The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod

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 strengthens the hypothesis that arthropodized biramous appendages evolved before full body arthrodization.

The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited

The origin of mandibulates, the hyperdiverse arthropod group that includes pancrustaceans and myriapods, dates back to the Cambrian. Bivalved arthropod groups such as hymenocarines have been argued to be early mandibulates, but many species are still poorly known, and their affinities remain uncertain. One of the most common and globally distributed Cambrian bivalved arthropods is Tuzoia. Originally described in 1912 from the Burgess Shale based on isolated carapaces, its full anatomy has remained largely unknown. Here, we describe new specimens of Tuzoia from the Canadian Burgess Shale (Wuliuan, Cambrian) showcasing exceptionally preserved soft tissues, allowing for the first comprehensive reconstruction of its anatomy, ecology and evolutionary affinities. The head bears antennae and differentiated cephalic appendages. The body is divided into a cephalothorax, a homonomous trunk bearing ca 10 pairs of legs with heptopodomerous endopods and enlarged basipods, and a tail fan with two pairs of caudal rami. These traits suggest that Tuzoia swam along the seafloor and used its spinose legs for predation or scavenging. Tuzoia is retrieved by a Bayesian phylogenetic analysis as an early mandibulate hymenocarine lineage, exemplifying the rapid diversification of this group in open marine environments during the Cambrian Explosion.

Ordovician opabiniid-like animals and the role of the proboscis in euarthropod head evolution

A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia. These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.

The origin and early evolution of arthropods

The rise of arthropods is a decisive event in the history of life. Likely the first animals to have established themselves on land and in the air, arthropods have pervaded nearly all ecosystems and have become pillars of the planet's ecological networks. Forerunners of this saga, exceptionally well-preserved Palaeozoic fossils recently discovered or re-discovered using new approaches and techniques have elucidated the precocious appearance of extant lineages at the onset of the Cambrian explosion, and pointed to the critical role of the plankton and hard integuments in early arthropod diversification. The notion put forward at the beginning of the century that the acquisition of extant arthropod characters was stepwise and represented by the majority of Cambrian fossil taxa is being rewritten. Although some key traits leading to Euarthropoda are indeed well documented along a diversified phylogenetic stem, this stem led to several speciose and ecologically diverse radiations leaving descendants late into the Palaeozoic, and a large part, if not all of the Cambrian euarthropods can now be placed on either of the two extant lineages: Mandibulata and Chelicerata. These new observations and discoveries have altered our view on the nature and timing of the Cambrian explosion and clarified diagnostic characters at the origin of extant arthropods, but also raised new questions, especially with respect to cephalic plasticity. There is now strong evidence that early arthropods shared a homologous frontalmost appendage, coined here the cheira, which likely evolved into antennules and chelicerae, but other aspects, such as brain and labrum evolution, are still subject to active debate. The early evolution of panarthropods was generally driven by increased mastication and predation efficiency and sophistication, but a wealth of recent studies have also highlighted the prevalent role of suspension-feeding, for which early panarthropods developed their own adaptive feedback through both specialized appendages and the diversification of small, morphologically differentiated larvae. In a context of general integumental differentiation and hardening across Cambrian metazoans, arthrodization of body and limbs notably prompted two diverging strategies of basipod differentiation, which arguably became founding criteria in the divergence of total-groups Mandibulata and Chelicerata. The kinship of trilobites and their relatives remains a source of disagreement, but a recent topological solution, termed the 'deep split', could embed Artiopoda as sister taxa to chelicerates and constitute definitive support for Arachnomorpha. Although Cambrian fossils have been critical to all these findings, data of exceptional quality have also been accumulating from other Palaeozoic Konservat-Lagerstätten, and a better integration of this information promises a much more complete and elaborate picture of early arthropod evolution in the near future. From the broader perspective of a total-evidence approach to the understanding of life's history, and despite persisting systematic debates and new interpretative challenges, various advances based on palaeontological evidence open the prospect of finally using the full potential of the most diverse animal phylum to investigate macroevolutionary patterns and processes.