Isoxys

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Isoxys
Temporal range: Atdabanian to Paibian
Artistic reconstruction of Isoxys curvirostratus.jpg
Isoxys curvirostratus from the Chengjiang Biota
Isoxys acutangulus 2.jpg
Fossil of Isoxys acutangulus from the Burgess Shale
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Order:† Isoxyida
Family:† Isoxyidae
Genus:† Isoxys
Walcott, 1890
Type species
Isoxys chilhoweanus
Walcott, 1890
Species

See text

Isoxys (meaning "equal surfaces") is a genus of extinct bivalved Cambrian arthropod; the various species of which are thought to have been freely swimming predators. [1] It had a pair of large spherical eyes (which are the most commonly preserved feature of the soft-bodied anatomy), [2] and two large frontal appendages used to grasp prey. [1]

Contents

Description

Diagram of I. curvirostratus showing frontal (b) and biramous appendages (c,d) (c) and (d) represent first four limb pairs and remaining posterior limbs, respectively Figure 3. Morphological reconstruction and phylogenetic position of Isoxys curvirostratus (cropped).jpg
Diagram of I. curvirostratus showing frontal (b) and biramous appendages (c,d) (c) and (d) represent first four limb pairs and remaining posterior limbs, respectively

Species of Isoxys have roughly semicircular bivalved carapaces, which vary in morphology between species. The front and rear edges of the carapaces bear forward and posterior facing spines, respectively which in some species are greatly elongated. [3] The carapaces of Isoxys are typically 1.1 to 3.3 centimetres (0.43 to 1.30 in) in length (with the juveniles of some species being as small as 0.5 centimetres (0.20 in) [4] ), excluding the spines, though some species are known to reach over 6 centimetres (2.4 in). In long-spined species when including spine length, some specimens exceed 10 centimetres (3.9 in). [5] The opening angle of the carapace was close to vertical, giving it a narrow profile when viewed from above. [3]

The head had a pair of large spherical stalked eyes, as well as a pair of upward-curling frontal appendages, which have a varying number of podomeres/segments, depending on the species. Most podomeres on the frontal appendages have upward-facing endite spines, with the number and placement of spines varying between species. The last podomere of the frontal appendage is a curved terminal claw. The trunk lacks clear segmentation (arthrodization). Along the body are pairs of biramous (divided into two parts) appendages, the counts of which differ between species (Isoxys curvirostratus has 14, while Isoxys auritus has 11). In Isoxys curvirostratus the endopods (the inner, leg-like parts of the biramous limbs/appendages) have well defined segments/podomeres. The first four biramous limb pairs of Isoxys curvirostratus were shorter than the remaining pairs, with their endopods having borne well developed endite (inward facing) spines and ended with a curved subchela (claw), with these features absent in the endopods of the more posterior limbs. The exopods (outer part of the biramous limbs) of Isoxys curvirostratus had thick paddle-shaped lamellae which projected perpendicular to the limb axis. [6] Isoxys volucris from Greenland had paddle-shaped exopods suggested to have been fringed with setae. [7] The end of the trunk has paired telson flaps. [6]

Ecology

Species of Isoxys are thought to have been actively swimming predators, using its frontal appendages to capture soft-bodied prey, with the frontalmost pairs of biramous limbs aiding in food processing. [6] The various Isoxys species are thought to have occupied a variety of niches, from swimming just above the seafloor (nektobenthic) to open ocean swimmers (pelagic). [3] Swimming was likely accomplished by rhythmic movement of the legs. [6] Eyes of different specimens appear to have been adapted to different light intensities; one specimen of I. auritus was either crepuscular in shallow water, or lived in waters around 140 m below the sea surface; whereas another was morphologically adapted to a diurnal light intensity in shallow waters. [8] Isoxys species with elongated carapace spines are likely to have engaged in vertical migration up and down the water column, like many modern marine invertebrates. [3] The elongate carapace spines, particularly the forward-facing ones, are thought to have improved hydrodynamic lift, as well as possibly serving as a defense against predators. Some pelagic Isoxys species qualified as being zooplankton. [4] Specimens of Isoxys minor have been found with eggs adhered to the inner surface of the carapace, indicating they engaged in brood care. The brood size was large, with approximately 300 eggs, each 0.5 millimetres (3⁄128 in) across per (presumably female) individual. Egg bearing individuals were only around half the maximum size, suggesting that individuals continued to grow beyond sexual maturity. [9] Isoxys species lacked a distinct planktic larval stage. At least some Isoxys species carapace morphology changed between juveniles and adults, which likely improved their hydrodynamic efficiency. [4]

