Titanokorys_disc.tnt 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, <i>Titanokorys gainesi</i>, 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 <i>Titanokorys</i> 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 <i>Pahvantia</i>. Despite distinct carapaces, <i>Titanokorys</i> shares similar rake-like appendages for sediment-sifting with <i>Cambroraster,</i> a smaller but much more abundant sympatric hurdiid from the Burgess Shale<i>.</i> 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.

放射齿动物（Radiodonts）是起源于寒武纪大爆发的干群真节肢动物（stem-group euarthropods），是古生代早期体型最大、多样性最高的捕食者类群之一。该类群动物地理分布广泛，占据了多样的生态位：从底栖觅食者至浮游滤食者，再到顶级捕食者均有涵盖。本文记述了目前已知最大的寒武纪赫德虾类放射齿动物（hurdiid radiodont）新属新种*Titanokorys gainesi*，其化石发现于不列颠哥伦比亚省库特尼国家公园大理石峡谷的布尔吉斯页岩（Burgess Shale）地层中。该新种体长估计可达半米，拥有硕大的卵圆形中央背甲，具显著的短后侧突与前端棘刺。几何形态测量学分析揭示了赫德虾类背甲形态的高度多样性，并显示*Titanokorys*填补了长背甲与短背甲类群之间的形态学空白。然而，背甲形态易发生趋同演化，且与营养生态位并无稳定关联，这一点通过新数据（包括对鲜为人知的*Pahvantia*的重新评估）得到了证实。尽管背甲形态迥异，*Titanokorys*却与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*Cambroraster*拥有类似的耙状附肢以用于沉积物筛分。这两个物种在同一沉积层理面中的共存，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。