Ozarcus mapesae is just barely a Palaeozoic shark

Updated December 14, 2020
because so many more chondrichthyes and chimaera have been added to the LRT.

Updated April 25, 2020
with a DGS coloring of the Ozarcus skull (Fig. x) and this taxon added to the LRT nesting alongside Falcatus, in the most primitive stem clade leading to bony fish, not sharks and chimaera.

Figure x. Skull of Ozarcus colored using DGS methods and tetrapod homologs. Note the tiny teeth inside the large tooth basin. The narrowness of the skull is partly due to crushing.

Figure x. Skull of Ozarcus colored using DGS methods and tetrapod homologs. Note the tiny teeth inside the large tooth basin. The narrowness of the skull is partly due to crushing.

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, then 1472 taxa, now 1673 taxa), Ozarcus nests basal to Falcatus, not far from another big-eyed taxon, Doliodus. All of these are stem osteichthyans, a sister taxon to the shark + chimaera clade.

FIgure 1. Ozarcus from Pradel et al. 2019 animated to show the mandible/gill elements opening and closing.

FIgure 1. Ozarcus from Pradel et al. 2019. Here an eyeball is added and the mandible is animated to show the mandible/gill elements opening and closing. This is a derived taxon after having lost its teeth, not the origin or genesis of jaws. That happened in the Early Silurian, much earlier than the Mississippian portion of the Carboniferous period.

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 jaws. However, until now little was known about visceral arches in early gnathostomes, 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 characters. 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 a stem-osteichthyan, not a shark. The ancestors and sister of sharks include Rhincodon, the whale shark and various chimaeras (ratfish), like Heterodontus.

Figure 5. Shark skull evolution according to the LRT. Compare to figure 1.

Figure 5. Shark skull evolution according to the LRT. Compare to figure 1.

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.

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 ancestral morphotype of jawed vertebrates. Bony fishes and stem chondrichthyans may have more to tell us about our first jawed ancestors than do living sharks.”

A wide gamut trait-based phylogenetic analysis
is essential before coming to conclusions. For the origin of jaws and teeth go back to LoganielliaManta and Rhincodon. Ozarcus is not far from these traditionally overlooked  taxa.


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

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