NEWS & VIEWS
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PAL AEONTO LO GY
Hallucigenia’s head The finding of pharyngeal teeth and circumoral mouthparts in fossils of the Cambrian lobopodian animal Hallucigenia sparsa improves our understanding of the deep evolutionary links between moulting animals. See Letter p.75 X I A O YA M A
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ossils provide direct evidence of evolutionary history, and their unique morphological combinations can reveal crucial evolutionary links between extant taxa1. Most major animal phyla first appear in the fossil record during the Cambrian period, 541 million to 485 million years ago, and this early flowering of animal life has been termed the Cambrian explosion. Therefore, Cambrian fos sils are particularly important for understand ing the origin and early evolution of major animal groups. In this issue, Smith and Caron2 (page 75) redescribe one of the most celebrated Cambrian animals, Hallucigenia sparsa, and document several new features of this species, including its pharyngeal teeth and circumoral elements, which are suggested to be two of the few morphological characters uniting all groups within the Ecdysozoa. The Ecdysozoa is far and away the richest animal group3. It is composed of eight extant phyla that shed their cuticle periodically to accommodate growth4 — nematode worms and crustaceans are familiar examples. The two commonly recognized subgroupings of ecdysozoans, Cycloneuralia and Pan arthropoda, have distinctly different body plans (Fig. 1). Cycloneuralia unites wormlike organisms (Nematoda, Nematomorpha, Priapulida, Kinorhyncha and Loricifera) that have a non-segmented body terminating in a mouth that can turn inside out (eversible) and has a ring of nerves behind it — their brain. By contrast, panarthropods (Arthropoda, Onychophora and Tardigrada) are all seg mented, with paired legs, and have a dorsal (upper side) brain in front of the mouth. These great morphological disparities have made it difficult to illuminate the last common ances tor of the Ecdysozoa and to fully understand the evolutionary relationships between its phyla, particularly between Cycloneuralia and Panarthropoda. Early ecdysozoan fossils are crucial for addressing these questions. Among the earliest Cambrian fossils, ecdyso zoans are the most diverse and abundant group. They are best shown in exceptionally preserved Cambrian fossil localities, such as the Cheng jiang biota in China (around 518 million years old) and the Burgess Shale in Canada (about
Nematoda Nematomorpha Cycloneuralia
Eximipriapulus globocaudatus
Priapulida Loricifera Kinorhyncha
Ecdysozoa
Tardigrada Hallucigenia sparsa
Lobopodians Panarthropoda
Onychophora Radiodontans
Anomalocaris canadensis
Arthropoda Fuxianhuia protensa
Figure 1 | The Ecdysozoa. This large animal group comprises eight extant phyla and two informal extinct groups, the lobopodians and the radiodontans. According to their body plans, ecdysozoans are commonly recognized as two distinct subgroups, Cycloneuralia and Panarthropoda. The fossil record of both can be traced to the earliest Cambrian period, and some members, such as priapulids and arthropods, have changed little over 500 million years of evolution. Cambrian lobopodians and radiodontans represent crucial evolutionary links between Cycloneuralia and Panarthropoda. Smith and Caron2 present an analysis of the lobopodian Hallucigenia sparsa that includes details of its head structures. (Illustrations by David Baines.)
508 million years old). The body plans of some of the organisms represented have not changed much over 500 million years of evolution, such as priapulids (commonly known as penis worms) and arthropods (jointed-legged inver tebrates with an exoskeleton and a segmented body, such as insects and spiders). Other Cam brian groups are extinct but represent crucial evolutionary stages, such as lobopodians (an informal group of worm-like animals with non-jointed legs) and radiodontans (a group of animals characterized by possessing a pair of frontal appendages at the anterior part of the head and a ventral (lower side) mouth sur rounded by radial tooth plates). Cambrian lobopodians are assigned to Panarthropoda on the basis of their segmented body and paired legs, but they also share a worm-shaped soft body and a terminal mouth with cycloneuralians. These unusual character combinations make Cambrian lobopodians particularly relevant for understanding the evolutionary links between the two major ecdysozoan groups.
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Hallucigenia sparsa from the Burgess Shale is certainly the most famous Cambrian lobo podian animal. It was originally reconstructed upside down5 and considered to be one of the most bizarre Cambrian creatures until it was recognized as a lobopodian animal armed with dorsal spines6. However, owing to lack of evidence of clear head structures, the front and rear ends of H. sparsa have been a subject of debate. Smith and Caron’s redescription includes a new set of anatomical features that once and for all clarifies the anterior–posterior orientation of H. sparsa. They show that the animal had an elongated head with a pair of dorsal eyes. It also had hardened, lamellae-like structures surrounding its mouth opening (circumoral elements), and the front part of its foregut (its pharynx) was lined with teeth. Although pharyngeal teeth and circumoral mouthparts have been reported in other Cambrian lobopodians7,8, Smith and Caron have provided the most convincing evidence yet of equivalent structures in this extinct group. These findings amplify the transitional
NEWS & VIEWS RESEARCH status of Cambrian lobopodians, because the pharyngeal teeth of H. sparsa most closely resemble those of Cambrian priapulids, whereas circumoral structures are also a key characteristic of Cambrian radiodontans. More crucially, H. sparsa is now regarded2,9 as an ancestor of living onychophorans (com monly known as velvet worms), so the find ing of H. sparsa mouthparts suggests that the absence of circumoral elements and pharyn geal teeth in extant onychophorans is probably the result of secondary loss. Thus, this com bined structure is now reported for all major ecdysozoan groups. Smith and Caron further notice the simi larities of ecdysozoan mouthparts (see Supplementary Note 1, transformation series 9 and 13 of the paper2), and suggest that all phar yngeal teeth and circumoral structures across ecdysozoan groups share a single origin from the last common ancestor of ecdysozoans. This provides new anatomical features to unite the Cycloneuralia and Panarthropoda.
