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Arthropoda


Abstract

This page presents a brief systematic overview of the Arthropoda, from their origins to the present.

Keywords: Arthropoda, arthropod, Ecdysozoa, Tactopoda, Onychophora, Euarthropoda, Tardigrada, Arachnomorpha, Trilobita, Chelicerata, Crustacea, Mandibulata, Atelocerata, Hexapoda, segmentation, ecdysis

Introduction

Arthropods are bilaterally symmetrical segmented animals with paired and usually jointed appendages, on some or all of the body segments, with chitinous claws. The body is surrounded by a tough organic or organic-mineral cuticle, incorporating a-chitin, which functions as an exoskeleton. In order for the animals to grow, the exoskeleton must be moulted regularly. The mixocoel includes metanephridia and, except in tardigrades, an ostiate heart. Segments are added from a posterior growth zone during ontogeny.

At least half of the described species of living animals are arthropods (mostly insects), but arthropods are less common as fossils. The most familiar group of fossil arthropods is undoubtedly the trilobites. Fossil groups have been incorporated into arthropod classification systems at least since the 18th century, but important new finds - especially of very early stem group organisms, such as those of the Chengjiang and Sirius Passet lagerstätten - are profoundly influencing views of early arthropod evolution.

Recent technological advances, notably the cladistic analysis of genetic sequences, have revolutionised arthropod systematics: long-standing problems are being actively re-examined and some cherished beliefs challenged.

An early milestone was a consensus on the placement of the Onychophora at the base of the arthropod family tree (see fig. 1) rather than with the annelids. This led to the 'lobopod' concept (Snodgrass 1938; Budd 1997, 1999) and helps to define arthropod apomorphies, the unifying characteristics of the group. Subsequent elucidation of the Tardigrada (e.g. see Dewel & Dewel 1997 and references therein) has contributed to our understanding in a similar manner.

Another development has been the ecdysozoan concept, which hypothesises a clade of organisms sharing the characteristic of ecdysis, or molting, under the influence of ecdysteroid hormones. If true, the closest relatives of arthropods are priapulids and nematodes, rather than molluscs and annelids as held by the more traditional view (e.g. see Nielsen 2001, p. 119 for discussion).

Despite the advances, however, arthropod systematics is a science undertaking a flurry of development, and it will still be some time before widespread agreement on most issues is reached.

 
 

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  • For a somewhat different view, Berkeley has its usual excellent page.
 
 

Morphology

The criteria which have traditionally been employed to "define" the arthropoda are often somewhat problematic, especially when taken individually. They are segmentation (e.g. see Budd 2001a); externally jointed appendages (though not for the Onychophora); an exoskeleton (varyingly sclerotised) which necessitates periodic molting to allow growth (a characteristic shared with other phyla); and a body commonly divided into head, thorax, and abdomen (though these parts vary in distinctness and in some the different regions may be fused together).

Phylogeny and Evolution

Affinities

Traditionally, arthropods have been considered a sister group to the annelids, the two of them in turn a sister group to the Mollusca, primarily on account of the serial repetition of some structures, or segmentation. However, as Budd 2001a has pointed out, "segmentation" is not an easily defined concept, and may not represent an apomorphy for this traditional phylogeny at all.
Arthropod stem-group studies suggest that full arthropod segmentation has a different derivation from that of the annelids. In line with other recent analyses, this suggests that the 'Articulata' of Cuvier is not a true clade, and the arthropods considered to be a group of protostomes which are phylogenetically distinct from the classic spiralians. Arthropod affinities may rather lie with the other moulting animals, in the so-called 'Ecdysozoa' of Aguinaldo et al. (1997).
The clade Ecdysozoa Aguinaldo et al. 1997 has been proposed to embrace all organisms sharing the characteristic of ecdysis, or molting, under the influence of ecdysteroid hormones. In addition to the arthropods, this clade is supposed to include priapulids and nematodes. Proponents believe it to be a monophyletic group, i.e. that these animals are related and form a natural clade.
The ecdysozoan hypothesis has not been universally adopted, however. Nielsen (2001, p. 119) mentions some critical 18S rDNA studies which have produced different phylogenies and concludes that the discrepancies will have to be resolved through further study.

