Phylogeny of extant and fossil Juglandaceae inferred from the integration of molecular and morphological data sets

It is widely acknowledged that integrating fossils into data sets of extant taxa is imperative for proper placement of fossils, resolution of relationships, and a better understanding of character evolution. The importance of this process has been further magnified because of the crucial role of fossils in dating divergence times. Outstanding issues remain, including appropriate methods to place fossils in phylogenetic trees, the importance of molecules versus morphology in these analyses, as well as the impact of potentially large amounts of missing data for fossil taxa. In this study we used the angiosperm clade Juglandaceae as a model for investigating methods of integrating fossils into a phylogenetic framework of extant taxa. The clade has a rich fossil record relative to low extant diversity, as well as a robust molecular phylogeny and morphological database for extant taxa. After combining fossil organ genera into composite and terminal taxa, our objectives were to (1) compare multiple methods for the integration of the fossils and extant taxa (including total evidence, molecular scaffolds, and molecular matrix representation with parsimony [MRP]); (2) explore the impact of missing data (incomplete taxa and characters) and the evidence for placing fossils on the topology; (3) simulate the phylogenetic effect of missing data by creating "artificial fossils"; and (4) place fossils and compare the impact of single and multiple fossil constraints in estimating the age of clades. Despite large and variable amounts of missing data, each of the methods provided reasonable placement of both fossils and simulated "artificial fossils" in the phylogeny previously inferred only from extant taxa. Our results clearly show that the amount of missing data in any given taxon is not by itself an operational guideline for excluding fossils from analysis. Three fossil taxa (Cruciptera simsonii, Paleoplatycarya wingii, and Platycarya americana) were placed within crown clades containing living taxa for which relationships previously had been suggested based on morphology, whereas Polyptera manningii, a mosaic taxon with equivocal affinities, was placed firmly as sister to two modern crown clades. The position of Paleooreomunnea stoneana was ambiguous with total evidence but conclusive with DNA scaffolds and MRP. There was less disturbance of relationships among extant taxa using a total evidence approach, and the DNA scaffold approach did not provide improved resolution or internal support for clades compared to total evidence, whereas weighted MRP retained comparable levels of support but lost crown clade resolution. Multiple internal minimum age constraints generally provided reasonable age estimates, but the use of single constraints provided by extinct genera tended to underestimate clade ages.     

Cladistic analysis of Albian heteromorph ammonites

A computer-based parsimony analysis of Albian heteromorph ammonites from the Tethyan and European faunal provinces is demonstrated. The results indicate that whilst Anisoceratidae, Baculitidae, Scaphitidae and Turrilitidae are monophyletic, Hamitidae is not. Previous studies of ammonite phylogenetics have used the coiling mode, ornamentation of the shell and the suture line as sources of characters. Comparison of the consistency and retention indices of these character suites indicates that whereas ornamentation is relatively consistent within clades, suture line morphology is highly homoplastic. The earliest known Sciponoceras , S. skipperae sp. nov., is described.     

Cladistic information in phylogenetic definitions and designated phylogenetic contexts for the use of taxon names

A phylogenetic definition of a taxon name associates that name with a clade through its reference to a particular ancestor and all of its descendants. Depending on one's perspective, phylogenetic definitions name either clades on the one true, but unknown, phylogeny, or components on cladograms (clades on hypotheses regarding the true phylogeny). Phylogenetic definitions do not contain enough information to identify components without a reference cladogram. As a result, (1) if clades are equated with components on cladograms, a phylogenetic definition may associate a taxon name with different clades on different cladograms, and (2) the inclusiveness, diagnostic synapomorphies, and distribution in time and space of the clade with a particular name can differ markedly depending on the phylogenetic hypothesis one chooses to adopt. This potentially unacceptable lability in the clade to which a name refers can be avoided by using a taxon name in conjunction with only phylogenetic hypotheses on which specific taxa are related in a particular fashion. This designated phylogenetic context can be described in an n-taxon statement that would be appended to the phylogenetic definition. Use of the taxon name would be considered inappropriate in conjunction with cladograms on which the relationships contradict those in the n-taxon statement. Whereas phylogenetic definitions stabilize the meaning of taxon names, designated phylogenetic contexts would stabilize the usage of those names.     

