Article: A muscle attachment proposal for septal function in Mesozoic ammonites

The unusual septal surface typical of lytoceratid ammonites is described from unique Wester Australian specimens of the late Cretaceous Indopacific species Pseudophyllites indra (Forbes). Median dorsal septal recesses and inner septa combine to form a septate tunnel lying within the phragmocone; their free margin are complexly fluted like that of the septal periphery in contact with the outer shell wall. Functional analysis of the fluted septal recesses and inner septa suggest that they were not related to phragmocone strength but facilitated the attachment of adductor muscles. By analogy, a muscle attachment function is argued for the flute septal periphery of P. indra and for the septal periphery of Mesozoic ammonites in general. The role of septal design in contributing necessary strength to phragmocone construction is re-evaluated and considere subordinate.Muscle attachment translocation during growth, a special problem for ectocochleate cephalopods, was accomplished by the release of muscle attachment and rapid forward movement of the ammonite animal in it shell. Muscles were re-attached along a narrow zone at the free margin of a newly formed septum, convolution of which enlarged the attachment surface. It is argued that connecting rings of the siphuncle were preformed in the body chamber prior to movement of the animal; location of the siphuncle, details of its construction, and the nature of associated structures are consistent with this proposal. The muscle-attachment hypothesis is further supported by shell microfabrics known for Mesozoic ammonites, including new data for Sciponoceras. Gross differences in shell form and ornamentation which separate Mesozoic ammonites and nautiloids are thought to be due to differences in growth style, necessitated by the manner in which muscle attachments were translocated in members of the two groups.