Relationships Within the Arthropoda

"The interrelationships of major clades within the Arthropoda remain one of the most contentious issues in systematics, which has traditionally been the domain of morphologists. A growing body of DNA sequences and other types of molecular data has revitalized study of arthropod phylogeny and has inspired new considerations of character evolution" (Giribet et al. 2001, p. 157).

This renewed interest notwithstanding, there is still no universally accepted view of arthropod phylogeny at the highest level. However, to be able to discuss the subject at all, it is useful to have some kind of a model. For the purposes of this discussion, that depicted in fig. 1 has been adopted. It is consistent with the "mandibulate hypothesis" (i.e. that the Crustacea form a clade with the Atelocerata rather than with the Trilobita + Chelicerata – the so-called APT hypothesis) and draws heavily on the phylogeny presented in Wheeler 1997.

Concordance with some other phylogenies (or parts thereof) include the following:

Broadly consistent with:

  • Dohle 1965 (myriapoda + Hexapoda)
  • Dunlop & Selden 1997 (Chelicerata)
  • Giribet et al. 2001 (Mandibulata)
  • Grimaldi & Engel 2005 (most groupings, except Myriapoda)
  • Snodgrass 1935 (Mandibulata)
  • Wheeler 1997 (Crustacea + Atelocerata)

Not consistent with:

  • Giribet et al. 1996 (Crustacea + Hexapoda & Myriapoda + Arachnida)
  • Giribet et al. 2001 (monophyletic myriapoda)
  • Manton 1977 (Uniramia)
  • Wills et al. 1997 (Schizoramia)

Priority of names above the family level is not regulated by the ICZN, so a profusion of names has been published, many of them synonymous, polyphyletic, or both. To try to contain the confusion, the remainder this article generally prefers the first published of any synonyms, and attempts to show synonymy clearly. Assuming this scheme, the following high-level taxa, as originally defined (or reasonably extrapolated) are considered -

Monophyletic:

  • Antennata (jnr. syn. of Atelocerata)
  • Arachnomorpha Heider 1913 = Trilobita + Pycnogonida + other Chelicerata
  • Atelocerata Heymons 1901 (syn. Tracheata) = Chilopoda + Progoneata + Hexapoda
  • Chelicerata Heymons 1901 = Pycnogonida + Xiphosura + Eurypterida + Arachnida
  • Euarthropoda Cuenot 1949 = Arachnomorpha + Mandibulata
  • Euchelicerata Weygoldt 1986 = Xiphosura + Eurypterida + Arachnida
  • Mandibulata Snodgrass 1935 = Crustacea + Atelocerata
  • Panarthropoda Nielsen 1995 (jnr. syn. of Arthropoda)
  • Progoneata Pocock 1893 = Symphyla + Diplopoda + Pauropoda
  • Tracheata (jnr. syn. of Atelocerata)

Poly- or paraphyletic:

  • Cormogonida Zrzavý et al. 1997 = Xiphosura + Arachnida + Crustacea + Atelocerata
  • Merostomata Dana 1852 = Xiphosura + Eurypterida
  • Myriapoda Latreille 1796 = Chilopoda + Symphyla + Diplopoda + Pauropoda
  • Pancrustacea Zrzavý & Štys 1977 = Crustacea + Hexapoda
  • Proarthropoda Vandel 1949 = Trilobitoidea + Trilobita
  • Schizoramia Hessler & Newman 1975 = Arachnomorpha + Crustacea
  • Trilobitomorpha Størmer 1944 = Trilobitoidea + Trilobita
  • Uniramia Manton 1972 = Onychophora + Atelocerata

Most would agree that the use of polyphyletic 'concept' names, such as Uniramia, should be discontinued. However, it is less clear what to do when the simple removal of one or a few unrelated taxa from a formerly polyphyletic taxon, leaving the concept more or less intact, and leaving a monophyletic taxon as well. Such is the case with the name Trilobitomorpha (also see below) for example.

There are also those who argue that taxonomic names should not be allocated to paraphyletic groups - which would potentially rule out such useful terms as "fish", "reptile" and probably "sponge" as well, if carried to its ridiculous extreme.

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Fig. 1: Cladogram showing the arthropod phylogeny adopted herein. (Rotated view.)

