Aglaspidida is an extinct order of marine arthropods known from fossils spanning the Middle Cambrian to the Upper Ordovician. Initially considered chelicerates, modern anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within this group,[1] and that they lie instead within the Artiopoda, thus placing them closer to the trilobites,[2] being placed in the artiopod subgroup Vicissicaudata.[3]

Aglaspidid fossils are found in North America (upper Mississippi valley, Missouri and Utah), Europe, Australia, and China. Certain other artiopods are believed to be closely related to the aglaspidids, including the order Strabopida, which includes the genera Strabops, Paleomerus, Parapaleomerus, and possibly Khankaspis.

Description

The exoskeletons of Aglaspidida are moderately phosphatized, though they are much less mineralised than those of trilobites.[3] The headshields typically have a pair of unstalked eyes attached to the upper surface, though some species appear to have lost their eyes entirely. The underside of the headshield (cephalon) had either four or possibly five pairs of attached limbs (including a pair of antennae[2]). The trunk segments (tergites) were freely articulating (with the exception of the posteriormost one[4]), and bore outwardly projecting pleurae.[4] The body ends with a tailspine.[3] A distinctive feature of aglaspididans are "postventral plates", a pair of two flat sclerotized plates located on the underside of the posterior final few segments of the body, covering the base of the tailspine.[2]

Members of the family Agaspididae (which constitute the majority of the group) are additionally defined as having a flat, wide body, with the posterior outer corners of the headshield having an acute to spinose shape, with the trunk ancestrally composed of 12 tergites, with the tailspine being elongate and fused to the posteriormost 12th trunk segment/tergite.[3]

Evolution

Aglaspidida first appeared in Laurentia (what is now North America) during the Guzhangian stage of the upper Miaolingian (Cambrian Series 3), before undergoing a major radiation during the Furongian (Upper Cambrian), reaching a worldwide distribution during this period. During the Ordovician, remains are known from Avalonia (modern Wales), Gondwana (Morocco) and South China, with the youngest representative being known from the Katian stage of the Upper Ordovician in Morocco.[3]

List of genera

Cladogram after Jiao et al. 2021.[5]

Vicissicaudata

References

  1. ^ Ortega-Hernández, J.; Braddy, S. J.; Jago, J. B.; Baillie, P. W. (2010). "A new aglaspidid arthropod from the Upper Cambrian of Tasmania". Palaeontology. 53 (5): 1065–1076. Bibcode:2010Palgy..53.1065O. doi:10.1111/j.1475-4983.2010.00974.x.
  2. ^ a b c Ortega-Hernández, J.; Legg, D. A.; Braddy, S. 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. PMID 34814371.
  3. ^ a b c d e Lerosey-Aubril, Rudy; Zhu, Xuejian; Ortega-Hernández, Javier (2017-09-11). "The Vicissicaudata revisited – insights from a new aglaspidid arthropod with caudal appendages from the Furongian of China". Scientific Reports. 7 (1): 11117. Bibcode:2017NatSR...711117L. doi:10.1038/s41598-017-11610-5. ISSN 2045-2322. PMC 5593897. PMID 28894246.
  4. ^ a b Ortega-HernáNdez, Javier; Van Roy, Peter; Lerosey-Aubril, Rudy (May 2016). "A new aglaspidid euarthropod with a six-segmented trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco". Geological Magazine. 153 (3): 524–536. Bibcode:2016GeoM..153..524O. doi:10.1017/S0016756815000710. ISSN 0016-7568.
  5. ^ Jiao, De-Guang; Du, Kun-Sheng; Zhang, Xi-Guang; Yang, Jie; Eggink, Daniel (May 2022). "A new small soft-bodied non-trilobite artiopod from the Cambrian Stage 4 Guanshan Biota". Geological Magazine. 159 (5): 730–734. Bibcode:2022GeoM..159..730J. doi:10.1017/S0016756821001254. ISSN 0016-7568.
  • Hesselbo, SP. 1992. Aglaspidida (Arthropoda) from the Upper Cambrian of Wisconsin. Journal of Paleontology 66(6)885-923.
  • Raasch, GO. 1939. Cambrian Merostomata. Geological Society of America Special Paper 19, 146p.
Allikas: http://en.wikipedia.org/wiki/Aglaspid
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