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Kulindroplax
''Kulindroplax perissokomos'' is a Silurian mollusk, known from a single fossil from the Coalbrookdale Formation fauna of England. It lived during the Homerian Age (about 425 million years ago). It is considered a basal aplacophoran. Unlike all modern aplacophorans, which are shell-less, ''Kulindroplax'' has a chiton-like shell, and it is considered a transitional fossil in the evolution of molluscs. The only known specimen, described in 2012, was conserved at the Oxford University Museum of Natural History. Morphology ''Kulindroplax'' is about wide and long. It is the first known mollusk showing an unambiguous combination of valves, or exterior shells, and a worm-like body. It bears seven similar, unarticulated valves, with a shorter head valve and a taller caudal one, lacking ornaments and also several densely packed, long spicules. It has no discernible foot, and the radula is not preserved. A gill array is present, along with a respiratory cavity opening posteriorly ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phthipodochiton
''Phthipodochiton'' is an extinct genus of molluscs, known from several fossils from the upper Ordovician fauna of the Lady Burn Starfish beds of Girvan, Scotland. It shows a mixture of aplacophoran body plan and polyplacophoran-like valves, and it is an informative fossil in the evolution of aculiferan mollusks. It was previously classified under the genus ''Helminthochiton'', but it has been reassigned to its own genus in 2012. Morphology ''Phthipodochiton'' body is worm-like, with eight polyplacophoran-like valves but no true foot. Head and tail valves are slightly smaller than the intermediate ones. The only ornaments on the valves appear to be growth lines. The body is also covered by a sheet of spicules ; no radula has been preserved. Life habits ''Phthipodochiton'' was carnivorous, feeding on crinoids, as shown by a fossil preserved with gut contents. In contrast with modern chitons, ''Phthipodochiton'' probably did not creep on its foot but had a locomotion style sim ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Coalbrookdale Formation
Coalbrookdale Formation, earlier known as Wenlock Shale or Wenlock Shale Formation and also referred to as Herefordshire Lagerstätte in palaeontology, is a fossil-rich deposit ('' Konservat-Lagerstätte'') in Powys and Herefordshire at the England–Wales border in UK. It belongs to the Wenlock Series of the Silurian Period within the Homerian Age (about 430 million years ago). It is known for its well-preserved fossils of various invertebrate animals many of which are in their three-dimensional structures. Some of the fossils are regarded as earliest evidences and evolutionary origin of some of the major groups of modern animals. Roderick Murchison first described the geological setting of Coalbrookdale Formation by which he gave the name Silurian in 1935, referring to the Silures, a Celtic tribe of Wales. It is assigned to the Wenlock Group in 1978 based on the age of crustacean fossils found around the region. Robert J. King of the University of Leicester discovered the fi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aplacophora
Aplacophora is a presumably paraphyletic taxon. This is a class of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world. MolluscaBase eds. (2021). MolluscaBase. Aplacophora. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=411 on 2021-04-13 All known modern forms are shell-less: only some extinct primitive forms possessed valves. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear. Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than ; some species, however, can reach a length of . Habitat Caudofoveates generall ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aplacophora
Aplacophora is a presumably paraphyletic taxon. This is a class of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world. MolluscaBase eds. (2021). MolluscaBase. Aplacophora. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=411 on 2021-04-13 All known modern forms are shell-less: only some extinct primitive forms possessed valves. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear. Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than ; some species, however, can reach a length of . Habitat Caudofoveates generall ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Acaenoplax
''Acaenoplax'' is an extinct worm-shaped mollusc known from the Coalbrookdale Formation of Herefordshire, England. It lived in the Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ... period. It was a couple of centimetres long and half a centimetre wide, and comprises serially repeated units with seven or eight shells, and rings of 'spines'. Some of its characters are reminiscent of the polychaete worms, and the character combinations do not place it obviously in the stem of any modern mollusc group, but although it was originally interpreted as a polychaete, this position is untenable for a number of reasons. Morphology The organism resembles a bristled worm, but bears a number of shells on its upper surface. The first shell is cap-like, whereas the others are saddl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a large surface area to the external environment. Branchia (pl. branchiae) is the zoologists' name for gills (from Ancient Greek ). With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians. Semiterrestrial marine animals such as crabs and mudskippers have gill cham ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Conchifera
Conchifera is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, such as clams, tusk shells, ammonites, and monoplacophorans. The other subphylum is Aculifera. Non-monoplacophoran conchiferans emerged within the once-widespread Monoplacophora. The only descendant which retains its ancestral shape is the Tryblidiida. This taxonomic term is used mostly by paleontologists, and not by scientists who study the living molluscs. Classes Classes within the Conchifera include: * Monoplacophora * Bivalvia * Gastropoda * Scaphopoda * Cephalopoda * Archinacelloidea† * Rostroconchia† * Helcionelloida Helcionelloida is an extinct group of ancient molluscs (phylum Mollusca). These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was ...† See also * Evolution of Mollusca References External links BioLib info {{Authority c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Least Squares Inference In Phylogeny
Least squares inference in phylogeny generates a phylogenetic tree based on an observed matrix of pairwise genetic distances and optionally a weight matrix. The goal is to find a tree which satisfies the distance constraints as best as possible. Ordinary and weighted least squares The discrepancy between the observed pairwise distances D_ and the distances T_ over a phylogenetic tree (i.e. the sum of the branch lengths in the path from leaf i to leaf j) is measured by : S = \sum_ w_ (D_-T_)^2 where the weights w_ depend on the least squares method used. Least squares distance tree construction aims to find the tree (topology and branch lengths) with minimal S. This is a non-trivial problem. It involves searching the discrete space of unrooted binary tree topologies whose size is exponential in the number of leaves. For n leaves there are 1 • 3 • 5 • ... • (2n-3) different topologies. Enumerating them is not feasible already for a small number of leaves. Heuristic search me ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aculifera
Aculifera is a proposed clade of molluscs incorporating those groups that have no conch or shell, that is, the Polyplacophora, Caudofoveata (= Chaetodermomorpha) and Solenogastres (= Neomeniomorpha). For a summary, see It is sister to the Conchifera. The oldest known aculiferan is '' Qaleruaqia'', which was found in the Aftenstjernesø Formation in Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ..., which dates back to the Cambrian Stage 4. References Mollusc taxonomy {{mollusc-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, the equivalent Latin term ''cladus'' (plural ''cladi'') is often used in taxonomical literature. The common ancestor may be an individual, a population, or a species (extinct or extant). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over the last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic. Some of the relationships between organisms ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cephalopoda
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology. Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by ''Nautilus'' and ''Allonautilus''. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Tw ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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