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.

Robots take to the land, with old swimming circuitry.

The origin of tetrapods and the accompanying changes from swimming to walking require numerous morphological and biological changes. However, a new study published in Science demonstrates that the neurological signals for walking and swimming are remarkably similar. Using a robotic salamander, the team wired the circuitry of the robot like the modern salamander and lamprey, where signals for locomotion are fired off in traveling waves along the axis of the body. The robot swims well (although slower than the living salamander), but when placed on land to get the robot to walk the researchers just had to lower the frequency of the signals to slow down to the rhythm of walking. Robots are an important innovation in the study of vertebrate motion and evolution.


Ijspeert, A.J., A. Crespi, D. Ryczko, and J-M. Cabelguen. 2007. From Swimming to Walking with a Salamander Robot Driven by a Spinal Cord Model. Science 315 (5817), 1416