The Middle Tithonian (Upper Jurassic) Ammonoid fauna of Cañadón de los Alazanes, southern Neuquén-Mendoza Basin, Argentina

The Middle Tithonian rock and ammonite successions of Cañadón de los Alazanes (southern Neuquén-Mendoza Basin, Argentina), sampled for the first time, correlate almost exactly with those of the close localities Mallín de los Caballos and Mallín Quemado, but showing a comparatively reduced thickness. The studied fauna comprises: Pseudolissoceras zitteli (Burckhardt) [M&m], Neochetoceras? sp., Pseudhimalayites subpretiosus (Uhlig) [M, = P. steinmanni (Haupt)], Torquatisphinctes proximus (Steuer) transient αand transient β, and Corongoceras cf. alternans (Gerth) [M]. This fauna shows strong Tethyan affinities enabling close time-correlation with the European standard chronostratigraphic scale. The type specimens of P. zitteli, P. planiusculum (Zittel), P. rasile (Zittel), C. alternans and C. lotenoense (Spath) are figured. The Haploceratid genus Pseudolissoceras Spath most probably originated in Early Tithonian Hybonotum Zone populations of Haploceras carachtheis of Crussol (France), widely expanded its biogeographic distribution in the Semiforme Zone in the form of P. zitteli, prior to the diachronous disappearence of local adaptations in the Neuquén-Mendoza basin [P. pseudoolithicum (Haupt)] and northern Central Tethys (P. concorsi Donze and Enay = P. bavaricum Barthel). P. subpretiosus occurs in the lower Proximus Biozone (approximately upper Semiforme-lower Fallauxi Zones), although it appears to range, with its local microconch Simocosmoceras adversum andinum Leanza and Olóriz, through the upper Zitteli and lower Proximus Biozones in other localities of the basin. T. proximus occurs abundantly, differentiated in transients α and β, the latter giving gradually origin to the early Andean himalayitids [e.g., Windhauseniceras internispinosum (Krantz)] via Torquatisphinctes? windhauseni (Weaver sensu Leanza), a form close to "Burckhardticeras" peroni (Roman). Both transients of T. proximus are mainly confined to the Proximus Biozone and their sexual dimorphism remains unrecognized.