Competition and constraint drove Cope's rule in the evolution of giant flying reptiles

Abstract

The pterosaurs, Mesozoic flying reptiles, attained wingspans of more than 10 m that greatly exceed the largest birds and challenge our understanding of size limits in flying animals. Pterosaurs have been used to illustrate Cope’s rule, the influential generalization that evolutionary lineages trend to increasingly large body sizes. However, unambiguous examples of Cope’s rule operating on extended timescales in large clades remain elusive, and the phylogenetic pattern and possible drivers of pterosaur gigantism are uncertain. Here we show 70 million years of highly constrained early evolution, followed by almost 80 million years of sustained, multi-lineage body size increases in pterosaurs. These results are supported by maximum-likelihood modelling of a comprehensive new pterosaur data set. The transition between these macroevolutionary regimes is coincident with the Early Cretaceous adaptive radiation of birds, supporting controversial hypotheses of bird–pterosaur competition, and suggesting that evolutionary competition can act as a macroevolutionary driver on extended geological timescales.

Adaptive evolution toward larger size in mammals

“…Taken together, our results demonstrate that mammals have consistently evolved toward larger size, almost certainly reflecting an adaptive response to new selective circumstances, such as competition (48), climate changes (7,49), or dietary specialization (11). These results are not compatible with purely passive explanations for trends through time (24,50).…”

Geographic range did not confer resilience to extinction in terrestrial vertebrates at the end-Triassic crisis

“…It is reasonably common to observe discrete macroevolutionary patterns in different higher taxa or across different major habitats. Notable examples include the incongruence between terrestrial and marine Phanerozoic diversity curves [45][46][47] , variations in the apparent force of Cope's rule sensu lato in different higher taxa [48][49][50] , and variations in the relationships between body size, population density and fecundity across clades 51,52 . However, the differences between patterns observed at major extinction events may result from differences ARTICLE between the particular driving forces of the crises themselves.…”

A Medium-Sized Robust-Necked Azhdarchid Pterosaur (Pterodactyloidea: Azhdarchidae) from the Maastrichtian of Pui (Haţeg Basin, Transylvania, Romania)

“…Moreover, in the latest Cretaceous deposits of Romania, pterosaur fossils belonging to medium or small size classes are actually more abundant than are very large or giant ones, with the latter comprising less than 30% of the sample of more than 50 confidently identified specimens (Vremir et al, 2011;Dyke et al, in prep.). We subsequently question whether the increasing rarity of smaller pterosaurs throughout the Cretaceous is actually a taphonomic effect caused by an apparent increase in the terrestrialization of pterosaur faunas (Buffetaut et al, 1997;Butler et al, 2013;Benson et al, 2014). If true, this taphonomic artifact may imply the existence of many small pterosaur species in habitats usually not conducive to fossilization of their thinwalled bones.…”

“…As discussed above, both the bone texture of this element and its well-defined tubercles strongly suggest that it comes from a morphologically mature individual. It has been argued that small pterosaurs are largely or entirely absent from the Late Cretaceous (e.g., Butler et al, 2013;Benson et al, 2014), a factor that may have played a part in pterosaur extinction at the end of the Mesozoic (Unwin, 2006;Witton, 2013). R.2395 augments the 3 m wingspan (or smaller) Eurazhdarcho and the 2.5 m span Montanazhdarcho minor (McGowen et al, 2002;Vremir et al, 2013) from the Campanian-Maastrichtian, suggesting that smaller pterosaurs may not be as rare in this interval as has been suggested (Benson et al, 2014).…”

“…It has been argued that small pterosaurs are largely or entirely absent from the Late Cretaceous (e.g., Butler et al, 2013;Benson et al, 2014), a factor that may have played a part in pterosaur extinction at the end of the Mesozoic (Unwin, 2006;Witton, 2013). R.2395 augments the 3 m wingspan (or smaller) Eurazhdarcho and the 2.5 m span Montanazhdarcho minor (McGowen et al, 2002;Vremir et al, 2013) from the Campanian-Maastrichtian, suggesting that smaller pterosaurs may not be as rare in this interval as has been suggested (Benson et al, 2014). Moreover, in the latest Cretaceous deposits of Romania, pterosaur fossils belonging to medium or small size classes are actually more abundant than are very large or giant ones, with the latter comprising less than 30% of the sample of more than 50 confidently identified specimens (Vremir et al, 2011;Dyke et al, in prep.).…”