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Hallucigenia's onychophoran-like claws and the case for Tactopoda
Abstract and Figures
The Palaeozoic form-taxon Lobopodia encompasses a diverse range of soft-bodied 'legged worms' known from exceptional fossil deposits. Although lobopodians occupy a deep phylogenetic position within Panarthropoda, a shortage of derived characters obscures their evolutionary relationships with extant phyla (Onychophora, Tardigrada and Euarthropoda). Here we describe a complex feature in the terminal claws of the mid-Cambrian lobopodian Hallucigenia sparsa-their construction from a stack of constituent elements-and demonstrate that equivalent elements make up the jaws and claws of extant Onychophora. A cladistic analysis, informed by developmental data on panarthropod head segmentation, indicates that the stacked sclerite components in these two taxa are homologous-resolving hallucigeniid lobopodians as stem-group onychophorans. The results indicate a sister-group relationship between Tardigrada and Euarthropoda, adding palaeontological support to the neurological and musculoskeletal evidence uniting these disparate clades. These findings elucidate the evolutionary transformations that gave rise to the panarthropod phyla, and expound the lobopodian-like morphology of the ancestral panarthropod.
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... Euarthropods are members of Ecdysozoa, a clade composed of Scalidophora (Kinorhyncha, Lorcifera and Priapulida), Nematoida (Nematoda and Nematomorpha) and Panarthropoda (Euarthropoda, Onychophora and Tardigrada). Conventionally, molecular [2][3][4][5][6] and some morphological [7][8][9][10][11][12][13] phylogenetic analyses have supported the Lobopodia hypothesis (=Arthropoda of [1]) in which Euarthropoda and Onychophora are closest relatives; however, this has been challenged by morphologybased phylogenetic analyses that instead support a sister-group relationship between Euarthropoda and Tardigrada (Tactopoda hypothesis) [10,[14][15][16][17][18][19][20]. The Protarthropoda hypothesis (a clade of onychophorans and tardigrades) is a third rival that has been supported by both molecular [21,22] and morphological [19,23] data. ...
... Since support for Tactopoda is rooted in morphology and attempts to resolve bodyplan evolution require integrated phylogenetic analysis of living and fossil taxa, here we explore support for these competing phylogenetic hypotheses within morphological datasets that have recovered Lobopodia [8,9,13] and Tactopoda [16][17][18]. Morphology-based phylogenetic analyses are particularly sensitive to taxon and character sampling, as well as methods of phylogenetic inference, principally because of their small size. Through application of parsimony, maximum likelihood and Bayesian phylogenetic inference methods as well as standard statistical tests of phylogenetic support, we show that morphological datasets cannot discriminate among the three competing phylogenetic hypotheses of panarthropod relationships. ...
... The Aria dataset is composed of 111 taxa and 276 characters, including 36 extant euarthropods, plus Nematoda and Priapulida as the outgroup; the clades of onychophorans and tardigrades are distinguished as 'Onychophora' and 'Tardigrada'. As an exemplar Tactopoda-supporting dataset, we used Yang et al. [18], updated from Yang et al. [17] and Smith & Ortega-Hernandez [16] (henceforth 'Yang dataset'). The Yang dataset is composed of 50 taxa and 95 characters, including two extant euarthropods, three extant onychophorans and five extant tardigrades, plus Tubiluchus troglodytes as an outgroup. ...
Panarthropoda, the clade comprising the phyla Onychophora, Tardigrada and Euarthropoda, encompasses the largest majority of animal biodiversity. The relationships among the phyla are contested and resolution is key to understanding the evolutionary assembly of panarthropod bodyplans. Molecular phylogenetic analyses generally support monophyly of Onychophora and Euarthropoda to the exclusion of Tardigrada (Lobopodia hypothesis), which is also supported by some analyses of morphological data. However, analyses of morphological data have also been interpreted to support monophyly of Tardigrada and Euarthropoda to the exclusion of Onychophora (Tactopoda hypothesis). Support has also been found for a clade of Onychophora and Tardigrada that excludes Euarthropoda (Protarthropoda hypothesis). Here we show, using a diversity of phylogenetic inference methods, that morphological datasets cannot discriminate statistically between the Lobopodia, Tactopoda and Protarthropoda hypotheses. Since the relationships among the living clades of panarthropod phyla cannot be discriminated based on morphological data, we call into question the accuracy of morphology-based phylogenies of Panarthropoda that include fossil species and the evolutionary hypotheses based upon them.
