Rhombicalvaria, Helmetid Arthropod from Chengjiang

Rhombicalvaria acantha

Arthropoda Hemetidae

Geological Time: Early Cambrian (~525 million years ago)

Size (25.4mm=1 inch): 25 mm long and 14 mm across on a 30 mm by 45 mm matrix

Fossil Site: Chengjiang Maotianshan Shales - Quiongzhusi Section, Yu’anshan Member, Heilinpu Formation, Mafang Village, Anning, Kunming County, Yunnan Province, China

Rhombicalvaria acanthaDescription: This unusual arthropod is known as Rhombicalvaria acantha. The species is quite rare (this is the first example I have been able to secure).. With the discovery of the Chengjiang Biota in 1984 a window on the Cambrian Explosion in China was opened.

The diversity of soft-tissue fossils is astonishing: algae, medusiforms, sponges, priapulids, annelid like worms, echinoderms, arthropods (including trilobites), Rhombicalvariahemichordates, chordates, and the first agnathan fish make up just a small fraction of the total. Numerous problematic forms areknown as well, some of which may have represented failed attempts at diversity that did not persist to the present day.

It bears some resemblance to the younger Burgess Shale genus Helmetia, which has led some to place them in the Helmetidae along with several other similar arthropods, such as Kuamaia and Panlongia. The body shape suggests that the helmetids were benthic animals which were possibly carnivorous. The species is only known from the Chengjiang Biota.

click fossil images to enlarge

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