774
Journal of Paleontology 92(5):768–793
Figure 5. Early− Middle Ordovician paleogeography map (modified from Cocks and Torsvik, 2002 and Popov et al., 2009), showing global distribution of the genus Ahtiella (stars). Paleogeographic map. ATA=Armorican Terrane Assemblage; CAB=Central Andean Basin; CNF=Central Newfoundland (placement based on Neuman, 1984).
remnant ocean not large enough to prevent faunal dispersion (Benedetto et al., 2003; Benedetto, 2004). It seems likely that brachiopod dispersion from Famatina to Cuyania was facilitated by the gradual approximation of the Cuyania terrane to the Gondwana margin combined with a generalized sea-level rise (Carrera and Astini, 1998; Cañas, 1999; Astini, 2003). An interesting feature is that diversification of the sub-
family Ahtiellinae was centered mainly in Avalonia, Cuyania, and Baltica (Fig. 5). The Welsh Treiorwerth Formation yielded Inversella (Reinversella) monensis Bates, 1969 (Neuman and Bates, 1978), whereas the Central Newfoundland Summerford Group contains the ahtiellins Schedophyla potteri Neuman, 1971, Inversella sp., and the endemic Guttasella gutta Neuman, 1976. In the Cuyanian Precordillera Basin, Ahtiella argentina co-occurs with I. (R.) arancibiai Herrera and Benedetto, 1987 (Benedetto et al., 2008) and the endemic ahtiellin Sanjuanella plicata Benedetto and Herrera, 1987. In Estonia, Ahtiella lirata Öpik, 1932 is approximately coeval with I. (Inversella) borealis Öpik, 1933. Outside the Baltic and Celtic faunal provinces, the only ahtiellins reported are Borua Williams and Curry, 1985 from Ireland, and two species of Schedophyla Neuman, 1971 from southern China (Xu and Liu, 1984; Zhan et al., 2006). However, as noted below, the placement of Schedophyla among the ahtiellinis requires further confirmation. The Norwegian Rutrumella Harper in Bruton and Harper, 1981 is a poorly known genus that has been referred questionably to the sub- family (Cocks and Rong, 2000).
The Andean region as a center of origin
As Gutiérrez-Marco and Villas (2007) previously noted, and regardless of the chosen paleogeographic scenario, it is apparent that Ahtiella originated along the proto-Andean Gondwana
margin. Several recent paleontological discoveries provided evidence supporting that both the Central Andean Basin and the arc-related Puna-Famatina Basin operated simultaneously as centers of evolutionary radiation (‘centers of origin’) and spe- cies pump regions (sensu Harper et al., 2013) from which new taxa spread to neighboring areas (Benedetto and Sánchez, 2003; Muñoz and Benedetto, 2016; Benedetto and Muñoz, 2017). Such temperate Gondwana basins acted as sites of origination, as did the equatorial shallow-water shelves of Gondwana and peri-Gondwanan terranes, which have been identified by Bassett et al. (2002) as the main source of the precursors to the Ordo- vician radiation. For instance, the earliest known punctate orthide Lipanorthis Benedetto in Benedetto and Carrasco, 2002 from the upper Tremadocian of northwestern Argentina was not an immigrant from the tropical belt, as Harper et al. (2004) suggested, but probably originated from a Protorthisina-like plectorthoid ancestor inhabiting the Central Andean Basin in the latest Cambrian (Benedetto, 2013). Furthermore, based on cla- distic analysis, Benedetto and Muñoz (2017) showed that plec- torthoids not only underwent an important diversification in the Central Andean Basin during the Tremadocian and Floian but also could have been a source for the heterorthids, which through the Ordovician spread along the western Gondwanan shelves (Peru, northern Africa) and peri-Gondwanan terranes (Avalonia, Armorica). The Puna-Famatina volcanic arc (Fig. 5) was another sig-
nificant center of origin during the Early to Middle Ordovician. As it has been already noted, its shelly faunas exhibit a high level of endemicity, in particular bivalves (Sánchez and Babin, 1993; Sánchez, 1997) and brachiopods (Benedetto, 2003b; Benedetto and Sánchez, 2003). Volcanic islands and archipe- lagos have long been recognized as important evolutionary centers of modern biota (e.g., MacArthur and Wilson, 1967), but their role in promoting faunal diversification in the past was not fully acknowledged until Neuman (1984) proposed that the distinctive Celtic faunas from the Ordovician volcaniclastic rocks of the Caledonian-Appalachian folded belt inhabited intra-Iapetus volcanic islands. Also relevant was the subsequent study by Webby (1992) on the low-latitude Ordovician faunas from the volcaniclastic rocks of New South Wales. Harper et al. (2009) emphasized the role of such volcanic chains as cradles and centers of origin contributing to the increase of γ-diversity during the Great Ordovician Biodiversification Event.
Current ideas about the origin of Plectambonitoidea
The general statement that the order Strophomenida evolved from the early to middle Cambrian Nisusiidae of the class Kutorginata (Williams and Hurst, 1977) or, alternatively, from an ancestor similar to Billingsella Hall and Clarke, 1892 at the Cambrian-Ordovician transition, has been based essentially on the presence in all these groups of an apically perforated pseu- dodeltidium (Cocks and Rong, 1989; Williams et al., 1996). However, no further compelling evidence has been presented to support such ancestor-descendant relationships for all members of the order. According to Bassett et al. (2001), bilingsellides and kutorginates share the well-developed perforate pseudo- deltidium and the lack of dental plates, but differ in that sockets and socket plates have a different origin in billingsellides and
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207
