Discussion

Ammonite touch marks record the impact of the shells as they bounced and rolled on a firm mud bottom in relatively shallow water. The impressions are preserved on the basal surfaces of siltstones and fine sandstones. The waterlogged shells were

Fig. 18.8 Touch mark, USNM 534427, showing five straight, narrow, closely spaced ribs, from the upper part of the Dakota Formation near Delta in westcentral Colorado (loc. 13, Fig. 18.1). This impression was made by a large ammonite such as the middle late Cenomanian species Dunveganoceras conditum Haas, 1951 (Fig. 18.9B), although this species has not been recorded in Colorado west of Pueblo.

Fig. 18.8 Touch mark, USNM 534427, showing five straight, narrow, closely spaced ribs, from the upper part of the Dakota Formation near Delta in westcentral Colorado (loc. 13, Fig. 18.1). This impression was made by a large ammonite such as the middle late Cenomanian species Dunveganoceras conditum Haas, 1951 (Fig. 18.9B), although this species has not been recorded in Colorado west of Pueblo.

probably resuspended and carried along by bottom currents associated with storms. As the storms subsided, the ammonites were "the first particles to touch bottom, so that their markings could be immediately cast by the sand, in whose suspension they had been transported" (D. Seilacher, 2005, personal communication).

This scenario requires a substrate firm enough to retain impressions of the shells. The firmness of the bottom may have been due to the cohesive properties of the sediments, perhaps related to the presence of bacterial films or mucus. The

Fig. 18.9 A. Touch mark, USNM 534428, from the Hartland Shale Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 15, Fig. 18.1), showing fairly closely spaced ribs bearing inner and outer ventrolateral tubercles, suggesting the outer whorl ofthe phragmocone of an adult specimen of Dunveganoceras conditum Haas, 1951, which is the only ammonite like it known from this stratigraphic unit. X1. B. Paratype of D. conditum, AMNH27686, left lateral view, illustrated for comparison, Frontier Formation, T39N R83W, Wyoming. X 0.33.

Fig. 18.9 A. Touch mark, USNM 534428, from the Hartland Shale Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 15, Fig. 18.1), showing fairly closely spaced ribs bearing inner and outer ventrolateral tubercles, suggesting the outer whorl ofthe phragmocone of an adult specimen of Dunveganoceras conditum Haas, 1951, which is the only ammonite like it known from this stratigraphic unit. X1. B. Paratype of D. conditum, AMNH27686, left lateral view, illustrated for comparison, Frontier Formation, T39N R83W, Wyoming. X 0.33.

ammonite impressions were subsequently covered by the sediments entrained in the current. Afterward, the bottom must not have experienced any significant erosion that would have removed the impressions, or any large-scale bioturbation that would have obliterated them.

Fig. 18.10 A. Touch mark, USNM 534429, showing broad, thick, alternating ribs, attributed to Calycoceras aff. C. canitaurinum (Haas, 1949) as defined by Cobban (1988a), from the Hartland Shale Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 14, Fig. 18.1). Ammonites in the underlying Lincoln Member include C. canitaurinum. X1. B. Calycoceras aff. C. canitaurinum, USNM 376912, right lateral view, illustrated for comparison, USGS Mesozoic loc. 23154, Frontier Formation, Wyoming (Cobban, 1988a: pl. 5). In central Wyoming, Calycoceras aff. C. canitaurinum occurs in the Frontier Formation just above C. canitaurinum, as at Pueblo. X 0.33.

Fig. 18.10 A. Touch mark, USNM 534429, showing broad, thick, alternating ribs, attributed to Calycoceras aff. C. canitaurinum (Haas, 1949) as defined by Cobban (1988a), from the Hartland Shale Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 14, Fig. 18.1). Ammonites in the underlying Lincoln Member include C. canitaurinum. X1. B. Calycoceras aff. C. canitaurinum, USNM 376912, right lateral view, illustrated for comparison, USGS Mesozoic loc. 23154, Frontier Formation, Wyoming (Cobban, 1988a: pl. 5). In central Wyoming, Calycoceras aff. C. canitaurinum occurs in the Frontier Formation just above C. canitaurinum, as at Pueblo. X 0.33.

