The origin of placoderms = the convergent origin of jaws in vertebrates

Never averse to change,
the large reptile tree (LRT, 2110 taxa) just moved most of the placoderms down to the poraspids, primitive jawless Silurian armored fish (Fig 1). Near the base, Poraspis has only a few skull bones. These subdivide in Drepanaspis and some reconnect in placoderms. I didn’t pay close enough attention to these two jawless taxa earlier. These insights shed new light on the dual origin of jaws in vertebrates and resolve earlier chronological problems when the LRT nested placoderms with catfish. Any time mistakes can be corrected comes as welcome news. Making mistakes is part of the process so long as corrections are eventually made.

Figure 1. Placoderm evolution (see figure 3 for cladogram) illustrated with taxa.

Prior to today,
placoderms nested with catfish in the LRT. Presently some Late Devonian putative placoderms (e.g Platysomus, Campbellodus and Materpiscis) still do. Those taxa had a taller than wide skull cross-section and fewer dermal bones. These can no longer be considered placoderms because they no longer nest with the other placoderms. The rest of the placoderms (Fig 1) move to nest with more primitive Ordovician and Silurian taxa often with much lower, wider cross-sections. Basal forms were jawless. That means placoderms developed jaws convergent with other vertebrates, like toothless Chondrosteus (Fig 2) following sturgeons and ostracoderms, preceding sharks.

Figure 2. Origin of jaws from the ostracoderm, Hemicyclaspis, Thelodus, Acipenser (sturgeon) and Chondrosteus.
Figure 2. Second origin of jaws from the ostracoderm, Hemicyclaspis, Thelodus, Acipenser (sturgeon) and Chondrosteus.

More on this topic later.
Glad to have the chronology of placoderms at last fall into line. That was a problem, now resolved.

Figure 3. Subset of the LRT focused on basal vertebrates and placoderms.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. This has been one of the largest shifts in tree topology since I can’t remember when.

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
Li Q et al. 2021. A new Silurian fish close to the common ancestor of modern gnathostomes. Current Biology 31:3613–3620. https://doi.org/10.1016/j.cub.2021.05.053
wiki/Bianchengichthys
wiki/Gnathostomata
wiki/Placodermi

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.