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Major Events in the Evolution of Life


Abstract

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

Introduction

Life on earth has a long and rich history, dating back almost to the time when the Earth’s crust solidified. Prior to that, the planet was unrecognisable. Catastrophic bombardment by meteors likely re-melted the entire planetary crust repeatedly; any life that may have evolved would have been wiped out. It is even possible this may have happened more than once.

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. The oldest fossils known to date, from the Apex Chert of Western Australia, are dated to around 3.5 Ga (3,500 million years ago). However, prior to the 1960s, little was known about the long Precambrian history of life. As far as was widely known, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and diversity: the so-called Cambrian explosion of animals.

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 during the Ordovician. 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.

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 point prior to the late Jurassic, the earliest birds diverged – according to the most widely held view – from theropod dinosaur stock.

In the Mesozoic, possibly as early as the Jurassic, 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.

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 lineages (which species evolved into which) 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 (at least of animals). Indeed, Charles Darwin (Origin, 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” (Darwin 1859, p. 307).

He was quite correct, on both counts.

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).

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 marvellous diversity: the so-called “Cambrian Explosion” (see below). 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.

Molecular Clock Estimates

Some molecular clock analyses suggest life may have originated as long ago as the Hadean Eon: Hedges 2002 estimates the divergence of Bacteria and Archaea at >4 Ga, noting however, that “the fidelity of genetic replication and repair systems in the early history of life is unknown, and the different environment of early Earth might have affected rates of molecular change. It is for these reasons that we have less confidence in the time estimates for the earliest splitting events” (p. 842). A phylogenetic tree constructed from highly conserved portions of the iron/manganese superoxide dismutase enzyme sequence (Kirschvink et al. 2000, p. 1404) suggests an age for this divergence of 3 to 4 Ga.

Oldest Traces of Life

 
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).

Oldest Body Fossils

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, pp. 717-8).

Martin & Sousa 2016, p. 9, “embrace” what they believe to be “about the surest thing we can say about early evolution, namely, that it occurred in the absence of molecular oxygen. If some molecular oxygen arose in the atmosphere through photolysis of water or other means, there was so little of it that it was irrelevant for microbial physiology and evolution, which was not occurring in the atmosphere anyway. In early evolution, O2 can be neglected. Early microbial evolution was the age of anaerobes.”

“The issue of cyanobacterial antiquity is directly related to the question of Earth’s atmospheric history. Most students agree that prior to the evolution of cyanobacterial photosynthesis there could have been only trace amounts of oxygen in the atmosphere, perhaps 10-10 atm 02, (Holland 1984). Higher oxygen concentrations must have been generated photosynthetically. The presence of hematite iron formations has been used as prima facie evidence for cyanobacterial oxygen production, but it is possible that these sedimentary deposits were generated by the photooxidation of ferrous iron dissolved in anoxic early Archean oceans” (Knoll 1996).

“… Despite low global pO2, relatively high concentrations of oxygen could have accumulated locally in association with high cyanobacterial productivity; however, oxygen oases would have been transient in time and space. Therefore, obligately aerobic organisms could not have evolved until pO2 reached stable and global levels of 1-2% PAL [present atmospheric level] (Chapman & Schopf 1983)” (Knoll 1996).

Around 605 to 590 Ma, the Varanger glaciation occurred; possibly the most profound planet-wide glaciation in our history. During the event, the earth biota suffered its first “mass extinction” – primarily of acritarchs.

First Metazoans

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 may hint at an arthropod s.l. presence, although the enigmatic Yilingia spiciformis is apparently not an arthropod (Chen et al. 2019).

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:

  • Sponges are widely recognised as the most primitive of living metazoans, occupying a basal position in metazoan phylogeny, as a sister group to all other Metazoa (e.g. Nielsen 2001, p. 30, 506-507, but see Dunn et al. 2008, Schierwater et al. 2009, and Ryan et al. 2013 for alternative possibilities). Thus their first occurrence in the fossil record is a metric of particular interest. However, only rare occurrences of Precambrian sponges have been reported. The earliest record is of presumed sponge remains from the Doushantuo phosphates, dated around 570 Ma (Li et al. 1998), and the earliest described species is Paleophragmodictya reticulata from the ?555 Ma Ediacara locality. However, sponges could have occurred earlier and not been recognised; spicules are not necessarily diagnostic, even in living sponges (Dr. Allen Collins, pers. comm.)

  • Fossils of the Twitya Formation are generally presumed to be cnidarians, or at least as metazoans of cnidarian grade. “Interpretation as colonial aggregates of prokaryotes (e.g. Nostoc-like balls) is possible but is difficult to reconcile with the morphology and relatively high relief of the remains, their occurrence at the bottom of turbidite beds, and the lack of a carbonaceous film outlining them, particularly in view of the of the fact that carbonaceous compressions are present in the formation” (Hofmann et al. 1990, p. 1202). Of principal significance is this occurrence of cnidarian-grade metazoans in pre-Varanger sediments, since the Varanger glaciation is sometimes cited as an evolutionary ‘bottleneck’ which arrested metazoan evolution. At least a dozen of the later Ediacaran genera are considered probable cnidarians.

