|Peripatus Home Page Evolution >> Major Events in the Evolution of Life||Updated: 06-Nov-2018|
This page provides a high level review of some of the main threads of the evolution of plants and animals, from the origin of life to the present day.
Keywords: evolution, metazoan radiation, Ediacaran, Cambrian, Embryophyta, tetrapods, Permian, extinction, dinosaurs, angiosperms, mammals, humans
Life on earth has a long and rich history – dating back almost to the time when the Earth’s crust solidified. This was a time when the planet was unrecognisable; when catastrophic bombardment by meteors likely wiped out the first microbes to evolve, again and again.
Much of this earliest life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known – even among the scientific community – the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called Cambrian Explosion. Whether the “explosion” was real or an artifact of preservation, by the end of the Cambrian all readily fossilisable modern animal phyla except the bryozoans (Entoprocta + Ectoprocta) were represented in the fossil record.
Plants had existed since the earliest fossils were formed, but did not achieve a high degree of organisation until vascular plants evolved on land, perhaps around the Silurian. There, they modified the environment, promoted the formation of true soils, and established new ecological niches without which animal life on land could scarcely have flourished. In the Mesozoic, possibly as early as the Triassic, the angiosperms (flowering plants) diverged from the Gnetales, or possibly the Bennettitales, leading to an enormous radiation of this group beginning in the mid-Cretaceous. The angiosperms are the most diverse group of land plants living today.
The chordates appear to have been present from at least the mid Cambrian, and diversified throughout the early Paleozoic, giving rise to fishes and, later, to the tetrapods which began to invade the land in the Devonian and Carboniferous. The first of these creatures were amphibians, tied to the water, but the evolution of the amniote egg allowed subsequent groups to become truly terrestrial. By the close of the Paleozoic, reptiles, dinosaurs, and mammal-like reptiles had taken possession of the land.
The Mesozoic was famously the “age of dinosaurs”, although their early days were perhaps more tentative than previously thought (Brusatte 2018). An exotic collection of specialised reptiles, including the giant aquatic reptiles and the pterosaurs, also dominated their respective environments at this time. At some time prior to the late Jurassic, the earliest birds diverged – according to the most widely held view – from theropod dinosaur stock.
Mammals also became established and diversified throughout the Mesozoic, but their great radiation, and the establishment of the modern families would not occur until the Paleocene. Of particular interest to us, human ancestry can be tentatively traced back into the late Miocene, although the lineages remain rather unclear until at least the Pliocene.
Origins and Early Development
As recently as the 1960s, as far as was widely known, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and diversity. Indeed, Charles Darwin (Origin of Species – 2nd ed. Chapter IX) recognised that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection. He suggested that “before the lowest [Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the [Cambrian] age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.”
He was quite correct, on both counts.
The oldest rocks known are those of the Isua Supracrustal Group of southwestern Greenland, surviving from around 3700 Ma but, unfortunately, preserving no fossils. The Isua rocks are strongly metamorphosed; although some sequences have been demonstrated to be of sedimentary origin, and may have once contained fossils, the heat and pressure to which they have been subjected will have destroyed them. Isotopic signatures of carbon recovered from these and similarly aged rocks provides indirect evidence that life may have existed in these remote times (Rosing 1999, Brack 2001, Ueno et al. 2006).
It should, however, be noted that a number of less enthusiastic commentators advise approaching such an interpretation with extreme care (e.g. van Zuilen et al. 2002).
Life may have evolved quite soon after the formation and cooling of a solid crust with liquid water, after the worst of the planet-forming bombardment from space had ceased. It has been suggested that life possibly originated more than once, but was wiped out by heavy bombardment which melted the crust again. It seems unlikely that we can ever know about that.
“Several authors have suggested that comets or carbonaceous asteroids contributed large amounts of organic matter to the primitive Earth, and thus possibly played a vital role in the origin of life. But organic matter cannot survive the extremely high temperatures (>104 K) reached on impact, which atomize the projectile and break all chemical bonds. Only fragments small enough to be gently decelerated by the atmosphere – principally meteors of 10-12–10-6 g – can deliver their organic matter intact. The amount of such ‘soft-landed’ organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only ~0.006 g cm-2 intact organic carbon would accumulate in 108 yr, but at the higher rates of ~4 x 109 yr ago, about 20 g cm-2 may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on Earth” (from Anders 1989, Abstract).
