Origins 9(2):67-74 (1982).
Related page — |
IN A FEW WORDS |
Fossilized footprints are a record of animal activities in the remote past. They can provide some types of information that fossil bones do not provide.
Fossil skeletons contain a wealth of information about animals that
have become extinct long ago. If most of he skeleton is available, the general structure
and appearance of the animal can be reconstructed with reasonable accuracy. From the
characteristics of the teeth we can deduce something about the food habits of the animal.
Bones and teeth can even provide much information beyond that, but can we be sure that the
animal lived and died at the place where we found its fossil bones? In many cases the
answer is no. When an animal dies its bones may be scattered by scavengers. If it dies
near water it may be washed far downstream, or it may float in a body of water for some
time before it sinks and is buried (Behrensmeyer and Hill 1980; Schafer 1972). After it is
buried it may still not be able to rest in peace. At some later time the sediments in
which it is buried may be eroded out, and the bones and sediments may be washed down to
another basin and deposited again.
An animal's footprints in the mud cannot go through any of these
processes and still be preserved. Consequently when we find a fossil animal footprint we
know that an animal walked on that very spot at some time in the distant past. This is one
of the characteristics of fossil footprints that makes them helpful in analyzing the
history of life on earth. The study of fossil footprints and other fossilized evidence of
animal behavior is called ichnology, and each type of footprint, believed to represent the
tracks of one species of animal, is given a genus and species name and is called an
ichnospecies.
METHODS
We quantitatively analyzed the stratigraphic distribution of fossil
vertebrate footprints as revealed by data from about 800 published papers and from
additional specimens in the American Museum, U.S. National Museum, Yale University Peabody
Museum, and the Raymond Alf Museum. Much of the published footprint literature dates from
the 19th and early 20th century, and contains considerable duplication — several
different workers giving different names to the same type of fossil tracks. As far as
possible we avoided this excess of ichnospecies for reptiles and amphibians by following
the taxonomy of Haubold's (1971) taxonomic revision, supplemented by more recent papers.
There is a large degree of uncertainty concerning the number of species
of birds and mammals represented by fossil footprints, because of the difficulty of
accurate identification and the lack of any comprehensive taxonomic study. Consequently in
our graphs of bird and mammal footprint diversity we indicate maximum and minimum
diversity that can be derived from the literature plus observed museum specimens.
In addition to the factors noted above, there are likely to be many
undescribed specimens scattered in various museums that we have not seen. However, the
sample reported here seems to be adequate to demonstrate reliable trends in distribution
and diversity. The patterns illustrated in this paper were evident early in the data
collection process, and further literature and museum study increased the numbers of taxa,
but did not significantly change the relative trends.
We compared the diversity of fossil footprints with diversity of body
fossils (bones and teeth) at successive stratigraphic levels. The body fossil data are
from Harland (1967).
COMPARATIVE DIVERSITY OF FOOTPRINTS AND BODY FOSSILS
Overall distribution of bird and mammal footprints correlates well with the distribution of bird and mammal body fossils (Figure 1). Mammal footprints and bones are both most abundant in upper Tertiary deposits. The mammal footprint fauna includes one small mammal taxon in the Jurassic and one in the Miocene or Pliocene. The other mammal tracks are predominantly carnivores, ungulates (hoofed animals), and elephants. Bird footprints are never very diverse, and they are nearly limited to Tertiary deposits. The bird footprint fauna is dominated by shore bird types and large ground-dwelling birds. However, many are just identified in the literature as "bird tracks."
FIGURE 1. Comparison of diversity of fossil footprints and body fossils of birds and mammals. Footprint diversity is given in number of ichnospecies. Various taxonomic levels are represented in the body fossil data (from Harland 1967), with families being most common. Maximum and minimum figures for the number of footprint taxa indicate the range of uncertainty resulting from the incomplete taxonomic information for many of these fossils.
In contrast to the bird and mammal footprints, the stratigraphic distribution pattern of amphibian and reptile footprints is very different from the distribution of body fossils (Figure 2). Amphibian footprints are rare after the early Permian, and reptile footprints, with the exception of dinosaur footprints, which are the most diverse and conspicuous group of tracks, are most abundant in late Triassic and early Jurassic rocks (Figure 2). The only Cretaceous reptile footprints identified in the literature are about a dozen types of large dinosaur footprints. In contrast, amphibian and reptile body fossil diversity is greatest in the Cretaceous and Tertiary, when corresponding footprints are rare or nonexistent.
FIGURE 2. Comparison of diversity of fossil footprints and body fossils of amphibians, all reptiles, dinosaurs, and (*) reptiles exclusive of dinosaurs, and of indeterminate tetrapod footprint taxa. Footprint diversity is given in number of ichnospecies. Various taxonomic levels are represented in the body fossil data (from Harland 1967), with families being most common.
CONCLUSIONS
If the geologic column represents sediments that have accumulated
over many millions of years, and the fossils from each geologic period are the remains of
animals living in successive time periods, it would be reasonable to expect that the
stratigraphic patterns of footprint diversity should roughly parallel the patterns of
equivalent body fossil diversity — the periods with the most kinds of dinosaur bones
should have the most kinds of dinosaur tracks, for example. The bird and mammal fossil
record fits that expectation quite well, but the reptile and amphibian record definitely
does not. We will discuss two approaches to explaining this discrepancy.
