Ceriodaphnia sp.

Hegna, TA & Kotov, AA, 2016, Ephippia belonging to Ceriodaphnia Dana, 1853 (Cladocera: Anomopoda: Daphniidae) from the Lower Cretaceous of Australia, Palaeontologia Electronica 19 (3), pp. 1-9 : 3-5

Ceriodaphnia sp.

Figure 1.1-1.2 , Appendix 1

Material. NMV P332637, Museum of Victoria. The rock which contains this specimen also contains fossil anostracans (see Jell and Duncan, 1986, figure 73A). Additional specimens are figured in the Appendix 1.

Description. Ephippium shaped as half an oval bisected along the short axis, straight dorsal margin, length = 0.6 mm, height/length = 0.8, posterior margin is symmetrically curved with a possible faint rim preserved around the ventral margin. No ornamentation is preserved, as in some extant species of Ceriodaphnia ( Figure 2.1 ). Single oval-shaped egg, long axis of the egg oriented parallel to the dorsal margin. Maximum egg length/max ephippial length = 0.5 (assuming that the carbonaceous material represents the true size of the egg). The ephippium is preserved mostly as an impression, with a small amount of carbonaceous material preserved medially — this may represent the remains of the resting egg.

Discussion. The fossils discussed herein were originally identified as daphniid indet by Jell and Duncan (1986) and were discussed alongside the putative cladoceran body fossils. In their remarks, they speculated that the ephippia they examined possibly belonged to either Simocephalus or Ceriodaphnia ( Jell and Duncan, 1986, p. 193) . Fryer (1991) revisited Jell and Duncan’s (1986) material and demonstrated the presence of Simocephalus - type ephippia based on their single egg and asymmetrical lateral outline. A closer examination of the cladoceran ephippia examined by Jell and Duncan (1986) demonstrate that the genus Ceriodaphnia is also present.

Daphniidae in general have a strongly developed, firm, well-elaborated ephippia (in contrast to chydorids and macrothricids, in daphniids and moinids the ephippiual portion of the female caparace is significantly modified, thickened, supplied with air loculi between internal and external wall), and bearing only 1-2 resting eggs ( Kotov, 2013). The symmetrical (in lateral view), half-oval shape of the ephippium coupled with the presence of one extremely large resting egg is characteristic of Ceriodaphnia (see Figure 2.1 for a SEM image of a modern female Ceriodaphnia with ephippium prior to moulting). Other lineages within Daphniidae have either different shaped ephippia, have ephippia with two resting eggs, or both (see the discussion in Kotov 2009b, 2013). Simocephalus , from the same locality, has a strongly asymmetric outline (in lateral view) that is like a teardrop shape bisected along the long axis. Among the other single-egg-ephippia cladocerans, chydorids have an ephippium with thin, slightly modified walls (as compared with parthenogenetic female) and curved dorsal margin ( Vandekerkhove et al., 2004), and moinids have much more rectangular to elliptical outline with stronger sculpture ( Goulden, 1968; Fryer, 1991; Lu, 2001).

In extant daphniids, the ephippium length is normally more than half of the female length, such a relationship would predict an adult female length of about 1 mm. This size is characteristic of many recent species (i.e., C. dubia Richard, 1894 , C. reticulata ( Jurine, 1820) , amongst others, see Flössner, 2000) and is roughly equivalent to the size of the undiagnostic circular fossils identified by Jell and Duncan (1986) as the body fossils of daphniid cladocerans described also above (although plausible as fossil cladocerans, the main cited similarity between the fossils and cladocerans is their size and their circular shape).

The fossil Ceriodaphnia ephippium does not preserve any evidence of sculpture. Among recent species ( Hudec, 2010; Kotov, 2013) ephippial sculpture is variable—there are both taxa with ephippia covered by a complicated sculpture and those without any sculpture. Therefore, the ephippium from Koonwara is within the range of variation known from recent species.

Occurrence. Same as above.

Figures

FIGURE 1. Fossil diplostracan remains from Koonwarra. 1, An image of a fossil ephippium belonging to Ceriodaphnia sp. from the Koonwarra Fossil Beds (photographed with obliquely directed light), NMV P332637, 2, Interpretive drawing of Figure 1.1, scale bar for 1-2 equals 0.1 mm. 3, Diplostracan indet, NMV P109450 (figure 70G and 71C in Jell and Duncan, 1986), See Figure 1.4 for scale bar. 4, Diplostracan indet, NMV P109462 (figure 70H and 71E in Jell and Duncan, 1986). Scale bar in 1.4 applies to 1.3 as well. Scale bar for 1.3-4 equals 1 mm. Abbreviations: aII - antenna II; hd - head; tl – thoracic limbs; vv - valves.

FIGURE 2. Modern Ceriodaphnia sp. 1, Scanning electron microscope image of a modern ephippium-bearing Ceriodaphnia sp. female (collected from Glubokoe Lake, European Russia) prior to releasing the ephippium during molting, ephippium tilted slightly away from angle of photograph, scale bar equals 0.1 mm. 2, Scanning electron microscope image of the mandibles of Daphnia magna, scale bar equals 0.1 mm.