Paleozoic Era Paleobiology


Paleozoic Era Paleobiology

(544 to 245 mya)

"The Age of Trilobites"

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The Paleozoic Periods: Cambrian Ordovician Silurian Devonian Carboniferous Permian

The Paleozoic (meaning "time of ancient life)" Era lasted from 544 to 245 million years ago, and is divided into six periods. These 300 million years of the Paleozoic era realized many critical events in evolution, including the development of most invertebrate groups, life's conquest of land, the evolution of fish, reptiles, insects, and vascular plants, the formation of the supercontinent of Pangea. Fish and fish-like vertebrates arose in the early Paleoozoic and comprise more than half of the diversity of vertebrates that inhabit the world today. Also importantly, there were also no less than two ice ages in the Paleozoic. The Paleozoic was ended by the greatest mass extinction event in geologic history, the Permian/Triassic extinction, when some 95% of all marine species met extincttion.

Cambrian Period (544 to 505 mya) - Most major animal groups appear (Cambrian Fossils)

The name Cambrian derives from Cambria, the Roman name for Wales, where rocks of this age were first studied. Hard-shelled animals appeared in great numbers for the first time during the Cambrian, significantly because shallow seas flooded the continents. Gondwana formed near the South Pole.

Wiwaxia Inigmatic Soft Body Animal Fossil of the Cambrian ExplosionThe Cambrian truly is an astonishing period in evolution of life on earth. Most major groups of animals first appear in the fossil record, an event popularly and scientifically called the "Cambrian Explosion". The name largely derives from the hypothesized explosion of diversity of life that occurred very rapidly, but that this actually occurred is not a consensus among scientists. Darwin and others once believed that the Cambrian rocks contained the first and oldest fossil animals. We now know that these occur in the earlier Vendian strata.

Many marine metazoans having mineralized exoskeletons flourish in the Cambrian, including sponges, corals, molluscs, echinoderms, bryozoans, brachiopods and arthropods. It is commonly believed that there were no organisms at the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. This is, however, remains in dispute. The first shelled metazoans that are characteristic of the Cambrian occur well after the earliest complex trace fossils. This suggests that hard parts evolved later. Hence, trilobites, archaeocyaths, and small shelly animals did not evolve before the middle part of the Early Cambrian. The evolution of shelled metazoans is reflected by the appearence of successively more advanced shelly fossils.

Trilobites dominate the Cambrian fossil record, and these arthropods actually attained their peak number of families near the end of the Cambrian. It is believed there were some 15,000 species that evolved during the Paleozoic. Hence, the Paleozoic is sometimes called the age of trilobites. Modern times are sometimes called the age of insects (that are also arthropods), and it is believed there may be some 10,000,000 species of insects on Earth today (with beetles predominating).

The first detailed record of vertebrates appears during the Cambrian as fossils of jawless fish. These bottom-dwellers, some of which had skeletons made of cartilage rather than bone, first appeared some 500 million years ago. Many were covered in plate-like armour.

When one thinks of the migration of animals from the marine environment to land, Devonian-time comes to mind. But, in fact, the fossil record has substantial evidence that this critical adaptation had already begun in the Cambrian. One of these fossils, Climactichnites is almost as enigmatic today as it was during the time of Darwin.

Ordovician Period (505 to 440 mya) - Massive marine life diversification (Ordovician Fossils)

The Ordvician is named after a Celtic tribe called the Ordovices, and was a time that life diversified and specialized. Owing to continental separation, trilobites drifted apart genetically taking on new, location-dependent forms, some quite exotic. The first planktonic graptolites evolved, and other graptolite species became extinct. Most profound perhaps was the colonization of land. Terrestrial arthropod fossils occur in Ordovician strata, as do microfossils of the cells, cuticle, and spores of the early land-based plants.

Hoplolichas tricuspidatus Spectacularly Spinose Lichid  Russian trilobiteOrdovician strata are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Remains of Ostracoderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils.

