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Gondwana Research 14

Origin, diversification, and relationships of Cambrian lobopods

Jianni Liu a,⁎, Degan Shu a,b, Jian Han a, Zhifei Zhang a, Xingliang Zhang a

a Early Life Institute, The Key Laboratory of Continental Dynamics, Northwest University, Xi'an, 710069, PR Chinab School of Earth Sciences and Resources, China University of Geosciences (Beijing), 100083, PR China

Received 30 April 2007; received in revised form 25 September 2007; accepted 9 October 2007Available online 25 October 2007

Abstract

As an extinct group, the origin, diversification and relationships of Cambrian lobopods have long been one of the most hotly-debated subjects.Lobopods used to be regarded as closely related to modern onychophorans and tardigrades, but the similarities are based on the few species ofCambrian lobopods and often on a fairly general level. With the discovery of new creatures and based on new observations on fossil lobopods, weconsider that Cambrian lobopods show great diversity and reveal a close relationship between Cambrian lobopods and arthropods. In addition,comparison between the dorsal spines of Cambrian lobopods and small shelly fossils suggest that Cambrian lobopods might have originated fromthe Meishucun radiation and diversified in the Qiongzhusi radiation.© 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Cambrian lobopods; Arthropods; Small shelly fossils; Origin; Diversification

1. Introduction

The supercontinent of Rodinia is described as tectonicallystable and coherent between 1000 and about 750 Ma (Hoffman,1991). The configuration of Rodinia has been vigorouslyexamined by scientists around theworld and numerous alternativemodels have been proposed (Yoshida et al., 2003; Maruyamaet al., 2007). It is generally accepted that the Laurentian, Baltica,Siberia and Gondwana blocks were certainly separated by thesplit-up of Rodinia in the Neoproterozoic and moving apartduring the Ediacaran–Early Cambrian interval (McKerrow et al.,1992). The geographic distribution of lobopods in the Cambrianincluding south China, Siberia, Greenland and North America,indicates that this type of faunal community had dispersedworldwide in the interval from the Late Tommotian through theAtdabanian. The first lobopod to be described from the Cambrianwas Aysheaia pedunculata (see Briggs et al., 1994, Fig. 87, 88,89) from the Burgess Shale, named by Walcott (1911). Later,

⁎ Corresponding author. Early Life Institute and Department of Geology,Northwest University, 229#, Taibai Road, Xi'an, 710069, PR China. Tel.: +8629 88303706; fax: +86 29 88302128.

E-mail addresses: eliljn@nwu.edu.cn, liujianni@126.com (J. Liu).

1342-937X/$ - see front matter © 2007 International Association for Gondwana Rdoi:10.1016/j.gr.2007.10.001

ConwayMorris (1977) described a second lobopod,Hallucigeniasparsa (see Ramsköld and Chen, 1998, Fig. 3.1), from theBurgess Shale. As the two undisputed lobopods discovered in theMiddle Cambrian, these two taxa have been widely cited anddiscussed (Ramsköld, 1992; Hou and Bergström, 1995; Rams-köld andChen, 1998; Bergström andHou, 2001; Hou et al., 2004;Zhang and Aldridge, 2007).

Since the Lower Cambrian Chengjiang fauna was discov-ered in 1984 (Hou et al., 1991), Hou et al. have describedsix species of lobopods before the end of last century,they are Microdictyon sinicum (Fig. 1a) Chen et Hou1989; Luolishania longicruris (Fig. 1i) Hou et Chen 1989;Onychodictyon ferox (Fig. 1d) Hou, Ramsköld et Bergström1991; Cardiodictyon catenulum (Fig. 1b) Hou, Ramsköld etBergström, 1991; Hallucigenia fortis (Fig. 1c) Hou etBergström 1995 and Paucipodia inermis (Fig. 1e) Chen,Zhou et Ramsköld 1995; together with the other fourlobopods Xenusion auerswaldae (see Dzik and Krumbiegel,1989, Fig. 1, 4) Pompeckj (1927) from the Lower Cambrian ofnorthern Europe; Pambdelurion whittingtoni (see Budd, 1997,Fig. 11.3, 11.4) Budd, 1997, Kerygmachela kierkegaardi (seeBudd, 1993, Fig. 1, 2, 3) Budd, 1993 andHadranax augustus (seeBudd and Peel, 1998, plate I, II and text-fig. 2) Budd and Peel,

esearch. Published by Elsevier B.V. All rights reserved.

