Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade; the name "halwaxiid" was formed by combining the names of two members of the proposed group and Wiwaxia. The group was defined as a set of Early to Mid Cambrian animals that had: a "chain mail" coat of three concentric bands of small armor plates that are called sclerites; some scientists are unhappy with this loose definition, arguing that such traits may have arisen convergently rather than being inherited from a common ancestor. This objection implies that the group may not be monophyletic, that their similarities are not biologically equivalent but evolved separately. In fact the originators of the term "halwaxiid" prefer an evolutionary "family tree" in which the halwaxiids are not monophyletic.
The possibility of convergence is reinforced by the presence of a scleritome in a derived gastropod mollusc. Other fossil organisms became involved in the debate as there were plausible arguments for considering them related to the halkieriids or Wiwaxia or both. Orthrozanclus, first described in 2007, looked an intermediate between the halkieriids and Wiwaxia as it had a shell like that of halkieriids, unmineralized sclerites and long spines like those of Wiwaxia – in fact the article which first described Orthrozanclus introduced the term "halwaxiid". New finds of Odontogriphus, reported in 2006, put this animal into play as well – despite its lack of sclerites or shells, its feeding apparatus looks like Wiwaxia’s; the siphogonuchitids, a Early Cambrian group known only from isolated sclerites among the small shelly fossils appear in analyses as their sclerites suggest this group may have been close to the ancestors of halkieriids. Halwaxiid sclerites were not able to grow once they had been formed, but must rather have been either moulted or resorbed to make way for new, larger sclerites.
The new sclerites cannot have poked between existing sclerites. Since 1995 several phylogenies, or evolutionary "family trees", have been suggested for these organisms; the position of Wiwaxia is highlighted, since the position of this organism has been central to the debate since 1990. Not depicted: Sun et al. propose Halkieria as a stem brachiopod, Wiwaxia as a stem mollusc. Wiwaxia and Halkieria did not thrive for a long period of time and were extinct before the Cambrian was over. A possible reason for this is that during the Cambrian substrate revolution, the microbial mat's disappearance triggered an alternating pattern in the fauna that were present for the Halwaxiids to eat. Since the debate started in 1990, the most intense part has centered round Wiwaxia; until 2006 only one, poor-preserved fossil of Odontogriphus had been described. Conway Morris dismissed the earliest classification of Wiwaxia as a polychaete worm, because he thought there was little structural similarity between a polychaete's scale-like elytra and Wiwaxia’s sclerites, because the arrangement of the sclerites, with quite different numbers in each band, showed no sign of the regular segmentation, a feature of polychaetes.
Instead he thought Wiwaxia was similar to shell-less aplacophoran molluscs, that it must have moved on a mollusc-like muscular foot, that its feeding apparatus looked like a primitive form of the molluscan radula, a tooth-bearing chitinous "tongue". Hence he classified it as a "sister" of the molluscs; when he described the first articulated specimens of Halkieria in 1990, Conway Morris wrote of "the halkieriid-wiwaxiid body plan" and that the halkieriids might be close relatives of molluscs. Shortly after this in 1990 Butterfield published his first paper on Wiwaxia, he argued that, since Wiwaxia’s sclerites appeared to be solid, they were not similar to the hollow sclerites of halkieriids. In fact he thought they were more similar to the chitinous bristles that project from the bodies of modern annelids and in some genera form leaf-like scales that cover the back like roof tiles - in composition, in detailed structure, in how they were attached to the body via follicle-like pockets in the skin, in overall appearance.
He contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as to be mounted in two parts on the sides of the "head", an arrangement, common in polychaetes. He therefore classified Wiwaxia as a polychaete. Conway Morris and Peel accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes; however they argued that Wiwaxia was closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller, more numerous and hollow. They wrote that in 1994 Butterfield had found Wiwaxia sclerites that were hollow, they noted that one specimen of Wiwaxia showed traces of a small shell a vestige left over from an earlier stage in the ani
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
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
Vernanimalcula guizhouena is an acritarch dating from 600 to 580 million years ago. Vernanimalcula means "small spring animal", referring to its appearance in the fossil record at the end of the Marinoan Glaciation and the belief upon discovery it was an animal; the Vernanimalcula fossils were discovered in the Doushantuo Formation in China. This formation is a Konservat-Lagerstätte, one of the rare places where soft body parts and fine details are preserved in the fossil record; the Vernanimalcula fossils were interpreted as showing a triploblastic structure, a coelom, a differentiated gut, a mouth, an anus, paired external pits that were believed possible sense organs, making it the earliest known member of the Bilateria. The appearance of Vernanimalcula so early in the fossil record was believed to have had important implications if it were bilaterian; the radiation of animals into many phyla would have occurred before any animal became much larger than microscopic size, making the sudden appearance of many animal phyla in the Cambrian explosion an illusion and represented a sudden increase in size and the development of fossilised body parts by species in existing phyla.
The description of Vernanimalcula as bilaterian has been challenged. Other workers in the field have claimed that Vernanimalcula is a taphonomic artefact generated by phosphate growth within a spherical object such as an acritarch, thus Vernanimalcula was not an animal, let alone a bilaterian. Chen et al. defended their interpretation of Vernanimalcula against the claims of Bengtson and Budd. Petryshyn et al. examined additional fossils resembling Vernanimalcula and concluded that the fossils are "likely biogenic in nature." Snowball Earth Spriggina Kimberella
Olenellus is an extinct genus of redlichiid trilobites, with species of average size. It lived during the Botomian and Toyonian stages, 522 to 510 million years ago, in what is North-America, part of the paleocontinent Laurentia. Olenellus means small Olenus, after a genus belonging to the Ptychopariida, to which the type species O. thompsoni was assigned. The name Olenus refers to a mythological figure, turned to stone by the gods; the names of the species have the following derivations. Agellus comes from the Latin word for hamlet. Chiefensis refers to the Chief Range, which includes the Ruin Wash section, that holds the last of the Olenellina. Fowleri was named in honor of Ed Fowler, whose quarrying skills exposed the type locality of this species. Getzi is called after Noah L. Getz, on whose Lancaster estate several Olenellus species were first collected. Nevadensis refers to the state of Nevada. Parvofrontatus means ‘small front’ from the Latin words parvus and frontatus, indicating the short distance between the anterior border and the glabella in this species.
Roddyi was named in honor of Dr. H. Justin Roddy, an internationally known naturalist and professor of geology. Romensis refers to the Rome Formation. Terminatus is from the Latin terminus, meaning final, indicating the demise of the Olenellus lineage. Olenellus is the only genus recognised in the subfamily Olenellinae; the sister group called. "Paedeumias" was regarded as a genus related to Olenellus or a subgenus being part of Olenellus. Recent analysis shows that there is a group of species assigned to Olenellus nested within Olenellus. However, this group is more related to the majority of the remainder of Olenellus species than to O. agellus and O. romensis. This implies that either two new monophyletic subgenera need to be erected, or Olenellus and Olenellus need to be dropped as subgenera, the latter being proposed by Lieberman. O. thompsoni is found in the middle Upper Olenellus-zone of Vermont. O. Agellus is present in the middle Upper Olenellus-zone of Vermont. O. Chiefensis has been collected from the final layer of the Upper Olenellus-zone of Nevada.
O. Clarki is found in Upper Olenellus-zone of California. O. crassimarginatus has been collected in the middle Upper Olenellus-zone of Vermont. O. fowleri has been collected from the final layer of the Upper Olenellus-zone of Nevada. O. Getzi is found in Upper Olenellus-zone of Pennsylvania. O. Howelli occurs in the final layer of the Upper Olenellus-zone of Nevada. O. Nevadensis has been collected in the Upper Bristolia-zonule of California, it occurs in the Bristolia-zonule, Upper Olenellus-zone of Nevada. O. parvifrontatus has been collected in the Olenellus-zone of the Yukon Canada. O. Puertoblancoensis was found in the Botonian/Toyonian Olenellus-zone of the Caborca Region, Mexico O. robsonensis occurs in the? Middle Olenellus-zone of British Columbia, Canada. O. Roddyi occurs in the Olenellus-zone of Pennsylvania. O. Romensis occurs in the middle Upper Olenellus-zone of Virginia. O. Terminatus has been collected from the final layer of the Upper Olenellus-zone of Nevada. O. Transitans has been collected from the middle Upper Olenellus-zone of Vermont.
