The Tetraconata concept: hexapod-crustacean relationships and the phylogeny of Crustacea

A growing body of evidence indicates that Crustacea and Hexapoda are sister groups, rather than Hexapoda and Myriapoda. Some recent molecular data even suggest that Mandibulata is not monophyletic, with Myriapoda and Chelicerata instead being sister groups. Here, arguments for homology of the mandible throughout mandibulate arthropods and for a monophyletic Mandibulata will be presented, as well as arguments supporting the taxon Tetraconata (i.e. Crustacea + Hexapoda). The latter include molecular data (nuclear and mitochondrial ribosomal RNAs and protein coding genes), and morphological characters such as ommatidial structure, the presence of neuroblasts and a very similar axonogenesis of pioneer neurons. However, crustaceans are insufficiently sampled for the molecular data, and studies of neurogenesis are lacking for many crustacean taxa. Remipedia, Cephalocarida and Maxillopoda are particularly problematic. This is important for the entire problem, because monophyly of the Crustacea has not yet been proven beyond doubt and several molecular analyses suggest a paraphyletic Crustacea. Here, arguments for the monophyly of the Crustacea are reviewed and two alternatives for the relationships between the five higher taxa Remipedia, Cephalocarida, Maxillopoda, Branchiopoda and Malacostraca are discussed: the Entomostraca concept sensu Walossek with Malacostraca as sister group to Cephalocarida, Maxillopoda and Branchiopoda, and the Thoracopoda concept sensu Hessler with Cephalocarida, Branchiopoda and Malacostraca forming a monophylum.     

Phylogenetic position of the Pentastomida and (pan)crustacean relationships

Pentastomids are a small group of vermiform animals with unique morphology and parasitic lifestyle. They are generally recognized as being related to the Arthropoda; however, the nature of this relationship is controversial. We have determined the complete sequence of the mitochondrial DNA (mtDNA) of the pentastomid Armillifer armillatus and complete or nearly complete mtDNA sequences from representatives of four previously unsampled groups of Crustacea: Remipedia (Speleonectes tulumensis), Cephalocarida (Hutchinsoniella macracantha), Cirripedia (Pollicipes polymerus) and Branchiura (Argulus americanus). Analyses of the mtDNA gene arrangements and sequences determined in this study indicate unambiguously that pentastomids are a group of modified crustaceans probably related to branchiurans. In addition, gene arrangement comparisons strongly support an unforeseen assemblage of pentastomids with maxillopod and cephalocarid crustaceans, to the exclusion of remipedes, branchiopods, malacostracans and hexapods.     

Ovary structure and early oogenesis in the remipede, Godzilliognomus frondosus (Crustacea, Remipedia): phylogenetic implications

Remipedia are enigmatic crustaceans of uncertain phylogenetic position with the general consensus that they are crucial for understanding the crustacean/arthropod evolution. It has been demonstrated previously that the features of the ovary organization and subcellular aspects of oogenesis are useful in resolving phylogenetic relationships in arthropods such as hexapods and onychophorans. The structure of the female gonads in Remipedia remains largely unknown; therefore, we examined the gross morphology and ultrastructural details of the ovary in a remipede, Godzilliognomus frondosus, with special emphasis on characters relevant to phylogenetic reconstructions. The ovaries of G. frondosus are located in the anterior part of the body and are composed of a single anterior proliferative zone (the germarium) and paired ovarian tubes (the vitellarium). The oocytes undergo subsequent stages of development within the lumen of the ovarian tubes, hence the remipede ovaries can be classified as endogenous. During oogenesis, each oocyte is enveloped by a set of characteristic somatic follicular cells, which results in the formation of distinct ovarian follicles. Here, we demonstrate that Remipedia share significant similarities in the ovary organization with Cephalocarida, including the anterior location of the ovary, the anterior-most position of the germarium and the endogenous type of oocyte development. Phylogenetic implications of our findings are discussed.     

