Sõda

MEEDIAVALVUR: algab „sõjalise erioperatsiooni“ teine etapp nimega „SÕDA“

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


Kommenteeri