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Early growth‐stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia

2006; Wiley; Volume: 39; Issue: 2 Linguagem: Inglês

10.1080/00241160600623749

1502-3931

Björn Kröger,

Paleontology and Stratigraphy of Fossils

LethaiaVolume 39, Issue 2 p. 129-139 Early growth-stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia Björn Kröger, Björn Kröger Björn Kröger [bjoekroe@gmx.de], Museum für Naturkunde an der Humboldt Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, GermanySearch for more papers by this author Björn Kröger, Björn Kröger Björn Kröger [bjoekroe@gmx.de], Museum für Naturkunde an der Humboldt Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, GermanySearch for more papers by this author First published: 12 March 2007 https://doi.org/10.1080/00241160600623749Citations: 22AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Five apices of orthoceridan cephalopods from the early Middle Ordovician Holen Limestone of Öland, Sweden that where collected in the late 19th Century by G. Holm provide information on cephalopod evolution in the early Palaeozoic. The apices belong to specimens of the genus Hedstroemoceras Foerste, 1930 and Archigeisonoceras Chen, 1984. The apices are small in comparison with apices of other cephalopods of the Ordovician; the initial chambers of the shells of both genera are hemispherical and approximately 1 mm and 1.5 mm in cross-section diameter, respectively. The apical 2–3 mm of the shell are free from growth-lines and possess no cicatrix, though distinct longitudinal wrinkles are present. There is a slight variability of siphuncle position during early growth in Archigeisonoceras. It can be shown that the structure of the connecting ring of Hedstroemoceras is similar to that of other Orthocerida. Additionally, the hemispherical apex of Lituites perfectus Wahlenberg, 1821 gives evidence for the orthoceridan affinity of lituitidans. The investigation shows that early Middle Ordovician Orthocerida display a characteristic connecting ring structure, a characteristic apex morphology and variable siphuncular positions that differs significantly from other cephalopods of the Ordovician. Based on this evidence it is concluded that a small spherical apex is an autapomorphy of the Orthocerida. Moreover, this evidence supports a splitting of the order Orthocerida in two taxa of different affinities. The Orthocerida sensu stricto comprises orthocones with a tubular siphuncle nearly without endospiphuncular deposits, and a spherical apex. Embryonic shell, orthoceridan ancestry, orthoceridan classification. Citing Literature Volume39, Issue2June 2006Pages 129-139 RelatedInformation