An artist's impression of Pambdelurion whittingtoni

An artist's impression of the head of the ancient arthropod Pambdelurion whittingtoni. Today arthropods form a huge group of animals including insects, crustaceans and spiders. © Bob Nicholls

Arthropod ancestor had the mouth of a penis worm

New research reveals a 515-million-year-old mouth with rings of sharp teeth belonged to an ancient arthropod, giving clues to the ancestral origins of this feature.

Until recently, the fossilised mouthpart was thought to belong to a priapulid - or penis worm - as this species also has a round mouth with sharp teeth.

But in the new study, the authors show the mouth more strongly resembles that of Pambdelurion whittingtoni, a primitive arthropod from the Cambrian Period (541 to 485 million years ago).

'The mouth is so similar to that of P. whittingtoni that they have to be from the same kind of animal,' says Dr Greg Edgecombe, Museum palaeobiologist and one of the study's co-authors.

'But it still closely resembles the mouth you find on a penis worm - right down to details of the rings of teeth and plates.

'This suggests these circular mouths were inherited from the two groups' last common ancestor, which would have lived some 540 million years ago.'

Penis worm

Penis worms, such as Priapulus caudatus, also have circular mouths with rings of teeth © Jakob Vinther and Fredrik Pleijel


Size matters

When scientists discovered the fossilised mouthpart in 1994 in Chengjiang County, China, it was thought to belong to a species from the ancient arthropod family Anomalocarididae.

However, it was later reinterpreted as the mouth of a new penis worm species, Omnidens amplus.

The huge size of the mouth, which contained individual parts up to 47 millimetres long, indicated that O. amplus was around 2 metres long - the largest penis worm in history.

This new research, led by researchers at the Natural History Museum and the University of Bristol, compares the mouth to fossils of P. whittingtoni unearthed in Greenland, and suggests that both assessments were slightly off.

'The mouth from Chengjiang and the mouths found on the fossils from Greenland are so similar, it's hard to argue that the animal we know as Omnidens was anything other than a Pamdelurion­­-like creature,' says Dr Edgecombe.

While fossil records hint that P. whittingtoni could have grown to 55 centimetres in length, the sheer size of the Chengjiang mouth suggests it belonged to a creature around 1.5 metres long - making it the largest animal around during the Cambrian Period.

'It had a large pair of spiny appendages on its head, and flaps running down its body,' explains Dr Edgecombe. 'The mouth worked with the appendages to allow it to capture and consume prey.

'At a time when the diversity of life was expanding rapidly, these features gave the species a competitive edge.'

Fossilised Pambdelurion mouth and diagram

The fossilised Pambdelurion whittingtoni mouth (left) and an illustration of what it may have looked like (right)


Early ancestor

The team's analysis means the fossilised mouthpart of P. whittingtoni has once again been identified as belonging to an arthropod.

Since similar round mouths are found in many groups related to arthropods - such as tardigrades and penis worms - this suggests the structure of a radial mouth is ancient, passed down from the ancestor of all these groups.

According to the fossil record and DNA analysis, this creature would have lived around 540 million years ago as the ancestor for the entire super-phylum Ecdysozoa, which includes all animals that shed their exoskeleton.

'It's an exciting finding,' says Dr Edgecombe. 'Now we know that this mouth belonged to an arthropod species, we can point to examples of radial mouths right across these groups.

'This strongly suggests that these are inherited from the ancestor that unites them all - the ancestral ecdysozoan.'

The findings are published in the journal Palaeontology.

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