“The early Archean record tells us that life was present at least 3500 Ma ago. Microbial ecosystems were driven by autotrophy, most likely photoautotrophy, and oxygenic cyanobacteria may already have appeared. Heterotrophs included prokaryotes and, possibly, primitive amitochondrial eukaryotes capable of feeding by phagocytosis. Depending on the amount of O2 available, the biota could also have included aerobic prokaryotes and mitochondrion bearing eukaryotic heterotrophs (but perhaps not eukaryotic algae; see ... Knoll & Holland 1995). Although impossible to test empirically, the possibility that early communities included organisms unlike anything represented in the modern biota cannot be excluded. Clearly, early Archean ecosystems remain poorly understood” (Knoll 1996, p. 55).
The oldest fossils known to date, derive from the Apex Chert, a formation of the Pilbara Supergroup occuring in northwestern Western Australia, and dated at 3,465 Ma (± 5 Ma, see Schopf 1999, pp. 88-89). However, the fossils occur in fragments of rock within the chert; thus they are even older, though by how much, is unknown. The organisms themselves are filamentous, composed of distinct, organic walled cells occurring as a uniserial string, and are interpreted as cyanobacteria.
Also occurring in the Pilbara Craton of western Australia, the Strelley Pool Chert (SPC) includes structures interpreted as stromatolites. A 2006 study of these concludes that the evidence “strongly indicates that organisms flourished on a broad peritidal platform 3.43 Gyr ago in the Pilbara, rapidly taking hold and creating a reef-like build-up in shallow waters as surfaces became submerged. The variety of stromatolites present indicates that the SPC may contain not only some of Earth’s earliest fossils but also a diverse fossil ‘ecosystem’, sustained by shallow seawater free of terrigenous influx – ideal conditions for phototrophism” (Allwood et al. 2006, p. 717-718).
Another ancient cyanobacteria collection is reported from the Nauga Formation, Prieska, South Africa, between 2,588 ± 6 and 2,549 ± 7 Ma (Kazmierczak & Altermann 2002).
The occurrence of cyanobacteria of this age is consistent with the belief that aerobic metabolism evolved independently in all three lineages, after the diversification of the superkindoms, yet prior to the widespread introduction of molecular oxygen into the anaerobic biosphere at approximately 2.2 Ga. “The fact that enzyme distribution in aerobic pathways was largely incongruent with organismal speciation suggests that adaptation to molecular oxygen occurred after the major prokaryotic divergences on the tree of life. This is supported by data from geological and molecular evolutionary analyses, showing that all three domains of life, and many phyla within these domains, had appeared by the time that oxygen became widely available.... The relatively late onset of atmospheric oxidation argues against the invention of O2-dependent enzymes or pathways in the last common ancestor of modern organisms, suggesting that adaptation to molecular oxygen took place independently in organisms from diverse lineages exposed to O2” (Raymond & Segrè 2006).
At 2,000 Ma we find possible eukaryotes, and by 1,750 Ma we are confident of this identification. The current “standard” view of the origin of eukaryotes, revolutionary in its time, is Margulis’ endosymbiont hypothesis, which describes how eukaryotes improved their metabolic capacity by engulfing certain prokaryotes and converting them into intracellular organelles, principally mitochondria and chloroplasts.
“The bacterial endosymbiosis which led to the development of the mitochondria probably occurred very early in eukaryotic evolution. Phylogenetic analyses using different molecular markers place at the base of eukaryotes various protist lineages (the archamoeboe, metamonads, microsporidia and parabasalia) known as the Archezoa. The Archezoa are not a true clade of organisms but rather a collection of species united by their lack of mitochondria, anaerobic metabolism, simple cell morphology and bacteria-like ribosomes” (Brown 2001).
There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed from early in the Ediacaran. Although some traces are simple, rather featureless, winding trails, “others display transverse rugae and contain pellets that can be interpreted as of fecal origin. The bilaterian nature of these traces is not in dispute. Furthermore, such traces must have been made by worms, some of which had lengths measured in centimetres, with through guts, which were capable of displacing sediment during some form of peristaltic locomotion, implying a system of body wall muscles antagonized by a hydrostatic skeleton. Such worms are more complex than flatworms, which cannot create such trails and do not leave fecal strings” (Valentine 1995, p. 90). Sets of paired hypichnial ridges strongly hint at an arthropod s.l. presence.
Unfortunately, it is equally true that the relatively few body fossils known from the late Precambrian do not shed much light on the sequence of evolutionary advances that led to the famously diverse Cambrian taxa. There are a few sign-posts, however:
Mineralised skeletons of uncertain affinity – the ‘small shelly fauna’ – appear just before the beginning of the Cambrian, ~550 Ma, increasing in numbers and diversity towards the Tommotian. The most common skeletal materials are calcium carbonate (aragonite or calcite) and varieties of calcium phosphate. Many of the latter may originally have been carbonates, phosphatized during preservation.
Life on earth has a long and rich history – predating the beginning of the Cambrian by a factor of six or seven. However, much of this early life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known even among the scientific community, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called “Cambrian Explosion.”