Fig. 1. Early Cambrian lobopods from Chengjiang Fauna. a) Microdictyon sinicum ELI-JSMS0001; b) Cardiodictyon catenulum ELI-JSC0002A;c) Hallucigenia fortis ELI-JSH0007A; d) Onychodictyon ferox ELI-JSO0011A; e) Paucipodia inermis ELI-JSP0025A; f) Megadictyon cf. haikouensis HolotypeELI-JSMH008;g) Jianshanopodia decora Holotype ELI-J007; h) Miraluolishania haikouensis Holotype ELI-JSM0020A; i) Luolishania longicruris, the CameraLucida drawing of ours. Note that the smallest units in scale bar represent 1 mm.

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1998 from the Lower Cambrian of Greenland. These animals areregarded as closely related with each other and they have certainsimilarities tomodern onychophorans, tardigrades and arthropods(Hou and Bergström, 1995; Budd, 1993, 1997).

With the new century coming, three distinct formsMiraluolishania haikouensis (Fig. 1h) Liu et Shu 2004;Jianshanopodia decora (Fig. 1g) Liu et Shu 2006 andMegadictyon cf. haikouensis (Fig. 1f) Liu et Shu 2007 have been

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discovered by the Early Life Institute (abbreviated ELI) team,adding significant data to the group. This is not only because of alarger number of specimens and species, but in particular becausethese three taxa bear mosaic features of both lobopods andarthropods. Consequently, these Cambrian lobopods are nowregarded as the intermediate creatures between lobopods andarthropods (Xiao, 2004; Liu et al., 2004, 2006a, 2007).

The possible origin of the extinct lobopods has rarely beendiscussed (Dzik and Krumbiegel, 1989; Budd, 2001a,b). Dzikand Krumbiegel (1989) suggested an origin from priapulid–palaeoscolecid-like worms through the development of seg-ments and limbs with, at the same time, a change in the mode oflife from priapulid-style burrowing within the bottom sedimentto the crawling on the sea bottom of lobopods. Budd alsodiscussed the relationship of Cycloneuralia, lobopods andEuarthropoda based on the segmentation and Ecdysozoa concept(Budd, 2001a,b). Although both hypotheses offer hope thatlobopods have a close relationship with priapulid–palaeoscole-cid-like worms, no intermediate creatures have been discovered tosupport this idea. Fortunately, the discovery of complete specimensof Facivermis yunnanicus gives direct fossil evidence for thishypothesis (Liu et al., 2006b).F. yunnanicus, not only possesses thecharacters of a priapulid, but also bears features typical of lobopods.The mode of life of F. yunnanicus is also just between burrowingand crawling. Therefore, we considered that lobopods might haveoriginated from priapulid–palaeoscolecid-like worms. Besides,there are some lobopod plates discovered from the MeishucunStage of Cambrian, such as Zhijinites (Qian, 1984, 1989) andCambroclavus (Qian, 1984, 1989), which are probably the plates oflobopods (Qian et al., 2000). And the similarity between smallshelly fossils and lobopods plates are hotly discussed these years(Dzik, 2003; Hou and Aldridge, 2007). Thus, we suggest that thelobopods originate as early as the Meishucun radiation or even inthe Precambrian and then diversify in the Cambrian.