The Pentastomida are omitted from the diagram. They are sometimes considered to be crustaceans though in the view of Maas & Waloszek (2001) this group represents an early offshoot from the euarthropodan lineage, falling between the tardigrades and the trilobites.

Fossil Record

The assemblage of enigmatic Precambrian fossils known as Ediacarans has long been controversial. Some researchers even dispute that they are metazoans at all, proposing instead that they are some, failed, 'other' form of life. However, a growing body of evidence suggests that the earliest arthropods are to be found among the Ediacaran fossil record. If so, they represent fossil occurences at around the 555 Ma (million year) mark. It has been speculated that some of the Ediacaran forms, such as Diplichnites and Parvancorina (fig. 2), are primitive arthropods.

Another possibly Precambrian arthropod is the supposed onychophoran Xenusion auerswaldae, known from only a single specimen recovered from a glacial erratic boulder, found in Sweden.

Various arthropods are thought to be represented in the "small shelly fauna," an assemblage of phosphatised, disarticulated microfossils of mostly uncertain affinity, which first appears in the latest Precambrian. Some of the Cambrian components, notably the onychophoran Microdictyon, have subsequently been discovered fully articulated.

One of the earliest Cambrian arthropods is an embryo, Markuelia secunda, known from the basal Pestrotsvet Formation (Lower Cambrian, Lower Tommotian) in the classical Dvortsy section on the Aldan River in southern Yakutia, Siberia. More commonly, however, it is larger fossils which have been found and described. Two famous Lower Cambrian lagerstätten, Chengjiang and Sirius Passet, in particular, have elucidated the diversity of the ancient arthropods, and provided us with likely stem group forms.

The earliest known uniramous arthropod may be the rather poorly known Cambropodus Robison 1990, described from the famous Wheeler Formation in Utah.

Earliest evidence of arthropod activity on land is provided by trace fossils. Among the oldest are Late Cambrian to no younger than Arenig (Early Ordovician) tracks made by multiple ~50 cm-sized, many legged animals preserved in an eolian sandstone in the Nepean Formation (Potsdam Group) near Kingston, Ontario. However, these track-makers were probably amphibious arthropods - possibly euthycarcinoids - which only left the sea for a limited time, rather than fully terrestrial animals (MacNaughton et al. 2002).

The earliest identifiable terrestrial body fossils are arthropod fragments from the Late Silurian. However, these organisms possessed sensory and respiratory structures fully adapted for life on land, indicating an earlier history of terrestrial habituation (Shear & Selden 2001).

 
Parvancorina





        minchami (38329 bytes)

Fig. 2: Parvancorina minchami – A candidate arthropod, possibly a trilobite (see Fortey et al. 1996). In this scenario, the central axial ridge and the strongly arched anterior ‘lobes’ may be analogous to the midgut and gastric diverticulae. The scale bar is in centimetres. [Image and interpretation courtesy of Chris Nedin, Department of Industry, Science and Resources, Canberra.]

From the Middle Cambrian onwards, fossil arthropods become increasingly common and increasingly familiar.

The oldest insect fossils are the primitively wingless ("apterygote") springtails (order Collembola), which first appear in the Early Devonian Rhynie Chert. Pterygote insects appear much later, in the mid Carboniferous, but by the end of the Paleozoic they had aquired the ability to fold their wings over their bodies, and to metamorphose, and had assumed a near-modern ecological spectrum of plant-feeding activity - more than 100 Ma before the spread of flowering plants (Jarzembowski 2000).

Origins

Traditionally, zoologists have regarded molluscs and annelids as the closest relatives of arthropods. Uniquely among protostomes, arthropods and annelids share the characteristic of adding segments from a posterior growth zone during ontogeny. Arthropods and annelids share with molluscs the additional combination of coelomic cavities with metanephridia, which also function as gonoducts, and a haemal system.

However, in 1997 that idea was challenged when Aguinaldo et al. (1997) proposed a clade they named Ecdysozoa, characterised by ecdysis, or molting, under the influence of ecdysteroid hormones. The ecdysozoans are supposed to include arthropods, priapulids and nematodes.

