The origin of the mammal jaw joint revisited in 2024

From the Rawson et al 2024 abstract:
“Some probainognathian clades independently acquired ‘double’ craniomandibular contacts, with mammaliaforms attaining a fully independent dentary–squamosal articulation with a conspicuous dentary condyle and squamosal glenoid in the Late Triassic. The dentary–squamosal contact, which is traditionally considered to be a typical mammalian feature, therefore evolved more than once and is more evolutionary labile than previously considered.”

The authors reported,
“we used microcomputed- tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major.”

None of these are mammals in the large reptile tree (LRT, 2322), so their headline “oldest mammalian jaw joint” is misleading. Brasilodon is four nodes away from mammals in the LRT. Riograndia is three nodes away from mammals. Oligokyphus is also three nodes away from mammals.

Availability
tends to focus a taxon list. A cladogram should be the source for creating a taxon list approaching the taxa in focus.

Repenomamus
would have been a more interesting case of convergence with mammals (Fig 2). Tradition nests this taxon somewhere within Mammalia – because the convergence is not academically recognized. All it takes is another LRT to confirm, modify or refute this nesting. Repenomamus is not mentioned in the text.

Pre-mammal fossils are often crushed flat, which makes examination of the dentary and post-dentary in medial view often difficult, but not impossible, especially with µCT scanning.

Mammaliaformes definition:
“the most recent common  ancestor of Morganucodonta and the crown group mammals.”

Unfortunately, in the LRT Morganucodon is a basal marsupial, so it is an ingroup taxon, not an outgroup to the Mammalia. None of the three taxa listed above and studied by the authors are members of the Mammaliaformes on their cladogram.

More phylogenetic issue
Wikipedia -Repenomus reports, “Repenomamus is a genus of triconodonts, a group of early mammals with no modern relatives.”

Does anyone else see the problem with this statement?

Phylogenetically mammals are defined by
mammae = milk producing tissue found only in living taxa, including egg-laying monotremes. The last common ancestor of all living mammals is also a mammal. It’s nearest ancestors (= its parents) cannot be mammals. If a taxon has no modern relatives, it is a pre-mammal, even if Wikipedia (and the derivation of its name Repenomamus = reptile-mammal) says it is a mammal.

Paleontologists don’t always follow their own rules. And other paleontologists are sometimes reticent about correcting their colleagues who referee manuscripts.

Figure 1 in Rawson et al shows
several cynodont mandibles in medial view and many more listed in their cladogram. Several taxa close to mammals in the LRT are not listed or illustrated there. To their credit the authors used DGS colors to identify bones in their diagram, obviating the need for labels everywhere.

The authors concluded,
“3D μCT data of Brasilodon and Riograndia illustrate homoplastic evolution of the dentary–squamosal contact within probainognathian cynodonts and reveal the oldest occurrence of this feature in stem mammals. Although only Mammaliaformes evolved a fully developed dentary condyle and squamosal glenoid, the presence of an extensive lateral dentary–squamosal contact in Riograndia and the thickened, more condyle-like posterior rim of the articular process in more derived Jurassic tritheledontids demonstrates that at least two cynodont lineages were evolving features of the jaw articulation that are typically associated with mammals.”

This is misleading and old news. It’s been known for several decades that pre-mammals developed side-by-side jaw joints as transitional structures prior to losing the old jaw joint (as the bones kept shrinking and becoming more integral to the auditory series, Fig 1) while further developing the new jaw joint in mammals creating a replacement robust axle for chewing and biting. In other words, this study duplicates and confirms studies from the 1980s.

In a Nature promotional article
Luo 2024 shows the Late Triassic cynodont Riograndia has a dentary-squamosal joint in concert with large post-dentary bones that also form a jaw joint. He reported, “One hallmark of mammals is the jaw hinge that forms between the enlarged back end of the tooth-bearing dentary bone (namely the dentary condyle) on the lower jaw and the squamosal bone of the skull.”

