Tiny Romundina nests between arthrodire and ptyctodontid placoderms

Here, again, a picture of three taxa
tells the story of gradual evolution, this time from the small arthrdire placoderm, Coccosteus to Campbellodus (Fig 1), a ptyctodontid placoderm. Tiny Romundina nests between them in the large reptile tree (LRT, 2172 taxa).

Ptyctodontids had taller, narrower skulls
with reduced facial armor. The multi-part dorsal shield elevated like a spine in Campbellodus, but it was probably on the leading edge of a tall, discoidal torso, following a trend that had its origin in Romundina. In this way ptyctodontids, like Campbellodus, converged with extant queen trigger fish (Balistes, Fig 2), including the narrow, but robust nibbling mouth parts formed from the nasal in Campbellodus and from the premaxillary teeth in Balistes.

Why scale bars are important.
As we’ve seen many times before, phylogenetic miniaturization was present at the genesis of many new clades. This time it appears that miniaturization occurred between arthrodire placoderms and ptyctodontid placoderms (Fig 1).

These are only a few of the many changes
happening in the LRT during the last two months of housekeeping the fish clades. Spiny sharks are becoming more and more interesting.

References
Dupret V et al (4 co-authors) 2014. A primitive placoderm sheds light on the origin of the jawed vertebrate face. Nature. 507 (7493): 500–503.
Ørvig T 1975. Description, with special reference to the dermal skeleton, of a new radotinid arthodire from the Gedinnian of Arctic Canada. Colloque International due CNRS 218:41–71.

wiki/Arthrodira
wiki/Placodermi
wiki/Ptyctodontida
wiki/Romundina