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Paleobotany and evolution


 

 

Fossil conifers are found all over the Earth, from the arctic to the Antarctic, in many places that house no conifers today as well as in their present regions of abundance and diversity. The record consists of a wide range of different kinds of fossils, including pollen grains, wood, pollen and seed cones, and foliage preserved in three rather different manners. Impressions show shapes of leaves and various surface patterns but preserve no original material. Compressions preserve a carbonized film, and this frequently includes the resistant cuticle and thus reveals the detailed structure of the epidermis, which has been shown to differ considerably among different conifers. The cuticles are separated from the rock matrix and mounted on microscope slides for examination with light microscopy or on stubs for scanning electron microscopy. Finally, petrified material can preserve the internal structure of the fossilized organs, sometimes including fine cellular details. These can be revealed either by grinding slices of the fossil so thinly that they transmit light or by etching the rock with acid and trapping the resistant cell walls that stick up in a thin film of transparent plastic. Unfortunately, the different organs and types of preservation require different environmental and geological conditions, so it is extremely rare to have a complete picture of the external morphology and internal anatomy of all the organs of a fossil conifer. The relatively few whole plant reconstructions, however, are gradually increasing in number as paleobotanists painstakingly piece together the scattered bits. Even without these most completely known fossils, however, numerous individual organs are available and open to interpretation so that a fairly coherent picture of the origin and evolution of conifers has emerged. New fossils are discovered and described all the time, continuing to refine and enrich this picture.

The earliest known conifers date from the late Carboniferous period, some 300 million years ago, the period of the great coal age swamp forests, but they were not part of these forests. Growing on uplands surrounding the swamps, their remains rarely entered the fossil record, and when they did, they were often in the form of charcoalized fragments that had washed down after forest fires. Nothing is known of their reproductive structures, but their foliage, like that of most later ancient conifers, was reminiscent of shoots of Norfolk Island pine (Araucaria heterophylla) and other similar araucarians. Conifers became much more abundant in the drier Permian period that followed the Carboniferous at the end of the Paleozoic era. The great coal swamps, with their forests of tree-sized horsetails and club mosses, were replaced by dry-ground forests, and these were dominated, in part, by conifers. None of these belonged to any of the six still extant families but to extinct families, like Majonicaceae, Emporiaceae, and Utrechtiaceae, in which the dwarf reproductive shoots in the axils of the seed cone bracts still consisted of separate individual sterile and seed-bearing leaves and were not yet organized into definite, unified seed scales. Following the catastrophic Permian extinctions and the beginning of the Mesozoic era of reptiles, conifers continued to diversify during the Triassic period, showing increased variety in foliage and reproductive structures. By about 225 million years ago, late in the Triassic, fossils ascribed by some authors to several extant families – Araucariaceae, Cupressaceae, and Podocarpaceae – are present. Others are considered to be precursors to Pinaceae, if not members of that family. It is not at all certain that any of these early attributions to extant families are correct, but during the course of the following Jurassic period, fossils belonging to Araucariaceae and Taxaceae become part of the conifer floras that also included representatives of now extinct families, such as Cheirolepidiaceae and Miroviaceae. Conifers, in fact, were dominants of these early and mid-Mesozoic forests, but the first incontrovertible evidence of fully characteristic members (referred to as the crown groups) of the remaining extant families – Cupressaceae, Pinaceae, Podocarpaceae, and Sciadopityaceae – are only recorded in the last period of the Mesozoic, the Cretaceous, which began about 145 million years ago. Even then, very few extant genera are recorded from the Cretaceous, and most of those that are, like Pinus, first appeared late in that period. The vast majority of the living genera for which we have a fossil record are first identified in sediments dating from the early or mid-Tertiary period, after the catastrophic close of the Mesozoic 65 million years ago. Despite the much greater time depth for conifers as a whole than for flowering plants, the extant conifer genera are thus comparable in age to many familiar genera of broad-leaved trees, like poplars (Populus), oaks (Quercus), maples (Acer), and birches (Betula). The records of individual conifer genera are briefly reviewed in the discussion for each genus, but a few general remarks are appropriate here. There is no particular association between the age of a genus and the number of species is has today. The oldest extant genera, for example, those dating from the Cretaceous, run the gamut from presently monotypic (single species) ones, like Sciadopitys and Sequoia, to the biggest of all, Pinus, with about 100 species. On the other hand, relatively recent genera, while they might be expected to be small, like the monotypic, narrowly endemic (very localized) Microbiota, can also be quite large, like Juniperus, with its more that 50 species.

Much more captivating to the imagination is the fact that some genera that today are narrowly endemic monotypes, or even on the verge of extinction, were much more widely distributed in the past, and some were dominants over vast areas. Throughout the early and mid- Tertiary, volcanic activity and other tectonic processes streams, creating vast swamps over many areas of the northern hemisphere. These swamps, in contrast to those of the Paleozoic coal age, were dominated by conifers, particularly three genera of Cupressaceae with much more restricted ranges today. There are now two species of bald cypress (Taxodium) in southern North America, from the southeastern United States to northern Guatemala, while the closely related Chinese swamp cypress (Glyptostrobus) and the more distant dawn redwood (Metasequoia) are monotypic genera with even more limited natural distributions in historic times, the former in swampy parts of southern China and adjacent Vietnam, particularly near the coast, and the latter in a local region of central China, where it grows in ravines and along the floodplains of broader stream valleys. A few other genera that were widespread during the Tertiary but are much more restricted today include Cathaya and Pseudolarix of the Pinaceae, Tetraclinis of the Cupressaceae, and notably, Wollemia of the Araucariaceae, all of which are monotypic today. Even some genera with a number of species today were still more widely distributed in the past, including Araucaria, which extended into the northern hemisphere as well as more widely in the southern during the Mesozoic. A fairly common pattern among northern hemisphere genera with species in North America and eastern Asia today was to have been present in Europe as well during the Tertiary and to have been eliminated from there as the climate bot colder late in the period or during the upheavals of the Pleistocene epoch ice age. Arborvitaes (Thuja) and nutmeg yews (Torreya) are examples of this kind of pattern, while other genera also disappeared from North America during the Tertiary, like plum yews (Cephalotaxus). 

