Pérez et al.—Aspidostomatidae (Bryozoa) from the Cenozoic of Patagonia


433


Figure 1. Aspidostoma giganteum (Busk, 1854), autozooids and avicularium (MACN-In No. 32318, Burdwood Bank). Note the large size of A. giganteum zooids compared to those of its epibiont, the cheilostome Hippothoa flagellum. Scale bar = 200 µm.


was originally included in the Cellariidae due to its similarity to Melicerita Milne-Edwards, 1836 (Gordon and Taylor, 1999), but was later transferred to the Aspidostomatidae (Gordon and Taylor, 2015). The aims of this study are: (1) to analyze the relationships


between the fossil material assigned to Aspidostoma giganteum by Ortmann (1900, 1902), Canu (1904, 1908), and Conti (1949) and the species presently living in the Magellanic Region; (2) to redescribe A. incrustans Canu, 1908; and (3) to record the presence of Melychocella in the Miocene of South America.


Figure 2. Map of the localities mentioned in the text. Geologic setting


Patagonian Cenozoic marine units outcrop from northern Río Negro Province to Tierra del Fuego Province, Argentina. They were formed by transgressive events generated by tectonic controls and cyclic eustatic changes. At least five cycles of increased mean sea level have occurred since the Maastrichtian stage (Malumián, 1999). The San Julián Formation is a stratigraphic unit consisting


of ~80 m of greenish-grey limestones, yellow and brownish sandstones, thin conglomerates, and fossil beds (Erdmann et al., 2008). The depositional environment was a coastal plain to a sandy shallow platform (Náñez et al., 2009). This unit has been dated as late Oligocene using 87Sr/86Sr isotopic techniques (Parras and Casadío, 2002; Parras et al., 2012). The material analyzed by Ortmann (1900, 1902) comes from Oven Point and the mouth of the Santa Cruz River. The Monte León Formation outcrops only in Santa Cruz


Province. Its invertebrate biodiversity is very high, with the bryozoans being a common component of the fossil assem- blages. All the material analyzed here comes from ‘Cabeza de León’ (Fig. 2). At this locality, this section includes almost 47m of siliciclastic rocks in medium to fine sandy to silty beds. Most of the Monte León strata seems to have been deposited in a relatively low-energy shelf setting, with a gradual upward shallowing trend and an abundant supply of volcanic ash (Malumián, 1999). Isotopic studies by Parras et al. (2012) yielded 87Sr/86Sr ages of 22.12 Ma at the base and 17.91 Ma at


the top, which appears to be consistently early Miocene (Aqui- tanian to early Burdigalian). The Chenque Formation is well represented around the city


sion consisting of bioturbated fine to medium sands with interspersed layers of mud and containing an abundant fauna of marine inverte- brates and fragments of fish bones. Basal units have been interpreted as formed in an offshore low-sedimentation habitat, while the top of the sequence is regarded as a littoral environment characterized by tidal channels and tidal sand waves (Scasso and del Río, 1987; del Río et al., 2001). The bryozoanmaterial analyzed in this study comes from Playa Larralde on the Valdés Peninsula (Fig. 2). A diverse fauna ofmarine invertebrates, including bryozoans, is exposed in the uppermost levels at this site. Using 87Sr/86Sr, this stratigraphic unit has been dated as middle Tortonian (10 ± 0.3 Ma; Scasso et al., 2001). This isotopic age is close to the end of the Neogene climatic optimum (Casadío et al., 2005), when warmwaters occurred off the Atlantic coast of southern South America.


of Comodoro Rivadavia, Chubut Province. It is composed of a siliciclastic succession of sandstones and mudstones, with layers of marine invertebrates and oyster reefs. This unit has a thickness of around 300m (see Cuitiño et al., 2015) and repre- sents a shoreface to inner foreshore setting (Paredes and Colombo, 2001). The colonies of Aspidostoma were found at the lowest levels of the formation. The isotopic studies by Cuitiño et al. (2015) yielded 87Sr/86Sr ages of 19.69 Ma at the base to 15.37 Ma at the top (Burdigalian to early Langhian). Finally, the Puerto Madryn Formation is a siliciclastic succes-


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