On the origin of bactritoids (Cephalopoda)

There is a high probability that bactritoids represent a paraphylum or polyphylum. The initial chambers or protoconchs of the Early-Middle Devonian Bactrites Sandberger, Devonobactrites Shimansky, and Lobobactrites Schindewolf are elongated spheres with a diameter of 0.3–1.0 mm. The initial chambers are larger in diameter than the slender, smooth shaft located adorally to the initial chamber. Similar apices occur in a number of Late Silurian sphaerorthoceridans with central siphuncles. Sphaerorthoceridans with a bactritoid-like apex and an eccentric siphuncle are known from the Early Devonian. The earliest questionableBactrites occurs in the Pragian (middle Early Devonian). By Emsian time bactritoids are common elements of cephalopod faunas.Bactrites-like orthocones of the Middle Ordovician and Late Silurian are homeomorphs with clearly different early growth stages. Thus, the time interval between the first appearance ofBactrites and the origin of ammonoids can be narrowed down to the Pragian to Early Emsian. The placement of the siphuncle in a ventral marginal position has been used as one of the critical morphologic features in defining the bactritoids. However, the displacement of the siphuncle from subcentral or eccentric positions toward the conch margin occurred at least three times during the Ordovician — Early Devonian evolution of the Orthocerida. Thus, there is a high probability that a marginal shift of the orthocerid siphuncle occurred in post-Emsian times, too.