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Post-glacial Hirnantian (Upper Ordovician) bryozoans from Western Argentina: Implications for...
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POST-GLACIAL HIRNANTIAN (UPPER ORDOVICIAN) BRYOZOANS FROM 1
WESTERN ARGENTINA: IMPLICATIONS FOR SURVIVAL AND EXTINCTION 2
PATTERNS 3
KAREN HALPERN1,2 AND MARCELO G. CARRERA1,2 4
1Centro de Investigaciones Ciencias de la Tierra, Consejo Nacional de Ciencia y Técnica y 5
Universidad Nacional de Córdoba. Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, 6
Argentina. [email protected]: [email protected] 7
2Centro de Investigaciones Paleobiológicas: Facultad de Ciencias Exactas Físicas y 8
Naturales, Universidad Nacional de Córdoba. Av. Vélez Sarsfield 299, 5000, Córdoba, 9
Argentina. 10
Páginas: 26, Figuras: 3, Tablas: 2. 11
Cabezal: HALPERN AND CARRERA: HIRNANTIAN BRYOZOA FROM ARGENTINA 12
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Abstract. Two bryozoan taxa occurring in the Hirnatian (Upper Ordovician) deposits in 13
western Argentina document a first postglacial community associated with a mid- to high- 14
latitude brachiopod assemblage, known as the typical Hirnantia fauna, in the Argentine 15
Precordillera. Helopora fragilis (Hall, 1852) and an indeterminate phyloporinid occur 16
within a thin but conspicuous shell bed that overlies diamictitic deposits from the lower 17
member of the Don Braulio Formation. The abundance of well preserved specimens of 18
bryozoans together with the dominance of suspension feeders suggests a mid-shelf setting 19
(offshore transition) with an intermediate to low sedimentation rate, low turbidity, and 20
nutrient-rich conditions. Hirnantian bryozoan assemblages identified from tropical and 21
subtropical regions are rather rare, and this assemblage represents a first high-latitude 22
Hirnantian record. The low diversity of bryozoans may be related to high-latitude location 23
of Argentina during the Late Ordovician. Helopora occurs mainly in Laurentia with several 24
species occurring in the Katian and with a few Hirnatian occurrences in Gondwana. This 25
broad distribution may imply that this genus had a broad environmental tolerance. It is one 26
of the genera that successfully crossed the Ordovician/Silurian boundary. However, 27
although this genus shows a post-extinction diversification and a more widespread 28
distribution in Laurentia (with numerous species recorded in the Silurian and Devonian of 29
Canada, Russia and China), it became extinct in Gondwana. We hypothesize that 30
environmental conditions after the Ordovician/Silurian boundary may have prevented the 31
settlement of bryozoans and caused the extinction of Helopora in the Argentine 32
Precordillera. 33
Keywords: Bryozoans. Upper Ordovician. Hirnantian. Argentine Precordillera. 34
Paleoecology. Paleobiogeography. 35
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Resumen. BRYOZOOS POSGLACIALES DEL HIRNANTIANO (ORDOVÍCICO 36
SUPERIOR) DEL OESTE DE ARGENTINA. Se describen dos briozoos del Ordovícico 37
Hirnantiano de la Formación Don Braulio en la Precordillera de San Juan, Argentina. Estas 38
formas se encuentran asociadas a la típica fauna de Hirnantia y comprenden la primera 39
asociación fosilífera registrada luego del evento glacial registrado en la Precordillera 40
Argentina. Helopora fragilis Hall, 1852 y un Phyloporinidae indeterminado aparecen 41
asociados en concentraciones fosilíferas registradas justo por encima de los depósitos de 42
diamictitas de la Formación Don Braulio. Las características paleoecológicas de la 43
asociación sugieren una depositación en ambientes de plataforma media con tasas 44
intermedias a bajas de sedimentación y baja turbidez. Los pocos registros de briozoos 45
Hirnantianos reconocidos hasta ahora provienen de latitudes tropicales a subtropicales, 46
mientras que la fauna de la Precordillera corresponde al primer registro de briozoos 47
Hirnantianos en altas latitudes. La baja diversidad que muestra esta asociación de briozoos 48
en Argentina podría ser atribuida, como principal factor, a bajas temperaturas como 49
