Prehistoric Periods

Hadean

The Hadean refers to the geologic eon before the Archean. It extends back to the Earths formation, and ended roughly 3.8 billion years ago (3800 million years ago), though the date varies according to different sources. The name ‘Hadean’ derives from Hades, Greek for ‘unseen’ or ‘Hell’ and suggesting the underworld or referring to the conditions on Earth at the time.

Archean

Archean refers to the time before the Proterozoic, 2500 million years ago. At the beginning of the Archean, the Earths heat flow was nearly 3 times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. The Earths crust was not only thinner than it is today, but probably broken up into many more plates, with numerous hot spots, rift valleys and transform faults. There were no large continents until late in the Archean; small protocontinents were the norm. The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in the Earths history.

The immediate cause of extinction appears to have been the continental drift of a significant landmass into the south polar region, causing a global temperature drop, glaciation, and consequent lowering of the sea level, which destroyed species’ habitats around the continental shelves.

The Ordovician Period is the second period of the Paleozoic Era and is broken into subdivisions: Lower (Tremadocian and Arenig) Middle (Llanvirn – subdivided into Abereiddian and Llandeilian, and Llandeilo) Late (Caradoc and Ashgill) This important period saw the origin and rapid evolution of many new types of invertebrate animals which replaced their Cambrian predecessors. Primitive plants move onto land which until then were totally barren. During the early Ordovician period the first vertebrate fish have been found. Graptolites and corals also flourished.

Sea levels were very high during this period. The Early Ordovician climate is thought to have been quite warm, at least in the tropics. Ordovician rocks contained abundant life and contain major oil and gas reservoirs in some regions.

In North America and Europe shallow continental seas that were rich in life. Trilobites (an extinct class of Arthropods) and Brachiopods (one of the major animal phyla) in particular were rich and diverse. The first Bryozoa (tiny creatures with a ring of tentacles surrounding the mouth) appear in the Ordovician as do the first coral reefs. Solitary corals date back to the Cambrian at least. It was long thought that the first true vertebrates (fish appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian period.

Now, extinct marine animals called graptolites thrived in the oceans. The southern continents were collected into a single continent called Gondwana during the Ordovician. Gondwana started the period in equatorial latitudes and drifted toward the South Pole during this period. As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician.

By the end of the period, Gondwana had neared or approached the pole and was largely glaciated – a glacier is a large, long-lasting flow of ice that is formed on land and moves in response to gravity. The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.

Silurian Period – 438-408 Million Years Ago

Silurian The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 443 million years ago, to the beginning of the Devonian period.

As with other geologic periods, the rock beds that define the periods beginning and end are well identified, however, the exact dates are uncertain by 5 – 10 million years. During this period, the Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses.

Jan Palonka, CC BY 3.0, via Wikimedia Commons

Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. Silurian witnessed a relative stabilization of the Earths general climate, ending the previous pattern of erratic climatic fluctuations.

Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces. Later in the Silurian, the climate cooled slightly, however, in the Silurian-Devonian boundary, the climate became warmer.

Subdivisions of Silurian:

Llandovery

The Llandovery epoch lasted from 443 million years ago to 428.2 ± 2.3 million years ago, and is subdivided into 3 stages: the Rhuddanian, lasting until 439 million years ago, the Aeronian, lasting to 436 million years ago and the Telychian. The epoch is named for the town of Llandovery in Carmarthenshire, Wales.

Wenlock

The Wenlock, which lasted from 428 million years ago to 422 million years ago, is subdivided into the Sheinwoodian (to 426 million years ago) and Homerian ages. It is named after the Wenlock Edge in Shropshire, England. During the Wenlock, the oldest known tracheophytes of the genus Cooksonia, appear. The complexity of slightly younger Gondwana plants like Baragwanathia indicates either a much longer history for vascular plants, perhaps extending into the early Silurian or even Ordovician.

Ludlow

The Ludlow, lasting from 422 million years ago to 418 million years ago, comprises the Gorstian stage, lasting until 421 million years ago, and the Ludfordian stage. It is named for the town of Ludlow in Shropshire, England.

