Attack of the Mysterious Soft-bodied Ediacaran Trilobite

This morning, just before leaving to go to breakfast, I had a quick flick through a couple of pages of Invertebrate Palaeontology and Evolution 4th Ed. by E.N.K. Clarkson, a textbook I recommend for anyone interested in invertebrate palaeontology. As expected there are a few pages on the Ediacaran fauna, including a picture with familiar organisms such as Spriggina and Dickinsonia. Also on the picture was an organism labelled "soft-bodied trilobite". This stuck in my head a little bit through the day (to the point where I mentioned it to someone). Later on I was round at a friend's house and looked in the book Invertebrates 2nd Ed. by Brusca and Brusca where I found the same soft-bodied trilobite image (one I sadly cannot find online). Well, I had to know more.

I was sure that there were no trilobite fossils in the Ediacaran, so I have had another look through Clarkson and found both a reference to a paper and the mention that several fossils had been found resembling trilobites and with several stages of growth. Thankfully Clarkson states that further evidence is needed to assess whether it is a trilobite ancestor. I am a bit busy so I cannot research it into much depth, but what I have found suggests that these fossils are unnamed and simply labelled "soft-bodied trilobite" wherever mentioned. I cannot find a link to the paper mentioned in Clarkson online, but here is the reference:

Jenkins, R.J.F. (1992) Functional and ecological aspects of Ediacaran assemblages, in Origin and Early Evolution of the Metazoa (eds J.H. Lipps and P.W. Signor). Plenum Press, New York and London, pp. 131-176.

The "soft-bodied trilobite" is thought to resemble the Redlichiida (a group of early trilobites) yet has affinities with other Ediacaran forms such as Archaeaspinus (left), Dickinsonia and Vendia which are not thought to be connected to the arthropods.

Claims of putative Ediacaran arthropods are not uncommon, though scrutiny tends to find that either that interpretation meets some difficulties or that there is not enough evidence available. They are almost always connected with the trilobites when claimed as protoarthropods. Among them is Spriggina (right), though its symmetry is not strictly bilateral like that of arthropods as the two sides are offset. I have previously discussed Bomakellia. Others include Marywadea and Parvancorina. The latter is a very interesting one as it may indeed be connected to the arthropods.

Parvancorina  is perhaps too primitive in body plan to be considered arthropodan. There are fine lines on some specimens which have been interpreted by some researchers as paired limbs, but without this data a connection is tentative at best. The biggest issue with identifying an ancestor is the lack of leg preservation (this does not mean they lacked jointed legs, as trilobite fossils rarely have them too). Parvancorina  is thought to resemble the Cambrian arthropod Primicaris, which in turn is thought to resemble a Cambrian trilobite protaspid (the early stage of trilobite growth in which there are no segments) suggesting that heterochrony (changes in developmental timing) may have played a large part in early arthropod evolution (though it should be noted that the growth of Parvancorina is different to that of arthropods).

The earliest known trilobites are Eofallotaspis (left) and Protofallotaspis. Both are fully formed trilobites, though there are Cambrian arthropods such as Skania  and the aforementioned Primicaris  which may even be intermediate forms. Below is an image showing a potential evolutionary sequence from Parvancorina through to an early trilobite:

 On the left is Parvancorina, unsegmented and showing simple metamorphosis (each branch is demonstrating ontogeny). Next is Primicaris showing incipient cephalisation. Naraoia comes next, with complete cephalisation and a still unsegmented body. Then we see Kuamaia  (a helmetid) which shows a clear division between cephalon, thorax and pygidium. Finally there is Redlichia  which has additional trilobite features. This image is not a case of simply lining up fossils and hoping they fit together in a sequence; it is based off of trilobite ontogeny and makes a strong case for Parvancorinomorphs being a basal clade of the arthropods. 

What was intended to be a bit of a rant about Precambrian soft-bodied trilobites instead taught me a fair bit about the origins of arthropods, which is a good way to end the day I think.