2. Origin of lobopods

As noted above, the origin of lobopods is still obscure. Dzikand Krumbiegel's (1989) hypothesis of an origin frompriapulid–palaeoscolecid-like worms through the developmentof segments and limbs is the main contender. In lobopodevolution, whatever the origin, the mode of life changed frompriapulid burrowing to lobopod-style crawling. Conway Morris(1997) thought that this transformation would have beendifficult to complete; however, it has its own reason based onthe Ecdysozoa concept. Similar to the ideas of Dzik andKrumbiegel, Budd also offers hope that lobopods have a closerelationship with priapulid–palaeoscolecid-like worms based onthe segmentation and the Ecdysozoa concept (Budd, 2001a,b).Furthermore, the discovery of complete specimens ofF. yunnanicus gave direct fossil support to this hypothesis.F. yunnanicus, not only possesses the characters of priapulid, butalso bears features of typical lobopods. The affinity of Facivermiswith the lobopods is supported by (i) the lobe-like appendages; (ii)the appendages having strong attachment with the trunk andattachment areas forming a round plate at a base; (iii) a fine canalrunning centrally through the entire length of the each appendage;

and (iv) as in the rare lobopod Miraluolishania, each appendageofFacivermis bears two rows of setae on its anterior and posteriormargins. Besides the affinities ofFacivermiswith lobopods, if weturnF. yunnanicus upside down and interpret the pear-shaped endas the anterior of F. yunnanicus rather than the posterior, it is verysimilar to the pear-shaped anterior proboscis of priapulidXishania longiusula (Hu, 2001). The similarities are: (i) thesame ratio of width/length, about 1/20; (ii) the pear-shapedanterior proboscis; (iii) two or three circles of hooks which arelocated in the same place; (iv) the anterior projection of proboscis;(v) the clear annulations and thin spines of the body. Furthermore,the mode of life of F. yunnanicus is intermediate betweenburrowing and crawling. We think that most part of the body ofF. yunnanicus rested in the sediment, and that the anterior bodyand appendages protruded out above the sea floor to catch prey.And in terms of the posterior morphology of Facivermis, weconceive that the enlarged pear-shaped trunk ends with two orthree circles of hooks could have helped to fix the animal like ananchor of a ship when it protruded the five pairs of appendagesand made purchase to allow it to prey upon its victim.

As noted above, there are certain taxa of small shelly fossilssuch as Microdictyon that are now regarded as lobopod plates.One of these,Zhijinites, as Qian et al. (2000) indicated, has featuresin common with the plates and spines of both Hallucigenia(Conway Morris, 1977) and Onychodictyon (Ramsköld and Hou,1991). These similarities include: (1) in gross morphology, bothstructures are composed of a sub-round plate as a base and a spine;(2) as with Zhijinites, the outer surface of spines of Hallucigeniaare finely granulated; (3) especially, there usually occurs in thesame number and symmetry of plates in one sample, and thissituation is in accord with those of dorsal spines of HallucigeniaandOnychodictyon. Therefore, we are inclined to regard Zhijinitesas likely isolated dorsal plates of lobopods; if accepted, then there isevidence that lobopods originated during or before the early Meis-hucun Stage.

3. Diversification

During the last few years, data on the range of Lower Cambrianlobopods has been greatly increased (Budd, 1993, 1997; Hou andBergström, 1995; Budd and Peel, 1998; Ramsköld and Chen,1998; Liu et al., 2004, 2006a, 2007), while a few additionalMiddleand Upper Cambrian taxa are now known (Collins, 1986; Maasand Waloszek, 2001). All these forms and the living onychophor-ans are generally included in the taxon Lobopoda, following thesuggestions of Simonetta and Delle Cave (1981). Although themajority of Cambrian lobopods sport spines or other sclerites,however, as several authors indicated (Briggs, 1994; Wills et al.,1994; Delle Cave et al., 1998), there is a fairly large morphologicalvariation within the group, even within a single Lagerstätte.Indeed, all the Cambrian forms were probably already stronglyderived, although more or less closely related, radiating from acommon source and this trend can be easily recognized: Pauci-podia (Chen et al., 1995a) is the simplest lobopod with six pairs ofappendages and a proboscis-like head area. Microdictyon(Ramsköld and Hou, 1991) bears pairs of sclerites with a seriesof perforated platelets;Hallucigenia (ConwayMorris, 1977), with

