Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warszawa, Poland; Institute of Evolutionary Biology, Faculty of Biology, Biological and Chemical Research Centre (CNBCh UW), University of Warsaw, Warszawa, 02-089 Żwirki i Wigury 101, Poland.
Arthropod Struct Dev. 2021 Mar;61:101026. doi: 10.1016/j.asd.2020.101026. Epub 2021 Jan 25.
A fossil larva lacking segmentation of the calcified carapace, closely resembling the trilobite protaspis, has been found associated with other skeletal elements of an angarocaridid Girardevia species in the mid Darriwilian of central Siberia. The presence of protaspis larvae in the angarocaridids, generally believed to represent a branch of the Aglaspidida, supports their proximity to trilobites and proves a low position on the arthropod phylogenetic tree but does not necessarily contradict the chelicerate affinity. The cephalic appendages of angarocaridids bore massive gnathobases with detachable spines, closely similar to those known in extant xiphosurans and in their probable Cambrian relatives. The stratigraphic succession of the angarocaridids, their phosphatized cuticle pieces being abundant in the Ordovician strata of Siberia, shows a gradual improvement of mechanical resistance of their carapaces, eventually resulting in a honeycomb structure. The associated benthic mollusc assemblage is dominated with the bellerophontids showing high mortality at metamorphosis and only the limpet-like Pterotheca, infaunal bivalves, and scaphopods being able to survive this in a substantial number. This suggests a strong selective pressure from predators equipped with well-skeletonised oral apparatuses able to crush mineralized body covers of their prey. Possibly, these were some of the associated conodonts of appropriate size and co-evolving towards their ability to crush more and more resistant cuticle. Less likely candidates for durophagy are endoceratid or orthoceratid cephalopods. Also the angarocaridids themselves, equipped with robust gnathobases of cephalic appendages, apparently predated on benthic shelly animals.
一种缺乏钙化甲壳分段的化石幼虫，与西伯利亚中部中达里威尔阶的一种安吉拉卡里达物种的其他骨骼元素密切相关，其形态非常类似于三叶虫原头节。安吉拉卡里达生物中存在原头节幼虫，一般认为它们代表了 Aglaspidida 的一个分支，这支持了它们与三叶虫的亲缘关系，并证明了节肢动物系统发育树的低位置，但不一定与螯肢动物的亲缘关系相矛盾。安吉拉卡里达生物的头部附肢具有带有可分离刺的大型颚基，与现存的剑尾目动物和它们可能的寒武纪亲缘动物非常相似。安吉拉卡里达生物的地层序列表明，它们的磷质外骨骼在西伯利亚的奥陶纪地层中非常丰富，其外壳的机械阻力逐渐提高，最终形成了蜂窝状结构。与之相关的底栖软体动物组合以贝尔罗芬蒂德为主，它们在变态时死亡率很高，只有类似于石鳖的 Pterotheca、穴居双壳类和箭石类能够在大量个体中幸存下来。这表明，具有良好骨骼化口腔器官的捕食者对它们的猎物进行了强烈的选择性压力，这些捕食者能够压碎猎物的矿化体壳。可能，这些是一些具有适当大小的相关牙形石动物，它们共同进化以增强压碎越来越具抗性的外骨骼的能力。不太可能是内壳头足类或正旋头足类头足类进行硬壳捕食。此外，安吉拉卡里达生物自己也装备了坚固的头部附肢颚基，显然以底栖有壳动物为食。
