Sunella

Sunella is an extinct genus of Cambrian stem group arthropod. It was named by Huo Shicheng in 1965. It is the type genus of the family Sunellidae. It has 4 spec

Sunella

Sunella is an extinct genus of Cambrian stem group arthropod. It was named by Huo Shicheng in 1965. It is the type genus of the family Sunellidae. It has 4 species, including its type, S. grandis.

Classification

When it was first found by Huo Shicheng in 1965, Sunella was thought to be an Ostracod-like arthropod;[1][2][3] then, in later studies it was thought to be an arthropod of uncertain affinities;[3][4][5][6] then it was interpreted as a bradoriid[1][7][8][9][10][11] and then as again an arthropod of uncertain affinities;[1][12] then as a species of Isoxys along with other Sunellids in Vannier & Chen 2000;[5][4][3][6][13] in later studies such as Zhang and Shu, 2007 and Sun et. al. 2021[3][6] it was yet again thought to be an arthropod of uncertain affinities possibly related to Tuzoia and Silesicaris,[3][1][14] a relationship that was later disproved 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;[15][3][12] and then in Sun et al. 2021 it was again considered an arthropod of uncertain affinities,[6] though similarities with Isoxys were noted and a relationship with Isoxys was suggested, as in Zhang and Shu, 2007, but due to limited preservation of soft tissue,[3] a the relationships of Sunella remained uncertain;[3][6] in 2025 it was recovered as a sister group to Isoxys by Collantes & Pates, 2025;[4] which in turn makes Sunella an upper stem group (ancestral) arthropod and among the most ancestral Deuteropods.[4][16]

Sunella is related to Isoxys (Walcott, 1890),[4][17][18][19] an image of the species I. volucris (Williams et. al. 1996)[4][12] from the Sirius Passet Lagerstätte [12][20][21][22] is pictured above.

Species

Sunella grandis (Huo, 1965) is the type species of Sunella.[3][11][23] Possible sexual dimorphism (when individuals of a species of different sexes are differentiated) is known.[3][4]

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

Sunella nanchengensis

Sunella nanchengensis (Huo, 1965) is a possible sexual dimorph of S. grandis and is now thought to be a synonym of S. grandis.[3][4]

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 moved to its own genus.[3][6] Probable sexual dimorphism has been observed.[3] It is the only species where soft parts are known.[3][4]

Caudicaella (Sunella) bispinata

A species that used to belong to Sunella, S. bispinata,[24][25] and was moved to a new genus, Caudicaella (Sun et al. 2021)[6] as Caudicaella bispinata (Cui and Huo, 1990).[6][24] Synonyms include Isoxys bispinata (Zhang et al., 2018) (not to be confused with Isoxys bispinatus)[5] Isoxys sp. (Sun et al. 2021) (for the specimens from the Heatherdale shale, not the Shuijingtuo formation)[5][6]

Tuzoia? (Sunella) parva

One former species is Sunella parva (Melnikova, 1988),[11][26] which may be the larval form of a new species of Tuzoia,[15][26][6] which was found not to be a species of Sunella by Melnikova 1998[11][26] and may need to be assigned to its own genus.[11][3][15][26] It has also been suggested to be a bradoriid (Melnikova, 1998)[11]

Synonyms

Sunellidae, which Sunella belongs to originally included two other genera, Chiella (Huo, 1965) and Luella (Huo, 1965),[2] 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 possible sexual dimorph of Sunella.[3][7][4]

Distribution

Sunella fossils have been found in the Chengjiang biota (exact locality cannot be determined;[3] dated to no older than 518 Ma),[27][23] the Shuijingtuo formation (dated to around 526.5 Ma),[28][24][26][6][29] the Qingjiang biota (dated to ~518 ma),[30] 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)[31][32] and the Guojiaba formation (tentatively assigned to Cambrian Stage 3),[3][33] all of which are in China.[3][6] Similiar 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.[4][5]

