DIPLOSTRACA Gerstaecker, 1866

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 : 2-3

Subclass DIPLOSTRACA Gerstaecker, 1866 ? Diplostraca indet.

Figure 1.3-1.4

Material. NMVP 109450 and NMVP 109462 from the Museum of Victoria, Australia.

Description. Only unclear impressions are found. Some traces of the head ( Figure 1.3 ; hd), valves (vv), postabdomen (pd), antennae II (aII), and thoracic limbs (tl) are observable, but no detail could be unambiguously identified. In some impressions, there are obvious mandibles ( Figure 1.4 ; md) of the branchiopod-type, asymmetrical, with bent distal end supplied with mastiscatory surface ( Figure 2.2 ). No further details are recognizable. In addition, the specimens seem to be strongly deformed, and may represent either molted exoskeletons or carcasses that had undergone significant decay of soft tissue prior to preservation.

Discussion. Jell and Duncan (1986) reported on several fossil specimens identified as daphniid body fossils. However, they list no daphniid-specific synapomorphies to support their identifications. Furthermore, many of their interpretive drawings suggest a chydorid-like body shape (round with a pointed rostrum) rather than a daphniid-like shape (more elliptical, rounded head shield, often with a posterior apical spine). Size and shape are really the only features that can be unambiguously described from these specimens— and they are nearly impossible to translate into unambiguous synapomorphies. The specimens described above could also belong to Ceriodaphnia (family Daphniidae ), but no real evidence for this could be found; the impressions are too deformed and badly preserved. The specimens could easily belong to other anomopod genera, ctenopod cladocerans, or possibly even juvenile clam shrimp (Spinicaudata, Laevicaudata, or Cyclestherida).

Occurrence. Lower Cretaceous Koonwarra Fossil Bed, Strzelecki Group, south Gippsland, Victoria, Australia. The age of this bed is constrained to a maximum age of between 115±6 Ma and 118±5 Ma (Aptian) by fission track radiometric ages derived from volcanogenic apatites isolated from above and below the fossil-bearing strata ( Lindsay, 1982, see Drinnan and Chambers, 1986). This is consistent with age estimates derived from palynological data ( Dettmann, 1986; Drinnan and Chambers, 1986).

Superorder CLADOCERA Latreille, 1829

Order ANOMOPODA Sars, 1865

Family DAPHNIIDAE Straus, 1820

Genus CERIODAPHNIA Dana, 1853

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.