Archigeisonoceras Chen, 1984

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 66

Genus Archigeisonoceras Chen, 1984

Type species: Archigeisonoceras elegatum Chen, 1984 .

Diagnosis.— Geisonoceratidae with a smooth shell. The suture is straight, the siphuncle slightly eccentric. The septal necks of the juvenile parts of the conch are suborthochoanitic, in later growth stages short orthochoanitic. The connecting ring slightly expands within the chambers. The annulosiphuncular deposits are relatively small and slight. Thin episeptal and hyposeptal deposits occur.

Remarks.— Chen et al. (1984) placed the genus in the family Geisonoceridae , which is correct according to Sweet (1964) and Balashov and Zhuravleva (1962). Zou (1988) defined the Early Ordovician genus Gangshanoceras Zou as a member of the family Proteoceratidae Flower, 1962 . Gangshanoceras Zou , and Archigeisonoceras Chen are virtually identical except for the expansion of the conch and the shift of the siphuncle from the shell wall toward the centre in Gangshanoceras . Important is that both genera develop suborthochoanitic septal necks in their juvenile septa but short orthochoanitic ones in the following septa (a feature known from primitive ellesmerocerids). Consequently both genera fall within the Geisonoceridae as well as Proteoceras Flower , which is the type genus of the family Proteoceratidae . The family Proteoceratidae is therefore in conflict with the scope of the Geisonoceratidae . Flower (1962) did not accept the family Geisonoceratidae as defined by Zhuravleva (1959) because of the weak definition of the boundary from the family Michelinoceratidae Flower, 1945 (inclusive criterion: tubular siphuncle). Later the originally wide scope of the Michelinoceratidae sensu Flower (1945) and Flower (1962) was restricted (e.g., Sweet 1964), and the boundaries of the Geisonoceratidae are defined by exclusion: Pseudorthoceratidae (exclusive cyrtochoanitic necks), Michelinoceratinae (without endosiphuncular deposits). Nevertheless, the early Middle Ordovician Archigeisonoceras , Proteoceras , and Gangshanoceras are intermediate between the Pseudorthoceratidae and the Geisonoceratidae . Chen (1984) illustrated four subadult specimens of Archigeisonoceras . The adult living chamber and the apical chambers of Archigeisonoceras are known only from the Baltoscandic material. Chen (1984) mentioned that Archigeisonoceras differs from Virgoceras Flower in its less developed annulosiphuncular deposits. But this difference may be visible only in the certain juvenile parts of the phragmocone, because strong and fusing annulosiphuncular deposits occur in the apical parts of the phragmocone in A. picus , A. folkeslundense , and A. repplingense . Virgoceras , which is mainly known from the Middle Silurian of Bohemia, differs from Archigeisonoceras , which is known only from the Middle Ordovician, in its stronger inflated siphuncle and in its ornamentation. Distinct growth lines as in Virgoceras are not known in Archigeisonoceras . It may be concluded that Archigeisonoceras and Gangshanoceras , which was also found in the late Arenig of Öland ( King 1999), are the earliest known Geisonoceratidae .

Species included.— Archigeisonoceras elegatum Chen, 1984 , Archigeisonoceras robustum Chen, 1984 , Archigeisonoceras picus sp. nov., Archigeisonoceras folkeslundense sp. nov., Archigeisonoceras repplingense sp. nov.