With no fossil evidence linking the arthropods to one group or another, and the genetic and morphological evidence being, at best, equivocal, we can say very little more except that the arthropods appear to have arisen well before the Cambrian, and probably before the peak of the "classical" Ediacaran faunas (~555 Ma) too.

Kerygmachela and Pambdelurion are two highly problematic taxa, being known from a single locality, the Sirius Passet location in northeastern Greenland, and occupying an uncertain position intermediate between the onychophorans, anomalocaridids, and euarthropods. Graham Budd, author of both taxa, interprets them to imply that at least the biramous arthropods actually arose from within the anomalocaridids, the biramous limb having evolved before full cuticular sclerotisation.

Evolution

Maas & Waloszek 2001 considers three "pro-arthropod" taxa considered basal to the euarthropods, namely the Onychophora, Tardigrada and Pentastomida, concluding that these groups lack or partly lack some characteristic features of the Euarthropoda. "This is first seen in details of the head formation that has not reached a stage including the segments of antennae, in the sense of multi-segmented feelers, and three more segments with limbs. None of the limbs of early derivatives of the euarthropodan stem lineage possessed a rigid basipod nor an endopod or a paddle-shaped exopod" (Maas & Waloszek 2001, p. 457).

Damen et al. 1998 presents an intriguing evolutionary hypothesis, also based on transformations of the head region ### after Remane. (A) An annelid-like ancestor is assumed in which the archicerebrum (fig. 3A, darkest shading; top) is connected with the eyes, and the prosocerebrum is located behind the stomodaeum (indicated by a black triangle). (B) More advanced bauplan in which the prosocerebrum has moved anterior to the stomodaeum and has fused with the archicerebrum. The deutocerebrum also has moved to fuse partially with the prosocerebrum but has retained a post-oral commissure. Remane originally proposed this as a hypothetical intermediate step toward the crustacean/insect bauplan whereas our present results suggest that this is the situation in spiders. (C) The crustacean/insect bauplan in which the deutocerebrum has fused fully with the prosocerebrum and the tritocerebrum has fused partly with the deutocerebrum, retaining a post-oral commissure. The position of the tritocerebral ganglion with its commissure and its transformations is shown darkly enhanced in each stage.

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Fig. 3: Reproduction of fig. 5 from Damen et al. 1998. Evolutionary transformation of the head region in arthropods after Remane. (A) An annelid-like ancestor is assumed in which the archicerebrum (darkest shading; top) is connected with the eyes, and the prosocerebrum is located behind the stomodaeum (indicated by a black triangle). (B) More advanced bauplan in which the prosocerebrum has moved anterior to the stomodaeum and has fused with the archicerebrum. The deutocerebrum also has moved to fuse partially with the prosocerebrum but has retained a post-oral commissure. Remane originally proposed this as a hypothetical intermediate step toward the crustacean/insect bauplan whereas our present results suggest that this is the situation in spiders. (C) The crustacean/insect bauplan in which the deutocerebrum has fused fully with the prosocerebrum and the tritocerebrum has fused partly with the deutocerebrum, retaining a post-oral commissure. The position of the tritocerebral ganglion with its commissure and its transformations is shown darkly enhanced in each stage.

Extinction

Arthropods are among the most successful groups of organisms living today although, of course, numerous representatives including whole classes, and even higher taxa, have gone extinct in the past. The well-known trilobites are an example.

Gould 1989 advanced the intriguing view that arthropod diversity in the Middle Cambrian Period, some 500 million years ago, was far greater than at any subsequent time. (In fact, he went further and maintained that many of the problematic organisms living at that time were not even arthropods, but representatives of other, now extinct, phyla. However, that view is hardly tenable today.) His contention was that effectively random extinction, rather than natural selection, was responsible for selecting the variety of arthropods (indeed, all organisms) which lived subsequently. Of course there is an element of truth in all of this, but in the final analysis Gould did not establish a convincing argument for his ideas.

