Per verificare se la legge di Cope si sia dimostrata realmente valida per i dinosauri, e in quale misura, Gene Hunt (curatore del Dipartimento di Paleobiology nel National Museum of Natural History di Washington), Richard FitzJohn (University of British Columbia), Matthew Carrano (National Museum of Natural History di Washington) hanno usato il femore di alcuni dinosauri come misura di riferimento e hanno effettuato analisi statistiche.
E allora perché i piccoli mammiferi non hanno sentito il bisogno di crescere? Ma perché mammiferi di piccola taglia, onnivori, con elevata capacità riproduttive e in grado di tollerare un'ampia varietà di condizioni climatiche, meno soggetti ai grandi fenomeni di estinzione erano anche meno soggetti alla pressione evolutiva. In altre parole stavano bene com'erano.
Hunt, FitzJohn e Carrano presenteranno i risultati della loro ricerca il 4 Novembre nel corso della conferenza annuale della Società Geologica Americana a Charlotte (North Carolina, USA). Dalle prime anticipazioni della loro ricerca si evidenzia che esistono dei limiti alla crescita dei teropodi (bipedi carnivori, come il T-Rex) mentre non esistono in sauropodi (quadripedi erbivori, come il diplodocus) e ornitopodi (bipedi erbivori, come gli iguanodonti).
Andrea Mameli www.linguaggiomacchina.it 2 Novembre 2012
Testing Cope's rule and the existence of an upper bound for body size in non-avian dinosaurs
Geological Society of America Abstracts with Programs. Vol. 44, No. 7, p.138
Gene Hunt, Richard FitzJohn, Matthew Carrano
Abstract
Although their body masses span at least five orders of magnitude, non-avian dinosaurs have only recently attracted the attention of scientists interested in the dynamics of body size evolution. Using femoral length as a proxy for overall body size in dinosaurs, we apply several statistical models to explore its evolution in a phylogenetic context. Targeted comparisons among models can be used to shed light on aspects of body-size evolution, and here we focus on two issues: (1) the pervasiveness of directional trends, and (2) the presence of detectable upper limits for body size. Directionality was assessed by comparing the fit of the non-directional model of Brownian motion (BM) to that of BM with an underlying trend. Results indicate support for a trend of increasing body size (Cope’s rule) in some, but not all dinosaur clades. There is little indication that body-size trends vary systematically across time intervals, but they do vary substantially across dinosaur subclades. To assess the macroevolutionary evidence for an upper limit to dinosaur body size we compared the fit of the BM model to that of BM in the presence of an upper reflecting boundary, a scenario similar to Stanley’s classic explanation of Cope’s rule as a passive trend. This model was fit using a novel likelihood function, and its improvement in fit relative to BM was judged using a likelihood ratio test with the null distribution generated by simulation. Results support the existence of a macroevolutionary upper limit to body sizes in theropods, but not in sauropods or ornithopods.
Geological Society of America Abstracts with Programs. Vol. 44, No. 7, p.138
Gene Hunt, Richard FitzJohn, Matthew Carrano
Abstract
Although their body masses span at least five orders of magnitude, non-avian dinosaurs have only recently attracted the attention of scientists interested in the dynamics of body size evolution. Using femoral length as a proxy for overall body size in dinosaurs, we apply several statistical models to explore its evolution in a phylogenetic context. Targeted comparisons among models can be used to shed light on aspects of body-size evolution, and here we focus on two issues: (1) the pervasiveness of directional trends, and (2) the presence of detectable upper limits for body size. Directionality was assessed by comparing the fit of the non-directional model of Brownian motion (BM) to that of BM with an underlying trend. Results indicate support for a trend of increasing body size (Cope’s rule) in some, but not all dinosaur clades. There is little indication that body-size trends vary systematically across time intervals, but they do vary substantially across dinosaur subclades. To assess the macroevolutionary evidence for an upper limit to dinosaur body size we compared the fit of the BM model to that of BM in the presence of an upper reflecting boundary, a scenario similar to Stanley’s classic explanation of Cope’s rule as a passive trend. This model was fit using a novel likelihood function, and its improvement in fit relative to BM was judged using a likelihood ratio test with the null distribution generated by simulation. Results support the existence of a macroevolutionary upper limit to body sizes in theropods, but not in sauropods or ornithopods.
