Phylogeny of the Nautiloids

A semiquantitative method of the phylogeny reconstruction is applied to study of fossil Nautiloidea. Morphology of calcareous deposits, appearing first in the phragmocone of the baItoceratid Ellesmeroceratina, supports a concept of their secretion from the cameral liquid. This model explains either the lack of both types of deposits in Ellesmeroceratina with the siphon retreating from the older parts of the phragmocone (as indicated by formation of diaphragms), and the lack of cameral deposits in the Endoceratina , whose long septal necks prevented an effective exchange of cameral liquid between the siphuncle and camerae. The baltoceratid Cochlioceras was ancestral to both the Endoceratina, which originated before the earliest Arenigian with elongation of the septal necks, and the Orthoceratina, which originated with a shift of the siphuncle from a ventral to central position in the phragmocone. The Discosorida evolved before the L1andeilian from unidentified, endogastrically curved. breviconic ellesmeroceratids probably through a swelling of the connecting ringsl They developed radial lamellae in the siphuncle and a complexly shaped adult shell aperture but the general shape of their endogastricalIy curved, compresse d shelI persisted until their extinction. The Oncoceratida evolved before the latest Arenigian from exogastrically curved ElIesmeroceratina. They differ from the ancestors of the Tarphyceratida in having a short body chamber. Oncoceratids do not differ from the Discosorida in structure of the connecting rings. The Actinoceratida evolved from some as-yet-unknown Late Arenigian sactoceratid Orthoceratina. The radial structure of siphuncular deposits, recognized commonly as typical of the actinoceratids, also occurs in some Orthoceratina (Clinoceras), whereas there are no radial blocks in the siphuncle of some Actinoceratina (Eushantungoceras). The Lituitina evolved from Late Arenigian Orthoceratina with a relatively breviconic shell, and elongated spetal necks. They are characterized by connecting rings disappearing during ontogeny and/or by the narrow ventral sinus of the shelI aperture. The main trend in their evolution was toward an exogastric shell with a deep apertural sinus, the coiling of which begins in the juvenile stages. It is suggested that L1andeilian Centrocyrtocerasis the earliest representative of the Nautilida, and links them with kionoceratid Orthoceratina. The retractor attachment scar typical of the Nautilida appeared only in the Silurian Lechtritrochoceras(Kosovoceras) due to a shift of the main attachment area from the ventral to lateral sides of the shell. The large protoconch appeared in the main nautilid stock as late as the Carboniferous. A modified systematics of the subclass Nautiloidea is proposed. 137 nautiloid species, mostly from the Paleozoic and Mesozoic of Poland, are described and/or illustrated. The following new taxa are erected: Lituitina, new suborder; Oncotoceratidac, new family; Weberoceratidae, new family; Arionoceratidae, new family; Cochlioceras roemerisp. nov.: Lysagoraceras kielcense sp. nov.: and Gonatocyrtoceras longissimum sp. nov.