Fossilized remains of an arthropod from the Cambrian period provide an unusual example of preservation of the brain and nervous system, and shed new light on when and how these tissues evolved. See Letter p.258
Even to palaeontologists, the fossil record can resemble the chaotic attic of an eccentric relative, stacked with ancient bric-a-brac of dubious usefulness. But the record has recently been throwing up some surprises that are bringing new order to this jumble. Our concept of dinosaurs, for example, has evolved from what were essentially bolted-together lumps of bone into living creatures covered in graceful feathers — and in colour too1. Other fossil finds have brought changes to the scale of our understanding of evolution. For example, the discovery of exceptionally well-preserved fossil muscle fibres throughout the record2 and fossilized embryos from at least the Cambrian period3, some 500 million years ago, have provided remarkable insight into the fine-scale evolution of these tissues and life stages. Now, on page 258 of this issue, Ma and colleagues4 describe preserved nervous tissue from the Cambrian — a find that grants palaeontologists access to the exclusive zoological club of those who study the brain and nervous system.
Certain tissue types, such as bone and shell, are much more likely to be preserved in the fossil record than others, owing to their mineralized and thus relatively refractory nature. Soft tissues, such as muscle or even the cuticle of arthropods (joint-legged invertebrate animals), which is largely proteinaceous and therefore liable to decay, are less likely to withstand oxidation, scavenging, mechanical damage and all the other vicissitudes that conspire to make the work of a palaeontologist so testing. Tissues also preserve differently under different conditions — 'bog people', for example, are often all leather and no bones because of the acidity of the peat they were preserved in. This variable survival is captured by the idea of preservation potential, and understanding this concept is critical when trying to reconstruct ancient organisms, because it places important limitations on what a difficult-to-interpret structure in a fossil might be. It is highly unlikely, for example, that delicate blood vessels are going to leave a trace in a fossil found in sandstone deposited in a high-energy environment.
The concept of preservation potential has recently been receiving empirical support from decay experiments — a useful, if rather smelly, approach to understanding the fossil record5. These studies have taught us that descriptions of fossils that portray kidneys and gonads rightfully raise eyebrows. Yet, somewhat surprisingly, they also suggest that the brain and nervous system might have a decent chance of preservation, at least in vertebrate animals5. The situation in invertebrates seems less clear, although possible examples have been reported6,7. Ma et al., however, have now provided convincing evidence for the preservation of the brain and partial nervous system in fossils of a Cambrian arthropod, and one of the most controversial and interesting arthropod species to boot, Fuxianhuia.
The taxonomic position of Fuxianhuia has been the topic of some dispute, but it is generally considered to lie close to the most recent common ancestor of the living arthropods8. As such, this fossilized material is ideally placed to contribute to neurophylogeny, the relatively new exploration of brain and nervous-system architecture in a phylogenetic context9. What does the most informative of these Fuxianhuia specimens tell us? Starting at the top, the fossil displays two large, faceted eyes on stalks (see Fig. 1 of the paper). Within the stalks, a dark, iron-rich material traces what Ma and colleagues interpret to be three neuropils (concentrations of nervous tissue) from which optic nerves lead towards the brain. Here, the preserved material is divided into three regions that correspond to the classical arthropod brain ganglia of the protocerebrum, deutocerebrum and tritocerebrum (a ganglion is a mass of nerve cells). The authors suggest that all three regions lie, at least in part, in front of the stomodeum — the part of the gut that leads directly to the mouth. From the deutocerebrum and tritocerebrum, stout nerves lead to the antenna and to what is presumably another appendage, respectively. This latter structure has proved problematic to describe in other Fuxianhuia fossils, and its very existence has sometimes been questioned8.
This fossil provides the most convincing, and certainly the oldest, description of nervous-system tissue in a fossil arthropod. So what do these remarkable results imply? The most striking feature, as the authors themselves stress, is how similar the tripartite brain of Fuxianhuia is to that of most modern arthropods, including crustaceans and insects. The exception to this are the branchiopods — the best-known example of which is Daphnia, the elegant water flea — which have a considerably simpler brain, in that they lack both complex optic neuropils and a prostomodeal tritocerebrum. If Fuxianhuia belongs in the stem group of all arthropods, then it follows that the organism's complex brain organization evolved relatively early within this phylum. If this is the case, the authors suggest, then the predecessors of extant arthropods with less complex brains (which includes not only the branchiopods, but also spiders, scorpions and their relatives) must have at some stage simplified their arrangements.
However, there are two potential alternatives to this far-reaching conclusion. It is possible that the arrangement in Fuxianhuia is convergent to that in the modern crustaceans or insects; in other words, similar brain assemblies to that reported for Fuxianhuia evolved again in later arthropods. Or it may be that we need to rethink the systematic position of Fuxianhuia. That latter option would entail a substantial rearrangement of our present understanding of early arthropod evolution — not least in the highly vexed issue of the 'great appendage problem'8. This refers to the controversial identity of a large anterior appendage found in many Cambrian arthropods, and seemingly also in the Fuxianhuia specimen described here. Discovering which part of the brain this structure is innervated from will add vital information to this debate. Either way, Ma and colleagues' findings will prompt hasty re-examination of many old specimens, and quite possibly some recasting of recent theories.
Li, Q.-G. et al. Science 335, 1215–1219 (2012).
Martill, D. M. Nature 346, 171–172 (1990).
Dong, X.-P. et al. Nature 427, 237–240 (2004).
Ma, X.-Y. et al. Nature 490, 258–261 (2012).
Sansom, R. S. et al. Proc. R. Soc. B 278, 1150–1157 (2011).
Conway Morris, S. Spec. Papers Palaeont. 20, 1–95 (1977).
Bergström, J. et al. GFF 130, 189–201 (2008).
Budd, G. E. Palaeontology 51, 561–573 (2008).
Harzsch, S. Integr. Comp. Biol. 46, 162–194 (2006).
About this article
Current Biology (2015)
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution (2013)