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Research Articles

Vol. 3 No. 1 (2026)

The largest fossil tanaidacean is not a tanaidacean – Cretitanais giganteus (Malzahn, 1979) is a junior synonym of Urda stemmerbergensis (Malzahn, 1968)

DOI
https://doi.org/10.26034/la.opal.2026.9328
Submitted
23 January 2026
Published
15-06-2026

Abstract

Tanaidacea and Isopoda are relatively closely-related groups of crustaceans. Representatives of both groups can have a quite similar appearance. Extant species of Tanaidacea differ from isopods by their chelipeds, appendages with distinctive, often large, chelae. Also, their head and the cheliped segment form a compound structure (carapace). A single fossil from the Lower Cretaceous of Germany – Cretitanais giganteus (Malzahn, 1979) – has received much attention in discussions about the fossil record of Tanaidacea and was often described as morphologically aberrant. A µCT scan of a resin cast of the C. giganteus holotype was acquired and a high resolution surface model was constructed. The morphological reinterpretation revealed that, previously, crucial morphological features have been overlooked and misinterpreted. Unlike all known tanaidaceans and unlike previously described, the fossil has not six, but seven pereon tergites and the alleged chela is a merus of a further posterior leg. From this, it is apparent that the fossil is not of a tanaidacean. Instead, the fossil shows stark resemblance to species of Urda Münster, 1840 and is herein interpreted to be conspecific to Urda stemmerbergensis (Malzahn, 1968), known from the same locality and stratum, making Cretitanais giganteus a junior synonym of it.

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