A Cambrian fossil from the Chengjiang fauna sharing characters with gilled lobopodians and radiodontans


 Background: Lobopodians are worm-like animals with simple legs. Probably representing a grade of organization, rather than an explicit clade, some lobopodians are thought to have given rise to both Euarthropoda and Onychophora (velvet worms). Another subset has been referred to as gilled lobopodians, and are characterized by flap-like appendages along the trunk and large, raptorial forelimbs. These animals probably include the ancestors of another important Cambrian group, the Radiodonta: large predatory or filter-feeding stem-arthropods such as Anomalocaris.Results: Parvibellus atavusgen. et sp. nov.from the Early Cambrian Chengjiang fauna of China is a small fossil preserving a distinct cephalic region bearing a pair of lateral projections and a circular, ventral mouth. The trunk bears eleven pairs of flap-like appendages and a short pair of terminal projections. A circular ventral mouth is also seen in Radiodonta and in some gilled lobopodians. Parvibellus atavus, gilled lobopodians and radiodontans also share the character of flap-like appendages along the trunk. However, the new fossil differs from radiodontans and gilled lobopodians by its small size and the absence of enlarged and/or raptorial frontal appendages. It also differs from gilled lobopodians in lacking ventral lobopod limbs, and from radiodontans in lacking stalked eyes. Conclusions: Parvibellus atavus expresses a unique combination of characters among Cambrian arthropods, and could be part of an early radiation of nektonic stem-Euarthropoda. Lobopodians have emerged as a diverse grade of proto-arthropods ('worms with legs'), walking on the substrate of the early Palaeozoic seas. The new fossil hints at a similarly diverse fauna of nektonic (swimming) stem-group arthropods in the Cambrian, from which gilled lobopodians and radiodontans may have evolved.


Background
Cambrian fossils offer unique insights into the early stages of animal evolution. Two groups of particular signi cance, both of which rst appear in the Cambrian, are lobopodians and radiodontans. Lobopodians have been characterised as 'worms with legs' and are thought to include the ancestors of both the modern velvet worms (Onychophora) and probably arthropods too (Euarthropoda). For overviews see, e.g.
Liu & Dunlop [1] and Smith &Ortega-Hernández [2], and references therein [1,2]. A remarkable lineage belonging to the euarthropod stem-group is the Radiodonta, which is usually characterised by a ring-like mouth (or oral cone)-albeit absent in some taxa [3,4]-a pair of large and sometimesraptorial frontal appendages, and a series of aps along the body which are presumed to have been used for swimming [5]. Some radiodontans achieved body lengths of more than two metres [6], rendering them among the largest Early Palaeozoic animals. Based on the armature of the frontal appendages and evolutionary trends from active predation through to suspension feeding has been proposed [7,8].
Other genera of Cambrian stem-group euarthropods such as Opabinia Walcott, 1912 from the Burgess Shale of Canada, and Kerygmachela Budd, 1993 and Pambdelurion Budd, 1997 from the Sirius Passet of Greenland, have been referred to as gilled lobopodians. They could represent transitional forms between Page 3/13 the lobopodian grade of organisation and the radiodontans [9][10][11][12]. Here, we describe a new fossil from the Early Cambrian Chengjiang fauna of China revealing a combination of characters consistent with a lineage which may have been close to the origins of gilled lobopodians and/or radiodontans.

Diagnosis
Cephalic region subcircular with paired lateral projections and a large circular ventral mouth. Cephalic region clearly differentiated from trunk, which bears eleven pairs of suboval lateral aps generally decreasing in sized from anterior to posterior; trunk terminating in a short pair of projections.

Description
Complete fossil in ventral view. Total body length 5.25 mm. Body clearly divided into a subcircular cephalic region (length 1.18 mm, maximum width1.38 mm) and a longer trunk (length 4.07 mm). Cephalic region with paired projections emerging from anterior third and directed laterally and slightly posteriorly. Lateral projections0.51 mm long, basal width 0.26 mm, tapering slightly from proximal to distal. Ventral surface of cephalic region dominated by large, circular mouth region. Mouth area consists of two circles, both composed of rings of plates; diameter of outer ring 0.6 mm, of inner ring 0.18 mm (Figs. 2,4B).About ten plates in outer ring and several tiny plates in inner ring, but plates are not well preserved. Trunk width ca. 1.03 mm, preserving hints of transverse segmentation and possible median gut trace. Trunk mostly straight, bending slightly to the left posteriorly ( Fig. 1). Trunk bears eleven pairs of ap-like projections along its entire length. Individual aps suboval to subtriangular in outline; transition from trunk to ap indistinct, but ap length ranging from ca.

