Extinct animals and plants

Japanese: 古生物 - こせいぶつ（英語表記）extinct animals and plants

Organisms that lived in the geological era. Their remains, footprints, burrows, feces, and other traces of life (traces) remain in the strata as body fossils and trace fossils, respectively. Paleontological organisms include extinct and extant species. There are about 130,000 species of paleontological organisms described as fossils, but this number is only 7.4% of the total number of known extant organisms, which is about 1.75 million. However, most paleontological species are morphospecies, which are conceptually different from biological species based on reproductive relationships. The total number of paleontological species that lived in the geological era, estimated from the average life span of species, is estimated to be about 1 billion species, and the fossil record is extremely incomplete. Except for special cases such as the ice-soaked mammoths discovered in the permafrost of Siberia and the fossils of insects sealed in amber, it is rare for the remains of paleontological organisms to remain as fossils in their original form.

Generally, after the death of an organism, the soft tissue decays and decomposes rapidly due to the action of decayers and bacteria, leaving only the hard chitinous and mineralized skeleton and tissue as the body fossil. However, in very rare cases, the soft tissue is replaced by minerals such as apatite before decay and decomposition, and exceptionally well-preserved fossils remain. A good example of this is the fossil group of the Burgess Shale, a mid-Cambrian rock formation in the Rocky Mountains of Canada. However, in general, biopolymers such as DNA and proteins in the remains of organisms decompose during the fossilization process and are preserved in the strata as chemically stable low-molecular-weight organic matter such as petroleum and coal (called chemical fossils). Chemical fossils that can be used to identify the ancient organism from which they originated are called biomarkers (biological indicator compounds).

Paleontology is concerned with all past life phenomena, and in order to do so, it is necessary to study fossils, which remain as remains or traces, in detail with reference to knowledge of comparative anatomy, comparative histology, ecology, etc., to reconstruct the body structure and ecology of lost paleontological organisms. When reconstructing the body structure, it is important to distinguish whether the form, structure, and material of the fossil are original or secondary while comparing them with living organisms. The field of research that examines the formation process of such fossils is called taphonomy.

[Tanabe Kazunari]

History of life on Earth

The diverse organisms on Earth today all have DNA and follow the same basic laws of genetics, so it is believed that all organisms evolved over a long period of time from a common ancestor. Organisms were once divided into plants and animals, but research using electron microscope technology and molecular biology techniques has broadly divided the living kingdom into three: archaea, eubacteria, and eukaryotes. The first two are called prokaryotes because they do not have a distinct nucleus or intracellular organelles. Animals, plants, fungi, and protists (including amoebas, foraminifera, and radiolaria) are included in the eukaryotes.

The Precambrian Era is the 4 billion years from 4.6 billion years ago when the Earth was born to 542 million years ago, and is distinguished from the Phanerozoic Era that followed. Most of the living organisms in the Precambrian Era were characterized by microorganisms such as Archaea and Eubacteria. Therefore, in the study of the history of life in the Precambrian Era, not only body fossils but also trace fossils and chemical fossils are important research subjects. The oldest existing strata on Earth (approximately 3.8 billion years old) contain carbon that is thought to be of biological origin, so it is believed that the first life appeared between approximately 4.6 and 3.8 billion years ago. Tiny "fossils" in spherical or ribbon-shaped forms have been reported as cyanobacteria (cyanobacteria, or blue-green algae) and fungi from strata approximately 3.5 billion years old in Western Australia and South Africa, but it has been pointed out that they may not have been derived from life. In highly saline tidal flats such as Shark Bay in southwestern Australia, the remains of cyanobacteria with weak photosynthetic ability and sand and mud deposits are piled up in stripes to form dome- or columnar-shaped calcareous structures called stromatolites. The oldest stromatolite fossils have been reported from strata in Western Australia dating back to about 3.4 billion years ago, but the fossil record is mostly found in strata from the mid-Precambrian period (about 2.7 to 1.2 billion years ago). Green algae, eukaryotes with high photosynthetic ability, appeared around 2.7 billion years ago and flourished explosively from the mid- to late Precambrian period.

