San Juan Formation. This unit isa muddy fossil-bearing carbonate sequence develo-ped over a Cambrian to Lower Ordovician carbonateplatform succession (Cañas, 1999).
Earliest conodonts recorded from the formationdemonstrate that the age of the lower levels is latestTremadocian (Paltodus deltifer Zone, Keller et al.,1994; Albanesi et al., 1998). The top of the San JuanFormation has been dated at several localities. In thenorthern sections, conodonts and graptolites rende-red a lower Middle Ordovician age (Isograptus victo-riae maximus) for the transitional beds between theSan Juan Formation and the black shales of the overl-ying Gualcamayo Formation (Ortega et al., 1985),whereas elsewhere conodonts have proved a Darri-wilian 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 (Da-rriwilian). These horizons are included in the bra-chiopod Ahtiella Biozone (Herrera and Benedetto,1991) that partly corresponds to the Lenodus variabilisconodont Biozone (Albanesi and Ortega, 2002). Thebryozoan material was mainly collected at two typi-cal sections of the Argentine Precordillera where su-perb 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 theP. horridus Subzone of the L. variabilis Biozone (Al-banesi and Ortega, 2002; Albanesi et al., 2006).
Taphonomic and paleoecologic studies (Sánchezet al., 1993, 1996; Carrera, 2000) reveal the autochtho-nous nature of the fossil assemblages.
General sedimentological features of the bryozo-ans-bearing limestones indicate a shallow subtidalenvironment in middle to distal ramp settings (Ca-ñas, 1999). A detailed biofacies study of the San JuanFormation was carried out by Carrera et al. (1999) andCarrera (2001).
Materials and methods
The present investigation is based on ten samples,from which 17 thin sections were produced. All spe-cimens are deposited in the Centro de Investigacio-nes Paleobiológicas CIPAL, Universidad Nacional deCórdoba (CEGH-UNC).
Bryozoans were observed in thin sections using atransmitted light microscope. The following morpholo-gical characters were measured and used for statistics:Autozooecial Aperture Width, Autozooecial ApertureSpacing, Acanthostyle Diameter, Wall Thickness inExozone, Autozooecial Diaphragm Spacing, Meso-zooecia Width, Mesozooecial Diaphragm Spacing. Thespacing of structures is measured as a distance betweentheir centers. Additional quantitative characters are theNumber of Mesozooecia and Acanthostyles surroun-ding each autozooecial aperture. Statistics were sum-marized using arithmetic mean (X), sample standarddeviation (SD), coefficient of variation (CV), and mini-mum (MIN) and maximum (MAX) value.
Systematic palaeontology
Phylum BRYOZOA Ehrenberg, 1831Class STENOLAEMATA Borg, 1926
Order TREPOSTOMIDA Ulrich, 1882Suborder AMPLEXOPORINA Astrova, 1965Family 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 colo-nies. Autozooecia curving gently from the endozoneinto the exozone. Autozooecial apertures oval torounded with well developed cingulum. Diaphragms
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Darriwilian bryozoans from Argentina 345
absent to rare in autozooecia. Abundant mesozooeciawith abundant diaphragms, thick-walled, restrictedto exozone. Mesozooecial apertures completely sea-led by laminated skeleton. Acanthostyles abundant,often arranged around autozooecial cingulum or inmesozooecial walls. Walls thin in endozone, thicke-ned in exozone, displaying obliquely laminated mi-crostructure with dark zooecial boundaries. Maculaeand monticulae may be present.Comparison. Lamottopora Ross, 1963, differs from Eri-dotrypa Ulrich, 1893, by having abundant mesozooe-cia, well developed acanthostyles and rare to absentautozooecial diaphragms. Occurrence and age. Two species were previouslyreported: Lamottopora duncanae Ross, 1963, andLamottopora pauca Brown, 1965, from the Middle Or-dovician of Vermont and Kentucky, respectively. Thepresent material is described from the upper levels ofthe San Juan Formation (Darriwilian), San Juan Pro-vince, Argentina.
Lamottopora multispinosa sp. nov.Figures 2.1-8; 4.1; Table 1
2003a. Hallopora sp. 1; Carrera, p. 162, pl. 8, figs. 4-7.