... Morphological analyses recover Tardigrada, Arthropoda, and Onychophora as a monophyletic group within Ecdysozoa referred to as Panarthropoda. All possible interrelationships of these lineages have found support in morphological analyses (Caron & Aria, 2017;Howard et al., 2020;Legg et al., 2013;Nielsen et al., 1996;Peterson & Eernisse, 2001;Smith & Ortega-Hernández, 2014;Waggoner, 1996;Wu et al., 2023;Yang et al., 2016). Analyses of mitochondrial sequences, phylogenomic analyses, investigations of the phylogenetic distribution of microRNA molecules, and presence/absence of orthologous genes have all recovered Tardigrada as the sister-group of an Arthropoda + Onychophora lineage (Campbell et al., 2011;Howard et al., 2022;Rota-Stabelli et al., 2010;Yoshida et al., 2017). ...
... Lobopodians have an extensive Cambrian fossil record and typically exhibit many more segments than a tardigrade. Several phylogenetic analyses have recovered Tardigrada as nested within lineages that include lobopodians, that is, Tardigrada is resolved as more closely related to some lobopodians than others in these studies (Caron & Aria, 2017;Howard et al., 2020;Kihm et al., 2023;Smith & Ortega-Hernández, 2014;Yang et al., 2016). Although these phylogenetic studies disagree on the exact relationship of tardigrades to lobopodians, their recovered topologies all suggest that the limited segment number characteristic of Tardigrada is a derived state of this lineage, as predicted by the model based on analyses of AP axis patterning genes. ...
Tardigrada is an ancient lineage of miniaturized animals. As an outgroup of the well‐studied Arthropoda and Onychophora, studies of tardigrades hold the potential to reveal important insights into body plan evolution in Panarthropoda. Previous studies have revealed interesting facets of tardigrade development and genomics that suggest that a highly compact body plan is a derived condition of this lineage, rather than it representing an ancestral state of Panarthropoda. This conclusion was based on studies of several species from Eutardigrada. We review these studies and expand on them by analyzing the publicly available genome and transcriptome assemblies of Echiniscus testudo , a representative of Heterotardigrada. These new analyses allow us to phylogenetically reconstruct important features of genome evolution in Tardigrada. We use available data from tardigrades to interrogate several recent models of body plan evolution in Panarthropoda. Although anterior segments of panarthropods are highly diverse in terms of anatomy and development, both within individuals and between species, we conclude that a simple one‐to‐one alignment of anterior segments across Panarthropoda is the best available model of segmental homology. In addition to providing important insight into body plan diversification within Panarthropoda, we speculate that studies of tardigrades may reveal generalizable pathways to miniaturization.
... This has led to several studies on the phylogenetic relationships within panarthropods, the main goal of which was to understand the morphological origination of the crown groups. For example, the Cambrian lobopodians, Kerygmachela kierkegaardi, Pambdelurion whittingtoni, and radiodontans were interpreted as stem groups of Euarthropoda, based on a pair of frontal appendages on the head and the paired gut-diverticula (17,18), while Hallucigenia sparsa was considered to be a stem-onychophoran based on the presence of stacked elements in sclerites of claws and dorsal spines (19). However, tardigrades have received little attention in studies of panarthropod phylogeny. ...
