The Pleco is a group of a aquarium fish that looks seriously primitive, it wouldn’t look out of place amongst the armour plated fish of the Devonian Seas. It has a triangular body. Most interestingly of all, it is armour plated with an external skeleton like the primitive placoderm fish of the Devonian.

So is it a direct modern link to the placoderms? The answer turns out to be “close, but no cigar”.

The placoderms are most closely related to a group of catfish Clarias, the walking catfish, is a living placoderm

The bones of the living walking catfish, Clarius, are directly analogous to those the of the placoderm Entelognathus.

The plecos are … Here’s an evolutionary tree.
As placoderm sisters, is there an armored catfish?

The Clarius, closest relation of the placoderms, are very closely related to Plecos, shown on the chart as Ictalurus. But the skeletal bones of Plecos are less closely related to Entelognathus.