Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint.

Abstract

The acquisition of the load-bearing dentary-squamosal jaw joint was a key step in mammalian evolution^1-5. 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 Mammaliaformes^6-8, 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 period^10-12. Brasilodon, contrary to previous descriptions^6-8, 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.