Gill function in an early arthropod and the widespread adoption of the countercurrent exchange mechanism.

Rising but fluctuating oxygen levels in the Early Palaeozoic provide an environmental context for the radiation of early metazoans, but little is known about how mechanistically early animals satisfied their oxygen requirements. Here we propose that the countercurrent gaseous exchange, a highly efficient respiratory mechanism, was effective in the gills of the Late Ordovician trilobite . In order to test this, we use computational fluid dynamics to simulate water flow around its gills and show that water velocity decreased distinctly in front of and between the swollen ends, which first encountered the oxygen-charged water, and slowed continuously at the mid-central region, forming a buffer zone with a slight increase of the water volume.

The trilobite upper limb branch is a well-developed gill.

Whether the upper limb branch of Paleozoic "biramous" arthropods, including trilobites, served a respiratory function has been much debated. Here, new imaging of the trilobite shows that dumbbell-shaped filaments in the upper limb branch are morphologically comparable with gill structures in crustaceans that aerate the hemolymph. In , the upper limb's partial articulation to the body via an extended arthrodial membrane is morphologically comparable to the junction of the respiratory book gill of and differentiates it from the typically robust exopod junction in Chelicerata or Crustacea.

Early Cambrian fuxianhuiids from China reveal origin of the gnathobasic protopodite in euarthropods.

Euarthropods owe their evolutionary and ecological success to the morphological plasticity of their appendages. Although this variability is partly expressed in the specialization of the protopodite for a feeding function in the post-deutocerebral limbs, the origin of the former structure among Cambrian representatives remains uncertain. Here, we describe Alacaris mirabilis gen.

Onychophoran-like musculature in a phosphatized Cambrian lobopodian.

The restricted, exclusively terrestrial distribution of modern Onychophora contrasts strikingly with the rich diversity of onychophoran-like fossils preserved in marine Cambrian Lagerstätten The transition from these early forebears to the modern onychophoran body plan is poorly constrained, in part owing to the absence of fossils preserving details of the soft anatomy. Here, we report muscle tissue in a new early Cambrian (Stage 3) lobopodian, Tritonychus phanerosarkus gen. et sp.

A predatory bivalved euarthropod from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China.

Bivalved euarthropods represent a conspicuous component of exceptionally-preserved fossil biotas throughout the Lower Palaeozoic. However, most of these taxa are known from isolated valves, and thus there is a limited understanding of their morphological organization and palaeoecology in the context of early animal-dominated communities. The bivalved euarthropod Clypecaris serrata sp.

Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda.

Panarthropods are typified by disparate grades of neurological organization reflecting a complex evolutionary history. The fossil record offers a unique opportunity to reconstruct early character evolution of the nervous system via exceptional preservation in extinct representatives. Here we describe the neurological architecture of the ventral nerve cord (VNC) in the upper-stem group euarthropod Chengjiangocaris kunmingensis from the early Cambrian Xiaoshiba Lagerstätte (South China).

A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora.

We describe Collinsium ciliosum from the early Cambrian Xiaoshiba Lagerstätte in South China, an armored lobopodian with a remarkable degree of limb differentiation including a pair of antenna-like appendages, six pairs of elongate setiferous limbs for suspension feeding, and nine pairs of clawed annulated legs with an anchoring function. Collinsium belongs to a highly derived clade of lobopodians within stem group Onychophora, distinguished by a substantial dorsal armature of supernumerary and biomineralized spines (Family Luolishaniidae). As demonstrated here, luolishaniids display the highest degree of limb specialization among Paleozoic lobopodians, constitute more than one-third of the overall morphological disparity of stem group Onychophora, and are substantially more disparate than crown group representatives.

Development and trunk segmentation of early instars of a ptychopariid trilobite from Cambrian Stage 5 of China.

Many three-dimensionally preserved exoskeletons found from the middle Cambrian (Stage 5) Gaotai Formation in Guizhou, southern China, have been assigned to the ptychopariid trilobite Gunnia sp. They represent mainly a series of early instars, exhibiting some delicate structures and morphological variation associated with their trunk segmentation and early development. Morphometric and statistical analyses indicate that the transverse joint appears to occur with the full growth of the third axial ring of the protopygidium, which increases in size much more rapidly than its corresponding protocephalon with growth.

Articulated Wiwaxia from the Cambrian Stage 3 Xiaoshiba lagerstätte.

Wiwaxia is a bizarre metazoan that has been interpreted as a primitive mollusc and as a polychaete annelid worm. Extensive material from the Burgess Shale provides a detailed picture of its morphology and ontogeny, but the fossil record outside this lagerstätte is scarce, and complete wiwaxiids are particularly rare. Here we report small articulated specimens of Wiwaxia foliosa sp.