consecuencia de su posición latitudinal. El género Helopora muestra una distribución 50
principalmente lauréntica con varias especies en el Katiano y luego muy pocos registros en 51
el Hirnantiano principalmente en localidades gondwánicas. Es uno de los pocos géneros 52
que cruzan exitosamente el límite Ordovícico/Silúrico, mostrando una diversificación post-53
extinción con numerosas especies, exhibiendo una distribución más amplia. A pesar de ser 54
un sobreviviente de la extinción, las condiciones ambientales en la transición Ordovícico-55
Silúrico de la Precordillera Argentina no permitieron su desarrollo en sedimentos más 56
jóvenes. 57
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Palabras clave: Briozoos. Ordovícico Superior. Hirnantiano. Precordillera Argentina. 58
Paleoecología. Paleobiogeografía. 59
INTRODUCTION 60
AVAILABLE taxonomic and biogeographic data on Ordovician bryozoan from Argentina 61
have increased remarkably over the past few years, including new occurrences found in the 62
Lower and Middle Ordovician units of the Argentine Precordillera (Carrera, 1995, 2003; 63
Carrera and Ernst, 2010). Recently published Upper Ordovician occurrences (Ernst and 64
Carrera, 2008; Ernst and Carrera, 2012) provide a more complete view on distribution 65
patterns of the Ordovician bryozoans. 66
On one hand, although Hirnantian deposits were intensely explored (Benedetto, 67
1986, 1990; Sánchez et al., 1991, 1993; Astini and Benedetto, 1992), bryozoans were 68
considered to be rather rare components of benthic assemblages in such deposits. On the 69
other hand, abundant bryozoan specimens were found in new fossil collections and their 70
occurrence has been briefly described by Carrera and Halpern (2011). This contribution 71
describes two bryozoans from the Upper Ordovician (Hirnantian) of the Don Braulio 72
Formation exposed in the Argentine Precordillera. These bryozoans are associated with 73
mid- to high-latitude brachiopods assemblages that belong to typical components of the 74
Hirnantia fauna (defined by Rong and Harper 1988, [for an updated biogeographic 75
interpretation of the Hirnantia Fauna see Benedetto et al., 2013]) and belong to a relatively 76
early community state that flourished after the glacial event (Carrera and Halpern, 2011). 77
Bryozoans can be pioneer organisms that rapidly colonize recently exposed glacial-78
marine coasts when ice retreats (Barnes and Griffiths, 2008), and it is possible that similar 79
pioneer-like organisms colonized habitats exposed after the retreat of ice during the 80
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Hirnantian in the Precordillera. Their presence may have facilitated the development of the 81
later postglacial associations. Available data suggest a nutrient-rich environment occurring 82
in mid-shelf settings (i.e., between storm wave base and fair-weather wave base), partly 83
coinciding with previous paleoecological interpretations (Sánchez et al., 1991). This 84
bryozoan assemblage is one of the few examples of high-latitude Ordovician bryozoans and 85
has significant paleoecological and paleobiogeographical implications. The goal of this 86
study is to describe bryozoan taxa surviving the end-Ordovician extinction in Gondwana 87
and assess their paleobiogeographic and paleoecologic patterns in the aftermath of this 88
extinction. 89
GEOLOGICAL SETTING AND STRATIGRAPHY 90
Bryozoans described in this study were found at the type section of the Don Braulio 91
Formation (Baldis et al., 1982) on the eastern flank of the Sierra de Villicum in San Juan 92
Province (Fig. 1.1–2). Sedimentological studies at this section have been carried out by 93
Astini (1992, 1993, 2001), Astini and Buggisch (1993), and Buggisch and Astini (1993). 94
Figure 1 about here 95
The Don Braulio Formation uncomformably overlies the La Cantera Formation 96
(Darriwillian–Katian) and underlies the Mogotes Negros Formation (Ludlovian–Pridolian) 97
(Baldis et al., 1982). The Don Braulio Formation consists of fine-grained sandstones and 98
siltstones that conformably overlie glaciogenic deposits (Peralta and Carter, 1990; Astini 99
1999). The Hirnantian age of this unit has been established on the basis of Normalograptus 100
