The different conditions of the premaxillary in primitive actinopterygians are individualized, or fused to adjacent bones. Their dental field can also be divided and fused to neighbouring bones, the premaxillaries may be separated across the midline, or, finally, they have disappeared. This separation and absence can result in a rostral notch observed in rhadinichthyids, and presumed in other groups such as the redfieldiiformes. It can be suggested that the plesiomorphous gnathostome dermal skeletal condition is micromeric, that the primitive actinopterygian larval snout condition derived from that condition was mesomeric, and that heterochronic changes during early actinopterygian evolution gave rise during development (1) of mesomeric adult primitive actinopterygians through neoteny, (2) of macromeric adult primitive actinopterygians through fusions of bones, (3) to the condition of actinopterygians lacking a premaxillary through its loss. The neopterygian condition may have arisen through paedomorphosis from either mesomeric primitive fossil actinopterygians, or directly by heterochrony from the larval primitive condition.
Vertebrata, Actinopterygii, comparative anatomy, evolutionary trend, premaxillary, dermal snout