James F Basinger. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997.
The dinosaurs experienced a world much different than our own. In addition to all the environmental consequences of a warmer global climate, the vegetation clothing the landscape was of fundamentally different character to that we see around us.
Perhaps that most significant difference is the dominion of the flowering plants on much of today’s land surface. This group, which numbers in the hundreds of thousands of species, evolved rather late in the Mesozoic. It is difficult to conceive of a world without them, and yet we must re-create this vegetation of the past in order to comprehend the world of the dinosaurs. Virtually all land ecosystems depend on energy stored by vascular plant photosynthesis. Large dinosaurian herbivores may have been the most conspicuous consumers of plants, but we commonly forget that the flesh consumed by the carnosaur was merely this energy taking a different form.
In our survey of Mesozoic vegetation, it is useful first to consider the evolutionary history of the major groups of land plants as told by the fossil record. It will then be possible to step back and briefly view Mesozoic environments.
The transition from the Paleozoic to the Mesozoic, the Permian-Triassic boundary, is marked by the most severe extinction of marine organisms in the history of life. The vast majority of species living in the Permian seas did not survive into the Triassic (see Erwin, 1993). Land animals, including reptiles, were not devastated to the same degree; neither were land plants. Nevertheless, global environmental changes occurring at this time strongly influenced the evolution of all land organisms.
The continents of the Paleozoic were somewhat distributed about the earth. Oceans surrounded these continents and commonly transgressed well inland to form great epicontinental seas. As the Paleozoic came to a close, motion of the earth’s crustal plates slowly drove the continents together so that they coalesced into a vast supercontinent called Pangaea. Pangaea was so large that its interior developed a severely continental, dry climate. To make matters worse, ocean levels fell and the continents became almost completely emergent. This further dried out the land and probably resulted in disastrous loss of habitat for marine life.
The drying out of the land surface occurred gradually, over a period of approximately 40 million years, from the mid-Permian through much of the Triassic. The earth’s land surface did not all become dry at the same time, but moist regions became gradually more restricted. Also, not all land surfaces became dry; the southern polar regions remained moist, although cool, and supported coal forests. Nevertheless, the vast coal swamps of the Carboniferous and Permian disappeared, and with them went the arborescent lycopsids (giant club mosses or scale trees) and sphenopsids (giant horsetails) and most of the archaic seed plants (cordaitalean conifers and early pteridosperms) and archaic ferns. The Triassic was a time of reorganization of land communities and the rise of the major groups of conifers, cycads, cycadeoids, ginkgophytes, later pteridosperms, and ferns that characterized global vegetation of the Jurassic and much of the Cretaceous.
The following overview of Mesozoic plant evolution is of necessity superficial because in diversity alone plants far exceeded the dinosaurs, the subject of the rest of this substantial encyclopedia. The balloon diagram presents a summary of the evolution of the major groups of land plants of the Mesozoic. Many groups of Paleozoic plants that became extinct near the Paleozoic-Mesozoic boundary have been omitted, and many minor groups have either been merged with related forms or entirely left off for sake of simplicity. Additional information on the plant fossil record can be found in the excellent paleobotanical textbooks by W. N. Stewart and G. W. Rothwell (1993) and T. N. Taylor and E. L. Taylor (1993).
Early land plants generally seem to fall naturally into two groups, the lycopsids and everything else. In the modern world, lycopsids occupy a variety of inconspicuous roles as forest, prairie, and aquatic plants herbs (nonwoody plants). Phylloglossum and Lycopodium (ground pines), Selaginella (club mosses), and Isoetes (quillworts) alone survive. The herbaceous lineages of the ground pines and club mosses can be traced well back into the Paleozoic and probably have changed little in appearance and habit throughout the Mesozoic and Cenozoic.
The quillworts, on the other hand, are the last of a long line of primarily woody plants that included the great lepidodenrids (scale trees) of the Carboniferous coal swamps. The scale trees died out in the Permian, and by the end of the Triassic only a handful of dwarf isoetaleans remained, persisting in shallow water habitats.
The evolution of sphenopsids is similar to that of the lycopsids. All sphenopsids possess the distinctive feature of whorled leaves and branches, as does the single surviving genus Equisetum (horsetails). Large calamite trees of the Carboniferous occupied significant roles in the coal swamps but are gone before the Mesozoic as the equatorial swamp habitats disappeared. A variety of unusual sphenopsids, all herbs to low shrubs, including several genera of horsetails, existed during the Triassic and Jurassic. It is possible that these plants were important colonizers of open habitats, areas typically now occupied by flowering plants. By the end of the Mesozoic, only Equisetum remained.
