THE GROWTH OF THE FORK STRUCTURE IN WALLISEROPS TRIFURCATUS INFORMED BY A TERATOLOGICAL EXAMPLE

The trilobite Walliserops trifurcatus (Phacopida: Acastidae: Asteropygine) is exceptional in possessing a precephalic projection divided into a three-tined fork. While outwardly appearing to be perfect trident, the fork tines are actually formed though intercalating bifurcation. The projection first bifurcates into two tines with one of those tines bifurcating again to form the middle tine. This structure also exhibits a form of “handedness” with the majority of the twenty-one specimens examined showing the left side bifurcating to form the central tine, with only about twenty percent forming the central tine from the right.

This suggests that the control of fork formation is either by asymmetrical promotion of bifurcation or suppression of bifurcation on one side of the structure relative to the other. Generally there would be no way to determine which was the case. However, the Houston Museum of Natural Science has a teratological example of Walliserops in their collections that can inform us on this control. This specimen developed a four-tined fork, with a strong lateral displacement of the left side fork, and upward and lateral displacement of the right. This is caused by a double bifurcation of the left side. In this case during growth, the left side bifurcated to intercalate the central tine, which then bifurcated again, adding a second center tine and displacing the right tine out and upward. The observation that the fourth tine is formed by a second bifurcation of the left (central) tine, rather than a bifurcation of the right tine would suggest that tine formation is controlled by promotion of bifurcation rather than suppression.