Origin of the Ostracoda and their maxillopodan and hexapodan affinities

There are Cambrian fossils attributed to the Ostracoda but the extant subclasses Podocopa and Myodocopa do not appear until the Ordovician. At this time the morphologically similar, free-living ancestors of the now sedentary Thecostraca (Ascothoracida, Acrothoracica and Cirripedia) may have still been extant, and from an ecological point of view it seems likely that, by and large, ostracods replaced them. However, living ostracods have an abbreviated, direct development, and some key aspects of their morphology, such as the nature of the maxillary segment and abdomen, are conjectural. Thus the affinities between these and related taxa remain uncertain; e.g., while some contemporary carcinologists place Ostracoda as a taxon coordinate with the Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Malacostraca, others tentatively or unequivocally ally them with the Maxillopoda (generally Mystacocarida, Copepoda, Tantulocarida and Thecostraca, and sometimes Branchiura and Pentastomida). Others, largely involved with fossils, have stretched the definition of the Maxillopoda even further, to the point where it seems even less likely a monophyletic taxon. Until recently cladistic analyses utilizing genetic (largely 18S rDNA) as well traditional morphological characteristics have given confusing results regarding the affinities between these taxa, and an important one suggested the Ostracoda might even be diphyletic. Furthermore, a very recent genetic study utilizing protein encoding genes places a podocopine ostracod among the most primitive of the extant crustaceans (Branchiopoda, Cephalocarida Remipedia and Mystacocarida), and then generally at the base of a lineage leading to the Malacostraca, a lineage giving rise to copepods and cirripeds along the way. This indicates these so-called maxillopodan taxa evolved independently from a malacostracan-like ancestor, and if so they are convergent. And finally, from genetic studies it is not only becoming well documented the Crustacea rather than Myriapoda gave rise to the Hexapoda, but it appears the Hexapoda stem from among the lower rather than the higher crustaceans, possibly even from the Ostracoda. Whether there were terrestrial ostracods at the time hexapods appeared in the Lower Ordovician is unknown, but the modest diversity of terrestrial ostracods today are podocopines which also first appeared in the Lower Ordovician. Thus, if current interpretations of living ostracodan and fossil hexapodan body plans are largely correct, it can be hypothesized the Ostracoda are close to the ancestor of the Hexapoda.     

Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic

Recent molecular analyses indicate that crustaceans and hexapods form a clade (Pancrustacea or Tetraconata), but relationships among its constituent lineages, including monophyly of crustaceans, are controversial. Our phylogenetic analysis of three protein-coding nuclear genes from 62 arthropods and lobopods (Onychophora and Tardigrada) demonstrates that Hexapoda is most closely related to the crustaceans Branchiopoda (fairy shrimp, water fleas, etc.) and Cephalocarida + Remipedia, thereby making hexapods terrestrial crustaceans and the traditionally defined Crustacea paraphyletic. Additional findings are that Malacostraca (crabs, isopods, etc.) unites with Cirripedia (barnacles, etc.) and they, in turn, with Copepoda, making the traditional crustacean class Maxillopoda paraphyletic. Ostracoda (seed shrimp)--either all or a subgroup--is associated with Branchiura (fish lice) and likely to be basal to all other pancrustaceans. A Bayesian statistical (non-clock) estimate of divergence times suggests a Precambrian origin for Pancrustacea (600 Myr ago or more), which precedes the first unambiguous arthropod fossils by over 60 Myr.     

The ovary structure and oogenesis in the basal crustaceans and hexapods. Possible phylogenetic significance

Recent large-scale phylogenetic analyses of exclusively molecular or combined molecular and morphological characters support a close relationship between Crustacea and Hexapoda. The growing consensus on this phylogenetic link is reflected in uniting both taxa under the name Pancrustacea or Tetraconata. Several recent molecular phylogenies have also indicated that the monophyletic hexapods should be nested within paraphyletic crustaceans. However, it is still contentious exactly which crustacean taxon is the sister group to Hexapoda. Among the favored candidates are Branchiopoda, Malacostraca, Remipedia and Xenocarida (Remipedia + Cephalocarida). In this context, we review morphological and ultrastructural features of the ovary architecture and oogenesis in these crustacean groups in search of traits potentially suitable for phylogenetic considerations. We have identified a suite of morphological characters which may prove useful in further comparative studies.     

