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In the Cerro Mallín Quemado area (Sierra de la Vaca Muerta) the three members of the Vaca Muerta Formation (Portada Covunco, Los Catutos and Pichi Moncol) can be recognized, including the whole of the Tithonian rock-record. The ammonite fauna does not show significant differences with respect to that of the nearby locality Pampa Tril, but the record of faunal horizons is patchier. Eighteen species of ammonites were recorded through the studied sections, covering the whole of the Andean Tithonian. The current chronostratigraphic zonation of the Andean Tithonian is briefly discussed, updated and correlated with the most recent literature. From the current succession of ammonite bio-horizons previously defined in the basin, only three (picunleufuense alpha, picunleufuense beta and falculatum) were recognized definitely. Four other bio-horizons (perlaevis, erinoides, internispinosum alpha and vetustum) were recognized only tentatively, because the typical assemblages of morphotypes (morpho-species) were not clearly or completely recognized. The current regional time-correlation chart dated by the ammonite bio-horizons of the Neuquén Basin along a 70°W transect is updated with the results of the present study and additional information recently obtained from other localities.

The recent recognition of the earliest Andean Tithonian ammonite fauna (Picunleufuense Zone) has allowed a balanced classification of the ataxioceratid ammonites, reflecting their phylogenetic relationships. The Picunleufuense Zone fauna, first recognized in the Neuquen Basin at the base of the Vaca Muerta Formation, is represented by numerous records of sparse ammonite faunas from Antarctica, the Austral Basin and the Tarapaca Basin (N. Chile and S. Peru). These Southeastern Pacific faunas include the Tithonian genera Indansites, Choicensisphinctes, Zapalia, Krantziceras (Tithonian–lower Berriasian) and Platydiscus. These five genera are grouped in a new subfamily established in this paper: Zapaliinae subfam. nov. (type genus: Zapalia). The origin of Indansites and Choicensisphinctes (possibly via Indanistes) from Lithacoceras is discussed. Zapalia must have been originated from Indanistes in the upper Proximus Zone. Platydiscus could have been originated from Krantziceras. The Tithonian Southeastern Pacific ataxioceratids (Zapaliinae subfam. nov.) form a group well separated from the Tithonian Indo-Madagascan Virgatosphinctinae.

Asphinctites tenuiplicatus [M] and Polysphinctites secundus [m] from the Asphinctites tenuiplicatus Zone (Early Bathonian), are usually considered as a sexual dimorphic pair, although authors describe them as separate species. We used statistical methods to test the sexual dimorphic correspondence between those morphospecies, based on a rather large sample of well-preserved macro- and microconchs derived from a single horizon of calcareous concretions in the Polish Jura. Our results indicate that both dimorphs or sexes have identical ontogeny up to a critical diameter, from which they diverge towards the characteristic morphology and sculpture of each dimorph. Thus, both dimorphs are described as a single species: Polysphinctites tenuiplicatus [M and m]. After review of the several nominal species usually assigned to the genera Asphinctites and Polysphinctites throughout their stratigraphic and biogeographic range in the Early Bathonian of the Tethys, it is concluded that they actually correspond to only two species of a single lineage. The corresponding name for the lineage should be Polysphinctites (= Asphinctites as a junior synonym).

The external area-to-volume ratio of the ammonite shell has been held to be related to morphology but never evaluated quantitatively. A dimensionless ratio, the Vogel number, was computed for large samples of Devonian to Cretaceous ammonites with a new method based on the ADA-model. The estimated ratios range from 2.4 to 3.4. The highest values are exhibited by uncoiled serpenticone ammonites, lowering in the sequence serpenticone-oxycone-spherocone. It is shown that the area-to-volume relationships are controlled by the involution (degree of overlapping) and the relative width of whorl section. The typical evolutionary trends serpenticone–spherocone and/or serpenticone–oxycone, broadly documented through the history of the Ammonoidea, could have been driven, at least in part, by the lowering of the area-to-volume ratio.

