Rivista Italian,r di Paleontologia e Stratigrafia volume I I u no. I J prs, pp. 141-149 Apflt luuî MIDDLE JURASSIC NAUTIT,OIDEA FROM \TESTERN FRANCE PATRICK BRANGER Receiz'ed Septetnber 19,2002; dccepted October 23, 2003 Kq, irords : Nautilida, Cephalopod, Middle Jurassic, Biostratig- Introduction raPh)', Poitou, Frrnce. \Well known for its rich ammonite fauna, the Mid- Abstract. The Middle shelf margin limestones and nrarly Jurassic dle Jurassic shelf margin limestones and marly limestones limestones of Vesrern France vield quite numerous Nautilida. This record from the Seuil du Poitou (\Western France) were inten- is here described for the first time in detaì1, nine genera and more than sively last years. spite fre- fortl' species, whose range rarely exceeds one or two ammonite biozones, studied during the forty In of constitute a rather precise tool to date Middle Jurassic beds. Each taxon quent discontinuities throughout the succession, most is placed in an accurate biostratigraphical chart; a new genus, Pictonautilus biozones recognised elsewhere in Western Europe have nov. gen., is described. The strxtìfgrphical nnge of Cenoceras, Digonioceras been found, except for the Middle Bathonian, where the and Gen. B n. ? (sensu Chirat 1997) is extended up to the Middle Callovian. Tulites subcontractus Morrisiceras morrisi Zones are In the Jurassic successions of Poitou, Nautilida always represent a mrnor ^nd missing (Mangold et 1971). Many biohorizons and p,rrt of the fossil record and thev ;lre nrore abundant in four levels: Mid- ^1. dle Aalenìan, base of Upper Bajocian, Lox,er Bathonian and top of Mid- subzones of large extension (Gabilly 1964; Gabilly et al. dle Bathonian. Ds'arf Nautilida (Paracenoceras) and species showing nrore 1921; Cariou 1974;1984) have been defined here. From fofded septa and a subventral siphuncle (Pseutlaganides) or strongly ribbed Aalenian to Callovian, these levels also yield quite nu- specinrens (C),matonautilus) are characteristic of stable open-shelf environ- merous Nautilida that have never been studied by these nrents (Callovian), whereas large shelled nautilus would be mostly present authors. few of these fossils were described by Tin- during deepening episodes. The geographical extension of many tîxa ìndi- Only cates a provincialisnr that fits with that of amn.ronites of the same period. tanr (1969; 1980; 1987; 1994). For the last twenty years, beside the other fossils, I carefully collected and identi- Riassunto. I calcari e le marne calcaree del margine di piattafor- fied more than forty species of Nautilida, mostly on the ma del Giurassico medio della Francia occidentale hanno fornito Nau- North Aquitaine border of the Seuil du Poitou. This pa- tilida abbastanza nunrerosi. Questa documentazione è qui descritta in per presents the first results of such a research. dettaglio per la prin.ra volta, nove !Ìeneri e più di quaranta specie, la cui distribuzione raranrente supera una o due biozone ad anrmoniti, e costi- tuisce uno strumento piuttosto preciso per datare gli strati del Giurassico medio. Ogni taxon è posto in un accurato diagramma biostratigrafico; Systematics and stratigraphic distribution viene descritto un nuovo genere, Pìctonautilus nov, gen. La distribuzione stratìgrafica di Cenoceras, Digonioceras e Gen. B n. ? (sensu Chirat) viene Most of the Middle Jurassic fossiliferous beds of esteso fino al Calloviano medio. Nelle successioni giurassiche del Poitou, Poitou contain Nautilida. Fig. 1 indicates the biostrati- i Nlutilida rappresentano sempre una parte minore della documentazlone graphical range for each genus and species, in a detailed fossile, e sono più abbondanti ìn quattro livelli: Aaleniano medio, base del Bajociano superiore, Bathoniano inferiore e sommità del Bathoniano biostratigraphical chart based on ammonites. Most Nau- nredio. I Nautilida nani (Paracenocer4J), le specie che mostrano setti piìl tilida are adults and always associated with ammonites. piegati ed un sifone subventrale (Pseudaganides), e gli esemplari forte- Their frequency increases throughout four levels: Lower nrenre cosrari (C),matonautilus) sono caratteristici di ambienti stabili di and Middle Aalenian, base of Upper Bajocian, Lower Ba- piattaforn.ra aperta (Calloviano), mentre i nautili con grandi conchiglie thonian and top of Middle Bathonian, which correspond sono nraggiormente presenti durrnte gli episodi di approfondimento. Lestensione geografica di molti t:rxa indica un provincialismo che coin- to transgressive units (Gonnin er al. 1,992; 1993) . Till now, cide con quello degli ammoniti durante lo stesso periodo. the Graphocerds concdaum (Late Aalenian), Hyperlioce- Faculté des Sciences, Universitó de Poitiers, Laboratoire de Géobiologie, Biochronologie et Paléontologie Humaine, ,lO Avenue du Recteur Pineau, 86022 Poitiers cedex, France. E-mail: patrick.branger@,n'anadoo.fr 142 P Branget .l :l'l I .ll'l ìta rl :lsli ':tÈt;t_= c ÈtìtÌ zot\fts Slil-rl: : | ì t,: F Èlì lÉ ,1, :lì Èlil'lj li \-l ' :l: l( lr -ìlu l!1. ililì i[H.l.