A middle Cambrian arthropod with chelicerae and proto-book gills
A middle Cambrian arthropod with chelicerae and proto-book gills
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DOI：
10.1038/s41586-019-1525-4
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发表时间：
2019-09-26
期刊：
影响因子：
64.8
通讯作者：
Caron, Jean-Bernard
中科院分区：
文献类型：
--
作者：
Aria, Cedric;Caron, Jean-Bernard
螯合动物是一种无处不在的物种，对现代陆地生态系统有相当大的生态影响——特别是作为昆虫的捕食者，同时，例如，作为分解者。化石记录表明，螯肢动物早在古生代海洋生态系统中就已多样化，至少在奥陶纪(2)。然而，螯合起源的时间和这一群体最早成员的身体计划类型仍然存在争议。尽管megacheirans（3-5）以前被解释为螯类动物，habeliidans(6)（包括Sanctacaris(7,8)）被认为属于它们的直系茎系，但用于诊断螯类动物的特殊摄食附属物（螯肢）的证据一直缺乏。在这里，我们使用了来自中寒武纪伯尔格斯页岩（加拿大不列颠哥伦比亚省大理石峡谷）保存完好的大量化石材料来证明Mollisonia plenovenatrix . . 11 .具有强壮但短的螯肢，它们位于非常前面的眼睛之间。这表明，螯虫很早就进化出了一种专门的进食功能，可能是对短触角的修改。头部还包括一对大复眼，其次是三对长长的、不知名的行走腿和三对粗壮的、咬牙的咀嚼附属物；这种结构将能手纲动物与中螯合纲动物（“真正的”螯合纲动物，不包括海蜘蛛）联系起来。躯干末端为四节的臀骨，有十一对相同的肢，每对肢由三个宽片状的外足瓣组成，内足瓣不是减少就是没有。这些重叠的外足瓣类似于易螯合的书鳃，尽管它们缺乏诊断性的盖(9)。此外，M. plenovenatrix的眼睛由三个视神经丸支配，这加强了一种观点，即所有冠状真节肢动物都有一个复杂的类似于马拉孔甲鱼的视觉系统（10,11）。因此，这些化石表明，螯肢动物与下颌骨一起作为底栖微型食肉动物出现在寒武纪大爆发的核心。
The chelicerates are a ubiquitous and speciose group of animals that has a considerable ecological effect on modern terrestrial ecosystems-notably as predators of insects and also, for instance, as decomposers(1). The fossil record shows that chelicerates diversified early in the marine ecosystems of the Palaeozoic era, by at least the Ordovician period(2). However, the timing of chelicerate origins and the type of body plan that characterized the earliest members of this group have remained controversial. Although megacheirans(3-5) have previously been interpreted as chelicerate-like, and habeliidans(6) (including Sanctacaris(7,8)) have been suggested to belong to their immediate stem lineage, evidence for the specialized feeding appendages (chelicerae) that are diagnostic of the chelicerates has been lacking. Here we use exceptionally well-preserved and abundant fossil material from the middle Cambrian Burgess Shale (Marble Canyon, British Columbia, Canada) to show that Mollisonia plenovenatrix sp. nov. possessed robust but short chelicerae that were placed very anteriorly, between the eyes. This suggests that chelicerae evolved a specialized feeding function early on, possibly as a modification of short antennules. The head also encompasses a pair of large compound eyes, followed by three pairs of long, uniramous walking legs and three pairs of stout, gnathobasic masticatory appendages; this configuration links habeliidans with euchelicerates ('true' chelicerates, excluding the sea spiders). The trunk ends in a four-segmented pygidium and bears eleven pairs of identical limbs, each of which is composed of three broad lamellate exopod flaps, and endopods are either reduced or absent. These overlapping exopod flaps resemble euchelicerate book gills, although they lack the diagnostic operculum(9). In addition, the eyes of M. plenovenatrix were innervated by three optic neuropils, which strengthens the view that a complex malacostracan-like visual system(10,11) might have been plesiomorphic for all crown euarthropods. These fossils thus show that chelicerates arose alongside mandibulates(12) as benthic micropredators, at the heart of the Cambrian explosion.