The Dicynodon-Lystrosaurus Assemblage Zone Boundary May Not Approximate the Marine-Defined Permo-Triassic Extinction Event

Wow, well this should definitely generate some discussion and a bit of research.... 

Gastaldo, R. A., Kamo, S. L., Neveling, J., Geissman, J. W., Bamford, M., and C. V. Looy. 2015 Is the vertebrate-defined Permian-Triassic boundary in the Karoo Basin, South Africa, the terrestrial expression of the end-Permian marine event? Geology (Advanced Online). doi: 10.1130/G37040.1
Abstract: The end-Permian extinction records the greatest ecological catastrophe in Earth history. The vertebrate fossil record in the Karoo Basin, South Africa, has been used for more than a century as the standard for understanding turnover in terrestrial ecosystems, recently claimed to be in synchrony with the marine crisis. Workers assumed that systematic turnover at the Dicynodon assemblage zone boundary, followed by the appearance of new taxa directly above the base of the Lystrosaurus assemblage zone, is the continental expression of the end-Permian event and recovery. To test this hypothesis, we present the first highprecision age on strata close to the inferred Permian-Triassic boundary. A U-Pb isotope dilution–thermal ionization mass spectrometry zircon age of 253.48 ± 0.15 Ma (early Changhsingian) is from a silicified ash layer ~60 m below the current vertebrate-defined boundary at Old Lootsberg Pass (southern South Africa). This section yields newly discovered plants and vertebrates, and is dominated by a normal polarity signature. Our collective data suggest that the Dicynodon-Lystrosaurus assemblage zone boundary is stratigraphically higher than currently reported, and older than the marine extinction event. Therefore, the turnover in vertebrate taxa at this biozone boundary probably does not represent the biological expression of the terrestrial end-Permian mass extinction. The actual Permian-Triassic boundary in the Karoo Basin is either higher in the Katberg Formation or is not preserved. The currently accepted model of the terrestrial ecosystem response to the crisis, both in this basin and its extension globally, requires reevaluation.

More Dates for the Manicouagan Impact Structure

van Soest, M. C., K. V. Hodges, J.-A. Wartho, M. B. Biren, B. D. Monteleone, J. Ramezani, J. G. Spray, and L. M. Thompson. 2011. (U‐Th)/He dating of terrestrial impact structures: The Manicouagan example. Geochemistry, Geophysics, Geosystems 12, Q0AA16, doi:10.1029/2010GC003465

Abstract - The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high‐precision dating by the U‐Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high‐temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U‐Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U‐Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2s) U‐Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2s), indicating variable postimpact helium loss due to low‐temperature thermal disturbance. Preimpact titanite crystals from a shocked meta‐anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2s), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2s), suggestive of postimpact helium loss. These results indicate that (U‐Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4He loss related to reheating subsequent to impact.

Direct U-Pb Dating of Fossil Bone Material and Does the Presence of Paleocene Dinosaurs Even Matter?

Interesting paper, the focus of which is twofold: 1) demonstate the applicability of new U-Pb dating technology on fossil bone material; and 2) support the hypothesis that some non-avian dinosaurs survived into the earliest Paleocene. I'll need someone with more background on radioisotopic dating techniques to evaluate this paper for me, especially regarding aspects of Pb loss, diagenetic recrystallization, etc., but the implications are exciting if indeed this technique can be used to directly date fossil materials.

I think the idea of Paleocene dinosaurs will be debated for a long time, but really what does it matter?  It is not as if non-avian dinosaurs made it into the Paleocene, that they made it very far in.  Our base geologic timescales are based upon stratigraphic ranges of marine organisms, which are only now starting to be well calibrated. Correlation of these intervals between the marine and terrestrial realms is tricky at best and I would not be surprised if as we get more precision and accuracy in our dating techniques we find that there is a little slop around boundary defining events (i.e. extinctions of major terrestrial groups) and the exact dates defining the marine-defined boundaries. 

But does it really matter if the last non-avian dinosaurs died out entirely by the exact last day of the Cretaceous, or if a few locally squeezed into the earliest Paleocene?  These dates are so similar in relation to the overall earth timescale that they are pretty much simultaneous and really have no bearing on our ideas regarding the extinction of non-avian dinosaur. Same goes for the Triassic, if we find a phytosaur in the very earliest Jurassic strata is this really significant other than for general bean counting? I for one would not be surprised one bit if such a specimen was found; however, these data would not offer much in resolving the real question, which is why these major extinctions occurred in the first place.

Fassett, J.E., Heaman, L.M., and A. Simonetti. 2011. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico. Geology 39:159-162. doi: 10.1130/G31466.1

Abstract - Vertebrate fossils have been important for relative dating of terrestrial rocks for decades, but direct dating of these fossils has heretofore been unsuccessful. In this study we employ recent advances in laser ablation in situ U-Pb dating techniques to directly date two dinosaur fossils from the San Juan Basin of northwestern New Mexico and southwestern Colorado, United States. A Cretaceous dinosaur bone collected from just below the Cretaceous-Paleogene interface yielded a U-Pb date of 73.6 ± 0.9 Ma, in excellent agreement with a previously determined 40Ar/39Ar date of 73.04 ± 0.25 Ma for an ash bed near this site. The second dinosaur bone sample from Paleocene strata just above the Cretaceous-Paleogene interface yielded a Paleocene U-Pb date of 64.8 ± 0.9 Ma, consistent with palynologic, paleomagnetic, and fossil-mammal biochronologic data. This first successful direct dating of fossil vertebrate bone provides a new methodology with the potential to directly obtain accurate dates for any vertebrate fossil.