Monday, 14 September 2015

Extending The Lineage Of The Lobopodians

A modern onychophoran or velvet worm, Peripatus
The arthropods are the largest and most diverse group of animals, possessing segmented bodies characterised by segmented limbs.

Their closest relatives are the onychophorans, better known as velvet worms. Their bodies are segmented, but the limbs are simple tubes, rather than the complex, jointed appendages of the arthropods. Both originated during the Cambrian Explosion.

Yet when palaeontologists re-examined fossils which recorded this event, they found something surprising: creatures which blended anatomical aspects from both groups.

Known as the lobopodians, they possessed distinctly arthropod-like bodies, but the limbs were simple stumps like those of the onychophorans. There is still a debate as to whether the lobopodians are a group of onychophorans on their way to becoming arthropods, a group of primitive arthropods or even a completely separate and intermediate phylum. What is clear, however, is that they represent a link between the two groups. Unlike the arthropods and the onychophorans, the lobopodians have no living descendants.

A-E is the Carboniferous long legged lobopodian Carbotubulus
while F-G is of the short legged lobopodian Ilyodes inopinata
The lobopodian fossil record is most extensive during the Cambrian, but a few groups survived into the Ordovician, as shown by recent discoveries of anomalocarids from the Ordovician of Morocco. Lobopodian with Cambrian body types endured even longer than was previously thought.

Uncovered from the Carboniferous Mazon Creek formation, the lobopodian possesses long limbs like those seen in its Cambrian forebears, in contrast to the short, stubby limbs of onychophorans and other lobopodians.

The Mazon Creek formation is known for the exceptional preservation of its fossils which are found within nodules of phosphate. The new lobopodian was similarly well preserved, giving excellent insight into its anatomy. Named Carbotubulus waloszeki, it possessed a worm-like body with ten pairs of long, tubular limbs. Its distinctive anatomy extends the record of long-limbed lobopodians by 200 million years into the latest Carboniferous 296 million years ago. While more study is needed, our broadened understanding of the temporal distribution of the lobopodians will impact the way we view the long term effects of the Cambrian Explosion, in particular the role of body plans to determine post-Cambrian ecological trends.