Discussion
The preserved morphology of Parvibellus atavus gen. et sp. nov. appears to be unique among the animals found so far at Chengjiang, and from comparable Cambrian to Ordovician Lagerstätte. In overview, the fossil is fairly small (length ca. 5 mm), with a distinct cephalic region bearing a single pair of tapering lateral projections (Fig. 1). The mouth is circular, possibly formed from circlets of plates, and located ventrally in the middle of the cephalic region (Fig. 2). Any plate dentition is equivocal. There is no evidence that the lateral projections represent stalked eyes, i.e. they are not bulbous at the tips, and these same projections are neither articulated into limb podomeres nor enlarged or raptorial with either spines or setae. A sensory function as antennae seems plausible. The trunk expresses at best only weak ventral segmentation, but clearly bears eleven pairs of ap-like appendages (Fig. 1A) along its length with the aps often overlapping slightly, especially towards the posterior end. In life these aps probably sloped below the ap behind it, a condition also seen in, e.g., the radiodontan Anomalocaris Whiteaves, 1892 where it may have been an adaptation for swimming [5] through enabling the entire lateral body margin function as a single n ap. In the new fossil there is no evidence for ventral leg-like appendages (i.e.lobopods) associated with these aps. The trunk terminates in a pair of short projections.
A possible interpretation of this fossil is an early instar of a radiodontan in which the circular ventral mouth and aps along the trunk are present, but the stalked eyes and raptorial frontal appendages have not yet developed. Arguing against this is the fact that unequivocal juvenile radiodontans such as the ca. 18 mm long specimen of Lyrarapax unguispinus Cong et al., 2014 described by Liu et al. [13] resemble adults (cf. [3]) and have fully developed stalked eyes and spiny frontal appendages. If the new fossil were a juvenile radiodontan, it would imply a fundamental shift in their morphology during early postembryonic development. Lobopodian a nities also appear unlikely as the fossil lacks an elongate, worm-like body. The closest match among the known lobopodians would be Aysheaia pedunculata Walcott, 1911 which also has laterally projecting (albeit here branching) frontal appendages and ten pairs of fairly broad lobopod limbs [1,14]. By contrast, the trunk appendages of our new fossil appear ap-like ( Fig. 1),with a degree of overlap in life typical for a nektonic animal (see above),and are thus inconsistent with a function as walking appendages. Furthermore, none of the Early Palaeozoic lobopodians are known to have a ventral mouth [1], although it should be noted that a mouth in this position is present in modern velvet worms (Onychophora).

Comparisons with gilled lobopodians and radiodontans
The circular ventral mouth consisting of two putative rings, together with the ap-like appendages along the trunk could support a nities with radiodontans. Budd [9] recognised an 'AOPK' group comprising Anomalocaris (a radiodontan), plus the gilled lobopodians Opabinia, Pambdelurion and Kerygmachela.These taxa share the presence of lateral lobes and enlarged frontal appendages. Several subsequent studies also recovered gilled lobopodians close to Radiodonta ( [7,15]; [6]: supplementary data), Ortega-Hernández [12] noted that the gilled lobopodians probably represent a grade, rather than a clade, and that within this assemblage the position of the mouth may have shifted from an anterior position (Kerygmachela) to a ventral position (Pambdelurion, Radiodonta).
Also important are the trunk appendages. Parvibellus atavus gen. et sp. nov. reveals eleven pairs of aps (Fig. 1A), exactly matching the count in Pambdelurion and Kerygmachela [9,16] and at least some radiodontans where the complete body is known [6,17]. Other radiodontans have been described with eight to ten pairs of trunk aps [3,8,18], while Opabinia has fteen. Additionally,Opabinia, Pambdelurion and Kerygmachela have all been interpreted as having both lateral aps and lobopod limbs [9, 10]hence the name gilled lobopodians-although at least in Opabinia the lobopod limbs are less obvious and have not been universally accepted [19]. Radiodontans were traditionally assumed to have lost the lobopod limbs and retained only the aps. Van Roy et al. [6], see also [11], proposed that radiodontans actually possessed both dorsal and ventral aps: the dorsal aps homologous with the aps of the gilled lobopodians, the ventral aps homologous with their stubby lobopodian limbs. In this hypothesis these dorsal/ventral structures may eventually have evolved into the typical arthropod biramous limb ([6]: Fig. 4).