Fossils of organisms thought to be multicellular animals have been reported from strata dating back to the end of the Precambrian period. In particular, the embryonic fossils of bilaterian animals discovered in strata dating back about 590 million years in southern China, and the fossil group resembling diploblastic radiolateria such as modern jellyfish and sea pens found in strata dating back about 570 million years around the world (called the Ediacara biota after the Ediacara in central-southern Australia where they were first found) are well-known. However, the exact taxonomic affiliation of these fossil groups has not been determined, and there are alternative theories that they are unicellular organisms or an unknown extinct group of organisms.

In the Cambrian period, at the beginning of the Phanerozoic epoch, most marine invertebrates with hard tissues made of chitin and minerals appeared all at once, and the number of species increased explosively. This major event in the biological world at the beginning of the Cambrian period is called the Cambrian explosion. Throughout the Phanerozoic epoch, the biological world underwent a total of five mass extinction events (at the end of the Ordovician, late Devonian, late Permian, end of the Triassic, and end of the Cretaceous), which caused repeated extinction and recovery after radiation, but over time the diversity of species increased and the structure of body parts (body plans) and ecosystems became more complex.

Taking marine fauna as an example, three evolutionary groups are recognized based on their taxonomic composition and patterns of diversity change: the Cambrian fauna, the Palaeozoic fauna, and the modern fauna. Of these, the Cambrian fauna includes trilobites (arthropods), inarticulate brachiopods, archaeococcids, and monoplacophorans (molluscs), and flourished mainly in the first half of the Palaeozoic era (540 million to 400 million years ago). Paleozoic faunas include brachiopods, crinoids (echinoderms), graptolites (hemichordata), tetracorals and tabular corals (cnidarians), nautiluses (molluscs), and bryozoans, and flourished greatly during the Paleozoic era from the Ordovician period onwards. However, they suffered a major blow from the mass extinction event in the Late Permian period (about 251 million years ago), and from the Mesozoic era onwards, their ecological niche was replaced by modern fauna such as foraminifera, bivalves, gastropods (molluscs), sea urchins (echinoderms), crustaceans, bony fish, and mammals.

Of the five great extinction events, the one at the end of the Cretaceous period (about 65.5 million years ago) saw the simultaneous extinction of ammonites (molluscs), marine reptiles, and land dinosaurs, but birds, which evolved from mammals and dinosaurs, escaped extinction and prospered greatly in the following Cenozoic era. The oldest fossils of the Hominidae family have been found in strata from the late Neogene period (about 6 to 7 million years ago) in Central Africa, and the oldest fossils of the Homo (human) genus, which includes modern humans, have been found in strata from about 2.4 million to 1.8 million years ago (early Quaternary period) in East Africa.

Meanwhile, it is said that plants moved onto land about 470 million years ago (mid-Ordovician). In the early Devonian, plants with conductive tissues that were not true vascular bundles (prevascular plants) and vascular plants flourished. In the late Devonian, in addition to the plants that reproduced by spores, plants that reproduced by seeds and plants with cambium in their stems appeared, and in the Carboniferous period, huge woody plants over 20 meters tall, such as calamites and linden, created large forests and became the source of high-quality coal. From the Devonian to the early Carboniferous periods, the concentration of carbon dioxide in the atmosphere was much higher than it is today, and it is believed that the Earth was in a warm period. However, as plants increased, they consumed carbon dioxide through photosynthesis, and the development of soil further absorbed carbon dioxide, causing a rapid decrease in the amount of carbon dioxide in the atmosphere in the late Carboniferous period, and the Earth cooled and entered an ice age. As a result, ferns, which had previously flourished mainly in wetlands, declined, and instead seed plants with superior reproductive abilities to ferns (such as Glossopteris, which flourished on the Gondwana continent) spread their distribution. In the Late Carboniferous Period, cycads and ginkgoes, which had motile sperm, appeared. In the Triassic Period of the Mesozoic Era, almost all of the ancestors of gymnosperms appeared, and in the Jurassic Period, conifers and angiosperms appeared in the Early Cretaceous Period.