Pridoli

The Pridoli, lasting from 418 million years ago to 416 million years ago, is the final and shortest epoch of the Silurian. It is named for the town of Pridoli in the Czech Republic.

The base of the Silurian is set at a major extinction event when 60% of marine species were wiped out. Silurian period sees the appearance of the first plants on land. Algae was in abundance in the seas, as well as marine invertebrates. Coral reefs made their first appearance during this time, built by extinct tabulate (an extinct form of coral) and rugose corals (are an extinct order of coral).

The first bony fish, the Osteichthyes (superclass of fish) appeared, represented by the Acanthodians (sometimes called spiny sharks) covered with bony scales. Fishes reached considerable diversity and developed movable jaws, adapted from the supports of the front 2 or 3 gill arches. A diverse fauna of Eurypterus (Sea Scorpions) – some of them several metres in length, prowled the shallow Silurian seas of North America, many of their fossils have been found in New York State. Brachiopods (also known as lamp shells), bryozoa (tiny colonial animals), molluscs (these range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus), and trilobites (Trilobites are extinct arthropods) were abundant and diverse.

Myriapods became the first proper terrestrial animals. The terrestrial ecosystems included the first multicellular terrestrial animals that have been identified, relatives of modern spiders and millipedes whose fossils were discovered in the 1990s. Myriapods – 4 groups of arthropod – the centipedes, millipedes, pauropods and symphylans – share a number of common features such as a similar body plan consisting of a head followed by an elongate trunk with many legs.

During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation. The southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.

The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include two phases of the Tethys – the Proto-Tethys and Paleo-Tethys – the Rheic Ocean, a seaway of the Iapetus Ocean (now in between Avalonia and Laurentia), and the newly formed Ural Ocean.

At the end of Silurian, a series of minor extinction events, including the Lau event, occurred. They were probably caused by climate change or impact events.

Devonian Period – 408-362 Million Years Ago

Devonian

The Devonian is a geologic period of the Paleozoic era around 408 million years ago also known as the Age of Fishes. The Devonian era is named after Devon, England, where rocks from this period were first studied. During the Devonian era, the planet was changing in appearance and the great supercontinent of Gondwana was heading slowly northwards away from the south pole. A second supercontinent began to form that straddled the Equator. Known as Euramerica, or Laurussia, it was created by the coming together of parts of North America, northern Europe, Russia and Greenland.

Devonian Times

The Hangenberg event sits on or just below the Devonian/Carboniferous boundary and marks the final spike in the period of extinction. The Hangenberg event was an anoxic period at the end of the Famennian epoch (late Devonian) associated with the Late Devonian extinction, marked by a black shale.

Stampfli & Borel 2000, Attribution, via Wikimedia Commons

Fammenian – South American glaciation begins.

The Kellwasser event is the term given to the extinction pulse that occurs near the Frasnian/Famennian boundary. There may in fact have been two closely spaced events here.

Frasnian – Frasnian time occurred between 385.3 million and 374.5 million years ago. The stage’s name is derived from the town of Frasnes in the Ardennes region of southern Belgium. The lower boundary point of the Frasnian is defined on the basis of the first occurrence of the conodont Ancyrodella rotundiloba.

Givetian – Givetian time spans the interval between 391.8 million and 385.3 million years ago. It was named for exposures studied near Givet in the Ardennes region of northern France and is characterized by a zone (a smaller subdivision of geologic time) whose rocks include the ammonite genus Maenioceras. Widespread shrubs and trees appeared.

Eifelian – Eifelian time spans the interval between 397.5 million and 391.8 million years ago. The name of the Eifelian Stage is derived from the Eifel Hills in western Germany, near Luxembourg and Belgium.

Emsian – Emsian time spans the interval between 407 million and 397.5 million years ago. The Emsian Stage was named for exposures studied in the region of the Ems River in western Germany, where it consists of wackes (dirty sandstone) noted for their rich fossil faunas.

