Impressions of the attachment of the soft body to the shell in late Cretaceous pachydiscid ammonites from the Western Interior of the United States. American Museum novitates ; no. 3273
Supplemental Materials
Date
1999
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Publisher
New York, NY : American Museum of Natural History
DOI
DOI
Abstract
Late Cretaceous pachydiscid ammonites Menuites oralensis Cobban and Kennedy, 1993 and Menuites portlocki (Sharpe, 1855) complexus (Hall and Meek, 1856) from the Western Interior of the United States show four kinds of markings on the phragmocone and body chamber. These markings are preserved on internal molds that retain traces of the original shell, mostly the inner prismatic layer. (1) Transverse lines appear on the surface of the inner prismatic layer and extend adorally as far as midway onto the adult body chamber. They consist of a sequence of regularly spaced iridescent lines that usually cross the venter with a marked adoral projection, forming a chev-ronlike pattern. The transverse lines do not follow the shape of the apertural margin. Viewed in close-up, these lines appear as narrow bands of nacre; the adoral edge of each band is ragged and the adapical edge thins out and disappears. (2) A longitudinal band occurs on the flanks and extends from the umbilicus to about two--thirds whorl height. It appears on the surface of the inner prismatic layer. (3) A mid-ventral band extends for several tens of millimeters adoral of the ultimate septum and terminates in an unpaired scar. It appears on the internal mold and is visible below the inner prismatic layer. (4) A pair of dorsal scars occurs on the internal mold several millimeters adoral of the ultimate septum. Each scar is boomerang-shaped and extends from the dorsal margin to just ventral of the umbilical shoulder. We hypothesize that the transverse line formed at the adapical margin of the zone of nacreous secretion in the middle of the body chamber. They may have represented narrow bands of mantle attachment. The longitudinal band demarcated a broad area of nacreous secretion on the flanks, which may have represented an additional area of mantle attachment. During growth, both the transverse lines and the longitudinal band were overlain by the inner prismatic layer (and are thus now visible on the surface of this layer on partially exfoliated shells). The mid-ventral scar and dorsal scars just adoral of the ultimate septum are the sites of attachment of the soft body at the adapical end of the body chamber. These sites developed on the inside surface of the inner prismatic layer (and are thus now visible on the internal mold below the inner prismatic layer, if present).
Description
31 p. : ill. ; 26 cm.
Includes bibliographical references (p. 30-31).
Includes bibliographical references (p. 30-31).
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