| Xandarellida Temporal range: | |
|---|---|
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| Fossils of Zhugeia | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| (unranked): | † Artiopoda |
| Subphylum: | † Trilobitomorpha |
| (unranked): | † Xandarellida Chen, Ramsköld, Edgecombe & Zhou in Chen et al, 1996 [1] |
| Type species | |
| † Xandarella spectaculum Hou, Ramsköld & Bergström, 1991 [2] | |
| Genera | |
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| Synonyms | |
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Xandarellida is an extinct order of artiopod arthropods known exclusively from the Middle Cambrian of China and Siberia.
Xandarellida was defined by Chen, Ramsköld, Edgecombe & Zhou in 1996. Is was often referred to by the name Petalopleura in historical publications, but this grouping was later considered to be more poorly defined than Xandarella, which is now used in preference to it. [3] Xandarellida is classified as a member of Artiopoda, the clade containing the trilobites and their close relatives, typically as members of Trilobitomorpha, which contains the closest relatives of trilobites within Artiopoda. [3] Xandarellida contains numerous species, most from the Maotianshan Shales of China. The only exception to this is Phytophilaspis , which is instead from the Sinsk Formation of Russian Siberia. [4] [3]
In terms of anatomy, xandarellids share numerous characteristics such as a natant hypostome, alongside a unique feature known as “segment decoupling” where the amount of limbs exceeds the amount of tergites. [3] They also share a posteriorly extended semicircular head shield and ventral stalked eyes. The genus Cindarella in particular has complex eyes, with over 2000 ommatidia in each. [4] Cindarella also has a large anterior spine on the fifteenth tergite. Xandarella has an unusual tripartite head shield, with ventral eye slits likely retained from the ancestral stalked eyes, alongside possibly lacking the anterior spine. [3] Phytophilaspis has an especially intricate head shield, sharing its tripartite nature with Xandarella alongside bearing several sutures all over its body and seemingly having almost completely fused tergites. [5] Luohuilinella has a large amount of tergites (with the type species having 27) but otherwise resembles other xandarellids save for an unfused three-segmented pygidium. [6] Sinoburius resembles Xandarella in having an elongate posterior spine and Phytophilaspis in having all but a few tergites fused, although it exhibits secondary loss of eye slits. Unusually, Sinoburius also has two small lateral spines near its posterior. [3] Zhugeia is relatively usual for the clade, however its median spine is very long and needle-like, in addition to having a relatively large pygidium. [7]
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