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long dorsal spines on the trunk and three pairs of appendages at theanterior end of the trunk, is regarded as a typical ‘armored lobopod’(Budd, 1997);Cardiodictyon (Hou and Bergström, 1995) bears 25pairs of appendages on the trunk and pairs of unique triangular-shaped plates; Onychodictyon (Ramsköld and Hou, 1991), withlarge papillae, has a strange appearance; Luolishania (Hou andChen, 1989a) is of generalized appearance, without specializedhead structures; Miraluolishania (Liu et al., 2004) is much morederived, with pairs of eyes and antennae on the head and the trunkshowing primitive tagmosis; Aysheaia (Whittington, 1978) isunique, without antennae but with a pair of grasping appendages infront of the first pair of trunk appendages; Xenusion (Dzik andKrumbiegel, 1989) is a large lobopod with pairs of dorsal plates;Hadranax (Budd and Peel, 1998) is an incomplete animal bearingstrong appendages and dorsal tubercles; Kerygmachela(Budd, 1993), a gilled lobopod, bears lateral lobes on the trunk;Pambdelurion (Budd, 1997), a typical lobopod, possesses laterallobes and lobe-like limes on the trunk; Jianshanopodia (Liu et al.,2006a) andMegadictyon cf. haikouensis (Liu et al., 2007) are bothlarge lobopods within the Chengjiang Fauna, with a pair ofgrasping appendages. However, Jianshanopodia bears complexbranched appendages and a sucking pharyngealmechanism, whileMegadictyon cf. haikouensis possesses ‘Peytoia’ mouthparts andcaecae-like structures.

Summing up, a striking variation in the number of morpho-logical characters indicates that Cambrian lobopods did undergoearly diversification, and this may give us the impression that theirshared ancestor probably lived far back into the Precambrian.

4. Mode of life

It is generally held that lobopods crawled on other organismsusing their hook claws because of notable associations betweencertain lobopods and particular animals, such as Aysheaia, whichis commonly found associated with sponges (Whittington, 1978)(see Briggs et al., 1994, Fig. 89). Similarly, Microdictyon wasdescribed as associated with Eldonia (Chen et al., 1991). Inreconstructions they have been illustrated as clinging to spongesand eldoniids, and Aysheaia probably sucked the juice of thesponges (Whittington, 1978) and that Microdictyon was pelagic(Chen et al., 1995b). In terms of the morphology of Aysheaia, wethink that there exists this possibility. Besides, we agree thatsucking predation is a possibility. Alternatively, we think that theassociation betweenMicrodictyon and Eldonia does not indicatethat Microdictyon was pelagic, because as Dzik et al. (1997)shown, eldoniids were shelled animals of bilateral organizationthat lived a gregarious life on the sea bottom. Deposition wassudden in the fossiliferous intervals and some animals most likelywere transported. Hou and Bergström (1995) think that the smalland light xenusions were not likely to have been sorted alongwiththe fairly large-shelled eldoniids as a result of transport. However,no association betweenMicrodictyon and Eldonia occur in our 45specimens. Indeed,Microdictyon, Paucipodia and Onychodictyonis often observed to be buried with burrowing worm priapulid(Fig. 2), which suggests that they live in similar environment.Priapulidswere burrowing under the sea bottom,whereas lobopodswere crawling on the sea bottom. Therefore, we agree with Dzik

et al. that the association betweenMicrodictyon and Eldonia occa-sionally occurs because of transportation.

Most of the lobopods, such as Hallucigenia, Onychodictyon,Miraluolishania, bear pairs of strong and sharp spines on theirtrunk, which not only could be used to protect themselves, but alsocould be actively used against prey. Besides,Miraluolishania eachpossesses a pair of eyes, which is significant for predators. Inaddition, the claws of the lobopods would help them to get a firmhold on the prey they were interested in.

The large lobopods, Jianshanopodia and Megadictyon bearpairs of frontal appendages similar to those of Anomalocaris(Chen et al., 1994), together with a strong body and the‘Peytoia’mouthparts ofMegadictyon. We thus suppose that theywere predators. The branched lobe-like limbs of Jianshanopodiasuggest that it could swim for only a short timewhen not crawlingon the bottom. The other large lobopods such as Kerygmachelaand Pambdelurion, which come from Sirrus Passet in northGreenland, also bear pairs of frontal appendages with long spinesor ‘Peytoia’ mouthparts, indicating that they are also hunters.

