Specialized appendages in fuxianhuiids and the head organization of early euarthropods

Abstract

The organization of the head provides critical data for resolving the phylogenetic relationships and evolutionary history of extinct and extant euarthropods. The early Cambrian-period fuxianhuiids are regarded as basal representatives of stem-group Euarthropoda, and their anterior morphology therefore offers key insights for reconstructing the ancestral condition of the euarthropod head. However, the paired post-antennal structures in Fuxianhuia protensa remain controversial; they have been interpreted as both 'great appendages' and as gut diverticulae. Here we describe Chengjiangocaris kunmingensis sp. nov. and Fuxianhuia xiaoshibaensis sp. nov. from a new early Cambrian (Stage 3) fossil Lagerstätte in Yunnan, China. Numerous specimens of both species show a unique 'taphonomic dissection' of the anterodorsal head shield, revealing the cephalic organization in detail. We demonstrate the presence of a pair of specialized post-antennal appendages (SPAs) in the fuxianhuiid head, which attach at either side of the posteriorly directed mouth, behind the hypostome. Preserved functional articulations indicate a well-defined but restricted range of limb movement, suggestive of a simple type of sweep feeding. The organization of the SPAs in fuxianhuiids is incompatible with the (deutocerebral) anterior raptorial appendages of megacheirans, and argue against the presence of protocerebral limbs in the fuxianhuiids. The positions of the fuxianhuiid antennae and SPAs indicate that they are segmentally homologous to the deutocerebral and tritocerebral appendages of crown-group Euarthropoda respectively. These findings indicate that antenniform deutocerebral appendages with many podomeres are a plesiomorphic feature of the ancestral euarthropod head.

Arthropod fossil data increase congruence of morphological and molecular phylogenies

“…Under this scheme, the raptorial appendages of stem-group euarthropods such as megacheirans ('great-appendage' arthropods), fuxianhuiiids and bivalved stem-group arthropods (for example, Odaraia, Canadaspis and Perspicaris) were considered homologous to the chelicerae of chelicerates, and any antenniform appendages anterior to this were considered segmentally homologous to the antennae of onychophorans, that is, protocerebral rather than deutocerebral. Recent studies of Fuxianhuia and other closely related taxa 25,26 , however, indicate that the antennae are in fact deutocerebral. This finding implies that their post-antennal appendages are not homologous to chelicerae, and that mandibulate antennae are homologous to the antennae of many or most members of the euarthropod stem group.…”

Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda

“…Within the context of extant representatives, the postcephalic CNS of C. kunmingensis exhibits notable similarities with the thoracic VNC of the notostracan Triops cancriformis, which consists of a homonymous series of separate ganglia linked by short connectives that become progressively smaller toward the posterior end (1, 3). The overall morphology of T. cancriformis further resembles C. kunmingensis-and generally other fuxianhuiids-in the presence of dorsoventral trunk segmental mismatch expressed as appendage polypody (23,25,26). A similarly unspecialized VNC is also found in crustaceans typified by a trunk region with undifferentiated limb-bearing segments, such as the remipedes (5, 7) (Fig.…”

“…Moreover, the unresolved phylogenetic relationships within Panarthropoda complicate accurate reconstruction of the CNS ground pattern (16)(17)(18)(19)(20)(21)(22). In this study, we demonstrate the exceptional preservation of postcephalic neurological features in the early Cambrian fuxianhuiid Chengjiangocaris kunmingensis, an upper stem-group euarthropod (17) from the Xiaoshiba Lagerstätte, South China (23). These fossils clarify the neurological organization of the VNC in early euarthropod ancestors, thereby polarizing the evolution of the panarthropod CNS.…”

“…S2 A and B and S3). This organization reflects the notable dorsoventral segmental mismatch that typifies the fuxianhuiid trunk (23,25).…”

An exceptionally preserved arthropod cardiovascular system from the early Cambrian

“…As described previously 8 , the cerebral ganglia are preserved as a contiguous bilaterally symmetric structure comprising three segments, each defined by its paired nerve tracts associated with the protocerebral optic lobes and the deutocerebral antennae (A1) and short tracts 8 , terminating above the location of the roots of the paired scimitarlike second post-ocular appendages (A2) (Fig. 3c) disposed ventrally beneath the head 1,14 . Ventral nerve cords in the related fuxianhuiid Chengjiangocaris kunmingensis are preserved as paired connectives linking segmentally discrete ganglia 14 .…”