Diania

''Diania'' is an extinct genus of lobopodian panarthropod 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 a jointed exoskeleton and ''Diania'' was suggested to be evolutionarily close to early arthropods, but many later studies have rejected this interpretation.

Discovery

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.

Name

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. The specific epithet ''cactiformis'' is based on its spiny, cactus-like appearance, which led to it being informally called the "walking cactus" by the research team working on the fossils.

Description

The largest complete fossils of ''Diania'' are about in full length. Both the trunk and legs were covered by a soft cuticle with a series of annulations (ring-like grooves) and spines. The appendages (lobopods) are arranged in a more much lateral position than those of other lobopodians, giving the animal a similar dorsoventral aspect.

The worm-like body of ''Diania'' is composed of 10 trunk segments, each associated with a series of median circular structures and a pair of legs. 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. The only sign of a head is in specimen ELEL-SJ102058, which shows a helmet-like structure, with a pair of lateral spines and median protrusion that is presumed to be the location of mouth opening. A pair of tiny, modified appendages are located immediately behind the head.

The most recognisable feature of ''Diania'' is the robust, spiny legs (walking lobopods) which led to it being nicknamed "walking cactus". While the leg base is narrow and simple, the remaining sections are subequally robust to the trunk, with 15 widely-spaced annulations and 4 rows of well-developed spines. Although the widen ring-like annulations superficially resembled the rigid, segmented exoskeleton of arthropod appendages (which had led to the previous misinterpretation as such), there are no signs of any hardened exoskeleton nor segmentation, with deformation from various fossil materials confirming their soft and unsegmented nature. Instead of terminal claws like most other lobopodians, the leg of ''Diania'' terminates with multiple (up to 3) spines similar to the preceding annulation.

Paleoecology

''Diania'' may had been a benthic scavenger, using its robust legs to walk on the sea floor. The absence of sedimental gut content and terminal claws suggest ''Diania'' was unlikely to be a sediment feeder nor a good climber. The modified appendages behind its head may had played a sensing and/or food-manipulating role, similar to the tentacle-like appendages of ''Hallucigenia''. Similar to the dorsal spines of other armoured lobopodians (e.g. ''Hallucigenia'' and ''Luolishania''), the spines of ''Diania'' may had provided protection against predators.

History of research

''Diania'' belongs to a group of extinct animals known as the lobopodians. 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, annulated (ring-like grooves), but not segmented appendages called lobopods. In the original description (Liu et al. 2011), ''Diania'' was thought to be a lobopodian which appears to have evolved the name-giving character of the arthropods: scleritzed (hardened exoskeleton) and segmented appendages, which led to the suggestion of ''Diania'' being close to the origins of arthropods. Based on this interpretation, Liu et al. 2011 also suggests that ''Diania'' is an evidence showing arthropodization (i.e. the appearance of exoskeleton and segmentation around the appendages) evolved before arthrodization (i.e. exoskeleton and segmentation around the body). The phylogenetic analysis provided by Liu et al. 2011 resolved ''Diania'' as the sister group of ''Schinderhannes'' and the remaining arthropods, although the possibility of ''Diania'' emerging before the evolution of stem-arthropods like Gilled lobopodians and radiodonts also had been discussed.

However, both the phylogenetic and morphological interpretations provided by Liu et al. 2011 were soon being questioned by multiple later studies. The data provided by Liu et al. 2011 are inconsistent with the suspected phylogenic relationships, as the analysis run by Mounce & Wills 2011 and Legg et al. 2011 could only resolve ''Diania'' as a lobopodian indirectly related to arthropods, even using the same method by Liu et al. 2011 and coding ''Diania'' as the bearer of arthropod-like appendages. Furthermore, the re-examination of Ma et al. 2013 and Ou & Mayer 2018 even clarified that the ring-like structures of ''Dianias leg were soft annulation shared by the lobopods of other lobopodians, lacking any characters comparable to those of an arthropod appendage (e.g. exoskeleton, segmentation, pivot joint and arthrodial membrane). This conclusion eventually rejected the evidence of arthropodization on ''Diania,'' as well as the link between ''Diania'' and the origin of arthropod appendages.

After a series of restudies, ''Diania'' remain as a unique lobopodian with uncleared, non-arthropod related phylogenetic position. Further phylogenetic analysis inconsistently placing ''Diania'' as a basal member of the onychophoran stem-group alongside ''Xenusion'', or a basal lobopodian branched before the total-groups of each panarthropod phylum.

Other than the lacking of arthropodization, Restudy of ''Diania'' by Ma et al. 2013 also revealed the structure thought to be a stout head/proboscis by Liu et al. 2011 was in fact one of the animal's leg, the maximum number of leg annulations is 15 instead of 17, and the terminal leg structures are spines instead of claws. A putative head was first revealed by Ou & Mayer 2018 from the specimen ELEL-SJ102058.

References

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{{taxonbar, from=Q16894752

Cambrian animals of Asia
Maotianshan shales fossils
Cambrian invertebrates
Fossil taxa described in 2011
Xenusia
Monotypic prehistoric protostome genera
Cambrian genus extinctions