Etymology. The species name refers to presence of abundant acan-thostyles. 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 ramo-se extensions; long autozooecia, having roundedapertures with distinct peristomes; 4-7 rounded to po-lygonal mesozooecia surrounding each autozooecialaperture, sealed by calcitic skeleton on colony surface;7-9 large acanthostyles surrounding each autozooe-cial aperture, locally absent maculae indistinct, con-sisting of irregularly arranged macrozooecia andclusters of mesozooecia.
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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.
10 m
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Darriwilian bryozoans from Argentina 347
Description. Encrusting colonies with branched ra-mose extensions. Encrusting sheets 1.32-2.34 mmthick. Branched colonies 3.1-3.9 mm in diameter,with 1.8-2.5 mm wide endozones and 0.65-0.70 mmwide exozones. Autozooecia long, growing at firstparallel to the branch axis in endozone; in early exo-zone bending towards the colony surface at angles of21º-37° and intersecting the colony surface at anglesof 72º-90°. Autozooecial apertures rounded, withwell developed cingulum. Cingulum 0.030-0.055 mmthick. Autozooecial diaphragms absent or rare. Me-sozooecia abundant, rounded to polygonal in crosssection, 4-7 surrounding each autozooecial aperture,restricted to the outer exozone, bearing abundant dia-phragms, often beaded in places of diaphragm atta-chment, sealed by calcitic skeleton on colony surface.Autozooecial walls granular, 0.005-0.010 mm thick inendozone; coarsely laminated, weakly to stronglythickened in exozone, 0.030-0.055 mm thick in exozo-ne. Acanthostyles locally abundant, large, having in-distinct cores, originating in the outermost exozone,7-9 surrounding each autozooecial aperture; locallylacking. Maculae indistinct, consisting of irregularlyarranged macrozooecia and clusters of mesozooecia. Comparison. Lamottopora multispinosa sp. nov. differsfrom Lamottopora duncanae Ross, 1963 in having smallerbranch 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 newspecies differs also from Lamottopora pauca Brown, 1965by 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 Ordovi-cian; Estonia.
Emended diagnosis. Ramose branched colonies,with non-uniform endozones. Autozooecia long, gro-wing parallel to the branch axis in endozones, cur-ving more or less gently in exozones. Autozooecialcross sections polygonal in endozones, becomingrounded-polygonal to rounded in exozones.Autozooecial diaphragms absent in endozone, few toabsent in exozones. Autozooecial walls thin in endo-zones, thickened in exozones, showing distinct finelamination. Mesozooecia common to abundant, par-titioned by both diaphragms and cysts in the samemesozooecium. Massive extrazooidal vesicles occu-rring sporadically or in aggregations. Acanthostylesfew to abundant.Comparison. Aostipora Vinassa de Regny, 1921, is si-milar to Trematopora Hall, 1852, but differs by the pre-sence of vesicles. Aostipora is also similar to BatostomaUlrich, 1882, but differs from it by presence of vesi-cles and by having rare diaphragms. Occurrence and geological age. This genus includesAostipora cystata (Bassler, 1911) and Aostipora kucker-siana (Bassler, 1911), both from the Middle Ordo-vician of Estonia (Kuckers Shale, C2).
Aostipora sanjuanensis sp. nov.Figures 2.9-11; 3.1-5; Table 2
Etymology. The species name refers to San Juan Province, Argen-tina.Holotype. Branched colony CEGH-UNC 23937.Paratype. CEGH-UNC 23938.Type horizon. Upper levels of the San Juan Formation (Darri-wilian).Type locality. Quebrada Ancha Section, Talacasto, Argentina.
