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Mar 2, - This is a true story about two friends who came across a helpless horseshoe crab that was wedged inside the beam of a wooden fence.
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A long work of observation as well as fossil preparations, which came from the skillful hands of Jean-Bernard, provided us with another set of chelicerae and nothing less than an almost full complement of the remaining seven pairs of head limbs. These limbs without a doubt made Mollisonia a close relative of Sanctacaris and Habelia , while, from a strictly taxonomic perspective, the chelicerae were giving her the medal of the oldest known chelicerate.

To crown this picture, limbs behind the head had this peculiar way of being assembled from several overlapping thin lobes.

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Finding chelicerae was one thing, but this brought Mollisonia even closer to the modern chelicerate body plan. In that, Mollisonia bridged an evolutionary gap. While it can be argued that Sanctacaris and her relatives had already brought the making of chelicerates within the realm of the Cambrian explosion, it became clear with these new fossils that chelicerates had more than just emerged in the aftermath of the explosion; they most probably had been a full part of it, much earlier within the Cambrian, in forms we might not suspect.

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If the fossil record may still hold surprises for us, what we can say, however, considering that Sanctacaris , Mollisonia and their kind were small predators roaming the sea floor, is that the first chelicerates might have used that relatively poorly exploited ecological niche to flourish at the time. Mollisonia is therefore a creature with a formidable legacy, leading the way to horseshoe crabs and arachnids.

An issue remained: these three nerve masses inside each eye that we first saw seven years ago are in fact absent in modern chelicerates, which only have two. Combined with the fossil evidence from China, this must therefore mean that the optic part of the arthropod brain evolved a complex form early, in the midst of the Cambrian explosion, before getting simplified in modern chelicerates which instead rely on secondary eyes and other sensory devices.

And quid of the trilobite-like tail?

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Just like the resemblances with Sanctacaris and Habelia , Mollisonia could not hide much of her pedigree, which of course is a bonanza or sometimes a curse for the systematists. There are indeed reasons to think that this trilobite-like tail, called a pygidium, is the same structure as in trilobites, or at least that it is formed from the same genetic program. This, consequently, gives horseshoe crabs more than a superficial resemblance to trilobites.

We worked with Canadian scientific illustrator Joanna Liang to bring Mollisonia plenovenatrix to life. This is a more tedious process than one might think, for which detailed measurements are taken that do not even enter the description of the species. The process also forces us to question our judgment, as piecing together all those separate, targeted observations in our mind can be cause for an Arcimboldo.


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Yes, it is you. Like all these fossils, Mollisonia was an adventure. From rocks to paper, I have been fortunate to live it that way, thoroughly. Long after now, perhaps through the glass of a display stand, she will still look at me, whispering her stories; staring, as if waiting for me to tell her whether I understood them. Perhaps not, but trying was the whole point. Through posts, discussion, image and video content, the community space can be used by members to communicate with each other, and with editors, about topics ranging from the fundamental science itself through to policy, society and the day to day life of the research community.

It is also a place to learn more about the activities of Nature Research ecology and evolutionary biology editors and the policies and practices of our journals. A community from Nature Ecology and Evolution. Contributor Nature. Behind the paper. A little arthropod from the Burgess Shale cuts short the hunt for a Cambrian chelicerate.

Like 0 Comment. Facebook Twitter LinkedIn. Copy the link. ROMIP This is a dorsal view, with the head displaying the eyes and their nervous tissues, the walking legs, as well as the tiny chelicerae at the very front of the animal.

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Photo credit: Jean-Bernard Caron. A view of the Marble Canyon quarry towards the end of the expedition. The evening is approaching, and some larger fossil slabs are rock-sawn so that specimens could be placed safely in our metal buckets for helicopter transport.


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Rich slabs that cannot be cut will be protected between wood boards and… pool noodles. Whoever decided for whatever reason to export them to mountainous British Columbia did, without knowing, an immense favour to palaeontology. Sledge-hammered wedges into the hard, calcareous claystone finally crack open the tasty fossil beds. Duly earned satisfaction. Going at the precious shale layers with our small jackhammer. Our precision power tool. With Robert Gaines spotting behind, his mind perhaps starting to assemble stratigraphic charts of the outcrop.

Desmond Collins proudly holding the holotype specimen of Sanctacaris uncata upon its discovery, in the summer of Photo credit: Royal Ontario Museum. Your email address will never be sold or distributed to a third party for any reason. Due to the high volume of feedback, we are unable to respond to individual comments. Sorry, but we can't respond to individual comments.

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