Sex determination of the Early Cretaceous clam shrimp Eosestheria middendorfii (Yixian Formation, China)

Abstract

Clam shrimps have been recognized as a key group for the study of reproductive system evolution, owing to the diversity of sexual systems in extant members. However, there are comparatively little data on fossil taxa. In this study, we reveal the sexual system of the Early Cretaceous clam shrimp Eosestheria middendorfii (Yixian Formation, China). This is the first study that restricts the analysis to a single cohort, minimizing the otherwise considerable impact of ecophenotypic variation within this species. In addition, the presence of egg clutches, which identifies some individuals as either female or hermaphroditic, serves as an independent indicator for sex prior to the statistical treatment of the data. Obligate sexuality (‘dioecy’) is the inferred reproductive system for E. middendorfii and sexual dimorphism accounts for about 10% of the adult shape variation. Carapace shape variation resulting from malformation and deformation is more pronounced than the underlying sexual dimorphism. Subtle sexual dimorphism, smaller and slightly more elongate females, lateral egg clutches, and egg diameters of about 140 μm indicate that E. middendorfii is closely allied with the extant family Cyzicidae.

Analysis of morphological variability in the clam shrimp Eosestheria middendorfii (Crustacea, Spinicaudata) from the Lower Cretaceous of China, and its implications for spinicaudatan taxonomy

“…Hethke et al . () showed that a close relationship between Eosestheria middendorfii and the extant Cyzicidae is well supported (egg size, lateral position of egg clutch, males larger than females), corroborating the validity of this superfamily. However, strict application of the International Code of Zoological Nomenclature renders the family Eosestheriidae a junior synonym of the subfamily Bairdestheriinae.…”

“…Comparison of left and right valves in the modern relative Cyzicus shows that the mirrored outline of the right valve matches that of the left perfectly (Hethke et al . ). The outlines were then formatted to be represented by 1500 xy ‐coordinates, using the image capture software tpsDig2 (Rohlf ; http://life.bio.sunysb.edu/morph/).…”

Ecological stasis in Spinicaudata (Crustacea, Branchiopoda)? Early Cretaceous clam shrimp of the Yixian Formation of north‐east China occupied a broader realized ecological niche than extant members of the group

“…Following the close affinities between eosestherioids and cyzicids (Hethke et al . ), it is likely that Eosestheria middendorfii was nektobenthic and closely associated with the lake floor.…”

“…The egg production of females (Hethke et al . ) was probably high, considering the production of 1–2 egg clutches per day in extant species (Weeks et al . ).…”

Microsculpture Study of Late Permian and Early Triassic Conchostracans From the Babii Kamen Section (Western Siberia)

“…Of special interest [1,[6][7][8][9][10][11][12][13][14][15] is the shell microsculpture of these small crustaceans -a strengthening skeletal formation of the thin valves with a certain type, or with several types, of ornamentation -, which is a possible diagnostic feature of conchostracan genera and species. The key role of shell microsculpture for conchostracan identification was first stressed by N.I.…”