Palæos:		Arthropoda
INVERTEBRATES		Arachnomorpha

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Arachnomorpha

(Trilobita + Chelicerata)

A comprehensive study by Wills et al. rejects the Mandibulata and posits instead an Arachnomorpha that includes chelicerates, trilobites, and their early stem groups form) as a sister to Crustacea, the two together constituting a revamped, biramous Schizoramia.

As such, "Arachnomorpha" or "Arachnata" constitutes a broad taxon that includes the Trilobites, the Chelicerata and a number of primitive Cambrian to Devonian forms that are usually included in the Trilobitomorpha or the Xiphosura.

Arachnomorph Relationships

"A 'big-hand' for the chelicerates?: The phylogeny of arachnomorph arthropods and the origins of the Chelicerata"

T. J. Cotton

"The arthropod clade Arachnomorpha includes all taxa more closely related to chelicerates than to crustaceans. A new hypothesis of the relationships between arachnomorph arthropods is presented based on a cladistic analysis of 34 taxa and 53 characters. This study is considered to be superior to previous studies in providing detailed discussion of primary hypotheses of homology and by including a more complete range of terminal taxa. This analysis provides, for the first time, convincing synapomorphies for the Arachnomorpha and suggests that the marrellomorphs are not arachnomorphs. The assignment of Cambrian 'great appendage' (or megacheiran) arthropods to the Arachnomorpha is confirmed and potential synapomorphies uniting them and chelicerates are discussed and tested. Principal amongst these are the loss of the first cephalic appendages (the antennae/antennulae), loss of the exopods of the second cephalic appendages and modification of the endopods of these appendages into spinose grasping organs. The relationships of trilobites within a clade including naraoiids, helmetiids, tegopeltids and xandarellids are more fully resolved than in previous studies. This phylogenetic hypothesis is used to re-examine claims that unusual evolutionary processes were operating during the 'Cambrian explosion' of arthropods. A new phylogenetic classification of the Arachnomorpha abandons the vast majority of previously erected higher taxa as superfluous."

from  The Systematics Association - 3rd Young Systematics Forum, 2000 (former site)

Class Marrellomorpha Raymond 1935

The Marrellomorpha are a group of small unusual short-bodied forms, previously relegated to the "trilobitomorph" wastebasket, but now considered to be basal cheliceratan (or Arachnomorph) forms. The taxon (ranked as a subclass) is defined in the Treatise as:

"Trilobitomorpha with cephalic shield prolonged into flat horns; trunk with numerous free tergites and small telsonic plate; two pairs of antennae, other appendages of trilobitic type"

This definition only applies to the most typical (and earliest) order, the Marrellida (represented by the well-known Burgess Shale species Marrella splendens). But since then several other taxa have been believed to be part of the marrellomorph radiation. The Synopsis of the described taxa of the World list gives the following classification of the Marrellomorpha.

Class: Marrellomorpha
          Order: Marreliina
          Order: Mimetasterida
          Order: Acercostraca
          Order: Cyclina

Subclass Merostomoidea Størmer 1944

Included around the base of the "Crown Group" Chelicerata are a number of primitive forms called "Merostomoidea" - which seem to be transitional between Trilobites and Chelicerates proper. This taxon is defined in the Treatise as:

"Trilobitomorpha with eurypterid or xiphosurid type of body, trilobation more or less distinct; tergites of trunk free or ankylosed into continuous shield, telson mostly styliform; 1st and last pair of trilobite appendages may be modified and others partly reduced"

Which doesn't really tell us much, because many of these characteristics are shared by a number of fairly unrelated Cambrian arthropods.

Leif Størmer, who authored the above section of the Treatise, includes four orders, all at the time known only from the Burgess Shale, in this subclass:

Class: Merostomoidea 
          order: Limulavida 
          order: Emeraldellida
          order: Nectaspida 
          order: Leanchoiliida

The classification according to Valid PMPD Orders (site no longer available) elevated the Merostomoidea to the rank of class and reduces the number of orders to three

Class: Merostomoidea 
          Order: Limulavida 
          Order: Emeraldellida
          Order: Cheloniellida 

The Aglaspida and tentatively the Leanchoiliida they place in the class Merostomata, along with the Xiphosurida, Chasmataspidida, and Eurypterida (the latter three everyone agree are all true Chelicerata)

While the Synopsis of the described taxa of the World list has the following arrangement.

