Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint

James R. G. Rawson (), Agustín G. Martinelli (), Pamela G. Gill, Marina B. Soares, Cesar L. Schultz and Emily J. Rayfield ()
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James R. G. Rawson: University of Bristol
Agustín G. Martinelli: Museo Argentino Ciencias Naturales “Bernardino Rivadavia”—CONICET
Pamela G. Gill: University of Bristol
Marina B. Soares: Museu Nacional, Universidade Federal do Rio de Janeiro
Cesar L. Schultz: Universidade Federal do Rio Grande do Sul
Emily J. Rayfield: University of Bristol

Nature, 2024, vol. 634, issue 8033, 381-388

Abstract: Abstract The acquisition of the load-bearing dentary–squamosal jaw joint was a key step in mammalian evolution1–5. Although this innovation has received decades of study, questions remain over when and how frequently a mammalian-like skull–jaw contact evolved, hindered by a paucity of three-dimensional data spanning the non-mammaliaform cynodont–mammaliaform transition. New discoveries of derived non-mammaliaform probainognathian cynodonts from South America have much to offer to this discussion. Here, to address this issue, we used micro-computed-tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes6–8, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major. We find homoplastic evolution in the jaw joint in the approach to mammaliaforms, with ictidosaurs (Riograndia plus tritheledontids) independently evolving a dentary–squamosal contact approximately 17 million years before this character first appears in mammaliaforms of the Late Triassic period10–12. Brasilodon, contrary to previous descriptions6–8, lacks an incipient dentary condyle and squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate–articular joint. We postulate that the jaw joint underwent marked evolutionary changes in probainognathian cynodonts. 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.

Date: 2024
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DOI: 10.1038/s41586-024-07971-3

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