Palæos:		Arthropoda
METAZOA		Arthropoda

Page Back	Unit Back	Metazoa	Cladogram	References	Taxon Index	Pieces
Page Next	Unit Next	Unit Home	Unit Cladogram	Unit References	Glossary	Time

The Arthropoda

animals with jointed exoskeletons

Protostomia
|--Arthropoda  
|  |--Trilobita
|  `--Euarthropoda 
|     |--Myriapoda
|     `--+--Chelicerata
|        `--Pancrustacea
`--Lophotrochozoa

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

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 Uniramia (especially insects) on land.

Phylogeny

Phylogeny of Crown-Anthropoda after Conway Morris and Hou & Bergström

<==o ARTHROPODA  [polyphyletic??]
   |?-+?- TARDIGRADA 
   |  `--+-- LOBOPODA 
   |     `-- UNIRAMIA (myriapods & insects)
   `--+--o †ANOMALOCARIDA
      `-- SCHIZORAMIA (crustaceans & chelicerates)

note: See Panarthropoda page for somewhat more detailed cladogram

The above presents a paleontological perspective, and retains the Uniramia. Recent work with molecular analysis presents a different picture, although there is still no firm agreement. From the 13 September 2001 issue of Nature:

"Most recent studies suggest a close link between insects and crustaceans, but the relationships of myriapods (centipedes and millipedes) and chelicerates (spiders and allies) with this 'Pancrustacea' group have been contentious. Hwang et al. present molecular evidence for a close link between myriapods and chelicerates. In contrast, Giribet et al. mix molecules and morphology in a bold 'total evidence' approach that produces a more traditional arrangement, with myriapods closer to insects and crustaceans, and chelicerates lying further away."

Alternative phylogeny of Arthropoda after Hwang, et al., 2001

<==o ARTHROPODA 
   |--+-- MYRIAPODA 
   |  `-- CHELICERATA 
   `--o PANCRUSTACEA
      |-- HEXAPODA 
      `-- CRUSTACEA 

Alternative phylogeny of Arthropoda after Giribet, et al., 2001

<==o ARTHROPODA 
   |-- LOBOPODA
   `--+-- TARDIGRADA 
      `--o EUARTHROPODA
         |-- PYCNOGONIDA 
         `--+-- EUCHELICERATA 
            `--o MANDIBULATA
               |-- MYRIAPODA
               `--o PANCRUSTACEA
                  |-- HEXAPODA 
                  `-- CRUSTACEA 

Links

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

Phylum Arthropoda

Arthropoda (jointed-leg animals) - Specimens database at Marine Biological Lab (MBL), Woods Hole

Giribet G, Carranza S, Baguna J, Riutort M, Ribera C - First molecular evidence for the existence of a Tardigrada + Arthropoda clade.- Mol Biol Evol 1996 Jan;13(1):76-84     Use of rDNA and rRNA sequencing to determine phylogenetic relationships.  The results are surprising.  From the abstract:

The results show that arthropods are monophyletic. Tardigrades represent the sister group of arthropods (in parsimony analyses) or they are related with crustaceans (distance analysis and PRN).  Arthropoda are divided into two main evolutionary lines, the Hexapoda + Crustacea line (weakly supported), and the Myriapoda + Chelicerata line. The Hexapoda + Crustacea line includes Pentastomida, but the internal resolution is far from clear. The Insecta (Ectognatha) are monophyletic, but no evidence for the monophyly of Hexapoda is found. The Chelicerata are a monophyletic group and the Myriapoda cluster close to Arachnida.