Creation by Evolution/The Story Told by Fossil Plants

by Edward Wilber Berry

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THE STORY TOLD BY FOSSIL PLANTS

By Edward Wilber Berry

Professor of Palaeontology, The Johns Hopkins University

Students of evolution are at present interested almost exclusively in experimental studies that may disclose its causes, an extremely difficult problem even with the simplest and most rapidly multiplying organisms. Such studies, however, afford the only logical approach to an answer to the question “Why?” The answer to the question “How?” is given best in the geological record. Paleobotany—the science of the world’s oldest plant life—has this advantage over all other methods of finding an answer to this question: the student, to borrow a simile from written history, is dealing with the original documents in so far as they are preserved—the fossil plants—and he finds them in their actual order of succession. Our main task here, then, is simply to tell the story of the procession of the myriad of plant forms across the stage of the past. Before that story is told, however, the general facts and principles illustrated by fossil plants may be very briefly set forth.

First among these is the fact that plants underwent a gradual transformation from simplicity to complexity and were differentiated in both structure and habit in successively higher groups, thus exemplifying the universal principle of evolution. The earliest plants grew in the water, but gradually the main theater of plant operations was transferred to the land.

Each successive group of plants that appeared upon the scene illustrates a second great principle, which is called “adaptive radiation”—that is, from time to time, by progressive modifications, certain groups became dominant, such as the club mosses, horsetails, and seed ferns of the Carboniferous

Fig. 1.—Diagram showing the successive numerical dominance of progressively more complex plants through the geologic eras and the progressive increase in the complexity of the more familiar floras.

The space assigned to the several eras (Archeozoic, Proterozoic, etc.) corresponds roughly with the length of time included in them. The Archeozoic is the earliest era, the Cenozoic the latest. The heavier lines show the increase or decrease in the number of the different kinds of plants through geologic time. The flowering plants are now (in late Cenozoic time) by far the most numerous, and their rise to dominance has been rapid; yet representatives of the older forms still persist, though they are less abundant than the flowering plants.

period, the cycads of the Mesozoic era, or the flowering plants of the Cenozoic era. The members of these groups became adapted to a great variety of environment and tended to occupy all the available places on the land, and some of them, such as the water ferns or the higher aquatic plants, became readapted to an aquatic existence. One group after another thus became dominant and then waned or became entirely extinct. The accompanying diagram (Fig. 1) illustrates the successive numerical dominance of different plant types and the increasing complexity of the vegetable kingdom as a whole.

Another principle is illustrated by the progressive loss of plasticity in organisms or organs as they became more complex and more highly specialized. The simpler organisms outlasted the complex or gave origin to new types, for the more complex lost adaptability to new conditions and perished during changes of environment. Most of the earlier forms of the successive groups of plants were synthetic or generalized in structure. The earliest ferns, for example, show combinations of features that subsequently became the property of different fern families, and the seed ferns combined the features of ferns and cycads.

The first simple plants, which grew in the water, probably lacked the substance commonly called leaf green (chlorophyll); they obtained their nitrogen from ammonia compounds and gained their energy by oxidation, in much the same way that some modern bacteria oxidize iron and sulphur. With the development of chlorophyll they were able to utilize directly the carbon dioxide of the air and build up complex organic compounds. The acquisition of this power of using inorganic material for food and of converting sunshine into energy marks the first progressive step in the history of plants. The second step was the occupation of the land. During the long history of land floras, covering millions of years, the two principal advances were the development of what is called secondary wood, such as forms the seasonal layers of the oaks or the pines, which enables them to increase in size for many years and carry aloft an ever larger canopy of leaves, and the development of seeds, which are a much more efficient means of reproduction than the simple single-celled spores of the lower plants.

Geologists construct their history in much the same way as any other historians, but instead of dealing with written documents or the handiwork of man they deal with the series of rocks that make up the crust of the earth, and especially with the fossils preserved in the rocks—the remains of the plants and animals that were alive when the rocks were being deposited as mud or sand. (See geological time table on page 160.)

