Quaternary intertidal and supratidal crabs (Decapoda, Brachyura) from tropical America and the systematic affinities of fossil fiddler crabs

Concentrations of fully articulated crabs are rare in the fossil record, especially for terrestrial and semi-terrestrial taxa, which tend to be represented by scarce, fragmentary and poorly preserved fossils due to preservational biases. A newly discovered fossiliferous locality at Bahia Bique, west of Panama City, Panama, yielded a collection of supratidal, intertidal and shallow subtidal invertebrates and vertebrates of mid-Holocene age. Notable discoveries include the first fossils of the sally lightfoot crab Grapsus, the first for the land crab Cardisoma in the Eastern Pacific and, remarkably, the most complete and abundant collection of fossil fiddler crabs, Uca, yet discovered. The abundance and exceptional preservation of fossil male, female, juvenile and adult individuals of Uca aff. ornata in eroded burrow infills suggest that rapid entombment and early diagenesis were crucial for their preservation. The habitat preference of extant U. ornata for soft muds of open intertidal mudflats indicates that part of Bahía Bique must have been a large estuarine mudflat with close proximity to freshwater influx, in contrast to the present-day gravel field where the fossils are found as ex situ boulders, cobbles and gravel-sized clasts eroded from rocks of the poorly known Pacific Muck. We examine the systematic relationships of fossil fiddler crabs from Bahía Bique via synthetic and cladistic approaches, and conclude that they were from an extinct population of the extant Uca ornata. The fidelity of living–death assemblages between the Bique faunule and extant faunas of the tropical Eastern Pacific confirm the Quaternary age of the assemblage, and stimulate a detailed discussion of the preservation and palaeoecology of terrestrial and semi-terrestrial crabs in tropical assemblages.

在化石记录中，完全发育的蟹类浓度极为罕见，尤其是在陆生和半陆生类群中，这些类群往往仅以稀少、破碎且保存状况不佳的化石来代表，这主要是由于保存偏倚所致。在巴拿马城西部的巴伊亚比基发现的一个新化石地点，出土了一批中全新世的潮上带、潮间带和浅海潮下带的无脊椎动物和脊椎动物。令人瞩目的发现包括首次发现的莎莉光脚蟹（Grapsus）化石、首次在东太平洋发现的陆蟹（Cardisoma）化石，以及迄今为止发现的最为完整和丰富的化石弹涂蟹（Uca）收藏。Uca aff. ornata的化石雄性、雌性、幼体和成体个体的丰富性和卓越的保存状态，特别是在侵蚀的洞穴填充物中，表明快速埋藏和早期成岩作用对于它们的保存至关重要。现存U. ornata对开阔潮间带软泥的栖息地偏好表明，巴伊亚比基的一部分可能是一个大型河口泥滩，靠近淡水流入，与现今天然界中作为异地岩石的砾石、卵石和砾石大小的碎屑所形成的化石场地形成鲜明对比。我们通过综合和系统发育方法研究了巴伊亚比基的化石弹涂蟹的系统关系，并得出结论，它们来自现存的Uca ornata的一个已灭绝种群。比基动物群与现存的东热带太平洋动物群之间的生活-死亡组合的一致性证实了该组合的更新世年龄，并激发了关于热带组合中陆生和半陆生蟹的保存和古生态学方面的详细讨论。