Species of Isoxys were preyed upon by other animals. [4] Carapaces of the Sirius Passet species Isoxys volucris have been frequently found as stomach contents of the giant stem-chaetognath (arrow worm) Timorebestia, [10] with this species also found as stomach contents of a Sidneyia -like arthropod known from the same deposit. [11] Likely coprolites (fossilised feces) found in Chengjiang biota deposits containing Isoxys carapaces are suggested to have been produced by radiodonts. [12]

Taxonomy

Specimen of Isoxys longissimus Isoxys longissimus.jpg
Specimen of Isoxys longissimus

Isoxys is thought to be one of the basalmost known stem-group arthropods, showing a combination of traits characteristic of more primitive stem-group arthropods like radiodonts, like lacking an arthrodized (sclerotized and jointed) trunk exoskeleton, with those of modern arthropods, like possessing sclerotized and jointed (arthropodized) biramous limbs. It is one of two genera within the family Isoxyidae, alongside Surusicaris . [6]

Cladogram after Zhang et al. 2023: [6]

Total group  Arthropoda

A close relationship to the bivalved arthropod Tuzoia had historically been proposed based on the similarities of some aspects of their carapaces, [13] but preserved soft tissues of Tuzoia described in 2022 suggest that they are not closely related. [14]

Species

20 species of Isoxys have been described, [3] which have a global distribution, having been found in North America, Siberia, Australia, China and Europe, spanning from Cambrian Series 2 into the Miaolingian. [15]

Indeterminate species are also known from the Spence Shale of Utah, dating to the Miaolingian, as well as the Kaili Biota in Guizhou, China, which also dates to the Miaolingian. [15]

