Helmetia is an extinct genus of arthropod from the middle Cambrian (Wuliuan). Fossils of the type species Helmetia expansa have been found in the Burgess Shale of Canada. A putative second species Helmetia? fastigata was found in the Jince Formation of the Czech Republic.[1]

Fossils are both rare and poorly known; the genus was described by Walcott in 1918 and has not been reexamined, though it was briefly reviewed in the 1990s and has been included in a number of cladistic analyses. Alongside other helmetiids (e.g. Kuamaia), it is a member of Conciliterga,[2] a group which had been resolved by multiple phylogenetic analysis as one of the closest relatives of trilobites within Artiopoda.[3][4][5]

The most complete specimen of Helmetia is 19 cm long. The whole animal is broad and flat with a thin exoskeleton.[6] The leaf-shaped dorsal exoskeleton (tergite) was divided into a trapezoid cephalon (head shield), a thorax with six segments, and a triangular pygidium (tail shield) with 5 marginal spines. Unlike trilobites, the margin of the head shield is concave, ending in a spine on each frontal corner. There is an oval anterior sclerite with two median eye-like frontal organs at the anterior center of the head shield, behind which are two stalked lateral eyes hidden underneath the head shield.[6][7] Other ventral structures are not well described, and due to that it is originally considered as swimming suspension feeder which only had filamentous limbs (exopods).[6] However, remains suggest that it had endopods (walking legs) like other artiopods, and possibly have around 15 pairs of them: 3 or 4 for head (the fourth pair are located at the cephalon-thorax boundary), 1 for each thoracic segment and at least 5 for pygidium.[2] The first pair of appendages (antennae) are yet to be discovered.[5]

References

  1. ^ Kordule Chlupáč (2002). "Arthropods of Burgess Shale type from the Middle Cambrian of Bohemia (Czech Republic)". Bulletin of the Czech Geological Survey. 77: 167–182.
  2. ^ a b Xianguang, Hou; Strom, Jan Berg (1997-12-22), "Arthropods of the Lower Cambrian Chengjiang fauna, southwest China", Arthropods of the Lower Cambrian Chengjiang fauna, southwest China, Fossils and Strata, Scandinavian University Press, pp. 1–117, doi:10.18261/8200376931-1997-01, ISBN 978-82-00-37693-4, retrieved 2025-03-02
  3. ^ Edgecombe, Gregory D.; Ramsköld, Lars (1999). "Relationships of Cambrian Arachnata and the Systematic Position of Trilobita". Journal of Paleontology. 73 (2): 263–287. Bibcode:1999JPal...73..263E. doi:10.1017/S0022336000027761. JSTOR 1306784. S2CID 84029615.
  4. ^ Ortega-Hernández, Javier; Legg, David A.; Braddy, Simon J. (2013). "The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda". Cladistics. 29 (1): 15–45. doi:10.1111/j.1096-0031.2012.00413.x. ISSN 1096-0031.
  5. ^ a b O'Flynn, Robert J.; Williams, Mark; Liu, Yu; Hou, Xianguang; Guo, Jin; Edgecombe, Gregory D. (2025-02-12). "The early Cambrian Kuamaia lata , an artiopodan euarthropod with a raptorial frontal appendage". Journal of Paleontology: 1–13. doi:10.1017/jpa.2024.33. ISSN 0022-3360.
  6. ^ a b c CARON, J.-B. and JACKSON, D. A. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258, 222–256.
  7. ^ Ortega-Hernández, Javier (2015). "Homology of Head Sclerites in Burgess Shale Euarthropods". Current Biology. 25 (12): 1625–1631. doi:10.1016/j.cub.2015.04.034.

Further reading


Allikas: http://en.wikipedia.org/wiki/Helmetia
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