[Tanabe Kazunari]

Phylogenetic classification

Like modern organisms, paleontologists classify organisms into phylum, class, family, genus, and species based on the characteristics of their physiography. The phylogenetic classification system advocated by paleontologists is based on the morphology of hard tissues and the geological distribution of each taxonomic group, and does not necessarily coincide with the views of modern biologists, who emphasize the anatomical characteristics, development, and genetic information of soft body parts. Therefore, in the classification of paleontologists, not only hard tissues but also well-preserved soft-body fossils are important. It is also necessary to pay attention to morphological differences due to individual mutations and developmental stages, convergence, and parallel evolution phenomena. Some paleontologists, such as the Paleococcidae (Cambrian period), have no close relatives in the present world, so their classification position is unclear. Trace fossils include burrows, footprints, feces, and feeding marks, but they are rarely found with the remains of the paleontologists that created them, i.e., their body fossils. Therefore, the classification position of many trace fossils is unclear, but they are given scientific names at the genus level based on their morphological characteristics.

[Tanabe Kazunari]

Environment and Evolution

Since the birth of life, living organisms have evolved while being deeply connected to the global environment. For example, in the middle of the Precambrian era, eukaryotic green algae with high photosynthetic ability flourished in addition to the cyanobacteria that existed until then. Their activity synthesized organic matter from water and carbon dioxide, and at the same time released a large amount of oxygen into the seawater, gradually increasing the oxygen concentration in the ocean and atmosphere. By the Silurian period of the Paleozoic era, the amount of oxygen in the atmosphere had increased sufficiently to form the ozone layer, which blocked ultraviolet rays and enabled living organisms to move onto land. In order to understand evolution as an interaction with the environment (adaptation), it is necessary to clarify in detail the organic and inorganic environmental factors through geochemical analysis of the strata containing fossils and analysis of the sedimentary facies, as well as paleoecological information on various paleontological organisms. In paleoecological research, it is also important to examine in detail the occurrence and preservation state of fossils to determine whether they are in situ fossils formed by deposition of remains in the place where they lived, or allochthonous fossils formed by transporting remains to another place. In order to infer the behavior, lifestyle, and diet of ancient organisms, the study of trace fossils and comparison of functional morphology with closely related modern species provide important clues.

Evolutionary biology, which focuses on living organisms, mainly studies changes in gene frequencies of populations or species over a short period of time (microevolution) through both experiments and theory, but advances in molecular biology have made it possible to investigate phylogenetic relationships between taxa of genus or higher (higher taxa) and evolution at the molecular level. Molecular biological techniques cannot be applied to most extinct species at present, except for species that have survived from geological times to the present and paleontological organisms that have been preserved under special conditions, such as mammoths. Therefore, paleontology mainly deals with the evolution of higher taxa over a long time scale (macroevolution), phylogenetic relationships, and the relationship between diversity changes and mass extinction events. The formation of species is an issue that interests both biologists and paleontologists. To date, two theories have been proposed: one hypothesis states that new species formed by slow and continuous differentiation from large populations of ancestral species (gradual lineage theory), and the other that states that new species formed from small surrounding populations with rapid morphological changes over a short period of time (punctuated equilibrium theory). However, looking at the fossil record, both theories are found, and it is difficult to say which is correct.

The life span of a species, which is one of the factors that determine macroevolution, is short, averaging 2 to 3 million years for mammals, trilobites, and ammonites, but long, 10 million years for marine bivalves and foraminifera. The extinction rate tends to be higher for species with shorter life spans. Living organisms that have survived for a long time and maintained a primitive structure for a long time are called relict species or "living fossils." Good examples are Lingula (since the Cambrian period) and Ginkgo (since the Triassic period) that live in the Ariake Sea. On the other hand, trilobites, ammonites, and fusulina, which have undergone rapid morphological changes and have short life spans, are used as standard stones to classify and compare strata.

[Tanabe Kazunari]