Diagnosis. Branched colonies with narrow exozones;long autozooecia with rounded-polygonal apertures;autozooecial diaphragms absent in endozones, rarein exozones; mesozooecia rare to common; 6-7 mode-rately large acanthostyles surrounding each autozoo-ecial aperture; maculae not observed; vesicles withrounded roofs common in exozones, covered by la-
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Figure 2. 1-8, Lamottopora multispinosa sp. nov. 1, paratype (CEGH-UNC 9339), oblique branch section, scale bar = 1 mm / paratipo, sec-tion transversal de una rama, escala = 1 mm. 2, paratype CEGH-UNC 23940, longitudinal branch section, scale bar = 1 mm / paratipo, sec-ción longitudinal de una rama, escala = 1 mm. 3-4, paratype CEGH-UNC 23941, longitudinal sections of encrusting overgrowths, showingautozooecia and mesozooecia, scale bars 1 mm for 3 and 0.2 mm for 4 / secciones longitudinales de sobrecrecimientos incrustantes, mostran-do autozoecios y mesozoecios, escala 1 mm en 3 y 0,2 mm en 4. 5, holotype CEGH-UNC 23939, longitudinal section showing mesozooecial di-aphragms and laminated skeleton of the exozone, scale bar = 0.2 mm / holotipo, sección longitudinal mostrando los diafragmas de los meso-zoecios y el esqueleto laminado en la exozona, escala= 0,2 mm. 6, paratype CEGH-UNC 23940, longitudinal section showing mesozooecia andlaminated skeleton of the exozone, scale bar = 0.2 mm / paratipo, sección longitudinal mostrando los mesozoecios y el esqueleto laminado en laexozona, 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 sectionshowing autozooecial apertures and acanthostyles, scale bar = 0.2 mm / holotipo, sección tangencial mostrando las aberturas de los autozoe-cios y los acantoporos, escala= 0,2 mm. 9-11, Aostipora sanjuanensis sp. nov., holotype CEGH-UNC 23937. 9, branch longitudinal section, scalebar = 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 / seccióntangencial de una rama, escala = 1 mm.
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minated skeleton, occurring in mesozooecia, locallybuilding massive accumulations.
Description. Branched colonies 1.9-2.5 mm in dia-meter, with narrow exozones. Autozooecia long,growing at first parallel to the branch axis in endo-zone; in early exozone bending to the colony surfaceat angles of 22º-32° and intersecting the colony sur-face at angles of 42º-50°. Autozooecial aperturesrounded-polygonal. Autozooecial diaphragms ab-sent in endozones, rare in exozones. Mesozooecia ra-re to common, rounded to polygonal in cross sec-tion, restricted to the outer exozone, bearing abun-dant diaphragms and locally vesicles, often beadedin places of diaphragm attachment, sealed by calciticskeleton on colony surface. Autozooecial walls gra-nular, 0.005-0.010 mm thick in endozone; coarselylaminated, strongly thickened in exozone, with dis-tinct serrated autozooecial boundaries, 0.055-0.100mm thick in exozone. Acanthostyles abundant, mo-derately large, having distinct cores and narrow la-minated sheaths, originating in the outermost exo-zone, 6-7 surrounding each autozooecial aperture.Maculae not observed. Vesicles with rounded roofscommon in exozones, covered by laminated skele-ton, occurring in mesozooecia, locally building mas-sive congregations.
Comparison. Aostipora sanjuanensis sp. nov. is similarto A. cystata (Bassler, 1911) from the Middle Ordovi-cian of Estonia, but differs in having fewer mesozoo-ecia and larger autozooecial apertures (0.10-0.16 mmvs. 0.08-0.10 mm in A. cystata, measured from Bassler,1911: fig. 160d).
Order CRYPTOSTOMIDA Vine, 1884Suborder PHYLLOPORININA Lavrentjeva, 1979
Family PHYLLOPORINIDAE Ulrich, 1887Phylloporinidae sp. indet
Figures 3.6-8; 4.2
Material. Single tangential thin section CEGH-UNC 23945 and azoarium CEGH-UNC 23946.
Description. Colonies of unknown shape consistingof slender, apparently unlinked branches. Branchbifurcation frequent. Branches 0.44-0.60 mm wide.Autozooecia tubular; arranged in 3-4 alternatingrows on branches; growing parallel to the reversebranch wall for a long distance, bending sharply inexozones. Autozooecial apertures oval, 0.07-0.10mm wide, spaced 0.31-0.41 mm from centre to cen-tre longitudinally. Autozooecial diaphragms abun-dant, curved distally. Acanthostyles and heterozoo-ecia absent.