Charles Darwin (Origin of Species – 2nd ed. Chapter IX) recognised that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection, “and may be truly urged as a valid argument against the views here entertained.” He suggested that “before the lowest [Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the [Cambrian] age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.”
Today, although the fossil record now extends back 3,465 Ma (± 5 Ma, to the Apex Chert, see Schopf 1999, p. 100) and diverse precursors to the Cambrian biotas are gradually becoming understood, it still appears as if a genuinely rapid diversification of form did occur among the Metazoa (“animals”) during the Cambrian. During this time, most extant body plans are suddenly found in the fossil record. “Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (Valentine et al. 1991)” (Wray et al. 1996). By way of contrast, “it appears that no [new] phylum-level body plans have arisen in the animal kingdom in the last 500 million years” (Arthur 1997, p. 7).
When this radiation began, and how rapidly it unfolded, is still the subject of on-going research, yet much has become clear since Darwin’s day.
Understanding of Cambrian fossils, and their evolutionary significance, has been hugely enhanced by the study of fossils from several famous lagerstätten (sing. Lagerstätte): fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both.
The most widely known of these is the Middle Cambrian Burgess Shale. Although this fossil site has been known since the early 1900s, very detailed study would not be accorded these fossils until the mid 1970s. The definitive popular account of this research, together with a contentious interpretation, is to be found in Stephen J. Gould’s book, Wonderful Life (Gould 1989). More recently, in the 1980s and ’90s, significantly older Early Cambrian lagerstätten have been located and studied at the Chengjiang and Sirius Passet localities in China and Greenland, respectively.
Sirius Passet: Located on the northern coast of Greenland, this lower Cambrian site has yielded a diverse though not spectacularly preserved fauna, including a Redlichiid trilobite, Buenellus higginsi, the halkieriid Halkieria evangelista, and the enigmatic lobopod Kerygmachela kierkegaardi. The age is around 515 to 520 Ma.
Chengjiang: Fossils from Chengjiang, near the city of Kunming in Yunnan Province, China, preserve a diverse biota of rather similar age to the Sirius Passet fauna; the differences between their fossil assemblages are attributed to environment and geography. The soft-bodied fossils include algae, medusiform metazoans, chondrophorines, sponges, chancelloriids, sea anemones, priapulid worms, hyoliths, possible ectoprocts, inarticulate brachiopods, annelid-like animals, lobopodians, trilobites and other arthropods, hemichordates, chordates as well as taxa that cannot definitely be assigned to any well established groups.
Emu Bay Shale (Kangaroo Island): The Early Cambrian Emu Bay Shale of Kangaroo Island, South Australia, has long been famous as a source of magnificent specimens of the trilobites Redlichia takooensis and Hsuaspis bilobata. It is additionally important as the only site in Australia so far to yield a Burgess Shale-type biota. As such it represents an important basis for comparison with other Burgess Shale-type assemblages, particularly Chengjiang, which is the closest paleogeographically, although somewhat older.
Burgess Shale: I may come up with my own material eventually; meanwhile, do take a look at The Burgess Shale Geoscience foundation.
The base of the North American Whiterock Series coincides with the start of the great Ordovician radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and virgellinid graptoloids. Many other groups first appear in the fossil record at or near this horizon. This also marks the point of transition between dominance of Sepkoski’s Cambrian Evolutionary Fauna, of which trilobites are the main component, and the rise of the Paleozoic Evolutionary Fauna composed of the radiating groups.
The Whiterock Fauna shows explosive and sustained radiation at exactly this point, conforming to the Paleozoic fauna diversity pattern, and indicating that trilobites were active participants in the Ordovician radiation.
“Cluster analysis grouped families according to their numbers of component genera in each of the four Ordovician stratigraphic series ... showing diversity histories of all families [fig. 1] indicate that the trilobites form two major clusters, termed faunas. The Ibex Fauna, named for the epoch during which it flourished, is characterized by Early Ordovician dominance followed by severe diversity reductions in the later Ordovician (conforming to a null hypothesis of cohort decay). The Whiterock Fauna, named for the epoch in which it radiated rapidly, displays a contrasting pattern of minimal Early Ordovician diversity, Middle Ordovician (Whiterock) radiation, and high Late Ordovician diversity. These alternate, disjunct patterns are pervasive and represent high-level macroevolutionary trends. Surviving Silurian families constitute a subgroup of the Whiterock Fauna, termed the Silurian Fauna. No member of the Ibex Fauna survived the end of the Ordovician. In contrast, nearly three-fourths of families (74%) of the Whiterock Fauna survived, and the Whiterock Fauna accounts for all post-Ordovician trilobites, with the exception of the unclustered harpetids.” (Adrain et al. 1998).
