Adding taxa updates the origin of placoderms

A year ago
when fish (= basal vertebrates) were first added to the large reptile tree (LRT, now with 1757+ taxa; subset Fig. 1), the extant walking catfish, Clarias, nested with the Silurian placoderm, Entelognathus, rather than any other extant bony fish when there were very few other bony fish to nest with. Since then, adding taxa has separated these two, but they still nest as charter members of the unnamed catfish-placoderm clade. That was a heretical hypothesis then, and it remains so today.

Traditional fish paleontologists
consider placoderms basal to sharks and ratfish + bony fish. Stensioella was considered the most basal placoderm by Carr et al. 2009, who did not list outgroup taxa. These hypotheses are not supported by the LRT (subset Fig. 1) where placoderms arise from coelacanths among the bony fish, far from sharks and ratfish.

The LRT divides placoderms into four clades;

  1. Arthrodira (open ocean predators like Dunkleosteus, Coccosteus, Fig. 3)
  2. Antiarchi (armored jawless bottom dwellers like DicksonosteusBothriolepis, Fig. 2)
  3. Ptyctodontida (chimaera-like taxa like Australoptyctodus, Fig. 2)
  4. Phyllolepida (tiny-eye taxa like Entelognathus, Cowralepis)

Several traditional placoderms nest elsewhere in the LRT.

  1. Rhenanida – nests with catfish in the LRT
  2. Wuttagoonaspis – nests with catfish in the LRT
  3. Stensioellida – nests with Guiyu-like lobefins in the LRT
  4. Brindabellspida – nests with the tetrapodomorph Elpistostege

Several traditional placoderms have not yet been tested in the LRT.

  1. Petalichthyida (includes Diandongpetalichthys)
  2. Acanththoraci (closely related to rhenanids, nesting with catfish)
  3. Pseudopetalichthyida (similar to rhenanids, nesting with catfish)

After testing
in the LRT (subset Fig. 1) placoderms are still bony fish close to catfish and this clade still arises from coelocanths.

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

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

The pertinent taxa in the first list
(Fig. 2) start with the small, Early Devonian spiny shark Diplacanthus and end with the rather flat nearly jawless placoderm, Dicksonesteus also from the Early Devonian. That tells us that every taxon between them was part of the Early Devonian fauna. That also tells us the radiation of taxa in figure 2 must have occurred much earlier, sometime in the middle of the mysterious Silurian, which preserves very few gnathostome fish fossils.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi. See figure 3 for the arthrodire clade within Placodermi. Robustichthys is basal to catfish and lacks a squamosal.

Phylogenetic miniaturization
occurs at the origin of placoderms with the smallest specimen in figure 2, Romundina, half the size of its predecessor, Eurynotus. In like fashion, the smallest placoderm in figure 3is the unnamed ANU V244 specimen, is also half the size of its predecessor, the aforementioned Eurynotus.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Phylogenetically, the lack of marginal teeth
in placoderms goes back to a late-surviving taxon from the Jurassic, the angelfish-mimic,  Cheirodus (Fig. 1). Note the hidden palatine teeth in Cheirodus that in the arthrodires, Coccosteus and Dunkleosteus become visible and act as marginal teeth/plates. The Silurian ancestors of Cheirodus may not have been so uniquely angelfish-like. That shape is apomorphic due to the separation in time.

Ptyctodonts, like Austroptyctodus,
(Fig. 2) do not nest with other traditional placoderms in the LRT, but nest closer to Cheirodus.  These are the sort of results the LRT recovers only because it tests more taxa.


References
Carr RK, Johanson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology. 270 (7): 775–804.
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
Miles RS and Young GC 1977. 
Placoderm interrelationships reconsidered in the light of new ptyctodontids from Gogo Western Australia. Linn. Soc. Symp. Series 4: 123-198.
Young GC 1980. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny: Palaeontographica 167A pp. 10–76. 2 pl., 27 fig.
Zhu et al. 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.
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/Entelognathus
wiki/Bothriolepis
wiki/Dicksonosteus
wiki/Romundina
wiki/Qilinyu
wiki/Parayunnanolepis
wiki/Lunaspis
wiki/Coccosteus
wiki/Mcnamaraspis
wiki/Dunkleosteus

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