Intraspecific variation in, and ontogeny of, Prionotropis woollgari and Prionocyclus wyomingensis. Bulletin of the AMNH ; v. 86, article 4

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New York : [American Museum of Natural History]
"Examination of about 450 specimens of Prionotropis woollgari Meek (? non Mantell) and about 40 of Prionocyclus wyomingensis Meek from various localities in the western interior of the United States proves a high degree of both intraspecific variation and ontogenetic changes in these two ammonite species, often quoted and repeatedly used as index fossils but hardly ever thoroughly studied since their establishment. The interaction of both factors causes an almost bewildering diversity of morphologic aspects, particularly in Prionotropis woollgari. Both species are considered polytypic. The original of Meek's (1876, pl. 7) figure 1a, b, is designated lectotype of the typical Prionotropis woollgari Meek. Six varieties: crassa, intermedia, regularis, tenuicostata, praecox, and alata, the last based on the individual figured by Meek (1876) on plate 6, figure 2, and plate 7, figure 1g, h, are separated from the typical form, with P. branneri Dickerson considered a seventh. In addition to Ammonites percarinatus Hall and Meek, Ammonites graysonensis Shumard is also included in the synonymy of Meek's species. The ontogeny of the forma typica as well as of the six new varieties is closely followed with respect to dimensions, length of body chamber and full size attained, whorl section, ornamentation, and sutures. This study shows that intraspecific variation is largely due to differentiation in speed of development. Particularly interesting are sculptural and sutural ontogenies, the former for the appearance, further development, vanishing, and eventual disappearance of the various elements of ornamentation and for its general trend toward diminishing fineness and density, the latter for the trend of the first lateral lobe to change from trifidity to bifidity (and frequently of the second lateral lobe to change simultaneously from bifidity to trifidity). White's (1880) specimens of Prionocyclus wyomingensis are recognized as being typical of this species of Meek's. Two varieties, elegans, based on Whitfield (1880), and robusta, are separated from the typical form, and two large fragments belonging or closely related to this species are discussed as 'incertae sedis.' The ontogeny is studied along the same lines as in Prionotropis woollgari, with similar results, especially as to sutural development. It can be shown that the change of the first lateral lobe from trifidity to bifidity can be achieved in two different ways (orthogenesis?). As both species are genotypes, the differences between them stand for those between their genera. These differences are discussed and found just sufficient to warrant -- by the standards of present taxonomy -- generic separation. The variety praecox of Prionotropis woollgari, however, is somewhat transitional between both genera. Homeomorphies between Albian Pervinquierinae and certain Upper Cretaceous Prionotropidae are exposed and illustrated by several examples, and an attempt is made to point out some sculptural and sutural differences between both groups"--P. 221.
p. 145-224, [14] p. of plates : ill. ; 27 cm.
Includes bibliographical references (p. 222-224).