persculptus Zone (Brussa et al., 2003) and brachiopods Hirnantia sagittifera, Dalmanella 101
cf. testudinaria, Eostropheodonta aff. hirnantiensis, Paromalomena polonica, Cliftonia 102
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oxoplecioides and Plectothyrella crassicosta, belonging to the cosmopolitan Hirnantia 103
fauna (Benedetto, 1986). 104
The Don Braulio Formation diamictite represents a glacial-marine deposit on the 105
basis of marked erosional base, striated pavements, and presence of dropstones (Fig. 1.3; 106
Peralta and Carter, 1990; Buggisch and Astini, 1993). Its lateral and vertical extent is 107
restricted; the diamictite is continuously developed for 3 km and is 46 m-thick at 108
maximum. 109
The Don Braulio Formation can be divided into two members (Astini and Buggisch, 110
1993). The lower member begins with approximately 10–15 m-thick mud-supported 111
diamictites that frequently contain facetted and striated clasts. These diamictites alternate 112
with channel-like deposits filled with sandstones and grain-supported conglomerates. The 113
upper member starts with a conspicuous pavement ˗interpreted as a ravinement surface by 114
Astini and Benedetto (1992)˗ with abundant macrofossils and rounded clasts. The upper 115
member consists of transgressive, shallow subtidal, fine-grained siliciclastic sediments 116
(Fig. 1.4). The succession begins with 10–12 m-thick greenish bioturbated mudstones and 117
silty sandstones. Some deposits contain carbonate cement and common macrofossils 118
(bryozoans, brachiopods, trilobites, bivalves, crinoids, gastropods, and graptolites). The 119
overlying 7–12 meters of yellowish fine-grained siltstones rarely contain benthic shells but 120
are rich in graptolites. A single oolitic ferruginous sandstones caps the sequence, showing a 121
sudden shallowing-upward environment. Three ferruginous layers interbedded with the 122
yellowish siltstones occur in other sections (e.g., La Pola section located 2 km to the south). 123
The latter were interpreted to be shallow marine bars, probably representing a parasequence 124
boundary (Astini, 1992). 125
7
MATERIALS AND METHODS 126
The stratigraphic column was examined at the type section and at lateral coeval 127
sections; according to the stratigraphic distribution of the fossils, samples were obtained at 128
accurately defined 30 cm intervals. In addition, taphonomic and lithological features of the 129
section were considered, confirming previous environmental interpretations. 130
The bryozoans were collected from fossiliferous lenticular siltstone beds and from 131
an underlying coquinal bed located at the base of the upper member of the Don Braulio 132
Formation (Fig. 1.4). Bryozoan taxonomic features were studied in randomly-oriented thin-133
sections using a binocular transmitted light microscope. Macroscopic samples were 134
examined with a binocular microscope to identify the fauna associated with bryozans to the 135
lowest taxonomic rank. 136
Counts used for paleoecological interpretations were made on the macroscopic 137
surface of samples, considering complete colonies, articulated specimens or isolated and 138
fragmented valves with a recognizable hinge. These counts were used to calculate 139
percentages of taxa occurring in the type section (see Appendix 1). 140
Photographed material is housed at the repository of the Centro de Investigaciones 141
Paleobiológicas (CIPAL), Facultad de Ciencias Exactas Físicas y Naturales, Universidad 142
Nacional de Córdoba, under the prefix CEGH-UNC. 143
SYSTEMATIC PALEONTOLOGY 144
Phylum BRYOZOA Ehrenberg, 1831 145
Class STENOLAEMATA Borg, 1926 146
Order CRYPTOSTOMIDA Vine, 1884 147
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Suborder RHABDOMESINA Astrova and Morozova, 1956 148
Family ARTHROSTYLIDAE Ulrich, 1882 149
Genus Helopora Hall, in Silliman, Silliman and Dana 1851 150
Type species. Helopora fragilis Hall, 1852; original designation.!Lower Silurian, North 151
America. 152
Helopora fragilis Hall, 1852 153
Figure 2.1–2.5 154
1851. Helopora fragilis Hall, p. 398 155
1852. Helopora fragilis Hall, p. 44 156
