Written by Graham | Created: Saturday 13th July 2019 @ 1138hrs | Revised: Thursday 1st October 2020 @ 1152hrs
The Paleozoic Era
547-251.9 million years ago.
After the major extinction event at the end of the Proterozoic Eon, during which much of the odd life which flourished through the Ediacaran Period died out, probably as a result of a huge drop in the amount of oxygen available in the ocean, the Cambrian explosion saw the development of most of the modern animal phyla which remain today. Among them were the Archaeocyatha, the first known reef-building animals, which appeared about 525 million years ago and lasted until the end of the Cambrian, and the Anomalocaridids, a group of shrimp-like animals which came to fill a wide variety of ecological niches, including that of apex predator (the earliest known example), and which grew to a length of about seven feet. One of the most iconic groups of fossil animals, the Trilobites, also appeared around the same time as the Archaeocyatha, their exoskeleta suggestive of further development of predator-prey relationships. Whilst the vast majority of animal life remained within the seas, some centipede-like animals were spending time on land, leaving fossilised footprints dating from around 530 million years ago.
The Cambrian also saw the coming and going of a whole host of weird and wonderful animals: these include the probable early filter feeder Aegirocassis benmoulai and the bizarre five-eyed Opabinia regalis - both probably relatives of the Anomalocaridids - as well as the spiny worm-like Hallucigenia. Spines also appear on the mollusc-like Wiwaxia.
Spines of a different sort were also beginning to develop: a structure known as a notochord is diagnostic of a group of deuterostomes known as chordates, represented by specimens such as Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis, known from the Maotianshan Shales (c.525 mya), as well as the slightly later Pikaia gracilens from the Burgess Shale (c.508 mya). The conodonts - important fossils in paleontology - also arose during the Cambrian. These evolved from the deuterostomes of the earlier Cambrian, represented by Saccorhytus coronarius (c.540 mya) and the Vetulicolia, and eventually gave rise to the Vertebrata - including us.
One major feature of Cambrian organisms was their increased use of burrowing as a means to escape predators, ambush prey and also for anchoring. Deeper burrows were needed for the latter purpose, as the microbial mats which typified Precambrian seabeds had been decimated by Ediacaran grazers. This in turn led to the development of a loose upper substrate rich in oxygen above a layered, sulphidic lower substrate.
This Cambrian substrate revolution (also known as the agronomic revolution) led to the extinction or adaption of animals reliant upon the microbial mats and contributed to the evolutionary radiation which occurred during this time.
The end of the Cambrian is marked by a series of minor extinction events, followed by another radiation of life, the Great Ordovician biodiversification event (GOBE), during which many of the classes of modern animals appeared to fill out the phyla which developed during the Cambrian. The GOBE was at one point thought to have resulted from the Ordovician meteor event (around 467.5 million years ago), though this is no longer the case. Among the animals which appeared during this time were the Eurypterids, better known as the "sea scorpions," whilst the Nautiloids, a group of molluscs represented today by the Nautilus, became common. Early forms include the Orthocerida, named for Orthoceras regulare, which prowled the oceans about 488 mya.
The Nautiloids were joined by the subclass Ammonoidea during the Devonian. This iconic fossil taxon, better known as the ammonites, were closely related to the ancestors of the modern-day groups of octopuses, squid and cuttlefish, and they survived until the end of the Mesozoic Era. The Belemnoidea lived throughout the same time frame as the ammonites and bore a closer resemblance to squid, and especially cuttlefish. One possible member of this group was the Jeletzkya douglassae from some 320 mya, which is considered a member of the crown-group of the squid. The Belemnoidea also includes the belemnites, which lived from the Triassic to the end of the Cretaceous.
The Ordovician also furnishes the first potential evidence of more complex land plants, known as polysporangiates, in the form of fossilised spores. Indeed, one potential contributor to the Ordovician-Silurian extinction event, a major dying-off which marked the end of this era, is the action of the primitive ancestors of mosses absorbing carbon dioxide and breaking down rocks, releasing phosphorus and iron into the sea, which caused algal blooms and eventually leading to polar glaciation. Other potential causes include a gamma ray burst from a hypernova.
