Kiisortoqia soperi, Stein, 2010

Stein, Martin, 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), pp. 477-500 : 480-484

KIISORTOQIA SOPERI SP. NOV. ( FIGS 4–15 )

Derivation of name: Named after N.J. Soper, who, together with A.K. Higgins, collected the first fossils from the Sirius Passet Lagerstätte.

Diagnosis: Euarthropod with a simple head shield, 16 trunk segments, and a small, semicircular tail shield. Antennula about one half to two thirds of body length, sturdy, composed of a peduncle and about 15 articles, with paired spines medially.

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total length (mm)

Holotype: MGUH 28942 .

Type locality and horizon: J.P. Koch Fjord, Peary Land, central North Greenland; base of the Buen Formation (Cambrian series 2, stage 3).

Material illustrated: MGUH 28943–28969.

Other material: There are 147 additional specimens.

Repository: Typus specimen and illustrated specimens are deposited in the Geological Museum, Copenhagen (with the MGUH prefix) .

Description: Dorsal morphology: the observed specimens of K. soperi gen. et sp. nov. range from 234 mm to 534 mm in total length, with an approximately normal distribution ( Fig. 3A ). All specimens in this range have a trunk with 16 tergites.

The general habitus is subelliptical, with a length of approximately twice the width, and with the maximum width in the anterior third of the body at the third to fifth tergite ( Fig. 4 ). The length of the head shield is, on average, one fifth of the total length ( Fig. 3B ). The trunk tapers from the fifth tergite backwards to a small tail shield. The trunk and tail shield are trilobate ( Figs 4–6 ).

The head shield is a simple, convex shield, without marked trilobation, and is parabolic in outline, and wider than long.

The trunk consists of 16 segments ( Figs 4 , 5 ). The tergites are short, and are about five times wider than long. Tergites 1–5 are of almost equal length; tergites 3–5 are widest. The width and length of tergites decrease posterior to the fifth tergite. Axially, the posterior border of each tergite overhangs the following tergite by approximately one fifth of its length, but by less abaxially. The axis is approximately half the width of the tergites. The first and second tergites curve towards the anterior abaxially, whereas tergites posterior to that are straight. The tergopleurae of the first tergite terminate bluntly posterolaterally, and those of the second tergite have a pointed posterolateral corner. The tergopleurae of tergites 4–16 are extended into posterolateral projections. The length of these projections increases posteriorly.

The tail shield is a semicircular plate, and is twice as wide as long; the anterior half to two thirds are trilobate ( Figs 4–6 ).

Ventral morphology: the head carries the large antennulae and three pairs of biramous limbs ventrally. The hypostome or eyes are not known.

Each antennula reaches a length of one half to two thirds the total length of the body: it consists of a peduncle and about 15 articles. The peduncle is at least twice as long as the proximal article, and is distinguished from the articles by the absence of armament. It inserts well below the head shield ( Fig. 7D ). The articles are cylindrical, with a flat to concave medial side ( Fig. 7D ). Articles 1–6 are of equal width, but increase in length distally. Articles 7–15 decrease in both width and length. Each article carries two spines, set wide apart, medially.

B

5 mm

All postantennular limbs are biramous. The sizes increase from the first postantennular limb to the first trunk limb, and decrease posterior to the fifth trunk limb.

The arthrodial membrane between basipod and limb insertion is medially supported by at least three more strongly sclerotized ridges ( Fig. 8C , arrows). The basipod is trapezoidal in outline and long ( Figs 8A, B , 9D ): its length from the limb insertion to the mediodistal end is almost one quarter of the tergal width, and about one fifth to one quarter of the total length of the limb. The median face carries a biserial armature of spines ( Figs 8C, D , 9A , 10C, D , 11D ). The number of spines in the rows varies between the basipods of the limbs. The basipods of the anterior trunk segments carry the highest number of spines, with up to twelve in the anterior series, and about ten in the posterior series. Spines in the anterior series are of roughly equal size; two or three larger spines are inserted in the distal half of the posterior series.

The endopod comprises nine podomeres ( Figs 9A , 10B ). The podomeres are cylindrical; the distal ones have a slight projection laterodistally, which may be drawn out into a small spine ( Figs 9A, B , 10B ). The first podomere is one third to one half the length of the basipod ( Fig. 9A ). The eighth podomere carries two spines distally, flanking the terminal podomere, which is a long, sturdy spine ( Fig. 9C , arrows).

