Monday, March 26, 2007

Anasibirites-Wasatchites a polymorphic species?


Placenticeras syrtale dimorphs (top) and Anasibirites and Wasatchites (bottom)

With the amount of intermediate forms between Anasibirites kingianus (Waagen) 1895, and Wasatchites perrini Mathews 1929, is it reasonable to consider them as a dimorphic/polymorphic species?


From the website: "Because of the variable morphology in some populations all specimens are a seperate species, or all are one species with variable morphology. Mathews (1929) described (a) 32 species of Anasibirites, 5 species of Gurlyeites, 9 of Hemiprionites "Goniodiscus" and 1 of Kashmirites, and (b) 4 species of Wasatchites, 3 of Kashmirites and 1 of Keyserlingites, from Cephalopod Gulch near Salt Lake City, Utah. Later workers placed alot of them (a) in synonomy with Anasibirites kingianus (Waagen) and (b) in Wasatchites. In the Anasibirites Beds (about 300mm thick) of the Confusion Range of western Utah. there are representatives of all these genera, they grade from Hemiprionites to Anasibirites to Gurleyites to Wasatchites with intermediates between each."
Silberling (in Hose and Repenning, 1959) idententified these as Anasibirites kingianus and Wasatchites meeki without reference to any intermediate forms. See the two papers by E.T. Tozer (1971 and 1994) to see how he deals with these genera.


For reference, the Santonian (Late Cretaceous) ammonite Placenticeras syrtale is a polymorphic species which has been referred to many names of many ages (Kennedy and Cobban 1991, Kennedy Landman & Cobban 2001, Kennedy, Hancock, Cobban & Landman 2004, Wolleben 1967), to find one by itself you would be unable to place it in time or one of a number of subspecies described by Wolleben. There are 2 basic morphotypes of P. syrtale, a smooth shelled form (P. planum) and an ornamented form (P. syrtale, P. guadalupae, P. pseudocostatum, P. sancarlosensis etc.) much like the forms of Anasibirites and Wasatchites. The species or subspecies of P. syrtale exist in time for about 8 my with known ancestors and descendants and those of Anasibirites and Wasatchites for maybe 1 my, and I know of no ancestors or descendants.


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References:


Kennedy, Hancock, Cobban and Landman, 2004, A revision of the ammonite types described in F. ROEMER’S ‘Die Kreidebildungen von Texas und ihre organischen Einschl├╝sse’ (1852), Acta Geologica Polonica, No. 4, pp. 433-445
Kennedy, W. J., and W. A. Cobban, 1991. Upper Cretaceous (upper Santonian) Boehmoceras fauna from the Gulf Coast region of the United States. Geol. Mag. 128: 167–189.

Kennedy, W. J., Landman, N. H., and Cobban W. A., 2001, Santonian Ammonites from the Blossom Sand in Northeast Texas, American Museum Novitates, No. 3332, 9 p.

Wolleben, J. A., 1967. Senonian (Cretaceous) Mollusca from Trans-Pecos Texas and northeastern Chihuahua, Mexico. J. Paleontol. 41:1150–1165.


Mathews, Asa A. L., 1929, The Lower Triassic Cephalopod Fauna of the Fort Douglas Area, Utah, Walker Museum Memoirs Vol.1 No.1 University of Chicago Press


Hose, R. K., and Repenning, C. A., 1959, Stratigraphy of Pennsylvanian, Permian, and Lower Triassic Rocks of the Confusion Range, West-Central Utah, A.A.P.G. Bulletin vol. 43, no. 9


Tozer, E. T., 1971, Triassic Time and Ammonoids: Problems and Proposals, Canadian Journal of Earth Science, 8


Tozer, E. T., 1994, Canadian Triassic Ammonoid Faunas, GSC Bulletin 467


1 comment:

  1. Maybe it would be best to just leave them separated at this time.

    ReplyDelete