Tetrapod Fossil Footprints, Polonium Halos, and the Colorado Plateau

Introduction

In his recent book Creation's Tiny Mystery, Robert V. Gentry has reiterated the long-held creationist claim that at least some of the formations within the Colorado Plateau "were deposited within a few months of each other only a few thousand years ago" (1988, p. 53). Gentry goes on to make the following assertions:

Although the flood itself lasted just a year, long-term geological effects may have lasted for hundreds of years thereafter. For example, while the sedimentary rock formations observed in the Grand Canyon are ascribed to the period of the flood itself the erosional processes that cut through the freshly deposited sediments may well have continued for a number of years after the flood. In my model the bulk of fossil-bearing sedimentary rocks would have formed during the opening and closing stages of the flood, with lesser amounts being formed during the long period of subsidence and run-off after the flood.

[p. 185; emphasis added]

Such ridiculous claims might easily be ignored, but Gentry purports to support them by citing his analysis of secondary polonium halos in samples of coalified wood from three of the geological periods—Triassic, Jurassic, and Tertiary (Eocene)—represented within the Colorado Plateau (Gentry et al., 1976). Polonium halos are microscopic spheres which form as a result of the alpha decay of polonium 210 atoms which accumulate in radiocenters composed of lead and selenium. Gentry (1988) argues that these halos formed as a result of the infiltration and subsequent decay of a single solution of radioactive uranium.

Since the half-life of polonium 210 is only 138 days, secondary polonium halos would have developed in the water soaked wood in less than a year. On this basis, Gentry claims that the vast depth of sedimentary rock forming the Colorado Plateau must have been deposited within a matter of months. As in his earlier Science article (Gentry et al., 1976), Gentry suggests—he does not provide proof—that the uranium solution which infiltrated the wood was the same in the three samples he studied: "The evidence obtained in these experiments suggested a common source for the uranium in all the coalified wood specimens. These data implied only one uranium solution had infiltrated the different wood specimens" (Gentry, 1988, p. 57; emphasis added).

Furthermore, he does not demonstrate that all the halos developed at the same time. He contends that these secondary halos could not have (or were very unlikely to have) developed repeatedly in the wood samples from the different geological periods because an "extraordinarily complex, interrelated series of geological events" are necessary for radioactive uranium to infiltrate wood (1988, p. 56). These requirements are:

1) water, 2) uprooted trees as a source of the logs and smaller wood fragments, 3) a rich uranium concentration near the wood, and 4) a compression event occurring after the uranium solution invaded the wood, but prior to its becoming coalified.

[1988, p. 56]

(Most of the polonium halos are elliptical, presumably the result of compression of the wood after the formation of the halo.) Gentry's claim that polonium halos within geologically disparate coalified wood are unlikely to have developed repeatedly is based upon the assumption that, since the conditions for the formation of secondary polonium halos "appear" to be remarkably complex, the chances of these conditions occurring many times is unlikely and therefore it is more reasonable to assume that they happened only once. Of the four requirements, only the third—a source of uranium—would appear to limit the wide distribution of secondary polonium halos in fossilized wood. I find no compelling reason to believe that the samples of polonium—halo-bearing coalited wood could not have formed independently at different times. There are many "extraordinarily complex" systems and unlikely events which develop repeatedly in nature. An examination of one such phenomenon—the formation of fossil footprints—will demonstrate the fallacy of Gentry's claims and show that he has ignored, for whatever reasons, a large number of publications on trackways of the Colorado Plateau and their bearing on the length of time and the manner in which the plateau was formed. (Ironically, Gentry charges that it is evolutionary scientists who neglect to consider data supporting the flood origins of formations within the Colorado Plateau [1988, p. 531]).

Tetrapod fossil footprints in the Colorado Plateau

The Colorado Plateau, parts of which lie in Utah, New Mexico, and Arizona as well as Colorado (see Figure 1, p. 9), is a vast expanse of primarily sedimentary rock characterized by predominantly horizontal strata ranging in age from the Precambrian to the Cenozoic. This block of the earth's crust has remained remarkably undistorted throughout its history, whereas much of the crust around the plateau has been tilted, buckled, and broken (Chronic, 1983). Good exposure of the relatively undistorted strata has allowed paleontologists and geologists to collect and document the occurrence of tetrapod fossil footprints in rocks which span five geological periods, from the Pennsylvanian to the Cretaceous. No fewer than thirteen formations within the Colorado Plateau are now known to preserve a large and diverse footprint fauna (see Table 1).

It is a little-known fact that many extinct animals are known only from fossil footprints. Remarkable as this may seem, fossil tracks are much more common than the fossilized skeletal remains of extinct animals (Mossman and Sarjeant, 1983).

