Ventral Attachments

The venter and ventrolateral shoulder of the ammonite are the most common places of attachment of Placunopsis on Quenstedtoceras (L.) lamberti. Of 655 known deformities caused by epizoa, 582 or nearly 89% have epizoa attached to the venter or on the ventrolateral shoulder. Obviously, the venter was a more ideal location for attachment without initial discovery, but it was not too favorable for Placunopsis. Checa et al. (2002) described ventral deformations on other Jurassic ammonites caused by similar epizoa, and the compensatory growth that was probably undertaken by the ammonites.

6.1.1 Protuberances

Deformity. This deformity is defined as consisting of one or more protuberances on the venter. These protuberances on the ammonite are generally thin, flattened swellings and/or elongated shapes (Figs. 16.3a-c, 16.4a-b). Keupp (2005: Fig. 16.7) illustrated a specimen of Quenstedtoceras (L.) lamberti from the Dubki Quarry with this same deformity. Similar appearing deformities, although caused by bites, were described by Keupp (1976) as forma inflata and Kröger (2000) as forma augata. Keupp (1976) showed identical multiple ventral protuberances on Amoeboceras alternans, which may or may not be due to epicoles. The deformities described as forma inflata by Keupp (1996, 2000) and Hengsbach (1996) were not caused by the ammonite growing over an attached epizoön, but rather were the results of bites that had caused a rupture of the mantle, similar to bites seen in scaphites from the US Western Interior (N. L. Larson, 2003).

Cause. This bizarre deformity occurred when a single or several Placunopsis attached onto the ammonite venter (Fig. 16.4c). Abnormalities of the ammonite occurred when the shell grew around and over the epizoön attached to the venter, leaving a large, flat, rounded protrusion. The ventral placement of the epizoön caused an unusual deformation to the ammonite and must have been quite terrible for the attached "guest" as well. Because the ammonite grew its shell completely over the epizoon, the epizoon must have died from starvation or suffocation.

Several specimens were taken apart to expose the attached epizoa (Fig. 16.5b-f). While some specimens revealed small and deformed bivalves, most of the bivalves were not very deformed. This additional weight (on the venter) also upset the center of gravity and caused the ammonite to grow off "normal" (Fig. 16.9a), generally resulting in the venter veering off from a straight line, or zigzag as described by Checa et al. (2002).

6.1.2 Hunchbacks

Deformity. A wide, low, broad, sometimes elongated distortion on the venter, which gives the ammonite the appearance of a "hunchback" (Fig. 16.6a-c). Checa et al. (2002) illustrated several deformities caused by epizoa and coined the term zigzag to describe the resulting ammonite growth in relation to the epizoa.

Keupp (2000: 126) illustrated a similar deformity in a specimen of Pavlovia sp. cf. P. iatriensis from Russia and showed the way that this ammonite shell was

Fig. 16.7 (a) Two Placunopsis (7 x 8.8mm and 11 x 13mm) on flank and venter of Quenstedtoceras (L.) lamberti (BHI-5362). (b) Close-up of Placunopsis (7 x 8.8 mm) (BHI-5362). (c) The effect of a Placunopsis on the venter of Q. (L.) lamberti; note the place of attachment to the right of the umbilicus (BHI-5382) (x0.6). (d) Q. (L.) lamberti with the "hunchback " effect, Placunopsis missing (BHI-5370) (x0.8). (e) Q. (L.) lamberti with the "hunchback " deformity, Placunopsis (10 x 11 mm) on flank and the venter (BHI-5307). (f) Q. (L.) lamberti with the "hunchback" deformity, Placunopsis (9 x 11 mm) still attached (BHI-5349).

Fig. 16.7 (a) Two Placunopsis (7 x 8.8mm and 11 x 13mm) on flank and venter of Quenstedtoceras (L.) lamberti (BHI-5362). (b) Close-up of Placunopsis (7 x 8.8 mm) (BHI-5362). (c) The effect of a Placunopsis on the venter of Q. (L.) lamberti; note the place of attachment to the right of the umbilicus (BHI-5382) (x0.6). (d) Q. (L.) lamberti with the "hunchback " effect, Placunopsis missing (BHI-5370) (x0.8). (e) Q. (L.) lamberti with the "hunchback " deformity, Placunopsis (10 x 11 mm) on flank and the venter (BHI-5307). (f) Q. (L.) lamberti with the "hunchback" deformity, Placunopsis (9 x 11 mm) still attached (BHI-5349).

Fig. 16.8 (a) Ventral "depression" on Quenstedtoceras (L.) lamberti (BHI-5454) (xl). (b) Several specimens of Q. (L.) lamberti showing the ventral "depression " (x0.8). (c) Note the ventral "depression" on Q. (L.) lamberti and the place where the Placunopsis rested on the venter to cause the deformity (BHI-5378) (xl.5).

Fig. 16.8 (a) Ventral "depression" on Quenstedtoceras (L.) lamberti (BHI-5454) (xl). (b) Several specimens of Q. (L.) lamberti showing the ventral "depression " (x0.8). (c) Note the ventral "depression" on Q. (L.) lamberti and the place where the Placunopsis rested on the venter to cause the deformity (BHI-5378) (xl.5).

deformed as it grew over an attached epizoon. Kroger (2000) described this deformation as forma augata and similar specimens from the Dubki Quarry were figured by Keupp (2005: Fig. 16.3).

Cause. This is the result of a larger epizoon attached to the venter, and perhaps, partially to the flank (Figs. 163.5b, 16.5d, 16.5f, 16.7a-f). The attached Placunopsis seemed to have grown in size before the ammonite grew over it, thus making this deformation quite different from the previous description. The placement and large size of the pelecypod resulted in a deformity that resembles kyphosis, which according to Webster's New World Dictionary (Guralnik, 1986) means "a hump, to bend or arch, an abnormal curvature of the spine resulting in a hump or humpback." This could not have been advantageous to either life form, the epizoon dying either before or after the ammonite shell grew over it, and the ammonite shell becoming disfigured in the process.

6.1.3 Depressions

Deformity. A deformity appearing as a depression or dip on the venter (Fig. 16.8a-b). Landman and Waage (1986) illustrated several specimens of Hoploscaphites nicolletii with similar ventral depressions. They referred to this as a "stretch pathology" related to the growth of the ammonite as it rapidly reached maturity. The depressions in Quenstedtoceras are not the same, because these dips or depressions occur far back on the phragmocone, whereas in H. nicolletii, they tend to occur on the shaft of the body chamber. This was not related to a bite or a disease, even though it is similar in appearance to what was figured by Keupp (1977) as forma aegra aptycha.

Cause. This deformity is the result of a very small bivalve attached to the venter (Fig. 16.8c), as described in the previous descriptions but with a slightly different distortion. The ammonite added extra shell and grew evenly over the small animal attached to the venter. The continued growth of the ammonite shell then rebounded, leaving a slight depression on the ammonite venter.

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