However, this conclusion is bound to provoke some controversy. Limited by the vagaries of preservation, it is difficult to determine the detailed morphology and sym metry of the pharyngeal teeth and circumoral elements of H. sparsa — such details are essen tial for further comparative studies. Although the homology of ecdysozoan pharynxes lined with teeth is well accepted, the evolutionary links between the circumoral structures of Cycloneuralia and Panarthropoda are less clear, because these differ substantially in their structure, relative position, construction and symmetry10. Therefore, a more complete understanding of the evolutionary origin and transformation sequence of these mouth parts depends on a more thorough compari son of their morphology, development and innervation across all ecdysozoan groups. For this, new fossil evidence showing tran sitional features of the mouthparts between cycloneuralians and panarthropods would be particularly enlightening. ■
NANOTECHNO LO GY
Colourful particles for spectrometry A smartphone camera, patterned with arrays of filters made from colloidal suspensions of coloured particles, has been transformed into a powerful tool for spectral analysis. See Letter p.67 NORM C. ANHEIER
PLASMACHEM
I
n 1857, Michael Faraday gave a wellattended lecture at the Royal Institution of Great Britain, during which he pre sented his pioneering experimental work on the interaction of light with matter1. Faraday’s study probed the fundamental properties of light related to its reflection and absorption by progressively smaller particles. During the presentation, very fine gold particles dis persed in liquid were shown to produce vivid colours not seen with larger particles. Faraday did not know that he had created suspensions of particles now known as colloidal quantum dots (CQDs), but, guided by insight, he con cluded that the distinct colours were due to the minute sizes of the gold grains. On page 67 of this issue, Bao and Bawendi2 describe how they have exploited the unique optical prop erties of CQDs to develop a compact optical spectrometer that could be integrated with a smartphone camera or used as a miniature, hand-held sensing tool. Faraday had glimpsed a special condi tion that allows a particle’s quantum nature to be expressed. His work set the course for
nanoscience and quantum theory, but it took 125 years before the physics of the phe nomenon that he observed was attributed to quantum size effects3. It is now known that, when CQDs are exposed to light, some of the electrons in these particles are excited as they gain energy from the photons. However, unlike large particles and bulk materials, the nanoscale dimensions of the quantum-dot
Xiaoya Ma is at Yunnan University, Kunming 650091, China, and at the Natural History Museum, London, UK. e-mail:
[email protected] 1. Edgecombe, G. D. & Legg, A. D. in Arthropod Biology and Evolution (eds Minelli, A. et al.) 393–415 (Springer, 2013). 2. Smith, M. R. & Caron, J.-B. Nature 523, 75–78 (2015). 3. Telford, M. J., Bourlat, S. J., Economou, A., Papillon, D. & Rota-Stabelli, O. Philos. Trans. R. Soc. London Ser. B 363, 1529–1537 (2008). 4. Aguinaldo, A. M. et al. Nature 387, 489–493 (1997). 5. Conway-Morris, S. Spec. Pap. Palaeontol. 20, 1–95 (1977). 6. Ramsköld, L. & Hou, X. Nature 351, 225–228 (1991). 7. Hou, X. G., Ma, X. Y., Zhao, J. & Bergström, J. Lethaia 37, 235–244 (2004). 8. Vannier, J., Liu, J., Lerosey-Aubril, R., Vinther, J. & Daley, A. C. Nature Commun. 5, 3641 (2014). 9. Smith, M. R. & Ortega-Hernández, J. Nature 514, 363–366 (2014). 10. Dewel, R. A. & Eibye-Jacobsen, J. Hydrobiologia 558, 41–51 (2006). This article was published online on 24 June 2015.
particles confine the electrons and change the energy difference between their excited and relaxed states. CQDs emit light when the electrons relax from a higher to a lower energy state (Fig. 1). The colour of the light depends on the states’ energy difference and is critically linked to the size of the par ticles, which can be controlled when pro ducing the CQDs. The physics that underpins this behaviour allows CQDs to be used for spectroscopy. The first simple spectrometer, consisting of a dispersive prism, was developed by Isaac Newton, who proved that white light is com posed of a spectrum of many colours4. These days, optical spectrometers have become indis pensable instruments used to measure the dis tribution of light’s colours (wavelengths) in a variety of complex scientific investigations. Astronomers use them to collect and analyse optical spectra of exoplanets that may have lifesupporting atmospheres5. Planetary scientists are using spectrometers on board rovers on the
Figure 1 | Colloidal quantum dots. When they are excited by ultraviolet light (pictured), colloidal suspensions of minuscule particles (known as colloidal quantum dots, or CQDs) fluoresce at different colours depending on the particle size. Bao and Bawendi2 have exploited the unique optical absorptive properties of CQDs to develop a compact spectrometer that serves as a powerful tool to analyse the spectral characteristics of light. 2 J U LY 2 0 1 5 | V O L 5 2 3 | N AT U R E | 3 9
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