Placoderms, armoured fish, first arose during the Silurian and became the earliest major group of vertebrates during the Devonian, supplying the first apex predators from among our lineage: the fearsome Dunkleosteous, a six-metre-long bone-plated leviathan which prowled the Late Devonian oceans. Another group which are first evidenced during the Late Silurian are the ancestors of modern sharks. By the Devonian, the early sharks were assuming forms weird and wonderful, like Stethacanthus (the "ironing board" shark), becoming even stranger during the Carboniferous with Helicoprion and its relatives, sharks boasting a singular "tooth-whorl."
Bony fish too emerged during this time, forming two groups: the Actinopterygii ("ray-finned fish") and Sarcopterygii ("lobe-finned fish"), likely evolving from one lineage of Placoderms: Entelognathus features a jaw - a defining feature of fish and tetrapods which evolved from gill arches during the Silurian - which resembles those of members of these groups. The former group includes most of the fish species known today. The earliest known member of this group was thought to be Andreolepis hedei, which lived around 420 million years ago in the Late Silurian. The Sarcopterygii includes the famous "Lazarus taxon" the Coelacanth, as well as the lungfish and, perhaps most notably, the ancestors of amphibians, reptiles, birds and mammals. This group's fossil record begins with Guiyu oneiros from about 419 million years ago. By 375 million years ago, one member of this group, Tiktaalik roseae, was possibly making its way from the sea onto land.
The Silurian saw further evolution among land plants, with the development of vascular tissue: Cooksonia, a genus which first appeared around 430 mya, and Baragwanathia from about five years later, represent early vascular plants. Asteroxylon and Drepanophycus, relatives of Baragwanathia, appear early in the subsequent Devonian, providing a rich habitat for the arthropod pioneers on land.
These plants initially had to play second fiddle to fungi, with Prototaxites, a gigantic tree-like fungus which lived about the same time as Cooksonia, reaching heights of 8 metres. Another probably early fungus is Tortotubus, as well as the dubious Ornatifilum. Furthermore, the mid-Devonian Spongiophyton is potentially a lichen. The earliest plant to achieve a similar grandeur about which we know was Wattieza, a relative of modern ferns and horsetails which lived around 385 mya in the Mid-Devonian. Archaeopteris, related to the ancestors of the spermatophytes (seed-bearing plants), straddled the Devonian-Carbonaceous boundary, and during this latter period, the atmosphere, vastly richer in oxygen than today, furnished an opportunity for massive growth. A group of lycopsids (clubmosses), the Lepidodendrales, took full advantage.
The Devonian expansion of plants on land included Runcaria heinzelinii, a transitional fossil precursor of the seed plants which developed throughout the remainder of the Mesozoic, giving rise to many of the plants we know today.
The Sarcopterygii diverged during the early Devonian, with the ancestors of the coelacanths evolving separately from the rhipidistians. The latter group includes the Dipnoi or lungfish, as well as the clade Tetrapodomorpha.
The latter - which includes the aforementioned Tiktaalik - first emerges about 410 mya, the earliest known representatives being Tungsenia paradoxa and Kenichthys campbelli. Tinirau clackae lived around 387 mya, and shares a series of features with the tetrapods. Panderichthys rhombolepis (380 mya) and Tiktaalik represent further links in the evolutionary chain, which would eventually yield creatures akin to Ichthyostega, an amimal which would have borne a closer resemblance to the earliest true "amphibians" than to its fish ancestors. Ichthyostega dwelt in swampy terrain about 360 mya.
By the early Carboniferous, this lineage had produced basal tetrapods belonging to a series of families: the Whatcheeriidae, which includes the genera Watcheeria and Pederpes; as well as Crassigyrinus.
