Frey et al. 2020 brought us
a new, small, Late Devonian shark with large eyes. Ferromirum (Fig. 1). In the large reptile tree (LRT, 1770+ taxa, subset Fig. 4) Ferromirum has a long, lost living relative, the megamouth shark, Megachasma (Figs. 2, 3), a taxon overlooked by Frey et al.
While Ferromirum may look toothless
so does Megachasma (Figs. 2, 3) at the same scale. Rows of tiny sharp teeth lined like cemetery headstones filled the jaws of both taxa.
From the Frey et al. abstract:
“The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and thus our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons.”
Not that poor in 2020. And we have living specimens to dissect. The taxon exclusion problem experienced by Frey et al. is of their own doing.
“Here, we report a previously undescribed symmoriiform shark, Ferromirum oukherbouchi, from the Late Devonian of the Anti-Atlas. Computed tomography scanning reveals the undeformed shape of the jaws and hyoid arch, which are of a kind often used to represent primitive conditions for jawed vertebrates.
Not true. Taxon exclusion is the problem here. That led the Frey et al. team to a misunderstanding of shark origins (Fig. 4) in which they made the placoderm, Entelognathus, the outgroup and the bony fishes Guiyu, Climatius, Acanthodes and Doliodus nested basal to sharks. In the LRT these taxa are all derived from sharks. Essentially the cladogram in Frey et al. is upside down. See below for details.
Except for size, Megachasma provides many clues
to the in vivo appearance and habits of Ferromirum due to its close nesting and phylogenetic bracketing.
The Frey et al. abstract continues:
“Of critical importance, these closely fitting cartilages preclude the repeatedly hypothesized presence of a complete gill between mandibular and hyoid arches. We show that the jaw articulation is specialized and drives mandibular rotation outward when the mouth opens, and inward upon closure.”
As in the megamouth shark, “so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family.” according to Wikipedia, which also suffers from taxon exclusion.
“The resultant eversion and inversion of the lower dentition presents a greater number of teeth to prey through the bite-cycle. This suggests an increased functional and ecomorphological disparity among chondrichthyans preceding and surviving the end-Devonian extinctions.”
It’s good to know the megamouth shark is no longer alone.
Basically the Frey et al. cladogram is upside down
due to taxon exclusion. In addition to the issues listed above, the authors nest the dogfish shark, Squalus, at a highly derived node. The LRT (Fig. 4) nests it close to the base. The authors nest Ferromirum, Cladoselache, Ozarcus, and Akmonistion within the Holocephali (chimaeras). The LRT nests these taxa with sharks. The last two nest as proximal outgroups to the derived clade Osteichthyes (bony fish), which is a clade basal to sharks in Frey et al. More taxa from a wider gamut resolves all such issues. Don’t rely on the work of others or tradition. Find all this out for yourself.
Frey L, Coates MI, Tietjen K, Rücklin M and Klug C 2020. A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans. Nature Communications Biology 3:681 | https://doi.org/10.1038/s42003-020-01394-2 | http://www.nature.com/commsbio