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Re: The Hills Have New Papers



David Marjanovic wrote:

Does that mean there's a carbon isotope spike and an osmium isotope spike at the boundary...?

There's a negative carbon excursion that's been correlated to the boundary (defined as the first appearance of the ammonite _Psiloceras planorbis_ or, more broadly, the lowest occurrence of ammonoides of the _Psiloceras tilmanni_ group); there is also some osmium enrichment that has been correlated to the boundary because ^192^Os is an unradigenic isotope indicative of igneous activity, so its enrichment had been thought to coincide with the onset of CAMP volcanism in the supergroup, which, in turn, has been correlated to the Tr-J boundary.


Bizarre. Does that mean the crurotarsan mass extinction happened at the Norian-Rhaetian boundary? Or what? ~:-|

Dunno why this is bizarre -- because extinction events don't line up? Lucas, Tanner, and a few others have been saying for a few years now that there is no such thing as the Triassic-Jurassic extinction event; newer, higher stratigraphic resolution evidence suggests that there _were_ several successive extinction events spread out through the Rhaetian (probably within a 1-2 million year span of time), but the "big single spike on a graph" thing (indicating a much shorter-term event) isn't accurate, more or less removing this "event" from the famous "Big 5" of extinction events, at least a a single event. (I vote we replace it with the present one.)


Other issues have warranted a perceptual change of the Newark Supergroup's stratigraphy and dating anyway; the change in the time scale in 2004, which moved the Tr-J boundary to 199.6 +/- 0.6 Ma means that the well-dated (avg. 201 Ma) Orange Mountain Basalt, to which much has been internally correlated within the supergroup, is, by definition, Rhaetian in age, not Hettangian, as it has long been used.

   Anyway, as for a crurotarsan extinction, I'll just quote from the paper:

"The Newark body fossil record of tetrapods is thus sparse across the TJB and therefore inadequate to evaluate a possible TJB tetrapod extinction, and the direct correlation of such an extinction (if it exists) to the marine TJB has not been demonstrated (Lucas and Tanner, 2007). Indeed, the most substantial extinction of tetrapods across the TJB is the crurotarsan extinction, which occurs well below the lowest CAMP basalt in the Newark Supergroup. This is the extinction of phytosaurs, aetosaurs and rauisuchians ("thecodonts") that has long represented the bulk of the supposed terminal Triassic tetrapod extinction.
...If the vertebrate fossil criteria listed here are applied to the Newark Supergroup, the highest occurrence of crurotarsan body fossils and the Brachychirotherium highest occurrence is below the lowest CAMP basalt sheet (Fig. 6). The lowest occurrence of Otozoum is above the Newark extrusive zone, and the lowest occurrence of Protosuchus is above the only CAMP basalt sheet present in the Fundy basin. This indicates that the TJB is between the highest occurrence of crurotarsans (below the oldest CAMP basalt) and the lowest occurrence of presumed Jurassic tetrapods, which is in and above the Newark extrusive zone. Vertebrate biostratigraphy thus suggests that the TJB is in the Newark extrusive zone."


In other words, yes, the crurotarsan extinction appears to predate the Tr-J boundary based on present evidence. See also:

Lucas, S.G., and Tanner, L.H. 2007. Tetrapod biostratigraphy and biochronology of the Triassic-Jurassic transition on the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 244(1-4):242-256. doi: 10.1016/j.palaeo.2006.06.030.

ABSTRACT: Nonmarine fluvial, eolian and lacustrine strata of the Chinle and Glen Canyon groups on the southern Colorado Plateau preserve tetrapod body fossils and footprints that are one of the world's most extensive tetrapod fossil records across the Triassic-Jurassic boundary. We organize these tetrapod fossils into five, time-successive biostratigraphic assemblages (in ascending order, Owl Rock, Rock Point, Dinosaur Canyon, Whitmore Point and Kayenta) that we assign to the (ascending order) Revueltian, Apachean, Wassonian and Dawan land-vertebrate faunachrons (LVF). In doing so, we redefine the Wassonian and the Dawan LVFs. The Apachean-Wassonian boundary approximates the Triassic-Jurassic boundary. This tetrapod biostratigraphy and biochronology of the Triassic-Jurassic transition on the southern Colorado Plateau confirms that crurotarsan extinction closely corresponds to the end of the Triassic, and that a dramatic increase in dinosaur diversity, abundance and body size preceded the end of the Triassic.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Jerry D. Harris Director of Paleontology Dixie State College Science Building 225 South 700 East St. George, UT 84770 USA Phone: (435) 652-7758 Fax: (435) 656-4022 E-mail: jharris@dixie.edu and dinogami@gmail.com http://cactus.dixie.edu/jharris/

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