Despite the appearance of coral fossils during this time, reef ecosystems continued to be dominated by algae and sponges, and in some cases by bryozoans. However, there apparently were also periods of complete global reef collapse due to global disturbances. Conodonts decreased in the North Atlantic Realm, but new lineages appeared in other regions. Seven major conodont lineages went extinct, but were replaced by nine new lineages that resulted from a major evolutionary radiation. These lineages included many new and morphologically different taxa. Sea level transgression persisted, causing the drowning of almost the entire Gondwana craton. The Ordovician fossils are the oldest complete vertebrates. They were jawless, armored fish with large bony shields on the head, and small plate-like scales covering the tail.

The Ordovician ended with a major extinction event that caused the demise of some 60% of marine genera. A Late Ordovician glaciation contributed to profound ecological disruption and mass extinctions. Reef-building fauna were broadly decimated. Nearly all conodonts disappeared in the North Atlantic Realm while only certain lineages became extinct in the Midcontinental Realm. Trilobites were greatly affected with the Agnostids and the vast majority of Asaphid trilobites meeting extinction, and many groups of echinoderms, brachiopods, bryozoans, graptolites, and chitinozoans also disappearing. The Atlantic Ocean closed as Europe moved towards North America. Climatic fluctuations were extreme as glaciation continued and became pervasive. Cold climates with icebergs abounded.

Silurian Period (440 to 410 mya) - Life gains a foothold on land (Silurian Fossils)

Scyphocrinites, a palagic Camerate crinoid was able to float long distancesThe Silurian, so named after a Celtic tribe called the Silures, realized additional marked changes for Earth that affected life significantly. Sea levels rose as the climate stabilized, at least compared to the prior millions of years. Coral reefs made their first appearance and expanded. Land plants evolved in the moist regions near the Equator. The Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws, which resulted from an adaptation of an anterior gill arch. The Silurian strata has fossils that are substantive evidence of life on land, particularly the arthropod groups. The fossils of the earliest of vascular plants are also prevalent. In the oceans, there was a widespread radiation of crinoids and a continuation of the expansion of the brachiopods.

Devonian Period (410 to 360 mya) "The Age of Fishes" - Colonization of the land (Devonian Fossils)

The Devonian was a time of great change across the Tree of Life. Reef ecosystems saw new and more varied forms, including the ammonoids and fish. It was also a time when life achieved the critical event of adapting to land. A time of great transition. Two major clades of animal moved ashore and rapidly radiated. Both the first tetrapods, or four legged Dunkleosteus sp., a Late Devonian Placoderm Armored Fish from Morocco This armour plated fish grew to 30 feet. The Placondermi family evolved in the Lower Devonian and perished the Lower Carboniferous, leaving no descendants living today. land-living vertebrates, and the first arthropods colonized the land, including wingless insects and the earliest arachnids. In the sea, ammonoids and fish evolve and quickly diversify. Primitive plants that gained a foothold in the Silurian went on to form forests. Arthropods and ultimately tetrapods were plodding the lands. The first insects, spiders, and tetrapods evolve.

In the Lower Devonian, plants were very tiny and primitive, generally lacking the leaf, root and vascular systems that would soon appear. But plant radiation was already progressing rapidly and led to the ferns, horsetails and seed plants. By the late Devonian earth had forests of tall rooted trees covered with Devonian Fossil Fish Osteolepis macrolepidotus from Scotland, an ancient tetrapod ancestorleaves. The lycophytes (Phylum Lycopodiophyta) are the oldest extant lineage of vascular plants e.g., club moss) and gave rise to all descending vascular plants in a major phylogenetic split. The Lycopods that reproduced by way of spores went on to form vast swamp forests during the Carboniferous period with the Lepidodendrales (e.g., Lepidodendron) reaching heights more than 100 feet. Sigillaria is another example of a lycopod tree. The seed-bearing Gymnosperms appeared near the end of the Devonian, an adaptation ultimately leading to propagation to dryer habitats.

The Devonian is often appropriately called the "Age of Fishes", since the fish took their place in complex reef systems containing nautiloids, corals, graptolites, blastods, echinoderms, trilobites, sponges, brachiopods and conodonts. With the many new forms of predators, trilobites continue to evolve their defensive strategies. During the Devonian, Placodermi (armored fish), Sarcopterygii (lobe-finned fish and lungfish) and Actinopterygii (conventional bony fish or ray-finned fish) evolved rapidly, many of which became huge and fierce predators. Until later in the Devonian the fishes were the only vertebrates, and gave rise to all other New predators provided the selective pressure for some trilobites to evolve some elaborate defensive spines. vertebrate lineages. The first sarcopterygiians, or the lobe-finned fish, appeared whose descendants were the first tetrapods that also evolved before by the Uper Devonian. The class Chondrichthyes, the cartilaginous fish, including skates, rays, and sharks appeared during the Devonian period.