The first approach assumes that much of the geologic column was
deposited during a global flood. This model suggests that during the early to middle part
of the flood large numbers of amphibians and reptiles were moving about, and thus
producing footprints. Later during the flood (upper Jurassic and Cretaceous) there were
very few live amphibians or reptiles to produce footprints, except for the large
dinosaurs. During the Cretaceous when the only footprints preserved were the large
dinosaur tracks, there were many amphibian and reptile bodies that were being buried to
produce the abundant Cretaceous body fossils. During the Cenozoic almost no amphibian or
reptile footprints were preserved.
This flood model suggests that during the flood the birds and mammals
were in the uplands, away from the depositional basins, because of ecological differences
and/or their more adaptable behavioral response to the unusual biological crisis caused by
the flood. Consequently they left almost no footprints. This model further suggests that
the upper Tertiary footprints were formed after the flood when geological processes were
more like those observed today.
If this flood model is correct, and the birds and mammals were living
contemporaneous with the Paleozoic reptiles, why aren't there at least a few bird or
mammal tracks in Paleozoic sediments? It appears that there may be a few. In carboniferous
deposits in Nova Scotia tracks were found that "superficially ... resemble the tracks
of some of the wading birds, but of course there is little probability of their having
been made by birds" (Sternberg 1933) (Figure 3). If these tracks had been found in
Cenozoic deposits it seems likely that they would have been described as bird tracks.
Another interesting track was found in the Permian Hermit Shale of the Grand Canyon
(Gilmore 1927). It looks precisely like a bird track, but since birds are not thought to
have evolved until the Mesozoic, this Permian track is just listed as an
"unidentified track" (Figure 4).
FIGURE 3. Unidentified, bird-like fossil footprint from Paleozoic strata. A trackway from the Carboniferous of Nova Scotia (after Sternberg 1933).
FIGURE 4. Unidentified, bird-like fossil footprints from Paleozoic strata. A track from the Permian Hermit Shale of the Grand Canyon (after Gilmore 1927).
Another question that arises is why reptile and amphibian footprints
are so abundant in Paleozoic and lower Mesozoic sediments and so rare in recent sediments,
since reptiles and amphibians are common today. Part of the answer could be that there
were many types of reptiles and amphibians living earlier in earth history that no longer
exist. We would suggest that another part of the answer is that the rapid sedimentary
processes during the global flood were usually conducive to the burial and preservation of
footprints before they were destroyed by weathering processes. Thus during the flood there
would have been a great diversity of reptiles and amphibians making tracks under
conditions uniquely suitable for preserving those tracks. In more recent times, with most
of the Mesozoic track-makers extinct and conditions not as suitable for preserving
footprints, reptile and amphibian footprints seem to be seldom preserved.
The second approach to explaining these data is based on the
conventional geologic model of sedimentation over long ages of time, coincident with the
evolution of animal life. This model must assume that the stratigraphic distribution of
amphibian and reptile fossil footprints is an artifact, and does not at all reflect the
actual relative amount of animal activity during different geologic periods. Several
factors have been suggested as potential contributors to producing this artifact:
1) In Mesozoic and Tertiary deposits containing larger, more
conspicuous tracks, smaller tracks may be more likely to be overlooked, and not collected.
This argument is weakened by the fact that small reptile tracks are abundant in Triassic
and Early Jurassic rocks, when dinosaur tracks are also at their peak of abundance. It
also does not explain the near absence of amphibian and reptile tracks in Early Tertiary
deposits, which have no dinosaur tracks and few bird and mammal tracks to divert attention
from the smaller tracks.
2) Smaller, shallower tracks would be destroyed more easily by
weathering or by slumping of waterlogged sediment than the larger tracks. However, this
factor should have affected the entire geologic record, not just the post-Jurassic
deposits.
3) Older rocks tend to be well-indurated (firmly cemented), while
younger rocks are more likely to be relatively unconsolidated, and thus less suited to
preserve footprints. However, many of the Cenozoic footprints are quite well-preserved,
including delicate bird tracks. Also this factor does not explain the sharp drop in
footprint diversity after the lower Jurassic.
These factors may play a role in biasing the published footprint
record, but they do not seem adequate to explain the sharp contrast between the abundant
footprints of amphibians and small reptiles in Permian to Lower Jurassic rocks, and their
near absence in younger rocks.
The only explanation that this model provides for the bird-like
Paleozoic tracks is that they must have been made by unknown types of Paleozoic reptiles
with bird-like feet.
We conclude that these data are most easily explained by the global
flood model. Does this mean that they should be taken as proof of the flood? No, it does
not mean that. Science rarely makes its decisions on something that could be called proof,
but it makes its decisions on the perceived weight of evidence. This is particularly true
when studying unobservable events from the past history of our earth. Our footprints can
neither prove nor disprove the flood model. The question is, into which model do the data
make the most natural fit? It appears that the footprint data make a very natural fit with
a global flood model.
REFERENCES
Front Cover: Numerous trackways from the Permian De Chelly Sandstone near Monument Valley, Arizona. See Brand-Florence article on p. 67.
Back Cover: Close-up of amphibian or reptile trackway from the De Chelly Sandstone.
All contents copyright
Geoscience Research Institute. All rights reserved.
| Home
| About Us
| Contact Us
|
Send comments and questions to
webmaster@grisda.org
| What's New
| Resources
| Search
| Links
|
