30-527: Mosasauria is a clade of aquatic and semiaquatic squamates that lived during the Cretaceous period. Fossils belonging to the group have been found in all continents around the world. Early mosasaurians like dolichosaurs were small long-bodied lizards that inhabited nearshore coastal and freshwater environments; the Late Cretaceous saw the rise of large marine forms, the mosasaurids , which are
60-525: A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
90-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
120-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
150-404: A molecular perspective should be abandoned entirely. Nevertheless, other scientists have attempted to resolve these problems. One approach was utilized by a 2015 study by Reeder et al. : it closely integrated morphological, molecular, and paleontological data in a large dataset to overcome previous conflicts, which revealed new morphological support for molecular results that recovered Mosasauria as
180-956: A monitor lizard relationship and one that supported a closer relationship with snakes. The proposition of a snake relationship was spearheaded by Cope, who first published such a hypothesis in 1869 by proposing that mosasaurs, which he classified under a group called the Pythonomorpha , was the sister group of snakes. Some scientists went as far as to interpret mosasaurs as direct ancestors of snakes. Many opponents of snake affinities argued that mosasaurs belong among monitor lizards in Anguimorpha . Within that group placement varied, from placing mosasaurs within Varanoidea or its sister taxa, or as true monitor lizards within Varanidae . These debates spawned higher taxonomic groups that were erected in attempts to classify
210-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
240-1258: A sister clade to Serpentes. Another approach was developed by R. Alexander Pyron in a 2016 study, which also recovered Mosasauria as a sister clade to the Serpentes. A 2022 study suggested that many of the characters uniting snakes and mosasaurians were based on ambiguous or misinterpreted characters, and suggested that a close relationship to Varanoidea within Anguimorpha was the best supported hypothesis. Other Squamata Iguania Shinisaurus Heloderma Xenosaurus Varanoidea Coniasaurus Pontosaurus Mosasauroidea Serpentes Other Squamata Iguania Shinisaurus Varanoidea Heloderma Xenosaurus Adriosaurus Pontosaurus Mosasauroidea Serpentes Other Squamata Shinisaurus Xenosaurus Heloderma Varanoidea Mosasauroidea Coniasaurus Adriosaurus Pontosaurus Serpentes Other Squamata Iguania Pontosaurus Adriosaurus Mosasauroidea Other Scleroglossa Shinisaurus Xenosaurus Varanoidea Serpentes Clade In biological phylogenetics ,
270-515: A sister clade to the monitor lizards. The frequent discrepancies are due to the high prevalence of convergent evolution in squamates, which creates much room for interpreting molecular and morphological data; many of these studies had results that often contradicted each other, for example in completely different phylogenetic results by simply adding more datasets or varying which taxa are represented, which resulted in various conflicts creating even more uncertainty. Due to this, some scientists argued that
300-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
330-518: A superfamily called the Mosasauroidea , which would be a sister superfamily to the Varanoidea. Camp's take on the subject virtually ended the snake-monitor lizard debate for approximately 70 years, with nearly all subsequent studies supporting a relationship with monitor lizards. However, many studies continued to support going further than Camp in the monitor lizard relationship, placing mosasaurs within
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#1732863072875360-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
390-471: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
420-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
450-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
480-553: The genera in Mosasauria are not part of the clade Dinosauria . The specific placement of Mosasauria within the Squamata has been controversial since its inception, with early debate focusing on the classification of the mosasaurs. Cuvier was the first scientist to deeply analyze their possible taxonomic placement through Mosasaurus . While his original 1808 hypothesis that the genus was a lizard with affinities to monitor lizards remained
510-451: The Mosasauria in a polytomy , or a clade with unresolved relationships, with monitor lizards and beaded lizards ; and a 2012 study by Gauthier et al. recovered Mosasauria as a clade basal to both monitor lizards and snakes. With the advent of molecular genetics during the 2010s, some scientists argued for the combining of molecular and morphological data to examine relationships between mosasaurs and living squamates. An early study
540-456: The Varanoidea. The debate was reignited with the publication of a 1997 cladistical study by Michael S. Y. Lee, which recovered the Mosasauroidea as a sister taxon to the snake suborder Serpentes and resurrected the argument for a snake relationship. Prior, there had never been a modern phylogenetic study specifically testing the relationships between mosasaurians or snakes. Lee also resurrected
570-435: The clade's best-known members. The clade is defined as all descendants of the last common ancestor of the mosasaur Mosasaurus hoffmannii and dolichosaurs Dolichosaurus , Coniasaurus , and Adriosaurus suessi . Its placement within the squamate tree is highly controversial. Two prominent hypotheses include the varanoid hypothesis, which holds that mosasaurians are most closely related to monitor lizards , and
600-406: The defunct Pythonomorpha and redefined it to unify the Mosasauroidea and Serpentes under one clade. Multiple subsequent studies conducted by scientists such as Lee, Caldwell, and Alessandro Palci refined this hypothesis, where in some, the Mosasauria clade was revived and repurposed. However, there still remained little consensus. For example, a large-scale phylogenetic study by Conrad (2008) recovered
630-451: The fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. 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
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#1732863072875660-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
690-440: The holotype skull. Thus, in-depth research on the placement of Mosasaurus was not undertaken until the discovery of more complete mosasaur fossils during the late 19th century, which reignited research on the placement of mosasaurs among squamates. In a span of about 30 to 40 years during the late 19th to early 20th centuries, paleontologists fiercely debated the issue, which created two major schools of thought: one that supported
720-590: 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 that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
750-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
780-521: The most popular, Cuvier was uncertain, even at the time, about the accuracy of this placement. He simultaneously proposed a number of alternative hypotheses, with one such alternative suggesting that Mosasaurus instead had closer affinities with iguanas due to their shared presence of pterygoid teeth. With the absence of sufficient fossil evidence, researchers during the early and mid-19th century had little to work with. Instead, they primarily relied on stratigraphic associations and Cuvier's 1808 research on
810-547: The placement of mosasaurs (although not all are compatible). One of these was the Mosasauria, initially a loosely-defined group erected by Marsh in 1880 but redefined to its current definition by Conrad (2008). In 1923, Charles Lewis Camp published Classification of the Lizards , in which he proposed through the review and rebuttal of previous arguments using his own anatomical observations that all taxa more closely related to Mosasaurus than Dolichosaurus should be classified into
840-418: The pythonomorph hypothesis, which argues for a sister relationship with snakes . A third ophidiomorph hypothesis argues that snakes are members of the Mosasauria as modern descendants of the dolichosaurs, while a fourth stem- scleroglossan hypothesis considers neither group to be related to the mosasaurians. Like other ancient marine reptiles, such as those in the orders Ichthyosauria and Plesiosauria ,
870-784: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Mosasauroidea Mosasauroidea is a superfamily of extinct marine lizards that existed during the Late Cretaceous . Basal members of this group consist of small semiaquatic forms with terrestrial limbs ("plesiopedal"), while laters members include larger fully aquatic paddle-limbed ("hydropedal") forms commonly known as mosasaurs. These were traditionally grouped within their own separate families ,
900-410: Was Lee (2009), which based on nuclear and mitochondrial DNA in living squamates and morphological data recovered mosasaurs as a stem group to snakes, which some later authors interpreted as placement of snakes within the Mosasauria clade itself. However, a 2010 study by Wiens et al. attempted to replicate Lee (2009) using a larger dataset but instead yielded results that recovered the Mosasauria as
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