The four geographic components just considered, eastern and western North America, Europe, and eastern Asia, together make up a Holarctic distribution today or an arctotertiary distribution in the past. Most of the conifer genera that had a full arctotertiary distribution in the past became extinct in one or more of the four regions so that only a few genera retain a Holarctic distribution today, including true firs (Abies), larches (Larix), spruces (Picea), and pines (Pinus), all of the Pinaceae, yews (Taxus) of the Taxaceae, and junipers (Juniperus) of the Cupressaceae. Note that four of these are among the five largest conifer genera. There appears to have been some safety in species numbers during Tertiary and Quaternary extinctions as those genera that experienced loss of one or more regions were all monotypic or had just a handful of species. A pattern similar to the remnant arctotertiary distributions of the northern hemisphere may be found with such southern temperate genera as Athrotaxis, Austrocedrus, Dacrycarpus, Fitzroya, Lepidothamnus, and Libocedrus, each of which was more widespread in the past, but the corresponding patterns there have been much less extensively worked out than those in the north. There are many reasons for this lower level of knowledge, including fewer paleobotanists having started work later on the fewer relevant deposits in the smaller southern landmasses. A little bit of it may also be due to a greater focus on an earlier southern hemisphere distribution pattern that resulted from the concentration of all the southern lands, including Antarctica, into the ancient continent of Gondwana, which housed forests during the Permian dominated by another extinct gymnosperm group unrelated to conifers, the glossopterids, with large, tongue-shaped leaves. In between the north and south temperate regions, the fossil record of conifers in the modern tropical zones is so poorly known that we cannot really discern anything useful about historical patterns of geographic distribution there during the Tertiary.

This history of regional extinctions in both the northern and southern hemispheres during the Tertiary and Quaternary periods might give an impression of decline in the conifer flora through the course of the Cenozoic era, but this would be somewhat misleading. While it is true that most regions now have lower conifer generic diversity than they had during the Tertiary, global diversity remained essentially unchanged since genera that disappeared from one place usually persisted elsewhere. There are only a few known cases of global extinction of a widespread genus during the Tertiary, including Cunninghamiostrobus, a genus related to Cunninghamia, and Pityostrobus, a “garbage can” morphogenus containing species not assignable to any of the extant genera of Pinaceae. In fact, taken at face value, the Tertiary record of conifers is actually one of steady progressive generic diversification during the earlier Paleogene and slowing down during the succeeding Neogene. About a dozen of each of the extant genera are first known from each of the Cretaceous, Paleocene, Eocene, and Oligocene epochs (the latter three making up the Paleogene), but only two in the Miocene and one in the Pliocene (together making up the Neogene). Fifteen of the extant genera have no known fossil record, but they are likely to be spread across the same time distribution, the way undecided voters often are, rather than being clumped in any one epoch. There are also many genera described from Paleogene deposits of each epoch that are now extinct, but these are almost all based on very little material from just a few places and hence of uncertain status. A number of these are members of the Podocarpaceae that are based on scraps of foliage of restricted distribution in space and time so that one cannot be sure that they are not unusual species of still extant genera. None of these poorly known genera was common or widely distributed. Likewise, the widely distributed genera that became extirpated in most regions were essentially either all monotypic or with few species. No extant genus that is known to have had numerous species during the Tertiary is now a locally restricted monotypic endemic.

Conifers were an important component of temperate ecosystems throughout the Tertiary, and they remain so today. The boreal forest, which is dominated by a few species of conifers, is the single largest forest formation on Earth today. The evolutionary roots of the conifers may lie deep in the past, but they are hardly a spent force. While suffering significant losses in global dominance after the rise of angiosperms during the Cretaceous, they have since held their own during the last 60 million years, and they continued to evolve new genera during that interval, just as have the flowering plants. The overall pattern can be viewed in an evolutionary context in terms of adaptation to the new environmental conditions that are always emerging in the world. Because reproductive structures (except pollen grains) are more transient and less numerous than vegetative structures, the history of the evolutionary diversification of pollination and seed-dispersal mechanisms is less clear than that of photosynthetic specializations. The forms of conifer foliage in various fossil floras often helps in interpreting the environments in which those conifers grew, especially in concert with clues from the rocks themselves, including evidence on the nature of the soils (paleosols) that supported them. While the range of known fossil conifer foliage is vast, understanding how the different foliage forms interrelate evolutionarily lags far behind. In particular, the steps leading to the more extreme foliage forms, such as the broad multiveined leaves of Nageia or the leaflike branching systems (phylloclades) of Phyllocladus, is worthy of further investigation using both fossils and the most powerful molecular tools available to developmental biologists. There is a great deal to learn still about conifers of the past and their evolution.

 

Written by James E. Eckenwalder

 

  • Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
  • Copyright © James E. Eckenwalder, Conifers Garden. All rights reserved.

 

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