Phylum Tardigrada (water bears), well known for their cryptobiosis, includes small invertebrates with four paired limbs and is divided into two classes: Eutardigrada and Heterotardigrada. The evolutionary origin of Tardigrada is known to lie within the lobopodians, which are extinct soft-bodied worms with lobopodous limbs mostly discovered at sites of exceptionally well-preserved fossils. Contrary to their closest relatives, onychophorans and euarthropods, the origin of morphological characters of tardigrades remains unclear, and detailed comparison with the lobopodians has not been well explored. Here, we present detailed morphological comparison between tardigrades and Cambrian lobopodians, with a phylogenetic analysis encompassing most of the lobopodians and three panarthropod phyla. The results indicate that the ancestral tardigrades likely had a Cambrian lobopodian-like morphology and shared most recent ancestry with the luolishaniids. Internal relationships within Tardigrada indicate that the ancestral tardigrade had a vermiform body shape without segmental plates, but possessed cuticular structures surrounding the mouth opening, and lobopodous legs terminating with claws, but without digits. This finding is in contrast to the long-standing stygarctid-like ancestor hypothesis. The highly compact and miniaturized body plan of tardigrades evolved after the tardigrade lineage diverged from an ancient shared ancestor with the luolishaniids.
... Downloaded from https://www.science.org on July 05, 2023 brain that were inherited from their deep bilaterian ancestors (12). However, if so, they are all contained within the anterior appendagebearing segment in at least onychophorans, and do not correspond to the overt anteriorposterior divisions of the CNS, which are clearly segmental in origin. ...
Strausfeld et al. (Report, 24 Nov 2022, p. 905) claim that Cambrian fossilized nervous tissue supports the interpretation that the ancestral panarthropod brain was tripartite and unsegmented. We argue that this conclusion is unsupported, and developmental data from living onychophorans contradict it.
... This system not only serves as a hydrostatic skeleton for locomotion 40,58,59 but also plays roles in respiration 60 , nutrition 5,61 , hormone dispersion 62-64 , immune response 65,66 , and excretion 5,57,67 . Its origin most likely dates back to the early Cambrian (~520 Mya), i.e., the time when an onychophoran-like body plan first appeared 48,[68][69][70] . ...
An antagonistic hemolymph-muscular system is essential for soft-bodied invertebrates. Many ecdysozoans (molting animals) possess neither a heart nor a vascular or circulatory system, whereas most arthropods exhibit a well-developed circulatory system. How did this system evolve and how was it subsequently modified in panarthropod lineages? As the closest relatives of arthropods and tardigrades, onychophorans (velvet worms) represent a key group for addressing this question. We therefore analyzed the entire circulatory system of the peripatopsid Euperipatoides rowelli and discovered a surprisingly elaborate organization. Our findings suggest that the last common ancestor of Onychophora and Arthropoda most likely possessed an open vascular system, a posteriorly closed heart with segmental ostia, a pericardial sinus filled with nephrocytes and an impermeable pericardial septum, whereas the evolutionary origin of plical and pericardial channels is unclear. Our study further revealed an intermittent heartbeat—regular breaks of rhythmic, peristaltic contractions of the heart—in velvet worms, which might stimulate similar investigations in arthropods.
... Monophyly of this clade has been established through phylogenetic analysis of both non-coding and protein-coding gene datasets , and morphological data sets (Legg et al., 2013), although it has been challenged by other recent morphological analyses that endorsed a rival sister group relationship between Euarthropoda and Tardigrada (e.g. Smith and Ortega-Hernández, 2014). Note the name Arthropoda in GenBank refers to what we consider Euarthropoda; there is no GenBank taxonomy ID for the clade comprising Euarthropoda and Onychophora. ...
