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Fish evolution: traditional vs the LRT

Tuesday, December 14, 2021 13:25

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According to traditional fish cladograms
(Fig. 1) placoderms with jaws are direct descendants of osteostracans without jaws. Never mind the sturgeons, sharks, acanthodians, lobefins, catfish and others that separate placoderms from osteostracans when more taxa are added in the large reptile tree (LRT, 2018+ taxa).

The simpler answer based on taxon exclusion
is preferred and taught in university textbooks and lectures.

Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Pseudoscaphorhynchus, a derived sturgeon. Chondrosteus, a fish with jaws, but no marginal teeth.

Traditional academic solution:
Do what the professor teaches out of the textbook: Pre-separate taxa into categories that will never mix. Keep sturgeons in the bony fish clade and remind yourself that their tubular mouth parts are derived from true jaws. Never let sturgeons enter a cladogram that includes osteostracans, Chondrosteus (Fig. 2) and sharks.

Figure 3. Eurynotus nests between Cheirodus and ptyctodont placoderms in the LRT.

LRT solution:
Let all taxa interact with all other taxa and let the clades sort themselves out naturally… even if this conflicts with university textbooks and lectures… even if this introduces an operculum (in sturgeons) prior to, during (in ratfish and spoonbills) and after gill slits (in sharks). There are no rules. Anything can happen. Reversals can happen. So let it happen. Don’t hobble yourself by assuming you know how things are related before the software tells you.

Figure 4. Late surviving Gregorius is basal to a all later bony fish, from the moray eels to ray fins to spiny sharks.

That’s why the LRT is here.
The LRT keeps adding taxa to a single growing cladogram.

So,
keep adding taxa and let your cladogram grow naturally. It might get weird. It might get heretical, but that’s okay.

Figure 5. Fish evolution from Hybodus to Amia documenting the shark to bony fish transition.

If you don’t like any part of the LRT tree topology for whatever reason,
repeat the experiment with a similar taxon list and your own character list and let us all know what sort of tree topology it recovers. It will help your hypothesis if you show your work (Figs. 2–5).

Source: https://pterosaurheresies.wordpress.com/2021/12/15/fish-evolution-traditional-vs-the-lrt/

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