Fig. 2

2BPLS plots of vertebral shape versus forelimb proportion (A–E) and percentage head mass (F–J). In C2 (A) and the last cervical vertebrae (E), elongation of distal forelimb elements is associated with increased intervertebral stability (decreases to centrum length and neural spine height). In the middle cervical vertebrae (B–D), longer distal forelimb elements are correlated with increases to intervertebral flexibility (increased centrum length, decreased neural spine height). Increases in relative head mass correspond to vertebral shape changes associated with intervertebral flexibility in C2-C50% (F–H) and increased stability in C75% and the last cervical vertebrae (I, J). The drawings below the X-axes display the vertebral shape change between end members along PLS block 1 for each individual cervical region for neck-forelimb (A–E) and head-neck (F–J) integration. In A–E, Y-axis drawings depict forelimb proportion change for end members across PLS block 2 for each individual cervical region. In F–J, Y-axis drawings display purely diagrammatical changes in head mass to visually display a change in head mass across the Y-axes. Representative species for end-member morphology for PLS block 1 are Topaza pyra and Grus leucogeranus (A), Phoebastria irrorata and Spheniscus mendiculus (B), Topaza pyra and Phoebastria irrorata (C), Sula dactylatra and Topaza pyra (D), Topaza pyra and Grus leucogeranus (E), Calyptomena viridis and Heleia goodfellowi (F), Lipaugus vociferans and Zavattariornis stresemanni (G, H, J) and Capito niger and Glareola pratincola (I). Representative end-member species for morphology for PLS block 2 are Phalacrocorax harrisi and Sula dactylatra