Dwykaselachus: no longer ‘an enigmatic chondrichthyan’

Coates et al. 2017 described a 3D, uncrushed Middle Permian
‘enigmatic chondrichthyan’ braincase belonging to Dwykaselachus oosthuizeni (Oelofsen, 1986; Fig. 1).

The authors reported,
“Externally, the braincase is that of a symmoriid shark and is by far the most complete uncrushed example yet discovered. Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits.”

Neither Oelofsen nor Coates et al. had access to the smaller, Late Devonian shark, Ferromirum (Frey et al. 2020; PIMUZ A/I 4806; Fig. 1) a nearly identical, but smaller, 3D skull with jaws described three years later.

In the large reptile tree (LRT, 2008+ taxa) Ferromirum and related symmoriform sharks, like Dwykaselachus, are not related to chimaeroid fishes (Holocephali).

“Despite only 47 described living species¹, chimaeroids are the focus of resurgent interest as potential archives of genomic data² and for the unique perspective they provide on chondrichthyan and gnathostome ancestral conditions.”

In the LRT chimaeroids arise from the derived shark, Heterodontus, and symmoriform sharks are not far from megamouth (Megachasma) sharks. Whale sharks (Rhincodon) and nurse sharks (Ginglymostoma) represent the ancestral condition.

“Chimaeroids are also noteworthy for their highly derived body plan. However, like other living groups with distinctive anatomies, fossils have been of limited use in unravelling their evolutionary origin, as the earliest recognized examples already exhibit many of the specializations present in modern forms.”

Be wary when authors use ‘the earliest recognized examples’. Academics don’t want to recognize pterosaur precursors, whale and turtle precursors, etc. recovered by the LRT. The LRT documents chimaeroid ancestors back to Ediacaran worms. Heterodontus is not generally ‘recognized’ as a chimaeroid ancestor and so tends to be omitted from taxon lists in analyses.

“Here we report the results of a computed tomography analysis of Dwykaselachus, an enigmatic chondrichthyan braincase from the ~280 million year old Karoo sediments of South Africa. Externally, the braincase is that of a symmoriid shark and is by far the most complete uncrushed example yet discovered. Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits.”

The LRT does not test internal traits. External traits nest Dwykaselachus far from chimaeroids, close to symmoriforms like Ferromirum.

“These results have important implications for our view of modern chondrichthyan origins, add robust structure to the phylogeny of early crown group gnathostomes, reveal preconditions that suggest an initial morpho-functional basis for the derived chimaeroid cranium, and shed new light on the chondrichthyan response to the extinction at the end of the Devonian period.”

Or not. Coates et al. were swayed by the invalid hypothesis that chimaeroids split from sharks at their mutual genesis. The LRT does not support that hypothesis because it includes more traditionally omitted taxa.

Colleagues, without a valid phylogeny (Fig. 2) that minimizes taxon exclusion, your conclusions are likely to be tinged with mistakes due to taxon exclusion. In the end Coates et al. thought they were shedding light on ‘the origin of chimaeroid fishes’ when the subject of their study was not even related to chimaeroid fishes.

Frey et al. 2020 reported,
“The general shape of the [Ferromirum] neurocranium resembles that of Ozarcus and Dwykaselachus , but the Ferromirum gen. nov. neurocranium has suffered post mortem compaction, possibly losing around 25% of its dorsoventral height.”

The Frey et al. team felt comfortable enough to substitute a model of the Dwykaselachus braincase in a model Ferromirum skkull to demonstrate a unique outward rotation of the mandibles as the jaws opened. Now one wonders whether that rotation trait appeared in the Late Devonian Ferromirum or 60 to 100 million years later in the Middle Permian Dwykaselachus as Frey et al. made a chimaera taxon by mixing two sister taxa.

References
Coates MI et al. (4 co-authors) 2018. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature 541:208–211.
Frey L et al. (4 co-authors) 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
Oelofsen BW 1986. A fossil shark neurocranium from the Permo-Carboniferous (lowermost Ecca Formation) of South Africa. Indo-Pacific fish biology: Proceedings of the Second International Conference on Indo-Pacific Fishes: 107–124.

wiki/Dwykaselachus
wiki/Symmoriiformes
reptileevolution.com/ferromirum.htm