Shell abnormalities in scaphitid ammonites

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Abstract

The kinds incidence of shell abnormalities were investigated in over 2000 adult Late Cretaceous scaphitid ammonites. Most are species of Hoploscaphites Discoscaphites from the Maastrichtian Fox Hills Formation; for comparison, four older species of Scaphites were also studied. Four kinds of abnormalities vary in their incidence among the Fox Hills taxa but display no general trends. (1) Morton's syndrome, shell asymmetry unrelated to external injury, occurs in 3 to 6% of the Fox Hills scaphites. (2) Suture wander. veering-off of the ventral lobe of the suture from the midline of the venter, occurs in most Fox Hills taxa with an incidence ranging from 1 to 11%. (3) Stretch pathology, an attenuation of the shell on the posterior part of the adult body chamber, is confined to Hoploscaphifes nicolleti. (4) Externally inflicted injury, indicated by repaired damage on mature shells, has an average incidence of 10% in Fox Hills scaphites except in the stratigraphically youngest H. nebrascensis H. cheyennewis which approach 25 40% respectively. Good correlation between percent injuries adult size ornamentation is obtained only in Discoscaphifes; all morphs of the co-occurring Hoploscaphites display nearly the same incidence of injuries regardless of their morphologic variation. The stratigraphically youngest H. nebrascensis becomes larger more nodose simultaneously sustains more injuries. However, its dimorph H. cheyennensis remains morphologically unchanged although its incidence of injuries quadruples. These injury data, in themselves, do not support the hypothesis that predators were contributing more heavily to the mortality of Maastrichtian ammonites precipitated their extinction.

Ancillary