Some short-lived Early Ordovician (Tremadocian) forms preceeded the principal Ordovician groups such as Seutelluina, Phacopida (Fig. 2) and Trinucleina. These principal forms were “all highly differentiated and diverse, most of them of crypogenic origin, and surviving for various periods thereafter. It is an interesting feature of trilobite evolution that after this great burst of constructional themes in the early Ordovician very few entirely new patterns of organisation arose; afterwards evolution in trilobites was largely a matter of [variations upon the Ordovician themes]” (Clarkson 1993, pp. 373-374).
The Ordovician trilobites were more successful at exploiting new environments, notably reefs, but they too suffered a crisis, during the mass extinction at the end of the Ordovician. Some distinctive and previously successful forms such as the Trinucleoidea and Agnostoidea became extinct.
Whiterock Fauna families, and in particular Silurian Fauna groups, can generally be traced only to the Early Ordovician, and in many cases they are entirely “cryptogenetic.” These clades certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were being developed very rapidly. Of the Silurian Fauna genera present during the Llandovery epoch, 78% also originated in that epoch, which demonstrates that extinctions were followed by a rapid post-extinction “rebound.” However, once this Llandovery rebound was completed, standing diversity returned to and was maintained at pre-extinction amounts. (After Adrain et al. 1998.)
Vertebrates underwent a major adaptive radiation during the Devonian, when jawed species (gnathostomes) and particularly placoderms (armoured fishes), became dominant. A Lochkovian peak of diversity is registered in various Lower Devonian series all around the Old Red Sandstone Continent and Siberia, for ostracoderms in general, and heterostracan pteraspidomorphs in particular. It occurs at different time slices in the Lochkovian, depending on the localities, and may be followed by another smaller peak in the Pragian” (Blieck 2017, Abstract).
“The relationship of the three living groups of sarcopterygians or lobe-finned fish (tetrapods, lungfish and coelacanths) has been a matter of debate. Although opinions still differ, most recent phylogenies suggest that tetrapods are more closely related to lungfish than to coelacanths. However, no previously known fossil taxon exhibits a concrete character combination approximating the condition expected in the last common ancestor of tetrapods and lungfish—and it is still poorly understood how early sarcopterygians diverged into the tetrapod lineage (Tetrapodomorpha) and the lungfish lineage (Dipnomorpha)” (Zhu & Yu 2002, p. 767, Abstract).
Zhu & Yu (2002) describe a new fossil sarcopterygian fish, Styloichthys changae, that “possesses an eyestalk and which exhibits the character combination expected in a stem group close to the last common ancestor of tetrapods and lungfish. Styloichthys from the Lower Devonian of China bridges the morphological gap between stem-group sarcopterygians (Psarolepis and Achoania) and basal tetrapodomorphs/basal dipnomorphs. It provides information that will help in the study of the relationship of early sarcopterygians, and which will also help to resolve the tetrapod-lungfish divergence into a documented sequence of character acquisition” (Zhu & Yu 2002, p. 767, Abstract).
Colonisation of the Land
The first major diversification of land plant life, such as rhyniophytes, zosterophylls, drepanophycaleans, lycophytes, trimerophytes, and (in the southern hemisphere) Baragwanathia, began as early as Late Ludlow (Late Silurian) in Australia through Gedinnian (Early Devonian) on Laurentia.
“The Devonian was pivotal in the evolutionary history of land plants: a diverse range of plant architectures evolved and plants radiated into a remarkable array of ecological niches (Gensel & Andrews 1984, Gerrienne et al. 2004, Stein et al. 2012). Numerous plant taxa have been reported from Devonian deposits around the world and phytogeographical provincialism began to develop in the global flora (see Raymond et al. 2006). However, plants with simple form and diminutive size, i.e., with axis widths less than 2 mm, from the Early–Middle Devonian, have been paid little attention (see Cai & Wang 1995, Wang & Berry 2003, Wang et al. 2004, 2007). Those basal euphyllophytes have quite distinct morphological characters in their branching patterns and sporangial attachment, and played key roles in the evolution of early land plants (Stewart & Rothwell 1993, Kenrick & Crane 1997)” (Xu et al. 2017, p. 524).
In contrast to megascopic plants, which appear to have colonized the land only once, many animal groups made the transition to terrestrial existence independently and overcame the problems of water relations in different ways. Early evidence for terrestrial animals is sparse, but by the Early Devonian exquisitely preserved arthropod faunas are known from several localities in North America, Germany and the United kingdom. These faunas document the appearance of diverse arthropod communities including centipedes, millipedes, trigonotarbids and their living relatives spiders, pseudoscorpians, mites (orbatids and endeostigmatids), arthropleurids (extinct arthropods), archaeognathans (primitive wingless insects), collembolans and possibly bristletails. Available evidence indicates that these animals were mainly predators and detritivores and, until the appearance of vertebrate herbivores in the latest Palaeozoic, most energy flow into animal components of early terrestrial ecosystems was probably through the decomposer pathway rather than direct herbivory. Indirect evidence for herbivory comes from wound responses in the tissues of some fossil plants, and perhaps also from fossil faecal pellets containing abundant spores (abridged from Kenrick & Crane 1997, p. 38).