Fig. 18.11 Roll mark collected as float, USNM534430, from the Lincoln Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 16, Fig. 18.1). Several species of acanthoceratid ammonites could have made this type of roll mark. The only acanthoceratid ammonites recorded from the Lincoln Member in the Pueblo area are Acanthoceras amphibolum Morrow, 1935, Calycoceras canitaurinum (Haas, 1949), and Tarrantoceras sp. (Cobban and Scott, 1972; Sageman and Johnson, 1985; see fig. 18.12B, C). Of these, Tarrantoceras is the most likely candidate, but none of the specimens of Lincoln age is known to be large enough to have produced a roll mark of this size. X1.

Fig. 18.11 Roll mark collected as float, USNM534430, from the Lincoln Member of the Greenhorn Limestone near Pueblo, Colorado (loc. 16, Fig. 18.1). Several species of acanthoceratid ammonites could have made this type of roll mark. The only acanthoceratid ammonites recorded from the Lincoln Member in the Pueblo area are Acanthoceras amphibolum Morrow, 1935, Calycoceras canitaurinum (Haas, 1949), and Tarrantoceras sp. (Cobban and Scott, 1972; Sageman and Johnson, 1985; see fig. 18.12B, C). Of these, Tarrantoceras is the most likely candidate, but none of the specimens of Lincoln age is known to be large enough to have produced a roll mark of this size. X1.

The ammonite impressions reflect the imprint of a curved surface on a flat bottom and as a result, do not always faithfully record the ornamentation. In addition, depending on the fragmentation of the shell, the strength and direction of the current, the firmness of the bottom, and the nature of the contact (resting, dragging, skipping), the impressions can be indistinct or distorted.

Fig. 18.12 A. Roll mark of an acanthoceratid ammonite, USNM 534431, such as Tarrantoceras Stephenson, 1955 (loc. 24, fig. 18.1). B, C. Holotype of Tarrantoceras sellardsi (Adkins, 1928), USNM 400760, illustrated for comparison, USGS Mesozoic loc. D12626, Tarrant Formation, Texas (Cobban, 1988b: pl.1, Figs. 6, 7). B. Ventral view. C. Right lateral view. D. Touch mark, USNM534432, attributed to Acanthoceras alvaradoense Moreman, 1942 (loc. 23, Fig. 18.1). This ammonite occurs a little below the Marker bentonite bed at other localities in New Mexico. E. Acanthoceras alvaradoense, left lateral view, illustrated for comparison, Tarrant Formation, Texas (Moreman, 1942: pi. 32, Fig 6). All figures X1.

Fig. 18.12 A. Roll mark of an acanthoceratid ammonite, USNM 534431, such as Tarrantoceras Stephenson, 1955 (loc. 24, fig. 18.1). B, C. Holotype of Tarrantoceras sellardsi (Adkins, 1928), USNM 400760, illustrated for comparison, USGS Mesozoic loc. D12626, Tarrant Formation, Texas (Cobban, 1988b: pl.1, Figs. 6, 7). B. Ventral view. C. Right lateral view. D. Touch mark, USNM534432, attributed to Acanthoceras alvaradoense Moreman, 1942 (loc. 23, Fig. 18.1). This ammonite occurs a little below the Marker bentonite bed at other localities in New Mexico. E. Acanthoceras alvaradoense, left lateral view, illustrated for comparison, Tarrant Formation, Texas (Moreman, 1942: pi. 32, Fig 6). All figures X1.

