Clam shrimps are large branchiopod crustaceans. They have laterally compressed shrimp-like bodies which are enclosed in chitinous bivalved carapace. This is the reason why they are called clam shrimps. Extant clam shrimps are widely distributed on all continents except for Antarctica, but fossil clam shrimps are widely distributed, and they were also found in Antarctica. According to the fossil records, clam shrimps extend back to the Devonian Period. The evolution of clam shrimps was initially centred on Europe, but in Mesozoic they diversified more rapidly in Asia  . During the Cenozoic they gradually declined with very scarce fossil records  , and resulted in both low abundance and diversity. Nowadays only fourteen genera in five families remain  .
2. Living Environment of Clam Shrimps
Clam shrimps inhabit seasonally astatic wetlands such as playas, vernal (rain and snow-melt) pools, rice field or fishless lakes  . This living environment is consistent with their relatively short life cycles. Their resting eggs are able to survive dormant for several years under dry conditions. All these special features make clam shrimps the very successful colonizers of ephemeral freshwater ecosystems  under a wet and dry alternating climate setting in the earth history  . This results in very abundant fossil records worldwide in the Mesozoic fine lacustrine deposits  -  .
3. Taxonomy of Fossil and Extant Clam Shrimps
Clam shrimp fossil records demonstrate that their fossilized soft parts are very rare. Most common cases are that they are preserved as phosphatized carapaces  , or the external or internal moulds of carapaces. Thus, the classification and taxonomy of fossil clam shrimps are mainly based on the morphological characters of their carapaces, such as the carapace outline, structure and the fine ornamentation patterns on growth bands  . While extant clam shrimps are classified based on the soft body morphological characters and molecular data. In order to solve this dilemma, a common language should be found for an integrated classification for fossil and extant clam shrimps  . During The Crustacean Society Mid-Year Meeting (May 2019 Hong Kong), fossil and extant clam shrimp specialists have carried out detailed discussion about the future research on clam shrimp taxonomy. Most participants agreed that clam shrimp carapaces could be the main object for searching for morphological features to discuss the relationship between fossil and modern clam shrimps.
Nowadays scanning electron microscopes (SEMs) are widely available and play a more important role in taxonomy of fossil clam shrimps  . The previous studies of fossil clam shrimps, mainly based on the observation under a light microscope, made un-precise descriptions of carapace ornamentation features, and thus taxonomic relationships cannot be determined clearly. A new SEM imaging of the type material of Neodiestheria dalaziensis from the Albian Dalazi Formation in northern China has revealed ontogenetically developing morphological patterns on growth bands of the juvenile stage of the carapace, which indicate that Neodiestheria is closely related phylogenetically to Triglypta  . This shows us that further investigation on the fossil and extant clam shrimp carapaces by the help of SEM could get more fruitful results to discuss their phylogenetic relationship.
The study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB26000000) and the National Natural Science Foundation of China (41572006, 41688103, 41972007). This is a contribution to UNESCO-IUGS IGCP Project 679.
 Shen, Y.B., Gallego, O.F., Buchheim, H.P. and Biaggi, R.E. (2006) Eocene Conchostracans from the Laney Member of the Green River Formation, Wyoming, USA. Journal of Paleontology, 80, 447-454.
 Brendonck, L., Rogers, D.C., Olesen, J., Weeks, S.C. and Hoeh, R. (2008) Global Diversity of Large Branchiopods (Crustacea: Branchiopoda) in Fresh Water. Hydrobiologia, 595, 167-176.
 Guériau, P., Robet, N., Clément, G, Lagebro, L., Vannier, J., Briggs, D.E.G., Charbonnier, S., Olive, S. and Béthoux, O. (2016) A 365-Million-Year-Old Freshwater Community Reveals Morphological and Ecological Stasis in Branchiopod Crustaceans. Current Biology, 26, 383-390.
 Li, G. and Matsuoka, A. (2015) Searching for a Non-Marine Jurassic/Cretaceous Boundary in Northeastern China. Journal of Geological Society of Japan, 121, 109-122.
 Boukhalfa, K., Li, G., Ben Ali, W. and Soussi, M. (2015) Early Cretaceous Spinicaudatans (“Conchostracans”) from Lacustrine Strata of the Sidi Aich Formation in the Northern Chotts Range, Southern Tunisia: Taxonomy, Biostratigraphy and Stratigraphic Implication. Cretaceous Research, 56, 482-490.
 Gallego, O.F., Monferran, M.D., Astrop, T.I. and Zacarias, I.A. (2013) Reassignment of Lioestheria codoensis Cardoso (Spinicaudata, Anthronestheriidae) from the Lower Cretaceous of Brazil: Systematics and Paleoecology. Revista Brasileira de Paleontologia, 16, 47-60.
 Schneider, J.W. and Scholze, F. (2016) Late Pennsylvanian-Early Triassic Conchostracan Biostratigraphy: A Preliminary Approach. Geological Society, London, Special Publications, 450, 450-456.
 Scholze, F., Golubev, V.K., Niedzwiedzki, G., Sennikov, A.G., Schneider, J.W. and Silantiev, V.V. (2015) Early Triassic Conchostracans (Crustacea: Branchiopoda) from the Terrestrial Permian-Triassic Boundary Sections in the Moscow Syncline. Palaeogeography Palaeoclimatology Palaeoecology, 429, 22-40.
 Stigall, A.L., Babcock, L.E., Briggs, D.E.G. and Leslie, S.A. (2008) Taphonomy of Lacustrine Interbeds in the Kirkpatrick Basalt (Jurassic), Antarctica. Palaios, 23, 344-355.
 Astrop, T.I. and Hegna, T.A. (2015) Phylogenetic Relationships between Living and Fossil Spinicaudatan Taxa (Branchiopoda Spinicaudata): Reconsidering the Evidence. Journal of Crustacean Biology, 35, 339-354.
 Li, G., Ohta, T., Batten, D.J., Sakai, T. and Kozai, T. (2016) Morphology and Phylogenetic Origin of the Spinicaudatan Neodiestheria from the Lower Cretaceous Dalazi Formation, Yanji Basin, North-Eastern China. Cretaceous Research, 62, 183-193.