Anyway, back to this morning: following my usual routine of looking on various journal sites for interesting papers, I happened to glance at the most recent issue of Geology (vol. 40(4)). This included three comments, and two replies entitled ‘Direct U–Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico’.
Now, as any seven-year-old will probably be able to tell you, the dinosaurs died out at the end of the Cretaceous, at 65.5 ± 0.3 Ma, then will probably go on the discuss meteorites and other things past the current scope of this post. All of these letters are in response to Fassett, Heaman and Antonio’s (2011) claim that they’ve been able to directly date a dinosaur bone from the Paleocene (after the K–P mass extinction) at 64.8 ± 0.9 Ma, after a test on a bone of ‘known’ age (73.6 ±0.9 Ma). Stop me if you’ve heard this before (Fassett 2009; open access :-))…
The method used by Fassett et al. to date their samples — uranium–lead isotope ratios — was the main criticism by the three comments (Koenig et al. 2012; Ludwig 2012; Renne and Goodwin 2012; all open access :-)). This largely centred around the inaccuracy of the isotopic ratios used (Ludwig 2012) and the seemingly arbitrary selection of results to give a 70–80 Ma result for one of their samples (Koenig et al. 2012; Ludwig 2012). The other major problem was that the intake of uranium or reworking post mortem was not considered, thereby making the calculated ages minimal estimates for the ages of the bones — which was already very close to the K–P boundary.
Basically, the comments disagree with the precision and accuracy of the age Fassett et al. proposed.
Fassett, Heaman and Simonetti’s replies (2012a, b; open access :-)) acknowledge that their full methodology was not included in the original article (Fassett, Heaman and Simonetti 2011) due to the space constraints of the journal itself, although these shall be discussed in a manuscript in prep. Fassett et al. also decry the statements of reworking — the samples are from large bones unlikely to have been reworked, and with mineral signatures unlike definite Cretaceous bones — and isotope ratio alteration — intake of isotopes would have likely been rapid: fossilisation within <100 ka.
So, for Paleocene (just) dinosaurs; the jury’s still out…no?
The second thing that arrive in my feeds, this time from Dave Hone’s Archosaur Musings, was cell biologist Prof Brian Ford’s article on why dinosaurs were aquatic — news that made it onto Radio 4’s Today programme. Here, Ford brings back the long dispelled notion that dinosaurs were too massive to walk on land, and so must have required water to support their bulky bodies. This is reminiscent of the story of the sauropods living in lakes (as seen in e.g. Fantasia) that almost all palaeontology textbooks use as an example of how scientists got it wrong.
On looking at this article, there are many problems, however I feel it best to say that you should read the comments at the end for more discussion from more knowledgeable people than me. See also here.
Well, following that foray in dinosaur tribulations; normal service shall be resumed eventually.
FASSETT, J. E. 2009. New geochronologic and stratigraphic evidence confirms the Paleocene age of the dinosaur-bearing Ojo Alamo Sandstone and Aminas Formation in the San Juan Basin, New Mexico and Colorado. Palaeontologia Electronica, 12, 146 p.
KOENIG, A. E., LUCAS, S. G., NEYMARK, L. A., HECKERT, A. B., SULLIVAN, R. M., JASINSKI, S. E. and FOWLER, D. W. 2012. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: comment. Geology, 40, e262.