|: -l il,l.!'l: Èl,l,l, -l -tlst t1 li l. l, l. l, lrl, l* l'1, lil. lil. È1. | | l:l z \\t Bl TIILIJI o I J \( I:!S l,'* rl r I Ir*' t l, t)t5( tt llkl:\11 kl I z t, c Nf)RRtSt t *rfrl"* I F lR()(iR\( lt ls o I I IIL l,' \R(t\sr)\l tî tl lr r î l? tf c { \tPtiRiliSt \Nl I I'r l*u l,li îtt z : IR \I)f()RDI:NSIS lr ut R( llts0\^t. il \Western Fi,, 1 Stratigraphic distribution of the Nautilida rvithin the Middle Jurassic of Frlnce, lccording to tl.re biozon;rl scheme proposed by the Groupe FranEais d'Etudes du Jurassique ( 1 997). ras discites (Early Bajocian) and Clydoniceras discus (Late show the characteristic sulcate venter and their ventral Bathonian) Zones have not yielded any specimen. The lobe is not deep enough. Inner whorls are involute, show- adopted classification is that proposed by Tintant & ing their close relationship with other Cenoceras. The last Kabamba f 1983). whorl becomes very evolute. The closest species for these fossils rs Cenoceras mazardrikense (Sarkar, 1964) from Baluchistan. Another interesting aspect of such specimens Cenoceratidae Tintant 8r Kabamba, 1983 is their perfect sin-rilarity with contemporary Procyma- Cenoceras Hyatt, 1883 toceras pictonicumTintanr (1969), the only difference be- ing the ribbing that is missin g on Cenoceras mazardribense Cenoceras, a world-wide taxon which appears in shells. As the lack of ribbing may depend on fossilisation, the Upper Triassic from New-Zealand (Kummel 1956), Procymatoceras pictonicum Tinrant is here regarded as a is here restricted to shells with rounded section, usually junior synonym for Cenoceras mazardrikense (Sarkar); wider than high; their suture line shows shallow ventral at most P. pictonicum could be kept as a subspecies of and lateral lobes. The body chamber section often widens Sarkar's taxon. Cenoceras survived at least until Middle and its ventral part flattens in the adult stage. Callovian (Reinecheia anceps Zone) (Fig. 1). Specimens D H h \(/ w U U $r/H 440 mm 180 mm 0.40 240 mm 0.54 110 mm 0.25 1.33 C. mazardrikense (P849) 200 95 0.47 120 0.60 40 0.20 1.26 380 175 0.46 200 u-)z 80 4.21 1.14 "P pictonicwm" (P865) 280 140 0.50 170 0.60 55 0.19 1.21 D: diameteqH: height,h: H/D,V: vidth, rv =V/D,U = umbilicusn,idth,u: U/D PB = collections of Patrick Br:rnger. In this taxon I also include large specimens up to Digonioceras Hyatt, 1894 45 cm in diameteq that are often found in the Lower and Middle Callovian and probably already in the Upper Recently rehabilitated by Chirat (1997), the genus Bathonian. These Nautilida cannot belong to the Parace- Digonioceras corresponds to Jurassic Nautilida (Toarcian noceras group because) even at a large size, they never to Bathonian) which section is usually wider than high Nautiloidea from western France t43 and with a wide umbilicus. Characteristic of this genus is The type species has only been figured by Lissa- the existence of an angular margin that borders the um- jous (1923) and cited by Tintant (198ab; 1987). Picto- bilical u'edge and appears more or less early during the nautilws extends from Upper Bajocian to Late Batho- ontogenesis. At present, Digonioceras has not yet been nian. recorded outside Europe and Arabia. The most inreresr- Discussion. Most of the Nautilida here considered ine specin.ren from Poitou (Pl. 1, fig.6) comes fron.r the as a new genus s/ere previously ranged into the genus Pa- lower part of the Ermnoceras coronatum Zone (Middle racenoceras (Tintant 1984b; 1987,p.92) fromwhich they Callovian) of Exoudun; it is very evolute (IJ/V : 0.29) differ sienificantly for their venter that is never grooved and densely septated (2a to 24 in the inner whorls). This and is bordered by sharp angular margins. The shape of new species represents :r still unnamed species and is the the venter is quite sirnilar to the one of Somalinawtilus first record of this genus within the Callovian (Fig. 1). Spath, 1,927, which is probably the closest genus. Nev- ertheless the latter differs mostly in the feature of the Oph ion autilus Spath, 1927 suture line that shows a lateral saddle according to the type species described by Dacqué (1905, pl. III, fig. 3a). Ophionautilus represents r minor part of the Nau- Only known by the type species, Somalinawtilazs is also tilida found in the Seuil du Poitou area. These larse and separated from Pictonawtilus by a important stratigraphic evolute shells are here represented by the single species gap (Kimmeridgian) (Fig. t). Opbionautilus burtonensis (Foord & Crick, 1890) from Species recognised as Pictonautilus the Upper Bajocian layers (Strenoceras niortense and Ga- - Pictonnutilus r;erciacense (Lissajous), Middle and Upper Bathonian rantiana garantiana Zones) (Fig. 1).