Hegna Thomas A, Astrop Timothy I
Department of Geology and Environmental Sciences, SUNY Fredonia, 203 Jewett Hall, 280 Central Avenue, Fredonia, NY 14063, USA. E-mail:
Fossil Forest Project, Blast Road, Brymbo, Wales, United Kingdom, LI11 5BT. E-mail:
Zool Stud. 2020 Aug 5;59:e43. doi: 10.6620/ZS.2020.59-43. eCollection 2020.
Clam shrimp (the paraphyletic assemblage of spinicaudatans, laevicaudatans, cyclestherids and the extinct leaiins) are small, bivalved branchiopod crustaceans that specialize in ephemeral freshwater habitats. They have a long fossil record (Devonian onward) that has often been overlooked. Here we briefly review the fossil record of the major groups of clam shrimp and clear up some misconceptions in the literature as to their origin. The dominant group of clam shrimp in the fossil record is the Spinicaudata, which have a diverse fossil record beginning in the Devonian. The clam shrimp suborder Laevicaudata are known 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 they represent crown group or stem group laevicaudatans. In contrast, the total group Spinicaudata have a rich record of mostly carapace fossils- the earliest from the Early Devonian. The leaiins are an enigmatic extinct diplostracan lineage thought to be closely related to the spinicaudatans. They have a record that extends from the Middle Devonian to the Permian. The Cyclestherida have a somewhat problematic fossil record: there are no examples of cyclestherids preserved with soft-parts, so the only character used to assign fossils to this lineage is the carapace shape. According to that metric, cyclestherids have a record that begins in the Middle Devonian. Exceptionally preserved clam shrimp are found in the Paleozoic and Mesozoic. Assessing holistically what is known about the clam shrimp fossil record along with carapace morphology, carapace ornamentation and examples of exceptional preservation will ultimately contribute to a synthetic paleontological and neontological understanding of the group, its systematics and evolution.
蚌虾（包括背甲蚌虾目、光尾蚌虾目、圆蚌虾科以及已灭绝的莱氏蚌虾类的并系类群）是小型的双瓣鳃足纲甲壳动物，专门栖息于临时性淡水生境。它们拥有悠久的化石记录（从泥盆纪至今），但常常被忽视。在此，我们简要回顾主要蚌虾类群的化石记录，并澄清文献中关于它们起源的一些误解。化石记录中占主导地位的蚌虾类群是背甲蚌虾目，其化石记录丰富多样，始于泥盆纪。光尾蚌虾亚目已知始于二叠纪，侏罗纪可能有软躯体保存。然而，由于这些化石特征较少，无法确定它们代表的是冠群还是干群光尾蚌虾。相比之下，整个背甲蚌虾目有丰富的主要为背甲化石的记录——最早可追溯到早泥盆纪。莱氏蚌虾类是一个神秘的已灭绝双甲目谱系，被认为与背甲蚌虾目关系密切。它们的记录从中泥盆纪延伸到二叠纪。圆蚌虾科的化石记录有些问题：没有保存软躯体的圆蚌虾科化石实例，所以用于将化石归入该谱系的唯一特征是背甲形状。据此标准，圆蚌虾科的记录始于中泥盆纪。在古生代和中生代发现了保存异常完好的蚌虾化石。综合评估关于蚌虾化石记录、背甲形态、背甲纹饰以及异常保存实例的已知信息，最终将有助于从古生物学和现代生物学角度对该类群、其分类学和进化形成综合理解。