  • One of the strongest candidates for a derived protostome among the Ediacaran taxa is the putative mollusc, Kimberella. If this taxon is indeed a mollusc, as suggested by Fedonkin & Waggoner 1997, or if the Ediacara/Zimnie Gory traces are correctly interpreted as radula scratches, we have evidence for derived protostomes at 555 Ma. Similarly, if Arkarua adami (from the Pound Subgroup, South Australia; Gehling 1987) is correctly interpreted as an echinoderm, we have evidence for a derived deuterostome of similar age. In either case, it follows that the protostome-deuterostome split must have occurred well before 555 Ma, which is in accordance with most ‘molecular clock’ studies.

  • The late Proterozoic–Cambrian ‘small shelly fauna’ (SSF) comprises millimeter-scale calcareous and phosphatic fossils that provide the earliest evidence of skeletal organisms. Some SFSs represent genuinely small organisms, while others are fragments of larger organisms. The SSF appears just before the beginning of the Cambrian, ~550 Ma, increasing in numbers and diversity towards the middle Cambrian. There is no noticable biotic turnover at the Proterozoic–Cambrian boundary. SSF deposits become less frequent towards the end of the Cambrian or early Ordovician, although there is some evidence they actually persist in sediment starved depositional settings, right through to the present day.

“In the 1970s, Ohno proposed that vertebrates arose through a process involving one or more genome-wide duplications. This hypothesis received early support from the discovery of multiple vertebrate Hox clusters compared with one invertebrate cluster and the finding of numerous vertebrate gene families with members distributed across multiple chromosomes. Further evidence came from the discovery of paralogous (that is, duplicated) blocks of linked genes on multiple chromosomes within the human genome, culminating in the discovery of widespread quadruply conserved synteny of the human genome. These studies support the so-called ‘2R’ scenario of two rounds of whole-genome duplication during vertebrate evolution” (Simakov et al. 2020, p. 820; references elided).

Simakov et al. (2020) present evidence for the remarkable idea that there were “two distinct ancient duplications based on patterns of chromosomal conserved synteny. All extant vertebrates share the first duplication, which occurred in the mid/late Cambrian by autotetraploidization (that is, direct genome doubling). In contrast, the second duplication is found only in jawed vertebrates and occurred in the mid–late Ordovician by allotetraploidization (that is, genome duplication following interspecific hybridization) from two now-extinct progenitors” (abstract).

Cambrian Radiation

Early Paleozoic Diversification

Trilobites

Few Paleozoic fossils are as iconic as the trilobites. This group of arthropods had its origins in the Cambrian – the earliest described species seems to be Profallotaspis jakutensis (Wood et al. 2019, p. 529) – but their major radiation occurred in the Ordovician.

 
“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 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.)



Fig. 1: Reproduction of figure 1 from Adrain et al. 1998, showing the two major clusters of Ordovician trilobite ‘fauna.’

Fish

The oldest vertebrates are Early Cambrian species are “fish-like” animals from China, undoubtedly craniates but having uncertain relationships to modern groups. The oldest armoured species (euvertebrates) are Ordovician in age. Vertebrates have their first adaptive radiation during the Silurian when jawless fish – “ostracoderms” – are dominant (after Blieck 2017).

Vertebrates underwent a major adaptive radiation during the Devonian, “when jawed species (gnathostomes) and particularly placoderms (armoured fishes), became dominant. A Lochkovian [earliest Devonian] 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 rise of jawed vertebrates (gnathostomes) throughout the Devonian ... and into the post-Devonian is one of the key episodes in vertebrate evolution.... This interval encompasses well-known early diversification events, including those of Osteichthyes (bony fishes: ray-finned Actinopterygii and lobe-finned Sarcopterygii, including tetrapods), Chondrichthyes (cartilaginous fishes: Elasmobranchii and Holocephalii), and Placodermi and Acanthodii (extinct groups of debated affinity to extant gnathostomes)” (Sallan & Coates 2010, p. 10131).

