Jurassic Ammonites

directly or indirectly each bed of the Les Fours à Chaux
section to units of the Bajocian standard biozonation of the
West Tethyan sector of the Sub-Mediterranean-Caucasian
Subrealm. Comparison with the sections of Bretteville-sur-
Odon and Maizet shows a differentiated evolution of the
Bajocian successions in the southern interior of Caen: the
lower part of the succession is similar and can be attributed
to eastward deepening carbonate ramp whereas the upper
Bajocian of the Fours à Chaux sector is reduced by higher
stratigraphic condensation and reflects the deposition in
coastal environments of the south-eastern sector of the
Armorican Massif. The comparison with the
contemporaneous levels present at Sherborne in Dorset
shows equivalencies; however the different criterion of the
taphonomic analyses carried on those successions prevents
any accurate parallelism. The importance of the Les Fours à
Chaux locality, over the taxonomy of ammonites and the
valorisation of the geological heritage, is highlighted.
1. INTRODUCTION

Thick successions of marine Middle Jurassic deposits rich in ammonites occur in the Jameson Land Basin in central East Greenland. The so-called Cranocephalites beds of this basin, comprising the Indistinctus–Pompeckji Zones were until now largely represented by a single collection. This was made by T. Harris during a 1927 excursion up the valley of Ugleelv to Katedralen, the type area of Cranocephalites pompeckji, which is the oldest named species of this genus. Revisits to this area in 1994 and 1996 by JHC resulted in a large bed-by-bed collection of Cranocephalites. The number of faunal horizons that could be distinguished grew from the nine previously recognized to 34 today. The zonal stratigraphy of the Cranocephalites beds encompasses, the Borealis, Indistinctus and Pompeckji Zones. The Pompeckji Zone is subdivided into four new subzones, reflecting four successive basic morphologies of Cranocephalites that should be recognizable more widely and are thus useful for subzonal correlations. The detailed zonation, that serves as the secondary standard zonation for the Boreal Province in the Middle Jurassic is thus highly improved. The biostratigraphic resolution obtained here is near the achievable limits. It allows a high-resolution study of the evolution of the ammonites which on this time-scale appears to be continuous. New species are described: Cranocephalites carolae sp. nov., Cranocephalites intermissus sp. nov., Cranocephalites episcopalis sp. nov.

A recent finding of Pleydellia buckmani Maubeuge, 1947 (Hildoceratoidea, Graphoceratidae, Dumortieriinae)  from  Monte  Nerone  (PU,  Italy)  is  here  reported  and  documented.  This graphoceratid is a marker of the Pleydellia buckmani Subzone (last subzone of the Pleydellia aalensis Biozone, uppermost Toarcian; see Cariou & Hantzpergue, 1997). Ammonites referred to this taxon are typical of late Toarcian successions from Luxembourg, Germany, France, Spain, Bulgary and Iran (Maubeuge, 1947; Rulleau et al., 2001; Schulbert, 2001; Weis et al., 2010; Di Cencio, 2014a) but are seldom reported in Italy. In fact, the taxon was signaled only once from the “condensed” succession of the Monte Nerone Area (Fosso del  Presale  section;  Cresta  et  al.,  1995)  and,  with  certainty,  it  was  reported  in  uppermost Toarcian sediments from Sicily (Pallini et al., 2004). The studied specimen was collected in well-bedded, hazelnut, nodular mudstone-wackestone strata (Calcari nodulari nocciola sensu Centamore et al., 1971) pertaining to the Bugarone Fm. (Jacobacci et al., 1974). P. buckmani is easily recognizable, differing from all other species of the genus in its several very fasciculated and sinuous ribs (Di Cencio, 2014b). The specimen here described sheds new light on the stratigraphy and biostratigraphy of the Monte Nerone
Area.  Several  aspects  of  the  Umbria-Marche  Domain  have  remained  still  unexplored,  and numerous  long-standing  problems  are  still  open  to  debate.  A  “renaissance”  of  ammonites studies can definitely play a fundamental role in this respect.

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Cresta S., Basilici G., Bussoletti A., Comas M.J., Elmi S., Fernandez-Lopez S., Ferretti A., Goy A., Henriques M.H., Kalin O., Jimenez A., Martinez  G.,  Sandoval  J.,  Ureta  S. &  Venturi  F.  (1995).  Relazione  sul Workshop  “Paleontologia  e  Biostratigrafia dei  sedimenti ammonitiferi  toarciano-bajociani  in  successioni  di  alto  morfostrutturale  dell‟Appennino  umbro-marchigiano:  l‟area  di  Monte Nerone. Correlazioni con altre regioni della Tetide occidentale” (Piobbico, 2-14 agosto 1993). Boll. Serv. Geol. It., 112: 129-139, 1 fig., 1 tav. 
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Di  Cencio  A.  (2014b).  The  systematic  and  taxonomic  revision  of  the  „Pleydellia-group‟  ammonites  from  the  Minette  ironstones  (Late Toarcian), southern Luxembourg. 9th International Symposium Cephalopods – Present and Past & 5th International Symposium Coleoid Cephalopods through Time, 4-14 September 2014, Zurich.
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Schulbert C. (2001). Die Ammonitenfauna und Stratigraphie der Tongrube Mistelgau bei Bayreuth (Oberfranken). PhD Thesis: 1-183, 31pls.
Weis R., Mariotti N., Di Cencio A., Faber A., Fuchs D. & Guérin-Franiatte S. (2010). Cephalopods from the Jurassic „Minette‟ Ironstone Formation of Luxembourg and Lorraine (NE Paris Basin): an update. 8th International Symposium Cephalopods  – Present and Past, 30 August – 3 September, Dijon.