Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University, Kunming 650091, People's Republic of China.
MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, Waihuan South Road, Chenggong District, Kunming 650500, People's Republic of China.
Proc Biol Sci. 2023 Apr 26;290(1997):20230335. doi: 10.1098/rspb.2023.0335. Epub 2023 Apr 19.
Early euarthropod evolution involved a major transition from lobopodian-like taxa to organisms featuring a segmented, well-sclerotized trunk (arthrodization) and limbs (arthropodization). However, the precise origin of a completely arthrodized trunk and arthropodized ventral biramous appendages remain controversial, as well as the early onset of anterior-posterior limb differentiation in stem-group euarthropods. New fossil material and micro-computed tomography inform the detailed morphology of the arthropodized biramous appendages in the carapace-bearing euarthropod from the early Cambrian Chengjiang biota. In addition to well-developed grasping frontal appendages, possesses two batches of morphologically and functionally distinct biramous limbs. The first batch consists of four pairs of short cephalic appendages with robust endites with a feeding function, whereas the second batch has more elongate trunk appendages for locomotion. Critically, our new material shows that the trunk of was not arthrodized. The results of our phylogenetic analyses recover isoxyids as some of the earliest branching sclerotized euarthropods, and strengthens the hypothesis that arthropodized biramous appendages evolved before full body arthrodization.
早期的泛节肢动物的进化涉及到从叶足动物样的分类单元到具有分段的、硬化的体干（体节化）和附肢（附肢化）的生物的重大转变。然而，完全体节化的体干和附肢化的腹肢的起源以及在干群泛节肢动物中前后肢分化的早期发生仍然存在争议。新的化石材料和微计算机断层扫描为来自早寒武世澄江生物群的具壳泛节肢动物的附肢化双分支附肢的详细形态提供了信息。除了发育良好的抓握前附肢外，还具有两批形态和功能不同的双分支附肢。第一批由四对短的头胸部附肢组成，具有具有摄食功能的强壮的内肢；而第二批则具有更长的用于运动的体干附肢。重要的是，我们的新材料表明的体干并没有体节化。我们的系统发育分析结果表明，等叶目是最早分支的具有硬化外骨骼的泛节肢动物之一，这加强了这样的假设，即附肢化的双分支附肢是在完全体节化之前进化而来的。