Fig. 18.13 A. Touch mark, USNM 534433, attributed to Pseudaspidoceras flexuosum Powell, 1963, from the P. flexuosum Zone in the Mancos Shale of southwestern New Mexico (loc. 22, Fig. 18.1; Cobban et al., 1989: 64). The wide umbilicus and prominent umbilical tubercles are characteristic of this species. X1. B. Holotype of Pseudaspidoceras paganum, Reyment, 1954, C. 47422, right lateral view, illustrated for comparison, lower Turonian, Pindiga, Bauchi Province, Nigeria (Reyment, l959: pl.4, Fig. 1). X0.75.

Fig. 18.13 A. Touch mark, USNM 534433, attributed to Pseudaspidoceras flexuosum Powell, 1963, from the P. flexuosum Zone in the Mancos Shale of southwestern New Mexico (loc. 22, Fig. 18.1; Cobban et al., 1989: 64). The wide umbilicus and prominent umbilical tubercles are characteristic of this species. X1. B. Holotype of Pseudaspidoceras paganum, Reyment, 1954, C. 47422, right lateral view, illustrated for comparison, lower Turonian, Pindiga, Bauchi Province, Nigeria (Reyment, l959: pl.4, Fig. 1). X0.75.

Ammonite touch marks can be used to derive information on current direction. The ventral touch marks of the scaphitid ammonites show no preferred orientation (Fig. 18.5A), perhaps implying variable current directions. In contrast, roll marks reveal the current track, but it is difficult to determine up from down current. The longer the shell rolled, the longer the roll mark, and the easier to follow the trace.

Fig. 18.14 A. Touch mark, USNM534434, ofthephragmocone ofFagesia catinus (Mantell, 1822) (loc. 22, fig. 18.1). Specimens of this Turonian species have been found in calcareous concretions at this stratigraphic level. B, C. A similarly sized specimen ofF. catinus, USNM425388, illustrated for comparison, USGS Mesozoic loc. D11009, Colorado Formation, southwestern New Mexico (Cobban et al., l989: Fig. 92GG, HH). B. Ventral view. C. Right lateral view. All figures X1.

Fig. 18.14 A. Touch mark, USNM534434, ofthephragmocone ofFagesia catinus (Mantell, 1822) (loc. 22, fig. 18.1). Specimens of this Turonian species have been found in calcareous concretions at this stratigraphic level. B, C. A similarly sized specimen ofF. catinus, USNM425388, illustrated for comparison, USGS Mesozoic loc. D11009, Colorado Formation, southwestern New Mexico (Cobban et al., l989: Fig. 92GG, HH). B. Ventral view. C. Right lateral view. All figures X1.

Fig. 18.15 Touch mark attributed to Protexanites bourgeoisianus (d'Orbigny, 1850), USNM534435, from the El Vado Member ofthe Mancos Shale of northwestern New Mexico (loc. 18, Fig. 18.1), because there is no other ammonite like it from this stratigraphic unit (see Fig. 18.16B). X1.

Fig. 18.16 A. Touch mark attributed to Protexanites bourgeoisianus (d'Orbigny, 1850), USNM 534436, from the sandy member of the Niobrara Formation of northeastern New Mexico (loc. 17, Fig. 18.1), a stratigraphic unit that has yielded an upper Coniacian assemblage of the Scaphites depressus Zone. The impression is slightly distorted and the ventrolateral tubercles are missing. Note the ribbing of a juvenile nearby, much like the specimen illustrated by Kennedy and Cobban (1991: pl. 8, Fig. 1). X1. B. A specimen of P. bourgeoisianus, USNM 433795, right lateral view, illustrated for comparison, USGS Mesozoic loc. 23100, Cody Shale, Wyoming (Kennedy and Cobban, 1991: Fig. 20). X0.75.