Morphology of the brain in Hutchinsoniella macracantha (Cephalocarida, Crustacea)

External morphological features of Cephalocarida have long been interpreted as plesiomorphic with regard to those of other crustaceans. Based on transmission electron microscopy and light microscopy, however, the brain in the cephalocarid Hutchinsoniella macracantha has been shown to contain a number of structures that are more difficult to interpret in an evolutionary context. These include the multi-lobed complex, a unique cluster of neuropils associated with the olfactory lobes. To establish a well-founded comparison of phylogenetically relevant, neuroanatomical data from Cephalocarida to other arthropods, we investigated the brain in H. macracantha using immunolabeling (acetylated α-tubulin, serotonin, RFamide, histamine) and nuclear counter stains of whole mounts and vibratome sections analyzing specimens with confocal laser scanning microscopy and computer-aided 3D-reconstruction. Other 3D-reconstructions were based on serial 1 μm semi-thin sections. The multi-lobed complex features a pedunculus and shows detailed homologies with the mushroom bodies of certain Insecta and Lithobiomorpha (Chilopoda), suggesting that the hemiellipsoid bodies in Remipedia and Malacostraca have derived from a cephalocarid-like pattern. Like the corresponding tracts in Insecta, the olfactory globular tracts linking the multi-lobed complex to the olfactory lobes are ipsilateral, probably constituting the plesiomorphic pattern from which the decussating tracts in Remipedia and Malacostraca have evolved. The olfactory lobes in H. macracantha are uniquely organized into vertical stacks of olfactory glomeruli whose exact shape could not be identified. Similarly to Malacostraca and Insecta, the olfactory glomeruli in H. macracantha are innervated by serotonin-like, RFamide-like, and histamine-like immunoreactive interneurons. This suggests homology of the olfactory lobes across Tetraconata, despite the different morphological organization. Although H. macracantha lacks elongated, unpaired midline neuropils known from the protocerebrum of other Arthropoda, the possible rudiment of a central-body-like neuropil that receives decussating fibers from anterior somata was revealed by the serotonin-like immunoreactive pattern.     

The ventral nerve cord in Cephalocarida (Crustacea): New insights into the ground pattern of Tetraconata

Cephalocarida are Crustacea with many anatomical features that have been interpreted as plesiomorphic with respect to crustaceans or Tetraconata. While the ventral nerve cord (VNC) has been investigated in many other arthropods to address phylogenetic and evolutionary questions, the few studies that exist on the cephalocarid VNC date back 20 years, and data pertaining to neuroactive substances in particular are too sparse for comparison. We reinvestigated the VNC of adult Hutchinsoniella macracantha in detail, combining immunolabeling (tubulin, serotonin, RFamide, histamine) and nuclear stains with confocal laser microscopy, complemented by 3D‐reconstructions based on serial semithin sections. The subesophageal ganglion in Cephalocarida comprises three segmental neuromeres (Md, Mx1, Mx2), while a separate ganglion occurs in all thoracic segments and abdominal segments 1–8. Abdominal segments 9 and 10 and the telson are free of ganglia. The maxillar neuromere and the thoracic ganglia correspond closely in their limb innervation pattern, their pattern of mostly four segmental commissures and in displaying up to six individually identified serotonin‐like immunoreactive neurons per body side, which exceeds the number found in most other tetraconates. Only two commissures and two serotonin‐like immunoreactive neurons per side are present in abdominal ganglia. The stomatogastric nervous system in H. macracantha corresponds to that in other crustaceans and includes, among other structures, a pair of lateral neurite bundles. These innervate the gut as well as various trunk muscles and are, uniquely, linked to the unpaired median neurite bundle. We propose that most features of the cephalocarid ventral nerve cord (VNC) are plesiomorphic with respect to the tetraconate ground pattern. Further, we suggest that this ground pattern includes more serotonin‐like neurons than hitherto assumed, and argue that a sister‐group relationship between Cephalocarida and Remipedia, as favored by recent molecular analyses, finds no neuroanatomical support. J. Morphol. 275:269–294, 2014. © 2013 Wiley Periodicals, Inc.     