"Introduction to Evolutionary Paleontology: Tracing the Evolution of Crustaceans, by Ikeya Masayuki and Yamaguchi Toshiyuki (1993, University of Tokyo Press)" ▽ "Introduction to Paleontology, by Mashima Ryuichi and Ikeya Masayuki (1996, Asakura Publishing)" ▽ "To Ultra-Darwinists: A Paleontologist's View of Evolution, by Niles Eldridge, translated by Nizuma Akio (1998, Springer-Verlag Tokyo)" ▽ "The Science of Paleontology 1: Overview and Classification of Paleontology, edited by Hayami Itaru and Mori Kei (1998, Asakura Publishing)" ▽ "The Science of Paleontology 2: Morphology and Analysis of Paleontology, edited by Tanabe Kazunari and Mori Kei (1999, Asakura Publishing)" ▽ "The Science of Paleontology 3: Life Histories of Paleontology, edited by Tanabe Kazunari and Ikeya Masayuki (2001, Asakura Publishing)" ▽ "Introduction to Modern Evolutionary Science" by C. Patterson, translated by Mawatari Shunsuke, Uehara Masumi, and Isono Naohide (2001, Iwanami Shoten)" ▽ "A 4 Billion Year History of Life" by Richard Forty, translated by Watanabe Masatake (2003, Soshisha) ▽ "The First 3 Billion Years of Life: Evolutionary Footprints Carved on the Earth" by Andrew H. Knoll, translated by Saito Takao (2005, Kinokuniya Shoten) ▽ "Paleontology" by Hayami Itaru (2009, University of Tokyo Press) ▽ "Field Paleontology: Deciphering the Footprints of Evolution from Fossils" by Oji Tatsuo (2009, University of Tokyo Press) ▽ "Encyclopedia of Paleontology, 2nd Edition, edited by the Paleontological Society of Japan (2010, Asakura Shoten)"

Odontocephalus aegeria (trilobite)
Hall, Devonian period, Paleozoic, body length approx. 7cm, from Pennsylvania, USA, photo/AIST Geological Survey of Japan (GSJ F13108)

Odontocephalus aegeria (trilobite)

Sugiyamaera carbonarium (Tetracoral)
Yabe and Minato Early Carboniferous period of the Paleozoic era Diameter: approx. 4cm Produced in Ofunato City, Iwate Prefecture Photo: National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan (GSJ F5793)

Sugiyamaella carbonarium (four-leaved sunflower)

Siringopora (tabletop coral)
sp. Early Carboniferous period of the Paleozoic era. Specimen width: approx. 12 cm. Produced in Sumita-cho, Kesen-gun, Iwate Prefecture. Photo courtesy of the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ F5795) .

Siringopora (tabletop coral)

Gaudriceras denseplicatum (ammonite)
(Jimbo) Late Cretaceous period, Mesozoic era, diameter approx. 5.5cm, from Obira-cho, Rumoi-gun, Hokkaido, Japan . Photo by National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan (GSJ F3242)

Gaudricellas denseplicatum (An…

Calamites (Horsetails)
sp. Carboniferous period of the Paleozoic era Length: about 20 cm Produced in Yorkshire, England © Photo courtesy of the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ F7504)

Calamites (Horsetails)

Ginkgoites (Ginkgo family)
sp. Late Triassic period of the Mesozoic era Leaf size: about 3 cm Produced in Mine City, Yamaguchi Prefecture © Photo courtesy of the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ F7559)

Ginkgoites (Ginkgo family)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地質時代に生存していた生物。その遺骸(いがい)や足跡、巣穴、糞(ふん)などの生活の記録（生痕(せいこん)）はそれぞれ体化石body fossil、生痕化石trace fossilとして地層中に残る。古生物には絶滅種と現生種が含まれる。化石種として記載された古生物は約13万種に達するが、その数は既知の現生生物の総種数約175万のわずか7.4％にすぎない。ただし、多くの古生物種は形態種であり、生殖関係を基礎とした生物学的種とは概念的に異なる。種の平均生存期間から推定される地質時代に生存した古生物種の総数は、約10億種と推定されることからみてもわかるように、化石の記録は著しく不完全である。シベリアの永久凍土層から発見された氷浸(づ)けのマンモスや琥珀(こはく)中に封じ込められた昆虫の化石のような特殊な場合を除くと、古生物の遺骸がそのまま化石として残ることはまれである。

　一般には生物の死後、軟部組織は腐食者やバクテリアの働きで急速に腐敗・分解してなくなり、堅いキチン質や鉱物質の骨格や組織のみが体化石となる。しかし、ごくまれには軟部組織が腐敗・分解する前に燐灰(りんかい)石などの鉱物に置換して例外的に保存のよい化石として残ることがある。その好例として、カナダ、ロッキー山脈の中部カンブリア系バージェス頁岩(けつがん)の化石群が挙げられる。しかし、一般には生物遺骸中のDNAやタンパク質などの生体高分子は、化石化の過程で分解して、石油や石炭などの化学的に安定な低分子の有機物（これを化学化石と称する）として地層中に保存される。化学化石のうち、その由来となった古生物が推定できるものを、バイオマーカー（生物指標化合物）biomarkerという。