Fossil age data and molecular sequences are increasingly combined to establish a timescale for the Tree of Life. Arthropods, as the most species-rich and morphologically disparate animal phylum, have received substantial attention, particularly with regard to questions such as the timing of habitat shifts (e.g. terrestrialisation), genome evolution (e.g. gene family duplication and functional evolution), origins of novel characters and behaviours (e.g. wings and flight, venom, silk), biogeography, rate of diversification (e.g. Cambrian explosion, insect coevolution with angiosperms, evolution of crab body plans), and the evolution of arthropod microbiomes. We present herein a series of rigorously vetted calibration fossils for arthropod evolutionary history, taking into account recently published guidelines for best practice in fossil calibration. These are restricted to Palaeozoic and Mesozoic fossils, no deeper than ordinal taxonomic level, nonetheless resulting in 80 fossil calibrations for 102 clades. This work is especially timely owing to the rapid growth of molecular sequence data and the fact that many included fossils have been described within the last five years. This contribution provides a resource for systematists and other biologists interested in deep-time questions in arthropod evolution.
ABBREVIATIONS
AMNH American Museum of Natural History
AMS Australian Museum, Sydney
AUGD University of Aberdeen
BGR Bundesanstalt fur Geowissenschaften und Rohstoffe, Berlin
BMNH The Natural History Museum, London
CNU Key Laboratory of Insect Evolutionary & Environmental Change, Capital Normal University, Beijing
DE Ulster Museum, Belfast
ED Ibaraki University, Mito, Japan
FMNH Field Museum of Natural History
GMCB Geological Museum of China, Beijing
GSC Geological Survey of Canada
IRNSB Institut Royal des Sciences Naturelles de Belgique, Brussels
KSU Kent State University
Ld Musee Fleury, Lodeve, France
LWL Landschaftsverband Westfalen-Lippe-Museum fur Naturkunde, Munster
MACN Museo Argentino de Ciencias Naturales, Buenos Aires
MBA Museum fur Naturkunde, Berlin
MCNA Museo de Ciencias Naturales de Alava, Vitoria-Gasteiz, Alava, Spain
MCZ Museum of Comparative Zoology, Harvard University
MGSB Museo Geologico del Seminario de Barcelona
MN Museu Nacional, Rio de Janeiro
MNHN Museum national d'Histoire naturelle, Paris
NHMUK The Natural History Museum, London
NIGP Nanjing Institute of Geology and Palaeontology
NMS National Museum of Scotland
OUM Oxford University Museum of Natural History
PBM Palaobotanik Munster
PIN Paleontological Institute, Moscow
PRI Paleontological Research Institution, Ithaca
ROM Royal Ontario Museum
SAM South Australian Museum, Adelaide
SM Sedgwick Museum, University of Cambridge
SMNK Staatliches Museum fur Naturkunde, Karlsruhe
SMNS Staatliches Museum fur Naturkunde, Stuttgart
TsGM F.N. Chernyshev Central Geologic Prospecting Research Museum, St. Petersburg
UB University of Bonn
USNM US National Museum of Natural History, Smithsonian Institution
UWGM University of Wisconsin Geology Museum
YKLP Yunnan Key Laboratory for Palaeobiology, Yunnan University
YPM Yale Peabody Museum
ZPAL Institute of Paleobiology, Polish Academy of Sciences, Warsaw.
... However, by combining extant morphology with the limited record of soft bodied annelid fossils from the Cambrian and the Palaeozoic yielded results more congruent with the molecular hypotheses (Chen et al., 2020;Parry, Edgecombe, Eibye-Jacobsen, & Vinther, 2016). Similar results have been found when analysing Cambrian molluscs (Vinther et al., 2017), arthropods (Legg, Sutton, & Edgecombe, 2013;Smith & Ortega-Hernández, 2014), and even comb jellies (Zhao et al., 2019). In the case of early animal evolution, paleontological evidence is notoriously hard to integrate with neontological evidence; in large part because Proterozoic (i.e. ...