The earliest known sharks are Silurian, and the group was highly diversified by the Devonian. Some of the Devonian sharks, such as the 1.8 m Cladoselache, were certainly apex predators, but perhaps the best known apex predator from this period, Dunkleosteus, belongs to quite a different group of fishes: the arthrodire placoderms. Placoderms were an extinct group of jawed fish, first appearing in the Late Silurian Period and rising to dominance in the Devonian. The front half of their bodies was covered in large bony plates, forming a rigid shield with a joint between the head and shoulder. There were two major groups of placoderms: the antiarchs and the arthrodires. The arthrodires had a ball-and-socket joint between the shoulder and head which permitted the head to be rotated back against the body, providing the mouth with a wide gape. Most arthrodires had large eyes and powerful jaws lined with sharp-edged plates of bone, and they were undoubtedly predators. The remainder of the body seems to have lacked scales; the only bony traces found have been vertebrae and rays supporting the medial fins. The structure of the pectoral fins is unknown, but large sockets on each side of the shoulder shield suggest great mobility and arthrodires are assumed to have been fast-moving, agile swimmers.

Early Tetrapods

“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).

The gnathostomes (jawed vertebrates) “underwent major changes over the Devonian-Mississippian divide.... Placoderms, sarcopterygians, and acanthodians are replaced by chondrichthyans, actinopterygians, and tetrapods, occupying a wider range of ecological roles and dominating all succeeding biotas.... This faunal transformation has been subjected to few analyses, and explanations have tended to focus on gradual replacement ... and competitive displacement…” (Sallan & Coates 2010, p. 10131).

“Most of our understanding about these crucial vertebrate events comes from only a few freshwater deposits from Euramerican localities, with outcrops in Greenland producing the most prolific stegocephalian remains. Elginerpeton and Obruchevichthys were first, between 385 and 375 MYBP, followed several million years later by a modest radiation that included Ventastega, Ichythostega, Acanthostega, Tulerpeton, Metaxygnathus, and Hynerpeton. Although most of these taxa possessed limbs (although this is not certain for Elginerpeton and Obruchevichthys), all of these taxa have been interpreted as fully aquatic, rather than terrestrial.... Their respiratory systems are poorly known, but osteology suggests that they were able to derive some amount of O2 from water instead of being entirely air breathing.... These stegocephalians disappeared soon thereafter, followed rapidly by rare appearances of limbed vertebrates” (Ward et al. 2006; citations omitted).

Colonisation of the Land

 

Plant life

Colonisation of the land by plants occurred in stages. The first, a bryophytic phase, lasting from the Ordovician to the Early Devonian, is evidenced by fossil spores and cuticles.

Next was the first major diversification of land plant life, such as rhyniophytes, zosterophylls, drepanophycaleans, lycophytes, trimerophytes, and (in the southern hemisphere) Baragwanathia, beginning as early as Late Ludlow (Late Silurian) in Australia through Gedinnian (Early Devonian) on Laurentia.

“Throughout Silurian times, only small rhyniophytes (or rhyniophytoids) and lycophytes are known…. Remarkably, even in these early phases of their evolution, plants had an almost worldwide distribution with records from North and South America, Europe, Africa, central Asia, China and Australia, presumably reflecting the wide dispersal potential of their spores and the lack of any competition. The global composition of these floras also appears to be fairly uniform, although this may in part be due to the problems of differentiating biological species within the plexus of these morphologically very simple plants” (Cleal & Thomas 2009, p. 203).

The first major diversification of land plant life, such as rhyniophytes, zosterophylls, drepanophycaleans, lycophytes, trimerophytes, and (in the southern hemisphere) Baragwanathia (Fig. 2, right), began as early as Late Ludlow (Late Silurian) in Australia through Gedinnian (Early Devonian) on Laurentia. Plants of this “rhyniophytoid phase” were small vascular plants having an axial organisation and terminal sporangia such as Cooksonia.

Cooksonia, the early or even primordial land plant, apparently played a key role: it has been described from a number of regions, ranges widely from the Silurian to the early Devonian, and is represented by several species such as C. pertoni, C. paranensis, C. banksii, and doubtfully C. cambrensis, C. hemisphaerica (Gonez and Gerrienne 2010a) with many other specimens having been described in open nomenclature. Cooksonia also occupies a special position in the colonization of terrestrial habitats, which is a major aspect of early plant research based on broad studies of associations, their successions and distributions (e.g. Edwards & Wellman 2001; Kenrick et al. 2012). This complex paleoecological approach has attracted considerable attention over the past two decades” Kraft et al. 2019, p. 144).

Kraft et al. 2019 envisage an “Initial Plant Diversification and Dispersal Event” to be one of the key steps in global terrestrialization. This first significant diversification and dispersal episode of vascular plants, which occurred in the Prídolí (latest Silurian) in several paleoregions representing wide paleolatitude and paleoclimatic ranges, such as Avalonia, Laurentia, Baltica, Gondwana, South and North China and Kazakhstania (e.g., Edwards & Wellman 2001, Wellman et al. 2013, and other references in Kraft et al. 2019).

(2)  
Fig. 2: Cooksonia cf. hemisphaerica, left, and Baragwanathia brevifolia (Kraft et al. 2019, fig. 4).