Usually the only part of Sunella that is found is the shell, but rarely (only twenty-two times in Sunella cf. shensiella and only six in the related Combinivalvula chenjiangensis) soft parts are also found. When this occurs, the carapace usually adopts a "butterfly" position, possibly due to the decay of the shell muscles that hold it together.[3] Only the soft parts of Sunella cf. shensiella and the related Combinivalvula chenjiangensis (Hou, 1987)[34][35][12] have been preserved.[3][4]

Sunella grandis had a bivalved[36] elongate roughly semicircular shell with an anterodorsal sulcus and short spines,[34] a typical bodyplan for a Sunellid,[5][6][4][3] while S. cf. shensiella had a more oval shaped shell, shorter cardinal spines and an anterodorsal sulcus pointing at a slightly lower angle.[3][6] 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.[4][37] Jinningella differs from Sunella by the presence of anterodorsal nodes.[3] possible sexual dimorphism has been recorded in both species.[3][4][6]

The carapace of S. grandis is typically 6.5-8.5 mm.[3][4][6] 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 spines compared to other sunellids.[6][4][5] 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).[4][38]

Distinguishing from similar arthropods

Isoxys has longer cardinal spines than Sunella, as seen in this fossil of I. longissimus.[4]

Sunella can be distinguished from Isoxys by the presence of an anterodorsal sulcus, shorter cardinal spines and different carapace shapes.[3][4] 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)[4][1][7][8][9][10][11][3] can be distinguished from Sunella by the possession of cardinal spines.[3][4]

Kunmingella.png
Surusicaris lacks spines.
Bradoriids, such as Kunmingella also lack spines.

Soft anatomy

The soft parts of S. cf. shensiella and Combinivalvula chengjiangensis show that Sunellids had non-biomineralized biramous limbs similar to the related Isoxyidae and a clearly segmented trunk.[3][5][4]S. cf. shensiella and the related Combinivalvula chenjiangensis preserve a possible median eye.[3][35] Neither the cephalic appendages nor proximal portions of trunk limbs as found in the related Isoxyidae are known.[4][3]