This thesis is about the complicated and multifaceted problem of the evolution of the
eumetazoan body plan and its key role in the emergence of organismal complexity in
the animal kingdom. As such, it draws on a very broad range of topics and discussions
including empirical research programmes in palaeontology, comparative morphology,
classical and genetic developmental biology, and morphological and molecular
phylogenetics; as well as theoretical/philosophical research around the conceptual
bases of phylogenetics, homology, scientific integration, and biological complexity and
individuality. In the following chapters, I first provide background on and an outline
of the thesis (chapter 1); I then propose an overarching conceptual framework for the
integration of different kinds of evidence in macroevolutionary biology (chapter 2),
with a special emphasis on the integration of morphological and developmental genetic
evidence in inferring morphological homology (chapter 3); followed by providing a
phylogeny of the major animal groups incorporating the two Ediacaran fossils
Dickinsonia and Yorgia (chapter 4) and building on this phylogenetic placement to
propose a novel evolutionary scenario for the evolution of key features of the
eumetazoan body plan—namely the gastric cavity and bilateral symmetry—in light of
Dickinsonia and other Ediacaran fossils (chapter 5). I finish with a discussion on the
evolution of complexity in the animal kingdom in light of preceding chapters and the
multilevel selection literature (chapter 6), and a brief final conclusion.
Lobopodians are an iconic and diverse group of animals from the Cambrian, which alongside radiodonts, present an important window into the evolution of arthropods and the development of Paleozoic ecosystems. Of these, a rare few species outside of Radiodonta possess lateral swimming flaps. The recent discovery of Utahnax provided much-needed insight into the evolution of swimming flaps, suggesting that the ventrolateral flaps of Kerygmachela evolved independently from other flap-bearing lobopodians and radiodonts. Here a new pelagic lobopodian species is described, Mobulavermis adustus new genus new species, the first lobopodian to be reported from the Cambrian-age Pioche Shale of Nevada. Mobulavermis adustus was large and possessed more ventrolateral flap pairs than any other known lobopodian or radiodont. It is found to be a close relative of both Kerygmachela and Utahnax , allowing the establishment of the new lobopodian family Kerygmachelidae new family. In addition, an indeterminate euarthropod fossil from the Pioche Formation is described in brief, and the recently described Chengjiang species Parvibellus avatus Liu et al., 2022, thought to have been related to the “gilled lobopodians,” is reinterpreted as a juvenile siberiid lobopodian.
UUID: http://zoobank.org/759c4eb9-ec60-4d5a-8b20-4f115ab79575
The Parsimony Ratchet1 is presented as a new method for analysis of large data sets. The method can be easily implemented with existing phylogenetic software by generating batch command files. Such an approach has been implemented in the programs DADA (Nixon, 1998) and Winclada (Nixon, 1999). The Parsimony Ratchet has also been implemented in the most recent versions of NONA (Goloboff, 1998). These implementations of the ratchet use the following steps: (1) Generate a starting tree (e.g., a "Wagner" tree followed by some level of branch swapping or not). (2) Randomly select a subset of characters, each of which is given additional weight (e.g., add 1 to the weight of each selected character). (3) Perform branch swapping (e.g., "branch-breaking" or TBR) on the current tree using the reweighted matrix, keeping only one (or few) tree. (4) Set all weights for the characters to the "original" weights (typically, equal weights). (5) Perform branch swapping (e.g., branch-breaking or TBR) on the current tree (from step 3) holding one (or few) tree. (6) Return to step 2. Steps 2-6 are considered to be one iteration, and typically, 50-200 or more iterations are performed. The number of characters to be sampled for reweighting in step 2 is determined by the user; I have found that between 5 and 25% of the characters provide good results in most cases. The performance of the ratchet for large data sets is outstanding, and the results of analyses of the 500 taxon seed plant rbcL data set (Chase et al., 1993) are presented here. A separate analysis of a three-gene data set for 567 taxa will be presented elsewhere (Soltis et al., in preparation) demonstrating the same extraordinary power. With the 500-taxon data set, shortest trees are typically found within 22 h (four runs of 200 iterations) on a 200-MHz Pentium Pro. These analyses indicate efficiency increases of 20×-80× over "traditional methods" such as varying taxon order randomly and holding few trees, followed by more complete analyses of the best trees found, and thousands of times faster than nonstrategic searches with PAUP. Because the ratchet samples many tree islands with fewer trees from each island, it provides much more accurate estimates of the "true" consensus than collecting many trees from few islands. With the ratchet, Goloboff's NONA, and existing computer hardware, data sets that were previously intractable or required months or years of analysis with PAUP* can now be adequately analyzed in a few hours or days.
