Onychophora as Missing Links
Velvet Worms, Water Bears, and Arthropods are Closely Related
Velvet Worms have not changed much for millions of years, and they are a reminder of the first land animals.
Velvet Worm Biology
Onychophorans (Velvet Worms) look rather like a cross between a caterpillar and an earthworm with legs. They are only found on land, usually in the damp moss and leaf litter of warm places.
- All are predatory, and they have an unusual way of capturing their prey – they squirt sticky glue at them and then devour the helpless victim with their sharp jaws.
- Most are around 5 cm long, but a few species can grow to four times this size – all are rare, and most are endangered. There are certainly over 100 species, and possibly a lot more yet to be described (because their rarity and habitat makes finding them difficult).
The first fossil Velvet Worms were marine, but they are all now extinct and the only survivors live on land. It is thought that the species that first made the transition from sea to land (initially by adapting to life between the tides) was the first animal to walk on land – long before the insects evolved.
Velvet Worms as Missing Links
Because the Onychophora are an ancient group (there are fossils over 500 million years old), and because the modern forms do not seem to have changed very much over all this time, Velvet Worms have often been hailed as a ‘missing link’. At one time they were thought to be an intermediate stage in the evolution of the arthropods (crabs, spider, insects etc.) from the annelids (the true worms). Much about them does look like a worm in the early stages of growing legs, but recent molecular studies have come up with a different story.
Molecular Studies and Velvet Worm Classification
DNA studies suggest that Velvet Worms are closely related to Water Bears (tardigrades), and that both are also close to the arthropods. The old suggestion (that these three phyla are close to the annelids) has fallen out of favour, and it is now believed that they are nearer the Round Worms (Nematoda) and Horsehair Snakes (Nematomorpha).
In the past it was only possible to infer relationships between different groups of animals by studying their body structure, and that of their fossil ancestors. Now it is becoming routine to look at the details of their chemistry (largely DNA and RNA, but also the proteins and other molecules that this genetic material codes for), and this new way of studying the living animals is throwing up surprises. Some of the simplest animals seem to have unexpected relatives, and it looks as if similar structures and body plans have sometimes been developed independently in unrelated groups (in much the same way that bats, birds, and insects have all got wings).
Evolution proceeds by moving towards better adaptation, and sometimes this involves the loss of complexity – what looks simple and ‘primitive’ often represents the loss of unhelpful structure and could be considered ‘advanced’.
Main Reference: New Zealand Onychophora – Landcare Research NZ
Velvet Worm
|
What do you think about this article?
More in Science & Nature
Latest Articles