ABSTRACT
The histological features of Dinosauria and its relatives (Pan-Aves, the stem-group for birds, comprising also pterosaurs and a variety of other basal avemetatarsalians) are surprisingly varied in tissue type and expression, notably with ontogenetic stage and with adult size of the taxon in question. As a group, compared to other archosauromorphs and other reptiles, this clade generally expresses higher growth rates, characterized by the predominance of the fibrolamellar complex, extensive resorption of inner cortical tissue (and hence generally larger medullary cavities), higher osteocyte density and higher density of vascular canals, with a predominance of radial and circumferential canals, frequently anastomosing, especially present at younger ontogenetic stages. Secondary (Haversian) osteons can be found, especially in smaller elements of the skeleton (metapodials, ribs) and in the inner cortices of larger elements (femur, tibia) at later stages of growth. Birds inherited the growth patterns of small theropod dinosaurs from which they arose and tended to evolve thinner bone walls and flight-adaptive bone tissues. An increase in growth rate, reflecting rapid development and skeletal maturation before the onset of reproductive maturity, appears to have evolved in the latest Cretaceous at or shortly before the origin of crown-group birds.
In describing the osteohistological patterns of Archosauromorpha for this book, we have divided the group into three stages, the first necessarily paraphyletic. We begin with “non-archosaurian archosauromorphs,” that is, those outside the crown-group Archosauria, which comprises birds and crocodylians plus all descendants of their most recent common ancestor. The crocodile stem-group is Pan-Crocodylia, called Pseudosuchia (Gauthier 1984), and the bird stem-group (Pan-Aves) was named Ornithosuchia by Gauthier (1984). Some objected that Ornithosuchia was an inappropriate name when the Ornithosuchidae was found to belong within Pseudosuchia, and the name Avemetatarsalia was suggested as a substitute. Within Avemetatarsalia, a node-taxon that unites Pterosauria and Dinosauria is called Ornithodira; the two stems of this group are Pterosauromorpha and Dinosauromorpha, the latter comprising dinosaurs (including birds) and several taxa close to dinosaurs. Most work has been done on the Ornithodira, particularly dinosaurs. We try to document, in a very preliminary way, some of the osteohistological patterns that seem to emerge from work that has so far been published on all these lineages.