The exopod is a paddle-shaped flap, fringed with setae. It reaches more than two thirds the length of the endopod ( Figs 9A , 10B ). Proximally, it articulates in a hinge joint along the slanting lateral edge of the basipod ( Fig. 11D ).

Figures

Figure 3. Statistics of selected morphometrics, based on 69 specimens that could be measured. A, size–frequency histogram based on the total length of the specimens. B, plot of length of the head shield against total length.

Figure 4. Holotype of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28942, arrows mark exsagittal mineralizations. B, drawing of MGUH 28942, heavy lines indicate non-biological features, such as cracks. Abbreviations: ant, antennula; app1 ex, exopod of first postantennular appendage; app2 en, endopod of second postantennular appendage; app2 ex, exopod of second postantennular appendage; bas spn, spines on the median edge of the basipod; cs, cephalic shield; gud, gut diverticulae; tg1, first trunk tergite; tg2, second trunk tergite; tg15, fifteenth trunk tergite; tg16, sixteenth trunk tergite; ts, tail shield.

Figure 5. Kiisortoqia soperi gen. et sp. nov. A, MGUH 28963, oblique entombment, showing the length of the limbs. B, MGUH 28964, horizontal entombent, showing the length of the antennula. C, drawing of MGUH 28964. Abbreviations: ant, antennula; app1 ex, exopod of first postantennular appendage; app2 ex, exopod of second postantennular appendage; bas spn, spines on the median edge of the basipod; cs, cephalic shield; en, endopod; tg1, first trunk tergite; tg2, second trunk tergite; tg15, fifteenth trunk tergite; tg16, sixteenth trunk tergite; ts, tail shield.

Figure 6. Kiisortoqia soperi gen. et sp. nov., posterior part of trunk and tail shield. A, MGUH 28966, parts of the exopods are represented by the margins with setae, and arrows mark the exsagittal mineralizations. B, drawing of MGUH 28966, exopods indicate that the posterior limbs were flipped backwards at the time of burial. C, MGUH 28962, in the posterior part of the trunk, an elongate hollowed-out structure potentially marks the position of the gut (gut?). D, drawing of MGUH 28962. Abbreviations: tg15, fifteenth trunk tergite; tg16, sixteenth trunk tergite; ts, tail shield.

Figure 7. Details of the antennula of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28948, the most completely preserved antennula, showing medial spines. B, drawing of MGUH 28948. C, MGUH 28953, showing pivot joints between articles (arrows). D, MGUH 28960, antennula rotated, showing peduncle and concave median surface of articles, arrows mark spines. E, MGUH 28954, showing widely spaced double rows of spines (arrows). Abbreviations: ant, antennula; app1 ex, exopod of first postantennular appendage; app2 en, endopod of second postantennular appendage; app2 ex, exopod of second postantennular appendage; cs, cephalic shield; ped, peduncle; tg1, first trunk tergite.

Figure 8. Details of the postantennular limbs of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28947, showing shape of basipod and first podomere, the arrows mark exsagittal mineralizations. B, drawing of MGUH 28947. C, MGUH 28945, showing sclerotized ridges in the arthrodial membrane between body and basipod (arrows). D, MGUH 28959, showing posteriorly directed basipod spines. Abbreviations: bas, basipod; bas spn, spines on the median edge of the basipod; cs, cephalic shield; en1, first podomere of endopod; en2, second podomere of endopod; tg1, first trunk tergite; tg2, second trunk tergite.

Figure 9. Details of the postantennular limbs of Kiisortoqia soperi gen. et sp. nov. A, close-up of MGUH 28946, showing endopods, exopod setae, and basipod spines (see Fig. 8B for an interpretative line drawing), the first postantennular limb is only marked by the basipod armature, arrows point to bulges that may represent the limbs of the opposing limb series folded under the axis. B, close-up of the postantennular limbs of MGUH 28942, arrows mark joints between the podomeres of the endopod. C, MGUH 28956, close-up of distal parts of endopods, showing the distal (ninth) podomere, which is a stout spine (arrows). Abbreviations: ant, antennula; app1-3 bas, basipods of first to third postantennular appendages; app1 ex, exopod of first postantennular appendage; app2 en, endopod of second postantennular appendage; cs, cephalic shield; en1–9, first to ninth podomere of the endopod; exs, exopod setae; tg1, first trunk tergite; tg2, second trunk.

