The Fossil Record of the Clam Shrimp (Crustacea; Branchiopoda)

摘要

Clam shrimp (the paraphyletic assemblage of spinicaudatans, laevicaudatans, cyclestheridsand the extinct leaiins) are small, bivalved branchiopod crustaceans that specialize inephemeral freshwater habitats. They have a long fossil record (Devonian onward) that hasoften been overlooked. Here we briefly review the fossil record of the major groups ofclam shrimp and clear up some misconceptions in the literature as to their origin. Thedominant group of clam shrimp in the fossil record is the Spinicaudata, which have adiverse fossil record beginning in the Devonian. The clam shrimp suborder Laevicaudata areknown from the Permian, with possible soft-part preservation from the Jurassic. However,owing the character-poor nature of these fossils, it is impossible to tell if theyrepresent crown group or stem group laevicaudatans. In contrast, the total groupSpinicaudata have a rich record of mostly carapace fossils— the earliest from the EarlyDevonian. The leaiins are an enigmatic extinct diplostracan lineage thought to be closelyrelated to the spinicaudatans. They have a record that extends from the Middle Devonian tothe Permian. The Cyclestherida have a somewhat problematic fossil record: there are noexamples of cyclestherids preserved with soft-parts, so the only character used to assignfossils to this lineage is the carapace shape. According to that metric, cyclestheridshave a record that begins in the Middle Devonian. Exceptionally preserved clam shrimp arefound in the Paleozoic and Mesozoic. Assessing holistically what is known about the clamshrimp fossil record along with carapace morphology, carapace ornamentation and examplesof exceptional preservation will ultimately contribute to a synthetic paleontological andneontological understanding of the group, its systematics and evolution.