Arthropods

The earliest evidence for terrestrial activity by animals is provided by trace fossils. Among the oldest are Late Cambrian to no younger than Arenig (Early Ordovician) tracks made by multiple ~50 cm-sized, many legged animals preserved in an eolian sandstone in the Nepean Formation (Potsdam Group) near Kingston, Ontario. However, these track-makers were probably amphibious arthropods – possibly euthycarcinoids – which only left the sea for a limited time, rather than fully terrestrial animals (MacNaughton et al. 2002).

Caradocian (Ordovician) trackways attributed to the ichnogenera Diplichnites and Diplopodichnus occur in the Borrowdale Volcanic Group of northwestern England. Such traces are consistent with tracks made by modern millipedes, although it is not possible to be certain that the trace fossils were left by fully terrestrial animals (Johnson et al. 1994).

Meandering, sub-vertical burrows in an Upper Ordovician paleosol from Pennslyvania show bilaterally symmetrical backfilling resembling that found today in burrows made in decaying wood by some millipedes. Again it is impossible to confidently assign these burrows to millipedes or even to arthropods, but subaerial burrowing is strong evidence for a fully terrestrial habit by some animal.

The earliest terrestrial body fossils are unassignable fragments from the Llandovery (Lower Silurian) Tuscarora Formation near Millerstown, Pennslyvania. Acid digestion of rock samples has recovered pieces of bristles or setae which may be arthropodan although they do not appear to be hollow, as are nearly all arthropod setae. Annelid setae are not hollow (Gray & Boucot 1994).

The earliest identifiable terrestrial body fossils are arthropod fragments from the Late Silurian. However, these organisms possessed sensory and respiratory structures fully adapted for life on land, indicating an earlier history of terrestrial habituation that has not yet been found.

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. Earliest evidence for terrestrial animals is sparse, but species interpreted to be fully terrestrial are known from before 400 Ma (Ward et al. 2006) and exquisitely preserved Early Devonian 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.

Although insects may have have diverged (from a common ancestor with fairy shrimps, Anostraca) as early as the Ordovician-Silurian boundary (Gaunt & Miles 2002, Glenner et al. 2006) the oldest proposed insect records, such as the Devonian taxon Strudiella devonica described from Belgium, remain controversial (e.g., Hornschemeyer et al. 2013, Haug & Haug 2017); uncontested insect fossils are much younger.

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).

Tetrapods

“The development of limbs from fins was a defining stage in the evolution of tetrapods (Clack 2002a). The earliest limbs with digits are thought to have been used to facilitate underwater bottom walking (Coates & Clack 1995), but the critical next step was their use in terrestrial locomotion. … [However,] the evolution of limbs enabling tetrapods to use quadrupedal gaits on land is still poorly understood” (Smithson & Clack 2018, p. 89).

“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 2002b, 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).

Romer’s Gap is an apparent gap in the tetrapod fossil record (or, more generally, the record of any terrestrial animal; see Ward et al. 2006) extending through the early part of the Carboniferous, lasting for up to 20 million years following the end-Devonian extinction. The name appears to have first been coined by Coates & Clack (1995).

“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).

Sallan & Coates (2010) explain Romer’s Gap as the post-extinction trough following the Hangenberg extinction at the end of the Devonian. “Corresponding lack of Tournaisian material from other terrestrial groups (e.g., insects) supports this conclusion..., and Devonian levels of faunal disparity are only regained in the Serpukhovian.... Even late Mississippian assemblages of vertebrate species from both sides of the newly formed Pangaea are strikingly similar (e.g., Bearsden and Bear Gulch localities...), likewise for associated invertebrate assemblages…. Extinctions remove characters from the pool of varying morphologies.... For example, digit number is known to be variable among late Famennian [the last age in the Devonian Period] tetrapods ... but stabilizes with a maximum limit of five among all later forms” (Sallan & Coates 2010, p. 10134, 10135).

The “gap”, however, has now been shown to be the result of collection failure (see Smithson et al. 2012, Clack et al. 2016, Smithson & Clack 2018).

Notwithstanding, 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 2002b, 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 2002b, 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.

The earliest terrestrial vertebrates are often informally referred to as “amphibians”. “Traditionally the name ‘Amphibia’ has been employed more broadly to refer to all tetrapods that are not amniotes, but this is rather misleading because most of these forms were not similar to extant amphibians and may have been different biologically. Among the Paleozoic and early Mesozoic ‘amphibians,’ most authors have distinguished two principal groups – Lepospondyli and Temnospondylli. The exclusively Paleozoic lepospondyls comprise a heterogeneous assemblage of small forms with holospondylous vertebrae. By contrast, temnospondyls are a remarkably diverse group that ranged from the Mississippian to the Early Cretaceous and include the stem forms of at least frogs and salamanders. … During the Triassic, temnospondyls were represented by a variety of lineages and were semi-aquatic or fully aquatic carnivores in many ecosystems. Most Triassic temnospondyls are referred to the Stereospondlyi,” a subgroup of which is the Capitosauroidea. Capitosauroids “include often large forms with a dorsoventrally flattened truck and head, and an elongated snout, which lends them a superficially crocodile-like appearance. A characteristic feature … is the presence of a deeply incised otic notch, or opening (which presumably accommodated the tympanic membrane), along the posterior margin of the skull roof” (Sues & Fraser 2010, p. 12).