Taxonomy

 

Phylum Arthropoda von Siebold & Stannius 1845

? 1829 Arthropoda Latreille
1845 Arthropoda von Siebold & Stannius 1845
1881 Gnathopoda Lankester
1938 Lobopodia Snodgrass
1995 Panarthropoda Neilsen
1997 Lobopodia Snodgrass 1938, Budd
1998 Haemopoda Cavalier-Smith
2001 Panarthropoda Neilsen, Neilsen
2001a Arthropoda, Budd

Description: Bilaterally symmetrical, segmented animals with paired and usually jointed appendages on some or all of the body segments; body surrounded by a tough organic or organic-mineral cuticle which functions as an exoskeleton; growth is by molting (ecdysis), or molting, the process regulated by ecdysteroid hormones.

Supersubphylum Protarthropoda Lankester 1904

1904 Protarthropoda Lankester, p. 565
1949 Pararthropoda Vandel
1954 Oncopoda Weber
non 1959 Protarthropoda Lankester 1904, Moore

Type: Peripatus Guilding 1825

Discussion: The original definition of the Protarthropoda assigns only the Onychophora to this group; inclusion of the Pentastomida and Tardigrada was assumed later, by Moore (1959, p. O18). However, Moore's view would make it a paraphyletic (non-natural) taxon: herein the term is restricted to the Onychophora only.

Subphylum Onychophora Grube 1853

1853 Onychophora Grube
2001 Phylum Onychophora Nielsen, p. 198

Type: Peripatus Guilding 1825

Discussion: Onychophorans share a number of characteristics with both annelids (segmented worms) and arthropods, although they are more closely related to the latter and are sometimes, as here, regarded as a subphylum of the Arthropoda. Other authors (e.g. Nielsen 2001) regard the Onychophora as a phylum.

Currently there are around 10 genera and 110 species recognised within two extant families: the Peripatidae (known from the circumtropical regions of Mexico, Central and northern South America, equatorial West Africa, and South East Asia) and the Peripatopsidae (found in Chile, South Africa, Australia including Tasmania, and New Zealand).

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Supersubphylum cf. Protarthropoda Lankester 1904

cf. 1904 Protarthropoda Lankester, p. 565

Discussion: Arthropod systematics has long been in a state of flux; no more so than now. The taxonomic hierarchy is quite incomplete, and here is one example. Whereas taxa having the rank of supersubphyla have been defined for the Onychophora and the euarthropods, no equivalents (that I am aware of) exist for the Dinocarida or Tardigrada.

Class Dinocarida Collins 1996

1996 Dinocarida Collins, p. 291

Type: (Order) Radiodonta Collins 1996

Description: As defined by Collins, Dinocarids are bilaterally symmetrical arthropods with a body divided into two principal tagmata, recalling the prosoma and opisthosoma of chelicerates, and a non-mineralised cuticle. The front part shows no external segmentation, bears one or more pre-oral claws, one or more pairs of prominent eyes, and a ventral mouth; differing from other arthropod classes in possessing no antennae and only one appendage or pair of pre-oral appendages on the prosoma, and in bearing gilled lateral lobes on the metameric trunk. The jaws vary from none to forms with both radiating teeth and teeth in rows.

Discussion: Collins included within the group the Anomalocaride (Anomalocaris and Laggania), Opabiniidae (Opabinia), Hurdia, Proboscicaris, Cassubia, and "three, possibly five, unnamed genera from China" within the Dinocarida, but was unconvinced of any close relationship between Anomalocaris and Kerygmachela which might more closely belong with the onychophorans.

Tactopoda Budd 2001b

2001a Tactopoda Budd (in press), Budd, p. 333
2001b Tactopoda Budd
2005 Tritocerebra Grimaldi & Engel

Discussion: Graham Budd first used the name Tactopoda in two papers published in 2001 (Budd 2001a & b, to accomodate the probable clade formed by the tardigrades and euarthropods.

In that paper, he did not specify a formal taxonomic rank, which would be difficult to do anyway, since the divisions at this level of the arthropod hierarchy are rather crowded. Tactopoda holds a rank intermediate between Arthropoda and Euarthropoda: If the Arthropoda are treated as a phylum, as here, then the rank is between phylum and supersubphylum. If the Arthropoda are considered a superphylum (a very reasonable alternative) then the Tactopoda might legitimately be treated as a phylum.

