Two Triassic perleidiforms: Perleidus (left) and Cleithrolepis (right).


Actinopterygian node N: Rudimentary nasal processes on premaxilla, equal number of radials in dorsal and anal fins, upper caudal fin elongated beyond muscular tail.


The basal actinopterygian clade kow as the perleidiforms mainly comprises of very small fish with a wide variety of overall body plans, encompassing forms with both elongated bodies (as the Perleidus above, left) and forms with deep and laterally compressed ones (such as Cleithrolepis, above right), and are best known from the Triassic.

The perleidiforms, as the peltopleuriforms and pholidopleuriforms with which they were contemporaneous, are inferred to stem from the lineage from which more advanced actinopterygians also evolved. This hypothesis is based on their sharing trends towards the neopterygian body plan: a shift in the organisation of the tail, becoming almost entirely ray-supported, while the muscular dorsal lobe - although still present - is greatly reduced in these fish. The structure of jaw bones also shows similarities with the more derived clades.