Amazing stuff! Apparently, evolution is a lot more complicated than textbooks suggest.
"Fossils found in Brazil are leading palaeontologists to re-write the evolution of mammals.
CT scans of mammal-precursor species Brasilodon quadrangularis and Riograndia guaibensis reveal that uniquely mammalian features in jaws and inner ears developed millions of years earlier than previously thought. ...
find ‘mammalian-style’ contact between the skull and lower jaw in Riograndia. This is 17 million years earlier than the previous oldest example of such a structure in the fossil record.
“What these new Brazilian fossils have shown is that different cynodont groups were experimenting with various jaw joint types, and that some features once considered uniquely mammalian evolved numerous times in other lineages as well,” ...
South America is fast becoming a central location for the study of early mammal evolution. ..."
From the abstract:
"The acquisition of the load-bearing dentary–squamosal jaw joint was a key step in mammalian 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 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 ... 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. ... 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."
Brazilian fossils reveal jaw-dropping discovery in mammal evolution (original news release) "The discovery of new cynodont fossils from southern Brazil by a team of palaeontologists from the University of Bristol, alongside colleagues from Argentina and Brazil, has led to a significant breakthrough in understanding the evolution of mammals."
Fig. 1: Time-calibrated relationships of Cynodontia.
Fig. 3: Anatomy of the lower jaws and jaw joint of R. guaibensis.
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