| Helicoplacoidea Temporal range: | |
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| The holotype specimen of Helicoplacus guthi [1] | |
| Scientific classification | |
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| Class: | †Helicoplacoidea |
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Helicoplacoidea is an extinct class within the Echinodermata. All known taxa were discovered in sediments dating back to the Cambrian. [1] [2]
Helicoplacoid skeletons consist of small platelets arranged like those of asteroids, lacking fused skeletons as in echinoids. Little is known about their ecology, besides that they were most likely sessile suspension feeders. Members of the group were able to expand and contract their bodies, and some have speculated that they may have largely fed at night. [3] Others say that they were adapted to the more stratified sediment of the Cambrian and that changes in the composition of this sediment resulted in their extinction. Most likely the extending and contracting played a role in gas exchange and nutrition intake as it generated a water current. [3] [4] They seem to be one of the oldest groups of echinoderms, and also one of the first to go extinct, as they first appeared some 530 million years ago and would go extinct around 516 million years ago. [1]
Most of the fossils are from White Mountains, Nevada. The rarity of fossils and general poor preservation of them has made it hard to determine any kind of phylogeny for them, though three genera ( Helicoplacus , Polyplacus and Waucobella ) are currently recognized. [5] Also due to this, it is difficult to establish a conclusive taxonomic classification of the Helicoplacoidea.
A list of helicoplacoid genera, as recognized by the Paleobiology Database, is as follows: [5]
Helicoplacus guthi, one of two species of Helicoplacus , was named after the fossil's discoverer, Peter Guth. However, Wilbur (2006) suggests that this specimen is actually a poorly preserved specimen of Helicoplacus gilberti. This fossil measured 3.3 cm, and comes from the Cambrian formation of Poleta, California. [6]
Archaeocyatha is a taxon of extinct, sessile, reef-building marine sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is believed that the centre of the Archaeocyatha origin is now located in East Siberia, where they are first known from the beginning of the Tommotian Age of the Cambrian, 525 million years ago (mya). In other regions of the world, they appeared much later, during the Atdabanian, and quickly diversified into over a hundred families. They became the planet's very first reef-building animals and are an index fossil for the Lower Cambrian worldwide.
Helicoplacus is the earliest well-studied fossil echinoderm. Fossil plates are known from several regions. Complete specimens were found in Lower Cambrian strata of the White Mountains of California.
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.
Hyposaurus is a genus of extinct marine dyrosaurid crocodyliform. Fossils have been found in Paleocene aged rocks of the Iullemmeden Basin in West Africa, Campanian–Maastrichtian Shendi Formation of Sudan and Maastrichtian through Danian strata in New Jersey, Alabama and South Carolina. Isolated teeth comparable to Hyposaurus have also been found in Thanetian strata of Virginia. It was related to Dyrosaurus. The priority of the species H. rogersii has been debated, however there is no sound basis for the recognition of more than one species from North America. The other North American species are therefore considered nomina vanum.
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.
Loxonepea is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Cambrian Period, which lasted from approximately 539 to 485 million years ago.
The Eocrinoidea are an extinct class of echinoderms that lived between the Early Cambrian and Late Silurian periods. They are the earliest known group of stalked, arm-bearing echinoderms, and were the most common echinoderms during the Cambrian.
Redondasaurus is an extinct genus of phytosaur from the Late Triassic of the southwestern United States. It was named by Hunt & Lucas in 1993, and contains two species, R. gregorii and R. bermani. It is the youngest and most evolutionarily-advanced of the phytosaurs.
The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 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 diversification in other groups of organisms as well.
Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.
Camptostroma roddyi is an extinct echinoderm from the Bonnia-Olenellus Zone the Early Cambrian Kinzers Formation near York and Lancaster, Southeastern Pennsylvania. In life, it would have resembled a cupcake, with the axial skeleton forming a star pattern on the upper surface. It was originally thought, on the basis of its medusoid shape, to be a jellyfish-like organism, but the fossils themselves clearly rule out the possibility of a gelatinous body - the stereom plates are clearly preserved and possess the calcitic cleavage pattern diagnostic of echinoderms. It has been placed in a class of basal echinoderms, the Edrioasteroids.
Eldoniids are an extinct clade of enigmatic disc-shaped animals which lived in the early to middle Paleozoic. They are characterized by their "medusoid" (jellyfish-shaped) bodies, with the form of a shallow dome opening below to an offset mouth supplemented by filamentous tentacles. Internally, they have a distinctive C-shaped cavity encompassing the gut, as well as hollow radial (radiating) structures arranged around a central ring canal. Most eldoniids are soft-bodied and can only be preserved in lagerstätten, but a few species may have hosted mineralized deposits. Historically, the affinities of eldoniids was enigmatic; recently, they been assessed as cambroernid deuterostomes. Their lifestyle is still an unresolved question; some authors reconstruct eldoniids as free-floating planktonic predators similar to jellyfish, while others argue that they were passive detritivores, embedded within the seabed for much of their life.
Pojetaia is an extinct genus of early bivalves, one of two genera in the extinct family Fordillidae. The genus is known solely from Early to Middle Cambrian fossils found in North America, Greenland, Europe, North Africa, Asia, and Australia. The genus currently contains two accepted species, Pojetaia runnegari, the type species, and Pojetaia sarhroensis, though up to seven species have been proposed. The genera Buluniella, Jellia, and Oryzoconcha are all considered synonyms of Pojetaia.
Camya is an extinct genus of early bivalve and is the only genus in the extinct family Camyidae. The genus is known solely from early Middle Cambrian fossils found in Europe. The genus currently contains a solitary accepted species, Camya asy.
Cincta is an extinct class of echinoderms that lived only in the Middle Cambrian epoch. Homostelea is a junior synonym. The classification of cinctans is controversial, but they are probably part of the echinoderm stem group.
Micromomyidae (Micromomids) is a family of extinct plesiadapiform mammals that include some of the earliest known primates. The family includes five genera that lived from the Paleocene epoch into the early Eocene epoch.
Ctenocystoidea is an extinct clade of echinoderms, which lived during the Cambrian and Ordovician periods. Unlike other echinoderms, ctenocystoids had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms, with their bilateral symmetry a trait shared with other deuterostomes. Ctenocystoids were once classified in the taxon Homalozoa, also known as Carpoidea, alongside cinctans, solutes, and stylophorans. Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during the Middle Cambrian, with one species surviving into the Late Ordovician.
Aquilolamna is an extinct genus of shark-like elasmobranch from the Late Cretaceous (Turonian)-aged Agua Nueva Formation of Mexico. It is currently known to contain only one species, A. milarcae, also known as the eagle shark, and it is classified in its own family Aquilolamnidae, which has been tentatively assigned to the mackerel sharks.
Yorkicystis is a genus of edrioasteroid echinoderm that lived 510 million years ago in the Cambrian aged Kinzers Formation in what is now Pennsylvania. This genus is important as it provides some of the oldest evidence of echinoderms losing their hard mineralized outer skeletons. Yorkicystis also shows that some echinoderms lost their skeletons during the Cambrian, which is a greatly different time as to when most other species lost theirs.
Eucryphia falcata is an extinct species of flowering plant. It belongs to the genus Eucryphia within the family Cunoniaceae.
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