Did shark skeletons evolve from bony ancestors?

Did bone precede cartilage in sharks? 
Or did shark-like cartilage precede bone in bony fish?

Good question.
A good answer will come from a cladogram that accurately mirrors evolutionary events.

Brazeau et al. 2020 bring us
a new, partial placoderm skull, Minjinia turgenensis (Fig. 1), that preserves a great deal of internal bone, and not a lot of dermal bone. Brazeau et al. think their specimen answers the above questions because they think placoderms phylogenetically precede sharks + bony fish.

From the Brazeau et al. 2020 text:
“Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). Phylogenetic analyses place this new taxon [Minjinia turgenensis] as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.”

What came first? The large reptile tree (LRT, 1733+ taxa; subset Fig. x) supports the hypothesis that the absence of endochondral bone in sharks and ratfish is a primitive trait retained from more primitive sturgeons (Pseudoscaphorhynchus) and paddlefish (Polyodon).

According to Wikipedia, sturgeons “are unique among bony fishes because their skeletons are almost entirely cartilaginous.”

According to the Caddo Lake Institute, “The only bone in the [paddle] fish’s body is the jawbone.”

What about placoderms? The LRT nests placoderms deep within one branch of osteichtheys close to catfish. The internal and external placoderm skeleton is made of strong bone. Not sure why this major item of evidence has been traditionally overlooked.

Brazeau et al. continue:
“The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion.”

Not really. Sturgeons are stem gnathostomes in the LRT (subset Fig. x). They are at the genesis of jaws, rather than derived taxa losing their jaws, as commonly thought.

Unfortunately,
extensive taxon exclusion ruins the basics of Brazeau et al. 2020.

Instead
the LRT nests Minjinia with the small, unnamed and better preserved bottom-feeding placoderm ANU  V244 specimen (Fig. 2), a more complete taxon not mentioned by Brazeau et al. 2020. Both nest between the more famous bottom-dwelling placoderms Entelognathus and Bothriolepis.

Considering the fact that sturgeons and paddlefish have so little bone,
sharks and ratfish don’t have that much bone to lose. We just have to remember to take sturgeons and paddlefish out of the clade of bony fish and put them where the LRT (Fig. x) indicates they nest.

Like other fish workers,
Brazeau et al. 2020 used an out-dated traditional cladogram missing so many pertinent taxa that placoderms nested basal to jawed fish. In the LRT (Fig. x) placoderms nest alongside catfish deep within one branch of the Osteichthyes.

The publicity for Minjinia has been extraordinary.
Sci-News.com reported,
“This discovery suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

While true, as shown by the LRT (Fig. x), the phylogenetic context of this placoderm fossil was greatly in need of additional taxa.

From cosmosmagazine.com:
“This 410-million-year-old fossil with a bony skull uncovered in Mongolia may force a rethink of how sharks evolved. Minjinia turgenensis, a new species, is an ancient cousin of both sharks and animals with bony skeletons, the researchers say – and that suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

Too few taxa mar this study. In the LRT Minjinia does nest with placoderms, but placoderms nest far from sharks, closer to catfish.

Co-author Martin Brazeau was reported as saying,
“Conventional wisdom says that a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us.”

Not related to sharks. Add taxa and placoderms move close to catfish.

“M. turgenensis belongs to a broad group of fish called placoderms, out of which sharks and all other jawed vertebrates – animals with backbones and mobile jaws – evolved.”

False. The loss of the mandible in one branch of the placoderms should not be confused with the genesis of the mandible in the clade Gnathostomata following sturgeons, a clade at the genesis of jaws in the LRT.

Again, from cosmosmagazine.com:
“The new find suggests the ancestors of sharks first evolved bone and then lost it again, rather than keeping their initial cartilaginous state for more than 400 million years, the researchers say.”

Not exactly true.  Sturgeons and paddlefish are more primitive and have very little bone. Placoderms, like Minjinia (Fig. 1) have lots of bone and nest deep within bony fish.

Sometimes scientists rush off to get publicity
BEFORE waiting a suitable amount of time for feedback (confirmation or refutation). In this case the peer-review process apparently failed because everyone was working from an old playbook. So did the publicity process.

References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

https://cosmosmagazine.com/nature/evolution/new-thoughts-on-how-sharks-evolved/
http://www.sci-news.com/paleontology/minjinia-turgenensis-08823.html