Elongation factor-2: a useful gene for arthropod phylogenetics.

Robust resolution of controversial higher-level groupings within Arthropoda requires additional sources of characters. Toward this end, elongation factor-2 sequences (1899 nucleotides) were generated from 17 arthropod taxa (5 chelicerates, 6 crustaceans, 3 hexapods, 3 myriapods) plus an onychophoran and a tardigrade as outgroups. Likelihood and parsimony analyses of nucleotide and amino acid data sets consistently recovered Myriapoda and major chelicerate groups with high bootstrap support. Crustacea + Hexapoda (= Pancrustacea) was recovered with moderate support, whereas the conflicting group Myriapoda + Hexapoda (= Atelocerata) was never recovered and bootstrap values were always <5%. With additional nonarthropod sequences included, one indel supports monophyly of Tardigrada, Onychophora, and Arthropoda relative to molluscan, annelidan, and mammalian outgroups. New and previously published sequences from RNA polymerase II (1038 nucleotides) and elongation factor-1alpha (1092 nucleotides) were analyzed for the same taxa. A comparison of bootstrap values from the three genes analyzed separately revealed widely varying values for some clades, although there was never strong support for conflicting groups. In combined analyses, there was strong bootstrap support for the generally accepted clades Arachnida, Arthropoda, Euchelicerata, Hexapoda, and Pycnogonida, and for Chelicerata, Myriapoda, and Pancrustacea, whose monophyly is more controversial. Recovery of some additional groups was fairly robust to method of analysis but bootstrap values were not high; these included Pancrustacea + Chelicerata, Hexapoda + Cephalocarida + Remipedia, Cephalocarida + Remipedia, and Malaocostraca + Cirripedia. Atelocerata (= Myriapoda + Hexapoda) was never recovered. Elongation factor-2 is now the second protein-encoding, nuclear gene (in addition to RNA polymerase II) to support Pancrustacea over Atelocerata. Atelocerata is widely cited in morphology-based analyses, and the discrepancy between results derived from molecular and morphological data deserves greater attention.     

Serotonin immunoreactive interneurons in the brain of the Remipedia: new insights into the phylogenetic affinities of an enigmatic crustacean taxon

ABSTRACT: BACKGROUND: Remipedia, a group of homonomously segmented, cave-dwelling, eyeless arthropods have been regarded as basal crustaceans in most early morphological and taxonomic studies. However, molecular sequence information together with the discovery of a highly differentiated brain led to a reconsideration of their phylogenetic position. Various conflicting hypotheses have been proposed including the claim for a basal position of Remipedia up to a close relationship with Malacostraca or Hexapoda. To provide new morphological characters that may allow phylogenetic insights, we have analyzed the architecture of the remipede brain in more detail using immunocytochemistry (serotonin, acetylated alpha-tubulin, synapsin) combined with confocal laser-scanning microscopy and image reconstruction techniques. This approach allows for a comprehensive neuroanatomical comparison with other crustacean and hexapod taxa. RESULTS: The dominant structures of the brain are the deutocerebral olfactory neuropils, which are linked by the olfactory globular tracts to the protocerebral hemiellipsoid bodies. The olfactory globular tracts form a characteristic chiasm in the center of the brain. In Speleonectes tulumensis, each brain hemisphere contains about 120 serotonin immunoreactive neurons, which are distributed in distinct cell groups supplying fine, profusely branching neurites to 16 neuropilar domains. The olfactory neuropil comprises more than 300 spherical olfactory glomeruli arranged in sublobes. Eight serotonin immunoreactive neurons homogeneously innervate the olfactory glomeruli. In the protocerebrum, serotonin immunoreactivity revealed several structures, which, based on their position and connectivity resemble a central complex comprising a central body, a protocerebral bridge, W-, X-, Y-, Z-tracts, and lateral accessory lobes. CONCLUSIONS: The brain of Remipedia shows several plesiomorphic features shared with other Mandibulata, such as deutocerebral olfactory neuropils with a glomerular organization, innervations by serotonin immunoreactive interneurons, and connections to protocerebral neuropils. Also, we provided tentative evidence for W-, X-, Y-, Z-tracts in the remipedian central complex like in the brain of Malacostraca, and Hexapoda. Furthermore, Remipedia display several synapomorphies with Malacostraca supporting a sister group relationship between both taxa. These homologies include a chiasm of the olfactory globular tract, which connects the olfactory neuropils with the lateral protocerebrum and the presence of hemiellipsoid bodies. Even though a growing number of molecular investigations unites Remipedia and Cephalocarida, our neuroanatomical comparison does not provide support for such a sister group relationship.     

