Geological Survey Professional Paper 669 The Colorado River Region and John Wesley Powell

Paleontology And Paleoecology

The problem of why traces of life are so uncommon in rocks of late Precambian age, especially when fossils are abundant and well developed in the Cambrian strata directly above, has intrigued geologists since the early days of geologic science. Attempts to discover recognizable plants or animals in these rocks have been many, yet results have been meager and mostly inconclusive. In the Grand Canyon, the normal difficulty of travel and of access to the upper Precambrian beds has been a deterrent to collecting material, but aside from this fact, study has been difficult because traces of life are both rare and obscure.

Presence of a fragmentary fauna in the Chuar Group of late Precambrian age was announced by Walcott in 1883 (p. 441) and in 1886 (p. 43); the evidence was later summed up (Walcott, 1895, p. 327) as "a minute discinoid or patelloid shell, a small Lingula-like shell (which may be a species of Hyolithes) and a fragment * * * of a trilobite * * *" The small discinoid shell was believed phosphatic and was subsequently as signed to a new genus and species, named by Walcott (1899, p. 234) Chuaria circularis. Still later, Walcott's Chuaria was discussed by White (1928, p. 599), who states that it "may be of plant origin, and can hardly be a bivalve." Additional specimens of this form have recently been found in the Chuar by Trevor Ford and W. J. Breed (unpub. data), who discuss the many uncertainties in classifying it and the differences of opinion on its interpretation. Meanwhile, the other questionable organic forms referred to by Walcott (1899, p. 235) have been largely discredited for various reasons.

Since Walcott's announcement, a few other reports of fossil animals from the Grand Canyon Precambrian have been placed on record. An impression in sandstone of the Nankoweap Group, attributed to a jellyfish medusa, was reported by Van Gundy (1937a, p. 314; 1937b, p. 304), discussed by Hinds (1938, p. 186), and later named and described by Bassler (1941, p. 522, pl. 64), who, however, stated that he "has become less certain of his first opinion as to the animal nature of the specimen * * *." Other "jellyfish-like impressions" have been reported by Alf (1959, p. 62) from the Unkar Group. The validity of the various claims is still in question, however, and many scientists remain skeptical. (See the opinion of G. Stiasny in Bassler (1941, p. 521).)

The evidence for a clearly defined flora in the form of stromatolites in the upper Precambrian rocks of Grand Canyon is much more definite than is the evidence for a fauna. Reference to "a stromatopora-like form [in the Chuar Group] that is probably organic" was made by Walcott in 1895 (p. 327). The structure of that form was regarded as possibly corresponding to that of Cryptozoon, and the form was referred to Cryptozoon? occidentale by J. W. Dawson (in Walcott, 1899, p. 233). When the algal Collenia was created (Walcott, 1914, p. 110), the form was placed in that genus. Recent studies by Trevor Ford, University of Leicester, England, and W. J. Breed, Museum of Northern Arizona, Flagstaff, Ariz., have shown that stromatolites, including those reported by Walcott, occur at three widely separated horizons in the Chuar Group (unpub. data).

In the Bass Limestone of the Unkar Group, near the base of the upper Precambrian sequence in Grand Canyon, four distinct forms of organic origin, including Collenia, and two or three others of problematic type, were collected and reported by White (1928, p. 598), but unfortunately they were never fully described. Studies of the Bass Limestone and its algal deposits are currently being undertaken by Michael O'Connor, Department of Geology, East Carolina University, Greenville, N.C.

The fauna of the Cambrian rocks of Grand Canyon has become well known through the work of C. E. Resser (in McKee and Resser, 1945, p. 171). He made an attempt to describe all Cambrian fossils that had been collected from the canyon up to 1945; so far as known, no additional species have been collected and described since that date. These fossils represent 76 localities, extending from the foot of Marble Canyon on the east to the Grand Wash Cliffs, more than 100 miles to the west. They represent many zones within the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone.

The Cambrian fauna of Grand Canyon consists of 47 species of trilobites, which constitute the most conspicuous and abundant element, 16 species of brachiopods, and several species of gastropods, mostly Hyolithes. Also included are numerous species of Conchostraca, sponge spicules, an Focrinus, worms (fig. 20F) and algal colonies of the form known as Girvanella. Distribution of trilobites clearly was controlled by facies; those in green shales are different from those in limestone. The Conchostraca are confined to red, ferruginous sandstones and most but not all species of brachiopods are in the medium- to coarse-grained, near-shore sandstones.

The fauna of the Devonian Temple Butte Limestone of Grand Canyon is poorly known; consequently, uncertainty exists about the environment of deposition of this formation. The armored placoderm known as Bothriolepis, found near the base of the formation both at Kanab Canyon (Walcott, 1883, p. 438) and Sapphire Canyon (Noble, 1922, p. 52), is considered to be a fresh water form. However, an assemblage of "Cyanthophylloid corals, casts of brachiopods, and gastropods," including some forms typical of a marine environment, has also been reported from the Devonian strata of Kanab Canyon (Walcott, 1883, p. 438). Probably on this basis, the suggestion is made by Denison (1951, p. 257) that "this is a near-shore facies, deposited in the advancing Chemung sea * * * the only fishes reported * * * must have been transported from a nearby land." Until additional diagnostic fossils are found, further speculation along these lines seems futile, but a rugged relief of as much as 100 feet at the formation base gives cause for question concerning a marine origin.

Knowledge of the faunas of the Mississippian Redwall Limestone has greatly increased during the past two decades. From one of the least known faunas in the formations of Grand Canyon, it has become perhaps the best known. This increased knowledge is primarily the result of a team effort in which eight paleontologists made detailed studies in their specialties, each contributing a chapter to a monograph on the Redwall Limestone (McKee and Gutschick, 1969). In addition, several other paleontologists examined fossils representative of minor groups and contributed data to the general report. Both stratigraphic and ecologic aspects of the various assemblages have been considered by these investigators.

