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Triassic Pangean desert may have blocked entry of dinosaurs into North America



From: Ben Creisler
bcreisler@gmail.com


A new paper not yet mentioned:

Dennis V. Kent, Paula Santi Malnis, Carina E. Colombi, Oscar A.
Alcober, and Ricardo N. Martínez (2014)
Age constraints on the dispersal of dinosaurs in the Late Triassic
from magnetochronology of the Los Colorados Formation (Argentina).
Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1402369111
http://www.pnas.org/content/early/2014/05/14/1402369111


A measured magnetozone sequence defined by 24 sampling sites with
normal polarity and 28 sites with reverse polarity characteristic
magnetizations was established for the heretofore poorly
age-constrained Los Colorados Formation and its dinosaur-bearing
vertebrate fauna in the Ischigualasto–Villa Union continental rift
basin of Argentina. The polarity pattern in this ∼600-m-thick red-bed
section can be correlated to Chrons E7r to E15n of the Newark
astrochronological polarity time scale. This represents a time
interval from 227 to 213 Ma, indicating that the Los Colorados
Formation is predominantly Norian in age, ending more than 11 My
before the onset of the Jurassic. The magnetochronology confirms that
the underlying Ischigualasto Formation and its vertebrate assemblages
including some of the earliest known dinosaurs are of Carnian age. The
oldest dated occurrences of vertebrate assemblages with dinosaurs in
North America (Chinle Formation) are younger (Norian), and thus the
rise of dinosaurs was diachronous across the Americas. Paleogeography
of the Ischigualasto and Los Colorados Formations indicates prolonged
residence in the austral temperate humid belt where a provincial
vertebrate fauna with early dinosaurs may have incubated. Faunal
dispersal across the Pangean supercontinent in the development of more
cosmopolitan vertebrate assemblages later in the Norian may have been
in response to reduced contrasts between climate zones and lowered
barriers resulting from decreasing atmospheric pCO2 levels.


==
Significance:
Uncertainties in reported 40Ar/39Ar dates from the Ischigualasto
Formation of Argentina allow its dinosaur-bearing fauna to be Norian
in age and possibly contemporaneous with some of the older U-Pb dated
dinosaur-bearing units in the Chinle Formation of the American
Southwest. Our magnetochronology of the previously undated Los
Colorados Formation, which also contains a diverse dinosaur
assemblage, constrains its age to the interval from 227 to 213 Ma
(Norian) and thereby largely restricts the underlying Ischigualasto
Formation to the Carnian. Rise of early dinosaurs was thus diachronous
across the Americas with their dispersal from the austral temperate
belt blocked until later in the Norian. The breakout may have resulted
from critically lowered climatic barriers associated with decreasing
atmospheric pCO2 levels.



Supplementary info:

http://www.pnas.org/content/suppl/2014/05/14/1402369111.DCSupplemental/pnas.201402369SI.pdf
==

News stories:


http://news.nationalgeographic.com/news/2014/05/140520-dinosaur-america-desert-climate-science/