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.

Palaeoecology and palaeogeography of Late Ordovician (Katian–Hirnantian) cephalopods of the Boda Limestone, Siljan district, Sweden

“…), are most common and diverse with a number of species of Isorthoceras . Small orthocerids with highly ornamented shells, such as Dawsonoceras Hyatt (); and Striatocycloceras Kröger & Isakar () are common. The strongly annulated and ornamented shell can be interpreted as a defence against durophagous predators (cf.…”

Cephalopods from reef limestone of the Vasalemma Formation, northern Estonia (latest Sandbian, Upper Ordovician) and the establishment of a local warm-water fauna

“…A comparison of the cephalopod record with brachiopod occurrence pattern provides an independent test of these paleogeographical and pelaeoecological models of faunal distribution. Few cephalopods of the Vasalemma Formation have been described in the past (Balashov 1953;Stumbur 1962;Kröger & Isakar 2006). Our own collections and existing museum collections provide a material of more than 300 specimens, which are described herein.…”

Colour patterns on Silurian orthocerid and pseudorthocerid conchs from Gotland – palaeoecological implications

“…The relatively thick siphuncle of Dawsonoceras resembles that of pseudorthocerids, but the spherical apex with missing cicatrix and growth lines developed on the apical part of the shell clearly suggests the position of the family Dawsonoceratidae Flower within Orthocerida (Kröger & Isakar 2006).…”