The end of the Devonian is followed by Romer's gap, a 15-million-year-long period which has yielded few fossils. After this hiatus, the fossil record reveals that the fish-like relatives of Tiktaalik and Ichthyostega were replaced by more advanced amphibians known as the Temnospondyls. The Carboniferous environment was typified by gigantic lycopsid trees, such as Lepidodendron and Sigillaria, horsetails, ferns and the conifer-like Cordaitales. Also present in the Carboniferous forests were the Pteridospermatophyta ("seed ferns"), an early type of spermatophyte which survived into the Cretaceous. By the Permian, the first Cycads - the most primitive of the modern conifers - had evolved, and were soon joined by the early Ginkgoales. Atmospheric oxygen was around 160% higher than today, enabling land invertebrates to grow to gigantic proportions. Famous members of these groups are the eurypterid Megarachne, the Late Carboniferous dragonfly Meganeura and an eight-foot long millipede, Arthropleura.
The Carboniferous rainforest collapse (CRC) occurred around 305 million years ago, changing the composition and size of the coal-forming forests and leading to the demise of many species. By the end of the Carboniferous, fungi had evolved the ability to break down dead trees, thus bringing to an end this period of fossil fuel production.
Towards the end of the Carboniferous, some early tetrapods developed the ability to lay eggs with a mineralised outer layer, i.e. a shell. This enabled them to remain on land permanently, rather than having to find water in which to lay spawn as their amphibian relatives did (and, indeed, continue to do). These early reptilians, known as Amniotes, soon branched into three distinct groups, differentiated by the number of temporal fenestra (openings behind the eye socket for the attachment of muscles) present on the skulls.
The first of these were the anapsids, who lacked any temporal fenestrae (a condition also present in Testudines, modern turtles and tortoises, although this likely arose within that group later). The anapsids include: the Mesosaurs, the earliest reptilians to return to the oceans during the Early Permian; the Pareiasaurs, large herbivores during the Late Permian; and the Procolophonidae, which survived into the Late Triassic, during which time they are represented by the herbivore Hypsognathus.
The second group are the synapsids, which are also known as stem mammals, as it is from this clade that the mammals arose. These proved to be the dominant group during the Permian and into the Triassic, before being eclipsed by the archosaurs. The synapsids had one temporal opening and are generally divided into two groups. Of these, the pelycosaurs include the fearsome Early Permian predator Dimetrodon, one of a number of members of the group to possess a sail-like appendage on its back, which may be interpreted as an early form of thermoregulation. These eventually gave way to the more mammal-like therapsids, a group including the Gorgonopsids and Cynodonts.
The Mesozoic Era
251.9-66 million years ago.
The transition between the Paleozoic and Mesozoic is marked by the most attritional of the Phanerozoic mass extinctions, the Permian-Triassic extinction event or "Great Dying." This holocaust was likely caused by massive flood volcanos, evidenced today by the Siberian traps, a large igneous province covering around 1 million mi3 of the modern Russian Federation.
The Cynodonts were, along with the Theriocephalia and Dicynodonts, one of the groups who survived into the Triassic, and this group eventually gave rise to mammals. The non-mammalian Cynodonts survived - depending on classification - as late as the Miocene, some 17.5 million years ago, in the form of the Gondwanatheria. Of the other groups, the Dicynodont Lystrosaurus was the dominant land animal in the wake of Great Dying. The largest known Dicynodont is also the latest, the elephant-sized herbivore Lisowicia.
The Mesozoic, however, came to be dominated by another group, the diapsids, more particularly the archosaurs. The Archosauromorphs first emerged in the Permian, around 260 mya, and the archosaurs radiated in the wake of the extinction, replacing the synapsids in many ecological niches. These include the include the Phytosaurs and Rauisuchids (the ancestors of today's crocodilians), members of the lineage pseudosuchia, as well as the Avemetatarsalia, the ancestors of the iconic dinosaurs (which includes our modern feathered friends) and flying pterosaurs.