Monophyly, phylogeny and systematic position of the †Synechodontiformes (Chondrichthyes, Neoselachii)

Klug, S. (2009). Monophyly, phylogeny and systematic position of the †Synechodontiformes (Chondrichthyes, Neoselachii). — Zoologica Scripta, 39 , 37–49.
Identifying the monophyly and systematic position of extinct sharks is one of the major challenges in reconstructing the phylogeny and evolutionary history of sharks in general. Although great progress has been accomplished in the last few decades with regard to resolving the interrelationships of living sharks, a comprehensive phylogeny identifying the systematic position of problematic or exclusively fossil taxa is still lacking. Fossil taxa traditionally assigned to synechodontiform sharks are very diverse with a fossil record extending back into the Palaeozoic but with uncertain inter- and intrarelationships. Here, phylogenetic analyses using robust cladistic principles are presented for the first time to evaluate the monophyly of this group, their intrarelationships and their systematic position within Neoselachii. According to the results of this study, taxa assigned to this group form a monophyletic clade, the †Synechodontiformes. This group is sister to all living sharks and displays a suite of neoselachian characters. Consequently, the concept of neoselachian systematics needs to be enlarged to include this completely extinct group, which is considered to represent stem-group neoselachians. The origin of modern sharks can be traced back into the Late Permian (250 Mya) based on the fossil record of †Synechodontiformes. The systematic position of batoids remains contradictory, which relates to the use of different data (molecular vs. morphological) in phylogentic analyses.     

A Total Evidence Phylogeny of the Arctoidea (Carnivora: Mammalia): Relationships Among Basal Taxa

A total evidence phylogenetic analysis was performed for 14 extant and 18 fossil caniform genera using a data matrix of 5.6 kbp of concatenated sequence data from six independent loci and 80 morphological characters from the cranium and dentition. Maximum parsimony analysis recovered a single most parsimonious cladogram (MPC). The topology of the extant taxa in the MPC agreed with previous molecular phylogenies. Phylogenetic positions for fossil taxa indicate that several taxa previously described as early members of extant families (e.g., Bathygale and Plesictis) are likely stem taxa at the base of the Arctoidea. Taxa in the “Paleomustelidae” were found to be paraphyletic, but a monophyletic Oligobuninae was recovered within this set of taxa. This clade was closely related to the extant genera Gulo and Martes, therefore, nested within the extant radiation of the family Mustelidae. This analysis provides a resolution to several discrepancies between phylogenies considering either fossil taxa or extant taxa separately, and provides a framework for incorporating fossil and extant taxa into comprehensive combined evidence analyses.     

Relationships among extant and fossil echimyids (Rodentia: Hystricognathi)

The echimyid rodents are the most diverse group of Neotropical hystricognaths, with approximately 40 extant and fossil genera. Craniodental characters are proposed in order to formulate hypotheses of phylogenetic relationships within the Echimyidae. A data matrix of 54 taxa and 50 characters is constructed and submitted to parsimony analyses using PAUP and WinClada programs. Analysis of the complete data set results in 47 448 most parsimonious trees 107 steps long. These trees are summarized in a strict consensus tree, which is taken as the main phylogenetic hypothesis resulting from this study. The monophyly of several currently recognized supraspecific taxa is not corroborated. These are: the subfamilies Eumysopinae, Echimyinae, Myocastorinae and Adelphomyinae; and the genera Proechimys , Echimys and Makalata . Conversely, the monophyly of Dactylomyinae and Trinomys is supported. New associations are proposed: (1) a clade comprising the extant Carterodon , Clyomys and Euryzygomatomys and the fossil Pampamys and Theridomysops placed at the base of the crown-group Echimyidae; (2) a clade uniting Proechimys , Hoplomys and Trinomys , which is the sister-taxon of (3) a clade including Mesomys , Lonchothrix , Myocastor and a clade with extant dactylomyines and echimyines and associated fossil taxa. Based on this phylogenetic hypothesis, patterns of tooth evolution in Echimyidae are discussed, and minimum ages for the divergence events within the family are estimated.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 142 , 445–477.     

