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

Abstract

Unresolved taxonomic issues regarding spinicaudatans, clam shrimps that formed the most abundant faunal element in the lacustrine Barremian to Aptian Yixian Formation, have hampered palaeoecological and evolutionary interpretations of this key fossil group. Here, we analyse morphological variability in East Asian clam‐shrimp taxa by quantifying: (1) size and shape; and (2) ornamental features (radial lirae distances). Intergeneric variability was examined using 51 specimens of various East Asian Mesozoic taxa, 16 of which were chosen for a taxonomic revision of the 10 species described from the Yixian Formation of western Liaoning. Geometric morphometrics proved to be effective for the separation of genera, while the quantification of ornamental features was species‐diagnostic. Various authors have previously indicated the possibility that East Asian clam‐shrimp species have been split excessively, a criticism that has been confirmed by our analyses. According to our results, four of the ten species described from the Yixian Formation are valid, of which one might represent a junior synonym of Diestheria jeholensis; five taxon names represent junior synonyms of Eosestheria middendorfii (Jones), and one a nomen dubium. According to lirae distances and analysis of shape, Eosestheria fuxinensis, the original type species of Eosestheria, is synonymous with the type of Diestheria. Therefore, a new type species for Eosestheria is warranted and here fixed as E. middendorfii. This taxonomic revision questions the high level of clam‐shrimp diversity recognized within the Yixian Formation, and indicates that many of the species described in fact represent various ontogenetic stages of E. middendorfii.

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

“…Specimens of the morphologically variable spinicaudatan Eosestheria middendorfii were identified based on ornamental features and shape according to Hethke et al . (). Several exceptionally large clam shrimp of horizon JSG H ( L = 20.6 mm) lack growth‐band crowding, and ornamentation on the last four growth bands differs from the species description in exhibiting coarse reticulation.…”

“…In the case of Lake Sihetun, a taxonomic revision demonstrated that five of the ten species reported from the Yixian Formation of the Sihetun area belong to a single species, Eosestheria middendorfii (Hethke et al . ). Excavation JSG yielded no evidence for co‐occurrences of this variable species with any other clam‐shrimp species.…”

“…Clam‐shrimp axes (Hethke et al . ) have been measured using a calliper with a precision of ±0.1 mm. Most of these measurements were taken in the field in Liaoning, where it was impossible to distinguish between closely adjacent, sub‐millimetric sedimentary layers, whose number has been subsequently counted in the lab.…”

“…In the present study, we re‐investigate the factors that determined the general high abundance and distribution pattern of the Early Cretaceous clam shrimp Eosestheria middendorfii (Jones ; Hethke et al . ).…”

The earliest example of sexual dimorphism in bivalves—evidence from the astartid Nicaniella (Lower Jurassic, southern Germany)

“…The two coefficients are independent variables that determine the shape of the outline and are evaluated by using PCA. For Fourier shape analysis, the programs Hangle, Hmatch, and Hcurve were used (Haines and Crampton, 2000;Schneider et al, 2010;Hethke et al, 2018). First, the outlines were transformed into 1000-2000 Cartesian coordinates using the program tpsDig2 (Rohlf, 2017).…”