Pragian – Hunsruck fauna – More than 260 animal species have been described from the Hunsruck Slate. The deposits occur in a strip some 15 kilometres wide and 150 kilometres long running from northwest to southeast. In the main depositional basins of Kaub, Bundenbach and Gemünden, echinoderms are concentrated in the southwestern area around Bundenbach, with brachiopods predominating in the northeast. The presence of corals and trilobites with well-developed eyes and the rarity of plant fossils from the central basin areas suggest a shallow-water environment. Other animal fossils include sponges, corals, brachiopods, cephalopods, cnidarians, gastropods, and worm trace fossils. Trilobites and echinoderms are relatively abundant in some horizons. Crinoids and starfish are the predominant representatives of the echinoderms, although holothurians (sea cucumbers) are also represented. More than 60 species of crinoids are described from the Hunsrück Slate. Both placoderm armoured fish and agnatha jawless fish have been discovered.

Rhynie chert – The Rhynie chert is an Early Devonian Lagerstatte found near the village of Rhynie, Aberdeenshire, Scotland, containing exceptionally preserved plant, fungus, lichen and animal material petrified in three dimensions by covering with fast-setting volcanic minerals. The bulk of the fossil bed consists of primitive plants (which had water-conducting cells and sporangia, but no true leaves), along with arthropods, lichens, algae and fungi. The bed lies under at least 1 metre of overburden, in a single small field near the village of Rhynie, so is effectively inaccessible to collectors.

Lockhovian – The beginning of the Devonian Period which lasts until the Pragian.

More About Devonian

The Devonian is usually broken into Early, Middle, and Late subdivisions. The rocks corresponding to these epochs are referred to as belonging to the lower, middle, and upper parts of the Devonian System. As the Paleozoic era continues, we see the first appearance of land creatures, such as mites, ammonites, crabs and the first arthropods like insects and spiders also started to colonize terrestrial habitats. During the Devonian Period the first fish evolved legs and started to walk on land as tetrapod’s. A recently discovered fossil creature from the Devonian has been hailed as a vital link between fish and the first vertebrates to walk on land. Found in the Canadian Arctic in 2004, Tiktaalik had a crocodile-like head and strong, bony fins that scientists think it used like legs to move in shallow waters or even on land. The fish showed other characteristics of terrestrial animals, including ribs, a neck, and nostrils on its snout for breathing air.

The first seed-bearing plants spread across dry land, forming huge forests. The vegetation of the early Devonian consisted primarily of small plants, the tallest being only a metre tall. In the oceans, fish diversified into the first primitive sharks and the first lobe-finned and bony fish appeared. Early placoderms fed on mollusks and other invertebrates, but later species developed into ferocious, fish-slicing monsters measuring up to 33 feet (10 metres) long.

The first ammonite mollusks appeared, and trilobites, the mollusc-like brachiopods, as well as great coral reefs were still common. The Late Devonian extinction severely affected marine life. Towards the end of the Devonian period, amphibians began to colonize the land. The first ferns also grew and true seed plants eventually emerged. The Devonian period also saw many new groups of fish evolve.

During the Devonian, there were three major continental masses: North America and Europe sat together near the equator, much of their current land underneath seas. To the north lay a portion of modern Siberia. A composite continent of South America, Africa, Antarctica, India, and Australia dominated the southern hemisphere. The Late Devonian extinction was one of five major extinction events in the history of the Earths biota (the total collection of organisms of a geographic region or a time period). A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when nearly all of the fossil agnathan fishes (jawless fishes) suddenly disappeared. A second strong pulse closed the Devonian period. Overall, 19% of all families and 50% of all genera went extinct.

Carboniferous Period – 362-290 Million Years Ago

Carboniferous

The Carboniferous is a major division of the geologic timescale that extends from the end of the Devonian period, about 359.2 million years ago, to the beginning of the Permian period, about 299 million years ago. As with most older geologic periods, the rock beds that define the periods start and end are well identified, but the exact dates are uncertain by 5 – 10 million years.