Isoxys possesses frontal appendages, as seen in this reconstruction of I. curvirostratus; which Sunella lacks.[3]
  1. ↑ 1.0 1.1 1.2 1.3 1.4 Xian-Guang, Hou (January 1999). "New rare bivalved arthropods from the Lower Cambrian Chengjiang fauna, Yunnan, China". Journal of Paleontology. 73 (1): 102–116. Bibcode:1999JPal...73..102X. doi:10.1017/S002233600002758X. ISSN 0022-3360. JSTOR 1306748 – via JSTOR.
  2. ↑ 2.0 2.1 Huo, Shicheng (1965). "The Early Cambrian ostracods from Shaanxi and Yunnan". Acta Palaeontologica Sinica (in Chinese and English). 13.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 3.31 3.32 3.33 3.34 3.35 Zhang, Xingliang; Shu, Degan (2007). "Soft anatomy of sunellid arthropods from the Chengjiang Lagerstutte, Lower Cambrian of southwest China". Journal of Paleontology. 81 (6): 1412–1422. Bibcode:2007JPal...81.1412Z. doi:10.1666/06-031R.1. Cite error: Invalid <ref> tag; name ":02" defined multiple times with different content.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 Collantes, Luis; Pates, Stephen (2025). "Isoxys carbonelli and the palaeoenvironmental disparity of Isoxys during Cambrian Stage 3". Historical Biology. 38 (4): 923–931. doi:10.1080/08912963.2025.2529395. ISSN 0891-2963. Cite error: Invalid <ref> tag; name ":3" defined multiple times with different content.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Chen, Fei-Yang; Betts, Marissa J.; Zhang, Zhi-Liang; Brock, Glenn A. (2025-06-01). "The bivalved arthropod Caudicaella aff. bispinata from the Heatherdale Shale (Cambrian Stage 3), South Australia". Palaeoworld. 34 (3): 100882. Bibcode:2025Palae..3400882C. doi:10.1016/j.palwor.2024.09.006. ISSN 1871-174X.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 6.15 6.16 Sun, Ao; Cong, Liu; Ma, Jiaxin; Fu, Dongjing (13 March 2021). "Taxonomy and ontogeny of bivalved arthropods from the lower member of the Shuijingtuo Formation, Series 2 and Stage 3, Eastern Three Georges Area, South China". Acta Palaeontologica Sinica. 60: 187–199. doi:10.19800/j.cnki.aps.2021019.
  7. ↑ 7.0 7.1 7.2 Huo, Shicheng; Shu, Degan (January 1985). Cambrian bradoriids of South China (in Chinese). China: Northwest University Press.
  8. ↑ 8.0 8.1 Jones, P.J.; McKenzie, K.G. (1980-01-01). "Queensland Middle Cambrian Bradoriida (Crustacea): new taxa, palaeobiogeography and biological affinities". Alcheringa: An Australasian Journal of Palaeontology. 4 (3): 203–225. Bibcode:1980Alch....4..203J. doi:10.1080/03115518008618932. ISSN 0311-5518.
  9. ↑ 9.0 9.1 Williams, Mark; Siveter, David J.; Popov, Leonid E.; Vannier, Jean M. C. (2007-05-14). "Biogeography and affinities of the bradoriid arthropods: Cosmopolitan microbenthos of the Cambrian seas". Palaeogeography, Palaeoclimatology, Palaeoecology. 248 (1): 202–232. Bibcode:2007PPP...248..202W. doi:10.1016/j.palaeo.2006.12.004. ISSN 0031-0182.
  10. ↑ 10.0 10.1 Melnikova, L. M. (1988). "Some Bradoriids (Crustacea) from the Botomian stage of Eastern Transbaikalia". Paleontologicheskii Zhurnal.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 L.m, Melnikova (1998). "REVISION OF SOME CAMBRIAN BRADORIIDS (CRUSTACEA) FROM THE SIBERIAN PLATFORM". Paleontologicheskii Zhurnal. ISSN 0031-031X.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Williams, Mark; Siveter, David J.; Peel, John S. (1996). "Isoxys (Arthropoda) from the Early Cambrian Sirius Passet Lagerstätte, North Greenland". Journal of Paleontology. 70 (6): 947–954. Bibcode:1996JPal...70..947W. doi:10.1017/S0022336000038646. ISSN 0022-3360. JSTOR 1306499.