As with most early trilobites, Olenellus has an flat exoskeleton, only thinly calcified, has crescent-shaped eye ridges. As part of the suborder Olenellina, Olenellus lacks dorsal sutures. Like all other members of the superfamily Olenelloide, the eye-ridges emerge from the back of the frontal lobe of the central area of the ceph
Burgess Shale type fauna
A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale type fauna" covers assemblages based on taxonomic criteria only; the fauna of the middle Cambrian has a cosmopolitan range. All assemblages preserving soft-part anatomy have a similar fauna though they span every continent; the wide distribution has been attributed to the advent of pelagic larvae. The fauna is composed of a range of soft bodied organisms; the major soft-bodied groups are sponges, palaeoscolecid worms, lobopods and anomalocaridids. Assemblages are diverse, with the most famous localities each containing in the region of 150 described species; the fauna of the Burgess Shale lived in the photic zone, as bottom-dwelling photosynthesisers are present in the assemblage. Sirius Passet is a lagerstätte in Greenland, formed about 527 million years ago, its most common fossils are arthropods. There are very few species with hard parts: trilobites, sponges, no echinoderms or molluscs.
Halkieria has features associated with more than one living phylum, is discussed below. The strangest-looking animals from Sirius Passet are Kerygmachela, they are regarded as anomalocarids because they have long, segmented bodies with a pair of broad fin-like flaps on most segments and a pair of segmented appendages at the rear. The outer parts of the top surfaces of the flaps have grooved areas which are thought to have acted as gills. Under each flap there is a fleshy leg; this arrangement suggests. There are several Cambrian fossil sites in the Chengjiang county of China's Yunnan province; the most significant is the Maotianshan shale, a lagerstätte which preserves soft tissues well. The Chengjiang fauna date to between 525 million and 520 million years ago, about the middle of the early Cambrian epoch, a few million years after Sirius Passet and at least 10 million years earlier than the Burgess Shale; the Chengjiang sediments provide what are the oldest known chordates, the phylum to which all vertebrates belong.
The 8 chordate species include Myllokunmingia a primitive agnathid and Haikouichthys, which may be related to lampreys. Yunnanozoon may be the oldest known hemichordate. Anomalocaris was a soft-bodied swimming predator, gigantic for its time. Unlike Kerygmachela and Pambdelurion, Anomalocaris had no legs, the grooved patches which are thought to have acted as gills were at the bases of the flaps, or overlapping on to its back; the two eyes were on long horizontal stalks. Amplectobelua found at Chengjiang, was similar, smaller than Anomalocaris but larger than most other Chengjiang animals. Both are thought to have been powerful predators. Hallucigenia looks like a long-legged caterpillar with spines on its back, certainly crawled on the seabed. Nearly half of the Chengjiang fossil species are arthropods, few of which had the hard, mineral-reinforced exoskeletons found in most marine arthropods. Many other phyla are found there: Porifera and Priapulida, Chaetognatha, Ctenophora, Hyolitha, Nematomorpha and Protista.
The Burgess Shale was the first of the Cambrian lagerstätten to be discovered, the re-analysis of the Burgess Shale by Whittington and others in the 1970s was the basis of Gould's book Wonderful Life, responsible for non-scientists' awareness of the Cambrian explosion. The fossils date from the mid Cambrian, about 515 million years ago and 10 million years than the Chengjiang fauna; the shelled fossils in the Burgess Shale are similar in proportions to other shelly fossil deposits. When organisms that were not preserved are entered into the equation, the shelly fossils represent about 2% of the animals that were alive at the time. Arthropods are the most abundant and diverse group of organisms in the Burgess Shale, followed by sponges. Many Burgess Shale fossils are unusual and difficult to classify, for example: Marrella is the most common fossil, but Whittington's re-analysis showed that it belonged to none of the known marine arthropod groups. Yohoia was a tiny animal with: a head shield.
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. 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. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving