Arthropod - Evolution and paleontology | dubaiairporthotel.info
Phylogenetic relationships of Annelids, Molluscs, and Arthropods evidenced from molecules and morphology. Article in Journal of Molecular Evolution. Molecular Biology and Evolution 12(4): Abstract: Phylogenetic relationships of annelids, molluscs, and arthropods evidenced from. Arthropod - Evolution and paleontology: The arthropods share many features with the phylum Annelida. Both arthropods and annelids are segmented, and.
Today, the prevailing opinion among modern taxonomists is that Onychophora and Arthropoda represent sister groups see Cladogram 1but the issue is far from settled. Velvet worms may or may not prove to be direct ancestors of the arthropods, but regardless of classification, it seems reasonable to assume that the very first arthropods were distinctly worm-like in structure and appearance.
Arthropod Evolution A classical controversy that still rages among invertebrate zoologists, including the entomologists, relates to the evolutionary pathway or pathways of adaptive radiation that arthropods followed as they gradually diverged from primitive ancestors.
The Monophyletic Hypothesis All arthropods differ from their annelid ancestors in several important ways: More about Trilobites The monophyletic classification scheme, proposed by Boudreaux Cladogram 2recognizes three evolutionary lineages within the phylum Arthropoda: Boudreaux's classification scheme excludes onychophorans because they lack a true exoskeleton, but other workers disagree.
A great deal of controversy surrounds the evolution of arthropod legs and wings. The Polyphyletic Hypothesis If we embrace the idea of polyphyletic origins, then arthropods are represented by as many as four major phyla -- each of which is presumed to have evolved separately from primitive annelid ancestors: Chelicerata -- 70, species -- including spiders, scorpions, mites, ticks, horseshoe crabs, and sea spiders.
Crustacea -- 30, species -- including shrimp, crabs, lobsters, woodlice, barnacles, amphipods, branchiopods, and copepods. We furthermore hope that coming investigations will consider the possibility the Clitellata is in fact an ingroup of the Polychaeta, which would make Annelida and Polychaeta synonymous.
Phylogeny of the Annelida and allies
Rearticulating with extra assumptions; a response to Eibye-Jacobsen and Nielsen. Zoologica Scripta 26 1 What is required is further evidence and study now that the possibility of the paraphyly of the Annelida has been raised. Journal of Natural History. The phylogenetic systematics of the polychaetes, i. The main competition is between two diametrically opposed hypotheses: A third hypothesis, which is based primarily on certain morphological peculiarities and the presence of exclusively monociliary cells in Owenia, and which postulates a sessile stem species, is mentioned only peripherally: From a decision in favour of Hypothesis 2 it would follow that the Clitellata should be considered the most primitive annelids, so that the possession of parapodia and many extremely differentiated chaetae, for instance, would be interpreted as a highly derived character state.
The consequence for the phylogenetic systematics of the Polychaeta is that oligochaete-like taxa would have to be considered more primitive than, for example, nereidid- like taxa. On the basis of Hypothesis 1, the evolution of these structures would have proceeded in the opposite direction, and polychaete systematics would have the reverse arrangement. The most important evidence for Hypothesis 2 comes from functional morphological considerations; namely the inference that metamerism has arisen from a burrowing mode of life.
It is shown here that 1 this hypothesis rests partly on ignorance of the close relationship between reproductive biology and morphology in the clitellates, 2 the notion that metamerism, and hence the stem species of the Articulata, originated from a burrowing life in the marine environment is unconvincing, and 3 the origin of metamerism can be explained quite differently with reference to modern ultrastructural findings. According to these findings, septa, which are the fundamental structural elements for annelid segmentation, evolved as a morphological prerequisite for the development of transversely running blood vessels; other purposes of septa e.
A highly complex blood vascular system may have been the consequence of the development of lateral parapodia-like appendages. Thus, parapodia are assumed to be part of the ground pattern of the Articulata and hence were present in the stem species of the Annelida.
This is consistent with the traditional interpretation of annelid systematics, which places the errant polychaete taxa at the base of the system Hypothesis 1. McHugh, Dahmnait Molecular evidence that echiurans and pogonophorans are derived annelids.
The Annelida, which includes the polychaetes and the clitellates, has long held the taxonomic rank of phylum. The unsegmented, mud-dwelling echiuran spoon worms and the gutless, deep-sea pogonophoran tube worms including vestimentiferans share several embryological and morphological features with annelids, but each group has also been considered as a separate metazoan phylum based on the unique characters they display. Phylogenetic analyses of DNA sequences from the nuclear gene elongation factor-1alpha place echiurans and pogonophorans within the Annelida.
This result, indicating the derived loss of segmentation in echiurans, has profound implications for our understanding of the evolution of metazoan body plans, and challenges the traditional view of the phylum-level diversity and evolutionary relationships of protostome worms.
Zoologica Scripta 26 2: In this paper, we first demonstrate the historical background for the current unsatisfactory state of systematics of the polychaetes. We then briefly discuss our knowledge of internal and external structures. A review of the polychaete families makes up the third section; 81 families are treated in detail.
Five families have been recently synonymized with others, and six families are too poorly known to be sufficiently characterized. Fossil polychaetes are briefly mentioned, with specific attention to problems associated with incorporating them in recent systematics. The traditional separation in 'errant' and 'sedentary' polychaetes has increasingly become recognized as being unsatisfactory; however, the current trend towards grouping the polychaetes in many orders without specifying the relationships among the orders, is no more satisfactory.
The lack of consistent morphological information is a major source of uncertainty.
Intensive morphological studies should remove terminological ambiguities and alleviate some of the problems. A series of cladistic analyses assess the status and membership of the taxon Polychaeta. As well as the polychaete families, non- polychaete taxa such as the Echiura, Euarthropoda, Onychophora, Pogonophora as Frenulata and VestimentiferaClitellata, Aeolosomatidae and Potamodrilidae are included in the analyses.
All trees are rooted using the Sipuncula as outgroup.
Characters are based on features where present such as the prostomium, peristomium, antennae, palps, nuchal organs, parapodia, stomodaeum, segmental organ structure and distribution, circulation and chaetae. A number of analyses are performed involving different ways of coding and weighting the characters, as well as the number of taxa included.
The First Arthropods
Transformation series are provided for several of these analyses. One of the analyses is chosen to provide a new classification. The Annelida is found to be monophyletic, though weakly supported, and comprises the Clitellata and Polychaeta.
The Polychaeta is monophyletic only if taxa such as the Pogonophora, Aeolosomatidae and Potamodrilidae are included and is also weakly supported. The Pogonophora is reduced to the rank of family within the Polychaeta and reverts to the name Siboglinidae Caullery, The new classification does not use Linnaean categories and the Polychaeta comprises two clades, the Scolecida and Palpata.
The Palpata has the clades Aciculata and Canalipalpata. The Aciculata contains the Phyllodocida and Eunicida. The Canalipalpata has three clades; the Sabellida including the Siboglinidae Spionida and Terebellida.
The position of a number of families requires further investigation. In the chapter, The Annelida. Rouse presents the same higher classification, incorporates the pogonophorans as Family Siboglinidae within the polychaetes, and has Echiura retained outside annelids meantime.
Kojima,Shigeaki Paraphyletic status of Polychaeta suggested by phylogenetic analysis based on the amino acid sequences of elongation factor-1 alpha. Molecular Phylogenetics and Evolution 9 2: In order to judge whether or not Polychaeta is a paraphyletic group, I determined almost the entire amino acid sequence of elongation factor-l alpha from thirteen polychaetes, two oligochaetes, two hirudineans, two vestimentiferans, and two molluscs.
Phylogenetic analysis by the neighbor-joining NJ method and the maximum likelihood ML method indicated the monophyly of Clitellata the oligochaetes and hirudineans.
In both the NJ and ML trees, vestimentiferans and clitellates were derived from polychaetes independently. The present results strongly suggest that Polychaeta is a paraphyletic group. Fauchald Recent views on the status, delineation and classification of the Annelida.
The Clitellata have also been proposed to be a member of the Polychaeta, potentially making this latter taxon synonymous with the Annelida.
The relationships within the traditionally formulated Polychaeta have never been investigated using cladistic methodology. Recent classifications of polychaetes show a large number of "orders" with no real attempts to relate the groups in a phylogenetic sense. In this paper a number of recent studies on annelid systematics and classification are reviewed.
Special emphasis is placed on the cladistic parsimony analyses of Rouse and Fauchaldwhere a comprehensive assessment of the relationships among the various polychaete and annelid groups was attempted.
A contrasting result by Westheide using a different methodology, is also outlined and discussed. Rouse Life history evolution of marine invertebrates: New views from phylogenetic systematics. Established theories on the evolution of the diverse life histories of marine metazoans, specifically invertebrates, were developed in the absence of rigorous phylogenetic methods. With improved estimates of evolutionary relationships for various marine invertebrate groups, based on phylogenetic systematics, we can now critically evaluate the assumptions upon which these theories are based.