Homoplasy and Heterochrony in an Early Cretaceous Mammal skeleton.

Homoplasy and heterochrony are two words you are likely not to encounter often, but a new primitive mammal named Yanoconodon allini nicely illustrates both terms. Homoplasy basically translates to convergence, or the acquisition of a particular trait in two or more species through separate evolutionary histories. The new skeleton exhibits lumbar ribs, a trait lost in more advanced mammals. However, the closely related Jeholodens lacks lumbar ribs indicating that the absence of the lumbar ribs in Jeholodens arose independently. Heterochrony basically translates to a developmental change in the timing of events. One type of heterochrony (neoteny) is the retention of early developmental features into adulthood. The lower jaw of Yanoconodon allini nicely illustrates this by retaining a connection to the middle ear bones by an ossified Meckel's cartilage. In adult monotreme and placental mammals the ossified Meckel's cartilage is reabsorbed in adulthood. Based on its position on the family tree (cladogram), Yanoconodon allini evolved from an ancestor that lacked a connection between the jaw and middle ear bones. Thus the connection arose independently by retention of an earlier developmental feature into adulthood.

Lou, Z, Chen, P., Li, G., Chen, M., 2007. A new eutriconodont mammal and evolutionary development in early mammals Nature 446, 288-293 (15 March 2007) doi:10.1038/nature05627.