　古生物学は過去のすべての生命現象を対象とするが、そのためには、遺骸や生痕として残された化石を比較解剖学・比較組織学・生態学などの知識を参考にして詳細に調べ、失われた古生物の体制や生態を復原する必要がある。体制の復原に際しては、現生生物と比較しつつ化石の形態、構造、および物質が本来のものか二次的なものかを識別することが重要である。このような化石の形成過程を調べる研究分野をタフォノミーtaphonomyという。

［棚部一成］

地球生命史

現在の地球の多様な生物はどれもDNAをもち、基本的に同じ遺伝法則に従うことから、すべての生物は共通の祖先から長い時間をかけて進化したと考えられる。生物はかつて植物と動物に分けられてきたが、電子顕微鏡の技術と分子生物学の手法を用いた研究から、生物界は古細菌、真性細菌、真核生物の三つに大別される。前二者ははっきりとした核や細胞内器官をもたないことから原核生物とよばれる。動物、植物、菌類、原生生物（アメーバ、有孔虫、放散虫を含む）は真核生物に含められる。

　地球が誕生した46億年前から5億4200万年前までの約40億年間を先カンブリア時代とよび、それ以降の顕生累代と区別される。先カンブリア時代の生物界のほとんどは古細菌や真性細菌などの微生物で特徴づけられる。したがって、先カンブリア時代の生命史の研究では、体化石だけでなく生痕化石や化学化石が重要な研究対象となる。地球上に現存する最古の地層（約38億年前）中には生命起源のものと思われる炭素が含まれていることから、最初の生命は約46億年前から38億年前の間に出現したと考えられる。オーストラリア西部や南アフリカの約35億年前の地層から球状ないしリボン状のかたちをした微小な「化石」が藍(らん)細菌（シアノバクテリアCyanobacteriaすなわち藍藻）や菌類として報告されているが、それらは生命由来ではない可能性が指摘されている。オーストラリア南西部のシャーク湾などの塩分の高い干潟では、弱い光合成能力をもつ藍細菌の遺骸と砂や泥の堆積物が縞状に積み重なってドーム状ないし円柱状の形をしたストロマトライトstromatoliteとばれる石灰質の構造をつくる。最古のストロマトライトの化石は西オーストラリアの約34億年前の地層から報告されているが、その化石記録は先カンブリア時代中ごろ（約27億年～12億年前）の地層に多い。光合成能力に富む真核生物である緑藻類は27億年前ごろに出現し、先カンブリア時代の中ごろから後期にかけて爆発的に繁栄するようになった。

　多細胞動物のものと思われる生物の化石は先カンブリア時代末期の地層から報告されている。とくに中国南部の約5億9000万年前の地層から発見された左右相称動物の胚化石や、世界各地の約5億7000万年前の地層から産した現生のクラゲやウミエラなどの二胚葉性放射相称動物に類似する化石群（最初の産地であるオーストラリア中南部のエディアカラにちなんで、エディアカラ生物群Ediacara biotaとよばれる）は有名である。ただし、これらの化石群の正確な分類学的所属については未確定で、単細胞生物や絶滅した未知の生物群とする異説もある。

　顕生累代初頭のカンブリア紀に入ると、キチン質や鉱物質の硬組織をもつ海成無脊椎(むせきつい)動物のほとんどが一斉に出現し、爆発的に種類を増やしていった。このカンブリア紀初頭の生物界の大変革事件はカンブリア爆発Cambrian explosionとよばれる。顕生累代を通じて生物界は計5回の大量絶滅事変（オルドビス紀末、デボン紀後期、ペルム紀後期、三畳紀末、白亜紀末）により放散後の絶滅と回復を繰り返しながら、時代とともに種の多様性を増加させ、また体の構造（体制）や生態系の構造を複雑化させていった。

