Today we’ll start a new series: “Freed from their primitive function” in which we’ll look at various parts of the vertebrate anatomy that started off doing one thing, but ended up doing another. These are excellent examples of evolution.
Today we’ll look at the posterior jaw bones that started off articulating with the skull and ended up transmitting sound in mammals. It was first recognized as early as Reichert (1837) 20 years before Darwin’s “On the Origin of Species’ (1859) and subsequently confirmed and elaborated by several workers (e.g. Allin 1975, Kemp 2007). Reichert employed embryology and comparative anatomy as he reported in the developing mammalian embryo that the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla. (Which reminds us that before these were jaw bones they were gill rims!)
Allin (1975) drew on the several paleontological examples then known to more fully fill in the story (Fig. 1) demonstrating graphically the gradual rise and expansion of the coronoid process along with the reduction of the postdentary jaw bones. This series also demonstrates the evolution of the canine and molar teeth. The evolution of the angular (lower postdentary jaw bone) into a thin flange that encircled an open space then continued to reduce until it ultimately framed the eardrum. Allin (1975) suggested at an early stage the angular framed a very large eardrum. Later, Kemp (2007) thought the postdentary bones were too large and still functioning as jaw bones to allow this, suggesting instead that the eardrum formed in the earliest mammals when the articular became very much more gracile and smaller. The unusual anatomy of the postdentary bones would have supported ventral jaw muscles acting to open the jaws.
The moment of direct contact between the dentary and squamosal marks the origin of the Mammalia and permitted the postdentary bones to continue their evolution to become ear ossicles. Note that even in primitive mammals, like Asioryctes (Fig. 1), the postdentary bones continued a tenuous connection to the dentary that was finally disconnected in eutherian mammals.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Reichert KB 1837. Über die Visceralbogen der Wirbelthiere im Allgemeinen und deren Metamorphosen bei den Vögeln und Säugethieren. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin: 120-222.
Kemp TS 2007. Accoustic transformer function of the postdentary bones and quadrate of a nonmammalian cynodont. Journal of Vertebrate Paleontology 27(2): 431-441.