Diania is an extinct genus of lobopodian 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 legs. Initially, the legs were thought to have jointed exoskeleton and Diania was suggested to be close to the origins of arthropods,[2] but many later studies have denied this interpretation.[3][4][5][1]
Fossils of Diania 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.[2]
The name Diania comes from "Dian" (Chinese: 滇), which is an abbreviation in the Chinese language for Yunnan; the province where the fossils were discovered.[2] The specific epithet cactiformis is based on its spiny, cactus-like appearance,[2] which led to it being informally called the "walking cactus" by the research team working on the fossils.
The largest complete fossils of Diania are about 6 centimetres (2.4 in) in full length.[2] Both the trunk and legs were covered by soft cuticle with series of annulations (ring-like grooves) and spines.[1] The appendages (lobopods) arranged in a more much lateral position than those of other lobopodians, giving the animal a similar dorsoventral aspect.[5]
The worm-like body of Diania compose of 10 trunk segments each associated with series of median circular structures and a pair of legs.[2][5] Due to the subequal morphology between each trunk segment and the usually poor preservation of both trunk ends, it is difficult to define the anterior-posterior axis of the animal.[5] The structure that is possibly a head is known only in specimen ELEL-SJ102058, showing a helmet-like structure with a pair of lateral spines and median protrusion that presumed to be the location of mouth opening.[1] A pair of tiny, modified appendages located immediately behind the head.[1]
The most iconic character of Diania is the robust, spiny legs (walking lobopods[1]), which led to it being nicknamed "walking cactus".[2][5] While the leg base is narrow and simple, The remaining sections are subequally robust to the trunk,[1] with 15 widely-spaced annulations[5] and 4 rows of well-developed spines.[2] Although the widen ring-like annulations superficially resembled the rigid, segmented exoskeleton of arthropod appendages (which had led to the previous misinterpretion as such[2]), there's no signs of any hardened exoskeleton nor segmentation, with deformation from various fossil materials comfirmed their soft and unsegmented nature.[5][1] Instead of terminal claws like most other lobopodians, the leg of Diania terminated with multiple (up to 3) spines similar to the preceding annulation.[5][1]