You might suppose that this record of the past would be so scanty and broken that we could not read it. It is, indeed, far from complete, but when we remember that even the formation of a single bed of sandstone or of clay consumed a long time we can see that innumerable plants and animals might have been covered up by accumulating sediments and so well preserved that we could use them in our study of the earth’s history.

The earliest chapter of the world’s organic history we call the time of ancient life, or the Palaeozoic era, which is saying the same thing in Greek. This Palaeozoic era we divide into periods, each marked by distinctive types of fossils. The names of the periods that make up the Palaeozoic era, given in order from the oldest to the youngest, are Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Some of these names are geographical, each derived from the name of some place where rocks of that age are exposed. Cambrian is from Cambria, the Latin name of Wales; Silurian is from the name of a tribe—the Silures—which in Roman times inhabited that part of Britain where the Silurian rocks

GEOLOGICAL TIME TABLE

(The older areas, periods, etc., are at the bottom; the later are at the top)

Cenozoic era Modern life: mammals and flowering plants (3 to 5 million years)	Quarternary period	Pleistocene epoch	Time of the Ice Age and of the ancestors of man. Extinction of many large animals and trees. Evolution of herbs. Elevation and extension of continents.
	Tertiary period	Pliocene epoch	Cosmopolitan forests.
		Miocene epoch	Zenith of development of forests.
		Oligocene epoch	Culmination of Eocene types.
		Eocene epoch	Modernization of flowering plants.
Mesozoic era Middle life: reptiles, cycads, and conifers (5 to 10 million years)	Cretaceous period	Earliest palms. Beginnings of forests of the ancestors of the flowering plants mixed with survivors of the older Mesozoic ferns, cycads, and conifers.
	Jurassic period	Widespread warm seas, marine mammals and terrestrial cycads and conifers. Toothed reptile-birds.
	Triassic period	Land extension and shallow seas and lagoons. Red deposits. First mammals.
Palaeozoic era Early life: fishes and flowerless plants (20 to 25 million years)	Carboniferous period	Permian epoch	Dwindling of ancient forms; rise of cycads.
		Swamps of the coal age. Ferns and seed ferns, giant club mosses, and horsetail rushes. Rise of primitive reptiles.
	Devonian period	First abundant fossil land plants. First amphibians.
	Silurian period	Rise of land plants, lung fishes, and scorpions.
	Ordovician period	Rise of shelled animals.
	Cambrian period	First abundant fossils. Marine plants. Dominance of trilobites.
Proterozoic era (25 million years)	Cellular plants and primitive, mostly soft bodied marine animals.
Archaeozoic era (50 million years)	The first life.

may be seen; Permian is from the province of Perm, in Russia. Or the name may have been suggested by the character of the rock, as Carboniferous, a term applied to the rocks of the coal age.

After this era of ancient life, the Palaeozoic, came the Mesozoic era, a time in which the forms were intermediate between the old and the new. The Mesozoic era is divided into three periods—the Triassic, the Jurassic, and the Cretaceous. The Triassic is so named because in that period three principal kinds of rock formations were deposited in southern Germany; the Jurassic is so named because the rocks of that period are very conspicuous in the Jura Mountains; and the Cretaceous gets its name from the fact that its characteristic rock is chalk (creta in Latin).

The Mesozoic era was followed by the Cenozoic, the time of modern life. In the rocks of this era we find the remains of warm-blooded animals and flowering plants, and in those of the later part of the era we find the skeletons and flint implements of ancient man.

Many fossil seaweeds are scattered through the older rocks, but the first land plants found in abundance as fossils lived in middle Palaeozoic time (Devonian). (See table on page 160.) Some of these may be considered transitional between seaweeds and true land plants (Fig, 2). Others were synthetic forms combining features of organization which during subsequent ages became segregated and characteristic of separate orders of plants. Such an ancestral plant is Hyenia (Fig. 4) which combines features of the later club mosses and horsetails. Others by their complexity indicate a long period of terrestrial existence. Some of these Devonian plants are true seed ferns (Figs. 3 and 4); others are arborescent club mosses, which combine the features of plants of later Palaeozoic time (Fig. 5). Another later Devonian type, widespread geographically, is a plant (Archaeopteris) whose classific place is uncertain, for we cannot yet be sure whether it is a true fern or a seed fern.