“The fossil record of early tetrapods has been increased recently by new finds from the Devonian period and mid–late Early Carboniferous period. Despite this, understanding of tetrapod evolution has been hampered by a 20-million-year gap (‘Romer’s Gap’) that covers the crucial, early period when many key features of terrestrial tetrapods were acquired” (Clack 2002, p. 72). “This period between the end Devonian and the mid-Viséan represents the time when tetrapods underwent a major diversification and acquired true terrestriality” (Clack & Finney 1999).
“The first half of the Mississippian or Early Carboniferous (Tournaisian to mid-Viséan), an interval of about 20 million years, has become known as “Romer’s Gap” because of its poor tetrapod record. Recent discoveries emphasise the differences between pre-“Gap” Devonian tetrapods, unambiguous stem-group members retaining numerous “fish” characteristics indicative of an at least partially aquatic lifestyle, and post-“Gap” Carboniferous tetrapods, which are far more diverse and include fully terrestrial representatives of the main crown-group lineages. It seems that “Romer’s Gap” coincided with the cladogenetic events leading to the origin of the tetrapod crown group” (Chen et al. 2018, Abstract).
The only articulated skeleton of a tetrapod yet found from the Tournaisian epoch (354–344 Ma) is Pederpes, “the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl” (Clack 2002, Abstract). With its later American sister-genus, Whatcheeria, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas” (Clack 2002, p. 72).
Chen et al. (2018) describe a partial right jaw ramus of a new tetrapod, Tantallognathus woodi, from the late Tournaisian or early Viséan of Scotland. The large and robust jaw exhibits a distinctive combination of characters. A phylogenetic analysis places Tantallognathus in the upper part of the tetrapod stem group, above Pederpes and Whatcheeria.
Early tetrapods evolved the amniote egg, thereby becoming truly terrestrial (Palmer 1999, p. 81).
Close of the Paleozoic
Of all Phanerozoic mass extinctions, the event at the end of the Permian was by far the most profound. At this time something like 60% of all species (and perhaps 90% of all marine forms) went extinct. The last few trilobites disappeared during this event although, as a class, they were already a very impoverished group by then.
No clear explanation is available and it is possible that the extinction was an emergent consequence of many factors which, individually, may have been too subtle to leave much evidence. However, “[w]idespread basaltic volcanism occurred in the region of the West Siberian Basin in central Russia during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the basin reveal that the basalts were erupted 249.4 ± 0.5 million years ago. This is synchronous with the bulk of the Siberian Traps, erupted further east on the Siberian Platform. The age and geochemical data confirm that the West Siberian Basin basalts are part of the Siberian Traps and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction” (Reichow et al. 2002, p. 1846, Abstract).
“Fundamental to understanding its cause is determining the tempo and duration of the extinction. Uranium/lead zircon data from Late Permian and Early Triassic rocks from south China place the Permian-Triassic boundary at 251.4 ± 0.3 million years ago. Biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the Changhsingian pulse of the end-Permian extinction, corresponding to the disappearance of about 85 percent of marine species, lasted less than 1 million years. At Meishan, a negative excursion in d13C [see Fig. 3] at the boundary had a duration of 165,000 years or less, suggesting a catastrophic addition of light carbon” (Bowring et al. 1998, p. 1039).
“The marine sediments of Jameson Land, East Greenland, preserve a unique record of the Permian-Triassic extinction event. High rates of sedimentation, controlled by active faulting, have produced a greatly expanded succession compared to other sections worldwide. In addition, the sediments have suffered remarkably little burial and thermal alteration and contain well preserved marine organisms and terrestrially derived plant remains. For the first time, it is possible to compare the terrestrial and marine fossil records of this extinction event using the same samples from the same sections. These studies have importance for palaeoecology as well as for correlation between marine and terrestrial sections worldwide. Marine ecosystem collapse is signalled by a sharp reduction in bioturbation, a disappearance of Permian taxa, a sharp decrease in the 13C curve (interpreted as an indication of productivity collapse) and the appearance of widespread oxygen restriction. This all occurs within approx. 50 cm at the top of the Schuchert Dal Formation. During the same interval, there is also a turnover in the terrestrial ecosystem from gymnosperm to pteridosperm dominated floras. Studies of sections in northern Italy also suggest that a similar synchronicity exists in the timing of ecosystem recovery in both marine and terrestrial realms during the late Lower Triassic” (Looy & Twitchett 1999).