Beyond evolving limbs capable of terrestrial locomotion, later tetrapods evolved the amniote egg, thereby becoming truly terrestrial, although it is not known exactly when. Most reptiles have soft parchment or membrane-like eggshells which are much less likely to fossilise than calcareous eggshells (Hirsch 1979, p. 1068). The oldest known fossil which might plausibly be an egg is Early Permian. The presumtive fossil egg was found by Llewellyn Price, having weathered free from the upper part of the Admiral Formation, Witchita Group, in the wonderfully-named Rattlesnake Canyon, Archer County, Texas. Romer & Price were unsure of the age at that time, whether latest Carboniferous or Early Permian, but the age was later established as Early Permian (Romer 1974).

The fossil itself is small – about 59 mm long by 36 diameter at its widest point – asymmetrically ovoid, somewhat blunt at both ends. The surface is studded irregularly with small, less than 1 mm, rounded swellings. It was first described by Romer & Price (1939) and re-examined some 40 years later by Hirsch (1979) who removed small pieces for sectioning and compositional analysis.

Hirsch noted that the outer “shell-like layer is not as uniform as described by Romer & Price and, contrary to their findings, thin wavy layers are visible throughout the specimen, at least to the depth to which the deepest cut was made (4.5 mm)…. Although the colored wavy layers which are visible to the naked eye can be seen in the polarized photo taken under the metallograph ... they are not discernible on the SEM. Thus it seems that these layers are not structurally significant but are rather colored or stained bands of the matrix. ... Closer observation of the shell-like layer shows very little similarity to calcified fossil eggshell” (p. 1071, 1075). But although he emphatically rejected the idea of a calcareous eggshell, Hirsch seems to have remained open to the idea that the fossil might still be a soft-shelled egg: “Phosphorous is relatively high and constant in the outer layers and seems to be less common toward the inside of the specimen. This phosphorous might be the residue of a soft, parchment-like organic shell layer. ¶ Unfortunately, the described specimen could only be compared with one nodular specimen of the same formation. This comparison showed that node-like structures are also found on nodules and confirmed the assumption that the phosphorous in the Romer and Price specimen is not a common occurrence” (Hirsch 1979, p. 1083). To the best of my knowledge there has not been any further study.

Four common animals are known from around the same stratigraphic horizon in the area: Diadectes, Dimetrodon, Edaphosaurus, and Ophiacodon. Romer & Price (1939, p. 829) note that the last of these is particularly abundant and “somewhat” favour it as the possible egg-layer.

Close of the Paleozoic

End Permian Extinction Event

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.

 
“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).

However, widespread “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).

“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.

Mesozoic Developments

Plant Life

Although the Triassic flora “included various holdovers from the Paleozoic, in addition, there were many new families as well as one new order, the Bennettitales. In fact, … the Triassic vegetation can be considered a mixture of ancient and modern. Indeed, some Triassic species are considered to be members of living genera, whereas others bear no resemblance whatsoever to modern-day taxa. There was a clear distinction during the Triassic between the land floras of the northern and southern hemispheres. Certain ferns grew in both hemispheres, but in general, there were relatively few taxa common to both the north and the south. … Although angiosperms … did not exist during the early Mesozoic, there were many other Triassic and Jurassic foliage taxa that would likely have formed dense ground cover. Herbaceous seed ferns and extensive stands of horsetails and ferns were probably common. There is certainly every reason to think that open, verdant hillsides, wooded and forested slopes, and hot and steamy forests and swamps were very much part of the Triassic landscape” (Fraser & Henderson (2006), p. 53, 54-55).

Lycopods were widely distributed, but these were herbaceous taxa similar to the modern day Lycopodium rather than the large, tree-like forms (Lepidodendron etc.) of earlier periods. Equisetales (horsetails) were also smaller than their Paleozoic ancestors, although many were still respectable trees. Having declined through the Permian, Filicales (true ferns) underwent a resurgence in diversity during the Triassic, including the appearance of some modern families such as the Osmundaceae, represented by the commonly found fossil, Cladophlebis. Cycads were more widespread than today, achieving a global distribution. Another very commonly found fossil, Taenopteris, belongs to this group. Bennettitales and Gingkoales were important floral taxa. Conifers, including araucarians (a sister group to the podocarps, such as the New Zealand kauri), included both smaller shrubs and large woody trees, such as the massive tree trunks of Araucarioxylon arizonicum, the state fossil of Arizona. (Summarised from Fraser & Henderson (2006), p. 55-60.)