Ferns (Pteropsida) experienced a great turnover from the Permian to Triassic. Few modern groups of ferns can be traced beyond the Triassic, and few of the Carboniferous ferns can be linked closely to Mesozoic forms (see Tidwell and Ash, 1994). Nevertheless, ferns of one type or another have occupied major roles in the forest understory and as colonizers of open habitats since the Paleozoic and continue to do so today.
The fern family Marattiaceae can be traced back into the Paleozoic with confidence. Typically with squat, thick stems and massive leaves, they are among the most spectacular plants. In the Carboniferous, Psaronius was abundant in the coal swamps. From the Permian through to the Jurassic, members of this family were diverse and ranged widely and can be found as fossils on most continents. They probably were very important in the forest understory. During the Jurassic they declined in importance, perhaps as a consequence of competition from the numerous other groups of ferns evolving at this time. Today, the family is a small group scattered throughout the tropics.
The Osmundaceae (cinnamon and royal ferns) is a primitive family of small ferns that can be traced back to the Permian and is allied with some of the Carboniferous ferns. The fronds may be borne on a small squat stem or a creeping rhizome and are commonly dimorphic; that is, they produce sterile leafy portions and fertile portions. When crushed, the fertile portion produces a reddish brown powder of spores and sporangia that looks like ground cinnamon. The Osmundaceae was probably very important ground cover in the Triassic and Jurassic, giving way to other groups of ferns in the Cretaceous. It persists today as very widely distributed ferns of moist woodlands and marshes.
Most other families of the filicalean ferns (including Schizaeaceae, Gleicheniaceae, Matoniaceae, Dipteridaceae, Dicksoniaceae, Cyatheaceae, and the extinct Temskyaceae) appear to have originated in the Mesozoic from poorly defined Permian-Triassic ferns. These families become diverse during the Jurassic and Cretaceous. Expansion of the role of these families during that time may have been at the expense of the more archaic marattiaceous and osmundaceous ferns as well as the sphenopsids. It is also likely that the return to moister climates worldwide during the Early to Middle Jurassic contributed to a vast expansion of habitat suitable for ferns.
Although ferns are unable to produce woody secondary tissue, some members of the Dicksoniaceae and Cyatheaceae have been able to achieve tree height by development of a massive and armored primary stem. Today, tree ferns of these families are important in many tropical and subtropical forests, even becoming part of the canopy. The abundance of large frond fragments in the fossil record of the Jurassic and Cretaceous indicates that tree ferns were abundant, possibly as forest margin and understory shrubs to small trees. In the modern world this habitat is most likely to be occupied by angiosperms. Decline of many of these families in the Late Cretaceous through the Tertiary is most likely attributable to the diversification of angiosperms. The Temskyaceae was driven to extinction by the end of the Cretaceous, and the Matoniaceae was reduced from a worldwide tropical distribution to only two genera surviving in Malaysia and Indonesia.
The Polypodiaceae, the most widespread and abundant of all modern groups of ferns and the group with which we are most familiar, has perhaps the poorest Mesozoic record. Diversification of this family appears to have begun in the Cretaceous and continues to the present day. In fact, the tremendous success of the Polypodiaceae parallels that of the angiosperms. Whereas the rise of the angiosperms doomed many groups of plants, the Polypodiaceae apparently prospered with them.
The seed ferns (pteridosperms) and ferns of the Paleozoic are excellent examples of convergent evolution. Members of both groups adapted to swamp and wetland habitats and in doing so came to resemble one another so closely that it may be impossible to distinguish fern from seed plant on the basis of fronds alone. These early seed ferns became extinct along with the scale trees and calamites at the close of the Paleozoic but were survived by innovative seed plants that likely played a key part in the evolution of angiosperms.
The earliest known of these “later pteridosperms” are the glossopterids of the Southern Hemisphere. Glossopteris has been found on all fragments of the southern supercontinent of Gondwana and was strong evidence for Wegener’s theory of continental drift. This group included large trees that dominated much of the southern mid- to high latitudes. The glossopterids diminish to extinction in the Triassic as the cycads and conifers diversify.
The Caytoniales includes an odd assortment of trees and shrubs of Late Triassic to Early Cretaceous age, some from the southern mid- to high latitudes and others from the north. Sagenopteris, a name applied to fossil leaves of the Caytonia family, is common in Jurassic to Early Cretaceous mixed forests of the northern mid to high latitudes. They may have had a similar appearance, and perhaps role, in these dominantly coniferous forests as broad-leaved hardwoods have in our modern mixed forests. There is some indication that the northernmost seed ferns were deciduous.
