Green Tea and Velociraptors

Reconstructing deep time trends in biodiversity remains a central goal for palaeobiologists, but our understanding of the magnitude and tempo of extinctions and radiations is confounded by uneven sampling of the fossil record. In particular, the Jurassic/Cretaceous (J/K) boundary, 145 million years ago, remains poorly understood, despite an apparent minor extinction and the radiation of numerous important clades. Here we apply a rigorous subsampling approach to a comprehensive tetrapod fossil occurrence data set to assess the group’s macroevolutionary dynamics through the J/K transition. Although much of the signal is exclusively European, almost every higher tetrapod group was affected by a substantial decline across the boundary, culminating in the extinction of several important clades and the ecological release and radiation of numerous modern tetrapod groups. Variation in eustatic sea level was the primary driver of these patterns, controlling biodiversity through availability of shallow marine environments and via allopatric speciation on land.

This is actually a hideously complicated paper involving a lot of statistics, a lot of discussion of methods, and the controls on the structure of the fossil record. As such, while I tried to write it in a way that a non-specialist would be able to understand, this proved immensely difficult given the short amount of space for writing. Nonetheless, I hope you find it enjoyable, thought-provoking, and learn a little about our understanding of the history of life on Earth!

Some of the key take-away points:

• A literal reading of the fossil record does not give an accurate picture of biological extinctions and radiations through time.
• There are numerous factors that need to be account for, such as the species-abundance distribution and the amount of rock record available for sampling.
• Sub-sampling protocols that apply an ‘equal coverage’ method to account for uneven sampling appear to account for these ‘sampling biases’ quite nicely.
• Don’t rely on just a single method for reconstructing diversity through time.
• Global patterns are often poor indicators of patchier regional patterns, and should not be used to look at large-scale patterns in life history.
• Sea level nonetheless appears to exert great control on the diversity of life through a combination of allopatric speciation and the availability of near-shore environments for habitation.
• The Jurassic/Cretaceous boundary 145 million years ago seems to have been important in hitting ‘reset’ on tetrapod life at the time, with many ‘basal’ forms going extinct and new groups radiating.
• This resulted in a ‘wave’ of extinctions and radiations, forming an ecological and taxonomic faunal turnover, and the origins of many modern groups.
• This occurred over many millions of years, and the timing and magnitude of this is still obscured in some groups due to a poor earliest Cretaceous fossil record.
• While not reaching the level of a ‘mass extinction’, some groups such as theropod dinosaurs did achieve around an 80% loss in diversity across the boundary.

The paper is a beast, and if anyone has any questions or queries, please do ask away! There’s a lot of code and data and discussion (see the SI files) to play with, and I hope that it proves to be a useful piece of research in advancing our field 🙂