Temporal range: Sirius Passet
The body of K. soperi consisted of a simple head shield, 16 trunk segments (tergites) and a tail plate. The species reached a length of 234–534 millimetres (9.2–21.0 in) and was in outward appearance almost elliptical, about twice as long as wide, with the widest point in the front third of the body at the 3rd to 5th tergite.
The head plate was simple, convex in shape, wider than long, and represented about 20% of the total body length. The tergites were short, about five times as wide as long, and the rear edge of each tergite to overshadow the following by about a fifth of its length. The tergites 1–5 all had about the same width, the following tergites have been coming back ever narrower. In the middle of an axis was clearly seen, which constituted about half the width of a tergite and gave him a three-lobed shape. At the lateral ends of each tergite existed rearwardly longer and longer spines. The tiny tail was semi-circular, about half as long as wide; and the front half had to two-thirds of the tail shield also has a three-lobed shape.
The first segment bore a ventral pair of large limbs, known as the antennule, which are about half to two-thirds as long as the body. They consisted of a cylindrical stem and about 15 segments. The segments had a flat outer side and two widely spaced spines.
The other limbs – three pairs in the head region shield and 16 on the body segments – composed of two branches. The basipod was long, trapezoid-shaped and had two rows with different numbers of spines. The exopods were paddle-like lobes, which were fringed with bristles. The length was little more than two thirds of the length of the endopodite.
Its gut is occasionally preserved in three dimensions, perhaps in phosophate.
It is believed that Kiisortoqia was a predatory swimmer. The large paddle-like exopods were probably suitable for swimming. With its robust antennule, K. soperi could capture prey, probably using its prickly basipods to bring them to its mouth.
The name of the genus is derived from the Greenlandic word kiisortoq, meaning "predator" or "hunter". The specific epithet is in honor of Norman John (Jack) Soper who, together with A. K. Higgins, discovered the Sirius Passet fauna and collected the first fossils from the locality.
More than 170 specimens of the species were recovered in the course of several expeditions between 1985 and 2006 from the Lower Cambrian Sirius Passet Konservat-Lagerstätte, in Peary Land, northern Greenland. The specimens are usually more or less completely preserved: disarticulated fossils were not discovered.
Kiisortoqia soperi possesses three crown-arthropod homologies: a head shield with three pairs of limbs, plus the antennula; postantennular biramous limbs; and flap-like exopods fringed with setae. An exact position within the arthropods, however, cannot be determined, due to the possible lack of eyes and the ambiguous shape of the tail plate. A possible synapomorphy of the antennules with the chelicerates is purely speculative. A later cladistic analysis resolved Kiisortoqia as sister to the rest of crown-group arthropods studied.
- Martin Stein (2010). "A new arthropod from the Early Cambrian of North Greenland, with a 'great appendage'-like antennula". Zoological Journal of the Linnean Society. 158 (3): 477–500. doi:10.1111/j.1096-3642.2009.00562.x.
- Martin Stein & Paul A. Selden (2012). "A restudy of the Burgess Shale (Cambrian) arthropod Emeraldella brocki and reassessment of its affinities". Journal of Systematic Palaeontology. 10 (2): 361–383. doi:10.1080/14772019.2011.566634.
- Data related to Kiisortoqia at Wikispecies