Sunella

Sunella is an extinct genus of bivalved arthropod known from the Cambrian of China. Named after paleontologist and geologist Sun Yunzhu, it was described by Huo Shicheng in 1965. It is the type genus of the family Sunellidae. The type species is S. grandis.

Classification

In the original description (Huo, 1965), Sunella was considered to be an ostracod. Most later studies have identified it as an arthropod of uncertain affinities, though some studies historically considered it a bradoriid during the 1980s. Sunella has often been suggested to be closely related to Isoxys, a genus of Cambrian arthropods with a superficially similar bivalved carapace, though true understanding of Sunella's affinities was for a long time hampered by poor knowledge of its soft tissue. A 2026 description of soft tissue remains of Sunellla by Liu and colleagues, suggested that Sunella represented one of the earliest diverging members of the arthropod clade Deuteropoda, and that while morphologically similar, Isoxys was more closely related to living arthropods than to Sunella. However, a later 2026 study by Zhai and colleagues reveal more details on the appendages, which suggest Sunella to be a pancrustacean mandibulate instead, related to ostracods.

Species

Sunella grandis Sunella grandis (Huo, 1965) is the type species of Sunella. It is known from the Chengjiang Biota of Yunnan, China. Probable sexual dimorphism has been observed.

Synonyms Sunella nanchengensis (Huo, 1965) Luella (Huo, 1965) Chiella (Huo, 1965)

Sunella nanchengensis Sunella nanchengensis (Huo, 1965) is a probable sexual dimorph of S. grandis and is generally considered a synonym of S. grandis. It was named after Nancheng, the previous romanization of Nanzheng, which is the type locality of the species.

Sunella shensiella Sunella shensiella (Huo,1965) is the only species of Sunella apart from S. grandis that is not considered a synonym of S. grandis or has been reassigned to a different genus. Probable sexual dimorphism has been observed. It is one of the species where soft anatomy has been preserved. Known from the Helinpu Formation of southwest China. Sunella dimorphismus Sunella dimorphismus (Liu et al, 2026) has preserved anatomy including trunk and appendages. Known from the Chengjiang Biota. However, the name was later pointed out to be nomen nudum as no type specimen was specified in the original description, at the same time the conspecific specimens was described as S. chimera by Zhai et al. 2026. Sunella chimera Sunella chimera (Zhai et al, 2026) is the new valid name applied to the aforementioned S. dimorphismus specimens. The new description providing more details on the body and appendages, especially the mouthparts, significantly rewrite the taxonomic position of suneliids.

Tuzoia? (Sunella) parva One former species is Sunella parva (Melnikova, 1988), which may be the larval form of a new species of Tuzoia, which was rejected from Sunella by Melnikova 1998 and may need to be assigned to its own genus. It has also been suggested to be a bradoriid (Melnikova, 1998)

Synonyms Sunellidae, which Sunella belongs to originally included two other genera, Chiella (Huo, 1965) and Luella (Huo, 1965), but Chiella was later considered a separate species within Sunella, but after Huo & Shu, 1985 Chiella was considered a junior synonym of Sunella and after Zhang & Shu, 2007 Luella was considered a probable sexual dimorph of Sunella.

Former species

Caudicaella (Sunella) bispinata A species that previously belonged to Sunella, S. bispinata, was moved to a new genus, Caudicaella (Sun et al. 2021) as Caudicaella bispinata (Cui and Huo, 1990). Synonyms include Isoxys bispinata (Zhang et al., 2018) (not to be confused with Isoxys bispinatus) Isoxys sp. (Sun et al. 2021) (for the specimens from the Heatherdale shale, not the Shuijingtuo formation)

Distribution Sunella fossils have been found in the Chengjiang biota (exact locality cannot be determined; dated to no older than 518 Ma), the Shuijingtuo formation (dated to around 526.5 Ma), the Qingjiang biota (dated to ~518 ma), the Niutitang formation ( generally Meishucunian (Cambrian Stage 2) to Nangaoan (Cambrian Stage 3) but may be up to as old as the Fortunian in some sections) and the Guojiaba formation (tentatively assigned to Cambrian Stage 3), all of which are in China. Similar fossils have been found at other formations in China but they are relatively uncommon and poorly studied and therefore cannot be confidently assigned to this genus or even Sunellidae. The former species Caudicaella bispinata is also known from the Heatherdale shale in Australia.

Preservation Usually the only part of Sunella that is preserved is the carapace, but rarely (only twenty-two times in Sunella cf. shensiella and only six in the related Combinivalvula chenjiangensis) soft tissue is also preserved. When this occurs, the carapace usually adopts a "butterfly" position, possibly due to the decay of the adductor muscles that hold it together. Only the soft parts of Sunella cf. shensiella and the related Combinivalvula chenjiangensis (Hou, 1987) have been preserved.

Morphology

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