New methods for parsimony analysis of large data sets are presented. The new methods are sectorial searches, tree-drifting, and tree-fusing. For Chase et al.'s 500-taxon data set these methods (on a 266-MHz Pentium II) find a shortest tree in less than 10 min (i.e., over 15,000 times faster than PAUP and 1000 times faster than PAUP*). Making a complete parsimony analysis requires hitting minimum length several times independently, but not necessarily all "islands" for Chase et al.'s data set, this can be done in 4 to 6 h. The new methods also perform well in other cases analyzed (which range from 170 to 854 taxa).
The integument of Peripatopsis moseleyi has been examined by light and electron microscopy with particular reference to the structure and formation of the cuticle. The evidence supports the idea that Peripatus is a true arthropod but not that it has direct affinities with the annelids. The characteristics of arthropod cuticle are present in their simplest form and pore canals and dermal glands are lacking. The cuticle is 1 or 2 µ, thick except in the hardened claws and spines. Above the procuticle (chitinprotein) is a thin 4-layered epicuticle. It is possible that the innermost of the 4 layers (prosclerotin) may correspond to cuticulin of other arthropods. In the claws and spines tanning in this layer extends to the procuticle. Hydrofuge properties of the cuticle probably depend on the outer layers of epicuticle, and it is suggested that the lamina concerned might consist of oriented lipid associated with lipoprotein (Dr. J. W. L. Beament). Wax and cement are absent. Non-wettability of the cuticle is probably ensured by the contours of micropapillae which cover the surface. Similar structures arise in Collembola and other terrestrial arthropods by convergence.
The formation of new cuticle before ecdysis is described. After the epicuticular layers are complete, the bulk of the procuticle is laid down in a manner probably common to all arthropods. Secreted materials originate in small vesicles derived from rough endoplasmic reticulum and from scattered Golgi regions. The latter contribute to larger vacuoles which rise to the surface of the cell and liberate material in a fluid state. This later consolidates to form procuticle. Vesicles may also open to the surface directly, and ribosomes probably occur free in the cytoplasm. At this stage the cell surface is reticulate, especially under micropapillae.
The ordinary epidermis has only one kind of cell, attached to the cuticle by tonofibrils disposed like the ribs of a shuttlecock, and to the fibrous sheaths of underlying muscle-fibres by special fibres of connective tissue. These features and the presence of numerous sensory papillae are associated with the characteristic mobility of the body wall. The appearance of epidermal pigment granules, mitochondria, the nuclear membrane, and a centriole are noted. No other cells immediately concerned in the formation of cuticle have been found. By contrast myriapods, which do not have wax either, possess dermal glands secreting far more lipid than is found in the Onychophora. The wax layer found in insects and some arachnids constitutes an advance of high selective value which emphasizes the primitive condition of the Onychophora. It is noted that the thick layer of collagen separating the haemocoel from the epidermis probably restricts the transfer of materials. It is suggested that since some features of cuticular structure and formation appear to be common to all arthropods, it is possible that some of the endocrine mechanisms associated with ecdysis may also be similar throughout the phylum.