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N X SD CV MIN MAX
Macrozooecial Aperture Width, mm 35 0.21 0.018 8.78 0.18 0.24
Autozooecial Aperture Width, mm 50 0.13 0.018 13.18 0.10 0.17
Aperture Spacing, mm 75 0.34 0.039 11.46 0.24 0.43
Mesozooecia Diameter, mm 75 0.11 0.033 29.26 0.05 0.22
Acanthostyle Diameter, mm 25 0.04 0.005 13.18 0.03 0.05
Acanthostyles per Aperture 10 8.1 0.994 12.28 7.0 10.0
Mesozooecia per Aperture 30 6.2 1.117 18.11 5.0 9.0
Mesozooecial Diaphragms Spacing, mm 40 0.10 0.026 26.25 0.04 0.15
Wall Thickness in Exozone, mm 12 0.039 0.008 19.50 0.030 0.055
Table 1. Descriptive statistics of Lamottopora multispinosa sp. nov. See text for abbreviations. / estadística descriptiva de Lamottopora mul-tispinosa sp. nov. Ver texto para las abreviaturas.
Figure 3. 1-5, Aostipora sanjuanensis sp. nov. 1-2, paratype CEGH-UNC 23938, branch longitudinal sections showing autozooecia with di-aphragms in exozone and vesicles (arrow), scale bars 0.2 mm / paratipo, secciones longitudinales mostrando los autozoecios con diafragmas enla exozona y vesiculas (flecha), escala = 0,2mm. 3, holotype CEGH-UNC 23957, branch longitudinal section showing vesicles, scale bar = 0.5mm / holotipo, sección longitudinal mostrando las vesículas, escala = 0,5 mm. 4-5, holotype CEGH-UNC 23937, tangential section showing au-tozooecia and mesozooecial apertures and acanthostyles, scale bars 0.5 mm for 4 and 0.2 mm for 5 / holotipo, sección tangencial mostran-do 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 6and 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 1mm en 6 y 7 y 0,5 mm en 8.
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Comparison. The present material is similar to thegenus Phylloporina Ulrich, 1887, in its internal morp-hology. However, Phylloporina has a reticulated colo-ny with anastomosed branches. Branch linkage couldnot be observed in the present material. Another si-milar genus, Pseudohornera Roemer, 1876, has a colo-ny consisting of unlinked branches. However, thepresent species differs in budding of autozooeciaalong the reverse side, whereas autozooecia in Pseu-dohornera bud along the median lamina. The taxono-mic position of this species remains uncertain untilsufficient material is studied.
Occurrence and geological age. Upper levels ofthe San Juan Formation (Darriwilian); Cerro ViejoSection, San Juan, Argentina.
Palaeoecology
The bryozoan bearing limestones have been thesubject of sedimentological (Cañas, 1995, 1999; Ca-rrera and Astini, 1998) and paleoecological (Sánchezet al., 1993, 1996; Carrera, 1997, 2001; Sorrentino et al.,2009) studies. In general, the upper interval of theSan Juan Formation is a muddy, fossil-bearing,shelf-ramp sequence dominated by skeletal-litho-clastic wackestone-packstones and nodular wackes-tones and mudstones. All these facies were deposi-ted in shallow to deep subtidal conditions, low-energy settings rarely affected by storms (thin layersof bio-lithoclastic grainstones). A detailed study ofbiofacies of the San Juan Formation was carried outby Carrera et al. (1999) and Carrera (2001). The bryo-zoan interval considered here was placed in the de-mosponge biofacies. Differences among coeval stra-tigraphic sections of this biofacies allowed recogni-tion of geographic differences in the distribution ofthe fauna (Sánchez et al. 1996; Carrera, 1997, 2000,2001).
Taxonomic diversity of the demosponge biofa-cies is the highest of all the intervals analyzed inthe San Juan Formation, and communities are do-minated either by sponges (24 species) or articulate
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N X SD CV MIN MAX
Autozooecial Aperture Width, mm 35 0.12 0.013 10.85 0.10 0.16
Aperture Spacing, mm 35 0.24 0.029 11.74 0.18 0.30
Acanthostyle Diameter, mm 20 0.036 0.006 15.76 0.025 0.050
Mesozooecia Diameter, mm 15 0.064 0.021 32.91 0.030 0.105
Wall Thickness in Exozone, mm 10 0.072 0.016 21.88 0.055 0.100
Table 2. Descriptive statistics of Aostipora sanjuanensis sp. nov. See text for abbreviations / estadística descriptiva de Aostipora sanjuanen-sis sp. nov. Ver texto para las abreviaturas.