The Czekanowskiales is another enigmatic group of trees or shrubs that inhabited the northern high latitudes of the Jurassic and Early Cretaceous. Like the Caytoniales, they were inclined to experimentation with the morphology of their reproductive organs. This, and the coincidence of the extinction of the later pteridosperms with the appearance of early flowering plants, has not gone unnoticed by those searching for the origins of the angiosperms.
Ginkgo biloba, the maidenhair tree of the East, is the sole living member of a lineage that extends back into the Paleozoic and was present in essentially its current form when the dinosaurs roamed. The maidenhair tree is truly a living fossil. We should consider ourselves fortunate to still possess this magnificent plant because not only was it decimated throughout its former range by Tertiary global climatic cooling but also it is believed now extinct in its native eastern China through human land use and habitat loss. It survives only in cultivation.
Ginkgophytes reached their zenith in the Jurassic and Cretaceous, at one time spanning the globe from pole to pole. Diversity was never great but included a number of plants with deeply dissected leaves in addition to those with the fan-shaped leaves typical of the living species. Ginkgos are especially conspicuous in high-latitude fossil floras, where they apparently filled a niche now occupied by broad-leaved deciduous hardwoods. Species of the northern deciduous forests persisted into the Tertiary as ginkgos elsewhere became extinct, probably through competition with angiosperms. During the Cenozoic, the range of ginkgos dwindled from circumpolar to a handful of sites in China. Ironically, it has by human intervention once more achieved worldwide distribution.
Only 10 genera of cycads exist now, scattered around the globe in tropical to subtropical climates. They typically have thick, short, sparsely or unbranched stems with a crown of pinnately divided leaves. Their large seeds are borne in cones that may be gigantic, weighing many kilograms. The primitive appearance is not deceiving because they are the remnants of a once diverse group that was probably second only to the conifers in many Mesozoic forests.
The origins of the cycads can be traced as far as the Carboniferous, but it is not until the Triassic that they assume a major role. Although they become diverse, there is little departure from the morphology typified by the few living relicts. Their short stature and slow growth would ill-suit them to the forest canopy, but they may have been successful understory plants and colonizers of open areas. Their stout stems and tough leaves are ideally suited to drought tolerance, and they were prominent in equatorial arid to semiarid regions. Their global distribution included even the high latitudes, where they may have been deciduous and frost tolerant, features not found in any living species. Cycads decline through the Cretaceous in direct contrast to the rise of the angiosperms. By the end of the Cretaceous, cycads were probably near their current level of diversity, and today they continue to tenaciously resist extinction.
That cycads were so common throughout much of the Mesozoic, and within reach of herbivorous dinosaurs, begs us to question their component of dinosaur diets. Leaves of most cycads appear to have been stiff and resistant to casual browsing. Nevertheless, the horny beaks and tooth batteries of ornithischians may have been more than a match for them. It may be that the sauropods were less well equipped to deal with most cycads. The growing tip of most cycads is deeply hidden by an armor of leaf bases and stiff bracts, perhaps as protection against predation. Unlike conifers, cycads are generally unable to branch and die if the shoot tip is destroyed.
Had dinosaurs depended on cycads in the Jurassic and Early Cretaceous, they must have had little difficulty in altering their diet to other plants because dinosaurs continued to enjoy their rule in the Late Cretaceous long after cycads had become scarce and largely replaced by angiosperms.
The cycadeoids paralleled the cycads in time, space, and appearance. Arising in the Triassic, they became abundant and worldwide in the Jurassic and Early Cretaceous. Although many became more highly branched and shrubby, others resembled cycads so closely that a microscopic examination is necessary to differentiate them. Their reproductive structures, however, differ vastly from the simple unisexual cones of cycads. Many had bisexual cones, an uncommon feature outside of the angiosperms, although the resemblance to flowers is purely superficial. Profound differences in reproductive organs between cycads and cycadeoids indicate an extremely remote relationship.
Similar vegetative appearance, and the common occurrence of both cycads and cycadeoids in the same deposits, indicates that the two groups occupied similar niches and may have had interchangeable roles in many environments. Both decline in the Late Cretaceous, but the cycadeoids failed to survive and became extinct well before the end of the Cretaceous.