  13. ↑ Vannier, Jean; Chen, Jun-Yuan (2000). "The Early Cambrian colonization of pelagic niches exemplified by Isoxys (Arthropoda)". Lethaia. 33 (4): 295–311. Bibcode:2000Letha..33..295V. doi:10.1080/002411600750053862. ISSN 1502-3931.
  14. ↑ "†class Thylacocephala Pinna et al. 1982 (crustacean)". Paleobiology database.
  15. ↑ 15.0 15.1 15.2 Izquierdo-López, Alejandro; Caron, Jean-Bernard (December 2022). "The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited". Royal Society Open Science. 9 (12) 220933. Bibcode:2022RSOS....920933I. doi:10.1098/rsos.220933. ISSN 2054-5703. PMC 9727825. PMID 36483757.
  16. ↑ Zhang, Caixia; Liu, Yu; Ortega-Hernández, Javier; Wolfe, Joanna M.; Jin, Changfei; Mai, Huijuan; Hou, Xianguang; Guo, Jin; Zhai, Dayou (2023-04-26). "Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution". Proceedings. Biological Sciences. 290 (1997) 20230335. doi:10.1098/rspb.2023.0335. ISSN 1471-2954. PMC 10113025. PMID 37072042.
  17. ↑ Paterson, John R.; García-Bellido, Diego C.; Edgecombe, Gregory D. (2012). "New Artiopodan Arthropods from the Early Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia". Journal of Paleontology. 86 (2): 340–357. Bibcode:2012JPal...86..340P. doi:10.1666/11-077.1. ISSN 0022-3360. JSTOR 41480196.
  18. ↑ "PBDB". paleobiodb.org. Archived from the original on 2022-02-03. Retrieved 2025-10-11.
  19. ↑ "PBDB Taxon". paleobiodb.org. Archived from the original on 2025-02-07. Retrieved 2025-10-11.
  20. ↑ Park, Tae-Yoon S.; Nielsen, Morten Lunde; Parry, Luke A.; Sørensen, Martin Vinther; Lee, Mirinae; Kihm, Ji-Hoon; Ahn, Inhye; Park, Changkun; de Vivo, Giacinto; Smith, M. Paul; Harper, David A. T. (2024-01-05). "A giant stem-group chaetognath". Science Advances. 10 (1) eadi6678. Bibcode:2024SciA...10I6678P. doi:10.1126/sciadv.adi6678. ISSN 2375-2548. PMC 10796117. PMID 38170772.
  21. ↑ Nielsen, Morten Lunde; Lee, Mirinae; Wilby, Philip R.; Park, Tae-Yoon S.; Nielsen, Arne T.; Vinther, Jakob (2025). "Negative ontogenetic allometry of cardinal spines in the early Cambrian arthropod Isoxys volucris indicates their defensive function". Palaeontology. 68 (4) e70017. Bibcode:2025Palgy..6870017N. doi:10.1111/pala.70017. ISSN 1475-4983.
  22. ↑ Daley, Allison C.; Peel, John S. (2010). "A Possible Anomalocaridid from the Cambrian Sirius Passet Lagerstätte, North Greenland". Journal of Paleontology. 84 (2): 352–355. Bibcode:2010JPal...84..352D. doi:10.1666/09-136R1.1. ISSN 0022-3360. JSTOR 40605523.
  23. ↑ 23.0 23.1 "Chengjiang Maotianshan Shales". www.fossilmuseum.net. Archived from the original on 9 March 2021. Retrieved 24 September 2025.
  24. ↑ 24.0 24.1 24.2 Cui, Zhilin; Huo, Shicheng (1990). "New discoveries of Lower Cambrian crustacean fossils from western Hubei". Acta Palaeontologica Sinica (in Chinese and English). 29 (3): 321–330.
  25. ↑ Vannier, Jean (2009-03-01). "L'Explosion cambrienne ou l'émergence des écosystèmes modernes". Comptes Rendus Palevol. 8 (2): 133–154. Bibcode:2009CRPal...8..133V. doi:10.1016/j.crpv.2008.10.006. ISSN 1631-0683.
  26. ↑ 26.0 26.1 26.2 26.3 26.4 Ma, Jiaxin; Lin, Weiliang; Liu, Cong; Sun, Ao; Wu, Yu; Wu, Yuheng; Fu, Dongjing (6 July 2021). "A new bivalved arthropod from the Cambrian (Stage 3) Qingjiang biota expands the palaeogeographical distribution and increases the diversity of Tuzoiidae". Journal of the Geological Society. 179 (1) jgs2020-229. doi:10.1144/jgs2020-229.
  27. ↑ Yang, Chuan; Li, Xian-Hua; Zhu, Maoyan; Condon, Daniel J.; Chen, Junyuan (2018-03-15). "Geochronological constraint on the Cambrian Chengjiang biota, South China". Journal of the Geological Society. 175 (4): 659–666. Bibcode:2018JGSoc.175..659Y. doi:10.1144/jgs2017-103. ISSN 0016-7649. Archived from the original on 2025-04-07. Retrieved 2025-09-14.
