Australasian Science: Australia's authority on science since 1938

The Rise of Arthropods

By John Long

Spectacular arthropod fossils have shed light on their early anatomy, and might one day help resolve the mystery of their distant origins.

Arthropods are joint-legged animals, one of the most successful animal groups on the planet. They include insects, crustaceans, spiders, scorpions, millipedes, centipedes and many other kinds of living and extinct animals, all of which share segmented jointed legs and an outer exoskeleton of a special kind of chitin, which they moult to grow.

Love them (prawns, crayfish) or hate them (spiders, ticks, cockroaches), they form an integral part of our daily lives, as much of our food depends upon their behaviour. One-third of all the food plants we eat relies upon pollination by bees, and at various times our crops are devastated by swarms of hungry locusts.

The arthropods account for approximately 80% of all known described species of animals, yet some scientists estimate there are perhaps 5–10 million undescribed forms. However, the really big question for evolutionary biologists is: when did arthropods first arise, and how?

The fossil record of arthropods is now very well-known, ranging from the many species of trilobites living in Palaeozoic seas through to superb 3-D insects preserved in amber.

Our earliest examples of arthropods come from the beginning of the Cambrian Period about 530 million years ago. This was a time when many arthropods, such as the first trilobites, had developed the hard chitinous exoskeleton, yet there was also a great diversity of other types of arthropods around.

“Soft-bodied” arthropods like Anomalocaris and its kin included filter-feeding giants as well as voracious predators with keen eyesight. Recent discoveries from Kangaroo Island in South Australia have shown that Anomalocaris had up to 32,000 lenses in their pear-shaped compound eyes, giving them acute vision as sharp as that of a modern dragonfly.

Other recent finds have shown that well-preserved arthropods like the vaguely shrimp-like Fuxianhuia from Yunnan, China, actually had carbonised outlines of the vascular system preserved (Nature Communications, doi:10.1038/ncomms4560). Earlier work by the same team identified the outline of the brain and optic lobes in the same species.

Such extraordinary detailed preservation of soft tissue anatomy in the earliest known arthropods shows they were highly complex animals, moreso than previously suggested, and that the standard arthropod body plan had fallen into place pretty soon after the group had first evolved.

Up until a few years ago the relationships of living arthropods were unresolved. A major molecular phylogeny by a US team was the first to show that all insects are a subgroup within the crustaceans (Nature, doi:10.1038/nature08742). This supports earlier ideas that primitive crustacean-like forms, such as the peculiar euthycarcinoid Kalbarria from Western Australia, were possible ancestors to the first insects. Insects invaded the land about 420 million years ago, much the same time as the first plants were colonising narrow areas of land alongside rivers, lakes and volcanic springs.

A recent review of the origin and early evolution of arthropods by Dr Greg Edgecombe of the Natural History Museum, London, and Dr David Legg of Oxford University (Palaeontology, doi: 10.1111/pala.12105) reconciles data from the most recent molecular studies with data from the palaeontological record, and concludes that divergence times for the origins of the group reside firmly back in the late Ediacaran period about 540–580 mya. There are currently no clear arthropod fossils from this age, although arthropod-like interpretations have been ascribed to some enigmatic forms in the Ediacaran fauna in South Australia – one of the oldest and most diverse assemblages of multicellular animals on the planet. We can only speculate they were there as small, non-chitinous forms, so they didn’t leave an adequate fossil record.

At a recent talk by Dr Jim Gehling of the South Australian Museum, who has been studying the Ediacaran sites for more than three decades, I heard talk of mobility in Ediacaran organisms. Jim showed us pictures of trails in the sediment made by little animals the size of rice grains. Most likely these trails were made by small worms or even early arthropods.

We won’t resolve this issue until unambiguous fossils are found from these beds that prove arthropods were present – a problem that molecular studies can’t shed any further light on. Continued field work and many trained eyes scanning Ediacaran age rocks in the field could one day provide the ultimate answer to this problem.

John Long is Strategic Professor in Palaeontology at Flinders University.