　海生動物群を例にとると、その分類学的構成や多様性の変動様式により、三つの進化動物群、すなわちカンブリア紀型動物群、古生代型動物群、および現代型動物群が認められている。このうち、カンブリア紀型動物群は三葉虫類（節足動物）、無関節型腕足動物、古盃(こはい)類、単板類（軟体動物）などを含み、おもに古生代前半（5億4000万年前から4億年前）に繁栄した。古生代型動物群は腕足動物、ウミユリ類（棘皮(きょくひ)動物）、筆石類（半索動物）、四放サンゴ類や床板サンゴ類（刺胞動物）、オウムガイ類（軟体動物）、コケムシ類を含み、オルドビス紀以降の古生代に大繁栄を遂げたが、ペルム紀後期（約2億5100万年前）の大量絶滅事変によって大打撃を受けて、中生代以降それらの生態的地位を有孔虫類、二枚貝類や腹足類（軟体動物）、ウニ類（棘皮動物）、甲殻類、硬骨魚類、哺乳類などの現代型動物群に置き換えられた。

　五大絶滅事変のうち白亜紀末（約6550万年前）の事変では、アンモナイト（軟体動物）、海生爬虫(はちゅう)類、陸生の恐竜類などが同時に絶滅したが、哺乳類や恐竜の仲間から分化した鳥類は絶滅をまぬがれて次の新生代に大繁栄を遂げた。最古のヒト科の化石は中央アフリカの新第三紀後期（約600～700万年前）の地層から、またわれわれ現生人類を含むホモ（ヒト）属の最古の化石は東アフリカの約240万～180万年前（第四紀前期）の地層からみつかっている。

　一方、植物が陸上に進出したのは約4億7000万年前（オルドビス紀中期）といわれている。デボン紀初期には、真の繊管束ではない通道組織をもった植物（前繊管束植物）や繊管束植物が繁栄した。デボン紀後半になると胞子で繁殖していた植物の仲間に加えて種子で繁殖するものや茎に形成層をもつものが現れ、石炭紀に入るとカラミテスやリンボクなどの高さ20メートルを超える巨大な木質植物が大森林をつくり、良質な石炭の原料となった。デボン紀から石炭紀前半にかけては、大気の二酸化炭素の濃度が現在よりかなり高く、地球は温暖な時期であったと考えられている。しかし植物が増えてくると、光合成により二酸化炭素を消費し、土壌の発達がさらに二酸化炭素を吸収することにより、石炭紀の後半には大気中の二酸化炭素が急減し、地球は寒冷化して氷河時代となった。その結果、これまで湿地を中心に繁栄していたシダ植物は衰えて、そのかわりにシダ植物より繁殖能力に優れた種子植物（たとえばゴンドワナ大陸に繁栄したグロッソプテリス）が分布を広げた。石炭紀後期には自分で運動できる精子をもったソテツ類やイチョウ類が出現した。中生代の三畳紀に入ると裸子植物の祖先群がほとんどすべて登場し、またジュラ紀には針葉樹が、前期白亜紀には被子植物が、それぞれ出現した。

［棚部一成］

系統分類

古生物は現生生物と同様に、体制の特徴に基づいて、門・綱・科・属・種の順で分類される。古生物学者の唱える系統分類体系は、硬組織の形態や各分類群の地史的分布に立脚するため、軟体部の解剖学的特徴・発生や遺伝情報を重視する現生生物学者の見解とはかならずしも一致しない。したがって古生物の分類では、硬組織のみならず保存のよい軟部化石が重要となる。また個体変異・発生段階による形態の違い、収斂(しゅうれん)や平行進化現象にも注意する必要がある。古生物のなかには、現世に近縁種がないため分類上の位置が未確定な古盃類（カンブリア紀）のような例もある。生痕化石には巣穴、足跡、糞(ふん)、捕食痕などがあるが、それらをつくった古生物の遺骸そのもの、すなわち体化石とともに産することはまれである。そのため生痕化石の分類上の位置は不明なものが多いが、形態学的特徴によって属のレベルで学名が与えられている。