The ammonite fauna of the Tithonian–Berriasian of the Vaca Muerta Formation in Pampa Tril has been recently described in detail. New important specimens and additional information are presented in this paper. The phyletic evolution of Choicensisphinctes, passing from C. platyconus to C. erinoides is confirmed, as well as the sexual dimorphic correspondence of this latter with C. mendozanus. A microconch of the genus Krantziceras is described for first time. New specimens of Substeueroceras koeneni identical to the paralectotype, along with material already described from the koeneni Hz. (Koeneni Zone), point to the fixation of this horizon as the type horizon of the species. New specimens of Subthurmannia boissieri from the Damesi Zone match clearly the range of variation of this species in Spain, thus providing an element for time-correlation with the Tethyan standard scale. Additional material from the internispinosum alpha Hz. confirms the origin of W. internispinosum from C. proximus by the inception of an evolutionary innovation in the juvenile ontogeny.

Seven previous proposals of aptychus (sensu stricto) function are reviewed: lower mandible, protection of gonads of females, protective operculum, ballasting, flushing benthic prey, filtering microfauna and pump for jet propulsion. An eighth is introduced: aptychi functioned to actively stabilize the rocking produced by the pulsating jet during forward foraging and backward swimming. Experiments with in-air models suggest that planispiral ammonites could lower their aperture by the forward shift of a mobile cephalic complex. In the experiments, the ventral part of the peristome is lowered from the lateral resting (neutral) position by the added “ballast” of a relatively thin Laevaptychus to an angle

New findings of the Early Tithonian genus Virgatosimoceras SPATH (Simoceratidae) from Hungary confirm the existence of a new chronospecies (Virgatosimoceras dunaii n. sp.) which fills the gap between V. albertinum and V. rothpletzi. The successive chronospecies demonstrate a well defined lineage through the Early Tithonian Darwini, Semiforme and Fallauxi zones, opening the possibility of enhancement of the time-correlation of the Submediterranean Province with adjacent regions. All figured records of Virgatosimoceras are revised. Micro- and macroconchs are tentatively distinguished for the first time. The similarities between Virgatosimoceras and some homoeomorphic perisphinctids are discussed. Based on the known records, is concluded that the genus was restricted to the western Tethys.

Ammonites are of outstanding importance in dating events of the Mesozoic and in the study of mechanisms, modes and timing of evolutionary processes. These applications rely on a detailed understanding of their morphology and the modes of variation. It has been known for a long time that their shape is composed of a number of highly correlated features. A new model, called the ADA-model, is introduced for the study of shell morphology (size and shape). The new model is based on classic dimensions which are stable parameters throughout ontogeny, giving very close agreement between predictions and actual observations. It was applied in the exploration of the morphospace occupied by the planispirally coiled and the regularly uncoiled Mesozoic Ammonoidea, based on two new reduced morphospaces introduced for the analysis. Results obtained expose close relationships between size and shape, and general patterns in the ammonite shell morphology and morphogenesis. (i) The relative apertural height of the whorl sect...

The Tithonian Ataxioceratids from Casa Pincheira (Mendoza) are described in the framework of a period of global relative high sea level stand, with Andean basins well communicated with Central and Western Tethys. The family Ataxioceratidae is interpreted including the subfamily Torquatisphinctinae Tavera (Katroliceras Spath, Torquatisphinctes Spath, and Pachysphinctes Dietrich). The original specimens of ‘Perisphinctes’ densestriatus Steuer, Paraulacosphinctes striolatus (Steuer), Paraulacosphinctes? mangaensis (Steuer), and Torquatisphinctes proximus (Steuer) are photographically refigured, and their types are designated. Type specimens are also designated for Euvirgalithacoceras malarguense (Spath) and Choicensisphinctes choicensis (Burckhardt). The genus Euvirgalithacoceras is used for the first time for lowermost Tithonian Andean forms, and Choicensisphinctes is suggested as a derivated genus. q 2003 Elsevier Ltd. All rights reserved.