Rapid Laurasian diversification of a pantropical bird family during the Oligocene–Miocene transition

Disjunct, pantropical distributions are a common pattern among avian lineages, but disentangling multiple scenarios that can produce them requires accurate estimates of historical relationships and timescales. Here, we clarify the biogeographical history of the pantropical avian family of trogons (Trogonidae) by re-examining their phylogenetic relationships and divergence times with genome-scale data. We estimated trogon phylogeny by analysing thousands of ultraconserved element (UCE) loci from all extant trogon genera with concatenation and coalescent approaches. We then estimated a time frame for trogon diversification using MCMCTree and fossil calibrations, after which we performed ancestral area estimation using BioGeoBEARS. We recovered the first well-resolved hypothesis of relationships among trogon genera. Trogons comprise three clades, each confined to one of three biogeographical regions: Africa, Asia and the Neotropics, with the African clade sister to the others. These clades diverged rapidly during the Oligocene-Miocene transition. Our biogeographical analyses identify a Eurasian origin for stem trogons and a crown clade arising from ancestors broadly distributed across Laurasia and Africa. The pantropical ranges of trogons are relicts of a broader Afro-Laurasian distribution that was fragmented across Africa, Asia and the New World in near coincident fashion during the Oligocene-Miocene transition by global cooling and changing habitats along the Beringian land bridge and North Africa.     

Molecular systematics of a speciose, cosmopolitan songbird genus: defining the limits of, and relationships among, the Turdus thrushes

The avian genus Turdus is one of the most speciose and widespread of passerine genera. We investigated phylogenetic relationships within this genus using mitochondrial DNA sequence data from the ND3, ND2 and cytochrome b genes. Our sampling of Turdus included 60 of the 65 extant species currently recognized, as well as all four species from three genera previously shown to fall inside Turdus (Platycichla, Nesocichla, and Cichlherminia). Phylogenetic trees based on maximum likelihood and maximum parsimony algorithms were congruent. Most of the Turdus taxa sampled fall into one of four clades: an African clade, a Central American-Caribbean clade, a largely South American clade, and a Eurasian clade. Still other taxa are placed either at the base of Turdus, or as links between clades. In no instance is any continent reciprocally monophyletic for the species distributed on it. A general lack of nodal support near the base of the phylogeny seems related to a rapid intercontinental establishment of the major clades within Turdus very early in the history of the genus. The monotypic genus Psophocichla is distantly related to, but clearly the sister of, Turdus rather than a constituent member of it.     

Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology

Higher‐level phylogenetics of Pycnogonida has been discussed for many decades but scarcely studied from a cladistic perspective. Traditional taxonomic classifications are yet to be tested and affinities among families and genera are not well understood. Pycnogonida includes more than 1300 species described, but no systematic revisions at any level are available. Previous attempts to propose a phylogeny of the sea spiders were limited in characters and taxon sampling, therefore not allowing a robust test of relationships among lineages. Herein, we present the first comprehensive phylogenetic analysis of the Pycnogonida based on a total evidence approach and Direct Optimization. Sixty‐three pycnogonid species representing all families including fossil taxa were included. For most of the extant taxa more than 6 kb of nuclear and mitochondrial DNA and 78 morphological characters were scored. The most parsimonious hypotheses obtained in equally weighted total evidence analyses show the two most diverse families Ammotheidae and Callipallenidae to be non‐monophyletic. Austrodecidae + Colossendeidae + Pycnogonidae are in the basal most clade, these are morphologically diverse groups of species mostly found in cold waters. The raising of the family Pallenopsidae is supported, while Eurycyde and Ascorhynchus are definitely separated from Ammotheidae. The four fossil taxa are grouped within living Pycnogonida, instead of being an early derived clade. This phylogeny represents a solid framework to work towards the understanding of pycnogonid systematics, providing a data set and a testable hypothesis that indicate those clades that need severe testing, especially some of the deep nodes of the pycnogonid tree and the relationships of ammotheid and callipallenid forms. The inclusion of more rare taxa and additional sources of evidence are necessary for a phylogenetic classification of the Pycnogonida. © The Willi Hennig Society 2006.     

Molecular phylogeny of Macaranga, Mallotus, and related genera (Euphorbiaceae s.s.): insights from plastid and nuclear DNA sequence data

Macaranga and Mallotus (Euphorbiaceae s.s.) are two closely related, large paleo(sub)tropical genera. To investigate the phylogenetic relationships between and within them and to determine the position of related genera belonging to the subtribe Rottlerinae, we sequenced one plastid (trnL-F) and three nuclear (ITS, ncpGS, phyC) markers for species representative of these genera. The analyses demonstrated the monophyly of Macaranga and the paraphyly of Mallotus and revealed three highly supported main clades. The genera Cordemoya and Deuteromallotus and the Mallotus sections Hancea and Oliganthae form a basal Cordemoya s.l. clade. The two other clades, the Macaranga clade and the Mallotus s.s. clade (the latter with Coccoceras, Neotrewia, Octospermum, and Trewia), are sister groups. In the Macaranga clade, two basal lineages (comprising mostly sect. Pseudorottlera) and a crown group with three geographically homogenous main clades were identified. The phylogeny of the Mallotus s.s. clade is less clear because of internal conflict in all four data sets. Many of the sections and informal infrageneric groups of Macaranga and Mallotus do not appear to be monophyletic. In both the Macaranga and Mallotus s.s. clades, the African and/or Madagascan taxa are nested in Asian clades, suggesting migrations or dispersals from Asia to Africa and Madagascar.     

