440


Journal of Paleontology 92(3):432–441


not fused between each other around the opesia-orifice, but connected in the marginal and proximal zones of the zooid. Opesia-orifice wider than long, 120 µm long × 260 µm wide, its proximal margin bent upward as a lappet consisting of three acute projections that may be lost during preservation. At both sides of the proximal margin there is an indistinct lateral notch with a small projection sinking in distobasal direction. Vicarious avicularia longer than the autozooids, 751 × 322 µm, located at the bifurcation of zooid rows. The acute rostrum covers one of the proximal corners of the distal zooid, the boundary V-shaped. Avicularian cryptocyst with small isolated granules and a pair of subequal, large reniform opesial foramina. Rostral foramen subelliptical, with a medioproximal projection. Lateral and distal walls with rounded, multiporous pore plates. Ovicells not observed.


Etymology.—From Latin bi, meaning two or double, and perforare, to bore; referring to the paired opesial foramina of the avicularium.


Other material.—Two fragments, MPEF-PI 6350, Punta del Marqués, Chubut Province. Chenque Formation.


Remarks.—We assigned this new species to Melychocella mainly due to the presence of vicarious avicularia with rostral and opesial foramina. It differs from M. cynura Gordon and Taylor, 1999, the type species of this genus, by lacking the pair of distal condyles in the opesia-orifice. M. biperforata n. sp. differs from the other species of the genus by having a pair of reniform opesial foramina in the avicularia.


Discussion


Aspidostoma giganteum can be easily distinguished from the new fossil species described in this study by the greater size of the autozooids. At around 1170 × 820 µm, the zooids are considerably larger than in most species of cheilostome bryozoans. Its gigantism might be related to its present distribution in cold-temperate waters of the Magellanic Region. However, several cheilostome species, mainly distributed in


Antarctica and in subpolar seas (Grischenko et al., 2002), are known with zooids that exceed the size of A. giganteum. Aspidostoma incrustans can also be set clearly apart from


other fossil species of Aspidostoma by the presence of huge pores piercing its cryptocyst.Onthe other hand, A. ortmanni, A. armatum, A. tehuelche,and A. roveretoi exhibit a common morphological pattern. Each is characteristic of a particular geological unit (Fig. 6) and can be distinguished by the morphological features summarized in Table 1. Aspidostoma ortmanni is characterized by having very small avicularia, A. armatum by its concave quadrangular process and its relatively large avicularia, A. tehuelche by its protruding avicularia and its avicularian cystides in contact with two auto- zooids, and A. roveretoi by its regularly hexagonal or rhombic zooids with strongly depressed central cryptocysts (Table 1). The fact that Melychocella biperforata was found by us in both


the Monte León and the Chenque formations, and that M. flammula and Aspidostoma incrustans were originally described by Canu (1908) from the Punta Borja (Chenque Formation) and were found during the present study in the Monte León Formation (Fig. 6), suggests that the upper levels of the Monte León Formation might correlate with the lower levels of the Chenque Formation. In addition to the species treated in this study, the Aspidos-


tomatidae of the southern tip of South America are represented by four Paleogene taxa: A. onychocelliferumCanu, 1911, A. globifera Canu, 1911 (Roca Formation, early Paleocene, Río Negro Pro- vince), A. hexagonalis Canu, 1904, and A. porifera Canu, 1904 (San Julián Formation, late Oligocene, Santa Cruz Province). Three species of Melychocella were already known fromthe


Paleogene of the Chatham Islands, New Zealand (Southwest Pacific):M. cynura Gordon and Taylor, 1999, M. obliqua Gordon and Taylor, 2015, and M. bilamellata Gordon and Taylor, 2015. No representatives of this genus had been recorded yet outside the Chatham Islands. The occurrence of two species of Melychocella in the Neogene of Patagonia expands the temporal range of the genus and lends further support to the hypothesis of a close link between the faunas of the southern tip of South America and the Australasian region during the early Miocene, as has been dis- cussed in detail in previous studies (Casadío et al., 2010; Pérez et al., 2015; López-Gappa et al., 2017).


Figure 6. Chronostratigraphic chart showing the age of the formations analyzed in this study and their species. The solid line within each rectangle indicates the approximate level of the bryozoan fossil associations. *Species recorded by Canu (1908) for Punta Borja, Chenque Formation.


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  |  Page 217  |  Page 218  |  Page 219  |  Page 220