Disc-head placoderms with tiny lateral eyes are anapsid mimics

Phyllolepid placoderms
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).

Figure 2. Cowralepis was first described as a growth series.

Figure 1. Cowralepis was first described as a growth series.

Anapsids are actually derived armored lancelets,
filtering food with oral cavities prior to the genesis of jaws.

Figure 1. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm.

Figure 2. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm. Red dots are resorted eyeballs.

In counterpoint,
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.

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 3. Minjina in 4 views, mirror-image added.

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.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

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.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the phyllolepid placoderms.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the 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)

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure x. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

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.


References
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.

wiki/Cowralepis
wiki/Entelognathus
wiki/Drepanaspis

 

 

 

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 )

Google photo

You are commenting using your Google 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.