Lytoceras

Abstract: Jurassic ammonites of late Bajocian age occur in the Cook Inlet region, Alaska, along the west side of Cook Inlet, and in the southeastern part of the Talkeetna Mountains. The dominant genera are Liroxyites, a subgenus of Oppelia, M egasphaeroceras, n.nd Dettermanites. These have not been recorded outside the Cook Inlet region. The ammonites also include a fair number of specimens of M acrophylloceras, Calliphylloceras, and Lissoceras, but only n, few specimens of Leptosphinctes. Lytoceras, Spiroceras?, and Sphaeroceras are represented by single specimens. These ammonites along the west side of Cook Inlet occur only in the lower part of the Bowser member of the Tuxedni formation. This lower part is overlain unconformably by the upper part, which is characterized by Cranocephalites of probable Bathonian age. The lower part is underlain abruptly but conformably by the Cynthia Falls sandstone member of the Tuxedni formation, which has furnished the middle Bajocian ammonites Chondroceras and Normannites from its lower and middle parts. Beneath the Cynthia Falls sandstone member follows an unnamed siltstone member that has furnished many ammonite genera characteristic of the European zone of Stephanoceras hurnphriesianum. The ammonites from the lower part of the Bowser member are dated as late Bajocian mainly because they include Sphaeroceras; Spiroceras?, and Leptosphinctes that are typically late Bajocian, becu.use they do not include any genera that are typical of the Bathonian or middle Bajocian of Eurasia and north Africa; and because they occur above beds that contain many middle Bajocian ammonites such as Chondroceras, Norrnannites, Telocert;ts, Stemmatoceras, Zemistephanus, Stephanoceras, and Witchellia. The presence of Leptosphinctes itself is good evidence for a late Bajocian age, but its association with Sphaeroceras and an uncoiled n.mmonite that is probably Spiroceras is excellent confirmatory evidence. Furthermore, the resemblance of the Alaskan Leroxyites u.nd Dettermanites to the European Oxycerites and Polyplectites, respectively, indicates an age not older than late Bajocian. Such an age for the lower part of the Bowser member is in line with the presence of Cranocephalites in the overlying beds, as that genus appears to represent much of the Bathonian, according to recent studies in Alaska, Montana, and Greenland. The dominance of the ammonites Liroxyites, ll1 egasphaeroceras, and Dettermanites in the upper Bajocian rocks of Alaska contrasts with their absence in the Tethyan region. Similarly the upper Bajocian rocks of the Tethyan region have furnished many ammonite genera that have 110t yet been found in Alaska. These differences indicate that faunal developments were somewhat different in the two regions during Bajocian time.

Abstract: (1) Introduction Dr. Trechmann, who had been favoured with identifications from Mr. S. S. Buckman, at first had the intention of dealing in the foregoing paper with some of the ammonites that he had collected in New Zealand ; but, at his request, I have undertaken, in this appendix, a description of a larger series of ammonites, incorporating therewith observations on certain :New Zealand specimens long since acquired by the British Museum (Natural History) and others sent recently by :Prof. P. Marshall and the Geological Survey of New Zealand. The examples originally forwarded by Dr. Trechmann included five belonging to the genus Psiloceras , of the Lower Lias, and five ammonites from the Upper Jurassic, representing the genera Phylloceras, Uhligites , and Aululacosphinctes 9. In addition there were an unidentifiable fragment of probably an Upper Jurassic ammonite, from Kohai Point, and several Senonian forms, already briefly mentioned elsewhere 1 ; further, a Jurassic Phylloceras and a Lytoceras belonging to Prof. Marshall. The Liassic ammonites in the British Museum, described below, belong to the genera Phiylloceras, Rhacophyllites , and Lytoceras,(Thysanoceras) . There is no detailed information available with these specimens, the matrix of which is a hard, dark-green rock, exactly like that of one of the Upper Triassic Pinacoceras recorded by Dr. Treehmann, 2 or of the Hettangian . Psiloceras . These had ,been bestowed among Neocomian ammonites, perhaps on account of the green matrix, the Lytoceras fragment being labelled 9 Crioceras . With them was the indeterminable impression of an ammonite in an unlocalized, similarly hard, compact, dark-green rock. Two .additional examples

Abstract: Bilotta, M., Venturi, F. & Sassaroli, S. 2010: Ammonite faunas, OAE and the Pliensbachian–Toarcian boundary (Early Jurassic) in the Apennines. Lethaia, Vol. 43, pp. 357–380. The critical Jurassic event known as Early Toarcian Oceanic Anoxic Event (OAE) has been the subject of several studies, but its palaeontological characterization is still problematic, and its dating is therefore debated. The rich ammonite faunas of the Mediterranean Tethys (Italy, Greece, Albania, North Africa, southern Spain, etc.) demonstrate that the OAE separates two very different assemblages, resulting from a strong post-OAE biological renewal. Faunas before the anoxic event include many taxa already present in late Pliensbachian (evolute Phylloceratida; Reynesocoeloceratinae, Protogrammoceratinae and Arieticeratinae), whereas after the anoxic event new forms radiated (Nodicoeloceratinae, Harpoceratinae and Hildoceratinae; afterwards also groups as Mercaticeratinae, Phymatoceratidae and Hammatoceratidae). As these assemblages show remarkable differences from their Northwest European equivalents, in the Apennine it is necessary to use a Mediterranean zonation, here formally established. The examined data suggest accurate boundaries for the OAE, which separates our first two Toarcian Zones, corresponding to Tenuicostatum and Serpentinum standard chronozones. Within this context, the Apennine assemblage placed immediately below the anoxic event is an endemic fauna with more than 15 ammonite genera, two of which (Secchianoceras and Petranoceras) are exclusive of the interval in question. Most abundant are Phylloceratidae (Phylloceras, Lavizzaroceras, Calaiceras and Harpophylloceras), Juraphyllitidae (Meneghiniceras) and Lytoceratidae (Lytoceras); Dactylioceratidae (Dactylioceras, Eodactylites and Secchianoceras) and Hildoceratidae are comparatively less numerous, but these latter show the largest taxonomic diversity (Fontanelliceras, Trinacrioceras, Protogrammoceras, Petranoceras, etc.). The description of this fauna improves the knowledge of a poorly understood interval immediately preceding the OAE, thus allowing better correlations in the Mediterranean area. □Ammonites, chronostratigraphy, Early Jurassic, OAE, Tethys.

Abstract: The Parkinsoni Zone of the Late Bajocian and the Zigzag, Aurigems and Retrocostatum Zones of the Bathonian have been identified on the basis of relatively highly diversified ammonite assemblages within seven ammonitico rosso sections belonging to the Czorstyn Unit, Pieniny Klippen Belt. The ammonite fauna has features in common with both the Mediterranean Province and the Sub-Mediterranean Province, containing abundant Phylloceratina and Lytoceratina on the one hand, but numerous Parkinsonia on the other. Ammonites of Arabian affinities belonging to the genus Micromphalites occur rarely in the Early Bathonian Zigzag Zone assemblage. Lytoceras joniaki , Lissoceras compressus and Cadomites ( Polyplectites ) minutus are proposed as new taxa.