A REVISION OF SCOMBRID FISHES (SCOMBROIDEI, PERCIFORMES) FROM THE MIDDLE EOCENE OF MONTE BOLCA, ITALY

Abstract:  A taxonomic revision of mackerel-like fishes (Scombridae, Scombroidei, Perciformes) from the Monte Bolca localities in Italy is presented as part of continuing research into phylogenetic relationships of fossil and Recent scombroids. Three new genera are proposed: † Godsilia , † Pseudauxides and † Thunnoscomberoides . In addition, species generally known as † Thynnus bolcensis , † Thynnus lanceolatus , † Scombrosarda propterygia , † Scomberomorus speciosus and † Scomberomorus tenuis are renamed † Thunnoscomberoides bolcensis comb. nov., † Godsilia lanceolata comb. nov., † Auxides propterygius comb. nov. and † Pseudauxides speciosus comb. nov., respectively. † S. tenuis , a nomen nudum , is a junior synonym of † P. speciosus . Provisional phylogenetic hypotheses of these Monte Bolca scombrids are as follows. † Auxides and † Pseudauxides are most likely primitive sister taxa of the mackerels Scomber and Rastrelliger with † Pseudauxides as the most primitive. † Godsilia may be placed between Scombrinae and Scomberomorinae. † Thunnoscomberoides seems to fit between Scombrinae and Thunnini.     

Molecular phylogenetics and the evolution of labral spines among eastern Pacific ocenebrine gastropods

Labral spines are sharp projections of the apertural lip found in some marine gastropods that are used to penetrate hard-shelled prey. The majority of gastropod genera that contain labral spine-bearing species are found in the subfamily Ocenebrinae (Gastropoda: Muricidae). To reconstruct the evolutionary history of labral spine-bearing and labral spine-lacking gastropods in the eastern Pacific (EP) Ocean, partial sequences of two mitochondrial genes (cytochrome oxidase I and 12S rRNA) were obtained from representative taxa. Despite high nucleotide bias, a variety of phylogenetic reconstruction methods produced the same tree topology. The traditional taxonomic view that all "Nucella-like" spine-bearing taxa in the EP belong to a monophyletic "Acanthina" is rejected due to nonmonophyly of this group. The more recently recognized "Acanthinucella" is also not monophyletic, and we therefore propose the new genus Mexacanthina for two Mexican species formerly assigned to Acanthinucella. The genus Ocinebrina, which first appears in the middle Eocene, is not a stem EP ocenebrine lineage and may also not be a monophyletic clade. Tracing the evolutionary history of labral spines among extant lineages indicates that the absence of a labral spine is ancestral for all EP ocenebrines. Ancestral conditions could not be resolved unambiguously for all nodes of the phylogeny based on extant taxa. However, by jointly considering both molecular phylogenetic relationships and the phylogenetic affinities of several extinct taxa, all remaining character state transformation can be inferred unambiguously. Based on this analysis, a labral spine likely evolved independently in at least four lineages of EP ocenebrines. Although homoplasy appears to characterize labral spine evolution among ocenebrine gastropods, the structural position of a labral spine was evolutionarily altered in one lineage, indicating that different types of labral spines do not necessarily reflect convergent evolution.     

Rooting Around the Eutherian Family Tree: the Origin and Relations of the Taeniodonta

Placing early groups into the overall phylogeny of eutherian mammals can be challenging, particularly when the group does not have extant members. We investigated the relationships of the Taeniodonta, an extinct group from the Late Cretaceous through Paleogene of North America. This group has a few purported close relatives, including Cimolestes, Procerberus, and Alveugena, that may form a sequence of ancestors and descendants. The leading hypothesis is that Procerberus gave rise to taeniodonts through Alveugena. We test this hypothesis and analyze relations to known stem and crown Eutheria to determine the place of taeniodonts in eutherian phylogeny. Cladistic analyses were performed using previously published characters and datasets, namely a taeniodont/cimolestid specific dataset and a reanalysis of Wible and colleagues (2009), with added taxa for both. Our studies suggest that taeniodonts arose from Cimolestes through Alveugena, that Procerberus is more distantly related to taeniodonts, and that taeniodonts and their relatives are stem eutherians. We diagnose the Taeniodonta based on these analyses. Other Paleogene groups, especially those allied with Cimolestes such as tillodonts and pantolestans, merit further study. Our findings indicate that stem eutherians such as the Taeniodonta, in addition to crown eutherians, continued to diversify during the Paleogene.     