The discovery of a fossil trackway demonstrates that the prints were made by a living animal capable of walking across a firm terrain, parts of which were covered by fine-grained and plastic sediments of varying water saturations which could conform to the contours of the animal's foot. Furthermore, this sediment would have to have sufficient body to hold the impression of the foot as it was withdrawn (Cole et al., 1985), and if the sediment were to lithify, the concavity of the impression would provide unequivocal evidence as to whether or not the strata had been inverted. Because it is possible to identify major taxonomic groups—and sometimes extinct species—by the configuration of the lithified impressions (Mossman and Sarjeant, 1983), a study of fossilized prints provides information on the diversity of extinct faunas and how they changed through time (Olsen and Padian, 1986: Haubold, 1986; Olsen and Gallon. 1984). Fossil trackways also provide information on such things as stride length, pace and pace angulation, trackway breadth. foot length, distance between the pectoral and pelvic girdles (and therefore some indication of the size of the animal), number of toes (the contour of the print may allow one to determine the number of phalanges per toe, which can help in determining what type of animal left the impression). posture, direction and speed of movement, behavior (whether the animals traveled in herds or singly), and paleopathology (deformities in the foot; Lockley, 1986b). Sometimes even the texture of the integument (skin) is preserved in the sediment. Fossil trackways can also furnish information on former shoreline trends and depositional cycles (Lockley, 1986a, 1987; Lockley et al., 1983; Gillette and Thomas, 1983).

Discussion

In order for fossil footprints to form, a relatively solid surface must exist over which a living animal can walk; trackways are not made by the carcasses of drowned animals. As the Noachian flood was supposed to have killed all terrestrial four-footed critters except those in the ark, creationists must explain the presence of fossil trackways at many levels within the Colorado Plateau, as well as how they were made while the flood waters were raging. Fossil footprints do not form spontaneously from suspended sediments swirling within flood waters. It is difficult to imagine how delicate footprints could have formed—not to mention how these footprints survived flood waters of creationist proportions.

In order to preserve a recognizable footprint, the sediment over which an animal passes must be fine-grained. If the sediment is not sufficiently fine-grained, a small footprint will not stand out against irregular undulations or deformities in the medium: for example. footprints don't preserve well in coarse sand or gravel. both of which provide poor resolution. The preservation of a footprint is directly proportional to the size of the animal; large and deeply impressed prints are not easily destroyed (Mossman and Sarjeant, 1983). In addition to a fine-grained sediment. the plasticity of the medium is crucial.

A sediment with too little or too much water will not preserve a recognizable footprint should the medium lithify (Brand, 1979). Subsequent to the formation of the print, it must be covered with additional sediment in order to eliminate the effects of wind, rain, and flowing water which would erase any trace of the impression over time. If the sediment which covers the impression is nearly identical in its composition to that holding the print, then it is unlikely that the print will ever be discovered because the layers will not readily separate to expose the fossil footprint (Mossman and Sarjeant. 1983). Finally, it is essential that the footprint-bearing sediment lithify if the print is ever to survive Clearly the quality of a fossil footprint depends upon a number of delicately balanced and interrelated factors. The fact that footprints often vary in the quality of resolution even over a few strides attests to the fortuitous and often precarious interplay of conditions necessary for their preservation. I think most people would agree from their own experience that footprints are ephemeral at best, and the remarkable fact that so many fossil trackways are preserved in the Colorado Plateau certainly demands the attention of creationists.

It is also important to note that fossil footprint species are not randomly distributed throughout the plateau. (The skeletal remains of the fossil footprint-maker may never be found: nevertheless, a footprint kind or "species" name can still be assigned to fossil footprints that exhibit unique morphological features.) Moving vertically from the Pennsylvanian to the Cretaceous, changes occur in the fossil footprint fauna of each formation. Pennsylvanian and Permian formations of the plateau preserve trackways of generally small, four- and five-toed amphibians and early reptiles (Lull, 1918; Gilmore, 1926. 1927: Baird, 1965; McKee, 1982). The footprint fauna changes more conspicuously in the Triassic formations with the advent of a suite of reptile prints, including relatively small three-toed dinosaur footprints (Peabody, 1948; Welles, 1971). Once into the Jurassic formations, the footprint fauna changes again with the addition of new types of dinosaur and other footprints, both large and small (Baird, 1980; Stokes and Madsen, 1979; Lockley, 1986a, 1987). The Cretaceous footprint fauna changes little from that of the preceding Jurassic period in terms of its major taxonomic constituents, but changes in the configuration of the footprints (which mirror anatomical changes) indicate that different species were responsible for making the trackways.

The presence of fossil footprints in the Colorado Plateau provides unequivocal evidence for the sequence in which the formations which make up the plateau were deposited. (Incidentally, fossil footprints do not sort hydrodynamically; the smallest footprints are at the bottom of the plateau, while the largest are at the top.) Pennsylvanian tetrapod trackways (the lowest currently known tetrapod tracks within the rocks of the plateau) were made at a time when the sediments forming those rocks occupied the outermost surface on the earth. Logically, no other younger sediments could have covered Pennsylvanian age sediments or the trackways would never have been made.