Arthropods radiated to become well-established on land in the Devonian, and in some cases attained impressive size. The earliest known Hexapods appear in the Devonian fossil record. The increasing biomass of land plants and higher oxygen levels by the end of the Devonian faciliated the adaption to terrestrial life of herbivorous animals. The arthropods colonized the land, including wingless insects and the earliest arachnids. Trilobites declined despite their newly evolved armaments and stealthy ways, possibly due to increasingly widespread and ever more skillful predators.

Carboniferous Period (360 to 286 mya) "The Age of Plants" - Reptiles and the amniotic egg appear (Carboniferous Fossils)

The Carboniferous Period derives its name from the massive deposits of coal found in U.K. and Western Europe. In North America, the Carboniferous is divided into the Mississippian Period and the Pennsylvanian Period. During the Carboniferous, the continents below the equator still formed the supercontinent Gondwana.

Pachlocrinus aequalais Lower Mississippian Crinoid from Crawfordsville IndianaLife flourished in the seas in the wake of the late Devonian Extinction. Ammonoids rediversified very quickly. Crinoids, blastoids, brachiopods and bryozoans and single-celled Eukaryotes fusulinids known as fusulinids became abundant. The ray finned fishes radiate enormously. However, the age of the trilobite was drawing to a close. After radiations Ordovician, Silurian, and Devonian periods, the nine trilobite orders had shrunk to one remaining in the Carboniferous, the Order Proetida, that too would go extinct at the end of the Permian.

Life on land really took root in the Carboniferous, setting the stage for huge coal deposits to be formed in low-laying swamps. Common in the coal producing swamps spore bearing Lycopod trees that grew to more than 100 feet tall, Sigillaria and both spore-bearing and seed ferns. The early wingless insect forms that appeared in the Devonian acquire wings, and continue their radiation filling ever-expanding environmental niches. The burial of organically produced carbon is believed to have caused atmosphereic oxygen to increase to concentrations 80% higher than today, and may have, in turn, led to gigantism in some insects and amphibians whose limited respiratory systems would have otherwise constrained thier size.

Despite the appearance of seeds, most Carboniferous plants continued to use spores from reproduction. The moist and swampy environments of the Carboniferous enabled the Lycophytes (i.e., scale trees and club mosses) Lepidodendron Bark Section of Clubmoss Treethat evolved during the late Silurian to early Devonian to continue to diversify and fourish throughout the Carboniferous. However, the dependency on a moist environment caused the extinction of most taxa during arid conditions that prevailed near the end of the Paleozoic. Similary, Calamites and ferns were other spore-bearing plants that appeared during the Devonian and thrived during the following Carboniferous period.

Reptiles first appear in the Pennsylvanian, following the appearance of amphibians in the Devonian. The amniote egg appears, an important evolutionary invent that set the stage for further colonization of the land by tetrapods. The ancestors of birds, mammals, and reptiles could then reproduce on land since the embryo no longer required an acqeous environment.

Permian Period (286 to 245 mya)

Dimetrodon , a Permian SynapsidThe Permian Period extends from about 286 to 245 million years ago, and is the last geological period of the Palaeozoic Era. The Permian was named in the 1840s by Sir Roderick Murchison, a British geologist, from the extensive Permian exposures near Perm in Russia. The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event, where some 90% to 95% of marine organisms and 70% of all terrestrial organisms became extinct.

Life on land included a diversity of plants, arthropods, amphibians and reptiles. The reptiles were mainly synapsids (Pelycosaurs and Therapsids) that appeared in the Upper Carboniferous, and were bulky, cold-blooded animals with small brains Towards the very end of the Permian the first archosaurs appear, the ancestors of the soon to follow Triassic dinosaurs. Permian marine environments were abundant in mollusks, echinoderms, and brachiopods.

Mesozoic Era Paleobiology Continued