A hypothesis is presented to explain the origin of a fundamental difference between the nervous systems of tardigrades and arthropods and to show how the hypostome of arthropods evolved. In arthropods the stomodeal nervous system is fused to a more posterior pair of trunk ganglia to form the tritocerebrum, whereas in tardigrades the two systems are not comparably linked. It is proposed that this difference resulted from a key transformation of the head in the stem group to Euarthropoda. The hypothesis specifies, based on a presumed plesiomorphic condition found in onychophorans, that an appendage-bearing metamere gave rise to the mouth cone of onychophorans, tardigrades and stem lineage arthropods. The metamere provided jaws and a circumoral ring of plates or sensory structures that in certain stem lineage arthropods is manifested as a "Peytoia." A simple model is developed to illustrate how in the stem group to Euarthropoda the mouth cone or "Peytoia" rotated, elongated in a posterior direction and became sclerotized on its anterior surface to form the hypostome. The hypothesis postulates further that backward rotation of the mouth cone brought the stomodeal system and a posterior pair of ganglia into juxtaposition. Thus the hypothesis offers a plausible explanation for the origin of the tritocerebrum and shows why the hypostome appears to be innervated by a neuromere of a more posterior segment. In general the model refines a construct introduced by LANKESTER (1904) that one of the distinctive features of arthropods is the formation of preoral segments by back migration of the mouth. The dramatic shift in orientation of the mouth in the stem group of arthropods appears to have been correlated with a change in feeding strategy. It is suggested that the complex evolutionary history of the hypostome, including derivation of the mouth cone or "Peytoia " from an appendage bearing segment and its subsequent rotation, elongation and scleroitization proposed here is concordant with this view. The hypothesis agrees with proposals that the hypostome is segmental but contradicts postulations that it formed from medially fused appendages. The model also reinterprets the absence of typical engrailed expression in the hypostome and predicts that remnants of a "labral" segment are confined to the areas innervated by the stomodeal system - regions previously considered to be nonsegmental. Finally, the origin of several extant taxa may be understood in terms of the transformation of the mouth cone. The presence of a biradially symmetrical cone in tardigrades suggests that they derived from the paraphyletic "Peyroia" group, even though they diverge basal to them in a recent cladistic analysis using a larger set of morphological characters (DEWEL & DEWEL 1997). Possession of a mouth cone argues also that the clade containing Tardigrada arose prior to the evolution of the hypostome, a structure that nonetheless predates the radiation of the arthropod crown group. Furthermore, it can be inferred from the location of all postantennal limbs caudal to the hypostome in several fossil arachnates (CHEN et al. 1997; RAMSKOLD et al. 1997; HOU & RERGSTROM 1997), that these taxa lacked a tritocerebrum as-defined for extant arthropods. The same seems to be true for modern Chelicerata. Molecular data suggest that the cheliceral segment is homologous to Al (TELFORD & THOMAS 1998; DAMEN et al. 1998). Thus the stomodeal system appears to be linked anatomically to homologues of the Al and not A2 ganglia, and hence the tritocerebrum, sensu stricto, may not be a synapomorphy for the crown group of Arthropoda.
This paper is a review of published information on fossil lobopodians, with addition of observations and ideas based on new material. It is also an analysis of the phylogeny of the group, and presents a new classification. A character shared by three or four families, yet not seen before, is a pair of enlarged sclerites covering the head. This forms an argument for re-orientingHallucigeniaonce again. This genus no longer being enigmatic, a corner-stone in Stephen Jay Gould's evidence for extinct phyla is therefore gone. It is suggested that the lobopodians, phylum Lobopodia, are arranged in two classes, the extinct Xenusia for marine forms, and the Onychophora for terrestrial forms. The marine lobopodians are morphologically much more diverse than the extant onychophorans, a condition expressed in the classification. New taxa are:Hallucigenia fortissp. nov.,the families Luolishaniidae, Cardiodictyidae and Onychodictyidae, and the new orders Archonychophora, Scleronychophora and Paronychophora.