Figure 4. Different colony forms and sizes of the San JuanFormation bryozoans / diferentes formas y tamaño de las colonias debriozoos de la Fromación San Juan. 1, A highly ramose colony ofLamottopora multispinosa sp. nov. (CEGH-UNC 0291) from the Ta-lacasto locality / colonia fuertemente ramosa de Lamottopora mul-tispinosa sp. nov. de la localidad de Talacasto. 2, A reticulate colonyof Phylloporinidae sp. indet (CEGH-UNC 23946) from the CerroViejo section / colonia reticulada de Phylloporinidae sp. indet de la sec-ción de Cerro Viejo.
Darriwilian bryozoans from Argentina 351
brachiopods (23 species). Trilobites, bryozoans,mollusks, and pelmatozoans occur in subordinateabundance. The high diversity values, and the highnumber of trophic types and guilds, indicate a sig-nificant increase in ecospace utilization (Sánchez etal., 1993; Waisfeld and Sánchez, 1996; Waisfeld etal., 2003). Bryozoans represent a small portion ofthis highly diverse assemblage. However, they areimportant components, together with crinoids, inthe epizoan association mainly attached to sponges(Carrera, 2000).
The top of the San Juan Formation in the Tala-casto section is represented by highly fossiliferousnodular wackestones and packstones where a diver-se fauna occurs, mainly including brachiopods andsponges. Huge palmate to saucer shaped sponges(Carrera, 1997, 2000) are the most distinctive feature.A significant number of ramose bryozoans (La-mottopora, Aostipora) were collected here. Completecolonies and fragments of these bryozoans appearembedded in matrix, while small bryozoan encrusta-tions occur on sponges within the same levels (Carre-ra, 2000). At this locality Lamottopora shows simple tohighly ramose forms three to six centimeters long (fi-gure 4.1), while Aostipora is a stick-like bryozoan ra-rely bifurcating and its colonies are smaller than tho-se of Lamottopora.
The Cerro Viejo assemblage is represented by ahigh percentage of elongated sponges indicative of lowenergy conditions; palmate to dish-shaped sponges arevery rare (Carrera, 1997). This locality is interpreted as
a middle to distal ramp setting rarely affected by stormwaves. Cañas (1995) described these upper levels in theCerro Viejo section as nodular wackestones and marls-tones deposited in more distal conditions.
Bryozoans in the Cerro Viejo assemblage occur inhigh percentage as epibionts, with bases attached tothe surface of sponges (Carrera, 2000). The encrus-ting bryozoans usually show a laminar base and anincipient erect zoarium. Some of these specimens re-semble the immature part of the ramose trepostomesrecorded in the matrix. Bryozoans collected in thematrix are scarce compared with those recorded inthe Talacasto section. Only a few fragments of the ra-mose bryozoans occur here, and two small fragmentsof Phylloporinidae sp. indet were recorded exclusi-vely at this locality. The erect, unifoliate form of thiszoarium resembles the reticulate morphologies. Frag-mentary preservation precludes observation of lin-ked branches; they are highly branched colonies butapparently unlinked.
Although reticulate or unlinked unifoliate colo-nies are generally assumed to inhabit quiet water en-vironments, some examples show that many can alsoinhabit areas of moderate to high energy conditions(McKinney and Gault, 1980; McKinney and Jackson,1989; Taylor, 1999). Anastomoses between branchesundoubtedly increase the rigidity of erect bryozoans,but apparently are a poor adaptation to strong cu-rrents (Taylor, 1999). Instead, there is a good correla-tion between branch diameter and habitat depth(Schopf et al., 1980), with a clear bias toward more ro-
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Figure 5. Palaeoecological reconstruction of the bryozoan-bearing interval in the Talacasto section. The association is dominated main-ly by sponges and brachiopods; bryozoans occur attached directly to the substrate or onto the sponges as epizoans (Carrera, 2000) / re-construcción paleoecológica del intervalo con briozoos en la sección de Talacasto. La asociación se encuentra principalmente dominada por esponjas ybraquiópodos, los briozoos aparecen adheridos al sustrato o sobre las esponjas como epizoos (Carrera, 2000).
M.G. Carrera and A. Ernst352
bust branched morphotypes in shallower environ-ments. Consequently, the slender and delicate formof our specimen (figure 4.2) suggests quiet water con-ditions, a feature that complements the evidence pro-vided by other paleoecological and sedimentologicalanalyses.