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. The angiosperms (flowering plants) are the most diverse group of land plants living today, comprising some 270,000 described species – more than all other groups of land plants combined – placed in about 380 families and 83 orders (Mayr 2001, p. 64) and dominating modern plant ecosystems. “In theior rise to ecological dominance angiosperms have exhibited extraordinary developmental and evolutionary plasticity. This has resulted in overwhelming morphological diversity and a great variety of adaptive types. Angiosperms are far more diverse in vegetative form and in the structure of their reproductive organs than any other group of land plants” (Friis et al. 2011, p. 1).

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: The most recent common ancestor of all living flowering plants is estimated to have existed perhaps as early as the Triassic or even the late Carboniferous (Qui et al. 1999) or, more conservatively, “between the Triassic and the Early Cretaceous (~247–136 million years ago” (Ramirez-Barahona et al. 2020). 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 (Norian; Late Triassic), and include the problematic taxa Furcula and Sanmiguelia. A phylogenetic study by Silvestro et al. (2021), calibrated using fossil occurrence data, dates angiosperm origins in the Jurassic and the origin of angiosperm families such as the Arecaceae as mid Cretaceous.

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. The oldest unequivocal angiosperm pollen grains first appear in the fossil record “during the Valanginian-Hauterivian; 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).

Some of the earliest “body” fossils are “small plants, possibly rooted aquatics or wetland herbs” (Wing 2004, p. 90). “Compression flowers in general, however, are rare, and it is usually only the large ones which are seen. Some years ago, sieving techniques used for Tertiary sediments were applied to the Cretaceous, and yielded a previously unimagined diversity of Cretaceous angiosperm flowers, from sites in North America, Sweden, Portugal, Kazachstan, and Japan. It is now becoming clear how lineages are related. The earliest pollen is 135 million years old, and many basal eudicot lineages were fully established by about 110 Ma. Insect pollination is overwhelmingly supported by the evidence, and was probably important for enhancing speciation rates. Once started, the radiation of angiosperms, especially in low latitudes, kept rising, and shows no sign of levelling off in the Tertiary” (Clarkson 1999, p. 53).

“The earliest unequivocal fossils assignable to angiosperms, probably representing small understorey plants thriving under warm climates, appear in the Early Cretaceous of northern Gondwana (~133–125 Ma), in areas roughly corresponding to the present-day Mediterranean region. Shortly thereafter, angiosperms experienced a major burst of morphological and ecological diversification, which by the middle Cretaceous (~115–100 Ma) had triggered the evolution of most extant lineages. Overwhelming palaeobotanical evidence indicates that, in many regions, the initial burst of diversification of angiosperms was not reflected in their ecological dominance” (Ramirez-Barahona et al. 2020; references elided).

“Until the 1970s not a great deal was known about fossil flowers. Since then our knowledge has grown explosively. For example, the mid-Cretaceous Archaeanthus, from Russell, Kansas, now one of the best-known early flowers, has been the subject of extensive research. The flower is borne terminally on a long axis, and the seeds can be macerated out. The stamens and tepals are known from scars, resin bodies are scattered in the fruit and tepals, and all these features, together with the morphology, indicate an evident relation to the extant Magnoliacea” (Clarkson 1999, p. 53).

Genetic evidence (Zanis et al. 2002) strongly suggests that the most ‘primitive’ (basal) living angiosperm is a little known shrub called Amborella trichopoda; 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).

“As part of this mid- to Late Cretaceous angiosperm diversification, several key modern plant families appear early in the fossil record, including palms (Arecaceae),which are represented by leaf fossils in late Coniacian–early Santonian deposits from eastern USA and lower Campanian sites in Austria and North America” (Greenwood et al. 2022).

(4) 

Fig. 4: Amborella trichopoda. [Photograph by Tim Stephens, courtesy of the Arboretum of the University of California, Santa Cruz.]

Reptiles

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.

Birds

Bird origins have been famously contentious from the earliest days of evolutionary theory. The three most plausible ideas to emerge from the debate are (1) that they evolved from some unknown group of basal archosaurs, probably in the Triassic Period; (2) that they are a sister group to the Crocodylians, perhaps arising from within the sphenosuchian crocodylomorphs in the Early Jurassic; and (3) certainly the most widely held view today, for an ancestry among the theropod dinosaurs, specifically the Maniraptora, in the Middle to early Late Jurassic.

