Preserved ammonitellas of Scaphites (Ammonoidea, Ancyloceratina). American Museum novitates ; no. 2815

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New York, N.Y. : American Museum of Natural History
"Ammonitellas, embryonic shells of ammonites, occur in the Upper Cretaceous (Turonian) of Colorado associated with adults of Scaphites ferroensis. They average 650 [mu]m in diameter and are similar to the embryonic whorls of the associated scaphite species to which they probably belong. They terminate at the primary constriction and accompanying varix but display variation in number of embryonic septa from shells with only a single prismatic proseptum and body chamber angles of 290 to shells completely filled with septa and body chamber angles less than 60. However, the lack of a body chamber during life is an impossibility and suggests that the primary varix must be a point of weakness for postmortem breakage. This, plus the presence of young postembryonic shells that extend beyond the primary constriction and reveal two to six septa, implies that variation in number of embryonic septa above the proseptum may be false. Based on functional morphology, the newly hatched ammonite may have led a planktic existence for some time after hatching"--P. [1]."
10 p. : ill. ; 26 cm.
Includes bibliographical references (p. 9-10).