Is Palliocerida a valid order within the Nautiloidea
Marek in 1998, established the Palliocerida as a separate order of cephalopods within the Nautiloidea for orthoconic and mildly cyrtoconic forms with incomplete connecting rings in the siphuncle such that the space within the camerae and siphuncle interior are contiguous, and in which the camerae, or chambers, contain organic calcite deposits.
The primary question is, do incomplete connecting rings justify the creation of a new order. In my view no.
First of all, open or incomplete connecting rings, that is with a gap or gaps open to the chambers, would greatly impair normal siphuncle function, which is to dewater the final chamber of the phragmocone after it's formed and to adjust buoyancy throughout during life for mobility. Open or missing connecting are more likely the result of decalification resulting in none preservation or some post mortem diagenetic process.
Second of all, open or incomplete connecting rings or not necessary in order to have cameral deposits Genera in found in different families and orders, such as Actinoceras and related genera in the Actinocerida; Michelinoceras in the Orthocerida; Pseudocyrtoceras in the Pseudorthocerida; Westonoceras in the Discosorida as well as Campbelloceras, Curtoceras,and Lituites, in the Tarphycerida, to name but a few, all have cameral deposits and except for some Lituites, well developed siphuncles will complete connecting rings.
Third, I see no phylogenetic reason for the order. Therefor considering the arguments given, it seems best that the forms included in the Palliocerida Marek 1998 , such as Plagiostomoceras be returned to the Orthocerida and the Palliocerida be abandoned
