Wagner and Chiu 2001
printed an X-ray image of a newt (genus: Triturus) foot with six toes (Fig. 1). They call the new one a pre-pollux.
Figure 1. Left to right: Original illustration from Wagner and Chiu 2001. Cleaned up and labeled. Placed on top of an in vivo pes with a phase shift movement of the metatarsals. Some toes are curled, so the graphic bones extend beyond the curl.I have been calling a similar medial manus digit
‘digit zero‘ for the last three years.
The problem I had with the Wagner and Chiu newt
was matching toes 1–5 to the toes in a photo of another specimen. It was impossible to do so while retaining the original connections to the distal tarsals (Fig. 1). Instead the toes had to be shifted medially with regard to the Wagner and Chiu tarsal images.
In other words, when digit zero is present
digits 4 and 5 arise from the lateral distal carpal and digit 1 arises from distal carpal 2.
When digit zero is absent
digits 4 and 5 each have their own distal carpal and digit 1 arises from distal carpal 1.
Still thinking about this one.
Not sure what the ramifications are with regard to newts and other tetrapods. Where is the error? Perhaps there is no error. It is what it is. There was a phase shift.
Earlier we looked at digit zero on the manus
of the theropod Limusaurus (which opened up the whole phase shift question in theropods), and the screamer, Chauna (which could have done the same with its ‘spike’, but never did).
Wagner GP and Chiu C-H 2001. The tetrapod limb: a hypothesis on its origin. Journal of Experimental Zoology (Mol Dev Evol) 291:226–240.