What are arthropods?

Arthropods are invertebrates with jointed legs. They make up about 75% of all animals on Earth and have a major role in maintaining ecosystems as pollinators, recyclers of nutrients, scavengers and food for other animals. They include many animals we come across in our gardens, such as spiders, ants, centipedes and slaters. Arthropods are divided into four major groups:

• insects;
• myriapods (including centipedes and millipedes);
• arachnids (including spiders, mites and scorpions);
• crustaceans (including slaters, prawn and crabs).

Arthropods Timeline

Arthropods have been around for a long time. Their ancestors originated 530 million years ago in Cambrian oceans - at a time that is still only incompletely understood. Arthropods exist today because they have successfully adapted to changing environments during this long period of time.

Arthropods Distribution

Although arthropods are everywhere, we do not know how the different groups are related. Scientists have proposed many conflicting ideas about how arthropods evolved and diversified. Researchers at the Australian Museum have been studying the relationships between groups of arthropods. They have attempted to resolve the problems of arthropod phylogeny by re-examining available information using new technology and by studying some recently discovered Cambrian fossils. The new technology used by the researchers included: DNA sequencing, electron microscopy and computer-aided phylogenetic analysis. The Australian Museum is in a unique position to tackle the problem of arthropod phylogeny because it has specialists in all major groups of arthropods.

Animals living today have a great deal of evolution behind them, so finding information about the deep-branches in the arthropod phylogenetic tree is difficult. To improve the chances of finding clues to these early branches, researchers study the phylogenies of each arthropod group and then choose only those animals that are most likely to provide information about their ancestors. Such animals include horseshoe crabs (or king crabs), scorpions, velvet worms, leptostracan 'shrimps' and wingless insects, such as silverfish.

Our understanding of arthropod phylogeny is quite unresolved and finding concrete answers to this problem will have important consequences for understanding biodiversity.

Key Definitions

Words to know:

• INVERTEBRATES - animals without a backbone.
• CAMBRIAN PERIOD- time between 544 and 500 million years ago.
• PHYLOGENY - an evolutionary history, often displayed graphically as a tree.