The Carboniferous is named for the extensive coal beds of that age found in Western Europe. The first third of the Carboniferous is called the Mississippian epoch, and the remainder is called the Pennsylvanian. Conifer trees were introduced in this important and well known time period. 

RoRo, CC0, via Wikimedia Commons

During this period snakes emerged as well as salamander type amphibians.  Large amphibians roamed the lands now and fresh water fish and sharks were now in abundance. Near the end of this period the first reptiles appeared on land.

More About Carboniferous

A global drop in sea level at the end of the Devonian reversed early in the Carboniferous; this created the widespread epicontinental seas and carbonate deposition of the Mississippian. There was also a drop in south polar temperatures; southern Gondwanaland was glaciated throughout the period, though it is uncertain if the ice sheets were a hold over from the Devonian or not. These conditions apparently had little effect in the deep tropics, where lush coal swamps flourished within 30 degrees of the northernmost glaciers. In the oceans the most important groups are the foraminifera, corals, bryozoa, brachiopods, ammonoids, echinoderms (especially crinoids), and Chondrichthyes (sharks and their relatives).

Many fish inhabited the Carboniferous seas; predominantly Elasmobranchs sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms.

Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.

Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole. Freshwater fishes were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus. Fossil remains of air-breathing insects, myriapods and arachnids are known from the late Carboniferous, but so far not from the early Carboniferous. Their diversity when they do appear however show that these arthropods were both well developed and numerous. Among the insect groups are the Syntonopterodea (relatives of present-day mayflies), the abundant and often large sap-sucking Palaeodictyopteroidea, the huge predatory Protodonata (griffinflies), the diverse herbivorous ‘Protorthoptera’, and numerous basal Dictyoptera (ancestors of cockroaches).

Many insects have been obtained from the coalfields of Saarbruck and Commentry, and from the hollow trunks of fossil trees in Nova Scotia. Some British coalfields have yielded good specimens: Archaeoptitus, from the Derbyshire coalfield, had a spread of wing extending to more 35 cm; some specimens (Brodia) still exhibit traces of brilliant wing colours. In the Nova Scotian tree trunks land snails (Archaeozonites, Dendropupa) have been found.

Cloudina

Carboniferous amphibians were diverse and common by the middle of the period, more so than they are today; some were as long as 6 metres, and those fully terrestrial as adults had scaly skin. They included a number of basal tetrapod groups classified in early books under the Labyrinthodontia. These had long bodies, a head covered with bony plates and generally weak or undeveloped limbs. The largest were over 2 metres long. They were accompanied by an assemblage of smaller amphibians included under the Lepospondyli, often only about 15 centimetres long. Some Carboniferous amphibians were aquatic and lived in rivers (Loxomma, Eogyrinus, Proterogyrinus); others may have been semi-aquatic (Ophiderpeton, Amphibamus) or terrestrial (Dendrerpeton, Hyloplesion, Tuditanus, Anthracosaurus).

One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. These included the earliest Sauropsid reptiles (Hylonomus), and the earliest known Synapsida (Archaeothyris). These small lizard-like animals quickly gave rise to many descendants.

The amniote egg allowed these ancestors of all later birds, mammals and reptiles to reproduce on land by preventing the desiccation, or drying-out, of the embryo inside. By the end of the Carboniferous period, the reptiles had already diversified into a number of groups, including protorothyridids, captorhinids, aeroscelids, and several families of pelycosaurs.

Permian Period – 290-248 Million Years Ago

Permian

The Permian Period, is the last period of the Paleozoic Era and famous for its ending epoch event, the largest mass extinction known to science. The Permian Period was named after the kingdom of Permia in modern-day Russia by Scottish geologist Roderick Murchison in 1841. The Permian period marked great changes in the Earths climate and appearance. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still at the grip of an Ice Age from the Carboniferous. Glaciers receded around the mid-Permian period as the climate gradually warmed, drying the continents interiors.

