Phosphatic connecting rings and ecology of an Ordovician ellesmerocerid nautiloid

Abstract

The phosphatic siphuncle of Bactroceras latisiphonatum Glenister, a Late Ordovician member of the ancestral order of nautiloid cephalopods, displays the outer ‘chalky’ and inner ‘horny’ tubes seen in the aragonitic connecting rings of Recent Nautilus and Carboniferous Orthocerida. The specimens were collected from the Malongulli Formation of central New South Wales. Bactroceras resembles Mesozoic ammonoids in having a marginal phosphatic connecting ring that is partly a product of diagenetic phosphatization. Both the septa and connecting rings were apparently able to resist greater hydrostatic water pressures than other ellesmerocerids and had a potential implosion depth comparable to that of mature Nautilus. The septal necks and siphuncle position resemble the morphology of Bactrites; but it is unlikely that Bactroceras and ‘Eobactrites’ were directly ancestral to bactritoids and ammonoids.