1983. Helopora fragilis Hall, 1852; Blake, p. 559, figs. 274, 1a–g 157
2007. Helopora fragilis Hall, 1852; Suttner and Ernst, p. 1500, pl. 6, figs. 1–2 158
Material. Four complete zoaria and several fragmented colonies, CEGH-UNC 25726–28 159
and 25730. 160
Description. Stick-like cylindrical colonies 1.7–2.3 mm in diameter. Zoarium erect; no 161
branching forms were found. The specimens have oval autozooecial apertures with a 162
maximum diameter of 0.22 mm and 0.12 mm wide (Table 1); these are arranged in 163
irregular rows. There are 8–9 autozoecia per 2 mm longitudinally and 16 diagonally over 164
the same distance. Diaphragms in autozoecia absent. Laminated zoecial walls in axial 165
region with thin weakly defined linear axis. Autozoecia extending from the base or the 166
median axis. Zoecial divergence from axis ranging from 25–30° near central axis. Living 167
chambers oriented from 60–90° to zoarial surface. Mesopores polygonal, small, and 0.03–168
9
0.12 mm in diameter, some with diafragms. Acanthostyles long, abundant, three to five 169
surrounding autozoecial apertures arising near the base of exozone. 170
Table 1 about here 171
Discussion. The main descriptive features of the specimens agree with those of the genus 172
Helopora Hall, particularly of H. fragilis Hall, 1852 (in Ulrich,1890; Blake, 1983), which 173
has the same zoecial dimensions and arrangement. 174
H. restricta (Goryunova, 1985) from the Lower Silurian of Estonia is very similar to 175
H. fragilis but the autozooecial apertures are more separated in the Estonian species. 176
H. fragilis described by Suttner and Ernst (2007) from the Upper Ordovician of 177
India has a very similar transversal section and the dimensions of autozoecia are similar. 178
Unfortunately, a better comparison cannot be made because the longitudinal sections have 179
not been preserved in the Indian material. 180
The genus Helopora was previously mentioned by Rusconi (1956) as a form found 181
in the Ordovician of Mendoza in the southern sector of the Argentine Precordillera. The 182
colony identified by Rusconi is a ramose bifurcated form that can be included in any kind 183
of Ordovician bryozoan genera. Unfortunately, no description or photograph was provided 184
by Rusconi. 185
Figure 2 about here 186
The colonies look similar to those described as Moyerella by Ernst and Carrera 187
(2008) in the late Katian Sassito Formation. This is not surprising in light of the apparently 188
close phylogenetic relationship between these genera (but see Ernst and Carrera, 2008). 189
However, Moyerella spinata Ernst and Carrera, 2008, shows autozoecia disposed in regular 190
diagonal rows and conspicuous tectitozoecia, which are absent in Helopora sp. 191
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Order Fenestrida, Elias and Condras, 1957 192
Suborder PHYLLOPORININA Lavrentjeva, 1979 193
Family PHYLLOPORINIDAE Ulrich, 1890 194
Phylloporinidae sp. indet. 195
Figure 3.1–3.2 196
Material. Two fragmented zoaria, CEGH-UNC 25728 and 25729 and several highly 197
fragmented branches. 198
Description. The specimens are represented by colonies of an unknown complete 199
shape and consist of slender, apparently unlinked branches. Branch bifurcation frequent. 200
Branches 0.45–0.55 mm wide. Autozooecia tubular, apertures oval, 0.20–0.24 mm in 201
maximum diameter, 0.9–0.12 mm wide, spaced 0.31–0.41 mm from centre to centre 202
longitudinally. Heterozoecia scarce, polygonal, 0.05–0.08 mm in maximum diameter. 203
Autozooecial diaphragms abundant. Acanthostyles absent. Small fragmented zoaria occur 204
associated with the previous forms in the longitudinal thin sections (Fig 3.2). They are 205
slender branches (0.50 to 0.7 mm) with three to four rows of autozoecia, 0.19 to 0.24 mm 206
in maximum diameter. 207
Figure 3 about here 208
Discussion. Phylloporinid specimens have been previously reported in Argentina in older 209
units (Phylloporina sassitoensis Ernst and Carrera, 2008 and Phylloporinidae sp. indet. 210
Carrera and Ernst, 2010). The preservation of the specimens as very disarticulated and 211
abraded fragments precludes a precise taxonomic placement. 212
11
