A new paper in open access:
D. Cary Woodruff, Denver W. Fowler, and John R. Horner (2017)
A new multi-faceted framework for deciphering diplodocid ontogeny.
Palaeontologia Electronica 20.3.43A: 1-53
Determining maturity in sauropod dinosaurs histologically is problematic as rapid growth leads to remodeling of Lines of Arrested Growth (LAGs). Although a complimentary system has been devised utilizing several factors including relative amounts of remodeling (Histologic Ontogenetic Stage [HOS]), most assessments of sauropod maturity are based on morphologic indicators. To better assess skeletal maturity and morphologic change through ontogeny, we examined cranial and postcranial material from over 20 diplodocid individuals (Apatosaurus and Diplodocus) from the Upper Jurassic Morrison Formation. Here we describe consistent combinations of morphologic and histologic features that can be used to ascertain maturity. Small diplodocids (femoral lengths ≤120 cm) display non- to weakly bifurcated cervical and dorsal neural spines, acamerate to camerate centra, two to six preserved LAGs in dorsal ribs, and a maximum femoral designation of HOS 7. Larger individuals (femoral length ~125 cm) have more developed internal pneumatic structures, greater neural spine bifurcation, preserve up to eight LAGs, and a femoral designation of HOS 9. In contrast, skeletally mature sauropods (femoral lengths >150 cm) have complex pneumatic structures, extended neural spine bifurcation (also within anterior caudals), and a femoral HOS between 11-13. Further, all of the preserved small diplodocid skulls exhibit a postparietal foramen (previously suggested to be an apomorphy of Dicraeosauridae), which is absent in large skulls (where preserved), suggesting that it is an ontogenetic character. These findings support the hypothesis of significant ontogenetic morphological change in diplodocid sauropods and suggest caution when describing new taxa on the basis of small-bodied holotypes.