“There is a widespread belief that ‘throughout their history fish have proved virtually immune to mass extinction events’” (Hallam & Wignall 1997, p. 71). During the Permian-Triassic extinction event fish diversity seems to have increased, whereas all other marine groups declined sharply. This presents an ecological problem: how can organisms at the top of a food chain survive a widespread collapse in productivity and biomass at lower trophic levels? Good Permian-Early Triassic fish faunas are known only from a few areas worldwide: Madagascar, East Greenland and Spitsbergen. One possibility is that in these areas the extinction was less severe than elsewhere and thus the food chain remained relatively intact. However, palaeoecological data from East Greenland show that conditions were just as bad here as elsewhere. Instead, it appears that fish were better preserved in the Lower Triassic than in the latest Permian. This is due to widespread benthic anoxia, high sedimentation rates and rapid concretion formation in the narrow, fault controlled basin of East Greenland. In contrast, benthic invertebrate groups show much worse preservation in the Lower Triassic than in the Late Permian (due to a dearth of silicified faunas). When the number of Lazarus taxa are taken into account, fish diversity shows a similar pattern through the Permian-Triassic interval as all other marine groups” (Twitchett 1999).
An analysis of “global Permian and Triassic plant data in a paleogeographic context show that the scale and timing of effects [on plants] varied markedly between regions” and that “the patterns are best explained by differences in geography, climate, and fossil preservation, not by catastrophic events” (Rees 2002, p. 827).
Fig. 3. Carbon isotope curve at Meishan section; from Bowring et al. 1998.
Early Cretaceous vegetation was broadly similar to that of Late Jurassic times, both in distribution and general composition. Low paleolatitudes at that time were arid, having desert and sub-desert conditions, and here the floras were dominated by cheirolepidiacean conifers and matoniacean ferns. Northern mid paleolatitude floras were more diverse, including ferns, bennettitaleans, cycads, conifers and some ginkgos. At higher latitudes, diversity declined again, the floras dominated by leptostrobaleans and ginkgos. Southern mid paleolatitudes were dominated by bennettitaleans and cheirolepidiacean conifers. (Adapted from Cleal & Thomas 2009.)
The most striking event in the evolution of plants during the Cretaceous was certainly the enormous radiation of angiosperms.
Appearance of the Angiosperms
The angiosperms (flowering plants) are the most diverse group of land plants living today, comprising some 270,000 described species, placed in about 380 families and 83 orders (Mayr 2001, p. 64).
The group may have evolved from either the Gnetales or possibly the Bennettitales (Willis & McElwain 2002, p. 184).
The enormous radiation of this group has largely occurred since the mid-Cretaceous, coevolving with a similar radiation of insects. However, the angiosperms must have arisen earlier: perhaps as early as the Triassic or even the late Carboniferous (Qui et al. 1999). Evidence supporting the earliest date estimates is mainly provided by calibrated genetic divergence studies, though fossil angiosperm-like pollen and leaves have been found dating back to the late Triassic. Several form-species of Crinopolles-type pollen possessing a tectate wall have been described, dating to perhaps 220 Ma. The oldest leaves are somewhat younger, perhaps 210 Ma, and include the problematic taxa Furcula and Sanmiguelia.
Some of the earliest “body” fossils are “small plants, possibly rooted aquatics or wetland herbs” (Wing 2004, p. 90). The oldest angiosperm fossils, however, are pollen grains, which first appear in the fossil record “during the Valanginian-Hauterivian [~139.8 to 129.4 Ma]; they spread out of the tropics in the Aptian and Albian [~125.0 to 100.5 Ma], and radiated in the Late Cretaceous” (Harris & Arens 2016, p. 640).
That being said, however, current orthodoxy is that the first true angiosperms evolved in the Early Cretaceous, probably in the Valanginian (~140 to ~133 Ma) or Hauterivian (~133 to ~129 Ma) ages.
Genetic evidence (Zanis et al. 2002) strongly suggests that the most ‘primitive’ (basal) living angiosperm is a little known shrub called Amborella trichopoda. Amborella is a small shrub with tiny greenish-yellow flowers and red fruit, native to the South Pacific island of New Caledonia. The Nymphaeales (waterlilies and their relatives) are also contenders for the distinction of being the most basal living angiosperms.
Perhaps the most basal fossil group yet to be well-delineated within the angiosperm clade is the Archaefructaceae, a family of herbaceous aquatic plants recovered from the Lower Cretaceous or possibly uppermost Jurassic Yixian Formation of western Liaoning, China. These plants had reproductive axes that lacked petals and sepals, and bore stamens in pairs below conduplicate (sharply folded together lengthwise) carpels. One combined morphological and molecular “total evidence” analysis places the Archaefructaceae as a sister group to all extant angiosperms, including Amborella and the Nymphaeales (Sun et al. 2002, p. 900).