The small slender fragments (Fig. 3.2) show forms and structures that can be also related to 213
commonly branched or bifurcate Arthrostylid genera: Nematopora, Heminematopora, 214
Pseudonematopora, or Glauconomella that could fit with the few characteristics reveal in 215
the thin-sections. However, preservation with only fragmented longitudinal sections 216
prevents a more precise identification, as a consequence, we prefer to keep these associated 217
fragments as small branches of phylloporinids. 218
TAPHONOMY AND PALEOECOLOGY 219
The lowermost fossil shell bed, above the diamitic deposits, consists primarily of 220
brachiopods, with less frequent bivalves and trilobites, and resembles the Hirnantia-221
Modiolopsis community, a shallow water fauna described from the Don Braulio Formation 222
by Sánchez et al. (1991). The assemblage also contains very common fragments and entire 223
colonies of bryozoans dominated by a stick-like, a few centimeters long cylindrical form 224
identified as Helopora fragilis. The other bryozoan taxon is represented by small and 225
fragmented colonies, which are identified as Phylloporinidae indet. 226
This shell bed is formed by carbonate lenses that consist of poorly sorted and 227
loosely packed valves embedded in a silty matrix with articulated valves of brachiopods 228
and bivalves, representing 15–20% of the rock fossil content. Fragmentation and 229
disarticulation degree increases with increasing packing of valves. The size of the 230
fragmented and complete bioclasts can be up to 5 cm. Nevertheless, complete identifiable 231
specimens are common. The valves are not preferentially oriented, although in some cases 232
they are convex-up as a result of current action (if they are convex-up - that is indicative of 233
currents). 234
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Taphonomic preservation depends on the taxonomic group. Calcitic skeletons are 235
preserved in most groups, whereas bivalves appear as internal moulds. Degree of 236
fragmentation in the shell bed is also variable. We find two types of erect bryozoan forms, 237
i.e., robust cylindrical (Helopora) and fragile ramose (Phylloporinidae indet.). The latter is 238
strongly fragmented, together with partly articulated and disarticulated complete bivalve 239
specimens as well as partly articulated multi-elemental skeletons. These parameters 240
indicate that the shell bed was formed under low/moderate-energy conditions (i.e.: for 241
preserving a silty matrix, despite the frequent articulated shells) and intermediate 242
sedimentation rates. Hence, this shell bed can correspond to a lag concentration associated 243
with a rapid transgression that began after the glaciation. Sedimentological studies from 244
other sections in the Argentine Precordillera indicate deepening-upward sequences, in the 245
overlying beds on the diamictite deposits. The shell bed corresponds to the base of the 246
deepening upward sequence and is used as a regional guide-bed (Astini, 2001; Astini and 247
Buggisch, 1993). 248
The recurrence of the taxa recorded in this assemblage throughout the rest of the 249
stratigraphic column and the fact that the bryozoan colonies are relatively well-preserved 250
implies that the shell bed was an in situ reworked para-autochthonous concentration. This 251
disagrees to some extent with its previous interpretation, as an autochthonous concentration 252
due to the high percentage (over 60%) of articulated valves and the presence of articulated 253
specimens of Modiolopsis cuyana (Sánchez et al., 1991). 254
The Hirnantia fauna has been interpreted as living in cold-to-temperate waters 255
(Rong and Harper, 1988; Astini and Benedetto, 1992; Rong and Li, 1999). It also occurred 256
over a fairly wide range of depths, although some genera, such as Hirnantia Lamont, 1935; 257
13
Eostropheodonta Bancroft, 1949, and probably Dalmanella Hall and Clarke, 1892, 258
exhibited preferences for inner-shelf settings (Brenchley and Cocks, 1982). 259
Biotic components found in the shell bed are arranged by their absolute abundance 260
in Appendix 1. Dalmanella testudinaria (Dalman, 1828) (58%) is one of the most eurytopic 261
