Invertebrates: Arthropods

The name, arthropod, means jointed foot, and all of the many and varied groups with Phylym Arthropoda are characterised by jointed limbs and jointed bodies. The body of an arthropod has a jointed exoskeleton that is mostly made up of chitin. There are more species of arthropods on Planet Earth, and more individual animals belonging to the phylum than any other group of animals. They occupy a huge range of habitats from the tops of the highest mountains to the deepest parts of the ocean and everywhere in between.

There are many different ways of classifying arthropods but we follow that used in Species 2000, the New Zealand Inventory of Biodiversity: Kingdom Animalia. Edited by Dennis Gordon and published in three volumes between 2009 and 2012, this inventory attempted to catalogue every plant and animal living in the New Zealand region, including estimates of organisms that have not yet been identified and described. In the Introduction to Arthropoda, the modern New Zealand fauna is estimated to include 3000 species living in marine environments, 1250 species in freshwater and a huge 18,000 occupying terrestrial habitats. Around 800 fossil arthropods are listed as of 2009; however, research on newly discovered spiders and insects housed in the Geology Museum collections at the University of Otago is steadily increasing this number.

We follow Species 2000 in assigning the fossils to one of four Subphyla: Trilobitomorpha (trilobites); Chelicerata (arachnids: spiders and mites); Crustacea (shrimps, crabs, lobsters, barnacles and their kin) and the biggest group by far, Hexapoda (insects of a myriad different kinds).

Arthropods have a very long fossil history, extending back to the earliest times of multicellular life. The earliest arthropod fossils found in New Zealand are the extinct trilobites which date back to the Cambrian.

Fossil Chelicerata in the Geology collections include several spiders, one selected as a treasure, a few pseudoscorpions and many minute mites, mainly trapped in amber, that are yet to be formally described.

The best-known fossil crustaceans are decapods (the word means 10-footed)  – examples are lobsters, crayfish, crabs and shrimps. A few live on land or in freshwater but most of the 10,000 living species occupy a wide range of environments in the ocean. About 500 living species of decapods are known from New Zealand and about 100 fossil species have been recorded.

The Geology Museum has fine examples of holotypes of four different groups of decapods. Those described and illustrated here include a freshwater crayfish (koura) from Central Otago, a species of hermit crab from Southland preserved in its gastropod home, a large crab, and and a palinuran or spiny lobster from South Canterbury.