Extant Clam Shrimp Egg Morphology: Taxonomy and Comparison with Other Fossil Branchiopod Eggs

Abstract

In examining the surface sculpture of the eggs of extant clam shrimps, chiefly from China, at least four types can be recognized. 1) Ridge-type eggshells have a variety of features, but all eggshell surfaces are ornamented with stout protruding ridges separated by deeply wide valleys. The surface can be textured with numerous minute pores. This pattern is unique to Limnadidae. 2) Reticular-type eggshells are spherical in aspect with large polygonal reticulations, which is seen in the taxa of Leptestheriidae (diameter 85-110 μm). 3) Hairy-type eggshells are covered by numerous hairy ornaments. Occasionally, crowded minute dots fill in among hairy structures (diameter 115-160 μm), and these types may belong to Cyzicus and Caenestheriella. 4) Ripple-type eggshells are spherical with a densely ripple sculpture (diameter 90-150 μm). This type is commonly seen in species of Eocyzicus (Cyzicidae) or Lynceus (Lynceidae). Therefore, the eggshell morphology is a useful character for identification at either the high taxonomic or the species level. In terms of egg diameter, the extant eggs are quite similar to those of fossil eggs from the Lower Permian (100-110 μm) of Germany and the Middle Jurassic (c.130 μm) of China. Likewise, the shape and size of the eggs of kazacharthrans from the Upper Triassic of China are somewhat similar to those of cyzicids. They are much smaller (150-160 μm) than that of extant notostracans (> 400 μm). In general, the anostracan eggs are larger than those of clam shrimps in size and less than in number, such as seen in a new taxon from the Middle Jurassic being 220-240 μm. However, Chirocephalus rasnitsyni from the Lower Cretaceous of Russia (c.200 μm) is similar to those of clam shrimps. Fossil cladoceran ephippia found from Early Permian to Quaternary ranges from 300 μm to 500 μm. They are easily distinguished from both extant and fossil clam shrimp egg forms.