Sunella
Extinct genus of stem group arthropod
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Sunella is an extinct genus of Cambrian stem group arthropod. It was described by Huo Shicheng in 1965. It is the type genus of the family Sunellidae. Type species is S. grandis.
| Sunella | |
|---|---|
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| CT scan of fossil specimen of Sunella dimorphismus | |
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| Life restoration of Sunella dimorphismus | |
Scientific classification  | |
| Kingdom: | Animalia |
| Clade: | Panarthropoda |
| Phylum: | Arthropoda |
| (unranked): | Deuteropoda |
| Family: | †Sunellidae |
| Genus: | †Sunella Huo, 1965 [3][6][4] |
| Type species | |
| †Sunella grandis | |
| Species | |
| Synonyms | |
Classification
In the original description (Huo, 1965), Sunella was described as an Ostracod-like arthropod;[7][6][3] then, in later studies it was identified as an arthropod of uncertain affinities;[3][1][8][4] then as a bradoriid[7][9][10][11][12][13] and then as again an arthropod of uncertain affinities;[7][14] then as a species of Isoxys along with other Sunellids in Vannier & Chen 2000;[8][1][3][4][15] in later studies such as Zhang and Shu, 2007 and Sun et al. 2021[3][4] it was yet again classified as an arthropod of uncertain affinities possibly related to Tuzoia and Silesicaris,[3][7][16] a relationship that was later rejected due to the fact that Silesicaris was reassigned to Thylacocephala and in 2022 Tuzoia was reassigned to Hymenocarina based on newly described soft parts showing that the similarities between the carapace shape between Isoxys and Sunella was only superficial;[17][3][14] and in Sun et al. 2021 it was again considered an arthropod of uncertain affinities,[4] though similarities with Isoxys were noted and a relationship with Isoxys was suggested, as in Zhang and Shu, 2007, due to limited preservation of soft tissue,[3] a relationship could not be confirmed with confidence;[3][4] in 2025 it was recovered as a sister group to Isoxys by Collantes & Pates, 2025;[1] which in turn makes Sunella an upper stem group arthropod and among the most basal Deuteropods.[1][18]
Species
Sunella grandis
Sunella grandis (Huo, 1965) is the type species of Sunella.[3][13][25] Probable sexual dimorphism has been observed.[3][1]
Synonyms
Sunella nanchengensis
Sunella nanchengensis (Huo, 1965) is a probable sexual dimorph of S. grandis and is generally considered a synonym of S. grandis.[3][1]
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.[3][4] Probable sexual dimorphism has been observed.[3] It is one of the species where soft anatomy has been preserved.[3][1]
Sunella dimorphismus
Sunella dimorphismus (Liu et al, 2026) have preserved anatomy including trunk and appendages.[26]
Caudicaella (Sunella) bispinata
A species that previously belonged to Sunella, S. bispinata,[27][28] was moved to a new genus, Caudicaella (Sun et al. 2021)[4] as Caudicaella bispinata (Cui and Huo, 1990).[4][27] Synonyms include Isoxys bispinata (Zhang et al., 2018) (not to be confused with Isoxys bispinatus)[8] Isoxys sp. (Sun et al. 2021) (for the specimens from the Heatherdale shale, not the Shuijingtuo formation)[8][4]
Tuzoia? (Sunella) parva
One former species is Sunella parva (Melnikova, 1988),[13][29] which may be the larval form of a new species of Tuzoia,[17][29][4] which was rejected from Sunella by Melnikova 1998[13][29] and may need to be assigned to its own genus.[13][3][17][29] It has also been suggested to be a bradoriid (Melnikova, 1998)[13]
Synonyms
Sunellidae, which Sunella belongs to originally included two other genera, Chiella (Huo, 1965) and Luella (Huo, 1965),[6] but Chiella was later considered a separate species within Sunella,[3] but after Huo & Shu, 1985 Chiella was considered a junior synonym of Sunella[3] and after Zhang & Shu, 2007 Luella was considered a probable sexual dimorph of Sunella.[3][9][1]
Distribution
Sunella fossils have been found in the Chengjiang biota (exact locality cannot be determined;[3] dated to no older than 518 Ma),[30][25] the Shuijingtuo formation (dated to around 526.5 Ma),[31][27][29][4][32] the Qingjiang biota (dated to ~518 ma),[33] 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)[2][5] and the Guojiaba formation (tentatively assigned to Cambrian Stage 3),[3][34] all of which are in China.[3][4] 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.[3] The former species Caudicaella bispinata is also known from the Heatherdale shale in Australia.[1][8]
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.[3] Only the soft parts of Sunella cf. shensiella and the related Combinivalvula chenjiangensis (Hou, 1987)[35][36][14] have been preserved.[3][1]
Morphology
Carapace
Sunella grandis had a bivalved[37] elongate roughly semicircular carapace with an anterodorsal sulcus and short cardinal spines,[35] a typical bodyplan for a Sunellid,[8][4][1][3] while S. cf. shensiella had a more oval shaped carapace, shorter cardinal spines and an anterodorsal sulcus pointing at a slightly lower angle.[3][4] Both species have shorter cardinal spines than Caudicaella bispinata also Caudicaella bispinata is more similar to Isoxys than other sunellids which led to its assignment to Isoxys in Zhang et al. 2018.[1][38] Jinningella differs from Sunella by the presence of anterodorsal nodes.[3] Probable Sexual dimorphism has been recorded in both species.[3][1][4]
The carapace of S. grandis is typically 6.5-8.5 mm.[3][1][4] The largest specimen is over 15 mm (1.5 cm)(the largest recorded for a sunellid), while Combinivalvula and Jinningella reached roughly 10 mm (1 cm) and Caudicaella typically reached 10–12 mm (1-1.2 cm) due to its long cardinal spines compared to other sunellids.[4][1][8] In comparison to other bivalved arthropods of the Cambrian, sunellids where medium-sized, larger than the bradoriids, which rarely reached above 10 mm (1 cm) and smaller than the Isoxyida which where often at least a few centimeters long with the exception of the dwarf I. carbonelli which grew to 11 mm (1.1 cm) and Surusicaris which grew to 14 mm (1.4 cm).[1][39]
Distinguishing from similar arthropods
Sunella can be distinguished from Isoxys by the possession of an anterodorsal sulcus, shorter cardinal spines and different carapace shapes.[3][1] Sunella be distinguished from the isoxyid Surusicaris by the lack of an anterodorsal sulcus and the lack of cardinal spines. Bradoriids, which share the possession of an anterodorsal sulcus (leading to the former assignment of Sunella to Bradoriida)[1][7][9][10][11][12][13][3] can be distinguished from Sunella by the possession of cardinal spines.[3][1]
Soft anatomy
The soft parts of S. cf. shensiella and Combinivalvula chengjiangensis show that Sunellids had non-biomineralized limbs similar to the related Isoxyidae and a clearly segmented trunk.[3][8][1]S. cf. shensiella and the related Combinivalvula chenjiangensis preserve a possible median eye.[3][36] Neither the cephalic appendages nor proximal portions of trunk limbs as found in the related Isoxyidae are known.[1][3]
