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 <i>Grapsus</i>, the first for the land crab <i>Cardisoma</i> in the Eastern Pacific and, remarkably, the most complete and abundant collection of fossil fiddler crabs, <i>Uca</i>, yet discovered. The abundance and exceptional preservation of fossil male, female, juvenile and adult individuals of <i>Uca</i> aff. <i>ornata</i> in eroded burrow infills suggest that rapid entombment and early diagenesis were crucial for their preservation. The habitat preference of extant <i>U. ornata</i> 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 <i>ex situ</i> 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 <i>Uca ornata</i>. 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.

完整铰接蟹类（fully articulated crabs）在化石记录中极为罕见，尤其是陆生与半陆生类群（terrestrial and semi-terrestrial taxa）——受埋藏偏差（preservational biases）影响，这类类群的化石通常稀缺、破碎且保存质量欠佳。巴拿马巴拿马城西侧的巴亚比克（Bahia Bique）新发现一处化石产出点，产出了一批中全新世（mid-Holocene）的潮上带（supratidal）、潮间带（intertidal）与浅潮下带（shallow subtidal）无脊椎动物及脊椎动物化石组合。此次发现的重要化石包括斜纹蟹属（*Grapsus*）的莎莉轻足蟹（sally lightfoot crab）化石，这是东太平洋地区首次发现地蟹属（*Cardisoma*）化石；尤为引人注目的是，本次发现了迄今为止已知最完整、最丰富的招潮蟹（fiddler crab）属（*Uca*）化石集合。采自侵蚀洞穴充填物中的近美观招潮蟹（*Uca aff. ornata*）化石涵盖雄性、雌性、幼体与成体个体，其丰度与极佳的保存状态表明，快速埋藏（rapid entombment）与早期成岩作用（early diagenesis）是这类化石得以保存的关键因素。现生近美观招潮蟹（*U. ornata*）偏好开阔潮间带软泥滩的栖息环境，这表明巴亚比克的部分区域曾是一处紧邻淡水输入（freshwater influx）的大型河口潮滩（estuarine mudflat）；而如今该化石产出点为砾石滩，化石以异地（ex situ）搬运的巨砾、圆砾与砾级碎屑形式产出，这些碎屑源自鲜为人知的太平洋腐殖泥岩层（Pacific Muck）。本研究通过综合分类与支序分类学（cladistic）方法，对巴亚比克产出的招潮蟹化石的系统发育关系进行了分析，结果表明这些化石隶属于现生近美观招潮蟹（*Uca ornata*）的一个已灭绝种群。巴亚比克小型动物群（faunule）与热带东太平洋现生动物群之间的生活-死亡组合（living-death assemblages）匹配度，证实了该化石组合的第四纪（Quaternary）年代，并为探讨热带组合中陆生与半陆生蟹类的保存与古生态学（palaeoecology）特征提供了详实的讨论基础。