New hypotheses concerning the earliest brachyurans (Crustacea, decapoda, brachyura)

Abstract

All Jurassic brachyuran taxa known to date are based solely upon dorsal carapaces, and only a limited number of Early and mid-Cretaceous crabs retain ventral parts. Therefore, all Jurassic taxa and many forms from the first half of the Cretaceous are carapace-based entities. All of them are considered to be “dromiaceans”, podotremes to be precise. The recent discovery of an exceptionally well-preserved male crab from the Upper Cretaceous (lower Cenomanian) of Chiapas (Mexico), Archaeochiapasa mardoqueoi Guinot, Carbot-Chanona & Vega, 2019, at first sight of a podotreme nature, has allowed a detailed description of its thoracic sternum and pleon, which revealed that it was actually a typical eubrachyuran, in need of a new family, Archaeochiapasidae Guinot, Carbot-Chanona & Vega, 2019. This has brought back to life one of my earlier ideas about the possible non-podotreme nature of certain enigmatic Late Jurassic and Cretaceous Brachyura previously placed in various “dromiacean” (i.e., podotreme) families and superfamilies. My investigations have led the me to formulate the present hypothesis that the extinct families Bucculentidae Schweitzer & Feldmann, 2009 (currently assigned to the Homolodromioidea Alcock, 1900), Lecythocaridae Schweitzer & Feldmann, 2009, Glaessneropsidae Schweitzer & Feldmann, 2009, Nodoprosopidae Schweitzer & Feldmann, 2009, and Viaiidae Artal, Van Bakel, Fraaije, Jagt & Klompmaker, 2012 (all four in Glaessneropsoidea Schweitzer & Feldmann, 2009) might, in fact (at least for some of them), be true eubrachyurans (Eubrachyura Saint Laurent, 1980). If correct, these assumptions would date the first “true crabs” as Jurassic, contrary to the currently held view that the earliest Eubrachyura (heterotremes) did not appear until the Cretaceous, and suggest that the evolutionary history of brachyurans started much earlier. This was unpredictable, at least for palaeontologists, but not so in view of a molecular estimate of decapod phylogeny that recovered the Majoidea Samouelle, 1819 as the oldest brachyuran lineage, with a divergence from other brachyurans from, at least, the Middle Triassic. The basal majoid family Oregoniidae Garth, 1958, which comprises only three extant genera, has several characters in common with Archaeochiapasidae; these leave little doubt about their close relationships.