Revision of annulated orthoceridan cephalopods of the Baltoscandic Ordovician

Abstract

The annulated orthoceridans of the Middle and Late Ordovician of Baltoscandia are described and their systematic frame is revised. The revision of these nautiloids, which are part of the Orthocerida and Pseudorthocerida, is based on the investigation of characters of the septal neck, the siphuncular tube, and the apex. An unequivocal terminology of these characters is suggested and applied. The shape of the septal neck and the siphuncular tube are described for the first time in Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum Eichwald, 1860, and Gorbyoceras textumaraneum (Roemer, 1861). Ctenoceras sweeti n. sp. is erected. The apex of Dawsonoceras barrandei Horný, 1956 is figured and described for the first time. The distribution of the character states of the apex and the septal neck support the emendation of the families Orthoceratidae, Dawsonoceratidae, and Proteoceratidae. The analysis shows also that the families Kionoceratidae, and Leuroceratidae must be refused because they represent not natural groups. However, it is also shown that the present knowledge is not sufficient to establish an unequivocal classification of the Middle, and Late Ordovician annulate cephalopods.Schlü sselwö rter: Cephalopoda, Orthocerida, Mittleres Ordovizium, Spä tes Ordovizium, Estland. ZusammenfassungDie orthoceriden Cephalopoden des Mittleren bis Spä ten Ordoviziums im Baltoskandium werden beschrieben und revidiert. Die Revision dieser Cephalopoden, welche zu den Orthocerida und Pseudorthocerida gehö ren, stü tzt sich auf die Untersuchung der Apikalenden, der Septalduten und der Form der Siphonalrö hre. Eine eindeutige Terminologie fü r diese Merkmale wird vorgeschlagen und angewandt. Die Form der Septalduten und der Siphonalrö hre von Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum, Eichwald, 1860 und Gorbyoceras textumaraneum (Roemer, 1861) wird erstmals beschrieben. Die Art Ctenoceras sweeti n. sp. wird aufgestellt. Der Apex von Dawsonoceras barrandei Horný, 1956 wird erstmals beschrieben und dargestellt. Die neu gefundenen Merkmale stü tzen eine Emendation der Familien Orthoceratidae, Dawsonoceratidae und Proteoceratidae. Es wird daher dafü r plä diert, die Familien Kionoceratidae (Hyatt, 1900) und Leuroceratidae (Sweet, 1964) nicht mehr zu verwenden, da diese keine natü rlichen Gruppen reprä sentieren. Die Untersuchung zeigt aber auch, dass es derzeit noch nicht mö glich ist die annulaten Cephalopoden des Mittleren und Oberen Ordoviziums zweifelsfrei zu klassifizieren.

The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden

“…The siphuncular segments are barrel-shaped; rapidly increasing in diameter close to the septal perforation, the side directed toward the conch center bends inward slightly, forming a shallow concave connecting ring segment ( Figs 13A, 14B). The asymmetrical shape of the connecting ring was noted in material previously described by Kröger & Isakar (2006). The septal necks are short and suborthochoanitic.…”

“…The errors must be corrected as follows: Striatocycloceras obliquum (Teichert, 1930) is figured in Kröger & Isakar (2006: figs 7k, o, 8d, 11), as correctly stated in the text, and Striatocycloceras undulostriatum (Hall, 1847) is also correctly figured (Kröger & Isakar 2006: figs 7l 8e, 10a); other species shown are Striatocycloceras romingeri (Foerste, 1932) (Kröger & Isakar 2006: figs 7p, 8r-t) and Striatocycloceras foerstei (Teichert, 1930) Kröger & Isakar (2006: figs 7q, 10h).…”

“…The diagnosis is adopted from Kröger & Isakar (2006). There the genus was revised and discussed in detail.…”

“…The external characters were described in detail by Kröger and Isakar (2006;see also Figs 16A-B). In specimen PMU 26706 additional details of the siphuncle are preserved.…”

“…This species was revised by Kröger & Isakar (2006) and its main features were described based on specimens from the Porkuni Formation of Estonia. Here, additional data regarding the variation of the external characters and features of the siphuncle are noted.…”