Pre-vertebral axial formulae were determined by somite counting and, if visible in the in situ hybridization wholemounts, the position of the first spinal ganglion.
In corn snakes and lizards, we used the amniote standard formula where the 1st pre-vertebra forms from somites 5 and 6.
In caecilian we determined (by Alcian blue staining and the position of the first spinal ganglion) that the 1st vertebra most likely forms from somites 3 and 4, which is the same as in Xenopus laevis (discussed in Handrigan and Wassersug (2007)) and Ambystoma mexicanum (Piekarski and Olsson, 2007), since we however did not study a complete developmental series it cannot be excluded that the relation between somite number and inferred anatomy is offset by 1 vertebra level.