species in the Hirnantia fauna, inhabiting a broad variety of substrates and depths (Rong 262
and Li, 1999), therefore, its dominance in the assemblage is not a conclusive 263
paleoenvironmental indicator. Paromalomena polonica (Temple, 1965) (17.5%) is also a 264
pedunculate species adapted to quiet and deeper waters (Rong and Li, 1999). Hirnantia 265
sagittifera (M’Coy, 1851) (12.1%) and Eostropheodonta hirnantiensis (M’Coy, 1851) 266
(3.75%) occur on mid-shelf fine-grained muddy bottoms, and scarcely occur on soft-bottom 267
substrates in deep-shelf (Rong and Li, 1999). 268
The presence, absence and diversity, as well as the relative abundance of bryozoan 269
growth forms provide useful environmental information for interpreting the 270
paleoenvironmental conditions (Hageman et al., 1997; 2000). The main environmental 271
factors that control the development of growth forms among bryozoan colonies are (1) 272
substrate, (2) rate of sedimentation and (3) water energy (McKinney and Jackson, 1991). 273
First, hard substrates or stable soft bottoms containing shell fragments and pebbles that 274
allow larval fixation are required for the settlement of bryozoan colonies. These conditions 275
can be found below the fair-weather wave base. Second, most bryozoans commonly grow 276
under low sedimentation rate (McKinney and Jackson, 1991; Smith, 1995), with the 277
exception of erect forms that can tolerate moderate sedimentation rates (Pedley, 1976). 278
Third, rigid erect forms are less frequent in shallow waters, apparently owing to breakage 279
by water movements (Holme and Wilson, 1985); we consider Helopora sp. to have shared 280
14
this kind of growth form (see fig. 5E in Nelson et al., 1988). Therefore, based on all 281
paleoecological data, we suggest that bryozoan assemblages inhabited mid-shelf settings 282
(offshore transition) with intermediate to low rate of sedimentation and low turbidity. 283
Moreover, the presence of bryozoans, bivalves and crinoids column fragments and the 284
dominance of brachiopods in this assemblage suggest a relatively nutrient-rich environment 285
for sustaining a community of suspension feeders (see Appendix 1). 286
A similar association in the Precordillera basin occurs in cool-water carbonates of 287
the Sassito Formation (Middle–Late Katian). Ernst and Carrera (2008) found a robust erect 288
cylindrical form (Moyerella) and a fragile ramose form (Phylloporina) together with a 289
brachiopod-dominated community. In spite of the differences between the depositional 290
settings and the taxonomic relationships of the taxa involved, the macroecological structure 291
of the local associations seems to have been persistent across the Katian/Hirnantian 292
boundary, when the first worldwide pulse of the Late Ordovician extinction occurred. 293
Additional paleoecological data from the Argentine Precordillera will improve the 294
understanding of the magnitude and causes of this biotic event at high-latitude shelves 295
because most data about the Late Ordovician extinction come from tropical and subtropical 296
regions. 297
PALEBIOGEOGRAPHY 298
The paleogeographic history of the Precordillera terrane shows that the Precordillera 299
shifted from equatorial to higher latitudes (Astini, 1998; Keller et al., 1998). Warm-water 300
carbonates were deposited during the Cambrian and Lower Ordovician, whereas Middle 301
Ordovician units were deposited at mid-latitude (30–35°) locations, including the Katian 302
Sassito limestones (Ernst and Carrera, 2008). The Hirnantian glacigenic rocks of the Don 303
15
Braulio Formation represent the last step in the shifting trajectory of the Precordillera 304
terrane (Astini, 1998; Benedetto et al., 2009; Benedetto et al., 2011). 305
Biogeographic analyses of trilobites, ostracodes, brachiopods, sponges and bivalves 306
that occur in the Argentine Precordillera were carried out to assess the changes in 307
paleogeographic position of this region (Benedetto et al., 1999; Sánchez et al., 2002, 308
Benedetto et al., 2009). These studies showed that, faunal affinities of the Argentine 309
Precordillera were essentially Laurentian during Cambrian and Early Ordovician times. 310
