Modelling growth, vital statistics and energy budget of sauropods: key features contributing to gigantism.
Zusammenfassung der Projektergebnisse
The first major goal of our project (R1 and E2 in FOR 533) was to assess aging, reproduction, maturation and growth in sauropods in order to understand their life history and to test several ecological hypotheses on their exceptional gigantism in size. As growth in vertebrates seems to be linked to metabolic rate, our second major goal was to assess the thermal physiology of dinosaurs. Burness et al. (2001) hypothesized that the body mass of the largest species and the size of land inhabited are proportional due to the decrease in per capita resources with decreasing area size, which results in a higher chance of extinction. Dinosaurs seem to be an exception from this rule. Their body size is much too large for the land mass inhabited. Our reanalysis of a dataset on extant mammals suggests that the land area inhabited by dinosaurs is much larger than needed to support a viable population of similar-sized scaled-up mammals. Janis and Carrano (1992) hypothesized that oviparous dinosaurs had a larger total potential reproductive output that in contrast to viviparous mammals does not decrease with increasing body size. This output enables dinosaurs to rapidly rebuild populations after environmental perturbations over evolutionary time. We corroborated this hypotheses by an analysis of the potential reproductive output (clutch/litter size and annual offspring number) of extant terrestrial mammals and birds (as "dinosaur analogs") and of extinct dinosaurs. In order to infer more information on dinosaur reproduction, which is needed to assess the fecundity rates of populations, we successfully established allometries between body mass and different reproductive traits (egg mass, clutch mass, annual clutch mass) for birds, crocodiles and tortoises. Reproductive traits of sauropodomorphs conform to those of scaled-up reptiles. Large sauropods had not exceptionally high annual egg numbers, a 75,000 kg sauropod laid maximal 850 eggs per year. Our next step was to model aging, maturation and growth in sauropodomorph individuals using the histological growth record preserved in the mid-shaft cortex of long bones. We estimated the ages at death of several sauropodomorph individuals and the ages at which they are/ would be fully-grown as well as the age at which these individuals reached sexual maturity and their maximum growth rates by growth curve fitting. All Sauropodomorpha studied reached full size within a time span similar to scaled-up modern mammalian megaherbivores and had similar maximum growth rates to scaled-up modern megaherbivores and ratites, but growth rates of Sauropodomorpha were lower than of an average mammal. Sauropodomorph ages at death probably were lower than that of average scaled-up ratites and megaherbivores. Sauropodomorpha were older at maturation than scaled-up ratites and average mammals, but younger than scaled-up megaherbivores. To estimate survivorship curves of Sauropodomorpha we developed a mathematical model for an age-structured stable population that is based on our estimated fecundities, ages at sexual maturity and lifespans of Sauropodomorpha. It predicts a survivorship curve for Sauropodomorpha that is very similar to that seen in birds and reptiles (Type 2, 3). In total, our analyses on the vital statistics and growth in Sauropodomorpha suggests that they had a life history intermediary to reptiles and birds, which is consistent with their taxonomy. The regression lines given in Case (1978) have often been used to compare dinosaurian maximum growth rates to those of extant vertebrates in order to derive whether the dinosaurs were endothermic or ectothermic. Our analysis of maximum growth rates in extant vertebrates suggests that growth rates of recent taxa are not unequivocally separated between endotherms and ectotherms, which makes it impossible to rule out any of the two thermoregulation strategies for dinosaurs. Maximum growth rates of sauropods were similar to those of recent ectothermic amniotes (reptiles), but not to endothermic rates (birds). Gillooly et al. (2006) used an equation predicting body temperature of dinosaurs from their body mass and maximum growth rate, and evidenced inertial homeothermy in Dinosauria as body temperature increases with body mass. They hypothesize that, due to overheating, the maximum body size in Dinosauria was ultimately limited by body temperature. Using a much larger dataset on much more reliable maximum growth rates in dinosaurs we were able to reject the hypothesis of overheating in large sauropods.
Projektbezogene Publikationen (Auswahl)
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(2010). Body size development of captive and free-ranging African spurred tortoises (Geochelone sulcata): High plasticity in reptilian growth rates. Herpetological Journal, 10: 213-216
Ritz J., Griebeler E.M., Huber R., Clauss M.
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(2011). Biology of the Sauropod Dinosaurs. The Evolution of Gigantism. Biological Reviews, 86(1): 117-155
Sander M., Christian A., Clauss M., Fechner R., Gee C.T., Griebeler E.M., Gunga H.-C., Hummel J., Mallison H., Perry S.F., Preuschoft H., Rauhut O.W.M., Remes K., Tütken T., Wings O., Witzel U.
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(2011). Reproductive Biology and Its Impact on Body Size: Comparative Analysis of Mammalian, Avian and Dinosaurian Reproduction. PLOS ONE, 6(12): e28442
Werner J., Griebeler E.M.
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(2011). The life-cycle of sauropod dinosaurs. Seiten 263-275 in: Biology of the Sauropod Dinosaurs: Understanding the life of giants (Hrsg. N. Klein, K. Remes, C. Gee, M. Sander). Life of the Past (Hrsg. J. Farlow) Indiana University Press
Griebeler E.M., Werner J.
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(2012). Reproductive investment in moa: a K-selected life-history strategy? Evolutionary Ecology, 26(6) 1391-1419
Werner J., Griebeler E.M.
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(2012): Dichotomy of eutherian reproduction and metabolism. Oikos, 121:102-115
Müller D.W.H., Codron D., Werner J., Fritz J., Hummel J., Griebeler E.M., Clauss M.
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(2013). Aging, maturation and growth of sauropodomorph dinosaurs as deduced from growth curves using long bone histological data: an assessment of methodological constraints and solutions. PLOS ONE, 8(6): e67012
Griebeler E.M., Klein N., Sander P. M.
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(2013). Body temperatures in dinosaurs: What can growth curves tell us? PLOS ONE, 8(10):e74317
Griebeler E.M.
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(2013). New insights into non-avian dinosaur reproduction and their evolutionary and ecological implications. PLOS ONE, 8(8):e72862
Werner J., Griebeler, E.M.
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(2013): Preliminary analysis of osteocyte lacunar density in long bones of tetrapods: all measures are bigger in sauropod dinosaurs. PLOS ONE
Stein K. W. H. & J. Werner
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(2014) Allometries of maximum growth rate versus body mass at maximum growth indicate that non-avian dinosaurs had growth rates typical of fast growing ectothermic sauropsids. PLOS ONE, 9(2):e88834
Werner J., Griebeler E.M.
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(2014). Online Comment on Grady et al. (2014), Science
Griebeler E.M., Werner J.
