Petrified Forest National Park: Plant Fossils

Arizona's Petrified Forest is known around the world for its spectacular display of countless great logs of semi-precious stone. These fossil trees are truly remarkable both from the standpoint of the variety of quartzfamily minerals included, and the infinite number of superlative "gems-in-the-rough" that assail the visitor's eye. Surely Petrified Forest National Park owes its existence and its world renown exclusively to these fossil giants of a bygone flora of 180 million years ago; and, just as surely, 99 out of 100 visitors to the park "fail to see the forest for the trees," seeing only fossils that represent a single species typified by preservation in vividly colored agate and jasper.

In addition to this most plentiful and spectacular species of tree fossil (Araucarioxylon arizonicum), two other trees of the Triassic forest of this area are common enough to be widely known and easily distinguished: Woodworthia arizonicum, and Schilderia adamanica. If this were the end of our story, our picture of this ancient forest would be but a vague and hazy scene. Fortunately, we have a great many more bits and pieces to add detail and interest to the picture, for at least eight other species of fossil woods of "tree size" plants have been found in the Chinle Formation of this vicinity. In most of these cases, our knowledge is based on but a single specimen; only a fragment of a single plant.

Such are the vagaries of plant fossilization that while some plant tissues may be abundantly preserved in their original and most minute detail, the wood of another plant may be incapable of fossilization except under a set of rarely-met-with ideal conditions. Hence it is unsafe to assume that our forest in life was made up of plants in the same ratio or proportion as is represented by the surviving fossils. Some of the rare species of fossil woods might well have been very common plants of the era.

While we may presume that other large plants that were common to the Triassic forest remain yet undiscovered, we must also assume that some quite common forms failed utterly to attain preservation in the fossil record.

For these reasons, we might compare our efforts toward a reconstruction of a Triassic forest scene to the process of assembling a tremendous jig-saw puzzle picture with the foreknowledge that a great many of the pieces have been lost beyond recall.

Although the wood fossils of which we have been speaking are the most enduring and easiest to be found among our jig-saw puzzle pieces, we have already seen how few they are in number, at best providing us with a sketchy outline to our picture. For interesting and beautiful details to add to our scene, we must look to a host of other sorts of fossils.

Undoubtedly, our richest source of information on just such details lies in the almost limitless quantities of very fine-grained shales in which are to be found beautifully preserved leaves, twigs, flowers and seeds, as well as microscopic grains and spores of ferns and fungi, each capable of precise identification because of the perfection of its preserved detail.

The paleobotanist, or student of fossil plants, like his colleagues in the other sciences, is a very cautious or conservative individual. In fact, leading paleobotanists throughout the world have adopted a set of international rules which require adherence to caution and against “jumping to conclusions” in the matter of naming plant fossils. As the various fossilized parts of a given plant are discovered, each will be described in detail and each is given a separate name. Thus it often happens that the leaves, wood, flowering parts, pollen spores and roots of a single plant may long be known under as many different scientific names until, at last, incontrovertible proofs are at hand by which to demonstrate that the many parts are, indeed, parts of a single species of plant.

While this seemingly confusing state of affairs does have its drawbacks, it makes the search for “proof of connection” between plant parts one of the exciting and fascinating, albeit time-consuming, parts of our project of restoring the Triassic forest scene. For instance, we presently have well over a score of species of fossil leaves, but in only a case or two do we have any indication of what leaf belongs to a certain plant stem!

At this juncture, let’s go back once again to consider-ation of the several species of tree fossils to see what sort of effort necessarily must precede our ability to "easily distinguish" one species from another. Despite the fact that our chosen fossil may be a log, perhaps one hundred and fifty feet in length, with a diameter of eight or ten feet, its precise identity is locked in microscopic details of growth habit and cell structure that require magnification often as great as 250 power for adequate study.

Since the important characteristics of cellular structure are oriented differently within the wood tissue, it will be necessary to prepare small specimens of our fossil so that no less than three different sections or planes may be studied under the microscope. These essential sections are: 1) a transverse section or cross section; 2) a radial section whose plane is along the grain of the wood and along a radius from the center to the outside; and 3) a tangential section which forms a plane at right angles to both the transverse section and the radial section.

Preparation of these sections is, in itself, a considerable undertaking, for our fossil of agate is appreciably harder than glass or common steel. We start by sawing a thin slice, perhaps 1/16 inch thick, from each of the needed planes. The sawing will be done on a diamond saw, a thin steel blade charged or studded about its perimeter with minute bits of crushed diamond. Our specimens need be only about the size of a postage stamp in area. The next step is to grind each of the three sections down, to a thickness which is only a minor fraction of the thickness of this printed page, so that it may be studied under light transmitted through the tissue-thin rock section.

Too often, at this point, the student will find it necessary to abandon some of his sections in preparation, because of unsatisfactory preservation of structure detail, and start over once again to produce a usable section. Not infrequently, the preparation of these three essential microscopic-slide sections will require literally weeks of careful and patient grinding before the student is ready for their study. Yet only by this process can we add, bit by tiny bit, the details which add interest and beauty to our "restored" but admittedly always incomplete picture of the Triassic forest as it looked 180 million years ago.

The diorama exhibit in the Museum of Rainbow Forest Visitor Center has been prepared as a means of conveying to the visitor the extent of our present knowledge of the Triassic forest. While each and every element of flora and fauna that is included is firmly based on fossil facts, it fails to put across one very important question: How great are the blank spaces in our jig-saw puzzle picture? We will add a detail here and there as future discoveries, future study and future research, provide the clues, but the picture can never be completed.