Fig. 1
From: The phantoms of a high-seven - or - why do our thumbs stick out?
Digit 0 in Xenopus tropicalis and putative relationships between digits and pre-axial and post-axial sides of the fin. a The left hindlimb of Xenopus tropicalis drawn after reference [8] with indication of the digit numbers. Digit 0 appears as an antero-ventral protrusion bearing a distinct claw. (Claws on digit 0-III are drawn in black). b Pectoral fin of lungfish and limbs of Ichthyostega and mouse. The metapterygial axis is indicated with a dashed line and runs through the post-axial part of the limb in Ichthyostega and mouse. Mednikov [13] recently hypothesized that the three most anterior digits in Ichthyostega derived from the pre-axial side of ancestral fins (indicated in red). Given the unique genetic position of the thumb, its identity as either ‘post-axial’ (blue) or ‘pre-axial’ (red) digit could be investigated