Leftover placoderm traits found in spiny sharks

Recent housekeeping
in the larger reptile tree (LRT, 2168 taxa) recovers the small putative placoderms ANU V244 and Entelognathus (Fig 3) as basal members of the clade Chondrichthys. The tiny placoderm, Shenacanthus, nests as a basal member of the clade Acanthodii. That means the split between bony fish and sharks did not occur after the end of the placoderms, but closer to the origin and radiation of placoderms in the Early Silurian.

That’s why tetrapods are members of the Placodermi, just as tetrapods are members of the Gnathostomata. This is what analysis recovers when taxa are not excluded.

While we’re at it,
the tiny Early Devonian whale shark, Ramirosuarezia (Figs 3, 4), shows what happened to the dermal skull bones of placoderms, like Entelognathus (Fig 3), during phylyogenetic miniaturization from placoderm ancestors to traditional members of the Chondrichthys (sharks and rays), distinct from the lineage of spiny sharks + bony fish (above).

Ramirosuarezia boliviana
(Pradel, Maisey, Tafforeau and Janvier 2009; Middle Devonian) is a tiny early whale shark (Rhincodon) after testing in the LRT. It was originally considered enigmatic. Here the loss of several lateral skull bones is shown in frame two of this reconstruction.

References
Pradel A, Maisey JG, Tafforeau P and Janvier P 2009. An enigmatic gnathostome vertebrate skull from the Middle Devonian of Bolivia. Acta Zoologica (Stockholm) 90
(Suppl. 1):123–133.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

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