H_all_ldk.tps from A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity

Radiodonts, stem-group euarthropods that evolved during the Cambrian explosion, were among the largest and most diversified lower palaeozoic predators. These animals were widespread geographically, occupying a variety of ecological niches, from benthic foragers to nektonic suspension feeders and apex predators. Here, we describe the largest Cambrian hurdiid radiodont known so far, Titanokorys gainesi, gen. et sp. nov., from the Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia). Estimated to reach half-a-meter in length, this new species bears a very large ovoid-shaped central carapace with distinct short posterolateral processes and an anterior spine. Geometric morphometric analyses highlight the high diversity of carapace shapes in hurdiids and show that Titanokorys bridges a morphological gap between forms with long and short carapaces. Carapace shape, however, is prone to homoplasy and shows no consistent relationship with trophic ecology, as demonstrated by new data, including a reappraisal of the poorly known Pahvantia. Despite distinct carapaces, Titanokorys shares similar rake-like appendages for sediment-sifting with Cambroraster, a smaller but much more abundant sympatric hurdiid from the Burgess Shale. The co-occurrence of these two species on the same bedding planes highlights the potential competition for benthic resources and the high diversity of large predators sustained by Cambrian communities.

放射齿类（Radiodonts）是演化于寒武纪大爆发时期的干群真节肢动物，曾是古生代早期体型最大、多样性最高的捕食类群之一。这类动物地理分布广泛，占据了多样的生态位：从底栖觅食者到游泳型滤食者，乃至顶级捕食者。本文记述了一种迄今已知最大的寒武纪赫德虾类（hurdiids）——产自不列颠哥伦比亚省库特尼国家公园大理石峡谷布尔吉斯页岩（Burgess Shale）的新属新种泰坦科里思虾（Titanokorys gainesi, gen. et sp. nov.）。该物种预估体长可达半米，拥有硕大的卵圆形中央背甲，带有显著的短后外侧突起与前棘。几何形态测量学分析显示，赫德虾类的背甲形态多样性极高，且泰坦科里思虾填补了长背甲类群与短背甲类群之间的形态学空白。不过结合新数据（包括对研究程度较低的帕万虾（Pahvantia）的重新评估）可知，背甲形态易受趋同演化影响，且与营养生态并无稳定的关联。尽管背甲形态差异显著，泰坦科里思虾与寒武耙虾（Cambroraster）拥有相似的用于分选沉积物的耙状附肢；后者是布尔吉斯页岩中体型更小但数量更为丰富的同域赫德虾类。二者在同一岩层中的共现现象，揭示了寒武纪群落中大型捕食者之间潜在的底栖资源竞争，以及当时大型捕食者类群的高度多样性。