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Bone Histology Presentation at SVP

At this year's meeting of the Society of Vertbrate Paleontology Kristi Curry Rogers presented these recent findings.

FROM THE SVP PRESS CONFERENCE
Thursday, Oct. 16, 10:30-11:30 a. m.

Kristi Curry Rogers, Gregory M. Erickson, and Mark Norell

Dinosaurian Life History Strategies, Growth Rates, and Character Evolution: New Insights Garnered from Bone Histology and Developmental Mass Extrapolation.

Did dinosaurs grow in a manner similar to living reptiles, birds, or mammals, or were they unique? Are the extremely rapid growth rates exhibited by modern birds a unique avian innovation, or were they inherited from dinosaurian precursors? Without a living dinosaur to observe, answers to these questions have remained intractable. Recent analyses of bone histology have called into question the longstanding assumption that dinosaurs were just scaled-up reptiles with slow growth rates. More recent work suggested that dinosaurs may have actually grown as fast as living birds, or may have been growing at intermediate rates. Most dinosaur bones are uniquely comprised of tissues with both slow-growing reptilian and rapid-growing avian/mammalian attributes, which complicate determination of growth rates and overall life history strategies.

The most common measure of developmental growth in living animals is derived from mass changes with respect to age, and can most often be represented by a sigmoidal curve. Comparing whole-body growth rates between Dinosauria and extant vertebrates requires similar data for dinosaurs. The recent merging of traditional bone histological analysis with scaling principles in a method termed Developmental Mass Extrapolation (DME) has provided the requisite tools and data to assess how dinosaurs really grew.

We analyzed genera of non-avian dinosaurs, including representatives from most major dinosaurian clades, as well as taxa ranging in size from tiny bipedal theropods to enormous quadrupedal sauropods. Notably, our sample also spans nearly the entire temporal range for non-avian dinosaurs, with taxa representing Early Jurassic to Late Cretaceous periods. Our analysis of histology and body mass in ontogenetic series from these dinosaurian genera revealed that sigmoidal equations accurately describe the growth data for six diverse dinosaur taxa. The onset of somatic maturity occurred between the ages of three and 13 years, with values positively correlated with increased body size. The results of our analyses do not support any of the earlier competing hypotheses regarding dinosaurian growth rates. The regression equation for Dinosauria indicates that all dinosaurs grew at rates more rapid than those of modern reptiles. Small dinosaurs tended to grow more slowly than large dinosaurs, at rates similar to those exhibited by marsupials. Mid-sized dinosaurs grew at rates comparable to those of precocial birds and eutherian mammals, and the largest sauropods approached the growth rates of modern whales. It is notable that no dinosaur in our sample attained the growth rates of extant altricial birds. These results demonstrate that growth in the Dinosauria was unique among major vertebrate groups, with rates spanning those known for all other tetrapods. Significantly, growth rates depend on the size of the dinosaur being considered. Birds clearly attained a portion of their elevated growth rates from their dinosaurian precursors, but how and when they surpassed the rates of non-avian relatives remains obscured. Our latest analyses focus on non-avian maniraptorans close to the dinosaur-bird transition, such as Velociraptor, and indicate that extremely rapid avian growth rates may have evolved only after the origin of Avialae.

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