Origin of teeth 2024

Grohganz et al 2024 reported,
“Teeth are a key vertebrate innovation; their evolution is generally associated with the origin of jawed vertebrates.

The authors were not aware of the results of the large reptile tree (LRT, 2328 taxa) which recovered several origins of jawed and almost jawed vertebrates. That means there were also several origins for the structures we call teeth – all by convergence.

Grohganz et al 2024 also reported,
“However, tooth-like structures already occur in jawless stem-gnathostomes; heterostracans bear denticles and morphologically distinct tubercles on their oral plates.”

Heterostracans include Hemicyclaspis (Fig 1), which does not include oral plates.

The authors looked at the plates of Protopteraspis, which were similar to the slightly mobile oral plates in Rhinopteraspis (Fig 2) animated here.

Grohganz et al 2024 also reported,
“We analysed the histology of the heterostracan denticles and plates to elucidate their morphogenesis and test their homology to the gnathostome oral skeleton. We identified a general model of growth for heterostracan oral plates that exhibit proximal episodic addition of tubercle rows. The distal hook exhibits truncated lamellae compatible with resorption, but we observe growth layers to be continuous between denticles. The denticles show no evidence of patterns of apposition or replacement indicating tooth homology.”

That’s an interesting observation/confession coming from a tooth origin paper.

“The oral plates and dermal skeleton share the same histological layers. The denticles grew in a manner comparable to the oral plate tubercles and the rest of the dermal skeleton. Our test of phylogenetic congruence reveals that the distribution of internal odontodes is discontinuous, indicating that the capacity to form internal odontodes evolved independently several times among stem-gnathostomes.”

See the LRT, remembering that gnathostomes evolved several times by convergence.

“Our results support the ‘outside-in’ hypothesis and the origin of teeth through the spread of odontogenic competence from extra-oral to oral epithelia and the subsequent co-option to a tooth function in gnathostomes.”

Since jaws developed several times by convergence, teeth developed several times by convergence and in different ways according to the LRT.

The authors concluded,
“Our results rather support the ‘outside-in’ theory, which argues for an evolutionary origin of teeth closely associated with the dermal skeleton; odontogenic competence spreading from extra-oral to oral epithelia and forming the denticles on the oral plates.”

Time to update the traditional cladogram of jaw and tooth genesis used by academics. The LRT can be a starting point. That’s what it was meant to be: a starting point.

References
Grohganz M, et al (3 co-authors) 2024. Morphogenesis of pteraspid heterostracan oral plates and the evolutionary origin of teeth. R. Soc. Open Sci. 11: 240836.
https://doi.org/10.1098/rsos.240836

Publicity
phys.org/news/2024-12-timetree-primates-evolutionary-history-life.html
frontiersin.org/news/2024/12/16/timetree-primates-evolutionary-history-of-life

Source: https://pterosaurheresies.wordpress.com/2024/12/26/origin-of-teeth-2024/