En este trabajo se estudia la estratigrafía, la fauna de amonites y la bioestratigrafía del Grupo Cuyo en la Sierra de Reyes (sur de la Provincia de Mendoza, Argentina). La sucesión alcanza un espesor de 230 m y está constituida, de base a techo, por las formaciones Bardas Blancas (fangolitas, fangolitas arenosas y areniscas finas), Los Molles (lutitas negras) y Lajas (areniscas gruesas hasta finas). Se reconocieron cuatro niveles con amonites. Los dos inferiores (I y II) se encuentran en la parte inferior de la Formación Bardas Blancas. El nivel I es monoespecífico, se compone de Dumortieria cf. pusilla y es atribuido al Toarciano superior-Aaleniano inferior. El nivel II se caracteriza por una asociación de Podagrosiceras cf. athleticum, Planammatoceras sp., Phylloceras sp. y Eudmetoceras? sp., que se asigna a la Zona estándar Malarguense del Aaleniano superior-Bajociano inferior. El nivel III, ubicado en la parte media de la Formación Bardas Blancas, posee una asociación del Bathoniano inferior compuesta porMorphoceras sp., Phylloceras sp. y Lytoceras cf. eudesianum. En el nivel IV, que aparece en el tramo inferior de la Formación Lajas, se han identificado Iniskinites crassus, Stehnocephalites gerthi and Lilloettia cf. steinmanni, taxa que representan a la Zona estándar Steinmanni, del Bathoniano superior. La fauna de amonites ha permitido establecer que la sucesión transgresiva en la región de estudio, representada por las formaciones Bardas Blancas y Los Molles, se desarrolló durante el lapso Toarciano tardío a Bathoniano temprano. El nivel IV del tramo inferior de la Formación Lajas indica que la reversión transgresión-regresión de la parte superior del Grupo Cuyo se produjo alrededor del Bathoniano temprano a tardío. Asimismo, los términos más jóvenes del Grupo Cuyo (parte superior fuertemente regresiva de la Formación Lajas) se asignan al Calloviano temprano. Este marco bioestratigráfico del Grupo Cuyo en la Sierra de Reyes y su comparación con el de regiones vecinas permiten tener un mejor conocimiento de la evolución secuencial del Jurásico Inferior alto a Medio en el sector central de la Cuenca Neuquina. This paper deals with the stratigraphy, the systematics of the ammonite faunas and the biostratigraphy of the Cuyo Group in the Sierra de Reyes (southern Mendoza Province, Argentina). The 230 m-thick marine siliciclastic succession of the Cuyo Group is composed, from base to top, of the Bardas Blancas Formation (mudstones, sandy mudstones and fine-grained sandstones), the Los Molles Formation (black shales) and the Lajas Formation (coarse- to fine-grained sandstones). Four ammonite levels have been recognised in the Cuyo Group. The first two levels (I and II) are located in the lower section of the Bardas Blancas Formation. The monospecific level I yielded Dumortieria cf. pusilla and is regarded as upper Toarcian-lower Aalenian, whereas level II is composed of an association of Podagrosiceras cf. athleticum,Planammatoceras sp., Phylloceras sp. and Eudmetoceras? sp. which can be assigned to the upper Aalenian-lower Bajocian Malarguense Standard Zone. The third level appears in the middle part of the Bardas Blancas Formation and containsMorphoceras sp., Phylloceras sp. and Lytoceras cf. eudesianum, and is assigned to the lower Bathonian. The younger level (IV) was identified in the lower section of the Lajas Formation and yielded Iniskinites crassus, Stehnocephalites gerthi and Lilloettia cf. steinmanni. This association corresponds to the upper Bathonian Steinmanni Standard Zone. The ammonite record of the Cuyo Group in the Sierra de Reyes allowed dating the transgressive succession represented by the Bardas Blancas and Los Molles formations as late Toarcian-early Bathonian. The ammonite fauna identified in the lower section of the Lajas Formation (level IV) indicates that the change from transgressive to regressive conditions recorded at the upper part of the Cuyo Group occurred around early-late Bathonian times. The uppermost strata of the Cuyo Group (upper section of the Lajas Formation), having a well defined regressive signature, can be dated as lower Callovian. This relatively well constrained biostratigraphic framework of the Cuyo Group in the Sierra de Reyes and its comparison with neighbouring areas allowed for a better understanding of the sequential evolution in central Neuquén Basin during the late Lower to Middle Jurassic.