On the Origin of Paleocene Dinosaurs, or the Preservation of Favoured Sauropods in the Struggle Against Extinction

If there is one thing (most) scientists agree on, it is the fact that around 65.5 million years ago, dinosaurs (at least the non-avian variety) went extinct. Whether it was the result of a massive bolide impact and its aftermath, long periods of intense volcanism, changes in global sea level and climate, or a combination of factors, life on Earth went through a major extinction event that wiped out around 75% of all species. It is believed that this extinction took place in a geologically short period of time, and left its physical mark across the globe in a band of iridium rich rock known as the Cretaceous-Paleogene (K-Pg) boundary (formerly referred to as the Cretaceous-Tertiary, or K-T boundary). The K-Pg boundary marks the end of the Cretaceous period, and with it, the end of the age of the dinosaurs. There are abundant dinosaur fossils found below the boundary, but none above, indicating that they all went extinct before the Paleogene period. This fact is further proven through relative and absolute dating techniques that can be used to determine the age of the rock layer in which the fossils are found. If a rock layer is 70 million years old, the fossils in it are probably around that age as well (barring the erosion, transportation, and re-deposition of the fossil to a geologically younger rock layer).

But for a long time, a handful of researchers have speculated that some non-avian dinosaurs may have been able to survive past the K-Pg boundary and the extinction event 65.5 million years ago. They theorized that there may have been areas where the effects of the extinction catastrophe were not as pronounced, and that some dinosaur populations may have been able to persist there for quite some time. But for how much time? Years? Decades? Millennia? Is it possible that some dinosaurs managed to make it through the end of the Cretaceous and into the Paleocene? According to a new paper in the journal Geology, some sauropods may have been able to survive in their "dino oasis" for quite a long time indeed; around 700,000 years.

As I mentioned above, scientists can determine the age of fossils because they can date the rocks that the fossils come out of. One of the most widely accepted methods of radiometric dating used today is argon-argon (⁴⁰Ar/³⁹Ar) dating. If I'm not mistaken, the K-Pg boundary date of 65.5 Ma was determined using this method. But in this new paper, Fasset et al. claim to have done something scientists have thus far been unable to do. They claim to have dated the actual fossilized bone (not the rock it was found in) using a new method of uranium-lead (U-Pb) dating (2011). They tested their new technique on to different samples. The first, used as a control, was a limb bone fragment known (at the time of this new publication) to have come from Cretaceous sandstone. The second sample came from a femur of the sauropod Alamosaurus sanjuanensis. And this is where it gets interesting. Using their new method of U-Pb dating, Fassett et al. determined that this bone was 64.8 ± 0.9 Ma (million years old) (2011). In other words, this femur is approximately 700,000 years younger than any other known dinosaur bone, or the K-Pg boundary. That means that the particular dinosaur it belonged to was a member of a long line of dinosaur survivors that made it through the K-Pg extinction event.

This is truly a phenomenal claim, and I honestly don't know if I believe it right off the bat. For starters, only two samples, both of which were touted as a "Paleocene dinosaurs" from the moment they were discovered, were used in this study. The control sample used was thought to have been Paleocene in origin by one of the same authors of this 2011 paper until that notion was put to rest by another group of researchers. And the new sauropod fossil was also described as being Paleocene in origin before the new technique was performed. I think it would have been a lot better for the researchers to start with less questionable material. Get samples from fossils of which there is no real debate over the age, and use your new method to determine if you can get a similar number. Having a large sample size in a situation like this is essential. Otherwise, there is no way to know if the new technique is capable of producing accurate dates in the first place. Not to mention the fact that there is no way of knowing when the uranium they are testing was present in the bone at the initial time of fossil deposition, or if it was deposited in the fossil after the fact. It is quite posible that the date of 64.8 ± 0.9 Ma is accurate timing for when the uranium was collected in the fossil, but that does not mean that the age reflects the time of the animal's death. And let's say, just for kicks, that this date is accurate, and that this dinosaur was alive 700,000 years after the rest of the dinosaurs went extinct. Is it possible that they survived, unchanged, in this isolated pocket of paradise for that long? I'd think a global catastrophe that wipes out the majority of planetary life would act as a rather strong selection pressure on any surviving dinosaurs. The idea that they would be able to remain there unchanged and unable to spread back out into other habitats is a little farfetched.

At the end of the day, I love the notion that some of bigger, better known Mesozoic beasties survived the K-Pg extinction event. And I think it is entirely possible that some of them did. There were two other major archosaur lineages that pulled it off, why not non-avian dinosaurs? But I just don't think at this point, the evidence is there to prove that it happened, particularly in the case of this new Geology paper. But hey, what do I know? Maybe some of the dinosaurs did make it into the Paleocene. Heck, maybe some have even managed to survive to the present day*. But while you're off exploring the globe in search of the Mokèlé-mbèmbé and other "living dinosaurs", I'll be looking out the window into the garden, watching the real living dinosaurs at the bird feeder.

*No, I do not really believe this.

References

Fassett, J. E., Heaman, L. M., Simonetti, A. 2011. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico. Geology, 39: 159-162.