Fig. 2.—Restoration of Psilophyton, an early Devonian terrestrial plant showing many features suggestive of algal ancestry.

During the remainder of Palaeozoic time the terrestrial vegetation consisted essentially of the coal plants—club mosses as large as trees, with woody stems and complex reproductive structure, tree-like horsetails, a great variety of seed ferns, some ancestral to the later cycads, and trees like the modern ginkgo. The most abundant of the latter were tall trees, somewhat like modern conifers but with a larger pith in the columnar trunk and large leaves like those of a corn plant. These had

Fig. 3.—Restoration of Eospermatopteris, the earliest known fern, from the middle Devonian. (After Goldring.)

curious flowers and seeds quite unlike those of modern plants. Their many varieties are collectively known as Cordaites, and a restoration of one is shown in Fig. 6. Near the end of the Palaeozoic era we find the coal plants dwindling in number, as a consequence of the changing conditions

Fig. 4.—Restoration of Hyenia, a middle Devonian plant, which combines features of the later club-moss and horsetail lines of evolution and suggests certain features of the fern line. (After Kräusel and Weyland.)

of Permian time, and new types making their appearance, such as cycads and coniferous trees, ancestral to Mesozoic forms. The late Palaeozoic rocks of Australia, India, South Africa, and South America give evidence of widespread glacial ice. The rigors of this time in these regions expelled many of the members of the earlier cosmopolitan flora and introduced a number of new types, known collectively as the Glossopteris flora. (Fig. 7.) The earlier part of the Mesozoic era was a time of widespread seas; the land deposits then laid down contain few fossil plants. The oldest Mesozoic rocks containing a representative flora are those laid down near the end of the Triassic period. In the long time that had elapsed since the Permian epoch many changes had taken place. A few surviving stragglers of the old order lingered on, but many of these older Mesozoic plants were the diversified descendants of the conifers, cycads, and ginkgos, though they included numerous ancestral representatives of most of the modern families of ferns. The Mesozoic has been

Fig. 5.—Restoration of Protolepidodendron, an upper Devonian ancestor of the subsequently differentiated forms of Lepidodendron and Sigillaria, the arborescent Palaeozoic club mosses. (After Berry.)

a, Lepidodendron-like leaf sears; b, Sigillaria-like leaf sears; c, leaf, about natural size.

called the age of gymnosperms (plants with naked seeds, such as the pines), but it may perhaps be more properly called the age of cycads (Figs. 8 and 9), for its rocks contain cycad-like plants in great abundance and variety.

The known Jurassic floras, whether of swamp or upland, consisted primarily of ferns, cycads, and conifers. The ferns were all forms of moderate size. None of the cycad-like forms that are so characteristic of that age of the earth’s history were tall; probably none were as tall as an old cycad of to-day. Rising above the general low level of these cycads were the various conifers, among which were the Jurassic forms of the maidenhair tree (Ginkgo) which is to-day represented by only a single species.

Fig. 6.—Restoration of Cordaites, a primitive conifer of the Carboniferous period. (After Scott.)

Fig. 7.—Typical members of the Glossopteris flora, a, Gangamopteris; b, Neuropteridium; c, d, Glossopteris.

Lower Cretaceous plants are found in the rocks of all the continents, and they are particularly abundant in North America and Europe. The two most extensive Lower Cretaceous floras are those preserved in the Potomac group of rocks of Maryland and Virginia and those of the rocks of the opposite side of the Atlantic, in southern Portugal. Comparisons of these floras shed light on the place of origin and the migrations of the various types. A third large

Fig. 8.—Restoration of Wielandiella, one of the best known branched cycads of the older Mesozoic. (After Nathorst.)

