Evolution of the thaumastocheliform lobsters (Crustacea, Decapoda, Nephropidae)

Deep-sea lobsters previously assigned to the family Thaumastochelidae Bate, 1888, the thaumastocheliforms, have very distinctive, greatly unequal first chelipeds, with the right side extremely elongate and pectinate, and in having short, quadrate pleonal pleura. Despite interesting morphology and a long taxonomic history, the phylogeny of the group has received little detailed analysis. Here, we conduct a species-level phylogenetic analysis of the thaumastocheliforms based on morphological and molecular data (three mitochondrial genes: COI, 16S rDNA and 12S rDNA; two nuclear protein-coding genes: H3 and NaK) to robustly reconstruct their evolutionary history and estimate divergence times. Separate and combined analyses of all data sources support thaumastocheliform monophyly, but as a clade deeply nested within the Nephropidae supporting recent synonymy of Thaumastochelidae with Nephropidae. Combined and molecular-only analyses support generic monophyly of all three thaumastocheliform genera and Dinochelus as sister to Thaumastochelopsis, fully corroborating the current, morphology-based taxonomy. In contrast, Thaumastocheles is recovered as paraphyletic in morphology-only analyses owing to minimal character support. The Cretaceous-Paleogene Oncopareia was recovered as a stem-lineage thaumastocheliform. The fossil record indicates that the thaumastocheliforms once lived in shallow, continental shelf depths, but moved into deeper water in the Cenozoic where they occur today. The thaumastocheliforms originated in northern Europe during the Mid-Late Cretaceous and later dispersed westwards to the south-eastern Pacific through the western Atlantic and eastwards to the western Pacific through the Indian Ocean. Thaumastochelopsis can be considered the most derived thaumastocheliform genus based on the degree of structural reduction relative to other thaumastocheliforms, its remote geographical occurrence (Australia) from the hypothesised place of origin (northern Europe) and its more recent estimated divergence than other genera (28 Mya for the MRCA of extant species of the genus).