The origin and early evolution of arthropods

Abstract

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 ...

The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited

“…Our phylogenetic analysis supports a 'deep-split’ scenario [ 70 ] using both datasets ( figure 10 ). In this scenario, the total group Mandibulata and the remaining arthropod groups originate soon after the origin of the first euarthropods [ 13 ].…”

The unbearable uncertainty of panarthropod relationships

“…Resolving evolutionary relationships among the panarthropod phyla is integral to understanding the evolutionary assembly of the arthropod bodyplans, from genomic, developmental and phenotypic perspectives. The pattern of character assembly for the arthropod bodyplans is perhaps the best resolved for all fossil groups, thanks to Cambrian fossil Konservat-Lagerstätten which are dominated by euarthropods [38][39][40]. This is largely a consequence of the biased preservation of their recalcitrant cuticles and because many key features of the euarthropod bodyplan are represented in their cuticular skeletons [41,42].…”

A possibly deep branching artiopodan arthropod from the lower Cambrian Sirius Passet Lagerstätte (North Greenland)

“…Artiopoda are inherently difficult to root due to the lack of informative synapomorphic characters, leaving both internal relationships as well as external euarthropod phylogeny unresolved (Aria 2022). Some phylogenetic hypotheses have proposed Squamacula clypeata as a sister taxon to all other artiopodans (Chen et al .…”