Pteraspis is an extinct genus of pteraspidid heterostracan agnathan vertebrate that lived in the Pragian epoch of the Devonian period in what is now Britain and Belgium. Like other heterostracan fishes, Pteraspis had a protective armored plating covering the front of its body. Though lacking fins other than its lobed tail, it is thought to have been a good swimmer thanks to stiff, wing-like protrusions derived from the armoured plates over its gills. This, along with the horn-like rostrum, made Pteraspis streamlined in shape. Pteraspis had some stiff spikes on its back an additional form of protection against predators, it is thought to have fed from shoals of plankton just under the ocean surface, is found in association with marine fossils. Pteraspis grew to a length of 20 centimeters. Parker, Steve. Dinosaurus: the complete guide to dinosaurs. Firefly Books Inc, 2003. Pg. 59
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174 to 163 million years ago. Fossil-bearing rocks from the Middle Jurassic are rare, but some important formations include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland, the Daohugou Beds in China, Itat Formation in Russia, the Isalo III Formation of western Madagascar. During the Middle Jurassic epoch, Pangaea began to separate into Laurasia and Gondwana, the Atlantic Ocean formed. Eastern Laurasia was tectonically active as the Cimmerian plate continued to collide with Laurasia's southern coast closing the Paleo-Tethys Ocean. A subduction zone on the coast of western North America continued to create the Ancestral Rocky Mountains; the Middle Jurassic is one of the key periods in the evolution of life on earth. Many groups, including dinosaurs and mammals, diversified during this time. During this time, marine life flourished. Ichthyosaurs, although common, are reduced in diversity.
Plesiosaurs became common at this time, metriorhynchid crocodilians first appeared. Many of the major groups of dinosaurs emerged during the Middle Jurassic. Descendants of the therapsids, the cynodonts were still flourishing along with the dinosaurs; these included the mammals. Mammals were diverse and numerous in faunas from around the world. Tritylodonts were larger, had an global distribution; the first crown group mammals appeared in the Middle Jurassic. A group of cynodonts, the trithelodonts, were becoming rare and became extinct at the end of this epoch. Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoes and ferns were common
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying the decomposition of both gross and fine biological features until long after a durable impression was created in the surrounding matrix. Lagerstätten span geological time from the Neoproterozoic era to the present. Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Devonian Hunsrück Slates and Gogo Formation, the Carboniferous Mazon Creek, the Jurassic Solnhofen limestone, the Cretaceous Santana and Tanis formations, the Eocene Green River Formation. Palaeontologists distinguish two kinds: Konzentrat-Lagerstätten are deposits with a particular "concentration" of disarticulated organic hard parts, such as a bone bed; these Lagerstätten are less spectacular than the more famous Konservat-Lagerstätten.
Their contents invariably display a large degree of time averaging, as the accumulation of bones in the absence of other sediment takes some time. Deposits with a high concentration of fossils that represent an in situ community, such as reefs or oyster beds, are not considered Lagerstätten. Konservat-Lagerstätten are deposits known for the exceptional preservation of fossilized organisms or traces; the individual taphonomy of the fossils varies with the sites. Conservation Lagerstätten are crucial in providing answers to important moments in the history and evolution of life. For example, the Burgess Shale of British Columbia is associated with the Cambrian explosion, the Solnhofen limestone with the earliest known bird, Archaeopteryx. Konservat-Lagerstätten preserve sclerotized and soft-bodied organisms or traces of organisms that are not otherwise preserved in the usual shelly and bony fossil record. In 1986, Simon Conway Morris calculated only about 14% of genera in the Burgess Shale had possessed biomineralized tissues in life.
The affinities of the shelly elements of conodonts were mysterious until the associated soft tissues were discovered near Edinburgh, Scotland, in the Granton Lower Oil Shale of the Carboniferous. Information from the broader range of organisms found in Lagerstätten have contributed to recent phylogenetic reconstructions of some major metazoan groups. Lagerstätten seem to be temporally autocorrelated because global environmental factors such as climate might affect their deposition. A number of taphonomic pathways may produce Lagerstätten; the following is an incomplete list: Orsten-type and Doushantuo-type preservations preserve organisms in phosphate. Bitter Springs-type preservation preserves them in silica. Carbonaceous films are the result of Burgess Shale-type preservation Pyrite preserves exquisite detail in Beecher’s trilobite-type preservation. Ediacaran-type preservation preserves moulds with the aid of microbial mats; the world's major Lagerstätten include: List of fossil sites Penney, D. 2010.