5. Relationships

The Cambrian lobopods have repeatedly been supposed tohave onychophoran and tardigrade affinities (Manton, 1977;Thompson and Jones, 1980; Bergström and Hou, 2001).Robison (1985) concluded that Aysheaia is related to modernonychophorans. However, although like those of onychophor-ans, the lobopods had strong terminal claws on their legs, othercharacters such as the location of mouth, the lobe-like limbs andthe ornament of the trunk are all different from those of modernonychophorans. Besides, there is a reasonable number oflobopods and their morphologies show a range of diversity, butonychophoran morphology does not show much diversification.Thus, we think that there is not close relationship between theCambrian lobopods and modern onychophorans.

A comparison between lobopods and tardigrades revealsnumbers of differences, which we would contend to show thatthey are unrelated. As indicated by Bergström and Hou (2001),lobopods and tardigrades share the presence of a terminal mouthwithout jaws (at least in Aysheaia and Paucipodia), but Robison(1985) stated that the location of mouth is different in differ-ent species of tardigrades; some species of lobopods, e.g.M. haikouensis, H. fortis, are observed with an antero-ventrallylocated mouth. Also, Bergström and Hou (2001) regarded thatlobopods share similarities with tardigrades in the presenceof feet with several claws. Unfortunately, most lobopods bearonly two claws at the end of their legs, such as Onychodictyon,Miraluolishania, Hallucigenia, Microdictyon and Cardiodictyon;claws at the distal extremity of the legs are not observed in somespecies such as Xenusion, Jianshanopodia and Megadictyon.Furthermore, as Robison (1985) stressed, the claws ofAysheaia andtardigrades are not comparable in structure and probably are nothomologous. Bergström and Hou (2001) also indicted that bothlobopods and tardigrades lack antennae. However,Miraluolishaniais reported to bear a pair of dorsal antennae. It was noted that thatwhen moulting, Xenusion left its old cuticle through a mid-dorsalsplit, while tardigrades do it anteriorly through the oral opening

Fig. 2. Lobopods are buried with priapulids. a) Paucipodia inermis is buried with priapulids; b) and c) Microdictyon sinicum is buried with priapulids;d) Onychodictyon ferox is buried with priapulids. Note that the smallest units in scale bar represent 1 mm.

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after they have expelled the cuticular elements of the buccal appa-ratus. Also, the diminutive size of tardigrades is strongly differentfrom lobopods.Walossek andMüller (1994) described fossils of thetardigradeBoeckelericambria pelturae discovered from theMiddleCambrian and suggested that tardigrades evolved their diminutivesize at the beginning of the Cambrian or in the Precambrian. Takentogether, the above differences strongly indicate that lobopods andtardigrades are undoubtedly unrelated.

Nowwe think that lobopods and arthropods are closely related.Lobopods are probably the ancestor of the arthropods, and someintermediate forms have been discovered. Miraluolishania, acreature with a mosaic of characters both lobopod and arthropod,gives strong support to the idea that arthropods should be rootedin the early lobopods. The lobopodian features ofMiraluolishaniaare its worm-like body design, the dorsal spines, and non-seg-

mented limbs or lobe-like legs. The arthropodian features ofMiraluolishania are its primary cephalization with paired eyes,paired antennae, and preliminary tagmosis. As is well known, theprimary cephalization and preliminary tagmosis are both nov-elties of ‘arthropodation’. The occurrence of a pair of eyes andantennae indicates (1): Both the protocerebrum and thedeutocerebrum of Miraluolishania have had some degree ofdevelopment. (2): The antennae would be innervated not laterthan the eye,which has been verified by developmental biology ofliving onychophorans (Eriksson et al., 2003). (3): The antennae ofMiraluolishania are probably the anterior-most limbs,which are different from the frontal limbs of other lobopods,e.g. Kerygmachela (Budd, 1993), Pambdelurion (Budd, 1997)andAysheaia. On the other hand, the antennae ofMiraluolishaniaare quite like the antennae of the stem-group euarthropods, such

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as Fuxianhuia,Naraoia, Xandarella (Bergström and Hou, 2001).The evidence strongly suggests thatMiraluolishania represents astep in the evolutionary ladder leading to the euarthropods (Xiao,2004).