Superclass Tardigrada Spallanzani 1776

1776 Tardigrada Spallanzani

Discussion: The tardigrades, or ‘water bears,’ are tiny, eight-legged, segmented creatures, sometimes accorded a phylum of their own (e.g. by Neilsen 2001). They live in water, although the ‘water’ may be the moisture held between the leaves of a moss, and they are capable of producing a thick-walled, protective resting cyst and surviving long periods of dessication (and, apparently, immersion in alcohol, freezing, boiling, vacuum and irradiation; Tudge 2000, p. 257).

The group comprises three classes: Heterotardigrada, Eutardigrada, and Mesotardigrada. For discussion and references, see Dewel & Dewel 1997.

Supersubphylum Euarthropoda Lankester 1904

1904 Euarthropoda Lankester

Description: Arthropoda distinguished by hardened body covering composed largely of chitin; body usually well-segmented and jointed externally, and commonly divided into head, thorax, and abdomen; with rather highly developed sensory organs, circulatory and nervous systems; sexes usually separate; young mostly passing through a number of larval stages before gradually or abruptly attaining adult form; growth accommodated by molting of the exoskeleteon (after Moore 1959, p. O21).

Further apomorphies, as noted in Maas & Waloszek 2001, include "a head tagma with one pair of antennae (first antenna, antennula of Crustacea) and 3 pairs of biramous limbs covered by a uniform shield, a segmented, limb-bearing body, and all post-antennular limbs comprising a well-sclerotised basipod carrying two rami. The inner ramus is 7-segmented (endopod) and the outer one is a seta-bearing flap (exopod)" (p. 453).

Discussion: Lankester 1904 introduced the "grade" Euarthropoda to include a list of "typical" arthropods, including particularly the classes Diplopoda, Arachnida, Crustacea, Chilopoda, and Hexapoda. The trilobites and "merostomes" (Xiphosura + Eurypterida) he included within the Arachnida.

The group is herein considered to include the sub-taxa Trilobitoidea, Arachnomorpha, and Mandibulata, wherein lie the great diversity of familiar spiders, scorpions, crustaceans, myriapods and insects, as well as many of the famously problematic stem group arthropods known only from fossils: the marrellomorphs, Burgessia, and so on.

Discussion: Whereas the phylogenies proposed up to this point are relatively uncomplicated, even if they are not universally agreed, relationships within the Euarthropoda are particularly fraught. "Novel hypotheses such as a crustacean-hexapod affinity were based on analyses of single or few genes and limited taxon sampling, but have received recent support from mitochondrial gene order, and eye and brain ultrastructure and neurogenesis. Here we assess relationships within Arthropoda based on a synthesis of all well sampled molecular loci together with a comprehensive data set of morphological, developmental, ultrastructural and gene-order characters. ... The optimal 'total evidence' cladogram supports the crustacean-hexapod clade, recognizes pycnogonids as sister to other euarthropods, and indicates monophyly of Myriapoda and Mandibulata" (Giribet et al. 2001).

Superclass Trilobitoidea Størmer 1959

1959 Trilobitoidea Størmer, pp. O28-29

Type: Marrella Walcott 1912*

Original Diagnosis: "Trilobitomorpha [earlier defined as 'Aquatic arthropoda with preoral antennae and remaining appendages of typical or modified trilobite type, biramous appendages characterised by presence of a lateral gill branch attached to very base of walking leg', p. O22] lacking distinct morphological features of Trilobita and, in addition, exhibiting divergent structural peculiarities of their own .... Postoral appendages less uniform than in Trilobita" (Størmer 1959, pp. O28-29).

Discussion: The Trilobitoidea was defined by Størmer to include a number of subclasses, of which the Marrellomorpha are the best known, primarily on account of the species Marrella splendens - most famously from the Burgess Shale. The other taxa he included within the Trilobitoidea - Burgessia, Sidneyia, Naraoia, and so on - are now thought to have affinities elsewhere within the Arthropoda.

* Størmer did not formally designate a type. However, the subclass Marrellomorpha (and so on down to the genus Marrella) is the first of several taxa listed immediately after.

Subphylum Arachnomorpha Heider 1913

1913 Arachnomorpha Heider
1944 Arachnomorpha Størmer
1973 Arachnata Lauterbach
1997 Lamellipedia Hou & Bergström
2000 Arachnomorpha Cotton & Braddy

Discussion: The Arachnomorpha is the clade comprising chelicerates and trilobites s.l., the latter in turn comprising the true trilobites (class Trilobita) plus some closely allied groups referred to an informal "trilobite clade" by Cotton & Braddy 2000.