［棚部一成］

環境と進化

生命の誕生以来、生物は地球環境と互いに深いつながりをもちながら進化してきた。たとえば、先カンブリア時代中ごろになると、それまでにいた藍細菌に加えて光合成能力の高い真核生物の緑藻類が繁栄を遂げた。その活動によって水と二酸化炭素から有機物が合成され、それとともに大量の酸素が海水中に放出されて海洋や大気の酸素濃度がしだいに増加していった。古生代のシルル紀までには大気中の酸素が十分に増加してオゾン層が形成された結果、オゾン層による紫外線の遮断が実現し、生物が陸上に進出することを可能にした。進化を環境との相互作用（適応）としてとらえるためには、古生物各種の古生態学的情報はもちろんのこと、化石を含む地層の地球化学的分析や堆積(たいせき)相の解析により、有機的・無機的環境要因を詳しく明らかにする必要がある。また、古生態学的研究では、化石の産状や保存状態を詳しく調べて、生きていた場所で遺骸が堆積してできた現地性化石であるか、それとも遺骸が別の場所に運搬されてできた異地性化石であるかを明らかにすることが重要である。古生物の行動・生活様式・食性などを推定するには、生痕化石の研究や近縁の現生種との機能形態の比較が大きな手掛りとなる。

　現生生物を対象とした進化生物学では、おもに短期間での個体群や種の遺伝子頻度の変化（小進化）を実験と理論の両面から研究するが、分子生物学の進展によって属以上の分類群（高次分類群）間の系統関係や分子レベルでの進化を調べることができるようになった。分子生物学的手法は、地質時代から現在まで生き延びた種やマンモスなど特殊な条件で保存された古生物を除き、大部分の絶滅種には今のところ適用できない。そのため、古生物学では主として長い時間軸での高次分類群の進化（大進化）、系統関係、多様性変動と大量絶滅事変との関係などを扱う。種の形成は、生物学者と古生物学者の両方が興味をもつ問題である。これまでに、新しい種は祖先種の大きな個体群からゆっくり連続的に分化しながら形成したとする説（系列漸進説）と、周辺の小個体群から短期間に急激な形態変化を伴いながら形成されたとする説（断続平衡説）が提唱されているが、化石記録をみると両方の場合があり、どちらが正しいかは一概にいえない。

　大進化を決める要因の一つである種の生存期間は、哺乳(ほにゅう)類や三葉虫、アンモナイトなどでは平均200万～300万年と短く、逆に海生二枚貝や有孔虫では1000万年と長い。絶滅率は、種の生存期間の短い類ほど高い傾向にある。種や属の生存期間が長く、原始的な体制を長期間維持してきた現生生物は、遺存種、または「生きた化石」とよばれる。有明海(ありあけかい)に生息するシャミセンガイLingula（カンブリア紀以降）やイチョウGinkgo（トリアス紀以降）がその好例である。逆に形態変化がすみやかで生存期間の短い三葉虫、アンモナイト、フズリナなどは標準化石として地層の区分や対比に利用されている。

［棚部一成］

『池谷仙之・山口寿之著『進化古生物学入門――甲殻類の進化を追う』（1993・東京大学出版会）』▽『間島隆一・池谷仙之著『古生物学入門』（1996・朝倉書店）』▽『ナイルズ・エルドリッジ著、新妻昭夫訳『ウルトラ・ダーウィニストたちへ――古生物学者から見た進化論』（1998・シュプリンガー・フェアラーク東京）』▽『速水格・森啓編『古生物の科学1　古生物の総説・分類』（1998・朝倉書店）』▽『棚部一成・森啓編『古生物の科学2　古生物の形態と解析』（1999・朝倉書店）』▽『棚部一成・池谷仙之編『古生物の科学3　古生物の生活史』（2001・朝倉書店）』▽『C・パターソン著、馬渡峻輔・上原真澄・磯野直秀訳『現代進化学入門』（2001・岩波書店）』▽『リチャード・フォーティ著、渡辺政隆訳『生命40億年全史』（2003・草思社）』▽『アンドルー・H・ノール著、斉藤隆央訳『生命最初の30億年――地球に刻まれた進化の足跡』（2005・紀伊国屋書店）』▽『速水格著『古生物学』（2009・東京大学出版会）』▽『大路樹生著『フィールド古生物学――進化の足跡を化石から読み解く』（2009・東京大学出版会）』▽『日本古生物学会編『古生物学事典　第2版』（2010・朝倉書店）』