A nities of Parvibellus
So where could Parvibellus atavus gen. et sp. nov. t into these evolutionary scenarios? A cephalic region bearing a single pair of appendages suggests it belongs to the lower stem-Euarthropoda sensu [12].As noted above, the circular ventral mouth and eleven pairs of lateral aps are strongly reminiscent of radiodontans, and similar aps are also seen in gilled lobopodians. A ventral mouth is also present in Pambdelurion [20].The cephalic projections in the new fossil are primarily orientated laterally and are not demonstrably raptorial. In this sense they differ from the more anteriorly directed and spiny frontal appendages of at least Pambdelurion, Kerygmachela and the radiodontans; Opabinia, by contrast, has a proboscis. There is no evidence in the new fossil for stalked eyes projecting laterally as in radiodontans. Pambdelurion and Kerygmachela have been reconstructed without eyes [9,16], while Opabinia has an unusual pattern of ve stalked eyes on the dorsal surface. The dorsal surface of the new fossil, and any eyes it may have borne, remains equivocal. Finally, at 5 mm in body length the new fossil is noticeably smaller than both the gilled lobopodians (Opabinia ca. 7 cm, Pambdelurionca. 29 cm, Kerygmachela ca. 18 cm) and the radiodontans (ca. 20 cm as adults up to more than 2 metres).
One solution would be to place the new fossil as sister-group to the clade encompassing the gilled lobopodians and the radiodontans: the 'AOPK' group sensu Budd [9] (Fig. 3A). This entire lineage could be de ned by the acquisition of lateral aps and, presumably, a shift from walking to swimming as the primary mode of locomotion. The gilled lobopodians and radiodontans could be further characterised by their larger body size and a probably predatory lifestyle facilitated by the development of a large, raptorial forelimbs. The implication would be that the lateral cephalic projections in the new fossil are homologous with the radiodontan frontal appendages. The problem with this hypothesis is the apparent absence of lobopodian limbs in Parvibellus atavus gen. et sp. nov. Either lobopod limbs were present in the new fossil, but have not been preserved, or there were at least two independent losses and/or transformations (sensu Van Roy et al. [6]) of these lobopod appendages. Placing the new fossil higher in the tree as, for example, sister-group to Radiodonta (Fig. 3B), could be reconciled with only a single loss of the lobopod limbs, but would require a reversal with respect to the enlargement and modi cation of the frontal appendages.

Conclusions
The present material does not allow us to unequivocally resolve the phylogenetic position of Parvibellus atavus gen. et sp. nov. Further discoveries may reveal other taxa with character combinations bridging the gap between lobopodians and radiodontans, and this may help in placing the new fossil more robustly. The important message here is that there were small, swimming arthropods in the Chengjiang biota ( Fig. 4) with a fairly simple grade of organisation from which other, more complex, body plans could have evolved. Just as lobopodians seem to have been a diverse grade of animals with walking appendages associated with several individual morphologies [1], so there may have been a similar nektonic grade of lower stem-group arthropods in the Early Cambrian whose diversity and relationships remain to be explored.

Methods
The new specimen was studied under a Leica steromicroscope M125and drawn with a camera lucida attachment. Both low angle light and immersion in alcohol proved useful for resolving details. Energydispersive spectroscopy (EDS) analyses of the specimen without coating were conducted on a ZEISS-Supra40VP environmental scanning electron microscope with an Inca(EDS) system and X-max 50 mm2 detector at the FU Berlin.Brightness/contrast and the tone of all images were re ned by optimizing the levels in Adobe Photoshop CC 2014. The gures were prepared with Coral Draw X7. Systematic terminology follows Ortega-Hernández [12]. Panarthropoda refers to a clade including Euarthropoda, Onychophora and Tardigrada. Euarthropoda sensu Lankester, 1904 consists of the clade including the most recent common ancestor of extant chelicerates, myriapods, and pancrustaceans and all of its descendants, to the exclusion of Onychophora and Tardigrada.
Abbreviations ce: cephalic region; cm: circular mouth; p: ap-like projections; fu: furcae; ir: inner ring; lp: lateral projections; or: outer ring; pl: plates; tr: trunk;ts, trunk segmentations. Figure 1 Parvibellus atavus gen. et sp. nov. Holotype (ELI-EJ 048A/B) and only known specimen of a new lower stem-euarthropod from the Chengjiang fauna of China preserved in ventral view, revealing a distinct cephalic region with lateral projections and a circular ventral mouth and a trunk with eleven pairs of aplike appendages. A and A1 Part of a near-complete specimen (ELI-EJ 048) and interpretative drawing. B and B1 Counterpart (mirrored) and interpretative drawing. Scale bars0.5 mm.