In embryonic vertebrates the skeleton develops initially as a cartilaginous structure, then ossifies (turns to bone) as the animal matures (except in sharks and rays which retain a cartilaginous skeleton). However two areas of bone, the skull and the clavicles (collar bones) develop not from cartilage but from membranes associated with the epidermis (skin). This presents challenges for growing vertebrates, as bone is inflexible, and unable to move to accommodate stresses during growth.
Three separate groups of vertebrates, Mammals, Birds and Teleost Fish, have independently evolved the same solution to this problem; during growth parts of the cranium secondarily chondrify (turn from bone to cartilage) enabling some flexibility in the growing skull.
In a paper in published in the journal PLoS One on 30 April 2012, Alida Bailleul and John Horner of the Museum of the Rockies and the Department of Earth Sciences at Montana State University and Brian Hall of the Department of Biology at Dalhousie University, discuss the results of an examination into the cranial development of Hypacrosaurus stebingeri, a large Cretaceous Hadrosaur (Ornithischian Dinosaur).
Bailleul et al. obtained a number of exceptionally preserved hatchling Hypacrosaurus skulls, which they cut into a series of thin sections (slices) enabling them to study the cell structure (chondrocytes, cartilage cells, are easily distinguished from osteocytes, bone cells) under a light microscope. This revealed that the young Dinosaurs had several patches of secondary cartilage development, roughly matching those seen in modern birds.
(A) Skull of a two-day-old chick, Gallus. (B) Skull of a post-hatching Hypacrosaurus. (C) Detail of (A). (D) Detail of (B). Blue and purple areas represent cartilage; blue at joints between bones, purple at points of muscle insertion. Hatched areas are on internal surfaces. Abbreviations: ang, angular; art fac, articular facet of Meckel’s cartilage; co, coronoid process; de, dentary; ju, jugal; ma, maxilla; pt, pterygoid; qj, quadratojugal; qu, quadrate; sq, squamosal; sur, surangular. Bailleul, Hall & Horner (2012).
This pattern of secondary cartilage development in an Ornithischian Dinosaur, only distantly related to the Birds, suggests that this trait originated early in the history of the group, and is likely to have occurred in all dinosaurs, and may have been one of the adaptations which contributed to the success and diversity of the group.
See also Head injury in a pachycephalosaur, A Heterodontosaurid Dinosaur from the Late Jurassic of Colorado, Torosaurus and Triceratops; one dinosaur or two? New description of an Ankylosaurid Dinosaur from Mongolia and Dinosaurs on Sciency Thoughts YouTube.
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