Figure 10. Details of the postantennular limbs of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28946, overview of an almost complete specimen, showing the length of the postantennular limbs and basipod spines. B, interpretative line drawing of anterior part of MGUH 28946, the endopod podomeres could only be traced on the actual specimen under oblique light from varying angles, see also Fig. 10A. C, MGUH 28949, basipod spines (arrow) point posteriorly (left). D, MGUH 28951, specimen in horizontal entombment, appendages are flipped anteriorly, as indicated by basipod spines. Abbreviations: ant, antennula; app1 ex, exopod of first postantennular appendage; app2 en, endopod of second postantennular appendage; app3 en, endopod of third postantennular appendage; bas spn, spines on the median edge of the basipod; cs, cephalic shield; en, endopod; exs, exopod setae; tg1, first trunk tergite; tg2, second trunk tergite; tg15, fifteenth trunk tergite; tg16, sixteenth trunk tergite; ts, tail shield.

Figure 11. Details of the postantennular limbs of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28950, showing the shape and size of the anterior exopods, the endopod of the third postantennular limb is covered by the exopod of the second. B, MGUH 28967, posterior part of trunk, the exopods are made visible by the marginal setae (arrows). C, MGUH 28952, posterior part of trunk, the exopods are made visible by the marginal setae (arrows) imprinted in tergites. D, MGUH 28969, showing the articulation of the exopod along the slanting lateral edge of the basipod (arrows), the joint between the basipod and the first endopod podomere is seen by a sharp fold, and the exopod is probably represented by the smooth area, with the setae probably being imprints of setae belonging to the exopod of the next anterior (left) limb. Abbreviations: app1 en, endopod of first postantennular appendage; app1 ex, exopod of first postantennular appendage; app2 en, endopod of second postantennular appendage; app2 ex, exopod of second postantennular appendage; app3 en, endopod of third postantennular appendage; asr, anterior spine row of basipod; en1, first podomere of endopod; exs, exopod setae; psr, posterior spine row of basipod.

Figure 12. Details of axial features of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28948, specimen in ventral aspect, close-up of posterior part of axial features, the smooth structures on top of the three-dimensional mineralizations may represent sternites. B, MGUH 28958, ventral aspect, showing transverse bars adjacent to anterior border of tergites. C, MGUH 28965, ventral aspect, showing transverse bars composed of at least two strands adjacent to anterior border of tergites. D, MGUH 28943, dorsal aspect, close-up of anterior part of trunk axis, showing transverse hollows in axial mineralizations. E, MGUH 28946, close-up of axis, ventral aspect, showing transverse bars and continuous patch of corrugated surface that may represent fossilized muscle tissue, the arrow marks the exopod setae.

Figure 13. Details of axial features of Kiisortoqia soperi gen. et sp. nov. A, MGUH 28955, dorsal aspect, close-up of anterior part of trunk and posterior part of head shield (left), showing putative muscle tissue in association with transverse hollows; B, MGUH 28966, ventral aspect, close-up of middle part of trunk, showing corrugated surface with transverse interruptions, the arrows point to exsagittal mineralizations. C, MGUH 28959, dorsal aspect, showing segmentally arranged, putative midgut diverticulae, the radial orientation of planar elements is seen in the anterior two trunk segments, and the sagittal orientation is seen in the posterior trunk segments. D, MGUH 28944, dorsal aspect, showing hollows after segmentally arranged axial mineralizations.

Figure 14. Potential gut traces of Kiisortoqia soperi gen. et sp. nov., all specimens in dorsal aspect. A, MGUH 28957, showing continuous trace between segmentally arranged hollows. B, MGUH 28968, close-up of posterior part of trunk and tail shield, showing hollow trace with transverse annulation. C, MGUH 28961, close-up of posterior part of trunk, showing a three-dimensionally preserved continuous trace with transverse annulation.

Figure 15. Reconstruction of Kiisortoqia soperi gen. et sp. nov. A, ventral view, mouth and anus are conjectural. B, lateral view. C, dorsal view. D, trunk limb in posterior view, the articulation between the exopod and the first endopod podomere, as well as the hinge dividing the exopod into two portions, are not known; E, trunk limb in oblique posterior view, with exopod swung backwards.