Like most people, I first became interested in paleontology as a child. But unlike others, it wasn't just T. Rex or Stegosaurus that drew me in; it was the realization that they, along with many other organisms, are gone. As I mucked my way through muddy creek beds around Cincinnati, I marveled at the pavement of shelled organisms the likes of which have not existed on this planet for hundreds of millions of years. It was the absence of these weird creatures from the modern fauna that truly captivated me. My research today focuses on answering questions surrounding the causes and consequences of extinction in the geologic past. My work is interdisplinary, incorporating varied paleontological research methods, at the intersection of geology and biology. Below I describe some of my research findings, unanswered questions, and the future directions that I hope to take my work.
Everyone has heard of mass extinctions, including the likely possibility that our planet is entering a 6th mass extinction due to the activity of humans. Most of us know that mass extinctions are distinctive because they are big - they have great extinction intensity. Instead of intensity, my research focuses on extinction selectivity - what went extinct. An unanswered question is whether extinction selectivity changes during mass extinctions. I have been using a method based on sampling probability to test for extinction selectivity in the fossil record. Traditionally, extinction selectivity is detected by looking for traits that correlate with extinction (or survival). However, if those traits are not preserved (a common problem for fossil species), then selectivity may go undetected. The method I employ, based on sampling probabity and without appeal to taxon traits, has revealed that the two best studed mass extinctions (the Permian/Triassic, P/Tr and the Cretaceous/Paleogene, K/Pg)also exhibit strong evidence for selectivity, despite some claims that mass extinctions may be less selective than normal extinction times. This observation makes sense since the change in fauna at the mass extinctions is greater than would be expected from increased extinction intensity alone. Biologists have observed that our modern extinction crisis is highly selective, which makes it much more devastating than it would be if extinction was randomly distributed among the biosphere, similar to the pattern observed at the P/Tr and K/Pg mass extinctions. Additionally, I am interested in how patterns in the fossil record affect our understanding of the intensity and selectivity of extinction events, as well as how to compare the extinctions of the past as recorded in the fossil record with the modern biodiversity crisis.
Few animals are as weird as the echinoderms, the sometimes-spiny distant cousins of vertebrates. And among echinoderms, crinoids are arguably the most bizarre. Most living echinoderms (sea stars, sea urchins, sea cucumbers, etc.) are free-living. Crinoids, which primitively possess long stalk for attachment to the seafloor (and contributing to their common name, "sea lilies"), are usually thought of as sessile. Somewhat surprising is that modern crinoids, even the stalked ones, are surprisingly mobile , with the the greatest mobility achieved by the crawling and swimming comatulid crinoids (the "feather stars"). I am interested in the diversification of the feather stars, which are a favorite subject of reef photographers, and contain most of the species diversity of modern crinoids. The feather stars have a particularly poor fossil record, so I have tried to determine when they diversified, while leaving their stalked cousins to the murky depths of the ocean. I am interested in when feather stars first began to swim, and whether improvements in mobility have contributed to their success, possibly in response to the more advanced predators that have proliferated in modern seas since the crinoid heyday of the Paleozoic. I approach these questions primarily through biomechanics, by contructing mathematical models of the physics of these crinoids. This research has led to observations of the traits that typically characterize a swimming crinoid. Among the more interesting future directions for this work are the many strange crinoids of the geologic past, many of whom have been suggested to have had the ability to swim, to float, to generate their own feeding currents, and much more; using conceptual physical models I am testing the plausibility of these claims of crinoid capability.
One of the as-yet-unresolved questions in crinoid paleobiology is the
relationship
of the survivors of the largest mass extinction in the history of life,
the P/Tr, to
their Paleozoic ancestors. It has been argued that post-Paleozoic
crinoids are all
descended from one or a few lineages, a seemingly unlikely, but
intriguing scenario. Not only did
crinoids recover from
the extinction event, they rediversified dramatically. While they
likely never
attained the same
species-level diversity that they had in the Paleozoic, the
post-Paleozoic crinoids
have come to occupy environments not previously seen in the Paleozoic.
And, as discussed
above, also have attained great strides in mobility, the likes of which
may not have
been possible in the Paleozoic. In my research, I am attempting to
construct crinoid
phylogenetic history across the Permian/Triassic boundary in order to
determine
whether the post-Paleozoic crinoids truly do descend from a single
common ancestor.
Additionally, reaching across my research themes, I am interested in
whether there
is any evidence for extinction selectivity for crinoids at the P/Tr, a
challenging
question to tackle since they were so dramatically hit (along with most
other life) at the P/Tr.