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"Trilobite clade" Cotton & Braddy 2000

1944 Trilobitomorpha Størmer
2000 "Trilobite clade" Cotton & Braddy

Discussion: Cotton & Braddy 2000 considers the arachnomorphs with particular regard to the fossil stem group organisms, many of which are notorious problematica, and derives a phylogeny with the marrellomorphs at the base of two large clades: a trilobite clade and a chelicerate clade. The trilobite clade includes organisms such as Xandarella, Tegopelte, the helmetids, Nectaspida, and the true trilobites.

A case could be made for resurrecting the name Trilobitomorpha (Størmer 1944) and recognising this concept with the formal rank of superclass.

Class Trilobita Walch 1771

1771 Trilobita Walch

Description: Marine arthropods characterised by a generally subelliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes, and having a distinct, ralatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium).

Discussion: The Euarthropoda were always traditionally subdivided into a few classes, of which the trilobites have nearly always been one.

(Read more.)

Chelicerata Heymons 1901

1901 Chelicerata Heymons
1904 Euarachnida Lankester

Discussion: Chelicerata possess chelicerae, though when considering fossils these are often not preserved and accessory characteristics, median eyes and some degree of post-abdominal differentiation, may be more useful diagnostics.

It is within an extended "Chelicerate-Allided Clade" that a recent study (Cotton and Braddy 2000) has placed some of the more famous Burgess Shale-type problematica, including Leancoila, Yohoia, Fortiforceps, Sidneyia, and Emeraldella. However, this conclusion is predicated, in part, on the the classical view that chelicerates "have lost the antennae, and the chelicerae are homologous to the second cephalic appendages of antennate arthropods."

This contrasts with the conclusion of the Hox gene study conducted by Damen et al. (1998) that head segmentation homology extends to the chelicerates; specifically that the homologue of the first antennal segment in insects is not missing from the chelicerates.

Class Pycnogonida Latreille 1810

1810 Pycnogonida Latreille

Discussion: Although the sea spiders are not clearly tagmatised, their appendages are believed to be homologous to the chelicerate pattern.

Superclass Euchelicerata Weygoldt 1986

1986 Euchelicerata Weygoldt

Discussion: "[Eu]Chelicerates ... always have been considered as an unquestioned, basic monophyletic group. They share a distinct and conserved bauplan, the major features of which include a prosomal-opisthosomal subdivision and lack of antennae. Moreover, the analysis of the brain ganglia and the innervation of the head appendages (the chelicerae and the pedipalps) have suggested that chelicerates lack the homologue of the first antennal segment" (Damen et al. 1998, p. 10665).

Class Xiphosura Latreille 1802

1802 Xiphosura Latreille

Discussion: The earliest horseshoe crab fossils are very ancient, dating to at least the Ordovician. Represented today by three or so genera, they are the only living chelicerates with opisthosoma bearing book gills.

Class Eurypterida Burmeister 1943

? 1866 Gigantostraca Haeckel
1943 Eurypterida Burmeister

Discussion: The earliest eurypterid fossils date from the Ordovician. The group persisted into the Permian, occupying both marine and freshwater environments, and, in some species, reaching lengths in excess of two metres.

"The most recent complete revision of eurypterid classification was done by V. P. Tollerton, Jr., in 1989. He recognized a dozen superfamilies based on the morphology of the legs - whether they were spiny, similar in appearance, and whether the sixth pair of appendages were modified as paddles. He recognized two suborders, the Pterygotina with greatly enlarged pincer-like chelicerae and simple walking legs, and the Eurypterina in which the chelicerae are small and the forward legs are usually spiny.

"Unfortunately, there are no published cladistic studies on the Eurypterida, and only a single unpublished thesis attempting to produce a phylogeny of all the taxa. It therefore remains uncertain which groups and what characters are truly primitive among sea-scorpions" (Systematics of the Eurypterida, University of California, Berkeley, web site).