Biodiversity of Fossils in Amber from the Major World Deposits. Siri Scienfic Press, Manchester, 304 pp. "Fossil Lagerstätten". Department of Earth Sciences, University of Bristol. 2003. Retrieved 2005-11-21. – A catalogue of sites of exceptional fossil preservation produced by MSc palaeobiology students at University of Bristol's Department of Earth Sciences. Orr, Patrick J.. "Three-dimensional preservation of a non-biomineralized arthropod in concretions in Silurian volcaniclastic rocks from Herefordshire, England". Journal of the Geological Society. 157: 173–86. Doi:10.1144/jgs.157.1.173. Retrieved 2006-10-26
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period. The Early Jurassic starts after the Triassic-Jurassic extinction event, 201.3 Ma, ends at the start of the Middle Jurassic 174.1 Ma. Certain rocks of marine origin of this age in Europe are called "Lias" and that name was used for the period, as well, in 19th-century geology. In southern Germany rocks of this age are called Black Jurassic. There are two possible origins for the name Lias: the first reason is it was taken by a geologist from an English quarryman's dialect pronunciation of the word "layers". There are extensive Liassic outcrops around the coast of the United Kingdom, in particular in Glamorgan, North Yorkshire and Dorset. The'Jurassic Coast' of Dorset is associated with the pioneering work of Mary Anning of Lyme Regis; the facies of the Lower Jurassic in this area are predominantly of clays, thin limestones and siltstones, deposited under marine conditions. Lias Group strata form imposing cliffs in southern Wales.
Stretching for around 14 miles between Cardiff and Porthcawl, the remarkable layers of these cliffs, situated on the Bristol Channel are a rhythmic decimetre scale repetition of limestone and mudstone formed as a late Triassic desert was inundated by the sea. There has been some debate over the actual base of the Hettangian stage, so of the Jurassic system itself. Biostratigraphically, the first appearance of psiloceratid ammonites has been used. If this biostratigraphical indicator is used technically the Lias Group—a lithostratigraphical division—spans the Jurassic / Triassic boundary. During this period, which had died out at the end-of-Triassic extinction, radiated out into a huge diversity of new forms with complex suture patterns. Ammonites evolved so and their shells are so preserved, that they serve as important zone fossils. There were several distinct waves of ammonite evolution in Europe alone; the Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the existing Rhaetian ichthyosaurs and plesiosaurs continuing to flourish, while at the same time a number of new types of these marine reptiles appeared, such as Ichthyosaurus and Temnodontosaurus among the ichthyosaurs, Eurycleidus and Rhomaleosaurus among the plesiosaurs.
All these plesiosaurs had large heads. In the Toarcian, at the end of the Early Jurassic, the thalattosuchians appeared, as did new genera of ichthyosaurs and plesiosaurs. On land, a number of new types of dinosaurs—the heterodontosaurids, scelidosaurs and tetanurans—appeared, joined those groups like the coelophysoids and the sauropods that had continued over from the Triassic. Accompanying them as small carnivores were the sphenosuchian and protosuchid crocodilians. In the air, new types of pterosaurs replaced those, but in the undergrowth were various types of early mammals, as well as tritylodont mammal-like reptiles, lizard-like sphenodonts, early lissamphibians. Late Triassic Toarcian turnover Davies, A. M. An Introduction to Palaeontology, Thomas Murby & Co. London House, M. R. Geology of The Dorset Coast, The Geologists' Association. Simms, M. J. Chidlaw, N. Morton, N. and Page, K. N. British Lower Jurassic Stratigraphy, Geological Conservation Review Series, No. 30, Joint Nature Conservation Committee, Peterborough.