Some authors present the origin of feathers as essentially the same phenomenon, on the reasonable premise that feathers are such a unique and diagnostic feature of birds, that it is inconceivable any other group of animals could possess them. In essence, anything with feathers is a bird, anything without them, is not. This view is not unreasonable, but it is not unquestionable either.

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; possibly not – see Wang & Zhou 2017 and update this bit) 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.

Birds has already evolved by the latest Jurassic, at least, so the frequently repeated talk of “protofeathers” and other clues to the origin of birds being found in Cretaceous rocks (the Jehol literature is rife with this sort of thing) is patently ridiculous. Nevertheless, birds continued to evolve throughout the Cretaceous, when many important novelties appeared.

The first known beak and pygostyle (the “parsons-nose” which is all that remains of the reptilian tail in modern birds) occur in a Chinese fossil dated at 130 Ma.

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.

Mammals

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).

A study by Meredith et al. (2011) derived the mean date for the split between placentals and marsupials as ~190 Ma, noting that this estimate accords well with the discovery of a stem eutherian from the Jurassic reported by Luo et al. 2011.

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).

“The diversity of living and extinct mammalian species is documented by the fossil record of ~220 million years and has evolved against the backdrop of radical alterations in terrestrial floras during the Cretaceous Terrestrial Revolution (KTR), the Cretaceous-Paleogene (KPg) mass extinction, continental rearrangements, and changes in key environmental parameters, such as average global temperature. However, the impact of these drivers on taxonomic diversification, particularly near the KPg boundary, remains controversial” (Meredith et al. 2011, p. 521). The results of the Meredith et al. (2011) study were “consistent with Benton’s hypothesis [Benton 2010] that the KTR (125 to 80 Ma), during which the angiosperm component of floras increased from 0 to 80%, was a key event in the diversification of mammals and birds” (p. 523).

“[O]ur results are consistent with the hypothesis that both the KTR and the KPg mass extinction played important roles in the early diversification and adaptive radiation of mammals. The KTR increased ecospace diversity [the reality of ‘ecospace diversity’ appears to be an unconscious assumption present throughout this paper], possibly precipitating interordinal diversification, whereas the KPg mass extinction made more of this ecospace available for mammals, promoting the emergence of crown-group orders with their distinctive morphological adaptations” (Meredith et al. 2011, p. 523).

The Cenozoic

Mammals

Both abundance and diversity of mammals were reduced severely by the Cretaceous/Paleogene mass extinction event (e.g. in the Hell Creek assemblage, only one out of 28 mammal species survived; Ward 2000, p. 173) with fewer taxa known from the Paleocene than from the Cretaceous. During the Cretaceous, mammals were mainly small, terrestrial-to-arboreal insectivores with low ecological disparity (Goswami 2012, Grossnickle & Polly 2013, Halliday et al. 2017), albeit with a few notable exceptions (Luo 2007).

The analysis by Meredith et al. (2011) suggests that “only 29 to 32 mammalian lineages, nearly all of which are stem branches leading to extant orders, may have crossed the KPg boundary…. This inference is consistent with the long-fuse model of mammalian diversification ... which postulates interordinal diversification in the Cretaceous, followed by intraordinal diversification that is mostly restricted to the Cenozoic ..., although conflicts do remain with the Cretaceous eutherian fossil record” (p. 523).

Whereas early placentals probably lived in the Mesozoic (although no Cretaceous eutherian mammal has been unambiguously resolved within the placental crown; Wible et al. 2009, Goswami et al. 2011, Halliday et al. 2017), 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). Paleogene placental mammals include the first large-bodied herbivores, specialised carnivores, and later, radiations of gliding, flying, and fully aquatic organisms, with a corresponding increase in diversity (Darroch et al. 2014, Halliday et al. 2017). “By far the largest component of the mammalian biota in the Paleocene is the collection of ‘archaic ungulates’ known as ‘condylarths’. While this grouping is almost certainly an anachronistic grade of largely terrestrial, bunodont, herbivorous-to-omnivorous mammals, there are several well-defined families which fall within ‘Condylarthra’” (Halliday et al. 2017, p. 525).

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.)

The diversity and range of mammals appears to have greatly increased after the Paleocene/Eocene boundary (about 55 million years ago), and new groups appeared on continents throughout the Northern Hemisphere. On the basis of primarily phylogenetic analyses, Asia has been suggested as a likely centre of origin (Bowen et al. 2002, p. 2028). A study by Bininda-Emonds et al. (2007) examined relations and divergence times among all living mammalian families and concluded that there was a dramatic upturn in mammalian diversification rates during the Eocene, ~55 to 50 million years ago. This idea has been challenged, however (see Meredith et al. 2011).

Primates

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.

“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).

