Science: How Land Animals Have Changed Through Geologic Time
Few groups of organisms have changed as dramatically through geologic time as have backboned animals that live on land. In this article we will examine changes reflected in fossils in the strata of the Colorado Plateau.
In the Colorado Plateau, the strata clearly lie on top of one another. There is no question of relative age. Bottom strata were deposited first; they are older than those on top. Second, these strata contain abundant evidence for the passage of time during deposition and for time lapse between the deposition of various strata. Third, the range of the geologic record in this area is outstanding. Each of the four major geologic periods is represented by strata miles long.
By limiting our examination to one geographical area, we limit the complexity, confusion and the uncertainty of time correlation that is often introduced when jumping from one geographic area to another. Staying within the Colorado Plateau makes the logic of the geologic record simpler and easier to understand. Once we prove the sequence of the fossils here, we can examine other geographic areas that contain better fossil records of select groups of animals.
Previous articles in this series have covered the topics of geographic location, strata sequence, depositional history and evidence for long time periods. The previous article of the series explored the changes observed in a few marine shellfish, land plants and freshwater fish.
Fossil land animals
As we examine changes in the land animals, we find patterns similar to those found in other fossil organisms. Most have become extinct. Some have been changed with time. Some have become extremely specialized. A few seem to have degenerated. And, as always, there are “living fossils” that have remained relatively unchanged for long periods of time. (Examples of these among land animals are turtles and crocodiles.) Perhaps the most prevalent pattern is that new forms show up continually as we progress up through the stratigraphic sequence, and older forms simply disappear from the record.
In other parts of the world, some land-living vertebrates are found in strata deposited about the same time as the Temple Butte Formation. In the Colorado Plateau, however, no land-animal fossils are found at this level. The strata being deposited then were primarily marine strata, so we would not expect to find remains of land animals.
The first continental strata (as contrasted to strata deposited in marine environments) after the Temple Butte are the Supai strata. Many land-animal tracks have been found in those strata, but so far no remains of the actual animals have been found. This is also true for the overlying Hermit and Coconino strata.
Amphibians and reptiles
The earliest known fossils of actual animals in the Colorado Plateau are in the Moenkopi Formation. The animals of the Moenkopi are reptiles and amphibians that have been extinct for a long time. Few of the 10 or so forms found there are common enough that you would expect to find them on display in a museum. One of these amphibians, however, was relatively common — Metoposaurus. It is unlike anything alive on earth today. Some of these extinct amphibians reached more than 10 feet in length. Definitely not your typical modern toad, frog or salamander! These large amphibians soon became extinct as early dinosaurs appeared.
The next formation up, the Chinle, contains many fossil land animals. Here are found a substantial number of fossil amphibians and reptiles that are much better known than those in the Moenkopi. In the Chinle we find the first dinosaur, Coelophysis. Even on a worldwide basis, it is among the earliest of the dinosaurs. This dinosaur has a simple design and, relative to later dinosaurs, is small — only 8 feet long from nose to tip of tail.
Living with the first dinosaurs was an alligator-like animal called a phytosaur. Superficially, it resembles an alligator or crocodile, but it is neither. The latter breathe through nostrils on the tip of their snouts. Phytosaurs breathed through a hole in the top of their skulls, between and a little in front of their eyes.
Two other reptiles are also found in the Chinle — Desmatosuchus and Placerias. Both are extinct, and quite peculiar. Metoposaurus, which we encountered in the Moenkopi Formation, continued to live in Chinle times.
Many of the formations between the Chinle and the Morrison contain some evidence for dinosaurs, particularly footprints. However, they are poor in actual skeletal material. Evidence of early crocodile fossils is reported from the Moenave. Of course, several of the formations were deposited in marine environments, so we would not expect to find land-animal fossils in them.
The Morrison Formation is an entirely different situation. It is a widely distributed formation composed of land-deposited sediments. Named for the small town of Morrison, Colorado, outside of Denver, it stretches from Montana to New Mexico. Extensive quarries of dinosaur bones occur in the Morrison Formation in Wyoming, Utah and Colorado. The variety is extensive.
The last dinosaur fossils in our strata are in the lower portion of the North Horn Formation. They are considerably different from those found in the Morrison Formation.
Mammals
The Morrison also contains some of the first evidence for fossil mammals. Full skeletons have not been found; only jaws and teeth reveal their existence. They were probably about the size of mice and rats.
In the upper portion of the North Horn are mammals that are larger than and far different from those found in the Morrison, but they would still be unrecognizable in terms of modern living mammals.
The next animals I will mention are from formations that are stratigraphically equivalent to the Colton Formation, though they are not specifically from the Colton. The first is Hyracotherium, more commonly known as “Eohippus.” This is the famous early “horse” with four hoofed toes on the front feet and three on the rear feet.
From the same time period is Phenacodus. This mammal superficially looks like a dog about the size of a collie. But it is not a dog — its teeth are much like those of “eohippus” and vastly different from those of a modern dog. It had five toes on each foot. Each toe had a small “hoof” on the end, not a claw like a dog.
