Abstract
Horseshoe crabs are classic “living fossils”, supposedly slowly evolving, conservative taxa, with a long fossil record back to the Ordovician. The evolution of their exoskeleton is well documented by fossils, but appendage and soft-tissue preservation is extremely rare. Here we analyse details of appendage and soft-tissue preservation in Yunnanolimulus luopingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable preservation of anatomical details including the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs. The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle. The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luopingensis indicates that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.
Highlights
A Middle Triassic horseshoe crab, Yunnanolimulus luopingensis, was recovered from a Middle Triassic fossil lagerstätte in Luoping, Yunnan, China[22], representing the first record of horseshoe crabs from China and the eastern Tethys
Based on the limb data, the Palaeozoic Offacolus, Dibasterium, Weinbergina and Venustulus are placed in the euchelicerate stem
The occurrence of book gills in fossil eurypterids suggests that expansive book gills originated before the divergence of xiphosurans and eurypterids[31]
Summary
A Middle Triassic horseshoe crab, Yunnanolimulus luopingensis, was recovered from a Middle Triassic fossil lagerstätte in Luoping, Yunnan, China[22], representing the first record of horseshoe crabs from China and the eastern Tethys. Further preparation of the described specimens and newly collected specimens have led to the discovery of well preserved anatomical details, including book gills, pusher legs, muscles, and setae. The horseshoe crab fossils were collected from the finely laminated micritic limestone of the middle part of Member II of the Guanling Formation around Dawazi Village, Luoping Town, Yunnan Province, China. The diversity of the Luoping biota makes it one of the most important Triassic fossil lagerstätten in terms of understanding the recovery and radiation of marine ecosystems after the profound Permo-Triassic mass extinction[23,25]. The details of pusher legs were revealed by further preparation of the described specimens (LPI-61734)
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