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Ammonoid fauna from the early Middle Jurassic of Mendoza Province, Argentina

1982; Cambridge University Press; Volume: 56; Issue: 1 Linguagem: Inglês

1937-2337

Gerd E. G. Westermann, Armando Riccardi,

Geology and Paleoclimatology Research

The classic outcrops at Cerro Tricolor bearing an important Ammonitina fauna of the Aalenian-Bajocian boundary, have been re-examined; a similar fauna has been sampled at Rio Po- timalal, 50 km farther south. Gerth's original assemblage (revised) contained Planammatoceras (P.) gerthi (Jaworski), P. (Pseudaptetoceras) tricolore n. sp., Eudmetoceras (E.) eudmetum jaworskii Westermann, Fontannesia (?) austroamericana Jaworski and, most importantly, Praeleptosphinctes jaworskii Westermann, the oldest known perisphinctid. We failed to find in the 5 m type section the extremely scarce Eudmetoceras s.s., Praeleptosphinctes and Fontannesia (?), but did collect in ad- dition to Gerth's taxa the zonal index Puchenquia malarguensis (Burckhardt), P?. (Gerthiceras?) mendozana n. subgen. n. sp., Planammatoceras aff. P. planinsigne (Vacek), Eudmetoceras (Euap- tetoceras) amplectens (Buckman), and phylloceratids. This assemblage bears specific affinities to that of the G. concavum and H. discites Standard Zones of the Mediterranean and Submediterranean provinces as well as to that of the time-equivalent upper E. howelli and D. widebayense Zones of south Alaska. This confirms our previous correlation of the P. malarguensis Zone. Even if the holotype of Praeleptosphinctes jaworskii, here re-figured after preparation, was found ex situ it could not be younger than P. singularis (-S. ovalis) Zone, since the highest level is E. giebeli (-W. laeviuscula s.s. + 0. sauzei) Zone in poorly fossiliferous sandstone facies. Generic dis- tinctness from other Leptosphinctinae lies in the complete absence of primary ribs. The small, probably incomplete assemblage of Rio Potimalal differs significantly (?) in the presence of Puchenquia (Gerthiceras) compressa n. subgen. n. sp. and in the (scarcity or) absence of P. malarguensis and Planammatoceras (Pseudaptetoceras) spp. Together with recent evidence from Chile, this suggests the basal P. malarguensis Zone.