Fig. 2
From: The ecological origins of snakes as revealed by skull evolution
Principal component analysis of skull shape variation in extant adult squamate species. a Plot of principal component (PC) scores showing the skull shape distribution of lizards (black squares), scolecophidians (green circles), alethinophidians (red circles), and tuatara (outgroup, orange square) for which 2D data were available. Numbers in brackets indicate the percentage of variance explained by each of the PC axes. The 3D-rendered skulls (with species names) corresponding to the observed extreme species in both positive and negative directions are shown. b For each PC, the extreme shapes at positive (+) and negative (−) values are depicted as skull outlines with vectors of landmark displacement (orange arrows) indicating the direction and intensity (reflected by vector length) of shape changes from the mean shape. Different regions of the skull are visible by black lines connecting landmark points: B, back of the skull; F, frontal; M, maxilla; N, nasal; P, parietal; PM, pre-maxilla; Q, quadrate. Parietal and quadrate bones, showing the greatest variations in size and shape, are shaded in blue color