Opabinia regalis Walcott is an enigmatic fossil from the Middle Cambrian Burgess Shale of uncertain affinities. Recent suggestions place it in a clade with Anomalacaris Whiteaves from the Burgess Shale and Kerygmachela Budd from the Greenlandic Sirius Passet Fauna; these taxa have been interpreted as 'lobopods'. Consideration of available Opabinia specimens demonstrates that reflective extensions from the axial region, previously thought to be either gut diverticula or musculature, can be accommodated in neither the trunk nor the lateral lobes that arise from it. They must therefore be external structures independent of the lateral lobes. On the basis of their sub-triangular appearance, size and taphonomy, they are considered here to represent lobopod limbs. Some evidence for the existence of terminal claws is also presented. The question of whether Kerygmachela, Opabinia and Anomalocaris constitute a monophyletic or paraphyletic grouping is considered. While they share several characters, most of these are plesiomorphies. Further, Opabinia and Anomalocaris share several arthropod-like characters not possessed by Kerygmachela. It is concluded that these three taxa probably form a paraphyletic grouping at the base of the arthropods. Retention of lobopod-like characters within the group provides important documentation of the lobopod-arthropod transition. A proper understanding of Opabinia and its close relatives, which may include the tardigrades, opens the way for a reconstruction of the arthropod stem-group. This in turn allows the construction of a speculative but satisfying scenario for the evolution of major arthropod features, including the origin of the biramous limb, tergites and arthropod segmentation. 'Arthropodization' may thus be seen not to be a single event but a series of adaptive innovations.
The integument of Peripatopsis moseleyi has been examined by light and electron microscopy with particular reference to the structure and formation of the cuticle. The evidence supports the idea that Peripatus is a true arthropod but not that it has direct affinities with the annelids. The characteristics of arthropod cuticle are present in their simplest form and pore canals and dermal glands are lacking. The cuticle is 1 or 2fj, thick except in the hardened claws and spines. Above the procuticle (chitin-protein) is a thin 4-layered epicuticle. It is possible that the innermost of the 4 layers (prosclerotin) may correspond to cuticulin of other arthropods. In the claws and spines tanning in this layer extends to the procuticle. Hydrofuge properties of the cuticle probably depend on the outer layers of epicuticle, and it is suggested that the lamina concerned might consist of oriented lipid associated with lipoprotein (Dr. J. W. L. Beament). Wax and cement are absent. Non-wettability of the cuticle is probably ensured by the contours of micropapillae which cover the surface. Similar structures arise in Collembola and other terrestrial arthropods by convergence. The formation of new cuticle before ecdysis is described. After the epicuticular layers are complete, the bulk of the procuticle is laid down in a manner probably common to all arthropods. Secreted materials originate in small vesicles derived from rough endoplasmic reticulum and from scattered Golgi regions. The latter contribute to larger vacuoles which rise to the surface of the cell and liberate material in a fluid state. This later consolidates to form procuticle. Vesicles may also open to the surface directly, and ribosomes probably occur free in the cytoplasm. At this stage the cell surface is reticulate, especially under micropapillae. The ordinary epidermis has only one kind of cell, attached to the cuticle by tono-fibrils disposed like the ribs of a shuttlecock, and to the fibrous sheaths of underlying muscle-fibres by special fibres of connective tissue. These features and the presence of numerous sensory papillae are associated with the characteristic mobility of the body wall. The appearance of epidermal pigment granules, mitochondria, the nuclear membrane, and a centriole are noted. No other cells immediately concerned in the formation of cuticle have been found. By contrast myriapods, which do not have wax either, possess dermal glands secreting far more lipid than is found in the Onycho-phora. The wax layer found in insects and some arachnids constitutes an advance of high selective value which emphasizes the primitive condition of the Onychophora. It is noted that the thick layer of collagen separating the haemocoel from the epidermis probably restricts the transfer of materials. It is suggested that since some features of cuticular structure and formation appear to be common to all arthropods, it is possible that some of the endocrine mechanisms associated with ecdysis may also be similar throughout the phylum.
The main features of the phylogeny program TNT are discussed. Windows versions have a menu interface, while Macintosh and Linux versions are command-driven. The program can analyze data sets with discrete (additive, non-additive, step-matrix) as well as continuous ...