Conifers arose in the Carboniferous but did not become common until the beginning of Mesozoic (see Miller, 1977; Beck, 1988). As a group, they were well suited to the extensive dry land habitats of the Triassic and soon came to dominate the forest canopies throughout the world. Permian and Triassic conifers belong to the Voltziales, the “transition conifers,” named so because they are perceived as a group transitional between early seed plants and living groups of conifers. All modern families of conifers, including the Araucariaceae, Podocarpaceae, Taxodiaceae, Cupressaceae, and Pinaceae, are first recognizable in the Late Triassic to Middle Jurassic and replace the Voltziales before the end of the Jurassic.
The voltzialean conifers would have looked much like many modern conifers: tall and highly branched, with needle-like leaves. Seed cones show considerable diversity but are all variations on the theme found in most modern conifers, in which seeds are borne on the surfaces of overlapping scales that are assembled into cones. Petrified woods of these trees are quite similar to woods of a number of living conifers, particularly members of the Araucariaceae and Taxodiaceae. The name Araucarioxylon has been used for many of these fossil woods, indicating their resemblance to wood of the modern Araucaria (e.g., Norfolk Island pine and monkey puzzle). Petrified conifer woods of the Triassic and Jurassic, including huge tree trunks found in so-called petrified forests of the western United States, are generally the remains of voltzialeans. As may be deduced, the distinction between voltzialeans and modern conifers is blurry.
Various members of the Voltziales were found on all continents. Aridity and global warmth during the Triassic to Early Jurassic appears, however, to have created an effective barrier to migration across the low, equatorial latitudes, tending to isolate groups adapted to the more humid mid- to high latitudes. This isolation was increased as Pangaea began to break apart, creating an oceanic barrier between the northern and southern continents. Many conifer families evolving in the Jurassic were more or less confined to either the northern or Southern Hemisphere, failing to effectively cross the equatorial barriers.
The Pinaceae (pine family) is entirely of the Northern Hemisphere. With a record extending well back into the Jurassic, it seems to have been most common in the mid latitudes of the Mesozoic. A number of now extinct genera flourished in the Cretaceous, but only a few of the living genera, such as pines and golden larch, can be traced this far. Although the pollen record of the family indicates that many of the living genera may have been present in the Cretaceous, most do not enter the macrofossil record until the Tertiary. This has led to speculation that many evolved in alpine regions during the Late Cretaceous and early Tertiary and did not become widespread until onset of Tertiary global climatic deterioration. Most members of the Pinaceae are now restricted to montane, boreal, and northern mixed forests of the mid- to high latitudes. The Pinaceae has been mistakenly portrayed as an archaic gymnospermous group evolutionarily eclipsed by the angiosperms. Although some conifers fit this description, the Pinaceae underwent diversification in parallel with the angiosperms, although on a more modest scale, during the Cretaceous and Tertiary. It is now diverse, widespread, and highly successful in a wide range of habitats and dominates vast tracts of boreal forest.
The Araucariaceae is now entirely restricted to the Southern Hemisphere. The Norfolk Island pines and monkey puzzles are familiar house and garden trees. Despite numerous reports of wood and leaves attributed to this family from the Northern Hemisphere, only a few are credible. It seems that most of these remains belong to other families, and that the Araucariaceae made few inroads into the Northern Hemisphere in the Jurassic but was extirpated there in the Cretaceous.
The Podocarpaceae is also a widely distributed Southern Hemispheric family. Members of this family are commonly called “yews” and “pines,” although there is no relationship to these Northern Hemispheric trees. The fossil record of the podocarps in the Northern Hemispheric record is limited to a few arguable macrofossil reports and a record of pollen grains that more probably belong to the Pinaceae. Its southern record, on the other hand, is rich and reveals it to have been a common feature of southern forests throughout the Jurassic and Cretaceous. It continues to occupy a variety of habitats, from tropical rainforest to alpine shrub, throughout much of the Southern Hemisphere.
The Taxodiaceae (redwood family) today consists of a handful of relicts of a once prominent group spanning both hemispheres. Nine genera, most of each with only a single surviving species, are limited to small areas of North America and eastern Asia; only a single genus survives in Australia. Included are the bald cypress (Taxodium), dawn redwood (Metasequoia), and the largest living trees, the redwood and giant sequoia (Sequoia and Sequoiadendron). During the Jurassic and Cretaceous, however, members of this family were major constituents of midand high-latitude forests, and they are common fossils in dinosaur-bearing rocks throughout the Northern Hemisphere. As a group they grow rapidly and tend to form large trees, ideally suiting them to canopy domination in moist forests. They may well have formed a part of the diet of high browsers. From limited evidence of their presence in coal deposits, members of this family may well have commonly formed swamp forests, thereby contributing significantly to the formation of Jurassic and Cretaceous coal reserves. Some, such as dawn redwood, bald cypress, and Chinese cypress (Glyptostrobus), evolved deciduousness and became abundant in the high northern deciduous forests. The family appears to have been decimated first by angiosperm competitors, then by Tertiary climatic deterioration.
