Misszhouia, Chen & Edgecombe & Ramsköld, 1997

Chen, Jun-Yuan, Edgecombe, Gregory D. & Ramsköld, Lars, 1997, Morphological and ecological disparity in naraoiids (Arthropoda) from the Early Cambrian Chengjiang Fauna, China, Records of the Australian Museum 49 (1), pp. 1-24 : 2-3

Misszhouia n.gen.

Etymology. In honour of Miss Zhou Gui-qing, for her skilled preparation of Chengjiang fossils.

Type species. Naraoia longicaudata Zhang & Hou, 1985 .

Diagnosis. A naraoiid with antennule directed anterolaterally, proximal part not strongly deflected outwards; four pairs of relatively small digestive caeca in cephalon, without ramifying anterior diverticula; trunk a single tergite, lacking articulations (shared with Naraoia ); gut of relatively small diameter; exopod "fan" with large surface area composed of long lamellar setae; exopod attached along entire length of basis and at least proximal part of first podomere of endopod; exopod shaft gently tapering distally; distal lobe of exopod lanceolate, not substantially expanded; endopod composed of seven podomeres including terminal claw.

Hypostome and frontal organs. Homologies between naraoiid and trilobite hypostomes appear complex, and some discussion is required in order to use a meaningful descriptive terminology for Misszhouia . The structure identified by Zhang & Hou (1985) as the hypostome of M. longicaudata is described here as the "ovate lobe of the hypostomal complex" ( Figs 2c, 3a , 4 ). This ventrally convex lobe is a moderately large sternite (less than 30 percent the length of the cephalon). Its margins are not strongly defined, indicating weaker sclerotisation than that of the tergites. It is longitudinally ovate, with maximum width posterior to midlength.

Anterior to the ovate lobe of the hypostomal complex are three ventrally-bulging organs, here called frontal organs, of which the medial of the three is set forward of the other two ( Figs 2c,d, 3 , 4 ). The medial frontal organ is elongate and teardrop-shaped, the lateral pair more or less ovate. All three are preserved protruding strongly ventrally with an anteroventral slope making their anterior edge vertical or even slightly protruding in front of the cuticle at its base. The frontal organs are set upon a low relief, boomerang-shaped platform that appears to represent a weakly sclerotised sternite. Some specimens show the frontal organs and ovate lobe to be surrounded and joined by a thin sheet of tissue (arthrodial membrane or ventral cuticle) that extends out to the bases of the antennules (see below). The frontal organs and ovate lobe are thus conjoined as a structural complex that may include two lightly sclerotised sternites. If the attachment site of the antennules is to be regarded as a homologous landmark between naraoiids and trilobites, it is plausible to regard the hypostomal region of Misszhouia as including the whole complex-frontal organ sternite, the ovate lobe, and the intervening field of arthrodial membrane-and extending out to the antennular bases. This may, however, be a dubious assumption, given Muller & Walossek's (1987: 47) observations on the lability of antennular position within Crustacea. The lateral and posterior margins of the hypostomal complex are poorly defined ( Fig. 2 b-d); it is not certain that the shield-shaped impression seen in some specimens (Fig. 5b) is an accurate outline of the hypostomal complex.

Whittington (1977) noted that the hypostome of Naraoia was lightly sclerotised, and it was not identified with certainty. Fortey & Theron (1995: Table 2, character 6) coded the Naraoia hypostome as conterminant (attached) but that of Misszhouia is, in their terminology, natant. The hypostome does not have a sutural attachment to cephalic doublure, and no rostral plate is present; hypostomal attachment is by imbedding in soft tissue (see discussion of Ventral Cuticle below).

The presumed homology of the frontal organs in Misszhouia is with parts of the median eye complex in other arthropods. The lateral pair of frontal organs may represent the paired ventral frontal organs of Crustacea

Figures

Fig. 3. Misszhouia longicaudata (Zhang & Hou, 1985). a, camera lucida drawing of ELRC 11558 (fig. 2c). b, camera lucida drawing of ELRC 11559 (Fig. 2a). See Appendix for abbreviations.

Fig. 4. Misszhouia longicaudata (Zhang & Hou, 1985). a, camera lucida drawing of head and anterior part of trunk of ELRC II076a, from locality MQA. The specimen was embedded in the sediment tilted to the right side, so that ventral structures are displaced to the left. The biramous limbs are numbered I to 5 from anterior on the exopod shafts. See Appendix for abbreviations; b, ELRC l1076a. Light from NW (see Fig. lOb for trunk).