Ozarcus mapesae is not a Palaeozoic shark

Pradel et al. 2019
report on a new big-eyed, ultra-short rostrum fish they name Ozarcus mapesae (AMNH FF20544, Fig. 1; Upper Mississippian). They called it, “a Palaeozoic shark with osteichthyan-like branchial arches.” In the large reptile tree (LRT, 1472 taxa), the clade of small fish that has a giant orbit and an ultra-short rostrum, the Acanthodii, nests between lobe fin and ray fin fish, not very close to sharks. So Pradel et al. start with a false premise and their charts suffer from taxon exclusion that we learned about here.

The authors report,
“The evolution of serially arranged, jointed endoskeletal supports internal to the gills—the visceral branchial arches—represents one of the key events in early jawed vertebrate (gnathostome) history, because it provided the morphological basis for the subsequent evolution of jaws1–5. However, until now little was known about visceral arches in early gnathostomes6–17, and theories about gill arch evolution were driven by information gleaned mostly from both modern cartilaginous (chondrichthyan) and bony (osteichthyan) fishes.

“New fossil discoveries can profoundly affect our understanding of evolutionary history, by revealing hitherto unseen combinations of primitive and derived characters18,19. Here we describe a 325 million year (Myr)-old Palaeozoic shark-like fossil that represents, to our knowledge,the earliest identified chondrichthyan in which the complete gill skeleton is three-dimensionally preserved in its natural position. Its visceral arch arrangement is remarkably osteichthyan-like, suggesting that this may represent the common ancestral condition for crown gnathostomes.”

Wrong premise. Wrong conclusion
The gill arches of Ozarcus mapesae are ‘remarkably osteichthyan-like’ because they belong to an osteichthyan, not a shark. We know the ancestors and sister of sharks. They are not acanthodians. They are angel sharks and their jawless kin.

The authors conclude,
“Ozarcus thus shows a novel combination of chondrichthyan and osteichthyan characters, thereby demonstrating that the most recent common ancestor of crown gnathostomes possessed an osteichthyan-like branchial apparatus.Our findings cast doubt on the traditional view of visceral arch evolution that modern chondrichthyans mirror the ancestralmorphotype of jawed vertebrates. Bony fishes and stemchondrichthyansmay havemore to tellus about our first jawed ancestors than do living sharks.”

A wide gamut trait-based phylogenetic analysis
is essential before coming to conclusions. In Pradel et al. taxon exclusion ruined an otherwise good study of a new taxon.

References
Pradel A, Maisey JG, Tafforeau P, Mapes RH and Mallant J 2014. A Palaeozoic shark with osteichthyan-like branchial arches. Nature doi:10.1038/nature13195