The Cupressaceae (cypress or cedar family) is the only conifer family that successfully diversified in both hemispheres. Branches of these plants typically bear scale-like leaves arranged in whorls or opposite pairs and are known from rocks as old as Late Triassic. Although widely distributed, cedars are rarely abundant in fossil assemblages and do not seem to have occupied the prominent position of other conifers. They are closely related to the redwoods and have been included with them.
The Cheirolepidiaceae is the only major family of Jurassic and Cretaceous conifers that failed to survive into the modern world. It was a diverse and enormously successful group that was united by its production of the peculiar Classopolis pollen. Leaves were needle-like to scale-like, and branches may resemble those of other families such as the Taxodiaceae and Cupressaceae. Many members of this family appear to have been well adapted to arid and semiarid conditions, and an abundance of Classopolis pollen is commonly interpreted as indicative of hot and dry climate. The overwhelming abundance of these plants throughout the equatorial regions during much of the Jurassic and Cretaceous, and the typical absence there of other types of conifers such as the Pinaceae, reveals strong latitudinal zonation of vegetation that was controlled by moisture.
From available evidence, the angiosperms originated in the Early Cretaceous but did not become particularly common or abundant until the Late Cretaceous (see Beck, 1976; Friss et al., 1987). There are no confirmed reports of any flowering plants from pre-Cretaceous rocks. Angiosperm fossils reveal a slow increase in diversity and abundance up to about the Lower-Upper Cretaceous boundary and then an explosive radiation that resulted in their common occurrence in fossil floras worldwide. Early angiosperms appear to have included shrubs and herbs that occupied an assortment of marginal habitats, possibly as weedy opportunists. The efficiency of their reproductive process, which included the development of the flower, was revolutionary. In concert with their exploitation of various pollinators, particularly insects, the angiosperms invaded virtually every possible habitat. They appear first in the low latitudes; by the end of the Lower Cretaceous they were found in both north and south polar regions.
Although some angiosperm families, such as the Platanaceae (sycamores and plane trees), are conspicuous and abundant in Late Cretaceous floras, most modern families are not recognizable. Unfortunately, great numbers of early reports on Cretaceous fossil plants list numerous modern genera and families of flowering plants. Most of these assignments are now considered inappropriate; the fossils actually representing extinct families, even orders, of angiosperms. Nevertheless, recent advances in the study of Cretaceous fossil flowers is revealing early members of some familiar families in the Cretaceous record as well as an assortment of unusual forms that seem to have left no descendants. This is currently one of the most exciting areas of paleobotanical research (Friis et al., 1987).
During the Late Cretaceous, angiosperms overwhelmed the equatorial regions and eventually entirely displaced the cheirolepidiaceous conifers, the cycadeoids, and, to a large extent, the cycads and many equatorial ferns. Unfortunately, the macrofossil record from low latitudes is poor, so we have little appreciation for the nature of the vegetation. The pollen record indicates the presence of a diversity of angiosperms of unknown affinity as well as an abundance of palms.
In the mid- to high latitudes of both hemispheres, the angiosperms flourished but did not entirely displace the gymnosperms. In the Northern Hemisphere the Taxodiaceae maintained a firm grip on many of the polar floras until mid-Tertiary climatic decline. The Pinaceae became diverse in the Tertiary and continues to be tremendously successful, particularly in boreal temperate regions. In the Southern Hemisphere, the podocarps and araucarian conifers are still significant forest constituents.
In considering the evolutionary implications of the angiosperms, it is important to keep in mind that many of the herbaceous flowering plants that we now associate with open areas and forest understory, including the entire grass family, the sedges, the aster family, and many others, are unknown from the Cretaceous. Grasslands did not exist until the mid-Tertiary. Open areas and semiarid regions would most likely have been covered with angiospermous shrubs, ferns, cycads, and other shrubby gymnosperms. Grazing was not a dinosaurian pastime.