Lower Cretaceous flora is that of the so-called Wealden of England, Belgium, and Germany. Other floras of this age are found in South Africa and eastern Asia, as well as in Spitzbergen, Australia, New Zealand, and Greenland. Although the known floras of the Lower Cretaceous epoch necessarily represent only a small percentage of the species that clothed the earth during that time, they furnish some suggestive data concerning the march of vegetation during the time in which the flowering plants first appeared, when the transformation was made from a Jurassic to an Upper Cretaceous and essentially modern flora. In the varying

Fig. 9.—Recent Japanese cycads, showing the character of the vegetation in Jurassic time. (After Wieland.)

proportions of its main types of plants, the Lower Cretaceous flora discloses local differences of soil, altitude, humidity, and precipitation. The dominant late Jurassic types—the ferns, cycads, and conifers—continued without marked change through early Cretaceous time. The early Cretaceous cycads were essentially the familiar types of later Jurassic time. They were abundant in genera, species, and individuals, and they were quite as dominant an element of the lower Cretaceous floras as they had been of those of late Triassic and Jurassic time. Before the end of the Lower Cretaceous epoch, however, most of these plants had become extinct. In rocks laid down near the end of that epoch we find preserved the first representatives of the flowering plants (the angiosperms—that is, plants having enclosed seeds, ,such as the walnuts, oaks, and maples), and during Upper Cretaceous time these plants gradually became predominant.

Although the seas were widespread in early Mesozoic time there were many large areas of land, but we know nothing about the floras of these areas, which may have been the scene of the evolution of the flowering plants. Certainly during late Cretaceous time they spread continuously southward in Europe, North America, and Asia, and almost everywhere the same forms occur, alike in Bohemia, Alabama, or Sakhalin Island, localities suggesting their northern origin. During Upper Cretaceous time they penetrated far into South America, reaching Argentina, and they even reached Antartica (Graham Land). These Upper Cretaceous floras invariably show a mingling of temperate and tropical types, indicative of a humid warm-temperate climate, and they all contain forms that are to-day largely confined to the Southern Hemisphere. Throughout Upper Cretaceous time new types continued to appear and the stragglers from older floras gradually died out, so that by the dawn of the Tertiary period most of the archaic forms had become extinct.

The flowering plants possess for us a profound interest, because they yield the concentrated foodstuffs that made possible the evolution during Tertiary time of the mammals—the horses, cows, hogs, sheep, etc.—on which depend our agriculture and consequently our civilization.

The earliest floras of the Cenozoic era—the age of mammals and of flowering plants—are marked by a great modernization of forms. They consisted in large part of ancestors of forms that exist today, and their chief scientific interest lies largely in the great differences in geographical distribution which they show in contrast with the present distribution of their descendants. The contrast between the continents was not so great as in earlier times, and the whole Northern Hemisphere was clothed with forests much like those that survive today in southeastern Asia and southeastern North America. Species of magnolia, sequoia, walnut, and sassafras were then native in Europe, and during early Cenozoic time the nipa palm, the date, the cinnamon, and the bread fruit tree lingered in our Gulf States.

Gradually these floras become more modern; herbaceous plants—those having no persistent woody stem—multiplied, and then came another change of climate, during the epoch known to geologists as the Pleistocene. Because of the widespread glaciation which gives this epoch a distinctive place in geological chronology, it is often called the Ice Age or the glacial epoch, although a similar period of climatic rigor, already mentioned, occurred in Permian time, and evidence of other glacial epochs in early Paleozoic and pre-Paleozoic time has been discovered. Pleistocene glaciation was contemporaneous with the evolution of the human stock and exercised a profoundly modifying influence on the noble races of mammals and forest trees of the Northern Hemisphere. It also modified greatly the topography, producing numerous lakes, ponds, and bogs. The freshness of the deposits it left—its moraines, its bowlder till, and its sand plains, all scarcely modified in the relatively few thousands of years that have elapsed since the last ice sheets disappeared—emphasize the nearness of the great glaciers to the period of human history.

