Palaeos		Arthropoda
Metazoa		Contents

The Arthropoda

animals with jointed exoskeletons

ECDYSOZOA 
|==Dinocaridida
|
`--ARTHROPODA
   |--Fuxianhuiida
   `--+--Candaspidida
      `--+--Marellomorpha
         |--Megacheira
         `--+-Chelicerata
            `--+--Trilobita
               `--Mandibulata
                  |--Myriapoda
                  `--+--Crustacea
                     `--Hexapoda

Taxa on This Page

1. Arthropoda

This page is still under construction and revision, but has been erected as a provisional and partial updating of the earlier Palaeos pages on this subject. Note that most of the links here will only work when the other pages are uploaded. MAK120515

The Arthropods

The arthropods constitute the most successful and diverse form of life on Earth, constituting millions of species in a diverse range of habitats, and varying in size from microscopic to quite large.

The name "arthropod" comes from the Greek, arthros jointed, and podes, feet.  Hence, jointed feet.  All arthropods possess a jointed rigid exoskeleton, which both supports the musculature and protects the animal.  The exoskeleton is formed by the cuticle, a protective layer composed mostly of chitin, long chained molecules of the sugar acetyl glucosamine, which when cross-linked form tough fibers embedded in a protein matrix

As the animal grows it has to shed its exoskeleton at regular intervals, a process known as molting.  During this time it is vulnerable to predators and disease, and also to desiccation if it is a  terrestrial form like an insect or spider.  Because the larger the animal the longer it takes for the new molt to harden, and hence the longer the time it is vulnerable, this process imposes limits on the size arthropods grow to.  Another limit with terrestrial forms like insects is that breathing is through pores in the side of the body.  Hence arthropods are mostly small, even microscopic, with very few growing to giant size.  Science fiction giant ants and the rest are simply physiologically impossible (though that never stopped sci fi writers in the past).

Primitive arthropods, like trilobites and some crustaceans, have many body segments and many pairs of similar legs.  In more advanced forms there is the tendency of the segments to fuse into functional groups, called tagmata (sing. tagma) such as the head and thorax of an insect or the carapace of a crab.

Taxonomy/Systematics

Arthropods are variously classified.  For example the "mandibulates" (e.g. insects) posses jaws (mandibles) and so are distinguished from the "chelicates" which developed pincers instead (e.g. spiders).  Conversely, there are those arthropods that possess two pairs of antennae (biramous) as opposed to those with one pair ("uniramous").  There are also embryonic differences as well.  crustaceans for example have a unique type of larvae, called the nauplius.

Because of all these differences there was for some time a controversy over whether the arthropods constitute a valid monophyletic group, or whether they evolved from separate, even unrelated. soft-bodied ancestors.  The late English zoologist S. M. Manton ref, [ref] argued persuasively that the arthropods consist of three distinct groups that evolved from separate ancestors - the Uniramia, Biramia (Crustacea), and Chelicerata, each of which constitutes a distinct phylum.  Others have argued on the basis of molecular and anatomical evidence that the arthropods did indeed evolve from a common ancestor.  Nowadays Manton's polyphyletic view of the arthropods is out of favor.  One of the more recent theories of arthropod relationships has even involved splitting up the Uniramia by aligning the Hexapoda with the Crustacea rather than the Myriapods (the Pancrustacea hypothesis) [2].

Today there is still little agreement regarding arthropod phylogeny between what could be called the minority paleontological and the majority molecular paradigms

Evolution and Fossil Record

The first arthropods appear in the early Atdabarian epoch (early Cambrian) although arthropod tracks are known from the Tommotian.  It is almost certain that the first arthropods evolved around the Poundian-Tommotian boundary ( some 545 million years ago).  For the first ten million years or so they were probably soft-bodied forms.  During the Atdabarian the first hard-bodied armoured trilobites appeared, but these were still a minority in relation to the soft-skinned forms.  The various lineages of arthropods underwent an explosive evolutionary radiation, but many of these types died out at the end of the Cambrian.  The survivors flourished and have made up the bulk of invertebrate faunas ever since - Trilobites and then Crustacea in the seas, and Chelicerates (especially mites and spiders) and myriapods and hexapods (especially insects) on land.

Descriptions

Arthropoda von Siebold and Stannius 1848 (= Euarthropoda Lankester 1904)

Range: Fr ECambrian

Phylogeny: Panarthropoda : Aysheaia + (Tardigrada + (Cambrian lobopodians + Onychophora + (more Cambrian lobopodians + (Siberiids + (Dinocaridids + (Schinderhannes + * : Fuxianhuiida + (Candaspidida + (Marellomorpha + Megacheira + (Chelicerata + (Trilobita + Mandibulata))))))))))

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).

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). - Chris Clowes

Links: Phylum Arthropoda: Glasgow University Zoology Museum, Scotland; Introduction to the Arthropods; Phylum Arthropoda - brief but useful overview of various groups, especially Crustacea - part of the MEER site; Arthropoda Phylum Overview; Arthropoda - Tree of Life project; Phylum Arthropoda Animal Diversity Web; Arthropoda (jointed-leg animals) - Specimens database at Marine Biological Lab (MBL), Woods Hole