For the most part, the transition from Paleozoic to Mesozoic floras occurred during the Late Permian to Middle Triassic as a result of profound changes to land environments, including widespread aridity. Throughout the Mesozoic, the equatorial regions appear to have been generally drier and the polar regions moister. The fossil record from the equatorial regions is, however, extremely poor. Most of our evidence from low latitudes comes from the pollen record of marine sediments. Because we do not know which plants produced many of the types of pollen grains, this is most unsatisfactory. The equatorial regions have been very difficult to explore, in part because too much vegetation obscures the rocks, a problem that the human population may soon remedy. More extensive work in these areas is of critical importance to our understanding of plant evolution.
There is currently much debate concerning the presence or absence of permanent ice anywhere on the earth, except at very high elevation. There is some indication of ice-related sediments at very high latitudes, but this is at odds with the record of fossil plants in the polar regions and with evidence for warm polar oceans throughout the Mesozoic. Antarctica was near or on the pole at that time, but its rock record is now concealed by ice, which is a great inconvenience. Much more data from the polar regions may help to answer this question. Our limited knowledge of plant diversity in the very high latitudes could also benefit from more extensive investigation of the Arctic and Antarctic, which are areas that are difficult, and expensive, to explore.
The Mesozoic may be conveniently divided into four packages of time for our purposes. Most of the Triassic is a time of floral change as the Paleozoic component declines and the Mesozoic floras are established. The Lower to Middle Jurassic finds climates becoming more humid in general; modern families of seed plants and ferns replace more archaic or transitional forms. From the Late Jurassic to Early Cretaceous, the Classopolis equatorial zone expands, signaling warmer global climates and perhaps a broader equatorial arid belt. Angiosperms evolve but have little impact on floras. The Late Cretaceous sees the onslaught of the angiosperms.
As a result of continuity, or at least close proximity, of land masses during much of the Mesozoic, plant dispersal was limited primarily by latitudinal temperature and moisture gradients. As a result, vegetation was rather uniform throughout climatic zones. The considerable floristic differences that now exist between continents straddling the same climatic zones appear to be a primarily Tertiary phenomenon.
The following is meant to be an imprecise view of global vegetation, drawing from the previous discussion of the evolution and distribution of major groups of plants, and with liberal interpretation. A recent English translation of V. A. Vakhrameev’s book, Jurassic and Cretaceous Floras and Climates of the Earth (1991), is a rich source of data on this topic. Three paleogeographic maps depict global vegetation patterns in the Early-Middle Jurassic, Late Jurassic, and Late Cretaceous.
During the Triassic, the arid equatorial interior of Pangaea was most likely host to drought-tolerant cycads and cycadeoids, with archaic ferns and voltzialean conifers occupying suitable habitats. In the southern mid- to high latitudes, for example, Australia and southern Africa, the glossopterid pteridosperms declined during this period, and caytonialean pteridosperms (especially Dicroidium), cycads, ginkgophytes, and voltzialean conifers became more important. North of the equatorial belt were cycads, voltzialean conifers, various ginkgos, and ferns.
During Early to Middle Jurassic times, the equatorial regions became dominated by cheirolepidiaceous conifers, cycads, and cycadeoids—assemblages commonly interpreted as indicative of hot and dry conditions. Aridity appears to have been a feature of the equatorial belt throughout much of the Mesozoic.
South of this belt, in Australia, southern Africa, India, southern South America, and Antarctica, existed a diverse flora of ferns, cycads, cycadeoids, ginkgophytes, pteridosperms, and araucarian and podocarp conifers. There is no evidence of a south polar vegetation zone at this time.
To the north, the midlatitude belt of northern North America and central and southern Europe and Asia experienced warm, mainly subhumid conditions and hosted forests of conifers, particularly Taxodiaceae and Cheirolepidiaceae. The Yorkshire Jurassic Flora, superbly described by T. M. Harris (1961, 1964, 1969, 1974, 1979), finely represents this northern mid- latitude vegetation zone. A tremendous diversity of cycads, cycadeoids, ginkgos, pteridosperms (especiallySagenopteris), ferns, and shrubby sphenopsids completes the picture of a rich, lush vegetation. Central and southern North America appear to have been more subequatorial at that time and experienced a hot, semiarid to subhumid climate, with perhaps an open forest and scrub vegetation.