Like Kerygmachela and Pambdelurion, Jianshanopodiaand Megadictyon cf. haikouensis are the two big lobopodswhich have been found to bridge lobopods and arthropods.Jianshanopodia, with the complex branched appendages, throwsnew light on the origin of biramous appendages. It is well knownthat the most obvious feature of arthropod diversity is manifestedin the number, morphology and function of their appendages.Antennae, mouthparts, walking legs, grasping and swimmingappendages are all modifications of the basic jointed-leg structurethat defines the phylum (Budd, 1996; Shubin et al., 1997). It isgenerally held that jointed legs evolved from simple unjointedappendages of Cambrian lobopods. Cambrian taxa with differenttypes of “arthropodization” and limbmorphology can be identified(Shu et al., 1995; Hou and Bergström, 1997; Zhang et al., 2003).Some paleontologists have hypothesized that fusion between thelateral lobes and ventral lobopodia might have given rise to thebiramous limb (Shubin et al., 1997; Budd, 1999). It is oneevolutionary possibility. However, comparison of Dll expressionand limb outgrowth in various types of crustaceans and insectssuggests that uniramy, biramy and polyramy are the products ofshifts in signals along the dorso-ventral axis of the body wall andthose additions or reductions in branch number might haveevolved readily (Panganiban et al., 1995). Correspondingwith thisidea, the complex branches of Jianshanopodia appendages arelikely to represent another variant, which seems to be consistentwith the Kukalova-Peck's (1992) viewpoint.

Megadictyon cf. haikouensis is another big lobopoddiscovered from the Early Cambrian Chengjiang Lagerstätte,it had a head bearing caecae-like structures similar to those ofthe arthropod Naraoia and Chelicerate, ‘Peytoia’-like mouth-parts and frontal appendages. The ‘Peytoia’-like mouthparts andthe frontal appendages are similar to those of the AOPK(Anomalocaris–Opabinia–Pambdelurion–Kerygmachela)group (Budd, 1997), together with Jianshanopodia, which notonly indicates that lobopods exhibit a much wider morpholog-ical diversity than the extant onychophorans, but also suggeststhat the frontal appendages is a homologous feature that appeardeep in the lobopod–arthropod phylogeny (Budd, 2002).Furthermore, the strong transversely-wrinkled frontal appen-dages of Megadictyon cf. haikouensis hint that they weresclerotized to some degree and the heteronomous annulations ofthe trunk indicate Megadictyon cf. haikouensis had gotthe primitive segmentation. In addition, the appendageswith appendicules show that Megadictyon cf. haikouensisis closely related to Onychodictyon. Therefore, Megadictyoncf. haikouensis is regarded here as another rare transitional formbetween lobopods and arthropods.

6. Conclusions

Based on the diverse morphologies of Cambrian lobopodsfound thus far, we regard that there is a riot of morphologicalinvention at the origin of metazoans. At the same time, analysis of

the relationships of Cambrian lobopods to other phyla indicatesthat lobopods have a much closer relationship with arthropodsthan Onychophora and Tardigrada. This indication providesfurther evidence that arthropods may not be as closely related tothe annelids as have been commonly thought. Furthermore, weprovide new evidences about the origin of lobopods, which notonly suggests that lobopods might have originated frompriapulid–palaeoscolecid-like worms, but also breaks down theview that the phyla appeared abruptly in the Cambrian with nohints of their forebears under theweight of the similarities betweensome small shelly fossils and the plates of lobopods.

Acknowledgments

This work was supported by the National Science Foundationof China (grants 40332016 and 30270207), the Program forChangjiang Scholar and Innovative Research Team in theUniversities, and Ministry of Sciences and Technology of China(PCSIRT, grant 2000077700). The authors are indebted to Dr.Susan Turner for improving the English language; Yanbing Ji andYanchun Yao for field work help. Special thanks are given toHuilin Luo and Shixue Hu in the Yunnan Institute of GeologicalScience for advice.

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