Later, the degree of endemicity increased during the isolation of the Argentine Precordillera 311
and finally, Gondwanic affinities arose after the Darriwilian. Late Ordovician to early 312
Silurian faunal affinities were mainly Gondwanic. Bryozoans, like other taxonomic groups, 313
also follow this paleobiogeographic pattern. 314
Bryozoans occured at different locations and stratigraphic positions in the Argentine 315
Precordillera during the Ordovician (Ernst and Carrera, 2008). Despite some Early 316
Ordovician collections (Carrera, 1995, 2003), most of the paleontological records are from 317
Middle to Upper Ordovician successions, including Darriwilian tropical carbonates to 318
Katian temperate water carbonates (Ernst and Carrera, 2008; Carrera and Ernst, 2010; Ernst 319
and Carrera, 2012). The new fossil assemblage described here provides a more complete 320
view of the local bryozoan diversity during the Ordovician. The uppermost Ordovician 321
bryozoan record is scarce worldwide, in spite of the numerous Upper Ordovician reports 322
from Baltica, Laurentia, Siberia, Avalonia, South China, North-western Africa and 323
Mediterranean regions (Ulrich, 1890; Brood, 1981; Ross 1985; Hu, 1986; Tuckey, 1990; 324
Buttler 1991; Destombes et al., 1971; Anstey et al., 2003; Buttler et al., 2007; Jiménez-325
16
Sánchez and Villas, 2010). Recent reviews consider that most of these localities are of 326
Katian age, so there are a few records of definitively Hirnantian bryozoans. 327
With the exception of high-latitude bryozoan assemblages reported here, the rest of 328
the Hirnantian records came from equatorial or subequatorial regions, i.e., Anticosti Island, 329
Canada (Ernst and Munnecke, 2009), the Hirnantian Girardeau Limestone in Missouri, 330
USA (in Jimenez-Sánchez and Villas, 2010; see age review in Bergström et al., 2006), 331
probably the Öljemyr beds in Gotland (Sweden) and Poland (‘Pirgu/Porkuni’ stages 332
[Brood, 1981; see age review in Rhebergen, 2009]) and the Late Katian to Silurian interval 333
in Northern India (Suttner and Ernst, 2007). Therefore, our finding shows that Hirnantian 334
bryozoans were not restricted to low latitudes only (Fig. 4). 335
Hirnantian bryozoan records in Anticosti (Ernst and Munnecke, 2009), Öljemyr 336
(Brood, 1981), Missouri (in Jimenez-Sánchez and Villas, 2010) and India (Suttner and 337
Ernst, 2007) show that the bryozoan assemblages were relatively diverse: each locality 338
bears 13, 12, 11 and 29 species, respectively. The high diversification found in the 339
Canadian and Indian bryozoan assemblages can be related to their latitudinal position (see 340
discussion in Powell, 2007). Both localities represent shallow tropical to subtropical areas; 341
the Anticosti bryozoans appear in a reef-related community (Ernst and Munnecke, 2009). 342
The Girardeau Formation is formed by a micritic limestone deposited in shallow water 343
(Bergström et al., 2006). The Öljemyr beds consist of erratic silicified boulders of unknown 344
origin, probably coming from a former carbonate shelf (Brood, 1981; Rhebergen, 2009). 345
There is a clear difference between these localities and the low diversity with only two 346
species recorded in the Don Braulio Formation. 347
Figure 4 around here 348
17
In spite that other environmental factors can influence diversity, temperature –349
related to a latitudinal gradient– has been considered a paramount environmental factor for 350
Ordovician bryozoan differentiation (Tuckey 1990; Jimenez-Sánchez and Villas, 2010; 351
Taylor and Sendino, 2010; Jimenez-Sánchez et al., 2012). In particular, the Katian 352
bryozoans from the Mediterranean province form cold-water and low-diversity associations 353
along the margin of Gondwana (Jimenez-Sánchez and Villas, 2010). The low diversity 354
found in the high-latitude Argentinean bryozoans could be attributed to this pattern. 355
The genus Helopora occurs in Middle to Late Ordovician (‘Richmondian’) 356
sequences of the United States of America in Ohio, Indiana, Michigan, Minnesota and 357
Tennessee (Ulrich, 1895, 1890; Ross, 1963) and of Estonia (Goryunova, 1985), and also in 358
