Pterosaurs reached only modest sizes in the Triassic-Jurassic. By contrast, the Cretaceous saw a trend toward large to giant size (2 m to >6 m wingspans), and while small-medium (<1 m to 2 m wingspans) sized forms are known from the Lower Cretaceous they are rare in the Upper Cretaceous. This pattern has been ascribed to the appearance of birds in the mid-Mesozoic, and their displacement of pterosaurs from niches previously occupied by small-medium sized forms. Here we show how new finds of small-very small pterosaurs (<1 m wingspans) from the mid-Cretaceous Kem Kem Group of Morocco point to several sampling biases of the data upon which these patterns are founded. Evidence for the size range of these pterosaurs strongly correlates with sample-size: as the sample increased (from <100 to >400 specimens) both very small and giant forms have been discovered. Histological analysis suggests that very small/small morphs are immature individuals rather than species in which adults were small-bodied. This new data shows that size distribution patterns based on all available specimens differ markedly from those based on a much more restricted sub-set of named taxa. Critically, this analysis reveals that pterosaur size ranges in the Cretaceous do not reflect a switch to large and giant size, but an extension of the size range from very small through to giant forms. Cretaceous niches previously occupied by small pterosaurs in the Triassic and Jurassic were increasingly occupied not by birds but by early ontogenetic stages of large and giant pterosaurs.
- Kem Kem Group