Description of Ammonite Touch Marks

Careful examination of the ammonite touch marks reveals the species that probably produced them (Figs. 18.4-21, arranged in taxonomic order). The main evidence for this determination is the impression of the ornamental features (ribs, tubercles, and keels), which form a distinctive pattern. We compared this pattern to specimens in our collections, bearing in mind that the pattern of ornamentation commonly changes during ontogeny. We examined those ammonites known to occur in the same strata as the impressions and in age-equivalent strata from elsewhere.

Most of the touch marks were produced by acanthoceratid ammonites including species of Collignoniceras, Calycoceras, and Tarrantoceras. These ammonites were widespread in the Late Cretaceous Western Interior Seaway. Other touch marks in our collection were produced by scaphitid ammonites.

The majority of touch marks are impressions of the venter or ventrolateral edge of the shell, as inferred from the kind and distribution of ornamental features. For example, the ventral impression of scaphitid ammonites consists of a series of 5-10 straight ridges resembling scratch marks (Fig. 18.5A), while the ventral impression of Tarrantoceras consists of three parallel rows of small bumps (Fig. 18.12A). The vent-rolateral impression of Collignoniceras woollgari (Mantell, 1822) consists of a series of clavate ridges (Fig. 18.17A), while the ventrolateral impression of Fagesia catinus (Mantell, 1822) shows a break in the shape of the ridges, corresponding to a change in the shape of the whorl section as it passes from the flanks to the venter (Fig. 18.14).

The ammonite touch marks show a broad range in size. Those attributed to Dunveganoceras conditum (Haas, l951) (Fig. 18.8) and Calycoceras aff. C. canitaurinum (Haas, 1949) (Fig. 18.10) are the largest, whereas those attributed to Scaphites corvensis Cobban, 1952 (Fig. 18.6B) are the smallest. In addition, the impressions vary in how faithfully they reproduce the original morphology. For example, the impression of the shell flanks attributed to Prionocyclus novimexicanus (Marcou, 1858) is very faithful (Fig. 18.19), whereas the ventrolateral impression attributed to Protexanites bourgeoisianus (d'Orbigny, 1850) is distorted, with twisted ribs (Fig. 18.16A). The impressions also vary in their degree of relief. For example, the touch mark attributed to Calycoceras aff. C. canitaurinum (Fig. 18.10) is very prominent, whereas that attributed to Pseudaspidoceras flexuosum Powell, 1963, is very faint (Fig. 18.13).

The touch marks of scaphitid ammonites are extremely abundant (Figs. 18.4, 18.5, 18.6A-D). For example, there are nine sets of impressions on a single slab 10 cm on each side (Fig. 18.5A). All of the impressions on this slab reflect the venter and show no preferred orientation.

The most spectacular ammonite impressions are roll marks or "tire tracks." Figure 18.11 depicts a "tire track" approximately 10 cm long and 4 cm wide, probably produced by an acanthoceratid ammonite like Tarrantoceras rolling on the seafloor. The track consists of five parallel rows of bumps and forms a slightly arcuate pattern. Other roll marks in our collection were produced by Collignoniceras woollgari. In Fig. 18.17C, the roll mark is slightly arcuate and represents the

Fig. 18.4 Scaphitid touch marks, USNM 534416, from the upper part of the Ferdig Member of the Marias River Shale in northcentral Montana (loc. 4, Fig. 18.1). The touch marks (arrows) and steinkerns can be assigned to Scaphites corvensis Cobban, 1952. Similar impressions are also present in the Ferdig Member elsewhere in northcentral (locs. 5-8), northwestern (locs. 1, 2), westcentral (loc. 9), and southcentral Montana (loc. 10). X1.

Fig. 18.4 Scaphitid touch marks, USNM 534416, from the upper part of the Ferdig Member of the Marias River Shale in northcentral Montana (loc. 4, Fig. 18.1). The touch marks (arrows) and steinkerns can be assigned to Scaphites corvensis Cobban, 1952. Similar impressions are also present in the Ferdig Member elsewhere in northcentral (locs. 5-8), northwestern (locs. 1, 2), westcentral (loc. 9), and southcentral Montana (loc. 10). X1.

Fig. 18.5 Scaphitid touch marks (arrows) attributed to Scaphites corvensis Cobban, 1952, Ferdig Member ofthe Marias River Shale in northcentral Montana (loc. 6, Fig. 18.1). A. USNM534417. B. USNM 534418. C USNM 534419. D. USNM 534420. All figures X1.

Fig. 18.6 A-D. Scaphitid touch marks attributed to Scaphites corvensis Cobban, 1952, Ferdig Member of the Marias River Shale in northcentral Montana (Fig. 18.1). A. USNM534421 (loc. 8, Fig. 18.1). B. USNM 534422. C. USNM 534423 (loc. 7, Fig. 18.1). D. USNM 534424 (loc. 7, Fig. 18.1). E-G. Holotype ofScaphites corvensis Cobban, l952, USNM106755, USGS Mesozoic loc. 20939, Cody Shale, Montana. E. Left lateral view. F. Ventral view of the hook. G. Ventral view of the shaft. All figures X1.

Fig. 18.6 A-D. Scaphitid touch marks attributed to Scaphites corvensis Cobban, 1952, Ferdig Member of the Marias River Shale in northcentral Montana (Fig. 18.1). A. USNM534421 (loc. 8, Fig. 18.1). B. USNM 534422. C. USNM 534423 (loc. 7, Fig. 18.1). D. USNM 534424 (loc. 7, Fig. 18.1). E-G. Holotype ofScaphites corvensis Cobban, l952, USNM106755, USGS Mesozoic loc. 20939, Cody Shale, Montana. E. Left lateral view. F. Ventral view of the hook. G. Ventral view of the shaft. All figures X1.

Fig. 18.7 A. Touch mark, USNM 534425, from a sandy bed in the Floweree Member of the Marias River Shale, Montana (loc. 3, Fig. 18.1), assigned to Metoicoceras mosbyense Cobban, 1953, because this is the only species of Metoicoceras known from this unit. B. A specimen of M. mosbyense, USNM 534426, photographed obliquely for comparison, to show the flattened venter bordered by ventrolateral clavi, USGS Mesozoic loc. 21487, Mosby Sandstone, eastcentral Montana. Both figures X1.

Fig. 18.7 A. Touch mark, USNM 534425, from a sandy bed in the Floweree Member of the Marias River Shale, Montana (loc. 3, Fig. 18.1), assigned to Metoicoceras mosbyense Cobban, 1953, because this is the only species of Metoicoceras known from this unit. B. A specimen of M. mosbyense, USNM 534426, photographed obliquely for comparison, to show the flattened venter bordered by ventrolateral clavi, USGS Mesozoic loc. 21487, Mosby Sandstone, eastcentral Montana. Both figures X1.

impression of the ventrolateral tubercles and serrated keel. In Fig. 18.17E, the impression of the tubercles and keel is smaller, reflecting a specimen at an earlier stage of ontogeny. These roll marks perfectly match those produced experimentally by Dzulynski and Sanders (1962), using specimens of C. woollgari.

Many ammonite touch marks are also associated with scour and tool marks of unknown origin. These marks may have formed before, after, or at the same time as the ammonite touch marks. Such marks are present, for example, in the Ferdig Member of the Marias River Shale (Figs. 18.5, 6A-D) and the Turner Sandy Member of the Carlile Shale (Fig. 18.21). Other structures may represent burrows that formed at the interface between the mud and the overlying sediments (Fig. 18.5A).

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