Paleontologists are constantly reminding themselves of the incompleteness of the fossil record. What has been preserved is only a small fraction of all the organisms and environments that have ever existed. This makes detecting evolutionary patterns a bit of a challenge. In a presentation given at this year's Society of Vertebrate Paleontology conference, Smithsonian paleontologist Matt Carrano dug into the long-standing question of whether changes in sea level triggered changes in dinosaur diversity.
Over the past few decades, paleontologists have produced a number of graphs depicting dinosaur diversity through time. They show a general trend toward increasing diversity from the Late Triassic through the end of the Cretaceous, but with a few fluctuations in between. The rise and the fall of the seas has been proposed as one of the drivers of these changes. Perhaps, it has been hypothesized, high sea levels might have favored dinosaur diversity by fragmenting some terrestrial habitats or isolating one area from another while simultaneously creating more environments where dinosaurs might be preserved. Then again, it has also been suggested that dinosaur diversity might go up when sea levels are low since there would be a larger land area. In order to detect whether any such trends existed, the scientists looked at the occurrence of about 749 dinosaur species through time and space, noting where paleontologists have gone looking for their bones, as well.
What the Carrano and his colleagues found was that the fluctuations in sea level did not influence dinosaur diversity as we know it today. Our perspective of dinosaur diversity is significantly shaped by where paleontologists have gone looking for fossils, the amount of effort expended there, and also by places that have yet to be extensively studied. Dinosaurs might be more plentiful and easier to find in Cretaceous rocks than Triassic ones, for example, which would account for why dinosaur diversity differs between the two time periods. Any scientific work proposing to look at dinosaur diversity has to take these sampling biases into account.
This is not to say that sea level change did not or could not have influence dinosaur diversity, though. Rising sea levels could have created island chains and other geographical pockets that could have driven dinosaur speciation, or low sea levels might have allowed dinosaur species to range more widely. (We know, for example, that the Western Interior Seaway caused Cretaceous dinosaurs to evolve in different ways in the eastern and western parts of North America.) Detecting these signals from the fossil record, however, will require in-depth sampling and a recognition of the way in which our search for dinosaurs skews the picture of their diversity. As stated by the authors of the paper that was the basis for the SVP presentation: "Considerable future work is required to establish how sampling biases may affect proposed long-term diversity trends and mass extinction events in the terrestrial realm." If paleontologists want to get at the big picture of dinosaur diversity, they need to look at these biases and get digging at places which are still poorly known.
Butler, R., Benson, R., Carrano, M., Mannion, P., & Upchurch, P. (2010). Sea level, dinosaur diversity and sampling biases: investigating the 'common cause' hypothesis in the terrestrial realm Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1754