External morphology of Lightiella monniotae (Crustacea, Cephalocarida) in the light of Cambrian 'Orsten' crustaceans

The species-poor meiofaunal Cephalocarida have played an important role in discussions of the phylogeny and evolution of Crustacea since their discovery in 1955. One reason may be that the morphology of cephalocarids includes some aspects of putatively ancient appearance, such as the simple roof-shaped head shield, the anterior three head appendages resembling those of a nauplius larva, or the trunk-limb-like second maxilla. Cephalocarida have even been suggested to represent the sister taxon to all other Eucrustacea. Presence of possibly plesiomorphic characters, however, does not necessarily point to a basal position in the system. Growing evidence demonstrates that the modification of the fourth post-antennular cephalic appendage, the ‘maxilla’, into a “mouth part” may have occurred independently in the different eucrustacean lineages, so a trunk-limb-like maxilla is an ancient feature that does not hold only for cephalocarids. Retention of its plesiomorphic shape and function in the Cephalocarida remains, however, noteworthy. Cephalocarids are still little studied and incompletely known, especially their external morphology. By examining several adults and one young specimen of Lightiella monniotae Cals and Delamare Deboutteville, 1970 from New Caledonia, we aimed to a) document as many details as possible, and b) compare these data with other species of Cephalocarida. We also aimed to reconstruct aspects of the ground pattern of Cephalocarida, which is a pre-requisite for any comparisons in a broader perspective of crustacean phylogeny. Among the new findings or conclusions are: (1) Lightiella is in need of a revision since several assumed differences between the species are questionable or subject to intra-specific variability; (2) the cuticle of the trunk-limb basipod is sub-divided into a number of smaller sclerotized areas as in various exceptionally 3D preserved fossil crustaceans from Cambrian ‘Orsten’ faunal assemblages; (3) a small transitional portion on the post-maxillulary limbs in the area where the endopod and basipod connect is discussed as either a reduced, proximal endopod segment or as an evolutionary new joint of the basipod to enhance its flexibility; (4) the so-called pseud-epipod is interpreted as an outer branch of the exopod; (5) compared to ‘Orsten’ crustaceans many characters of the Cephalocarida are more modified than previously assumed, including the morphology of the trunk-limb basipod, and the unique, ring-shaped appearance of the abdominal segments. Also the development is not as plesiomorphic as sometimes assumed, at least not compared to that of the strictly anamorphic series of the ‘Orsten’ eucrustacean Rehbachiella kinnekullensis. The application of SEM techniques has again proved to be especially appropriate because of the small size of these animals, and because it permits direct comparisons with other similarly small crustaceans and the ‘Orsten’ crustaceans and their larvae.     

