Placoderm jaw de-volution

About an hour ago, 
the question of pelvic girdles before jaws in vertebrate (more specifically, placoderm) evolution was reviewed in light of the LRT.

Now let’s re-examine
another tiny placoderm whose interrelationships were originally misinterpreted due to taxon exclusion.

Hu, Lu and Young 2017
studied jaw structure in a really tiny unnamed Devonian placoderm, ANU V244 (Fig. 1, shown 3x larger here) preserved in 3D. Unfortunately, Hu, Lu and Young followed tradition when they thought placoderms represented the genesis of jaw evolution, preceding the appearance of jaws in sharks and bony fish.

By contrast,
the large reptile tree (LRT, 1697+ taxa, subset Fig. 2) recovers sturgeons and Chondrosteus at the genesis of jaws, immediately preceding sharks + bony fish. In the LRT placoderms nest deep within bony fish, after the great dichotomy. Placoderms like the ANU specimen represent a reduction of jaw elements, not the acquisition. Placoderm precursors, like Cheirodus and Eurynotus, lose or fuse the maxilla. The ANU specimen also loses the premaxilla and reduces the mandible and dentary, which retains teeth.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on the catfish + placoderm clade, starting with a spiny shark, Diplacanthus.

The ANU specimen nests with the much larger Romundina (Fig. 2), a bottom feeder with reduced jaw elements and large cheeks.

The ANU specimen
is only one of several placoderms with reduced jaws (Fig. 3).

The LRT has been adding fish taxa over the past year, when the first catfish was nested with the first few placoderms. As it stands now, catfish are still closely related to placoderms in the LRT (subset Fig. 2).

The origin of placoderms would make a great PhD thesis, seen from all angles.

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 3. A sample of taxa related to Autroptyctodus with homologous skull bones color identified.

Once again,
a valid phylogenetic analysis that includes a sufficient number of pertinent taxa is key to understanding interrelationships. Don’t get turned around by using the traditional list of too few taxa. Don’t assume your predecessors and professors are correct. Test their hypotheses. Add taxa to provide, determine and validate the proper phylogenetic context in all cases. In can tell you from experience, it will be rewarding.


References
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

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