Class Arachnida MacLeay 1821

1821 Arachnida MacLeay

Discussion: The class Arachnida include the Araneae (spiders), Acari (ticks and mites), Scorpiones, Opiliones (harvestmen or daddy-longlegs), Thelyphonida (whip-scorpions), Pseudoscorpiones and others.

Arachnids are primitively terretrial (water mites have secondarily returned to an aquatic lifestyle); almost universally predatory or parasitic.

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Subphylum Mandibulata Snodgrass 1935

1935 Mandibulata Snodgrass

Discussion: The "mandibulate hypothesis" postulates a clade comprising the Crustacea together with the Atelocerata (insects plus the "myriapods"). The principal alternative view is the so-called APT hypothesis that the Crustacea form a clade with the Trilobita plus the Chelicerata. The former view is adopted here (also see fig. 1). 

Superclass Crustacea Pennant 1777

1777 Crustacea Pennant

Discussion: Crustacea have an extremely variable morphology, although all possess two pairs of antennae (at some stage in their life cycle). They are the dominant marine arthropods, and make up a significant portion of animal communities in all aquatic habitats.

The group includes the familiar lobsters, crabs, shrimp, and woodlice (pill bugs), as well as krill, barnacles, brine shrimp, copepods ostracods and many more.

Atelocerata Heymons 1901

1888 Antennata Lang
1893 Tracheata Pocock
1901 Atelocerata Heymons

Discussion: The three names, Antennata, Tracheata, and Atelocerata, are almost universally treated as synonyms; sometimes explicitly, sometimes not. The last, Atelocerata, seems to enjoy the widest usage, although it appears to have been the last name coined.

Occasionally the term Uniramia is also used in this context, as if it were a fourth sysnonym. However, Manton's (1972) definition of Uniramia included the Onychophora. Today, the Onychophora are almost universally regarded a belonging at or near the base of the arthropod clade; not within the Mandibulata (see fig. 1).

"The relationships of arthropods and their mode of head segmentation are the subjects of an on-going debate.... The major groups recognized are the chelicerates, myriapods, crustaceans, and insects. Traditionally, myriapods and insects have been grouped together into the Tracheata [= Atelocerata], but molecular phylogenies have suggested that the crustaceans should be seen as the sister group of insects instead" (Damen et al. 1998).< /p>< /p>< /p>< /p>< /p>< /p>

Class Chilopoda Latreille 1817

1817 Chilopoda Latreille

Description: Chilopoda (true centipedes) have bodies comprising many (up to 177) flattened segments; each segment, except the one behind the head and last two, bearing a single pair of appendages; the first pair of appendages on the trunk are modified into a pair of claws with poison glands, used for capturing prey (primarily invertebrates); the remaining appendages are legs, generally with a single claw at the end of each, which they walk on.

Superclass Progoneata Pocock 1893

1893 Progoneata Pocock

Discussion: As treated here (also in Zrzavý et al. 1997, p. 99; Giribet, Edgecombe and Wheeler 1999, fig. 4c) the Progoneata comprises the Symphyla, Pauropoda and Diplopoda. Edgecombe & Giribet 2002 notes, however, that "Pocock (1893) divided the Atelocerata into Opisthogoneata (Chilopoda + Symphyla + Hexapoda) and Progoneata (Pauropoda + Diplopoda) ... [implying] the ‘Myriapoda’ are paraphyletic if the opisthogoneate and progoneate groups are both themselves monophyletic, the former being especially doubtful" (p. 145).

Superclass Hexapoda Blainville 1816

1758 Insecta Linnaeus
1816 Hexapoda Blainville

Discussion: This is another case where the synonym which seems to have priority, Insecta L., is more frequently overlooked in favour of a later term. Although the ICZN does not govern names above the rank of family, nevertheless, it is usual practice to converse names and defer to priority.

The Hexapoda comprises five or six classes, several of which were formerly batched up in the paraphyletic "class" Apterygota.

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Burmeister 1943

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Grube 1853

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Latreille 1802

Latreille 1810

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Latreille 1829

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MacLeay 1821

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Manton 1977

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Pennant 1777

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Spallanzani 1776

Størmer, Leif 1944

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von Siebold; Stannius 1845

Walch 1771

Weygoldt 1986

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