Ordovician nautiloids of Tasmania

Of the 57 species of nautiloids known in Tasmania 26 are newly described and there are 17 new genera and one new family. The nautiloids in the Ordovician of Tasmania are divided into six biostratigraphic assemblages. The Piloceras-Machuroceras assemblage is Late Canadian in age and occurs at Adamsfieid in the Florentine Valley. It is composed of Pycnoceras adamsense n.sp., Machuroceras excavatwn, M. steanei, Suecoceras robustum, Piloceras tasmaniense, Metamanchuroceras wadeae n.gen.,n.sp.,and Allocotoceras insigne. The Whiterock Wutinoceras-Adamsoceras assemblage occurs in northern and western Tasmania and the Florentine Valley. It contains Wutinoceras paucicubiculatwn, W. multicubiculatum, Adamsoceras johnstoni, Karmbergoceras paucicubiculatum n.gen.,n.sp., and Endoceratidae n.gen.,n.sp. The Discoceras-Gorbyoceras assemblage is widespread in strata of Chazy to Blackriver age in Tasmania. It contains Mysterioceras australe, Gorbyoceras settlementense n.sp., Fitzgeraldoceras juneese n.~e~.,n.sp., Standardoceras burretti n.gen.,n.sp.,Orthoceras sp., Discoceras idaense, Beloitoceras sp., Centrocyrtocerina frizonense n.gen.,n.sp., Armenoceras cox‚Äöbetti n. sp ., Ormocerina asymmetrica n. gen., n. sp., Paramadiganella banksi n.gen.,n.sp., Florentinoceras calveri n.gen.,n.sp., Anmoceras lauriei n.gen.,n.sp., Octa,ceras uniquum n.gen.,n.sp., Felixoceras curtainense n. gen., n. sp ., and Probotryceras u.iestfieldense n. gen., n. sp .. The Tasmanoceras-Hecatoceras-Gouldoceras assemblage is the most widespread, occurring throughout Tasmania in the Early Trenton. It contains Anaspyroceras? anzaas, Discoceras reidi n.sp., Beloitoceras kirtoni, Mia~oceras bubsense n.sp., Zeehanoceras teicherti n.gen.,n.sp., Tasmanoceras zeehanense, T. pagei n.sp., Hecatoceras longinquwn, Gouldoceras synchonena, G. obliquum, G. benjaminense n.sp .. The Gordonoceras assemblage is of uncertain age, probably Eden, and occurs in the Florentine Valley and the Gor~on River. It contains only one species Gordonoceras bondi. The Westfieldoceras assemblage is Maysville in age and is only found in the Florentine Valley. It contains Westfieldoceras taylori n.gen.,n.sp., Tigeroceras florentinense n.gen.,n.sp., Armenoceras? sp., and Gouldoceratidae, n.gen.,n.sp.. Stromatoceras eximium, Ephippiorthoceras decorwn,Sinoceras? sp., Belo-Z:toceras? molense n.sp., and Centrocy11 tocerz.'.na sharplesi n.gen.,n.sp., cannot be assigned to an assemblage. The number of endemic genera in the Ordovician of Tasmania increases from a relatively low number in the Late-Early Ordovician to a very high number in the Middle and Late Ordovician. An origin for the Discosorida in the Ellesmeroceratida is suggested by the simple two-layered connecting rings present in the Gouldoceratidae (n.fam., Discosorida). The Gouldoceratidae evolved from the Central Australian genus Madiganella along two different trends. The first trend is towards more orthochoanitic septal necks and a relative increase in the size of the siphuncle. The second trend is towards more cyrtochoanitic septal necks and for a change in the slope of the siphuncular segments. Armenoceras corbetti has dorso-ventrally differentiated adnation similar to the Wutinoceratidae and supports an origin in that family for the Armenoceratidae. Centrocyrtocerina is a gyroconic cyrtocerinid which evolved from the Late Canadian Eothinoceras through Tangshanoceras. Kal'l1ibergoceras is an unusual member of the Endoceratidae as it has two endosiphotubes adorally forming a divided endosiphocone, the phylogenetic affinities of this genus are unclear. The shell structure is preserved in some specimens of nautiloids from Tasmania. They show that fossil nautiloids had a three-layered shell similar to that in Nautilus except that the oldest specimens known have only two-layers, suggesting that primitively nautiloids had only two layers and that the third (inner) layer evolved later.