After these Pennsylvanian tetrapod trackways were made, they were buried by sediment deposited by wind or a body of water (either fresh or marine) which covered the land. At a later date, the water receded, exposing the land over which terrestrial animals would once again walk. Then, repeating the cycle, the tracks were buried by wind-borne sediments or water. This same sequence of events occurred many times throughout the Pennsylvanian, Permian, Triassic. Jurassic, and Cretaceous periods, causing a thick sequence of sedimentary rock to accumulate. It is important to remember that thick layers of a variety of sedimentary rocks often separate succeeding trackway horizons; this indicates that these trackways (and the animals which made them) were separated in time by periods required for the intervening sediment (now rock) to be deposited. In other words, the trackways near the top of the plateau could not have been made before those near the, bottom, nor could any of the trackways have formed after all the sediments had been laid down. Unlike percolating groundwater and its constituent dissolved minerals (including radioactive uranium), footprints do not infiltrate sediments.

The presence of fossil footprints at different horizons within the Colorado Plateau proves that the conditions for the formation and preservation of footprints occurred many times. On the basis of their configuration, most trackways were made by animals moving along at a "relaxed" pace, not fleeing rising flood waters. The fine-grained nature of the sediment and the quality of preservation proclaim that calm environmental and meteorological conditions were prevailing, not those required for global catastrophe. It would not be unreasonable to assume that the extinct track-makers were surrounded by a thriving biological community, not a denuded landscape awash under millions of tons of sediment-laden flood water.

For these reasons, Gentry's claim that the formation of secondary polonium halos in samples of Triassic, Jurassic, and Tertiary (Eocene) coalified wood supports the hypothesis of the recent formation of the Colorado Plateau is refuted by the presence of footprint impressions at many levels within the plateau. Regardless of when and how the secondary polonium halos were formed, it would have been impossible for all of the footprints to have formed at the same time. Furthermore, fossil footprints are not subject to the same uncertainties that appear to plague the study of polonium halos, such as whether or not the ratio of uranium 238 to lead 206 has been altered by remobilization (uranium addition, lead removal, or both) through groundwater circulation after coalification, and whether or not the ratio is indicative of the formation time of the radiocenter. While the formation of secondary polonium halos remains contested and equivocal, fossil footprints and the trackways that they form are the actual record of an animal's activity at a specific location and time in the past. Gentry asserts that the sedimentary material that formed the rock of the Colorado plateau "could have been deposited both during the time when the waters were rising and again when they were receding" (1988, p. 53). Fossil footprints show this claim to be utter nonsense.

I have limited this discussion to tetrapod footprints of the Colorado Plateau because this is the area from which Gentry and his colleagues obtained their secondary polonium—halo-bearing coalified wood samples (Gentry et al., 1976). I should point out that there are many other types of ichnofossils in the Colorado Plateau that I have not considered, such as those made by burrowing invertebrates or scurrying arthropods. Of greater importance than this is the fact that ichnofossils are found worldwide (for a very small sample of the available literature, see Hitchcock, 1858; Frey, 1975: Hantzschel, 1975; Gillette, 1986).

In summary, secondary polonium halos in coalified wood samples from the Colorado Plateau must have developed repeatedly because fossil footprints, which are unequivocal in the nature of their origin, occur at many levels within the plateau. These multiple footprint horizons formed repeatedly over great periods of time whenever conditions favorable for their formation and preservation prevailed. Therefore, these fossil footprints (and all trace fossils) refute Gentry's claim that Triassic, Jurassic, and Eocene rocks of the Colorado Plateau "all originated at about the same time, in agreement with the flood-related scenario" (Gentry, 1988, p. 55). If the bulk of the Colorado Plateau was formed during the worldwide Noachian flood, then lithified trackways would occur—if at all—only in the lowermost levels of the plateau. The fact that this is not the case proves that the sedimentary rocks of the plateau arc not the result of a single global flood. This is but one example of how these sometimes neglected fossils can be used to refute the spurious claims of a young earth and highlight the egregious incompetence of some creationists. Gentry is welcome to believe that "the Genesis record of creation and the flood is the master key which unlocks all of Earth's geologic history" (1988, p. 185), but this belief is not supported by the evidence and therefore should not be taught as science.

Acknowledgements

I would like to thank B. Redifer for bringing the work of Robert Gentry to my attention. The quotes in this article were excerpted from Creation's Tiny Mystery by permission of the author. Furthermore, I am indebted to C. Hall and L. Johnson of the Tyrrell Museum of Palaeontology for providing literature on tetrapod trackways of the Colorado Plateau.