  28. ↑ Yang, Chuan; Bowyer, Fred T.; Condon, Daniel J.; Li, Xian-Hua; Zhu, Maoyan (2023-04-15). "New U-Pb age from the Shuijingtuo Formation (Yangtze Gorges area) and its implications for the Cambrian timescale". Palaeogeography, Palaeoclimatology, Palaeoecology. 616 111477. Bibcode:2023PPP...61611477Y. doi:10.1016/j.palaeo.2023.111477. ISSN 0031-0182.
  29. ↑ Zhao, Bingshuang; Long, Xiaoping; Chang, Chao (2024-03-01). "Early Cambrian sedimentary rocks in South China: A link between oceanic oxygenation and biological explosion". Earth-Science Reviews. 250 104708. Bibcode:2024ESRv..25004708Z. doi:10.1016/j.earscirev.2024.104708. ISSN 0012-8252.
  30. ↑ Fu, Dongjing; Tong, Guanghui; Dai, Tao; Liu, Wei; Yang, Yuning; Zhang, Yuan; Cui, Linhao; Li, Luoyang; Yun, Hao; Wu, Yu; Sun, Ao (2019-03-22). "The Qingjiang biota—A Burgess Shale–type fossil Lagerstätte from the early Cambrian of South China". Science. 363 (6433): 1338–1342. Bibcode:2019Sci...363.1338F. doi:10.1126/science.aau8800. PMID 30898931.
  31. ↑ Zhao, M.-S.; Wang, Y.; Tian, J. -C.; Lei, L. -F. (October 2013). "A sedimentary environment analysis of black shales based on fossil assemblage characteristics:A case study of cambrian niutitang formation in Changyang area, western Hubei". Geology in China. 40 (5): 1484–1492 – via Researchgate.net.
  32. ↑ "Niutitang Formation". chinalex.geolex.org. Retrieved 2025-09-27.
  33. ↑ Tan, Yineng; Xiao, Xianming; Gao, Ping; Meng, Guangming; Wang, Qiang; Liu, Wei; Feng, Yue (2025-06-15). "Sedimentary Environment and Organic Matter Enrichment Mechanism of the Lower Cambrian Shale in the Northern Margin of the Yangtze Platform". Journal of Marine Science and Engineering. 13 (6): 1175. Bibcode:2025JMSE...13.1175T. doi:10.3390/jmse13061175.
  34. ↑ 34.0 34.1 Hou, Xian-guang (1987). "EARLY CAMBRIAN LARGE BIVALVED ARTHROPODS FROM CHENGJIANG, EASTERN YUNNAN". Acta Palaeontologica Sinica. 26 (3): 272–285. Archived from the original on 2023-03-02. Retrieved 2025-10-10.
  35. ↑ 35.0 35.1 Fu, Dongjing; Zhang, Xingliang (2011). "A New Arthropod Jugatacaris Agilis N. Gen. N. Sp. from the Early Cambrian Chengjiang Biota, South China". Journal of Paleontology. 85 (3): 567–586. Bibcode:2011JPal...85..567F. doi:10.1666/09-173.1. ISSN 0022-3360. JSTOR 23020192.
  36. ↑ Castle Jones, Jack; M. Jackquet, Sarah; Chen, Feiyang; J. Betts, Marissa; Zhang, Zhiliang; Anthony Hall, Philip; A. Brock, Glenn; Klaebae, Robert (April 2025). "A new integrated lower Cambrian chronostratigraphy for the Normanville Group, eastern Stansbury Basin, with definition of the oldest small shelly fossil zones in South Australia". Australasian Palaeontological Memoirs. 57: 465–489 – via Researchgate.
  37. ↑ Zhang, Lei; Chang, Shan; Khan, Maliha Zareen; Feng, Qinglai; Danelian, Taniel; Clausen, Sébastien; Tribovillard, Nicolas; Steiner, Michael (2018-04-15). "The link between metazoan diversity and paleo-oxygenation in the early Cambrian: An integrated palaeontological and geochemical record from the eastern Three Gorges Region of South China". Palaeogeography, Palaeoclimatology, Palaeoecology. 495: 24–41. Bibcode:2018PPP...495...24Z. doi:10.1016/j.palaeo.2017.12.007. ISSN 0031-0182.
  38. ↑ Aria, Cédric; Caron, Jean-Bernard (2015). "Cephalic and limb anatomy of a new Isoxyid from the Burgess Shale and the role of "stem bivalved arthropods" in the disparity of the frontalmost appendage". PLOS ONE. 10 (6) e0124979. Bibcode:2015PLoSO..1024979A. doi:10.1371/journal.pone.0124979. ISSN 1932-6203. PMC 4454494. PMID 26038846.

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