Testing the phylogenetic position of Cambrian pancrustacean larval fossils by coding ontogenetic stages

The study of ontogeny as an integral part of understanding the pattern of evolution dates back over 200 years, but only recently have ontogenetic data been explicitly incorporated into phylogenetic analyses. Pancrustaceans undergo radical ontogenetic changes. The spectacular upper Cambrian “Orsten” fauna preserves phosphatized fossil larvae, including putative crown‐group pancrustaceans with amazingly complete developmental sequences. The putative presence and nature of adult stages remains a source of debate, causing spurious placements in a traditional morphological analysis. We introduce a new coding method where each semaphoront (discrete larval or adult stage) is considered an operational taxonomic unit. This avoids a priori assumptions of heterochrony. Characters and their states are defined to identify changes in morphology throughout ontogeny. Phylogenetic analyses of semaphoronts produced possible relationships of each Orsten fossil to the crown‐group clade expected from morphology shared with extant larvae. Bredocaris is a member of the stem lineage of Thecostraca or (Thecostraca + Copepoda), and Yicaris and Rehbachiella are probably members of the stem lineage of Cephalocarida. These placements rely directly on comparisons between extant and fossil larval character states. The position of Phosphatocopina remains unresolved. This method may have broader applications to other phylogenetic problems which may rely on ontogenetically variable homology statements.     

Pancrustacean phylogeny in the light of new phylogenomic data: support for Remipedia as the possible sister group of Hexapoda

Remipedes are a small and enigmatic group of crustaceans, first described only 30 years ago. Analyses of both morphological and molecular data have recently suggested a close relationship between Remipedia and Hexapoda. If true, the remipedes occupy an important position in pancrustacean evolution and may be pivotal for understanding the evolutionary history of crustaceans and hexapods. However, it is important to test this hypothesis using new data and new types of analytical approaches. Here, we assembled a phylogenomic data set of 131 taxa, incorporating newly generated 454 expressed sequence tag (EST) data from six species of crustaceans, representing five lineages (Remipedia, Laevicaudata, Spinicaudata, Ostracoda, and Malacostraca). This data set includes all crustacean species for which EST data are available (46 species), and our largest alignment encompasses 866,479 amino acid positions and 1,886 genes. A series of phylogenomic analyses was performed to evaluate pancrustacean relationships. We significantly improved the quality of our data for predicting putative orthologous genes and for generating data subsets by matrix reduction procedures, thereby improving the signal to noise ratio in the data. Eight different data sets were constructed, representing various combinations of orthologous genes, data subsets, and taxa. Our results demonstrate that the different ways to compile an initial data set of core orthologs and the selection of data subsets by matrix reduction can have marked effects on the reconstructed phylogenetic trees. Nonetheless, all eight data sets strongly support Pancrustacea with Remipedia as the sister group to Hexapoda. This is the first time that a sister group relationship of Remipedia and Hexapoda has been inferred using a comprehensive phylogenomic data set that is based on EST data. We also show that selecting data subsets with increased overall signal can help to identify and prevent artifacts in phylogenetic analyses.     

Upper Cambrian stem-lineage crustaceans and their bearing upon the monophyletic origin of Crustacea and the position of Agnostus

Walossek, D. & Müller, K. J. 1990 10 15: Upper Cambrian stem-lineage crustaceans and their bearing upon the monophyletic origin of Crustacea and the position of Agnostus. Lethaia , Vol. 23, pp. 409–427. Oslo. ISSN 0024–1164.
Three new arthropods in uncompressed condition have been discovered in Upper Cambrian limestone nodules (Orsten) of Västergötland, Sweden. Together with Martinssonia elongafa Müller & Walossek, 1986, they are recognized as descendants of early offshoots from the stem-lineage of Crustacea. Their morphology provides new insights into the evolutionary path and progressive development of ground plan characteristics along the stem-lineage and gives further support for the monophyletic origin of Crustacea s. str., which embraces all taxa with extant derivatives. Structures of the ventral morphology shared between these stem-lineage crustaceans and Agnostus lead to the consideration of alternatives for the currently assumed position of agnostids. ▭ Crustacea. ontogeny, phosphatization. phylogeny, stem-lineage, Sweden, 3 D-preseroation, Trilobita     

