Deformity. There are two types of deformities seen on either the flank or the venter, which are the results of a deep bite. One is characterized by the appearance of a large healed injury, regularly seen as a deep depression, remodeled and sculpted, devoid of normal ornamentation, and frequently with portions of the shell completely missing (Fig. 16.15a-f). Similar scarring was described as forma aegra aptychaby Keupp (1977) and illustrated by Bond and Saunders (1989).
The second deformity is typified by a long, shallow to deep depression that follows the spiral growth of the ammonite shell (Fig. 16.16a-d). This type of injury was classified by Holder (1956) as forma aegra verticata and figured by Keupp (1979, 1985). This type of scarring was also labeled by P. L. Larson (1984) and Landman and Waage (1986) as a "spiral furrow."
Cause. Both of these depressed scars are the result of severe bites or injuries to the body chamber and mantle during life. The first deformity resulted in the loss of some of the mantle, and as the ammonite attempted to heal the bite, it grew a smooth shell covering the bitten region. Because there was flesh and shell lost in the bite, there is a depression left that never grew back to the original form. If there is no ornament, this indicates that the injury happened somewhere behind the mantle margin, because only the lip of the mantle can create ribbing and ornament. If there is ornament, this indicates that the injury happened near or at the aperture, and that the ammonite managed to create new shell material with its damaged mantle.
The depressed line of scarring is the result of an injury to the lip (or edge) of the mantle in a still-growing ammonite. This type of injury resulted in a damaged and disfigured mantle that would have continued to produce a scarred shell throughout the lifetime of the ammonite, because some portion of the mantle was severely damaged. This type of deformity also has been observed in extant Nautilus (Fig. 16.16e, f).
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