Helodus: a skull without sutures

Decades prior to PAUP and MacClade,
Professor Moy-Thomas 1936 reasoned that Helodus simplex (Fig. 1; Agassiz 1838; Early Carboniferous, 300mya, 30cm long) ) was close to the ancestry of the clade Holocephalii (ratfish, chimaeras and kin; Fig. 2), which we looked at yesterday. In complete accord, the large reptile tree (LRT, 1641 taxa; subset Fig. 3) fully supports that nesting using modern software: PAUP and MacClade. So… belated well done, Professor Moy-Thomas!

Figure 1. Helodus skull drawings from xxx 1938 show no skull sutures. Colors are applied with blending edges to show were bones are based on tetrapod homologs.

Figure 1. Helodus skull drawings from Moy-Thomas 1936 show no skull sutures. Colors are applied with blending edges to show were bones are based on tetrapod homologs. Gill bars are missing from these diagrams, so were added in light blue here.

In Helodus
the skull bones are all fused together, so suture estimates are provided here (Fig. 1) based on phyogenetic bracketing. Note the tiny premaxillary teeth and complex maxillary teeth. Tabulars appear to be absent. Note the coosified cervicals / anterior dorsals extending to the notochord and first dorsal spine.

Moy-Thomas 1936
considered the anatomy of Helodus in detail. From the abstract:

  1. “The skull is found to be holostylic, and to have many characters in common with the skull of the Holocephali, but in some respects is less specialized.
  2. The pectoral fins, with their long metapterygium, small propterygium, and fused anterior radials, resemble very closely those of the Holocephali.
  3. The pelvic and unpaired fins, and general body shape are found to resemble those of the Holocephali.
  4. It is concluded that the Cochliodonts are almost certainly closely related to the ancestors of the Holocephali, and the relatively unspecialized condition of the teeth gives support to the view that the holostylic condition of the jaws is primitive for the group. It is suggested that all the Bradyodonts were holostylic, that the hyomandibular may never have been suspensory, and that they may have diverged from the true Selachii before the hyomandibular played a part in the jaw suspension.”
FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

FIgure 2. Ratfish (chimaera) and Heterodontus to scale.

 

Taking one phylogenetic step further back from Helodus,
yesterday we looked at Heterodontus (Fig. 2), the Chondrichthyes taxon phylogenetically ancestral to Helodus and the Holocephalii.

Figure 5. Subset of the LRT focusing on basal chordates, vertebrates and bony fish not related to tetrapods. Scomber and Istiophorus are new additions to the gold clade.

Figure 5. Subset of the LRT focusing on basal chordates, vertebrates and bony fish not related to tetrapods. Scomber and Istiophorus are new additions to the gold clade.

References
Agassiz L 1838. Recherches Sur Les Poissons Fossiles. Tome III (livr. 11). Imprimérie de Petitpierre, Neuchatel 73-140.
Moy-Thomas JA 1936. On the Structure and Affinities of the Carboniferous Cochliodont Helodus simplex. Cambridge University Press. 73(11):488–503.

wiki/Chondrichthyes
wiki/Chimaera
wiki/Cladoselache
wiki/Chondrosteus
wiki/Symmoriida
wiki/Horn_shark
wiki/Helodus not listed in English

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