The various groups of “stem” primates (for our purposes, ancestral primates which have not yet evolved all the characteristics of modern primate taxa) are collectively known as plesiadapiforms. This is not a “proper” taxonomic group; the relationships between its various members are poorly known and the group as a whole is probably paraphyletic. Of these composite taxa, the geologically oldest and most primitive known family is the Purgatoriidae, which includes the genus Purgatorius.

“A Cretaceous origin A Cretaceous origin of Pan-Primates has long been hypothesized by palaeontologists, in part based on the initial description of Purgatorius, including P. ceratops, which until more recently ... was considered latest Cretaceous in age.... A consensus later emerged that despite the lack of unambiguous records of Cretaceous plesiadapiforms, a pre-KPB origin of Pan-Primates was still within the realm of possibility.... Fox & Scott [Fox & Scott 2011] recently speculated that the early (Pu2) occurrence and derived characteristics of Purgatorius coracis imply that the ancestral purgatoriid was from the Late Cretaceous. Fossils reported here (i) derive from an older (Pu1) locality, less than 208 kyr and most likely to within 105–139 kyr post-KPB, and (ii) represent two sympatric species of Purgatorius, each with a uniquely accumulated suite of dental specializations that evolved following divergence from a common ancestor. These data provide even stronger evidence that the origin of plesiadapiforms, and in turn Pan-Primates, Euarchonta and Placentalia extends back into the Late Cretaceous” (Mantilla et al. 2021, p. 7).

The primate record, generally, and the human record in particular, is very incomplete. Probable primate ancestors – genera such as Purgatorius, Plesiadapis and Phenacolemur – date from the earliest Paleogene, approximately 66-65 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.

Human Origins

Origin

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 both humans and contemporary apes. 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 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.

Somewhat younger are “Ardipithecus kadabba from the Middle Awash area (Ethiopia, ca. 5.5 Ma) ... followed closely in time by the early Pliocene Ardipithecus ramidus (4.4 Ma), for which a very complete skeleton is available. The geographical and chronological distribution of these taxa strongly suggests that the earliest hominins evolved from an African Miocene ape” (Almécija et al. 2013, p. 2).

“In Africa, the fossil record for hominoids between 13 and 7 million years ago is relatively sparse. This has led some authors to postulate that the hominines initially diverged in Eurasia before migrating back into Africa (Begun et al. 2003, Stewart & Disotell 1998). However, recent discoveries and a growing appreciation of later Miocene hominoid diversity in Africa make this an untenable scenario” (Harrison 2010, p. 533.)

Miocene

“A diversity of hominoids also occurred in Asia during the middle and late Miocene, extending from Indo-Pakistan to Thailand. Of these, Ankarapithecus, Sivapithecus, Lufengpithecus, Khoratpithecus, and Gigantopithecus are all likely to be closely related to the extant orangutan (Begun 2007)” (Harrison 2010, p. 533).

“Gradual cooling during the middle Miocene led to greater seasonality in western and central Europe and a shift from subtropical evergreen forests to predominantly deciduous broadleaved woodlands. This shift was accompanied by a dramatic turnover of the mammalian fauna at 9.6 million years ago, termed the Mid-Vallesian Crisis, when most hominoids became extinct (Agustí et al. 2003). About 7 to 8 million years ago, uplift of the Tibetan Plateau and increased intensity of the Asian monsoon, together with the global expansion of C4 grasses, led to a further decline in the diversity of Eurasian hominoids. By 5 million years ago, hominoids had become extinct throughout Eurasia, except for those surviving in the present-day range of Asian hominoids (orangutans and hylobatids), extending from southern China to Southeast Asia.” (Rearranged and slightly adapted from Harrison 2010, p. 533.)

Pliocene

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.

Quaternary

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.

“The first hominin dispersal out of Africa is thought to have been when members of the species Homo erectus exited some 2 million years ago. The second wave of departures occurred when the ancestral species that eventually gave rise to Neanderthals moved into Europe around 800,000–600,000 years ago” (Delson 2019, p. 488).

This commentary may now be dated – needs checking: 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).

“The origin and early dispersal of Homo sapiens has long been a subject of both popular and scholarly interest. It is almost universally agreed that H. sapiens (modern humans) evolved in Africa, with the earliest known fossil representatives of our species dated to around 315,000 years ago in Morocco (at a site called Jebel Irhoud) and approximately 260,000 years ago in South Africa (at Florisbad). Stone tools comparable to those found with both of these fossils have been excavated in Kenya (at Olorgesailie) and dated to about 320,000 years ago…. Many key fossil discoveries from Israel document early examples of [human] dispersals. A fossil that includes the forehead region of a skull found there, at a site called Zuttiyeh, is dated to between 500,000 and 200,000 years ago, and analysis of the fossil’s shape indicates that it is either an early Neanderthal or from a population ancestral to both Neanderthals and H. sapiens” (Delson 2019, p. 487, 488).

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