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The Cambrian Period was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux. The period was established as "Cambrian series" by Adam Sedgwick, who named it after Cambria, the Latin name for 'Cymru' (Wales), where Britain's Cambrian rocks are best exposed. Sedgwick identified the layer as part of his task, along with Roderick Murchison, to subdivide the large "Transition Series", although the two geologists disagreed for a while on the appropriate categorization. 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 later periods.
The cloudinids, an early metazoan family containing the genera Acuticocloudina, Cloudina and Conotubus, lived in the late Ediacaran period about 550 million years ago. and became extinct at the base of the Cambrian. They formed millimetre-scale conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself remains unknown. The name Cloudina honors the 20th-century geologist and paleontologist Preston Cloud.
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters, as well as the shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus. Some animals, such as the tortoise and turtle, have both an endoskeleton and an exoskeleton.
A trace fossil, also ichnofossil, is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization. Ichnology is the study of such trace fossils and is the work of ichnologists.
The Doushantuo Formation is a fossil Lagerstätte in Weng'an County, Guizhou Province, China that is notable for being one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation is of particular interest because a part of it appears to cover the boundary between the enigmatic organisms of the Ediacaran geological period and the more familiar fauna of the Cambrian explosion where lifeforms recognizable as ancestors of later and recent lifeforms first emerged. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion.
Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
Namacalathus is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, N. hermanastes, was first described in 2000 from the Nama Group of central and southern Namibia.
Namapoikia rietoogensis is among the earliest known animals to produce a calcareous skeleton. Known from the Ediacaran period, before the Cambrian explosion of calcifying animals, the long-lived organism grew up to a metre in diameter and resembles a colonial sponge. It was an encruster, filling vertical fissures in the reefs in which it originally grew.
The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.
The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile, bag-like organisms with a soft skin armored with star-shaped calcareous sclerites from which radiate sharp spines.
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.
Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, 542 million years ago, includes:
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the Late Ediacaran and Early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversifications in other groups of organisms as well.
Ediacaran type preservation relates to the dominant preservational mode in the Ediacaran period, where Ediacaran organisms were preserved as casts on the surface of microbial mats.
Corumbella is an extinct genus of terminal-Ediacaran cnidarians. It is the only genus in the monotypic family Corumbellidae, and is represented by a single species Corumbella werneri. It possessed a carapace made up of thick polygonal rings in which plates with pores and papillae attest to the advent of skeletogenesis in latest Neoproterozoic metazoan. It was a sessile predator and somewhat resembles the later conulariids.
The Francevillian biota is a group of 2.1-billion-year-old Palaeoproterozoic, macroscopic organisms known from fossils found in Gabon in the Palaeoproterozoic Francevillian B Formation, a black shale province. The fossils are postulated to be evidence of the earliest form of multicellular life. The fossils were discovered by an international team led by the Moroccan-French geologist Abderrazak El Albani, of the University of Poitiers, France. While the fossils have yet to be assigned to a formal taxonomic position, they have been informally and collectively referred to as the "Gabonionta" by the Natural History Museum Vienna in 2014.
The Dengying Formation is an upper Ediacaran fossiliferous geologic formation found in South China. It was deposited on a shallow marine carbonate platform.
References
- ↑ Chen, Z.; Bengtson, S.; Zhou, C.-M.; Hua, H. & Yue, Z. (2008). "Tube structure and original composition of Sinotubulites: shelly fossils from the late Neoproterozoic in southern Shaanxi, China". Lethaia. 41 (1): 37–45. doi:10.1111/j.1502-3931.2007.00040.x.
- ↑ Bengtson, S. & Zhao, Y. (17 July 1992). "Predatorial Borings in Late Precambrian Mineralized Exoskeletons". Science. 257 (5068): 367–9. doi:10.1126/science.257.5068.367. PMID 17832833.
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