Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution

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 Isoxys curvirostratus from the early Cambrian
Chengjiang biota. In addition to well-developed grasping frontal
appendages. I. curvirostratus 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 I.
curvirostratus was not arthrodized. The results of our phylogenetic
analyses recover isoxyids as some of the earliest branching sclerotized
euarthropods, and strenghens the hypothesis that arthropodized biramous
appendage evolved before full body arthrodization.

早期真节肢动物的演化涉及从叶足类样类群向具有分节且高度骨化的躯干（arthrodization）与附肢（arthropodization）的生物的重大转变。然而，完全骨化的躯干与骨化的腹侧双枝型附肢的精确起源，以及干群真节肢动物中前后附肢分化的早期出现，至今仍存争议。新的化石材料与显微计算机断层扫描（micro-computed tomography）技术揭示了寒武纪早期澄江生物群中具甲壳的真节肢动物Isoxys curvirostratus的骨化双枝型附肢的详细形态。除发育良好的捕食性前附肢外，I. curvirostratus拥有两批形态与功能均不同的双枝型附肢：第一批为四对短小的头部附肢，具粗壮的取食功能内叶；第二批则是更长的躯干附肢，用于运动。关键的是，新化石材料显示I. curvirostratus的躯干并未发生骨化。系统发育分析结果表明，isoxyid类群是最早分支的骨化真节肢动物之一，这进一步支持了双枝型附肢的骨化先于全身骨化的假说。