Temporal range: Cambrian Stage 3
|Species:||† D. cactiformis|
Liu et al., 2011
Diania is an extinct genus of animal found in the Lower Cambrian Maotianshan shale of China, represented by a single species D. cactiformis. Known during its investigation by the nickname "walking cactus", this organism belongs to a group known as the armoured lobopodians and has a simple worm-like body with robust, spiny, and apparently jointed legs. Its significance is that jointed legs are the defining character of the arthropods and Diania may thus be very close to the origins of the most diverse group of animals on the planet.
Fossils of these animals were discovered independently by Jianni Liu from the Northwest University (China) in Xi’an, Qiang Ou from the China University of Geosciences in Beijing and Michael Steiner of the Free University Berlin. The fossils come from the famous Chengjiang deposit – or Maotianshan shale – of south-west China and are about 520 million years old. Specifically, they come from the Yu'anshan Formation, dated to the Cambrian Stage 3.
The name Diania comes from "Dian", which is an abbreviation in the Chinese language for Yunnan; the province where the fossils were discovered. The specific epithet cactiformis is based on its cactus-like appearance, which led to it being informally called the "walking cactus" by the research team working on the fossils.
Complete fossils of Diania cactiformis are about 6 centimetres (2.4 in) long and have a long, thin body. At the front end is a proboscis, presumably used in feeding. These animals have ten pairs of legs, and compared to the body these are quite robust and spiny. Because of this spiny appearance in the rocks the animal became known informally as the "walking cactus". The most important aspect is that legs appear to be jointed, with a hard exoskeleton divided into ring-like articles.
Diania belongs to a group of extinct animals known as the armoured Lobopodia. These have long been assumed to be related in some way to Arthropoda and it seems likely that arthropods evolved from somewhere within this group. However, all previously discovered lobopodians showed soft, rather ring-like, but not segmented legs. The significance of Diania is that it is a lobopodian which appears to have evolved the name-giving character of the arthropods: jointed legs.
In the original description of the fossil the authors suggested that Diania could be the sister group of Schinderhannes and the remaining arthropods, but discussed the possibility that it may be a more primitive form emerging before the evolution of creatures like Kerygmachela and Anomalocaris.
Diania also suggests that that arthropodization (i.e. the appearance of hard, ring-like joints around the legs) evolved before arthrodization (i.e. hard, ring-like segments, around the body). However, the phylogenetic analysis underlying this conclusion was flawed. 
- Mounce, R. C. P.; Wills, M. A. (2011). "Phylogenetic position of Diania challenged". Nature 476 (E1): E1. Bibcode:2011Natur.476E...1M. doi:10.1038/nature10266.
- Legg, D. A.; Ma, X.; Wolfe, J. M.; Ortega-Hernández, J.; Edgecombe, G. D.; Sutton, M. D. (2011). "Lobopodian phylogeny reanalysed". Nature 476 (7359): E1; discussion E3–4. doi:10.1038/nature10267. PMID 21833044.
- Jianni Liu, Michael Steiner, Jason A. Dunlop, Helmut Keupp, Degan Shu, Qiang Ou, Jian Han, Zhifei Zhang & Xingliang Zhang (2011). "An armoured Cambrian lobopodian from China with arthropod-like appendage". Nature 470 (7335): 526–530. Bibcode:2011Natur.470..526L. doi:10.1038/nature09704. PMID 21350485.
- Zoë Corbyn (February 23, 2011). "'Walking cactus' is arthropods' lost relative". Nature. doi:10.1038/news.2011.121.
- Colin Barras (February 23, 2011). "First animal to wear exoskeleton armour". New Scientist.
- Christine Dell'Amore (February 23, 2011). "Wormlike "walking cactus" fossil found". National Geographic News.
- Ed Yong (February 23, 2011). "Meet Diania the walking cactus, an early cousin of life's great winners". Not Exactly Rocket Science (Discover).