Modern squamates (lizards, snakes and amphisbaenians) are the world’s most diverse group of tetrapods along with birds and have a long evolutionary history, with the oldest known fossils dating from the Middle Jurassic period – 168 million years ago. Megachirella wachtleri is a lepidosaurian reptile from the Middle Triassic of the Italian Alps. Simoes et al. (2018) analysed X-ray computed tomography data to re-evaluate the diapsid phylogeny and present evidence that M. wachtleri is the oldest known stem squamate. Megachirella is 75 million years older than the previously known oldest squamate fossils, partially filling the fossil gap in the origin of lizards, and indicating a more gradual acquisition of squamatan features in diapsid evolution. Divergence time estimates using relaxed combined morphological and molecular clocks show that lepidosaurs and most other diapsids originated before the Permian/Triassic extinction event, indicating that the Triassic was a period of radiation, not origin, for several diapsid lineages.
Three possibilities for the origins of birds are still being credibly debated: The first is that they evolved from some unknown group of basal archosaurs, probably in the Triassic Period. Second, that they are a sister group to the Crocodylians, perhaps arising from within the sphenosuchian crocodylomorphs in the Early Jurassic. Certainly the most widely held view – though unfortunately misrepresented as the only credible modern view by the popular media – is for an ancestry among the theropod dinosaurs, specifically the Maniraptora, in the Middle to early Late Jurassic.
Despite intensive searching, the earliest known bird is still the famous Archaeopteryx, known from only seven skeletons and an isolated feather, all recovered from the Late Jurassic (Tithonian, ~152 to 145 Ma) Solnhofen Limestone of Germany. The small theropod dinosaur, Compsognathus, has also been recovered from the Solnhofen. This fossil record represents a difficult problem for advocates of the theropod hypothesis: birds (specifically Archaeopteryx) are supposed to be most closely related to the dromaeosaurids, which do not appear in the fossil record until Albian times (mid Cretaceous, about 110 Ma) yet Compsognathus, which is believed to have diverged from the theropod lineage long before the evolution of the dromaeosaurids, occurs alongside Archaeopteryx 40 million years earlier. At present, only the vagaries of the fossil record can be invoked to ‘explain’ the stratigraphic disjunction; our present understanding is unsatisfactory.
The first known beak and pygostyle (the “parsons-nose” which is all that remains in birds of the reptilian tail) occur in a Chinese fossil dated at 130 Ma. Feathers and bird bones have also been recovered from 110 Ma sediments in Victoria and Queensland.
Rahonavis is a primitive bird from 80 million-year-old rocks of Madagascar. Despite being more bird-like than Archaeopteryx, raven-sized Rahonavis retains some very distinctive theropod-like features. Other small primitive birds have been found elsewhere around the world. From Mongolia comes a large flightless bird, Mononykus, with wings replaced by a pair of single-digit hands that projected forwards. Another flightless bird is known from Patagonia. A sparrow-sized bird from Spain had a more modern shoulder joint than Archaeopteryx and a perching foot but it still had teeth.
Early mammals had a Pangean distribution, including Europe, Asia, Africa, Australia and the Americas. Mammals were presumeably abundant in the Mesozoic, though their fossil record is poor, probably due to their small size which makes the fossils fragile and difficult to find.
The earliest fossil mammals are early Mesozoic, the exact age being dependent upon which fossils one accepts as meeting the definition of ‘mammal.’ Conventionally, mammals are recognised by their jaw morphology: how the jaw articulates with the skull and incorporation of two small bones into the inner ear. In reptiles – including the mammalian ancestors – the jaw joint is hinged on two small bones; one (the quadrate) linked to the squamosal bone of the skull and the other (the articular) to the lower jaw itself (the dentary). In true mammals, the dentary is hinged directly to the squamosal; the quadrate and articular bones are incorporated into the mammals inner ear, becoming the incus and malleus respectively.
The Late Triassic morganucodontids exhibit an intermediate jaw morphology, neither completely reptilian nor yet fully mammalian: They had a jaw in which the dentary articulated with the squamosal but which still included articular and quadrate bones; these had not yet evolved to form the malleus and incus of the true mammalian inner ear. The morganucodontids are the oldest and most primitive of the triconodontans so are sometimes (e.g. Rich et al. 1996, p. 519) regarded as the first mammals.
“The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina” (Rauhut et al. 2002, p. 165).
