Ninmarooceras gen. nov., a unique cephalopod from the Cambrian-Ordovician transition of Australia

The Ellesmeroceratida contains some of the oldest known fossil cephalopods. The order is almost entirely restricted to the latest Cambrian to Early Ordovician and is crucial for understanding the origin and early radiation of cephalopods. Here, we describe the new ellesmeroceratid genus Ninmarooceras from the Cambrian-Ordovician transition of the Ninmaroo Formation at Black Mountain, western Queensland, Australia. This genus is unique among cephalopods in its peculiarly shaped diaphragms that are steeply inclined and branch towards the venter. Besides the straight to very slightly endogastrically curved type species Ninmarooceras ramosum gen. et sp. nov., we describe a second species, N. exoundulatum gen. et sp. nov. The latter species is slightly older and combines an exogastrically curved conch with shallow undulations. The combined occurrence of these features within a single genus calls into question their diagnostic potential in distinguishing between taxonomic groups above genus level of the Ellesmeroceratida and related early cephalopods. The variability of diaphragm morphology also supports the restriction of the Yanheceratidae Chen and Qi, 1979 as a family within the Ellesmeroceratida instead of its own order. We further discuss evolutionary and functional implications of the unique internal siphuncle structure of Ninmarooceras.