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（Cambrian explosion）时期的干群真节肢动物（stem-group euarthropods），为古生代早期（lower Palaeozoic）体型最大、多样性最丰富的掠食类群之一。该类群地理分布广泛，占据了多样的生态位，涵盖底栖觅食者（benthic foragers）、浮游滤食者（nektonic suspension feeders）乃至顶级捕食者（apex predators）。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 radiodonts）——新属新种（gen. et sp. nov.）*Titanokorys gainesi*，其标本采自不列颠哥伦比亚省（British Columbia）库特尼国家公园（Kootenay National Park）大理石峡谷（Marble Canyon）产区的布尔吉斯页岩（Burgess Shale）地层。该新物种体型预估可达半米，拥有极为庞大的卵形中央背甲（ovoid-shaped central carapace），带有显著的短后侧突（posterolateral processes）与前棘（anterior spine）。几何形态测量分析（geometric morphometric analyses）结果显示，赫德虾类的背甲形态多样性极高，且*Titanokorys*填补了长背甲与短背甲类群之间的形态缺口（morphological gap）。不过，结合本次研究的新数据（包括对鲜为人知的*Pahvantia*的重新评估），研究表明背甲形态易受趋同演化（homoplasy）影响，与物种的营养生态位（trophic ecology）并无稳定关联。尽管背甲形态差异显著，*Titanokorys*与布尔吉斯页岩中体型更小但丰度更高的同域分布赫德虾类寒武耙虾（*Cambroraster*）拥有功能相似的、用于沉积物筛选（sediment-sifting）的耙状附肢（rake-like appendages）。二者在同一地层层面（bedding planes）的伴生产出，揭示了寒武纪群落中大型掠食者对底栖资源的潜在竞争，以及当时大型捕食者类群的高度多样性。