In the north polar region, including northernmost North America and Europe and much of Siberia, floras were much lower in diversity. Here, the climate was humid, with moderate temperatures; even in the polar regions there was no severe frost, and forests could grow at the North Pole. However, continuous light in the summer and continuous darkness in the winter above the Arctic Circle creates seasonality so that plants had to become dormant even though the temperature was above freezing. It seems that relatively few plants could adapt to these conditions. One of the mechanisms evolved to deal with winter dormancy is deciduousness, a feature commonly found among trees in the polar regions. This zone typically includes land above approximately 60+N and in the Jurassic hosted deciduous ginkgos, the enigmatic deciduous conifer Podozamites, a number of archaic Pinaceae, the extinct deciduous pteridosperm Czekanowskia, ferns, and a few cycads.
Global climate apparently warmed through to the end of the Jurassic, with the result that the Cheirolepidiaceae-Classopollis equatorial arid zone expanded into the midlatitudes. This seems to have had the effect of pinching the old midlatitude warm and sub- humid climatic belt against the north polar zone.
North polar floras apparently changed little during this time, remaining dominated by early Pinaceae and deciduous ginkgos, Podozamites, and Czekanowskia. Northward movement of North America brought moderate and humid conditions to the northern half of the continent, and at the Jurassic-Cretaceous boundary thick coals developed in the northwestern United States and western Canada. The fossil floras of this area are similar to those of the circumpolar Siberian region.
The Early Cretaceous was a time of amelioration of global climate and a spread of more humid climate equatorward. The equatorial regions remained hot and dry, but the midlatitudes became more humid. Throughout central and northern North America and Siberia, coal deposits indicate that climate was humid. The flora of this region, above about paleolatitude 50°N, retained its Jurassic character in general. However, through the Early Cretaceous there is a considerable increase in the importance of the Taxodiaceae and a corresponding decrease in Czekanowskiales. Many of the taxodiaceous conifers seem to have adopted a deciduous habit and went on to play a major role in northern polar forests.
The floras of the subhumid to semiarid northern midlatitudes of the Early Cretaceous were much richer than the polar regions, with forests of various conifers, including principally Cheirolepidiaceae, Pinaceae, and Taxodiaceae. Cycads, cycadeoids (especially the barrel-shaped trunks of Cycadeoidea), and ferns were abundant.
Equatorial regions were apparently still primarily arid from Late Jurassic to Early Cretaceous times because they remained dominated by Cheirolepidiaceae, cycads, and cycadeoids. With the continuing breakup of the supercontinents, and the opening of seaways between continents, the low latitudes appear to develop a mosaic of arid to humid environments. Southern midlatitudes and polar regions experienced subhumid to humid climates. Dominant forest trees included primarily araucarian and podocarp conifers, with varying numbers of Cheirolepidiaceae, ginkgos, and a few other conifers. Ferns were diverse and abundant. Cycads and cycadeoids were also diverse and many show considerable similarity to Northern Hemispheric forms, reminding us of their very wide distribution. Primary differences between Northern and Southern Hemispheric floras of this time lay in the dominant canopy trees: The Pinaceae, Taxodiaceae, and Czekanowskiaceae were largely restricted to the Northern Hemisphere and the Araucariaceae and Podocarpaceae to the Southern Hemisphere. As noted previously, it appears that the floras of the very high southern latitudes did not differ much from those of the midlatitudes, unlike in the north, where a distinct north polar vegetation developed. This could be due to a lack of understanding of very high southern latitude vegetation because Antarctica was furthest south and is poorly explored for fossils. Also, perhaps southern plants were generally more tolerant of polar seasonality and were less constrained by latitude. It may also be that Antarctica had not yet positioned itself over the South Pole so that a true polar climate had not yet developed.
During the Early Cretaceous, angiosperms make their ominous appearance in the pollen and macrofloral records of the equatorial regions then spread to the higher latitudes toward the end of the Early Cretaceous.
A transformation of global vegetation occurred beginning in the Albian (latest Early Cretaceous) and continuing in the Late Cretaceous. From humble beginnings, the angiosperms evolved to overwhelm almost all terrestrial environments. Most of the known Cretaceous angiosperms were woody plants. Although many would have been large, canopy trees, angiosperms were especially important as shrubs. The role of angiosperms as herbs at this time is poorly known. Given the apparently poor representation of herbaceous angiosperms in both the macroflora and the microflora until the Tertiary, ground cover in open areas and under canopies may have consisted of mostly ferns.
The decline or extinction of numerous groups of ferns and gymnosperms appear to have been a direct consequence of habitat loss to the angiosperms. The most devastating effect was on the cycads, cycadeoids, and Cheirolepidiaceae of the equatorial regions. Most of our knowledge of this region comes from the pollen record, and unfortunately the affinities of these pollen grains to living angiosperms is generally unknown. The abundance of palm pollen in equatorial deposits has led some to refer to this as the Late Cretaceous palm zone.
