Download PDF by Kennedy W.J., Cobban W.A.: Aspects of ammonite biology, biogeography and biosratigraphy

By Kennedy W.J., Cobban W.A.

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TAXONOMIC PROBLEMS Variability. Ammonites, perhaps more than most groups, have suffered extensive taxonomic splitting, and authors have frequently divided intergrading populations into a whole range of species and even genera. Buckman (1892) divided morpho­ logically intergrading series into differing 'species', although admitting they probably belonged to what would now be termed a single biospecies. Spath (1923-1943) made similar statements and commented on the problems of identifying species when large numbers of specimens were available; whole monographs have thus been devoted to descriptions of what are now regarded as but a few species.

Text-fig. 7), a reflection of life habits that changed through ontogeny. AMMONITE Didymoceras nebrascense (Meek and Hayden) TEXT-FIG. cheyennense ECOLOGY 35 Didymoceras cheyennense (Meek and Hayden) 7. A r e c o n s t r u c t i o n o f the C a m p a n i a n n o s t o c e r a t i d h e t e r o m o r p h s Didymoceras ( M e e k a n d H a y d e n ) a n d D. nebrascense ( M e e k a n d H a y d e n ) , b a s e d o n material f r o m the Pierre S h a l e o f C o l o r a d o (after S c o t t a n d C o b b a n 1965).

B) A planktonic or nektonic life habit continued into post-larval life in many taxa, both normally coiled and heteromorph. Washings of sediments deposited many miles from shore often yield abundant juvenile ammonites (A. J. Lloyd, pers. ), and we know of many cases of huge concentrations of juveniles, complete with body chambers, preserved in pelagic deposits. For example we have a collection from the Marias River Shale (Santonian) of north-western Montana, which is a mass of juvenile Baculites codyensis Reeside and Clioscaphites vermiformis (Meek and Hayden).

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Aspects of ammonite biology, biogeography and biosratigraphy by Kennedy W.J., Cobban W.A.


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