Luo continued,
“Mesozoic mammals have the D-SQ joint in addition to the ancestral articular–quadrate joint inherited from cynodonts, which formed a double jaw hinge during a complex evolutionary transition towards having only a D-SQ joint, as is observed in living mammals.”

That statement is too generalized. Luo seems to be referring to multituberculates, which reversed to the more primitive pattern according to the large reptile tree (LRT, 2322 taxa). That’s why it is so important to follow the last common ancestor method of determining phylogenetic interrelationships, rather than rather than a defining character.

Luo continues,
“Rawson and colleagues have taken a great leap forwards towards understanding how
the mammal-like jaw joint transformed from the ancestral configuration, as observed
in non-mammalian cynodont sister clades.”

No. Rawson and colleagues have shown convergence (also see below). We knew how mammal jaw joints evolved since the 1970s.

Luo continues,
“The absence of a D-SQ contact in Brasilodon, even though it is in the closest branch on the evolutionary tree next to mammals, is in sharp contrast to the more mammalian-like D-SQ contact of Riograndia despite the fact that Riograndia represents a more distant branch of the mammalian evolutionary tree.”

Unfortunately, Brasilodon is not in the closest branch next to mammals in the LRT.

Luo continues,
“Rawson and colleagues propose that the D-SQ joint in Riograndia is an independent
experiment in iterative evolution before the appearance of the dentary condyle structure.”

That statement sums up Luo’s two-page review of Rawsom et al 2024. Now read what Soares wrote in 2011.

Soares et al 2011 reported,
“Riograndia shows a suite of important anatomical features quite derived among the non-mammaliaform eucynodonts, such as the partial closure of the medial orbital wall and braincase, extensive secondary osseous palate, wide primary palate, basicranium with jugular foramen separated from the periphery of fenestra rotunda, narrow zygomatic arch and much reduced postdentary bones.”  and “quadrate suspended by the squamosal; premaxilla” and “the articular process of the dentary in contact with the squamosal.”

So this revelation is 13 years old.

Getting back to Luo 2024, where he writes,
“In living mammals, the dentary condyle is developed differently from the rest of the
dentary. It forms from a process termed endochondral ossification of secondary cartilage
during bone development, and it is added onto the dentary bone that forms by what is called intramembranous ossification” (citing Anthwal and Tucker 2023).

Now that’s news. But it does not reflect evolutionary processes (Fig 1) in which the dentary simply changes shape. The dentary is not composed of two fusing bones in adult taxa basal to mammals. Anthwal and Tucker looked at a mouse embryo.

Anthwal and Tucker 2023 reported,
“A jaw joint between the squamosal and dentary is a defining feature of mammals…”

No, it is not. Repenomamus (Fig 2) and Riograndia (Fig 3) developed that trait independently. Sometimes it takes an independent researcher to point out little things like this that academics gloss over out of professional courtesy.

References
Anthwal N and Tucker AS 2023. Evolution and development of the mammalian jaw joint: Making a novel structure. Evolution & Development 25 (1), 3-14,
Luo Z-X 2024. A jaw dropping discovery about early mammals. Nature https://doi.org/10.1038/d41586-024-03038-5
Rawsom JRG et al (5 co-authors) 2024. Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint. Nature 1–10. https://doi.org/10.1038/s41586-024-07971-3
Soares MB, Schultz CL and Horn BLD 2011. New information on Riograndia guaibensis Bonaparte, Ferigolo & Ribeiro, 2001 (Eucynodontia, Tritheledontidae) from the Late Triassic of southern Brazil: anatomical and biostratigraphic implications. Anais da Academia Brasileira de Ciências. 83 (1): 329–354.

wiki/Mammaliaformes
wiki/Repenomamus
wiki/Riograndia
wiki/Brasilodon

Repenomamus is not a mammal, but close…

Source: https://pterosaurheresies.wordpress.com/2024/11/04/the-origin-of-the-mammal-jaw-joint-revisited-in-2024/