Re: The Hills Have New Papers

["Jerry D. Harris" <jharris@dixie.edu>]

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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