Discussion of Phylogenetic Relationships
The main characteristics supporting the nodes of this phylogeny are:
Node 1: Piston cartilage in lingual apparatus, loss of anal fin
Node 2: Horny plates or denticles on sucker
Node 3: Seven gill openings, gill pouches larger and more posteriorly placed, eel-shaped aspect.
The interrelationships of the ten extant genera is still unclear, but it is currently admitted that the organisation of the horny denticles of the sucker in Ichthyomyzon, Petromyzon and Caspiomyzon is primitive for the group. It is also quite similar to that found in one of the fossil lamprey, Pipiscius.
There are three fossil lampreys, Mayomyzon, Hardistiella, and Pipiscius, all from the Late Carboniferous of USA. Mayomyzon is the best known of them and resembles extant lampreys in many respects, except for the somewhat stouter body shape, smaller gill pouches, and coalescent dorsal and caudal fins. Mayomyzon possessed as piston cartilage and, thus, a complex "tongue"-like apparatus. Hardistiella may have retained a small anal fin and a more clearly hypocercal tail. Pipiscius is poorly known but possessed a rouded sucker armed with polygonal horny plates.
Other fossils, formerly referred to the Anaspida are now tentatively regarded as relatives of lampreys. Jamoytius (Early Silurian of Scotland) is a naked jawless craniate which may have possessed an annular cartilage. Euphanerops (Late Devonian of Canada) looks like a naked anaspid, with a strongly hypocercal tail, and also seems to have an annular cartilage. Some, however, still put this genus, as well as Endeiolepis among anaspids.
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