Phylum Arthropoda, Subphylum Crustacea, Class Ostracoda.
Genus: Nasunaris gen. nov.
Derivation of name: Latin nasus (nose) + naris (nostril); alluding to the prominent rostrum and rostral incisure. Gender, feminine.
Diagnosis: Cylindroleberididae with a well-developed elongate rostrum and ovoid rostral incisure and a prominent caudal siphon. Pedunculate lateral eyes and a medial eye bearing a short Bellonci organ.
Species: Nasunaris flata sp. nov.
Derivation of name: Latin flatus (breath of life); alluding to its respiratory system.
Diagnosis: as for the genus (monotypic).
Holotype: Oxford University Museum of Natural History OUM C.29 612; the only known specimen. A carapace and soft parts (d), reconstructed in three-dimensions (a–c,e–k,m–r).
Locality and stratigraphy: Herefordshire, England; Wenlock Series, Silurian.
The carapace is preserved with valves slightly agape. Maximum length (excluding rostrum), height and width (at mid-length) are 5050, 4850 and 3370 µm, respectively. The carapace is subcircular in lateral outline, with a long, tapering rostrum above an ovoid rostral incisure, and a short, tapering caudal siphon with an ovoid gape (a). In lateral outline, the position of the hinge is marked by a straight line running posteriorly from the highest point of the carapace, which is just in front of mid-length; in dorsal view the area adjacent to the hinge line is slightly depressed (i,j). Each valve is gently inflated, with a bulb immediately behind the rostral incisure, a shallow adductorial sulcus running obliquely posterodorsally from valve centre to just behind dorsal mid-length, and a narrow flattened admarginal zone from the rostral incisure to the caudal siphon. Along the rostral incisure the valve-free margin is flexed gently inwards (g).
The first antenna is long, slender, weakly tapered and projects forward adaxial to the second antenna (b,f,g,i,m). It consists of a long proximal part (podomere?) that is markedly geniculate with a slightly longer distal part (podomeres not discernible); the forked termination of the latter (evident in the right limb only) is presumed to be the proximal parts of two setae. A pair of pedunculate, elongate ovoid lateral eyes (presumed compound; ommatidia not discernible) arise abaxial and just above the base of the first antennae (b,f,h). Sagitally, a small medial eye bears a short, elongate, forward projecting structure, interpreted as a Bellonci organ (h). The second antenna (b,c,f,g,i,n) arises below the lateral eye; its large, globose, almond-shaped basipod is sited within the bulb of the carapace. The exopod is long, slender and curves anterolaterally through the rostral incisure and flexes backwards. It consists of two long subequal parts delimited at a weak flexure (a possible podomere boundary), the distal part bearing at least four closely spaced subparallel setae along the outer edge (more clearly evident on the right limb) of presumed separate podmeres. The endopod is curved, about one-quarter the length and one-third the width of the exopod; podomeres are not discernible. The gaps in the left exopod and right endopod represent missing data.
The mandible (c
) arises below and adaxial of the second antenna, and abaxial of the labrum. The limb base (presumed basipod and coxa) is elongate, but details of any enditic processes cannot be resolved. The presumed endopod is separated from the limb base by a geniculation; it is straight, short, elongate, tapering distally and it projects anteroventrally. A tiny narrow elongate structure, projecting dorsally from the limb base/endopod boundary, is interpreted as the proximal part of an exopod (it is better preserved on the right limb). The right first and second maxillae are intact, but only the base of each corresponding left limb is preserved. The first maxilla has a short limb base (presumed basipod and proximal endite) with a few poorly defined possible enditic processes, and a presumed endopod consisting of a proximal part (podomere?) that is geniculate with a shorter, tapering distal part (c
). The second maxilla (c
) lies aside the atrium oris and projects below the first maxilla. The limb base appears to bear a lateral lamelliform projection of equal size directed posteroventally (a presumed epipod), and an elongate ramus (conventionally, the exopod in myodocopids; reinterpreted as the endopod by Boxshall 1998
) consisting of a long distal part flexed at about 90° to a much shorter proximal part. The sixth limb has not been identified.
The body isthmus forms a posterodorsally inclined ridge coincident with the adductorial sulcus (a,c,f,i); a flattened area of the ventral part of the isthmus represents the adductor muscle attachment (most clearly evident on the left side; i). The seventh appendage is vermiform, about 4 mm long, slender, tapers weakly and may have a bifid-shaped termination (evident in right limb; f,i,r). This appendage arises below and posterior to the isthmus, curving behind the latter and over the posterodorsal body area. The furca is large and elongate, extending slightly outside the carapace ventrally (c,f,g); individual lamellae and claws are not discernible. The posteroventral body area behind the seventh appendage is flanked by delicate lamellae (c,f,i): parts of two lamellae with slightly thickened edges are preserved on the left side and one incomplete lamella on the right side (there may be multiple pairs, but this area is difficult to resolve). Such lamellae are presumed gills: they do not occur elsewhere over the body area and in position and morphology they are similar to the gills of the Silurian myodocopid Colymbosathon. The labrum is evident medially but its lateral margins are difficult to resolve. A well-developed atrium oris leads to a short oesophagus and a large, broadly curved stomach mostly in-filled with darker (phosphatized?) material—presumed food (e,f). The gut terminates bluntly in the presumed anus in front of the furca. Preservation of the ventral body area is not adequate to allow gender to be determined.