Distorted Shapes of Unknown Origin

Deformity. Webster's New World Dictionary (Guralnik, 1986) describes "anamorphism," as "an abnormal change of form which gives the appearance of a different species." The overall shape of the ammonite has become distorted with more shell growth on one side of the ammonite than on the other (Fig. 16.18a-c), making it look quite different when comparing both sides. The venter is commonly shifted to one side, so that one side is generally rounded with both sets of ventral tubercles, whereas the other side is commonly flat. This distortion, which can be viewed by

Fig. 16.18 (a) Kosmoceras with nonsymetrical venter (BHI-5607) (xl.2). (b) Trochospiral Quenstedtoceras (L.) lamberti "flattened" on one side, round on the other (BHI-5436) (xl). (c) Rurciceras (?) with a bent and curved venter (BHI-5606) (xl).

comparing one side of the ammonite to the other, was described as "forma cacoptycha" by Lange (1941) and illustrated by Keupp (1977, 1984, 2000), and Hengsbach (1996). Kröger (2002) described similar deformities in other ammonites calling them "Harpoceras-type", while Landman and Waage (1986) described this deformity as simply "local asymmetry." Checa et al. (2002) called the deformity "trochospiral growth," attributing the distortions to attachments of epizoa, which caused the ammonites to grow off center.

Cause. It is unknown, in any of the ammonites from this site, whether this distortion is the result of an attached, small organism (such as an epizoön) on the outer flank near the venter in an early growth stage of the ammonite, or the result of a nonlethal bite that damaged the mantle early in life, causing the ammonite to grow in a crooked or asymmetrical manner. Both possibilities are an option because this distortion is commonly observed in Quenstedtoceras (L.) lamberti from the Saratov site. Checa et al. (2002) attributed similar distortions to the attachment of epizoa during an early stage of life, causing tilting or trochospiral growth. Landman and Waage (1986) noted similar distortions in Maastrichtian scaphites from the Western Interior, which have never been reported to have deformities caused by epizoa but have a high percentage of healed bites (P. L. Larson, 1984; Landman and Waage, 1986; N. L. Larson, 2003).

This type of trochospiral growth is known to occur in ammonites of nearly all species, including those from the Late Cretaceous families Scaphitidae, Placenticeratidae, and Sphenodiscidae, and within these Late Cretaceous genera, the cause appears to be from a nonlethal bite earlier in life. This is theorized because attachments of epizoa on scaphites are still unknown. Keupp (1976: Fig. 16.4) noted a similar distortion in Pleuroceras and interpreted it as a healed bite.

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