A puzzling frog crab (Crustacea: Decapoda: Brachyura) from the Early Cretaceous Santana Group of Brazil: frog first or crab first?

Abstract

The evolutionary origin of frog crabs (Raninoida) remains puzzling partly due to their astonishing morphological disparity, ranging from broad and heavily ornamented ‘crab-like’ extinct families (necrocarcinids and allies), to elongate and smoother ‘frog-like’ extant ones (raninids and allies). However, an ancient Cretaceous clade (Palaeocorystidae) displays a combination of plesiomorphic and apomorphic traits that might advocate for either evolutionary scenario: from ‘crab-like’ to ‘frog-like’, or vice versa. This lack of agreement is partly fuelled by the scarcity of Early Cretaceous fossils, a time from which the first raninoidans are known. A close re-examination of an Early Cretaceous fossil from the Santana Group of Brazil, Araripecarcinus ferreirai Martins-Neto, 1987, combined with phylogenetic analysis including all main clades of podotreme crabs, reinforces its raninoidan condition, and rejects the initial hypothesis of a Portunoidea affinity. Furthermore, comparisons with other raninoidans support the hypothesis that a more ‘crab-like’ body plan is the plesiomorphic condition for raninoidans, and that the ‘frog-like’ architecture of Palaeocorystidae, and perhaps the Raninoidea as a whole, reflects a derived condition related to a specialized burrowing lifestyle. Phylogenetic analyses are fundamental to evaluate the position of Palaeocorystidae with respect to raninoidean and necrocarcinid-like families, helping to better resolve the Raninoida evolutionary tree of life, and to gain a broader understanding on their relatedness by common ancestry throughout geological time.

Keywords:

• Albian
• evolution
• Neotropics
• Raninoida
• Raninoidia
• South America

Acknowledgements

This work was supported by the Natural Science and Engineering Research Council of Canada Graduate Scholarship (NSERC CGS-D). My deepest thanks to Carlos Jaramillo (STRI), Daniéle Guinot (MHNH), Kecia Kerr (McGill University), A. Richard Palmer (University of Alberta) and Hiroaki Karasawa (Mizunami Fossil Museum, Japan) for intellectual support; to Paula Sucerquia (Universidade de Sao Paulo) for providing photos and a cast of the holotype and sole specimen of Araripecarcinus ferreirai; to Rodney M. Feldmann (Kent State University, USA) for providing photos of the cast of Araripecarcinus and the specimen of Cenomanocarcinus sp., plus valuable comments that improved an early version of the manuscript; to Barry Van Bakel (Oertijdmuseum De Groene Poort, Boxtel, The Netherlands) for thoughtful discussions and providing photos of Silvacarcinus; to Roger Portell (FLMNH) and the Invertebrate Paleontology Division, Florida Museum of Natural History, University of Florida, for the photo of Corazzatocarcinus; and to John Maisey, Lorraine Meeker and Chester Tarka (AMNH, USA) for the photos of the Cretaceous brachyuran larva preserved as stomach contents. I am very grateful to Carrie E. Schweitzer (Kent State University, USA), an anonymous reviewer, and the JSP editors for improving the quality of the manuscript. Partial funding for this project was provided by an NSERC Canada Discovery Grant A7245 to A. Richard Palmer.

Supplementary material is available online DOI: 10.1080/14772019.2013.871586