Sep 4, 2014

Hallucigenia – the claws have it sorted?


One of the most iconic of Cambrian age fossils from the famous Burgess Shale is a little (3.5 cm long) creature called Hallucigenia, which has been the subject of considerable controversy for several decades. Now, researchers, Martin Smith and Javier Ortega-Hernandez from the Department of Earth Sciences have reinvestigated this strange creature and its relationship to other animals.
Category: 2014
Posted by: Sarah

Hallucigenia sparsaHowever unlikely it might seem, their new analysis of its preserved anatomy firmly connects the extinct seabed dwelling Hallucigenia to another strange but living group of animals called the onychophorans or velvet worms. Sometimes described as ‘worms with legs’, today’s onychophorans are found living amongst the leaf litter of tropical forests and are widespread throughout the southern hemisphere.
The aptly named Hallucigenia is a very curious little worm-like fossil with legs and spikes along its back. It was first discovered by the American palaeontologist Charles Doolittle Walcott (1850-1927) over 100 years ago, in 505 million year old seabed strata of the Burgess Shale, which outcrop high in the Canadian Rocky Mountains of British Columbia. At the time Walcott thought that it really was a marine polychaete worm belonging to a genus called Canadia.
The name Hallucigenia was first given to these fossil remains by Simon Conway Morris back in 1977 when he was reviewing a number of Walcott’s ‘worm-like’ fossils because it had no resemblance to any living worms. As Conway Morris wrote, the name ‘refers to the bizarre and dream-like appearance of the animal’, which did not seem to fit into any known animal group. Like a number of other fossils from the Burgess Shale, Hallucigenia was seen as an ‘evolutionary misfit’, which appeared from nowhere and led nowhere.
However, over the intervening decades new and better preserved soft-bodied, worm-like fossils were discovered in the slightly older Cambrian strata of Chengjiang in China. One of these, called Microdictyon, has a caterpillar-like body with paired legs and pairs of armoured body plates along its back. In 1991, the Swedish palaeontologist, Lars Ramsköld and his Chinese colleague Hou Xianguang were first to suggest that this curious armoured ‘worm with legs’ (lobopodian) should be ‘grouped with the extant terrestrial velvet worms in the phylum Onychophora’. In addition, they suggested that if Hallucigenia be turned upside down and back to front, so that the stiff spiky legs became protective spikes along its back and the tentacles thought to protrude from its back became paired legs, then it bore a striking resemblance to Microdictyon and should also be seen as an onychophoran. The following year Ramsköld carried out some very careful ‘dissection’ of the original fossil specimen of Hallucigenia and show that what had looked like a single row of ‘tubercles’ was in fact a double row and could therefore be seen instead as Microdictyon-like legs.
Over the last two decades, many more fossil lobopodians have been discovered and the relative position of this extinct group to the wider grouping of the Panarthropoda, has been questioned. The panarthropods include the living Onychophora (velvet worms), Tardigrada (water bears) and the Euarthropods (the crown group that includes all living arthropods). Although Cambrian lobopodians, such as Hallucigenia, have previously been considered as onychophoran ancestors, the presence of primitive characters such as a non-segmented body and terminal claws on the walking legs, has suggested a deeper position in the early evolution of the wider arthropod grouping of the Panarthropoda.
The new study and analysis by Martin Smith and Javier Ortega-Hernandez has examined the terminal claws to the legs of some exceptionally preserved specimens of Hallucigenia from the Burgess Shale. Microscopic examination shows that the claws of the extinct Hallucigenia and those of the living onychophoran Euperipatoides have a closely similar (homologous) stacked growth structure, showing that the hallucigenid lobopodians are indeed stem onychophorans.
                         
An additional exciting outcome of the study is that ‘it turns our current understanding of the evolutionary tree of the arthropods upside down’, according to Javier Ortega-Hernandez. As he explains, whilst ‘most gene-based studies suggest that arthropods and velvet worms are closely related to each other…our results indicate that arthropods are actually closer to the water bears (tardigrades), a group of hardy microscopic animals best known for being able to survive the vacuum of space and sub-zero temperatures – leaving velvet worms as distant cousins’.
It is often thought that modern animal groups, such as the arthropods, which include spiders, insects and crustaceans, arose fully formed in an explosion of evolutionary diversity at the beginning of Cambrian times but as Martin Smith says ‘studies like ours show that evolution is a gradual process: today’s complex anatomies emerged step-by-step, one feature at a time. By deciphering ‘in-between’ fossils like Hallucigenia, we can determine how different animal groups built up their modern body plans.
                   
How and when exactly Hallucigenia’s descendants exchanged life on the seabed for life in tropical forests is not entirely clear. However, it is known that they had colonized the land by around 296 million years ago as fossil remains of landliving onychophorans have been found in the Carboniferous age Mazon Creek strata from America. These ancient terrestrial deposits are associated with numerous coal seams and were deposited in a swampy freshwater environment, which was periodically flooded with seawater. Like the Burgess Shale and Chengjiang sites, the Mazon Creek strata had unusual conditions of deposition and preservation, which fossilized soft bodied organisms.
A display of fossils from the Burgess Shale including images of Hallucigenia is to be seen in the Sedgwick Museum.
Websites: Canada’s Royal Ontario Museum (http://www.rom.on.ca/en) has a website dedicated to the Burgess Shale and its fossils, see http://burgess-shale.rom.on.ca/
Images:

1. Hallucigenia_sparsa NMNH198658_0262.g.tiff Fossil Hallucigenia sparsa from the Burgess Shale. Credit: M. R. Smith / Smithsonian Institute
2. Euperipatoides_kanangrensis_onychophoran_on_log DSCF0622.JPG The modern velvet worm Euperipatoides, walking on a eucalyptus log.
Credit: M. R. Smith
3. Onychophoran white.JPG The modern velvet worm Euperipatoides.
Credit: M. R. Smith
4. Euperipatoides_kanangrensis_onychophoran_claw_TLM 13001g1_conin_2.gw.tif
A claw of the modern velvet worm Euperipatoides, made up of three stacked elements (light micrograph).
Credit: M. R. Smith
5.Hallucigenia_sparsa_claw ROM 63051 94-0891A_0305.g.tif
Fossilized claw of Hallucigenia sparsa from the Burgess Shale, imaged under a scanning electron microscope.
Credit: M. R. Smith / Royal Ontario Museum
6. Reconstruction_of_Hallucigenia_Credit_Elyssa_Rider.jpg Reconstruction of the Burgess Shale animal Hallucigenia sparsa Credit: Elyssa Rider

Douglas Palmer



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