Gnathostome genesis reviewed by Zhu et al 2023

“How did Jawed Vertebrates Originate and Rise?”
is the question posed by Zhu et al 2023.  They wrote, “The emergence of the jaw
marked a substantial morphological shift of brains and facial organs in the earliest gnathostomes, leading to significant enhancement of feeding and respiratory efficiency, greatly increasing the evolutionary potential of vertebrates towards larger body sizes and diverse ecological niches.”

“Traditionally, jawed vertebrates can be divided into four distinctive groups: placoderms, acanthodians, osteichthyans, and chondrichthyans. Of these, only osteichthyans and chondrichthyans persist to this day. The most recent common ancestor of the latter two groups, and all of its descendants, constitute the crown-group gnathostomes.”

When authors say ‘traditionally’ they mean they are not aware of nor tested alternatives. They are going with the flow taught in university textbooks.

By contrast
in the large reptile tree (LRT, 2323 taxa), alternative cladograms are recovered simply by adding taxa. In the LRT vertebrates with jaws had two separate origins, as we learned earlier here.

Without this bedrock understanding of the vertebrate family tree, nothing else these authors have to say sits in the groove of understanding, as it once did and will continue to do so for decades, based on the snails’s pace at which paleontology tests and accepts new hypotheses.

Zhu et al wrote,
“Armored jawless fish, together with placoderms, constitute the gnathostome stem-group.”

Now these armored taxa constitute only half the gnathostome stem-group.
Our tetrapod to human ancestors were not amored taxa.

“Molecular biology provides crucial restrictive evidence as to when and how the jaws and jawed vertebrates originated.”

Molecular biology cannot and does not test fossils.

“Molecular clock studies attest to the fact that the last commonancestor of all modern gnathostomes emerged approximately 450 million years ago.”

That would be sometime in the Ordovician. In the LRT we have sturgeon ancestors, like Haikouichthys, in the Early Cambrian 538 mya, marking the separation from the conodont clade first appearing as fossils in the Late Cambrian, both derived from hagfish and lamprey-like ancestors that must have been present in the Ediacaran.

“Given the profound evolutionary gap between extant jawless cyclostomes and the gnathostome crown group, the detailed processes and mechanisms of the origin and early diversification of the jaw and jawed vertebrates are still largely to be extrapolated from investigations of early transitional fossils.”

Add taxa suggested by the LRT to confirm, refute or modify the LRT hypothesis. The ‘evolutionary gap’ is not profound. If an amateur can figure this out by simply adding taxa, professionals should be able to do this, too.

When one clade of gnathostomes separated from the other neither had jaws.
Be mentally open for this possibility.

Also be open to the possibility of gradual development of jaws in each case.

“The discovery and detailed studies of the Early Silurian jawed vertebrates, either preserved completely or disarticulated, filled in a 14 million-year gap in the earliest fossil record of jawed vertebrates.”

The authors refer to the discovery and studies of Shenacanthus (Fig 1`) and kin.

Zhu et al continue
“Despite these advances, how the jaw originated remains a persistent conundrum. Traditional theories suggesting a simple transformation from gill arches to jaw arches find little support in both fossil and extant vertebrates.”

The LRT provides no support for that (Fig 3) hypothesis.

Zhu et al wrote,
“The jawless galeaspids, which resemble jawed vertebrates in certain aspects such as having separate nasal sacs, provide an intermediate state supporting the heterotopic hypothesis of the jaw origin.”

Not in the LRT where galeaspids developed a buried niche with a dorsal oral slit not resembling jaws whatsoever.

“which group of jawless fish is the sister group of the jawed vertebrates, and how the jaw transformed from the jawless condition, remain unresolved.”

So Zhu et al have no idea. That’s why Zhu et al titled their paper with a question mark. Such papers are reviews of recent discoveries, not discovery announcements.

By contrast, the LRT recovered two origins for jaws, simply by adding taxa to analysis.

“Consensus has yet to be reached regarding the interrelationships of placoderms, the earliest jawed vertebrates.”

Waiting for consensus retards science according to Yale professor John Ostrom. Paleontologists as a group are encouraged to drag their heels because their lectures have to reflect textbooks that too often present out-dated ‘safe’ thinking that will sell textbooks to the greatest number of students directed by their ‘play it safe’ professors.

References
Zhu Y-A et al  2022. The Oldest Complete Jawed Vertebrates from the Early Silurian of China. Nature, 609(7929): 954–958. https://doi.org/10.1038/s41586-022-05136-8
Zhu M et al (5 co-authors) 2023. How did Jawed Vertebrates Originate and Rise? Journal of Earth Science 4(4): 1299-1301,

wiki/Gnathostomata

Source: https://pterosaurheresies.wordpress.com/2024/08/08/gnathostome-genesis-reviewed-by-zhu-et-al-2023/