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14 November 2019 New hypotheses concerning the earliest brachyurans (Crustacea, Decapoda, Brachyura)
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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 mardoqueoiGuinot, 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. Proposals made here are inevitably based on provisional assumptions, until the characteristics of the ventral parts and pereiopods prove or refute them, either entirely or in part. Our science, which is based on the observation of specimens and then on descriptive, explanatory and, above all, predictive concepts, especially where incomplete fossil animals are concerned, should be conceived as a step forward, rather than an achievement, each of these steps being, sooner or later, replaced by a better one, or considered to be such. That is why all species and the composition of the Jurassic and Early Cretaceous genera and families will need to be checked in light of new perspectives. In contrast to the presumed eubrachyurans (see above), the Tanidromitidae Schweitzer & Feldmann, 2008 and the apparently paraphyletic family Longodromitidae Schweitzer & Feldmann, 2009 are podotremes, within the Dynomeniformia Guinot, Tavares & Castro, 2013. The status and composition of the Goniodromitinae Beurlen, 1932 (in the Dromiidae De Haan, 1833), clearly paraphyletic, are briefly revised, while some genera, such as DistefaniaChecchia-Rispoli, 1917, are tentatively assigned to the Sphaerodromiinae Guinot & Tavares, 2003. A table summarises the changes in classification implied by these new proposals and research directions. Some remarks on the new section Callichimaeroida Luque, Feldmann, Vernygora, Schweitzer, Cameron, Kerr, Vega, Duque, Strange, Palmer & Jaramillo, 2019 are provided, as well as on the the putatively callichimaeroid-like family Retrorsichelidae Feldmann, Tshudy & Thomson, 1993.

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Danièle Guinot "New hypotheses concerning the earliest brachyurans (Crustacea, Decapoda, Brachyura)," Geodiversitas 41(1), 747-796, (14 November 2019). https://doi.org/10.5252/geodiversitas2019v41a22
Received: 15 February 2019; Accepted: 21 May 2019; Published: 14 November 2019
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