Arm hooks and structural features in the Early Permian Glochinomorpha Gordon,indicative of its coleoid affiliation

Data on the re‐examined‐type specimens of the Early Permian Glochinomorpha stifeli Gordon 1971 (Coleoidea, Cephalopoda) gives new insight into the evolutionary origination of gladius‐bearing coleoids in the Late Palaeozoic and suggests their parallel evolution with the belemnoids and phragmoteuthids. The iron‐oxidized arm hooks are found in front of a gladius on an un‐illustrated paratype (USNM 170614). They are dispersed in a maximum distance equal to 2.5 the gladius length from the incomplete anterior end of the rachis of the gladius. The hook height ranges from less than 1 mm to as much as 4 mm. They have a massive curved base, and a strongly curved shaft with a long thin distal part. The hooks confirm the cephalopod and the coleoid affiliation of G. stifeli that is widely rejected. The iron‐oxidized gladii (preserved length is less than 20 mm) show micro‐laminations and a fibrous ultrastructure; the fibre bundles are approximately 0.5 μm thick. Lack of calcium and the high content of nitrogen (up to 12% of total weight) confirm the originally non‐biomineralized (apparently chitin) composition of the gladii suggested by their fibrous ultrastructure. These data lead to the conclusion that in the evolutionary history of gladius‐bearing coleoids, the biomaterial (biochemical) development of the skeleton preceded its morphological transformation. This phenomenon resulted in a recombination of an advanced composition (being non‐biomineralized) with archaic features (sensu morphological structure of the posterior part) in the gladius of G. stifeli. The data cast doubt on the hypothesized origination of a gladius from a phragmoteuthid pro‐ostracum as well as the evolutionary origination of the gladius‐bearing coleoids from pro‐ostracum‐bearing phragmoteuthids or belemnitids.