FIGURE 1. Simplified cladogram depicting relationships between specimens of Mesozoic birds that have been histologically sampled (modified from Zhou et al., 2014). Red indicates lineages in which rapid growth evolved; the dashed line for Confuciusornis is to indicate that although growing relatively faster than other basal birds, this taxon did not achieve growth rates comparable to derived members of the ornithuromorph lineage. Stages 1-5 (grey numbers) refer to important evolutionary transitions in basal bird bone microstructure: 1, bone shows increased vascularity and a primarily woven matrix but growth is non-continuous; 2, the Confuciusornis lineage evolves at least a period of post-natal growth in which fibro-lamellar bone is formed and growth lines are only found in the outermost cortex indicating these birds approached adult size within the first year; 3, at least one lineage of Late Cretaceous enantiornithines evolves a slow and protracted growth strategy; 4, fibro-lamellar bone matrix indicative of more rapid growth appears in ornithuromorphs but growth lines persist; 5, derived ornithuromorphs evolve uninterrupted growth. Abbreviations: FLB, fibro-lamellar bone; LAG, line of arrested growth; PFB, parallel fibered bone; WB, woven bone.
FIGURE 3. Histological sections taken from Iteravis huchzermeyeri IVPP V18958: 1, ulna; 2, tibiotarsus; 3, close up of denoted area of the ulna ; 4, close up of the contact with the endosteal layer under cross polarized light showing presence of lamellae; 5, close up of denoted area of the tibiotarsus; 6, close up of the tibiotarsus under cross polarized light showing the endosteal layer is parallel fibered. Abbreviations: c, simple canal; icl, inner circumferential layer; po, primary osteon.