Ontogeny in the tube-crested dinosaurParasaurolophus(Hadrosauridae) and heterochrony in hadrosaurids

Abstract

The tube-crested hadrosaurid dinosaur Parasaurolophus is remarkable for its unusual cranial ornamentation, but little is known about its growth and development, particularly relative to well-documented ontogenetic series for lambeosaurin hadrosaurids (such as Corythosaurus, Lambeosaurus, and Hypacrosaurus). The skull and skeleton of a juvenile Parasaurolophus from the late Campanian-aged (∼75.5 Ma) Kaiparowits Formation of southern Utah, USA, represents the smallest and most complete specimen yet described for this taxon. The individual was approximately 2.5 m in body length (∼25% maximum adult body length) at death, with a skull measuring 246 mm long and a femur 329 mm long. A histological section of the tibia shows well-vascularized, woven and parallel-fibered primary cortical bone typical of juvenile ornithopods. The histological section revealed no lines of arrested growth or annuli, suggesting the animal may have still been in its first year at the time of death. Impressions of the upper rhamphotheca are preserved in association with the skull, showing that the soft tissue component for the beak extended for some distance beyond the limits of the oral margin of the premaxilla. In marked contrast with the lengthy tube-like crest in adult Parasaurolophus, the crest of the juvenile specimen is low and hemicircular in profile, with an open premaxilla-nasal fontanelle. Unlike juvenile lambeosaurins, the nasal passages occupy nearly the entirety of the crest in juvenile Parasaurolophus. Furthermore, Parasaurolophus initiated development of the crest at less than 25% maximum skull size, contrasting with 50% of maximum skull size in hadrosaurs such as Corythosaurus. This early development may correspond with the larger and more derived form of the crest in Parasaurolophus, as well as the close relationship between the crest and the respiratory system. In general, ornithischian dinosaurs formed bony cranial ornamentation at a relatively younger age and smaller size than seen in extant birds. This may reflect, at least in part, that ornithischians probably reached sexual maturity prior to somatic maturity, whereas birds become reproductively mature after reaching adult size.

Paleoneuroanatomy of the European lambeosaurine dinosaurArenysaurus ardevoli

“…The general form of the inner ear is similar to that described in other hadrosaurids (Brown, 1914; Langston, 1960; Ostrom, 1961; Evans, Ridgely & Witmer, 2009; Farke et al, 2013), and, as discussed in Evans, Ridgely & Witmer (2009), it resembles the condition in extant crocodilians. The three semicircular canals are oriented in approximately the three planes of space, where the anterior semicircular canal is slightly higher dorsoventrally and longer (Fig.…”

“…With regard to their ampullae, the lateral ampulla is larger than the posterior ampulla and the anterior ampulla, as in Parasaurolophus sp. RAM 14000 (Farke et al, 2013) and unlike in Hypacrosaurus altispinus ROM 702 and Lambeosaurus sp. ROM 758 (Evans, Ridgely & Witmer, 2009), where the anterior ampulla is the largest, followed by the lateral ampulla.…”

“…RAM 14000). These different angles are possibly a consequence of more strongly arched frontals in young individuals (Farke et al, 2013). In Arenysaurus the angle of the dural peak is close to 114° (Lautenschlager & Hübner, 2013; Farke et al, 2013).…”

“…To date, endocranial morphology is mainly known from North American specimens (Lull & Wright, 1942; Ostrom, 1961; Hopson, 1979; Evans, Ridgely & Witmer, 2009; Farke et al, 2013) and to a lesser extent from Asian remains (Young, 1958; Saveliev, Alifanov & Bolotsky, 2012; Godefroit, Bolotsky & Lauters, 2012; Lauters et al, 2013), including isolated individuals and ontogenetic series. In Europe, however, the cranial record of hadrosaurids is very scarce, and no paleoneurological analyses have yet been performed.…”

Ontogeny and the fossil record: what, if anything, is an adult dinosaur?

“…This is exemplified by the case of hadrosaurid dinosaurs, in which genus or species-diagnostic features are observable within juveniles of many taxa, despite major morphological changes through ontogeny (e.g. [49]). Nonetheless, the preponderance of 'primitive' features in juveniles still renders them difficult to place 'correctly' in a phylogeny (e.g.…”