Class: Merostomoidea
          Order: Limulavida
          Order: Emeraldellida
          Order: Naroidea (= Nectaspida)
          Order: Cheloniellida

and moving the Leanchoilida to the class Xiphosura

Phylogeny

Arachnomorpha after Dunlop and Selden 1997

According to Dunlop and Selden the Strabopida are the most primitive of the Arachnomorpha, and could have given rise to both chelicerates and aglaspidids. The Cheloniellida, Aglaspidida and Chelicerata represent three distinct arachnomorph taxa, with the Cheloniellida constituting the sister group to the Chelicerata.

<==o ARACHNOMORPHA 
         |-- †MARRELLOMORPHA 
         |-- †TRILOBITA 
         |-+-o †STRABOPIDA
         | | `-- †Strabopidae [Paleomeridae]
         | |  |-- †Xandarellidae
         | |  `-- †Almeniidae
         | `--o †AGLASPIDIDA
         |    `-- †Aglaspididae
         `--+--o †CHELONEILLIDA
            |  |-- †Neostrabops
            |  |-- †Duslia
            |  |-- †Triopus
            |  |-- †Pseudarthron
            |   `-- †Cheloniellon
            `-- CHELICERATA

Superclass Lamellipedia Hou & Bergström, 1997

Hou & Bergström, 1997 use the Superclass Lamellipedia, which seems to be pretty much the same as Arachnomorpha (Trilobita + Chelicerata + assorted basal forms). it is divided into the following classes:

Superclass: Lamellipedia 
          class Marellomorpha 
          class Artiopoda
          class Trilobita 
          class Chelicerata 

The Artiopoda include many of the "Trilobitomorpha" (primitive trilobite-like forms)

Phylogeny after Hou & Bergström, 1997

Here the Cheloniellida occupy a more primitive position, the Strabopida are more advanced (the sister group to the Chelicerata). There are also a large number of other taxa.

<==o LAMELLIPEDIA sensu Hou & Bergström, 1997
   |--o †MARRELLOMORPHA 
   |  |--o †MARRELLIDA
   |  |  `-- †Marrellidae
   |  `--o †MIMETASTERIDA
   |     `-- †Mimetasteridae?
   `--o ARTIOPODA 
      |--o †NECTOPLEURA 
      |  |--o †NECTASPIDA
      |  |  |-- †Orientellidae
      |  |  `--+-- †Liwiidae
      |  |     `-- †Naraoiidae
      |  |--o †VACHONISIIDA
      |  |  `-- †Vachonisia rogeri
      |  |--o †ACERCOSTRACA
      |  |--o †HALICYNA
      |  `--o †RETIFACIIDA
      |     `-- †Retifaciidae
      `--+--o †CONCILITERGA 
         |  `--o †HELMATIIDA
         |     |-- †Helmitiidae
         |     `--+-- †Skioldiidae
         |        `--+-- †Tegopeltidae
         |           `-- †Saperiidae
         |--o †PETALOPLEURA 
         |  |--o †XANDARELLIDA
         |  |  |-- †Xandarellidae
         |  |  `-- †Almeniidae
         |  |--o †CHELONEILLIDA
         |  |  `-- †Cheloniellidae
         |  `--o †SINOBURIIDA
         |     `-- †Sinoburiidae
         |-- †TRILOBITA 
         `--+--o †XENOPODA 
            |  |--o †EMERALDELLIDA
            |  |  `-- †Emeraldellidae
            |  `--o †LIMULAVIDA
            |     `-- †Sidneyiidae
            |--o †AGLASPIDIDA
            |  |--o †AGLASPIDIDA
            |  |  |-- †Aglaspididae
            |  |  `-- †Lemoneitidae
            |  `--o †STRABOPIDA
            |     `-- †Strabopidae [Paleomeridae]
            `-- CHELICERATA

Notes

The  Synopsis of the described taxa of the world list gives the following classification of the Chelicerata, including trilobitomorphs and Merostomoidea.

Class: †Euthycarcinoidea
Class: †Sanctacaria
Class: Pycnogonida
          Order: Pycnogonida
          Order: †Palaeopantopoda
          Order: †Palaeoisopoda
Class: Xiphosura
          Order: †Chasmataspidida
          Order: Xiphosurida
          Order: †Leanchoilida
Class: †Eurypterida
Class: †Merostomoidea
          Order: †Limulavida
          Order: †Emeraldellida
          Order: †Naroidea
          Order: †Cheloniellida
Class: †Marrellomorpha
          Order: †Marreliina
          Order: †Mimetasterida
          Order: †Acercostraca
          Order: †Cyclina
Class: †Pseudocrustacea
          Order: †Burgessida
          Order: †Waptiida
Class: Arachnida