Cichowolski and Rustán—Early Devonian bactritids from Argentina
distributed whereas other Malvinokaffric basins record stronger endemic signatures (Sánchez et al., 1995; Sterren et al., 2015). Brachiopods once cited as predominantly Malvinokaffric taxa also exhibit an influence from the Eastern American Realm into the Argentine Precordillera (Herrera and Racheboeuf, 1997; Herrera et al., 1998; Isaacson, 2007). Lower Devonian corals from Bolivia and Argentina do show complex paleobiogeo- graphic affinities (Fernández-Martínez et al., 2007). Middle Devonian ostracodes have a more definite Malvinokaffric imprint than Lower Devonian taxa (Salas et al., 2013; Salas, 2014). With a few exceptions, hyolithids (mainly known from Bolivia) also exhibit Malvinokaffric endemism (Malinky and Racheboeuf, 2011). Trilobites, however, present an overall endemic Malvinokaffric signature, although exceptions have been reported recently (Rustán and Vaccari, 2010, 2012; Holloway and Rustán, 2011). Ectocochleate cephalopods were planktonic, nektobenthic
or nektonic. The chambered shell imposed a limit to habitat or swimming depths, which probably is the reason for some endemism within the group (e.g., Cretaceous nautiloids at a specific level, Cichowolski, 2003). For some taxa, water temperatures might have played a role as a dispersion barrier, as is possibly the case for discosorids and ascocerids (e.g., Kröger, 2013). In ammonoids, the first ontogenetic stage was planktonic and prone to dispersal with oceanic currents. One of the characters that Bactritida and Ammonoidea share is a small embryonic conch, which suggests a planktonic initial stage (De Baets et al., 2012). In turn, bactritids were interpreted as planktonic vertical migrants as adults (e.g., Holland, 2003; Klug et al., 2010). The orientation of the conch would have been nearly vertical (Klug and Korn, 2004), with a slight inclination due to the ventral siphuncle position. The slender conch, small siphuncle, and absence of cameral and siphuncular deposits indicate a low energetic cost of buoyancy regulation. Bactritids would have been adapted to environments with low food availability (Kröger, 2008). In turn, based on the clearly developed muscle attachment scars (Kröger et al., 2005), they were probably capable of slow horizontal movements, but being inefficient swimmers, migrated mainly vertically or drifted passively (Klug et al., 2010). Therefore, froma paleoautoecological point of view, the bactritids were probably capable of migrating with currents during nearly every ontogenetic stage. Thus, the wide distribution of Bactrites gracilis and Devonobactrites,as confirmed by our new records (Fig. 6), indicates that cool waters from the extreme southern paleolatitudes of southwestern Gondwanan seas were not an effective biogeographic barrier for bactritids, in contrast with previous interpretations of Malvinokaffric endemism in co-occurring benthic groups (Boucot and Racheboeuf, 1993). Further studies on the entire cephalopod fauna of the Argentine Precordillera are in progress in order to revise these preliminary conclusions.
The origin and earliest records of bactritids.—The first appearance of bactritids in the fossil record has been repeatedly discussed (see Doguzhaeva, 2002; Holland, 2003; Kröger, 2008; Kröger and Mapes, 2007, and references cited therein). Erben (in Moore, 1964) suggested that the ancestor of bactritids may have been a member of the Orthocerida, which is the consensus opinion today. However, determining which
429
orthocerid group is the bactritid ancestor and when it occurred is an issue of ongoing discussion. The synapomorphies linking Orthocerida and Bactritida are a small subspherical to ovoid initial chamber, the straight to slightly bent conical shell, and the narrow siphuncle (Klug et al., 2015). The first proposal of an Ordovician origin of the Bactritida,
with Eobactrites Schindewolf (=Bactroceras Holm) as the main candidate (Schindewolf, 1932), was repeatedly refuted (e.g., Furnish and Glenister, 1964; Flower, 1964; Dzik, 1984), mainly on the basis of the extended temporal gap existing between these records and Early Devonian representatives of Bactrites. Moreover, Kröger and Mapes (2007), and more recently Aubrechtová (2015), demonstrated that similarities between Bactroceras and the Bactritida are superficial. Aubrechtová (2015) studied more than one hundred specimens of Bactroceras from the Ordovician of the Prague Basin and demonstrated that the protoconch of this genus is morphologi- cally different from that of bactritids from the Devonian. In Bactroceras, the protoconch is moderately large, hemispherical, with a small caecum and a constriction. This differs from those known in bactritids. Both taxa also differ in that bactritids have prominent hyponomic sinuses both in the suture and in the growth lines, and the septa are often inclined. In Bactroceras, the ventral lobe is considered to be a taphonomic artifact because the close proximity of the connecting ring to the phragmocone wall leaves no space for sediment, and conse- quently a lobe-like structure appears on the internal mold (Aubrechtová, 2015). Erben (in Moore, 1964) and other authors have placed the
origin of the Bactritida in the Silurian, based on the record of a single specimen of Bactrites from the upper Silurian of Morocco (Termier and Termier, 1950). Nevertheless, the specimen was poorly preserved and its illustration unconvincing and not diagnostic as a bactritid. Therefore it was excluded from the Bactritida (Holland, 2003; Kröger and Mapes, 2007). Subsequently, Ristedt (1981) described the species Bactrites bohemicus based on specimens from the Ludlow of Bohemia, but questioned whether that taxon was a direct ancestor of the Devonian bactritids instead of a homeomorph of Devonian Bactrites. The apical morphology of Devonobactrites, currently the oldest well-known Devonian bactritid, from the lower Zlíchovian at Filon Douze in Morocco, supports this opinion (Kröger and Mapes, 2007; Klug et al., 2008). The protoconch of Devonobactrites is elliptical, egg-shaped, and the conch adoral of the apical chamber is slender and displays a cross-sectional diameter less than that of the protoconch. This morphology is similar to that of the apex of Bactrites gracilis (Blumenbach, 1803) (=Orthoceras schlotheimii Quenstedt, 1845; Branco, 1885; Schindewolf, 1933; Kröger, 2008). Kröger and Mapes (2007) suggested an evolutionary
scenario for the bactritids with an origin in Sphaerorthoceras spp. described by Ristedt (1968) from the Upper Silurian, through the Protobactrites sp. from the Lower Devonian of Sardinia (Serpagli and Gnoli, 1977) because both taxa share similar initial chambers, but the siphuncle, which is central in Sphaerorthoceras, becomes eccentric in Protobactrites (Kröger and Mapes, 2007, fig. 8). These authors assumed homeomorphism in the migration of the siphuncle in two lineages: one leading to the Devonian Bactritida, and the other
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 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216