Stratigraphy and description of the rock matrix
 

The rocks in which Evan De Groot found the possible Xiphactinus were in a calcareous shale/clay unit 15 to 20 feet below the hard limestone (?equivalent to the Jetmore Member of Kansas) that caps the hills in this area and contain a lot of inoceramid bivalve shells.  The inoceramid shells were formerly called Inoceramus labiatus von Schlotheim, 1813 but now (Cobban, 1983) should be called Mytiloides mytiloides Mantell, 1822.  The clays underneath the hard limestone are still Greenhorn and not part of the underlying Graneros.  This is clear from the calcareous nature of the clays, their yellowish color and lack of fissile nature all of which differ from the lithology of the Graneros which tends top be less calcareous, fissile and usually bluish in color.  I originally thought these clays were a lower unit of the Greenhorn Formation that roughly corresponds to the lithologic description of what Hattin (1975) calls the Hartland Member in Kansas.  The assignment to the Hartland member is more tenuous since neither Iowa (Brener et al., 1981), South Dakota (Dennis Tomhave phone call of July 9 2001) nor Nebraska (phone call to Roger Pabian of July 9, 2001) subdivide the Greenhorn Formation into the four members that Hattin did in Kansas (Hattin, 1975; Hattin and Siemers 1987).  Nor is it even clear that all the distinctive lithologies used in Kansas to define the four members are present in the Greenhorn of Iowa. In Kansas the top of the Greenhorn includes a chalky shale (the Pfeifer Member) that is of similar lithology (Hattin and Siemers, 1987) to the bottom of the overlying Carlile shale (the Fairport Member). In Kansas the two are separated by an intervening nodular limestone (Fencepost limestone) but such a nodular limestone has not be defined or found in the Greenhorn of this area. In Kansas the bottom Lincoln member below the chalky shales has distinctive skeletal limestones that help define this bed. Again it is not clear that these skeletal limestones are even present in the equivalent portion of the Greenhorn of Iowa. Still it is helpful to distinguish between the harder limestones higher in the section and the calcareous muds/shales below.  This led me originally to a tentative assignment of the hard limestones to the Jetmore Member and of the clays to the Hartland Member since both correspond well lithologically to the descriptions of Hattin (1978).

After corresponding with Brian Witzke of the Iowa Geological Survey I now think the clay/shale unit may be younger than the Hartland equivalent of Kansas and would be better be classified for now as the Bridge Creek Member of the Greenhorn Formation, a classification also followed by Cobban (1983).  The Bridge Creek Member  is the equivalent to the Pfeifer and Jetmore Members of Kansas and Witzke says, "In Iowa the Bridge Creek interval is primarily characterized by chalky strata and calcareous (to chalky) shale, and it is notably more shaley and argillaceous than typical exposures to the west in Nebraska and Kansas.  In fresh unweathered exposures in Iowa this interval is light to medium gray in color, but weathered slopes typically show more pale colors (commonly with hues of yellow or orange) and the chalks may appear almost white." (personal e-mail of July 12, 2001).  This corresponds well to our exposure and given that Witzke has examined a lot more of the local Cretaceous exposures than I have, I think that classification of Bridge Creek member of the Greenhorn Formation is probably the best identification for now.  This would place the fossil right at the Turonian Cenomanian boundary and probably in the earliest Turonian time of the late Cretaceous (Watkins 1986).

Two other lithologic characteristics should be noted of the rocks that contained the fossil.   The Westfield exposure occasionally had fairly soft lenses of limestone.  One of these occurred about 5 inches above our fossil.  There was also at least one prominent (1-2 inches thick) bentonite bed about 11 inches above our fossil (Evan De Groot e-mail of July 10, 2001).  Since the bentonite is a fossil ash unit, it represents a time event.  Although there are a number of bentonite beds in the Greenhorn Formation, it may be possible after some analysis to tie this bed to specific bentonite beds elsewhere.  While it is premature to identify the bentonite, it is at the right level to suggest that it might be the HL-3 bentonite.

The unit in which the possible Xiphactinus was found, was generally devoid of obvious macrofossils. We did find one inoceramid shell (which corresponded generally to M. mytiloides1) just above the level of the Xiphactinus. There were also occasional plant axes sometime up to a couple feet long, but the preservation was not good enough ascertain if they were marine algal or terrestrial since no diagnostic branches or patterns were seen on the axes. There were also numerous scattered shark teeth (one day one of us EDG found 56 shark teeth) which generally appear to have eroded out of the clay/shale unit although some could have washed down from the overlying harder limestones. The shark teeth corresponded well to the illustrations of Witzke (1981, Plate 1, p.115) of Cretoxyrhina (Isurus in Witzke) mantelli, Cretolamna (Lamma in Witzke) appendiculata, Squalicarax falcatus, Scaponorynchus sp., and two forms of a clam eating shark (?Pytchodus)². I also found the guard of a Belemnites that appeared to have eroded out of the clay/shale unit about 5 feet above our Xiphactinus.
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¹ I am not an inoceramid expert and this tentative classification is based solely its general conformity to the M. mytiloides that are abundant in the harder cap limestone units.  If it is M. mytiloides that would be more reason to place the fossil above the Hartland and in the Bridge Creek Member.  Unfortunately, we did not collect or photograph this inoceramid and it could well be another species.

^{2 The updated names were suggested by some popular shark-teeth keys and seem to conform to Cappetta 1987 but should be considered tentative until confirmed by a shark expert}

References

Brenner, R.L., R F. Bretz, B.J. Bunker, D.L. Iles, G.A. Ludvigson, R.M. McKay, D.L. Whitley, B. J. Witzke. Cretaceous Stratigraphy and sedimentation in Northwest Iowa, Northeast Nebraska, and Southeast South Dakota. Iowa Geological Survey Guidebook Series Number 4. 172 p.

Cappetta, H. 1987. Chondrichthyes II: Mesozoic and Cenozoic Elasmobranchii 193 pp. Volume 3B of (Schultze editor) Handbook of Paleoichthyology

Cobban, W.A., 1983. Molluscan fossil record from the Northeastern part of the Upper Cretaceous Seaway, Western Interior.  In Cobban, W.A. and Merewether, E.A.  Stratigraphy and paleontology of Mid-Cretaceous rocks in Minnesota and contiguous areas. Geological Survey Professional Paper 1253 1-25 and 15 plates.

Hattin , D.E. 1975. Stratigraphy and depositional environment of Greenhorn Limestone (Upper Cretaceous) of Kansas. Kansas Geological Survey Bulletin 209 [1979 reprint edition] 128p.

Hattin, D.E. and C.T. Siemers [assisted by G.F. Stewart] 1987. Guidebook Upper Cretaceous stratigraphy and depositional environments of western Kansas. for Annual meetings of American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists [held at Oklahoma City, Oklahoma on April 1978]. Guidebook series 3 Kansas Geological Survey. [reprint with modifications of 1978 edition] 55p.

Watkins, D.K. 1986.  Calcareous nannofossil paleoceanography of the Cretaceous Greenhorn Sea.  Geological Society of America Bulletin 97:1239-1249.

Witzke B.J, 1981. Cretaceous vertebrate fossil of Iowa, and nearby areas of Nebraska, South Dakota, and Minnesota. in R. L. Brenner, R. F. Bretz, B. J. Bunker, D. L. Iles, G. A. Ludvigson, R. M. McKay, D. L. Whitley, B. J. Witzke. Cretaceous Stratigraphy and sedimentation in Northwest Iowa, Northeast Nebraska, and Southeast South Dakota. Iowa Geological Survey Guidebook Series Number 4. 172 p.