Last night, Elementary made TV history. It was the first police procedural to use the obscure paleo phrase “dead clade walking.” The CBS versions of Sherlock and Watson reveled in the jargon so much, in fact, that you’d think the network had paid rights to use the term and were trying to get their money’s worth.
Apparently based on the real-life case of a black market tyrannosaur that went up for auction, the Elementary episode used the fossiliferous phrase to set up a fierce rivalry between paleontologists. [Ye be warned, spoilers ahead.] Someone had smuggled an astonishingly rare infant tyrannosaur into the United States, and rumor had it that the fossil was found at a site above the K/Pg boundary – the literal line in the rock that marks the mass extinction which wiped out the non-avian dinosaurs and many other forms of life. The little tyrannosaur was evidence that some non-avian dinosaurs survived for at least a little while after the global catastrophe before ultimately going extinct. Afraid that the discovery would destroy his career-long insistence that such survivals didn’t happen, a nefarious paleontologist murdered two people and destroyed the fossil. And he would’ve gotten away with it too if it weren’t for that pesky Sherlock.
There have been fierce rivalries and controversies in the annals of paleontology. The Bone Wars, of course, and the charged days when all you needed to do to spark an academic dispute was say “punctuated equilibrium” to the wrong person. But if fossil experts really bashed each other on the head with rock hammers every time they were afraid of someone refuting their hypothesis, paleontologists would be extinct. Crime dramas need their villains, of course, but the idea of a “dead clade walking” doesn’t really have scholars of prehistory at each other’s throats.
University of Chicago extinction expert David Jablonski coined the term in 2002. “Because many survivors of mass extinctions do not participate in postrecovery diversifications, and therefore fall into a pattern that can be termed ‘Dead Clade Walking’ (DCW)”, Jablonski wrote, “the effects of mass extinctions extend beyond the losses observed during the event itself.” Global disasters leave a tapering tail of surviving species, some of which are never able to bounce back.
Jablonksi articulated the concept from patterns of extinction and recovery among marine invertebrates through the “Big Five” mass extinctions. In four of the five extinctions, for example, the number of invertebrate genera that disappeared in the aftermath of the disasters was significantly greater than the number that went extinct in the time prior to the major crisis. Ongoing ecological disturbance, the failure of survivors to adapt to new conditions, and other causes continued to eliminate lineages even after the harshest phase of the extinction had ended. “Simply surviving a mass extinction is no guarantee of success in the aftermath,” Jablonski concluded.
The concept that some lineages fizzle after mass extinctions rests on more than shells and squishy swimmers. In a study published last year, University of Lincoln paleontologist Marcello Ruta and coauthors looked at the evolutionary history of distant relatives of ours called anomodont therapsids. These archaic protomammals – which included tusked, barrel-bodied herbivores such as Lystrosaurus – thrived during the Permian, were cut back during the worst mass extinction of all time about 252 million years ago, and persisted through the following Triassic period before going totally extinct.
In terms of diversity, or the number of genera that existed, the anomodonts seemed to rebound after the extinction. The group continued to spin off distinct genera for millions of years. But, as Ruta and colleagues found, those genera were anatomically very similar to each other. Rather than undergoing a renewed radiation of new forms, or what paleontologists term an increase in disparity, anomodonts that survived into the Triassic might have been constrained in such a way that they simply couldn’t evolve new ways of feeding, moving, or otherwise surviving without drastic changes to their bodies. The anomodonts might have been “stuck” in an anatomical dead end, the variety of forms dwindling even as the number of genera fluctuated. Disparity as well as diversity counts in picking out a dead clade walking.
But dinosaurs don’t fit the pattern. Not yet. First of all, avian dinosaurs – birds – not only survived the mass extinction at the end of the Cretaceous, but have thrived to the present day. And as for all the rest, no one has yet found a non-avian dinosaur fossil that was unquestionably and originally laid down above the K/Pg boundary. There have been a few contenders, true, but those fossils appear to have been reworked from older strata. A delicate, articulated skeleton of just the sort Elementary‘s Sherlock pored over would make a much stronger case for non-avian dinosaurs that escaped the Cretaceous.
Such a skeleton might be found someday. There’s a possibility that some non-avian dinosaurs survived the end-Cretaceous catastrophe for a short time. The vast majority of what paleontologists know about the extinction on land comes from relatively limited swaths of exposed rock in western North America and Europe. How and when dinosaurs died out elsewhere around the planet relies on further evidence. The favored hypothesis is that all non-avian dinosaurs died within a short time of that devastating end-Cretaceous asteroid strike, but an unexpected skeleton could add more nuance to the tale. And though such a discovery would certainly be controversial, I doubt that anyone would turn into a Mesozoic Moriarty to halt the publication of a dead dinosaur walking.
Jablonski, D. 2002. Survival without recovery after mass extinctions. PNAS. 99, 12: 8139-8144
Ruta, M., Angielczyk, K., Fröbisch, J., Benton, M. 2013. Decoupling of morphological disparity and taxic diversity during the adaptive radiation of anomodont therapsids. Proceedings of the Royal Society B. 280, 1768: 20131071