Bryozoans afforded elevated sites on sponges thatcould have offered advantageous locations in the wa-ter column. Sponges provided the primary hardsubstrates for attachment; no other groups bear en-crusters. The large size and elevation above the subs-trate offered by sponges probably made them moresuitable substrates than brachiopods, gastropods, cri-noids or other skeletal alternatives.
Bryozoans very seldom settle directly onto sedi-ment, unless the grains are pebble-sized or larger.Settlement often occurs on elevated substrates(Taylor, 1999; Hageman et al., 2000). Paleozoic car-bonate environments, in which processes such asearly diagenesis and microbial production of sedi-ment are common features, can produce induratedsediments for attachment. However, by far themost abundant hard substrates in modern and an-cient seas are those provided by dead shells and ot-her bioclastic debris and, in some cases, by the ske-letons of the living biota as well (Wilson andPalmer, 1992). In both sections sponges are coloni-zed by epizoans, but the highly fossiliferous packs-tone facies of the Talacasto section also shows firm-ground surfaces that could have served as substra-tes for some of the large colonies found there (figu-re 5). The absence of such colonies in the CerroViejo section may be due to a more unstable sedi-ment-water interface (nodular mudstones and wac-kestones) where turbidity could have been slightlygreater. The absence of huge palmate to laminarsponges, commonly found with attached orga-nisms in the Talacasto section, cannot be ruled outas another possible cause of the difference.
The composition of the Argentinean limestoneassemblages corresponds to the typical tropical car-bonate environment recognized worldwide for thisperiod. Alberstad and Repetski (1989) included the-se carbonate environments in the sponge-algal fa-cies belt. And more recently, Waisfeld et al. (2003)included this carbonate biotic association in thebrachiopod-demosponge assemblage type.Bryozoans are rare components of these associa-tions or are at least volumetrically negligible untilthe Middle Ordovician. The Argentinean bryozo-ans in this interval are completely subordinate tobrachiopods and sponges and it seems to be the ca-se for all Lower Ordovician occurrences in shallowcarbonate environments. Unfortunately, there arevery few occurrences of bryozoan-rich depositsspecifying the relative abundance with respect to
other associated fossils; as a consequence, quantifi-cation of Middle to Upper Ordovician associationsremains uncertain. In the particular case of bryozo-an carbonates, they were most common in tropicaland subtropical areas in the Paleozoic (Taylor andAllison, 1998). The shallow carbonate location ofthe bryozoans described here is in agreement withthis pattern.
Few Lower Ordovician bryozoan records appearin the literature (Taylor and Ernst, 2004) comparedwith the highly diverse bryozoan associations thatoccur later in the Ordovician and younger periods.The earliest records of bryozoans occur in the Trema-docian, and their first radiation occurs in the Floian(Taylor and Ernst, 2004).
In the Argentinean limestones, only the laminarencrusting form Nicholsonella occurs in slightly olderlevels (Floian) of the San Juan Formation (Carrera,1995). The very low diversity recorded herein showsa slight decoupling with the global pattern. In the sa-me way, the first local radiation (only three genera)attributed to the Darriwilian bryozoans described inthis contribution, is minor compared with the impor-tant radiation that the phylum experienced at the sa-me time elsewhere (Taylor and Ernst, 2004). Later, inyounger units of the Precordillera (Sandbian-Katian,Carrera, 2003a, 2003b; Ernst and Carrera, 2008) bryo-zoans acquire levels of diversity comparable withthose of the global pattern.
Acknowledgments
The authors want to thank P.D. Taylor (London) and A.Jiménez-Sánchez (Zaragoza) for their helpful reviews and com-ments. Appreciation is extended to F.K. McKinney (Boone) and C.Buttler (Cardiff), for comments to the bryozoan taxonomy. MGCacknowledges support from CONICET (Consejo Nacional de In-vestigaciones Científicas y Técnicas), PIP 2009 (112-200801-00861)and ANCYPT (Agencia Nacional de Promoción Científica y Tec-nológica), PICT 2004-21857 (BID 1728 OC/AR). The authors wantto thank S. Druetta for the artistic reconstruction of the bryozoanassociation.
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Recibido: 26 de mayo de 2009.Aceptado: 11 de diciembre de 2009.
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