Cladistic relationships of extant and fossil hominoids

There is general agreement that the hominoid primates form a monophyletic group, that the extant great apes and humans form a second clade within that group with the gibbons as the sister group, and that the African apes and humans form a third clade. Although it has recently been proposed that humans and orang utans are sister taxa and also that the great apes form a clade to the exclusion of humans, our analysis, particularly of the molecular evidence, supports the existence of an African ape and human clade. The major problem in hominoid phylogeny at present is the relationships of the species within this clade: morphological data generally support the existence of an African ape clade which is the sister group to humans; some molecular data also support this conclusion, but most molecular evidence indicates the existence of a chimpanzee/human clade. We have cladistically re-analysed the DNA and protein sequence data for which apomorphic character states can be assessed. It is clear that there is a high degree of homoplasy whichever branching pattern is produced, with some characters supporting the existence of a chimpanzee/human clade and others supporting an African ape clade. When the cladistic analyses of morphological and molecular data are combined we believe that the most parsimonious interpretation of the data is that the African apes form a clade which is the sister taxon of the human (i.e., Australopithecus, Homo and Paranthropus) clade.This paper is not intended as a survey of all hominoid fossils but as a study of branching points in hominoid evolution and fossils are included which are relevant to this branching pattern. The analysis of fossil taxa in this study leads us to conclude that Proconsul is the sister taxon to the later Hominoidea. A number of middle Miocene forms such as Dryopithecus, Kenyapithecus, Heliopithecus and Afropithecus are shown to share derived characters with great apes and humans and provide evidence for the divergence of that clade from the gibbon lineage prior to 18 Ma. The position that Sivapithecus represents the sister group of the orang utan clade is supported here and shows that the orang utan lineage had diverged from the African ape and human lineage prior to 11·5 Ma. There is unfortunately no definitive fossil cvidence on branching sequences within the African ape and human clade, although a new specimen from Samburu, Kenya may be related to the gorilla.     

Avian higher level biogeography: Southern Hemispheric origins or Southern Hemispheric relicts?

A recent analysis of a comprehensive phylogenetic data set suggested Southern Hemispheric origins for various higher level taxa of neornithine (crown group) birds. These results contrast with hypotheses derived from the fossil record, with the occurrence of successively branching stem group representatives of many “Southern Hemispheric” bird groups in the early Cenozoic of Europe and North America suggesting relict extant distributions. However, a selective consideration of the fossil record and the merging of extant ranges may have resulted in a skewed picture of the past biogeographical history of birds. For future studies, it is proposed that multiple taxa of successively branching Northern Hemispheric stem group representatives are considered to narrow down the ancestral areas of the crown group representatives.     

Exclusive conservation of mitochondrial group II intron nad4i548 among liverworts and its use for phylogenetic studies in this ancient plant clade

Liverworts occupy a pivotal position in land plant (embryophyte) phylogeny as the presumed earliest-branching major clade, sister to all other land plants, including the mosses, hornworts, lycophytes, monilophytes and seed plants. Molecular support for this earliest dichotomy in land plant phylogeny comes from strikingly different occurrences of introns in mitochondrial genes distinguishing liverworts from all other embryophytes. Exceptionally, however, the nad5 gene--the mitochondrial locus hitherto used most widely to elucidate early land plant phylogeny--carries a group I type intron that is shared between liverworts and mosses. We here explored whether a group II intron, the other major type of organellar intron, would similarly be conserved in position across the entire diversity of extant liverworts and could be of use for phylogenetic analyses in this supposedly most ancient embryophyte clade. To this end, we investigated the nad4 gene as a candidate locus possibly featuring different introns in liverworts as opposed to the non-liverwort embryophyte (NLE) lineage. We indeed found group II intron nad4i548 universally conserved in a wide phylogenetic sampling of 55 liverwort taxa, confirming clade specificity and surprising evolutionary stability of plant mitochondrial introns. As expected, intron nad4i548g2 carries phylogenetic information in its variable sequences, which confirms and extends previous cladistic insights on liverwort evolution. We integrate the new nad4 data with those of the previously established mitochondrial nad5 and the chloroplast rbcL and rps4 genes and present a phylogeny based on the fused datasets. Notably, the phylogenetic analyses suggest a reconsideration of previous phylogenetic and taxonomic assignments for the genera Calycularia and Mylia and resolve a sister group relationship of Ptilidiales and Porellales.     

