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 inmammalian evolution. 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 thenon-mammaliaform cynodont–mammaliaform transition. New discoveries ofderived non-mammaliaform probainognathian cynodonts from South America have much to ofer 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 toMammaliaformes, the tritheledontid-related Riograndia guaibensis and the tritylodontid Oligokyphus major. We ind 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 yearsbefore this character irst appears in mammaliaforms of the Late Triassic period.Brasilodon, contrary to previous descriptions , lacks an incipient dentary condyleand squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate–articular joint. We postulate that the jaw joint underwent marked evolutionary changesin probainognathian cynodonts. Some probainognathian clades independentlyacquired ‘double’ craniomandibular contacts, with mammaliaforms attaining a fullyindependent dentary–squamosal articulation with a conspicuous dentary condyleand squamosal glenoid in the Late Triassic. The dentary–squamosal contact, which istraditionally considered to be a typical mammalian feature, therefore evolved morethan once and is more evolutionary labile than previously considered.