Fig. 18.16 A. Touch mark attributed to Protexanites bourgeoisianus (d'Orbigny, 1850), USNM 534436, from the sandy member of the Niobrara Formation of northeastern New Mexico (loc. 17, Fig. 18.1), a stratigraphic unit that has yielded an upper Coniacian assemblage of the Scaphites depressus Zone. The impression is slightly distorted and the ventrolateral tubercles are missing. Note the ribbing of a juvenile nearby, much like the specimen illustrated by Kennedy and Cobban (1991: pl. 8, Fig. 1). X1. B. A specimen of P. bourgeoisianus, USNM 433795, right lateral view, illustrated for comparison, USGS Mesozoic loc. 23100, Cody Shale, Wyoming (Kennedy and Cobban, 1991: Fig. 20). X0.75.

The shape of the component parts of the roll mark may help indicate the initial point of contact between the shell and the seafloor, i.e., the up current end.

The touch marks in the Ferdig Member of the Marias River Shale are remarkable because of their broad geographic distribution, covering an area of several hundred square kilometers in northcentral Montana (Fig. 18.3). They are confined to the upper Turonian Scaphites corvensis Zone of the Ferdig Member of the Marias River Shale. This unit consists of silty, noncalcareous shale with lenses of very fine grained sandstone (Mudge, 1972; Cobban et al., 1976; Lemke, 1977).

P.E. Cloud, in an unpublished report from the US Geological Survey (1959), characterized the paleoenvironment of this unit as follows: "a generally quiet marine water body deep enough to be spared strong wave or current action, shallow enough to be within reach of some standing waves, sediments anaerobic enough to

Fig. 18.17 Touch marks attributed to Collignoniceras woollgari (Mantell, 1822). A. USNM 534437, showing three sharply defined narrow ribs bearing inner ventrolateral tubercles, from part of the Mancos Shale of northwestern New Mexico (loc. 19, Fig. 18.1) that contains this species in limestone concretions. X1. B. A similarly sized specimen of C. woollgari, USNM 534438, photographed obliquely for comparison, USGS Mesozoic loc. D9896, Carlile Shale, Black Hills area, Wyoming. X1. C. Roll mark, USNM534439, from the Tropic Shale of southern Utah (loc. 21, Fig. 18.1), that contains this species in limestone concretions. X1. D. A similarly sized specimen of C. woollgari, USNM 534440, photographed obliquely for comparison, USGS Mesozoic loc. D3754, Carlile Shale, Black Hills area, South Dakota. E. Roll mark:, USNM 534441, from the Mancos Shale, New Mexico (loc. 25, Fig. 18.1). X1.5. F. A similarly sized specimen of C. woollgari, USNM 534442, photographed obliquely for comparison, Carlile Shale, Black Hills area, South Dakota or Wyoming. X1.5.

Fig. 18.17 Touch marks attributed to Collignoniceras woollgari (Mantell, 1822). A. USNM 534437, showing three sharply defined narrow ribs bearing inner ventrolateral tubercles, from part of the Mancos Shale of northwestern New Mexico (loc. 19, Fig. 18.1) that contains this species in limestone concretions. X1. B. A similarly sized specimen of C. woollgari, USNM 534438, photographed obliquely for comparison, USGS Mesozoic loc. D9896, Carlile Shale, Black Hills area, Wyoming. X1. C. Roll mark, USNM534439, from the Tropic Shale of southern Utah (loc. 21, Fig. 18.1), that contains this species in limestone concretions. X1. D. A similarly sized specimen of C. woollgari, USNM 534440, photographed obliquely for comparison, USGS Mesozoic loc. D3754, Carlile Shale, Black Hills area, South Dakota. E. Roll mark:, USNM 534441, from the Mancos Shale, New Mexico (loc. 25, Fig. 18.1). X1.5. F. A similarly sized specimen of C. woollgari, USNM 534442, photographed obliquely for comparison, Carlile Shale, Black Hills area, South Dakota or Wyoming. X1.5.

