Piersaloceras Teichert, 1930

Kröger, Björn, 2007, Concentrations of juvenile and small adult cephalopods in the Hirnantian cherts (Late Ordovician) of Porkuni, Estonia, Acta Palaeontologica Polonica 52 (3), pp. 591-608 : 601

Genus Piersaloceras Teichert, 1930

Type species: Piersaloceras gageli Teichert, 1930 , by monotypy. Lyckholm Stage , (= Nabala, Vormsi, and Pirgu Regional Stage), Late Ordovician, Piirsalu, Estonia .

Diagnosis.—Slightly cyrtocone graciloceratid with angle of expansion of 13 °. Piersaloceras differs from all other Graciloceratidae in having crenulated frills and longitudinal ribs. Cross section of conch is nearly circular or slightly depressed. Adult body chamber is tubular, less curved than juvenile growth stages. Similar to other Graciloceratidea Piersaloceras displays thin tubular siphuncle, positioned between centre and conch margin at convex side of curvature of growth axis. Sutures are straight or with very low lobe at flanks. Septal spacing is narrow, septal curvature shallow. Septal necks are achoanitic or very short suborthochoanitic. Siphuncle is slightly expanded within chambers. No endosiphuncular deposits are known (after Teichert 1930).

Remarks.—This monotypic genus was regarded by Dzik (1984) as a junior subjective synonym of Zitteloceras Hyatt, 1884 . However, the siphuncle of Piersaloceras is much further from the shell margin than that of Zitteloceras , and there are differences in siphuncular shape and length of septal necks that justify the generic distinction between Piersaloceras and Zitteloceras . The classification of Piersaloceras within the Oncocerida is disputed. The genus was originally classified within the Kionoceratidae by Teichert (1930) with regard to its characteristic ornamentation. Dzik (1984) also emphasised the ornamentation when he classified the genus within Zitteloceras and the Rutoceratidae . However, in the Rutoceratidae , a characteristic Devonian family, the siphuncle is invariably ventral and the septal necks are clearly longer (orthochoanitic−cyrtochoanitic, Sweet 1964). Following Balashov (1962) and Sweet (1964), Piersaloceras is classified within the Graciloceratidae . The characteristic features of the Graciloceratidae are an eccentric or marginal, thin, tubular siphuncle at the convex side of the conch, and a sculpture that consists of longitudinal and transverse elements. This is similar to Piersaloceras . The classification of the genus in consideration within the Graciloceratidae of the Oncocerida is thus fully justified.

A crenulated frilled shell similar to that found in Piersaloceras is developed in the Middle and Late Silurian oncoceridans Hercocyrtoceras Foerste 1928 , Corbuloceras Horný, 1965 , and Torquatoceras, Stridsberg, 1988 . Additionally, a scalloped or festooned sculpture that may represent an incipient stadium of crenulated frilled shells is present in Dawsonoceras Hyatt, 1884 and Cedarvilleoceras Shimizu and Obata, 1935 , Cyrtorizoceras Hyatt, 1900 , and Zitteloceras Hyatt, 1884 . These cephalopods, although similarly ornamented, belong to very different higher taxa of the Middle Ordovician–Middle Silurian time interval. Because of the obvious homeomorphy of this sculpture pattern, an adaptive merit of this pattern is discussed by Stridsberg (1988). However, fabricational noise or non−adaptional constraints are alternative explanations. A further discussion needs a thorough investigation of the specific sculpture pattern.

Occurrences.—Pirgu Regional Stage–Siuge Member, Porkuni Regional Stage, Late Ordovician of north Estonia.