Dimorphism in Placenticeras mintoi from the Upper Cretaceous Bagh Beds, central India

The Upper Cretaceous Bagh Beds yield many biota of which ammonoids are particularly well known. Previously Bagh ammonoids were grouped into many genera and species within several families. The present study, however, shows that failure to recognize dimorphism and a wide range of interspecific variation within the single genus Placenticeras perhaps results in taxonomic oversplitting. Specific dimorphism in P. mintoi has been established.     

Upper cenomanian ammonites from the environs of Saumur,and the provenance of the types of Ammonites vibrayeanus and Ammonites geslinianus

Ammonite faunas consisting of Neolobites vibrayeanus (d'Orbigny), Calycoceras (Calycoceras) naviculare (Mantell), C. (Lotzeites(?)) sp., Pseudocalycoceras lattense Thomel, Metoicoceras geslinianum (d'Orbigny) and Euomphaloceras septemseriatum (Cragin) from Saumur, within the type area of the Turonian stage, are described from the collections of the Château de Saumur. They allow the recognition of two Upper Cenomanian horizons at Saumur, the one equivalent to the Sciponoceras gracile/Metoicoceras geslinianum Zone as developed in the Sables à Catopygus obtusus/Sables de Bousse of the Cenomanian stratotype; the other, older assemblage equates with the Calycoceras naviculare/Eucalycoceras pentagonum Zone fauna known from the Marnes à Ostracées of the type Cenomanian. From lithological comparisons it is suggested that this area is the source of the types of both Metoicoceras geslinianum and Neolobites vibrayeanus, which are redescribed.     

Early Permian ammonoids from the Kaeng Krachan Group of the Phatthalung-Hat Yai area, southern peninsular Thailand

An Early Permian small ammonoid fauna consisting of Neocrimites sp., Agathiceras suessi Gemmellaro, A. girtyi Böse, Agathiceras? sp., and Miklukhoceras sp. was found in nodules of a fine sandstone bed exposed in the Phatthalung-Hat Yai area of southern peninsular Thailand. The ammonoid-bearing bed belongs stratigraphically to the uppermost part of the Kaeng Krachan Group, which is essentially a clastic-dominant, Late Carboniferous (?) to Early Permian stratigraphic unit, widely distributed in western and peninsular Thailand. This ammonoid fauna is considered to be of Bolorian (Kungurian) age and includes Agathiceras girtyi Böse, which is described for the first time from Thailand. The present discovery of Bolorian ammonoids suggests that the uppermost part of the Kaeng Krachan Group is slightly younger than previously considered and around the latest Early Permian. This further implies that the continental margin environment of the Sibumasu Block drastically changed at around Bolorian time from a cool, clastic-dominant shelf condition to a temperate to subtropical, carbonate platform due to rapid northward drift after middle Artinskian rifting.     

A revision of the ammonite faunas of the typeCenomanian. 2. The families Binneyitidae,Desmoceratidae, Engonoceratidae,Placenticeratidae, Hoplitidae,Schloenbachiidae, Lyelliceratidae and Forbesiceratidae

In this part of the revision of the ammonite faunas of the type Cenomanianwe deal with representatives of the genera Borissiakoceras, Austiniceras, Neolobites, Metengonoceras, Placenticeras, Hyphoplites, Schloenbachia, Stoliczkaia (Lamnayella), Stoliczkaia (Shumarinaia), Euhystrichoceras and Forbesiceras.     

The peculiar taphonomy of the streamlined late Campanian ammonite Metaplacenticeras subtilistriatum from northern Hokkaido, Japan

The peculiar taphonomy of the late Campanian ammonite Metaplacenticeras subtilistriatum is reconstructed on the basis of stratigraphic occurrence within the Hakobuchi Formation of northern Hokkaido (Japan), as well as of host rock and patterns of fragmentation. The species' taphonomy is closely linked to its streamlined, oxyconic shell with flat flanks and a smoothly tapered venter. Such shells were accumulated, together with numerous pebbles, by bottom currents (>0.25–0.5 m/s), in such a way that ventral portions of some shells fractured by collision with pebbles. Small holes and depressions in the external shell surface, presumably formed during compaction after final burial, are restricted to phragmocones. On this evidence, it is suggested that this species was nektobenthic, lived in a shallow environment (shoreface–inner shelf) where its streamlined shell allowed it to align with prevailing currents, similar to a wind vane.     

The ammonite fauna and genesis of a mid-Cretaceous siliceous oolite from the Alagoas Basin, Brazil

Acanthoceratid ammonites from near Maceió, in the State of Alagoas, provide evidence of a mid-Cretaceous marine incursion into the present onshore part of the Alagoas Basin. The ammonites Pseudocalycoceras sp. cf. P. harpax (Stoliczka, 1864) and Kamerunoceras sp. are assigned a late Cenomanian age. The rock is a siliceous oolite that occurs as derived nodules and fragments in late Cenozoic continental sediments. It is interpreted as an originally calcareous oolite formed in a near-shore, high-energy environment. Absence of primary cement suggests that silicification took place early in diagenesis. The silicified, and therefore more resistant material from the original Cenomanian sequence was reworked and redeposited with the Cenozoic sediments. The age, indicated by the ammonites, suggests that the original deposition was related to the global late Cenomanian—early Turonian sea-level rise.     

Mode of occurrence and taphonomy of the heteromorph ammonite Pravitoceras sigmoidale Yabe from the Upper Cretaceous Izumi Group,Japan

The heteromorph ammonite Pravitoceras sigmoidale Yabe, of the family Nostoceratidae, is the zonal marker of the upper Campanian P. sigmoidale Zone in southwest Japan, and is the main component of ammonite assemblages in this zone. We explain the taphonomic processes underlying the occurrence of P. sigmoidale in the Izumi Group, specifically in the Minato (Awaji Island), Anaga (Awaji Island), Koike–Omoizaki (Shikoku), and Hidonodani sections (Shikoku). The first two sections consist mainly of the non-turbiditic Northern Marginal Facies (NMF), while the latter two sections comprise the Main Facies (MF), a turbiditic facies deposited in waters deeper than those of the NMF. We recognise three modes of occurrence of P. sigmoidale, as follows: (1) In nodules crowded with P. sigmoidale (NCP); this mode occurs only in the NMF, and includes juvenile and adult specimens, together with other ammonites such as Solenoceras (Oxybeloceras) aff. humei (Douvillé), (2) In mudstone with isolated P. sigmoidale (MIP), which is found in all of the sections studied, (3) In sandstone with isolated P. sigmoidale (SIP), which occurs only in the MF. Adult individuals of P. sigmoidale are dominant in most of the sections, while juveniles were observed only in the Minato section, which originally was closer to land areas. The number of specimens of P. sigmoidale and Solenoceras spp. tends to increase in sections representing depositional environments proximal to terrestrial areas. Ammonite assemblages could have been transported from shallow- to deep-water settings by turbidity currents. The sorting of components by transport processes likely contributed to the formation of these fossil assemblages in different areas. This is an important clue to understanding the habitat of heteromorph ammonite life assemblages consisting mainly of P. sigmoidale.