In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Trilobites and other marine groups became extinct, and a group of small reptiles (diapsids) appeared. These were the ancestors to most modern reptiles and the ruling dinosaurs as well as pterosaurs and crocodiles. 

Stampfli & Borel 2000, Attribution, via Wikimedia Commons

Thriving also, were the early ancestors to mammals (synapdia), which included some large reptiles such as dimetrodon. Reptiles grew to dominate among vertebrates, because their special adaptations enabled them to flourish in the drier climate.

The warm zone spread in the northern hemisphere, where extensive dry desert appeared. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. The old types of plants and animals died out. The three primary subdivisions of the Permian Period are given below from youngest to oldest, and include faunal stages also from youngest to oldest.

• Lopin – Changhsingian Age / Wuchiapingian Age
• Guadalup – Capitanian Age / Wordian Age /Roadian Age
• Cisuralian – Kungurian Age / Artinskian Age / Sakmarian Age / Asselian Age

More About Permian

Sea levels in the Permian remained generally low, and near-shore environments were limited by the collection of almost all major landmasses into a single continent – Pangaea. One continent, even a very large one, has less shoreline than 6 to 8 smaller ones. This could have in part caused the widespread extinctions of marine species at the end of the period by severely reducing shallow coastal areas preferred by many marine organisms. Permian marine deposits are rich in fossil mollusks, echinoderms, and brachiopods. Fossilized shells of two kinds of invertebrates are widely used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist that is one of the foraminiferans, and ammonoids, shelled cephalopods that are distant relatives of the modern nautilus.

Terrestrial life in the Permian included diverse plants, fungi, arthropods, and various types of tetrapods. The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian there was a major transition in vegetation. The swamp-loving lycopod trees of the Carboniferous, such as Lepidodendron and Sigillaria, were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. Lycopods and swamp forests still dominated the South China continent because it was an isolated continent and it sat near or at the equator. Oxygen levels were probably high there.

The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. The ginkgos and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups. A number of important new insect groups appeared at this time, including the Coleoptera (beetles) and Diptera (flies). Permian tetrapods consisted of temnospondyli, lepospondyli and batrachosaur amphibians and sauropsids and synapsid (pelycosaurs and therapsids) reptiles.

This period saw the development of a fully terrestrial fauna and the appearance of the first large herbivores and carnivores. Early Permian terrestrial faunas were dominated by pelycosaurs and amphibians, the middle Permian by primitive therapsids such as the dinocephalia, and the late Permian by more advanced therapsids such as gorgonopsians and dicynodonts. Towards the very end of the Permian the first archosaurs appeared (proterosuchid thecodonts); during the following Triassic period these latter would evolve into more advanced types and eventually into dinosaurs. Also appearaing at the end of the Permian were the first cynodonts, which would go on to evolve into mammals during the Triassic.

Another group of therapsids, the therocephalians (such as Trochosaurus), appeared in the Middle Permian. During this time, amphibians remained common, including various Temnospondyli and Lepospondyli. Synapsids became the dominant type of animal, represented by the Pelycosaurs during the Early Permian and Therapsids during the Middle and Late Permian, and distingusihed by the appearance and possession of mammal-like characteristics (hence the old term, mammal-like reptiles). These were accompanied by Anapsids or Parareptiles, which included both lizard-like and large herbivorous forms, and primitive diapsids.

The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. It is also the only known mass extinction of insects. On an individual level, perhaps as many as 99.5% of separate organisms died as a result of the event. Recovery from the Permian-Triassic extinction event was protracted; on land ecosystems took 30 Million years to recover.

Next we have the start of the dinosaur periods. So head over to Triassic Period to read about this.

FURTHER READING & STUDY

• Harold J. E Peake (1940) The study of Prehistoric Times
• Springer Verlag (2004) Journal of World Prehistory
• Vladimir I. Davydov (2012) The Carboniferous Period
• Jonathan P. Wilson (2017) Dynamic Carboniferous tropical forests.
• Ralph T. Becker et al (2012) The Devonian Period
• Victoria Pease et al (2021) Precambrian research