Early Jurassic Period – The Lias epoch. Palaeos – overall presentation. Lecture 12 – Early Jurassic. Informative lecture notes by Dr. Paul Olsen
Quaternary is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy. It follows the Neogene Period and spans from 2.588 ± 0.005 million years ago to the present. The Quaternary Period is divided into two epochs: the Holocene; the informal term "Late Quaternary" refers to the past 0.5–1.0 million years. The Quaternary Period is defined by the cyclic growth and decay of continental ice sheets associated with Milankovitch cycles and the associated climate and environmental changes that occurred. In 1759 Giovanni Arduino proposed that the geological strata of northern Italy could be divided into four successive formations or "orders"; the term "quaternary" was introduced by Jules Desnoyers in 1829 for sediments of France's Seine Basin that seemed to be younger than Tertiary Period rocks. The Quaternary Period extends to the present; the Quaternary covers the time span of glaciations classified as the Pleistocene, includes the present interglacial time-period, the Holocene.
This places the start of the Quaternary at the onset of Northern Hemisphere glaciation 2.6 million years ago. Prior to 2009, the Pleistocene was defined to be from 1.805 million years ago to the present, so the current definition of the Pleistocene includes a portion of what was, prior to 2009, defined as the Pliocene. Quaternary stratigraphers worked with regional subdivisions. From the 1970s, the International Commission on Stratigraphy tried to make a single geologic time scale based on GSSP's, which could be used internationally; the Quaternary subdivisions were defined based on biostratigraphy instead of paleoclimate. This led to the problem that the proposed base of the Pleistocene was at 1.805 Mya, long after the start of the major glaciations of the northern hemisphere. The ICS proposed to abolish use of the name Quaternary altogether, which appeared unacceptable to the International Union for Quaternary Research. In 2009, it was decided to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.588 Mya and including the Gelasian stage, considered part of the Neogene Period and Pliocene Epoch.
The Anthropocene has been proposed as a third epoch as a mark of the anthropogenic impact on the global environment starting with the Industrial Revolution, or about 200 years ago. The Anthropocene is not designated by the ICS, but a working group has been working on a proposal for the creation of an epoch or sub-period; the 2.6 million years of the Quaternary represents the time during which recognizable humans existed. Over this geologically short time period, there has been little change in the distribution of the continents due to plate tectonics; the Quaternary geological record is preserved in greater detail than that for earlier periods. The major geographical changes during this time period included the emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which turned the Black Sea and Baltic Sea into fresh water, followed by their flooding by rising sea level; the current extent of Hudson Bay, the Great Lakes and other major lakes of North America are a consequence of the Canadian Shield's readjustment since the last ice age.
The climate was one of periodic glaciations with continental glaciers moving as far from the poles as 40 degrees latitude. There was a major extinction of large mammals in Northern areas at the end of the Pleistocene Epoch. Many forms such as saber-toothed cats, mastodons, etc. became extinct worldwide. Others, including horses and American cheetahs became extinct in North America. Glaciation took place during the Quaternary Ice Age – a term coined by Schimper in 1839 that began with the start of the Quaternary about 2.58 Mya and continues to the present day. In 1821, a Swiss engineer, Ignaz Venetz, presented an article in which he suggested the presence of traces of the passage of a glacier at a considerable distance from the Alps; this idea was disputed by another Swiss scientist, Louis Agassiz, but when he undertook to disprove it, he ended up affirming his colleague's hypothesis. A year Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects.
This idea led to the establishment of the Glacial Theory. In time, thanks to the refinement of geology, it has been demonstrated that there were several periods of glacial advance and retreat and that past temperatures on Earth were different from today. In particular, the Milankovitch cycles of Milutin Milankovitch are based on the premise that variations in incoming solar radiation are a fundamental factor controlling Earth's climate. During this time, substantial glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, all of Antarctica; the Great Lakes formed and giant mammals thrived in parts of North America and Eurasia not covered in ice. These mammals became extinct. Modern humans evolved about 315,000 years ago. During the Quaternary Period, flowering plants, insects dominated