Fig. 18.18 Touch marks attributed to Collignoniceras woollgari (Mantell, 1822), USGSMesozoic loc. D11022, southwestern New Mexico (loc. 26, Fig. 18.1). A. USNM534443. B. USNM534444. Both figures X1.

limit burrowing activity that would destroy lamination but rich in nutrients for surface feeders, in an area far enough away from a river system for its normal sediments to consist of silt from pulsatory system overflow but near enough to be within reach of heavy and rapid settling-out from occasional flood deliveries."

Most of the touch marks in the Ferdig Member reflect impressions of the venter of the body chamber near the point of recurvature. This implies that the shells were in a nearly vertical orientation with the phragmocone on top. Other impressions reflect the ventrolateral region where the ribs bifurcate. Some of these marks are relatively long, indicating that a broad area of the shell surface contacted the bottom (Fig. 18.5D). Fragments of actual shells, i.e., steinkerns, are also present at this site (Fig. 18.4), and probably represent the very specimens that made the touch marks.

It is possible that the touch marks in the Ferdig Member record a sequence in the taphonomic history of the ammonite shells. For example, the ventral impressions may have been produced during life. Scaphitid ammonites with hook-shaped body chambers were oriented with the phragmocone on top and the hook-like body chamber on the bottom (Landman, 1987). It is conceivable that these animals hovered just above the bottom and occasionally touched it. The ventrolateral and flank impressions may have been produced after death, during progressive stages of shell fragmentation and waterlogging. The steinkerns represent the final disposition of the empty shells.

On the other hand, these ammonite touch marks are also associated with tool marks of unknown origin. It is perhaps more parsimonious to argue that all of these impressions simply reflect transport of waterlogged shells and other debris during

Fig. 18.19 A. Touch mark, USNM 534445, attributed to Prionocyclus novimexicanus (Marcou, 1858) (loc. 20, Fig. 18.1). This unusual and rare preservation clearly shows an ammonite that briefly rested on its side. X1. B. A similarly sized specimen of P. novimexicanus, USNM 498417, right lateral view, illustrated for comparison, USGS Mesozoic loc. 21191, Carlile Shale, Butte County, South Dakota (Kennedy et al, 2001: Fig. 99F). X0.85.

Fig. 18.19 A. Touch mark, USNM 534445, attributed to Prionocyclus novimexicanus (Marcou, 1858) (loc. 20, Fig. 18.1). This unusual and rare preservation clearly shows an ammonite that briefly rested on its side. X1. B. A similarly sized specimen of P. novimexicanus, USNM 498417, right lateral view, illustrated for comparison, USGS Mesozoic loc. 21191, Carlile Shale, Butte County, South Dakota (Kennedy et al, 2001: Fig. 99F). X0.85.

Fig.18.20 A, B. Touch marks from the middle Turonian part of the Mancos Shale (loc. 12, Fig. 18.1) of northwestern Colorado attributed to Prionocyclus hyatti (Stanton, 1894) because this is the only prionocyclid ammonite known from the basal 3m of the Frontier Sandstone Member in this area. A. USNM 534446. B. USNM 534447. C. A similarly sized specimen of P. hyatti, USNM 534448, photographed obliquely for comparison, USGS Mesozoic loc. D14365, Semilla Sandstone, New Mexico. All figures X1.

Fig.18.20 A, B. Touch marks from the middle Turonian part of the Mancos Shale (loc. 12, Fig. 18.1) of northwestern Colorado attributed to Prionocyclus hyatti (Stanton, 1894) because this is the only prionocyclid ammonite known from the basal 3m of the Frontier Sandstone Member in this area. A. USNM 534446. B. USNM 534447. C. A similarly sized specimen of P. hyatti, USNM 534448, photographed obliquely for comparison, USGS Mesozoic loc. D14365, Semilla Sandstone, New Mexico. All figures X1.

periodic storm activity. The extent of waterlogging may have influenced how easily the shells were resuspended, and the kinds of impressions that they made.

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