The evolution of hermit crabs (Crustacea, Decapoda, Anomura, Paguroidea) on the basis of carapace morphology: a state-of-the-art-report

In our ongoing studies of both extinct and extant hermit crabs (Paguroidea Latreille, 1802), we have observed and recorded a number of morphological changes that have taken place between Early Jurassic times (c. 185 million years ago) and the present day. Species compositions of paguroid assemblages from marine Upper Jurassic and Lower Cretaceous rocks in Europe are here compared with those of modern marine settings. Basal paguroids with cylindrical carapaces predominated in reefal environments during the Late Jurassic, but were gradually replaced by hermit crabs with non-cylindrical carapaces. The most basal paguroids reveal a branchial groove, but branchial condensation towards a single (i.e., cervical) groove is here shown to have occurred early in their evolutionary history. In several extant, genera remnants of a branchial groove, in combination with several other basal features, can be seen. For this reason, these are here considered to be the most primitive members amongst extant paguroids. In this study, we resurrect the family Probeebeidae and erect a new family, Paguropsidae n. fam, to accommodate extant basal hermit crabs such as Paguropsis Henderson, 1888, Eopaguropsis Fraaije, Krzeminski, Van Bakel, Krzeminska & Jagt, 2012 and Paguropsina Lemaitre, Rahayu & Komai, 2018. Described are also one pair of Early Jurassic (Pliensbachian) and two pairs of Middle Jurassic (Callovian) isochelous paguroid chelae, all collected recently. A new specimen of Schobertella reveals some important morphological traits of the shield that had not been observed previously. The earliest known example to date of clearly heterochelous chelae in the fossil record of hermit crabs originates from upper Kimmeridgian/Tithonian strata in the Boulonnais (northwestern France). For the first time, a phylogenetic scheme of marine Paguroidea, as based on morphological data of carapaces, is presented.