The dinosaurs emerge around 243-233 mya, and are represented in the Triassic by Nyasasaurus, the early theropods Eoraptor and Coelophysis and the Herrerasaurids. Other Triassic reptiles include the mysterious Longisquama insignis, which bore a series of feather-like structures along its back, while the Thalattosaurs, Hupehsuchia and the ancestors of the dolphin-like Icthyosaurs became increasingly better-adapted to exploiting an aquatic environment, and the Kuehneosaurids exploited the aerial niche formerly inhabited by Coelurosauravus. Also present were the seal-like Nothosaurs, ancestors to the Pliosaurs and Plesiosaurs of the age of the dinosaurs.
The Triassic-Jurassic extinction event brought to a close the domination of the pseudosuchia, as well as causing the temnospondyl amphibians to go into terminal decline. The Jurassic was to be the first period of the age of the dinosaurs, with this group, along with the pterosaurs, representing the largest terrestrial creatures until the end of the Cretaceous, when the mighty Tyrannosaurus rex, taking a brief moment out of a titanic clash with a Triceratops or Ankylosaurus, roared impotently in the foreground as the Chicxulub impactor approached the earth to carry out its work of devastation 66 million years ago.
Among the dinosaurs were the mighty sauropods (descendants of animals similar to the Triassic Plateosaurus) and the diverse ornithischians, though it was a series of fearsome theropod groups which would fill the role of apex predator: Ceratosaurus, Megalosaurus, Allosaurus and the enormous Spinosaurus were among them. Another group, which were prevalent during the Cretaceous, were the dromeosaurs (perhaps better known as "raptors"), an unfriendly bunch of feathered carnivores with shearing claws. Their relatives included the troodontids, probably the most intelligent non-avian dinosaur group, and Archaeopteryx, a considerably earlier animal (dating from around 150 mya) which shows an even greater number of similarities with the only dinosaur group to make it above the K-T boundary: the birds. By the end of the Cretaceous, this new Avian lineage had produced several now-extinct varieties of primitive birds. Among these were: the Confuciusornithidae, the first birds with a pygostyle (a "parson's nose") rather than a more reptilian tail, during the early Cretaceous; the Enantiornithes, of which some 80 species are known; and species adapted to life in and around the seas, such as Ichthyornis and Hesperornis. All of these groups retained to some extent claws on their wings and teeth. More modern birds were represented by, for example, the duck-like late Cretaceous Vegavis iaai, the Paleogene relatives of which include the fearsome flightless bird Gastornis.
The cynodonts were also evolving throughout this period, though by now restricted to marginal lifestyles and small sizes. Already by the late Triassic, these were taking on forms which would be commonplace among later mammals. Early representatives of these mammaliaformes include Adelobasileus, Tikitherium (c.225 mya), Morganacudon (c.205 mya) and Megazostrodon (c.200 mya). Also dated to about this time, according to genetic studies, the Yinotheria, ancestors of the monotremes (surviving today in the form of the platypus and echidnas), branched off from the other mammals. Their earliest surviving members are the Shuotheriidae, around 165 mya.
These mammaliaformes would continue to develop into a number of distinct groups of mammals in the Jurassic and beyond. These include: the Allotheria (ancestors of the long-lived Multituberculata, which survived until 17.5 mya) potentially dates from the Triassic, if the order Haramiyida are included; the Triconodontidae (190-70 mya); the Docodonta (including Castorocauda, a semi-aquatic, beaver-like animal living about 165 mya); the gliding mammal Volaticotherium antiquum; the Dryolestoidea, which survived from the Jurassic into the Miocene; and the Cretaceous order Symmetrodonta. The Metatheria, the clade which includes modern marsupials, is first evidenced by Sinodelphys szalayi, from the Yixian Formation in China.
Among these is also the earliest known eutherian (the group encompassing placental mammals), Juramaia sinensis, which was found in China in rocks dating to around 160 mya. It would be Juramaia's successors which would, after the Chicxulub event, inherit the earth.
Also present in the Early Cretaceous was a plant, Archaefructus liaoningensis, which represents the earliest angiosperm (or flowering plant) known from the fossil record.