Rays, sawfish and skates have gill slits on their flat undersides.
White and mako sharks have gill slits on their lateral sides. Sturgeons, ratfish and most bony fish, have an operculum. Lampreys have a series of lateral gill holes. Ostracoderms have a series of holes on their flat ventral surface. Moray eels and their deep sea relatives have a single lateral hole without an operculum.
Those are the observations.
What do the evolutionary patterns tell us?
Put into a phylogenetic context,
(Fig. 1) patterns emerge, but reversals are apparent.
What you don’t want to do
is get caught “Pulling a Larry Martin” (= defining a clade by a short list of traits, like the type of gill openings present). Some clade members don’t follow all of ‘the rules’, but all clade members follow most of ‘the rules.’ If they don’t, they go to another clade.
What you do want to do
is let all the traits and all the taxa fight it out, out of sight, deep in the 0s and 1s of your unbiased software and see what patterns emerge.
For this situation I was curious to see
what patterns emerged, given the present cladogram. Sharp-eyed readers will note this portion has been corrected since the last time it was presented, and probably not for the last time as each new taxon sheds new light on various subsets of the LRT. It’s an ongoing project in real time. It’s never finished.
PS You’ve heard that ontogeny recapitulates phylogeny.
Here’s a series of paddlefish larvae at weekly intervals.
Note the shark-like stage
and the earlier bowfin-like stage, not so much recapitulating phylogeny, but predicting it in descendant taxa.
Thank you for your readership. Be good.