Throughout the early Mesozoic, mammals remained small, becoming more abundant, larger, and more diverse in the Cretaceous, which may have been a time of explosive radiation of Tribosphenida – early relatives of marsupials and placentals (Rougier 2002).
Whereas early placentals undoubtedly lived in the Mesozoic, the crown group radiation is thought to have occurred after the end of the Cretaceous (e.g., O’Leary et al. 2013, Halliday et al. 2017).
The earliest stem carnivorans, the group including cats, dogs, bears and others, are the genera Ravenictis and Pristinictis, known from the earliest Paleocene, and afrotherians (the group comprising elephants, dugongs, aardvarks, among others) from the Middle Paleocene. The first chiropteran (bat) fossils – already quite highly derived, capable of true flight although not echolocation – are known from the famous, Early Eocene Green River Formation of Wyoming. The earliest lagomorphs (rabbits, hares, etc.) are known from the mid Eocene of China. (After Halliday et al. 2017.)
“If the Cretaceous Indian genus Deccanolestes is, as some have suggested, closely related to purported euarchontans, such as nyctitheres ... or adapisoriculids ... then Deccanolestes would represent a Cretaceous occurrence of a euarchontan” (the group containing rodents and primates, among others; Halliday et al. 2017, p. 531, and references therein).
Notwithstanding, the primate record, generally, and the human record in particular, is very incomplete. The closest living relatives of primates may be the Dermoptera (colugos, arboreal gliding mammals native to Southeast Asia; suggested by genetic studies) or the clade of Dermoptera plus Chiroptera (bats; suggested by morphology). Any common ancestor to the three groups must have lived in the Cretaceous. Probable primate ancestors – genera such as Purgatorius, Plesiadapis and Phenacolemur – date from the earliest Paleogene, approximately 65 Ma. The oldest known true primates occur about 55 Ma (near the Paleocene/Eocene boundary).
Molecular evidence suggests that the evolutionary line of humans diverged from that of the other apes sometime between 10 and 5 million years ago. However, the discovery in Chad of a plausible human ancestor, Sahelanthropus tchadensis, in 6-7 million year old sediments suggests that the younger end of this range is unlikely and divergence between men and apes is older than indicated by most molecular studies. Moreover, the slightly younger Orrorin tugensis, discovered in ~6 Ma sediments from Kenya, hints at a diverse and perhaps geographically widespread homonid family tree living in the latest Miocene.
Molecular evidence suggests that the evolutionary line of humans diverged from that of the other apes sometime between 10 and 5 million years ago, but this timescale has been seriously challenged by discoveries of possible human ancestors in the older half of this range. Foremost among them are the 6-7 Ma Sahelanthropus tchadensis, discovered at Toros-Menalla in Chad, which is the oldest plausible human ancestor known to date, and, not much younger, the ~6 Ma Orrorin tugensis, discovered at Lukeino in Kenya. Together, the two fossil discoveries hint at a diverse and perhaps geographically widespread homonid ancestry, and an older divergence between men and apes than is indicated by most molecular studies.
The first recognisable apes had evolved by about 20 Ma ago with the appearance of Dryopithecus (formerly Proconsul) africanus, which may have been ancestral to humans. A great variety of forms appeared shortly after Dryopithecus, particularly in Africa, but with a few exceptions the fossil record becomes more than usually poor between about 14 and 4 Ma ago, and little is known from this period. Molecular evidence suggests that it was during this period, between 5 and 10 Ma ago, that the evolutionary line of humans diverged from that of the other apes.
The earliest fully bipedal human ancestor known is the 4 million year old Australopithecus afarensis, first recognised from the famous fossil known as Lucy. Subsequently, two main lines of pre-human evolution diverged: the australopithecines, which eventually became extinct, and those given the genus name Homo, beginning with an unnamed ~2.5 Ma old fossil, rapidly succeeded by Homo rudolfensis.
The earliest species assigned to Homo is the unnamed species appearing at the beginning of the Quaternary, approximately 2.5 million years ago. This species was rapidly succeeded by Homo rudolfensis. Then, two further species, Homo habilis and H. ergaster overlap through much of the interval 2 to 1.5 Ma; the latter seems more likely to have been the direct human ancestor.
Homo erectus is the only recognised representative of the genus between about 1.2 and 0.7 Ma; some interpretations place it on the direct ancestral line to modern humans, others consider H. ergaster to have been directly ancestral to H. heidelbergensis, which first appears about 0.6 Ma, and from there to modern H. sapiens. However, if H. erectus is not directly ancestral to H. heidelbergensis and modern man, we are left with a ~900,000 year gap in which the true intermediaries are unknown.
Anatomically modern H. sapiens first appears close to 200,000 years ago. The oldest well-dated fossils are the Omo I and Omo II fossils from Kibish, Ethiopia (McDougall et al. 2005).
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