the Upper Ordovician of India (Suttner and Ernst, 2007). The genus includes several 359
species showing a Laurentian distribution and a few Hirnatian records, with H. fragilis 360
occurring in Gondwana localities. Helopora can represent a highly tolerant and eurytopic 361
genus because it occurs in a variety of sedimentary facies around the globe. It is not 362
surprising, therefore, that is one of the genera that crossed the Ordovician/Silurian 363
boundary (Fig. 5). This genus further shows a post-extinction diversification and a more 364
widespread distribution, with numerous species from the Silurian and Devonian of Canada, 365
Russia, and China (Billings, 1866; Nekhoroshev, 1956; Yang and Hu, 1965). A similar 366
pattern, including migration routes for an entire Upper Ordovician–Silurian bryozoan 367
fauna, has been proposed by Ernst and Suttner (2007). 368
Figure 5 about here 369
Although the Silurian and Devonian rocks of Argentina have been extensively 370
studied, no bryozoans have been found until now. Helopora fragilis and the undetermined 371
18
phylloporinid were the last representatives of the phylum in the Argentine Precordillera. A 372
few representatives reappeared in the Carboniferous. 373
Despite the fact that Helopora is one of the survivors of the Late Ordovician 374
extinction, environmental conditions after the Ordovician–Silurian boundary may have 375
prevented the settlement of bryozoans and caused their extinction in the Argentine 376
Precordillera. Their differential survivorship could also be explained by lower rates of 377
colonization in the Precordillera terrane, which was located at high latitudes. 378
ACKNOWLEDGEMENTS 379
We thank J.L. Benedetto for his helpful comments on the draft of the manuscript. Also we 380
wish to thank Adam Tomašových, Andreij Ernst and Kamil Zagorsek, for their constructive 381
comments which substantially improved the manuscript. Authors wish to acknowledge the 382
assistance of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) 383
and the Universidad Nacional de Córdoba, both of which support facilities used in this 384
investigation (grant PIP 112-200801-00861). This article is a contribution to IGCP 591. 385
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FIGURE CAPTIONS 570
27
Figure 1. 1, Geological map and location of the studied section of the Don Braulio 571
Formation, Sierra de Villicum; 2, stratigraphic column of the Don Braulio Formation 572
(Hirnantian) in the studied section; 3, glacial diamictite and detail of striated boulder; 4, 573
shell-bed concentration with Hirnantian brachiopod and bivalve valves. 574
Figure 2. 1–5, Helopora fragilis Hall; 1, CEGH-UNC 25726, thin section of the coquina 575
bed showing a complete stick-like form. Scale bar= 2 mm; 2, CEGH-UNC 25726, 576
longitudinal section of H. fragilis, showing autozoecia and prominent acanthostyles. Scale 577
bar= 0.5mm; 3–4, CEGH-UNC 25726 and CEGH-UNC 25727a, tangential sections 578
showing elliptical autozoecia, polygonal heterozoecia and abundant acanthostyles. Scale 579
bar= 0.125mm; 5, cross-section, CEGH-UNC 25727b. Scale bar= 0.5mm. 580
Figure 3. 1–2, Phyloporinidae indet., fragmented ramose specimens; 1, CEGH-UNC 581
25728. Scale bar= 1mm; 2, CEGH-UNC 25729. Scale bar= 2mm. 582
Figure 4. Paleogeographic reconstruction of the Late Ordovician showing the distribution 583
of the glacial deposits in Africa and South America (modified from Cocks & Torsvik, 584
2006). The asterisk (*) points to the estimated position of the South Pole. Hirnantian 585
bryozoans’ records are restricted to Sweden (1), Poland (2), India (3), Canada (4), United 586
States of America (5) and Argentina (P). 587
Figure 5. Spatio-temporal distribution of Helopora. Note the thick line marking the 588
Ordovician-Silurian boundary and the distant geographical occurrences of this genus in the 589
Early-Middle Silurian. AC, Anticosti; AP, Argentine Precordillera; Ch, South China; Es, 590
Estonia; Gr, Greenland; In, India; Rs: Russia; Sp, Spain; UK, United Kingdom; US; 591
United States of America; *, Sandbian–Katian reports. 592
593
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