In the northern midlatitude zone, including central and southern North America, southern Greenland, most of Europe, and southern and eastern Asia, climate varied from subhumid to semiarid. Angiosperms were commonly abundant and apparently were dominant forest-forming trees in many areas, although conifers of the Taxodiaceae and Pinaceae are still numerous in many floras. The shrub component of the vegetation consisted of a mix of angiosperms, cycads, and ferns.
In the Northern Hemisphere, two floristic zones become established in the Late Cretaceous, recognized on the basis of their pollen floras. The Normapolles zone of the midlatitudes, particularly in the Atlantic region, was probably like the modern moist subtropics and tropics. The north polar regions were distinguished by the Aquillapollenites complex, with mixed vegetation most like that of modern midlatitudes. The modern affinities of these two pollen groups are unknown. Shifts in the boundaries between these two zones reveal climatic fluctuation during the Late Cretaceous.
In the north polar regions, including much of Canada, Alaska, and northern Asia, a diversity of angiospermous trees and shrubs invaded the northern forests but did not entirely displace the deciduous taxodiaceous conifers and ginkgos of the region. These gymnosperms were prominent members of the northern deciduous floras until the mid-Tertiary. The Czekanowskiaceae, formerly widespread in the north, was apparently driven to extinction by first the northern expansion of the Taxodiaceae during the Cretaceous and then the arrival of the angiosperms. The angiosperms of the northern forests were largely of unknown modern affinity but may have played a role in the evolution of many taxa of the Arcto- Tertiary circumpolar deciduous flora. The Pinaceae, now such an important part of northern forests, is not as conspicuous in the Cretaceous macrofossil record as it is in the pollen record. This may indicate that the Pinaceae was an important part of high-altitude floras before becoming widespread in high-latitude floras.
Southern Hemisphere floras were similarly enriched in angiosperms in the Late Cretaceous, although there were pronounced differences in the types of angiosperms present compared to those in the Northern Hemisphere. Araucarian and podocarp conifers continued to be common. By the end of the Cretaceous, the southern polar regions had developed a distinct vegetation dominated by podocarps and angiosperms, notably including Nothofagaceae (southern beeches), a prominent southern high-latitude and -altitude family.
Although never approaching the coolness of the modern earth, the global climates of the Mesozoic did vary considerably. Temperatures rose during the Late Cretaceous to a maximum in the Campanian, then fell gradually during the last few million years of the Mesozoic. On a local level, the effects of this change can be seen in floristic changes and in the southward shift in the humid, coal-forming zone in western North America from above 60+N to below 50+N. Nevertheless, the magnitude of climatic deterioration is not substantially greater than that at other times in the Mesozoic, and there is no evidence that even the polar regions experienced severe frost at any time during this interval. Although rates of plant extinction appear to increase toward the boundary, and continue at a relatively high level following the boundary, the plant record does not indicate a crisis as a consequence of global climatic change.
Global climate warmed again to a maximum several million years after the boundary. The Cretaceous-Tertiary boundary, then, roughly coincides with the climatic minimum between the Late Cretaceous and early Tertiary climatic optima. The boundary rather precisely coincides with the deposition of a worldwide iridium-rich fallout layer that was a consequence of a massive asteroid impact on the Yucatan Peninsula of Mexico. Detailed study of this boundary layer in western North America reveals instantaneous devastation of forests over huge areas, with recovery taking centuries. In some places a tremendous number of fern spores in the few millimeters above the impact layer indicates that the landscape was clothed by ferns for many years until forests reestablished themselves. Interestingly, there is no correspondingly abrupt extinction of plants because most types return shortly above the boundary. The terrestrial record at the boundary is poor elsewhere in the world but seems to indicate a less spectacular end to the Cretaceous on other continents than we see in North America. The role of this impact in the terminal Cretaceous extinction event is currently widely debated and is one of the most fascinating controversies in paleontology today.
The cumulative changes that we see occurring across the Cretaceous-Tertiary boundary create a transition from the Mesozoic floras to the Cenozoic. Unlike the Cretaceous, early Tertiary fossil plants are commonly recognizable as members of living families, and there was a rapid modernization of the global flora during the first 20 million years of the Tertiary. Perhaps this is a suitable boundary, then, for the plants as well as the dinosaurs, because it rightly recognizes the advent of the modern flora as well as fauna. Also, it properly places early angiosperms among the Mesozoic floras as an integral part of the world of the ruling reptiles.