Dinosaurs were extremely successful, dominating Earth for millions of years (not even counting the many more millions of years that birds have flourished), in many and glorious variants. But we have only sketchy notions of the growth and spread of these wondrous animals. Fossil evidence is sparse and irregular, as is geological evidence of ancient environments, so simple tabulation offers limited information about the origins and spread over time of dinosaurs.
A new study approaches this problem with a Bayesian model to infer dispersion paths for different dinosaur taxa . Using recorded finds for over 600 species, they project probable geographic locations through time, constructing a “path” representing the dispersion of the species. The model also incorporates some factors such as diet and gait, though these factors have little impact at this granularity (i.e., walking speed means little over many thousands of years).
The results show a rapid geographical dispersal (beginning in present day South America), which slowed over time. This is consistent with the notion that dinosaurs spread out into relatively unoccupied geographical areas, until eventually they filled the globe.
The researchers tie this pattern to the rate of speciation, which follows a similar trend. This is consistent with speciation due to invasion of new and geographically isolated environments. In contrast, later times would presumably be dominated more by sympatric speciation, i.e., competition within a (crowded) system.
The researchers characterize this pattern as a “geographical signature of an evolutionary radiation”. The suggest that this offers explanatory hypotheses for phenomena such as the diversity of Hadrosaur cranial decorations thought to be due to sexual selection, which would be a likely mechanism for sympatric speciation.
They also perceive the slowing rate of speciation in the Cretatceous as evidence that the radiation was ending, and the dinosaurs were in decline .
My own view is rather skeptical, if only because the statistics are based on such paltry data. There were millions and millions of dinosaurs, many of them tiny, and most probably unknown in the fossil record. For those we do have evidence for, the species identification is quite uncertain, as is the presumed taxonomic tree and behavior. However clever the model, it is based on extremely weak data.
In any case, the relationship between supposed movement and the rate of speciation is almost a tautology. I mean, what else could possibly happen over such long time periods? And how could these not be correlated in such a limited dataset?
I’m certainly not convinced that dinosaurs were in decline, whatever that means. Even if the rate of speciation was slowing (which I don’t think is evident), that doesn’t mean they are disappearing (which they weren’t). I suspect that if we had more evidence, we might find lots of interesting adaptation happening in the Cretaceous, though possibly not easy to see in the skeletal remains.
On a side note, I note that the BBC headline suggests that the finding is that “Dinosaurs ‘too successful for their own good’”. The actual paper doesn’t really say that, and, as far as I can tell, no one ever said that specific quote.
- Helen Briggs, Dinosaurs ‘too successful for their own good’, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-42945820
- Ciara O’Donovan, Andrew Meade, and Chris Venditti, Dinosaurs reveal the geographical signature of an evolutionary radiation. Nature Ecology & Evolution, 2018/02/05 2018. https://doi.org/10.1038/s41559-017-0454-6