like Cowralepis (Figs. 1, 2) and Minjinia (Fig. 3) have simple disc-like skulls and lack jaws. In this way they mimic older ‘jawless fish’ (= anapsida), like Drepanaspis (Fig. 4).
Anapsids are actually derived armored lancelets,
filtering food with oral cavities prior to the genesis of jaws.
Cowralepis and its Early to Middle Devonian allies had ancestors with jaws according to the large reptile tree (LRT, 1759+ taxa; subset Fig. x). Minjinia (Fig. 3) was originally considered a placoderm close to the shark/bony fish split. In the LRT Minjinia nests with the nearly blind phyllolepid placoderm bony fish, far from sharks.
Phyllolepid placoderms also reduce
their diphycercal tail, pelvic fins and lose their dorsal fins as they adapt to bottom feeding again, going through a process of reversal and convergence that could be misinterpreted without a wide gamut cladogram like the LRT.
Phyllolepid placoderms had their origins
with a Silurian placoderm, Entelognathus (Fig. 5), which has tiny eyes, but not yet a disc-like morphology. Entelognathus was originally misinterpreted (Zhu et al. 2013, 2016) as a placoderm at the genesis of jaws. By contrast, in the LRT Entelognathus was losing its jaws, a process that terminates with disc-like phyllolepid placoderms.
Entelognathus primordialis (Zhu et al. 2013, 2016; Late Silurian, 419 mya)
Drepanapis gemuendenensis (Schlüter 1887; Gross 1963; Early Devonian 405mya)
Minjinia turgenensis (Brazeau et al. 2020; Early Devonian)
Cowralepis mclachlani (Ritchie 2005; Carr et al. 2009; Middle Devonian)
Catfish also produced a similar morphology.
Wuttagoonaspis (Fig. 6) “is a genus of primitive arthrodire placoderms from Middle Devonian Australia” according to the fish workers posting in Wikipedia. In the LRT (subset Fig. x) it nests with the walking catfish, Clarias. Expand the taxon list, let catfish in, and see for yourself where “What-a-goon-aspis” nests.
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Broili F 1929. S. B. Bayer. Akad. Wiss., 1
Carr RK, Joahnson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology 270(7):775–804.
Ritche A2005. Cowralepis, a new genus of phyllolepid fish (Pisces, Placodermi) from the Middle Devonian of New South Wales. Proceedings of the Linnean Society of New South Wales 126:215–259.
Schlüter EF 1887. Panserfische, etc. Niederrhein. Ges., Bonn, 1887, 120.
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.
Zhu M et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310 (2016): 334-336.