See also

References

  1. 1 2 Vannier, J.; Garcia-Bellido, C.; Hu, X.; Chen, L. (Jul 2009). "Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas". Proceedings: Biological Sciences. 276 (1667): 2567–2574. doi:10.1098/rspb.2009.0361. ISSN   0962-8452. PMC   2686666 . PMID   19403536.
  2. ↑ García-Bellido, D.; Vannier, J.; Collins, D. (2009). "Soft-part preservation in two species of the arthropod Isoxys from the middle Cambrian Burgess Shale of British Columbia, Canada" (PDF). Acta Palaeontologica Polonica. 54 (4): 699. doi: 10.4202/app.2009.0024 . S2CID   53613716.
  3. 1 2 3 4 5 Pates, Stephen; Daley, Allison C.; Legg, David A.; Rahman, Imran A. (2021-06-30). "Vertically migrating Isoxys and the early Cambrian biological pump". Proceedings of the Royal Society B: Biological Sciences. 288 (1953): 20210464. doi:10.1098/rspb.2021.0464. PMC   8220267 . PMID   34157876.
  4. 1 2 3 4 Pates, Stephen; Ma, Jiaxin; Wu, Yu; Fu, Dongjing (December 2024). "Impact of ontogeny and spines on the hydrodynamic performance of the Cambrian arthropod Isoxys". Royal Society Open Science. 11 (12). Bibcode:2024RSOS...1140894P. doi:10.1098/rsos.240894. ISSN   2054-5703. PMC   11641431 . PMID   39677541.
  5. ↑ García-Bellido, Diego C.; Paterson, John R.; Edgecombe, Gregory D.; Jago, James B.; Gehling, James G.; Lee, Michael S. Y. (November 2009). "The bivalved arthropods Isoxys and Tuzoia with soft-part preservation from the Lower Cambrian Emu Bay Shale Lagerstätte (Kangaroo Island, Australia): Isoxys and Tuzoia From Emu Bay Shale (Australia)". Palaeontology. 52 (6): 1221–1241. doi: 10.1111/j.1475-4983.2009.00914.x . S2CID   129374179.
  6. 1 2 3 4 5 6 Zhang, Caixia; Liu, Yu; Ortega-Hernández, Javier; Wolfe, Joanna; Jin, Changfei; Mai, Huijuan; Hou, Xian-guang; Guo, Jin; Zhai, Dayou (19 April 2023). "Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution". Proceedings of the Royal Society B: Biological Sciences. 290 (1997). doi:10.1098/rspb.2023.0335. PMID   37072042.
  7. ↑ Stein, Martin; Peel, John S.; Siveter, David J.; Williams, Mark (2009-09-07). "Isoxys (Arthropoda) with preserved soft anatomy from the Sirius Passet Lagerstätte, lower Cambrian of North Greenland: Isoxys with soft anatomy from Greenland". Lethaia. 43 (2): 258–265. doi:10.1111/j.1502-3931.2009.00189.x.
  8. ↑ Schoenemann, B.; Clarkson, E. N. K. (2010). "Eyes and vision in the Chengjiang arthropod Isoxys indicating adaptation to habitat". Lethaia. 44 (2): no. doi:10.1111/j.1502-3931.2010.00239.x.
  9. ↑ Ma, Jiaxin; Pates, Stephen; Wu, Yu; Lin, Weiliang; Liu, Cong; Wu, Yuheng; Zhang, Mingjing; Fu, Dongjing (2023-05-25). "Ontogeny and brooding strategy of the early Cambrian arthropod Isoxys minor from the Qingjiang biota". Frontiers in Ecology and Evolution. 11. doi: 10.3389/fevo.2023.1174564 . ISSN   2296-701X.
  10. ↑ 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.; Nielsen, Arne T.; Vinther, Jakob (2024). "A giant stem-group chaetognath". Science Advances. 10 (1): eadi6678. Bibcode:2024SciA...10I6678P. doi: 10.1126/sciadv.adi6678 . PMC   10796117 . PMID   38170772.
  11. ↑ Peel, John S. (2017-04-03). "Mineralized gutfills from the Sirius Passet Lagerstätte (Cambrian Series 2) of North Greenland". GFF. 139 (2): 83–91. Bibcode:2017GFF...139...83P. doi:10.1080/11035897.2016.1260051. ISSN   1103-5897.
  12. ↑ VANNIER, J.; CHEN, J. (2005-02-01). "Early Cambrian Food Chain: New Evidence from Fossil Aggregates in the Maotianshan Shale Biota, SW China". PALAIOS. 20 (1): 3–26. Bibcode:2005Palai..20....3V. doi:10.2110/palo.2003.p03-40. ISSN   0883-1351.
  13. ↑ Ma, Jiaxin; Lin, Weiliang; Liu, Cong; Sun, Ao; Wu, Yu; Wu, Yuheng; Fu, Dongjing (January 2022). "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. Bibcode:2022JGSoc.179..229M. doi:10.1144/jgs2020-229. ISSN   0016-7649. S2CID   236289449.
  14. ↑ 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.
  15. 1 2 Liu, S.; Peng, J.; Wen, R.; Liang, B. (2018-06-30). "New data for Isoxys of the Balang Fauna (Cambrian Stage 4), South China". Bulletin of Geosciences: 147–162. doi: 10.3140/bull.geosci.1673 . ISSN   1802-8225. S2CID   134294843.
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