The new order Mixosiphonata (Cephalopoda: Nautiloidea) and related taxa; estimations of habitat depth based on shell structure

Abstract

The new order Mixosiphonata is characterized by its siphuncular structure. The order contains the ectocochleate cephalopods Boggyoceras centrale n.gen. et n.sp. (Carboniferous) and Zhuravlevia insperata (Cretaceous); both are orthocones. The bactritoid-like coleoid Ctenobactrites lesliensis (Carboniferous) has a similar siphuncular structure and is therefore, provisionally included in this new order. In the new order Mixosiphonata, the proximal part of the septal neck has the same structure as the septum from which it originates, whereas the entire distal part is composed of prismatic and spherulitic layers embedded into a chitinous substance. Also the connecting ring consists of prisms and spherulites embedded into chitinous substance, but the calcareous elements are smaller than those in the distal part of the septal neck. The structure of the connecting ring and septal neck differs from the 13 types of siphuncular structures in nautiloids, ammonoids and coleoids that are currently known. The siphuncular structure in the Ordovician cephalopod Bactroceras avus is described and compared with that in the mixosiphonates. The shell strength indexes derived from the extant Nautilus cannot be used for estimations of habitat depth in most, if not all, known externally shelled cephalopods because in the fossil cephalopods the siphuncular structures are different from that of extant Nautilus, and therefore, the mechanical strength of the siphuncles in the fossil shells and Nautilus shell is not comparable.http://zoobank.org/urn:lsid:zoobank.org:pub:6FE8CDBD-E99D-4055-93BA-22618E75CD52http://zoobank.org/urn:lsid:zoobank.org:act:56582CE7-7750-4937-98C0-5113CB0E896Ehttp://zoobank.org/urn:lsid:zoobank.org:act:7B9AF88B-4243-4418-957C-57EAF2CF2EDAhttp://zoobank.org/urn:lsid:zoobank.org:act:EB8874EB-C20F-41D7-8B70-6814378AFF13