From Martin Brazeau and the Imperial College London,
here’s a new YouTube video (53 minutes) on how and maybe why sharks lost their bony exoskeleton.
The phylogenetic context is wrong. Without testing, Brazeau et al. considered placoderms basal to sharks and bony fish.That’s a traditional mistake. In the large reptile tree (LRT, 1795+ taxa) placoderms are bony fish close to catfish. In the LRT sharks evolved from sturgeons (Fig. 2). Bony fish evolved from hybodontid sharks. The Silurian is when all this happened.
Unfortunately, as you’ll see
Brazeau et al. include only fossil taxa to determine which taxa were present in the Silurian.
(by reversal) anapsid-mimic placoderm, Minjinia (Fig. 3) was featured in Brazeau’s paper and video.
Brazeau illustrates his talk with an image of the exoskeleton and endoskeleton of the sturgeon Acipenser. which entered the LRT here. He reports the endochondral bone was lost in sturgeons. That is a traditional mistake as revealed by the LRT.
Brazeau correctly reports
the origin of bone precedes sharks and is lost in sharks. He just did not realize that placoderms are descendants of sharks, not their ancestors.
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