Brachiopods, corals, foraminifers, and crinoids are the most common fossils in the Redwall Limestone; gastropods, cephalopods, pelecypods, bryozoans, and blastoids are fairly common; trilobites, fish, ostracods, holothurians, and algae are also represented. Altogether, 17 groups of animals and one group of plants have been recorded from 500 stations. The nine fossil groups that represent the greater part of the entire fauna are distributed in distinctive combinations or associations within members of the formation. Typical associations are coral-brachiopod-crinoid, brachiopod-bryozoan, and coral-foraminifer-brachiopod.

Important data on the age and correlation of Redwall subdivisions have been furnished by studies concerning vertical zoning of the corals (W. J. Sando, in McKee and Gutschick, 1969) and studies of foraminifer zonation (Betty Skipp, in McKee and Gutschick, 1969). Much information on ecological and geographical features of the Redwall has likewise been provided by these studies. Interpretation of environmental features has been greatly furthered by studies of the mollusks by E. L. Yochelson, bryozoans by Helen Duncan, blastoids by D. B. Macurda, crinoids by J. C. Brower, and cephalopods by W. M. Furnish (in McKee and Gutschick, 1969). Data on the nature of bottom sediment, the turbulence of the water (energy factor), the turbidity, and the depth of water have been obtained from some of these faunal studies.

Scarcity of fossils in the red-bed sequence composed of the Supai Formation and the Hermit Shale has made practical subdivision of these rocks difficult. A moderately large flora from the Hermit Shale, consisting of 35 species of terrestrial plants, has been studied and described by David White (1929); these plants clearly indicate an Early Permian age. Only a few poorly preserved plants have been reported from the Supai, and these furnish little evidence of age. Other fossils in the red beds are a few trackways of vertebrate animals from both the Supai Formation and the Hermit Shale, two wing impressions of insects from the Hermit (Carpenter, 1927; 1928), and numerous deposits formed from algal growth, mostly in the Supai (White, 1927, p. 369). In some of the algal deposits plant forms and structures have been retained.

General time relations based on zones of marine fossils can be projected laterally in the walls of Grand Canyon, because red beds of the Supai intertongue westward with carbonate rocks commonly referred to the Pakoon Limestone of McNair (1951) and the Callville Limestone. Brachiopods from the lowest unit of the Supai throughout the western half of Grand Canyon establish the age of the basal Supai as Early Pennsylvanian (Morrow); Fusulinids, corals, and brachiopods from the Pakoon or uppermost limestone establish its age as Early Permian or Wolfcamp. Between these units of Morrow and Wolfcamp age, fossils are relatively rare and age determinations correspondingly less certain, but on the basis of both brachiopods and small foraminifers, the strata probably represent Des Moines or Virgil age or both. Determinations of these fossils have been made by R. C. Douglass and L. G. Henbest (foraminifers), R. E. Grant (brachiopods), and W. J. Sando (corals), of the U.S. Geological Survey.

The fauna of the Coconino Sandstone consists exclusively of tracks and trails; no skeletal parts or other organic remains have yet been found. On the basis of these footprints and trails, 17 genera and 22 species of animals have been described by Gilmore (1926, 1927, 1928). The tracks are considered to be mostly reptilian, and they represent a wide variety of quadrupedal forms. Some animals apparently were the size and shape of small lizards, others had large feet and long strides, and still others were short limbed and wide bodied. In addition, a few burrows of wormlike creatures and trails of invertebrates, probably insects, are locally present. The tracks and trails are clearly impressed; most footprints show toes, claws, and heel marks on the long, steeply-dipping surfaces of cross strata of which the Coconino is composed.

General composition of the marine faunas of the Kaibab Limestone and, to a lesser extent, the Toroweap Formation has been known a long time. Some of the common forms were collected and described by geologists of the earliest expeditions (Marcou, 1858; Newberry, 1861). In following decades, extensive lists of fossil determinations were prepared, most notably by G. H. Girty between 1910 and 1930, and published in the reports of many geologists. A summary of the paleontology and descriptions of the brachiopods of these formations was prepared by McKee in 1938 (pt. 2). Since then many additional forms, representing most of the faunal groups, have been reported and in some cases described; these additions have been listed and discussed by McKee and Breed (1969).

As currently recognized, the fauna of the Kaibab and Toroweap includes 7 genera of fish; 10, of crustaceans; 34, of brachiopods; 16, of bryozoans; 9, of corals; 34, of gastropods; 35, of pelecypods; and 13, of cephalopods. In addition, the Annelida, Porifera (fig. 20E), scaphopods, echinoids, and crinoids are represented. The distribution of most of these fossils was controlled by facies. The nature of this distribution was not recognized in the early days when the so-called Productus fauna and the Bellerophon fauna were considered indicators of different ages. Later, it was demonstrated through field relations that most of the brachiopods (Productus fauna) were restricted to relatively pure limestones and represented normal marine environments, whereas pelecypods and gastropods (Bellerophon fauna) were dominant in the near-shore sandstones and the magnesian limestones of contemporaneous age and represented brackish and (or) super-saline environments (McKee, 1938, p. 133).

Studies of the physical factors that have probably been responsible for the faunal distribution patterns in both the Kaibab and Toroweap formations have been made by a number of geologists (McKee, 1938, p. 134-142; Nicol, 1944, 1965; Chronic, 1952). The principal factors considered were bottom character, temperature, depth, marine currents, salinity, amount of light, and turbidity. For some of these factors, direct evidence is recorded in the rock, but for others, inferences had to be made from various types of evidence.