At the beginning of Pleistocene glaciation the flora of all three of the continents of the Northern Hemisphere was essentially similar. The retreat of the last ice sheet left an impoverished flora in Europe and two great asylums of survivors in eastern North America and eastern Asia. The explanation of this difference is, broadly speaking, very simple. In America and Asia, with their extensive coastal plains and north-south mountain chains, there were no insuperable barriers to the dispersal of plants southward, away from the frozen lands, but in Europe the mountain ranges (the Pyrenees, Alps, Carpathians, Balkans, Caucasus), which trend east and west, and many of which were themselves lofty enough to be local centers of glaciation, formed impassable barriers to plant migration, and branches of the sea effectually stopped the gaps between the mountain systems. Hence many of the plants of the Pliocene forests of Europe were unable to escape extinction.

Great sheets of ice accumulated over the land during at least four separate epochs. Each of these epochs lasted 10,000 to 20,000 years, and they were separated by long epochs of genial climate, known as interglacial epochs, each lasting for thousands of years, during which the floras spread northward, even to points beyond their present range. Many such interglacial floras are represented in deposits in Europe and have been diligently investigated in connection with the economic study of peat bogs. The best known interglacial flora of North America, where the extensive peat resources have been almost neglected, is that found in the Don Valley, near Toronto, Canada. Here are found impressions of leaves and other parts of the sycamore, maple, osage orange, and other types that do not to-day quite reach that latitude. Other traces of Pleistocene floras are found in cave deposits, associated with fossil remains of animals, some of which are now extinct. Swamp deposits that were overwhelmed by sand during changes along the coasts yield many species of plants, most of which still exist, such as the bald cypress, loblolly pine, sycamore, poplar, hickory, river birch, and several species of oaks. All these fossils show that the inter-glacial floras scarcely differed from those of to-day except in the details of distribution of the species. During the periods of glaciation these temperate forests retired southward and gave way along the ice front in this country to arctic willows and dwarf birches, which reached southward to about latitude 40°.

The post-glacial amelioration of the climate, the opening to occupation by plants of areas that had been covered with glaciers, the mixing of soils through the action of the ice, all combined to stimulate the evolutionary activity of plants, particularly the herbaceous forms. It seems probable that the herbaceous families that are characteristic of the Temperate Zone originated at this time.

Possibly more potent than natural causes in modifying the character and distribution of the existing vegetation has been the work of man, which includes the action of fire, the ax, and domesticated grazing animals. The forests are now waning. Human intercourse results in surprising feats of plant distribution, such as are shown in our familiar cosmopolitan weeds. Insect and fungal pests are similarly spread, both rapidly and widely, and tend increasingly to restrict or even to exterminate the native vegetation.

The long procession of changing forms has not yet come to a halt, and man, having learned some of Nature’s methods, has so applied them as to produce marvellous new varieties of flower and fruit, new habits of growth, and new adaptability to environment.

We have seen in our brief survey of the floras of the past that they illustrate the evolutionary principles set forth. We observe a gradual transformation from simple and generalized to complex and specialized forms. We see different groups becoming specialized in various ways and attaining dominance for a time, and eventually we see those that were less perfectly adapted to survive going down in competition with those that were more perfectly adapted. At one time it may be the Palaeozoic club mosses, whose trunks were mechanically defective as compared with the trunks of the contemporary exogenous conifers. At another time we see the seed ferns, with their large and complex seeds, replaced by plants having simpler and more efficient seeds. In one way or another the story repeats itself through millions of years of history.

REFERENCES

• Berry, Edward W. Paleobotany: A Sketch of the Origin and Evolution of Floras. Smithsonian Institution, Ann. Rept. for 1918, pp. 289–407, 1920.
• Berry, Edward Wilber. Tree Ancestors. A Glimpse Into the Past. Williams & Wilkins Co., Baltimore, 1923.
• Knowlton, Frank Hall. Plants of the Past. Princeton University Press, 1927.
• Scott, Dunkinfield Henry. Studies in Fossil Botany. 3rd ed., 2 vols. A. & C. Black, London, 1920–1923. Extinct Plants and Problems of Evolution. Macmillan & Co., London, 1924.
• Seward, A. C. Fossil Plants. 4 volumes. Cambridge University Press, 1898–1919.