Living with these two mammals was Coryphodon, a hippo-shaped herbivore with large dagger-like canine teeth. All three of these animals are found as fossils in strata of the same age in Europe as well as in other parts of North America. None of the three are like any living animal.
Green River
The next formation, the Green River, is primarily lake deposits. Fish fossils are extremely abundant in places. There are crocodiles and turtles, “living fossils” that lived with the dinosaurs and continue with little change even today. Fossil snakes are found here.
A few land animals are found as fossils, but they are not abundant. A notable one is the giant flightless bird Diatryma. It was nearly 7 feet tall! Its massive build and beak reminds one of a dinosaur. Similar fossil birds are found much later in the fossil record of South America.
Perhaps the most notable fossils from this formation are early primates much like modern lemurs (Notharctus) and tarsiers (Tetonius).
A variety — about 100 kinds — of early mammals are found in the Uinta Formation and the Duchesne River Formation. There is an “advanced” form of “eohippus” called Epihippus. It is quite similar to “eohippus,” with the major difference being changes in the teeth.
Rhino-sized beasts
Some fossils in the Uinta and Duchesne are similar to rhinos. One of the common larger animals is Dolichorhinus, a mammal from the Uinta Formation that belongs to an important and common group of this time period known as the titanotheres. There were considerable changes in this group from about the time of the Colton Formation until after the deposition of the Duchesne River. By the time of the deposition of the Brown’s Park, they were all extinct. A massive two-volume work on the group was completed by a leading American paleontologist early in this century.
The end of the titanothere line, a form known as Brontops, is found in Wyoming and South Dakota in strata deposited after the Duchesne River and before the Brown’s Park. Brontops is massive — 12 feet long and 8 feet tall!
Uintatherium, another large and common herbivore of the Uinta Formation, is also a giant and near the end of its line. Its skull is grotesque, covered with large bony bosses, and possessing huge saber-like canine teeth. The end of this group came a little later with a still larger form known asEobasileus. It stood 7-8 feet tall and was built like a rhinoceros.
Brown’s Park oddities
It should be emphasized that a significant time gap occurs between the deposition of the Duchesne River and the Brown’s Park. There is a major difference in the type of mammals. Though the record of the changes that occurred in mammal types is missing here, it is recorded in strata in other areas of the western United States.
The fossils of the Brown’s Park are sparse, and the ones that are found are considerably different from the earlier mammals. Three forms are quite interesting. Gomphotherium is one of the earliest members of the “elephant family” that is found in North America. It has four tusks, and its lower jaw not only has tusks, but the jaw is also much longer than that of an elephant. Its teeth are of a completely different design from those of the elephant. Its teeth are more like those of the later mastodons, yet still considerably different from theirs.
A curious creature found in the Brown’s Park is Moropus, a mammal the size of a modern horse and superficially resembling a horse. However, instead of hooves it has toes with large claws on them. Early fossil hunters thought that the claws came from an entirely different animal until they found undisputable evidence that the two features belonged together.
A more familiar mammal is also found in the Brown’s Park, the true rhino, Aphelops.
More modern mammals
After the deposition of the Brown’s Park, there is another very long time break with no strata being deposited or fossils preserved to record what was happening. The area was uplifted, and strata were, for the most part, being eroded away rather than being deposited. Later, during the time of the Ice Age, small deposits occur at various places, for example, in swampy areas around springs, in caves, in gravel pits, on river terraces and flood plains.
The fossils found in these places are a mixture of modern and recently extinct types. From the Colorado Plateau these include: camel (Camelops),Bison, mammoth (Mammuthus), ground sloth (Nothrotherium) and the modern horse (Equus). These are the more common animals, but they are only a sampling of the many mammals that are found during this time period.
Summary
The dominant land animals change as we progress through a long period of geologic time. The story starts with early amphibians and reptiles, then progresses through an extensive period when dinosaurs dominated the land fauna. The dinosaurs were not unchanging, however. They changed dramatically as time progressed. The variety is mind-boggling. It is in strata deposited during the dinosaur period that the first mammal fossils have been found.
Following dinosaur times was a long stretch of time in which mammals dominated and were gradually developed. At first the mammals were totally unlike any modern forms. As time progressed, more and more similarities to modern mammals show up in the fossils. Groups appear and are developed. Some of these groups continue today, but many have long since become extinct. By the time of the Ice Age, the mammals are essentially modern in design, with only a few strange forms still around.
The view this article gives is, of necessity, only a general one. It should, however, be sufficient to make one realize that there is much history to take into account before making conclusions about what has happened on earth in the past. The story is considerably more complex than may have been imagined or than is pictured in many “creationist” reconstructions.
In the next article, I plan to present some possible reasons for the sequence of changes observed in the fossil record. The facts presented in these articles should greatly expand our understanding of God’s creation process. This understanding should give us a much deeper appreciation for the design, patience and work involved in the development of this marvelous world we live in.
Author: Richard Burky