Global biodiversity and phylogenetic evaluation of remipedia (crustacea)

Remipedia is one of the most recently discovered classes of crustaceans, first described in 1981 from anchialine caves in the Bahamas Archipelago. The class is divided into the order Enantiopoda, represented by two fossil species, and Nectiopoda, which contains all known extant remipedes. Since their discovery, the number of nectiopodan species has increased to 24, half of which were described during the last decade. Nectiopoda exhibit a disjunct global distribution pattern, with the highest abundance and diversity in the Caribbean region, and isolated species in the Canary Islands and in Western Australia. Our review of Remipedia provides an overview of their ecological characteristics, including a detailed list of all anchialine marine caves, from which species have been recorded. We discuss alternative hypotheses of the phylogenetic position of Remipedia within Arthropoda, and present first results of an ongoing molecular-phylogenetic analysis that do not support the monophyly of several nectiopodan taxa. We believe that a taxonomic revision of Remipedia is absolutely essential, and that a comprehensive revision should include a reappraisal of the fossil record.     

Post-embryonic development of remipede crustaceans

During diving explorations of anchialine cave systems on Abaco Island, Bahamas, we collected five larvae that represent different developmental stages of remipede crustaceans. Based on four early naupliar stages and a post-naupliar larva, it is possible for the first time to reconstruct the postembryonic development of Remipedia some 25 years after their discovery. These specimens begin to fill in some critical gaps in our knowledge of this important group of crustaceans.     

Higher-level crustacean phylogeny: Consensus and conflicting hypotheses

This paper presents an overview of current hypotheses of higher-level crustacean phylogeny in order to assist and help focus further research. It concentrates on hypotheses proposed or debated in the recent literature based on morphological, molecular and combined evidence phylogenetic analyses. It can be concluded that crustacean phylogeny remains essentially unresolved. Conflict is rife, irrespective of whether one compares different morphological studies, molecular studies, or both. Using the number of recently proposed alternative sister group hypotheses for each of the major tetraconatan taxa as a rough estimate of phylogenetic uncertainty, it can be concluded that the phylogenetic position of Malacostraca remains the most problematic, closely followed by Branchiopoda, Cephalocarida, Remipedia, Ostracoda, Branchiura, Copepoda and Hexapoda. Future progress will depend upon a broader taxon sampling in molecular analyses, and the further exploration of new molecular phylogenetic markers. However, the need for continued revision and expansion of morphological datasets remains undiminished given the conspicuous lack of agreement between molecules and morphology for positioning several taxa. In view of the unparalleled morphological diversity of Crustacea, and the likely nesting of Hexapoda somewhere within Crustacea, working out a detailed phylogeny of Tetraconata is a crucial step towards understanding arthropod body plan evolution.     

A Review of Arthropod Phylogeny: New Data Based on Ribosomal DNA Sequences and Direct Character Optimization

Ribosomal gene sequence data are used to explore phylogenetic relationships among higher arthropod groups. Sequences of 139 taxa (23 outgroup and 116 ingroup taxa) representing all extant arthropod "classes" except Remipedia and Cephalocarida are analyzed using direct character optimization exploring six parameter sets. Parameter choice appears to be crucial to phylogenetic inference. The high level of sequence heterogeneity in the 18S rRNA gene (sequence length from 1350 to 2700 bp) makes placement of certain taxa with "unusual" sequences difficult and underscores the necessity of combining ribosomal gene data with other sources of information. Monophyly of Pycnogonida, Chelicerata, Chilopoda, Chilognatha, Malacostraca, Branchiopoda (excluding Daphnia ), and Ectognatha are among the higher groups that are supported in most of the analyses. The positions of the Pauropoda, Symphyla, Protura, Collembola, Diplura, Onychophora, Tardigrada, and Daphnia are unstable throughout the parameter space examined.