Molecular phylogeny of the Cricetinae subfamily based on the mitochondrial cytochrome b and 12S rRNA genes and the nuclear vWF gene

Despite some popularity of hamsters as pets and laboratory animals there is no reliable phylogeny of the subfamily Cricetinae available so far. Contradicting views exist not only about the actual number of species but also concerning the validity of several genera. We used partial DNA sequences of two mitochondrial (cytochrome b, 12S rRNA) and one partial nuclear gene (von Willebrand Factor exon 28) to provide a first gene tree of the Cricetinae based on 15 taxa comprising six genera. According to our data, Palaearctic hamsters fall into three distinct phylogenetic groups: Phodopus, Mesocricetus, and Cricetus-related species which evolved during the late Miocene about 7-12MY ago. Surprisingly, the genus Phodopus, which was previously thought to have appeared during the Pleistocene, forms the oldest clade. The largest number of extant hamster genera is found in a group of Cricetus-related hamsters. The genus Cricetulus itself proved to be not truly monophyletic with Cricetulus migratorius appearing more closely related to Tscherskia, Cricetus, and Allocricetulus. We propose to place the species within a new monotypic genus. Molecular clock calculations are not always in line with the dating of fossil records. DNA based divergence time estimates as well as taxonomic relationships demand a reevaluation of morphological characters previously used to identify fossils and extant hamsters.     

Cladistic analysis of Taxaceae s. l.

Taxaceae s. l. is a wider concept of classification treating five genera of Taxaceae s. str. and Cephalotaxus together. Cephalotaxus is morphologically very similar to the five genera of Taxaceae s. str. Various models of classification for six genera have already been published. However, the phylogenetic position and genuine relationships of these genera and species are still confusing. A cladistic analysis of Taxaceae s. l. has been carried out to resolve the problem existing in their phylogeny and to provide a new approach regarding the relationships of these six genera. Parsimony analyses were based on 28 characters and eight genera including two outgroups Agathis and Sciadopitys. The most parsimonious tree retained with branch length 38 and 194 rearrangement trials. Consistency index was 0.68 and retention index was 0.66. Principally, two main clades were found: one represented by Austrotaxus forming the base of the tree and another by the remaining five genera. Taxus + Pseudotaxus clade split after Austrotaxus, and Cephalotaxus was sister to Torreya + Amentotaxus clade. Taxus + Pseudotaxus clade was supported by the highest bootstrap value. Finally, cladistic analysis does not support existence of Cephalotaxaceae. Therefore, it would be better to classify Cephalotaxus within Taxaceae s. l. with the other five genera.     

Tardigrades as ‘Stem-Group Arthropods’: The Evidence from the Cambrian Fauna

The Cambrian fauna can now reasonably be seen as containing many taxa that lie in the stem-groups of the extant phyla. As such, these fossils suggest how both the ‘body plans’ of extant phyla were assembled, and also how various ‘minor’ phyla relate to the larger groupings of today such as the arthropods and annelids.

The various arthropod and lobopod taxa of the Cambrian faunas have been controversial and have generally been considered either as lying in the crown or (occasionally) stem groups of the euarthropods, onychophorans and tardigrades. However, phylogenetic analysis strongly suggests that many of even the most euarthropod-like taxa do not lie within the euarthropod crown-group but are more basal. Further, the commonly expressed view that Cambrian lobopods are in effect stem- or crown-group onychophorans also seems not to be well supported. Lobopods in the Cambrian appear to be diverse and not particularly closely related to one another, and certainly cannot be combined in a monophyletic clade.

Both these advances offer hope that the tardigrades (placed as the sister group to the euarthropods in many analyses of extant taxa, here collectively named the Tactopoda) may be more closely related to some of these Cambrian taxa than others. The challenge for both neontologists and palaeontologists is to refine the systematic analysis of both living and fossil taxa in order to maximise the usefulness of the (admittedly few) characters that unite tardigrades to their Cambrian forbears.