Evolution and taxonomy of Cambrian arthropods from...

ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2008

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 558

Evolution and taxonomy ofCambrian arthropods fromGreenland and Sweden

MARTIN STEIN

ISSN 1651-6214ISBN 978-91-554-7302-0urn:nbn:se:uu:diva-9301

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List of publications

I. Stein, M. 2008. Fritzolenellus lapworthi (Peach and Horne, 1892)from the lower Cambrian (Cambrian Series 2) Bastion Formation ofNorth-East Greenland. Bulletin of the Geological Society of Denmark56:1–10.

II. Stein, M. & Peel, J. S. 2008. Perissopyge (Trilobita) from the lowerCambrian (Series 2, Stage 4) of North America and Greenland. GFF130:71–78.

III. Stein, M., Waloszek, D., Maas, A., Haug, J. T. & Müller, K. J. 2008.The stem crustacean Oelandocaris oelandica re-visited. Acta Palaeon-tologica Polonica 53:461–484.

IV. Stein, M. (manuscript) A new arthropod from the Early Cambrian ofNorth Greenland with a ‘great appendage’ like antennula. Submitted toZoological Journal of the Linnean Society.

V. Stein, M., Peel, J. S., Siveter, D. J. & Williams, M. (manuscript)Isoxys (Arthropoda) with preserved soft anatomy from the Sirius PassetLagerstätte, lower Cambrian of North Greenland.

VI. Lagebro, L., Stein, M. & Peel, J. S. (manuscript) A new arthropodfrom the early Cambrian Sirius Passet Fauna of North Greenland.

Reproduction of the published papers is made with kind permission from thecopyright holders:

Paper I © Bulletin of the Geological Society of Denmark, Dansk GeologiskForeningPaper II © GFF, Geologiska FöreningenPaper III © Acta Palaeontologica Polonica, Instytut Paleobiologii PAN

Statement of authorship

Access to the material studied in papers I,–II, IV–VI was granted by J. S.Peel.Paper I: M. Stein performed the studies and wrote the manuscript.Paper II: M. Stein carried out the taxonomic studies and wrote the section onsystematic palaeontology. J. S. Peel provided the introduction and the sec-tions on material and stratigraphy.Paper III: M. Stein and D. Waloszek carried out most of the studies of thematerial. Initial reconstructions were created by M. Stein, D. Waloszek, andA. Maas. M. Stein created an early version of the three dimensional model ofthe largest stage and the animated sequence; J. T. Haug created the final ver-sion of the model of the largest stage and all models of the ontogenetic se-quence. K. J. Müller collected the material. Manuscript mainly by M. Steinand D. Waloszek with contributions by the other authors.Paper IV: M. Stein performed the studies, created the reconstructions andwrote the manuscript.Paper V: Initial studies were carried out by M. Stein and J. S. Peel. Detailedstudies were carried out by M. Stein, D. Siveter and M. Williams.Manuscript mainly by M. Stein with contributions by the other authors.Paper VI: L. Lagebro provided an initial study of the material. M. Stein per-formed detailed restudy and wrote the section on systematic palaeontology.J. S. Peel wrote the introduction.

Disclaimer

The papers presented here are for the purpose of public examination as adoctoral thesis only. They are not considered publications according to theICZN. Accordingly, all new taxonomic names and emendations in papersIV–VI are void. Authority for taxonomic work in papers I–III is by the origi-nal publications.

Front cover

Reconstruction of Kiisortoqia avannaarsuensis from the early Cambrian Sir-ius Passet Lagerstätte of Greenland.

Contents

Introduction ................................................................................................... 7

Trilobite biostratigraphy ................................................................................ 9Paper I ..................................................................................................... 10Paper II .................................................................................................... 12

Arthropod phylogeny ................................................................................... 14The current status of euarthropod phylogeny from a neontological per-spective ................................................................................................... 15The ground pattern and the importance of fossils ................................... 16The position of Trilobita ......................................................................... 19Paper III .................................................................................................. 21Paper IV .................................................................................................. 22Paper V .................................................................................................... 24Paper VI .................................................................................................. 25

Svensk sammanfattning ............................................................................... 27Trilobit-stratigrafi på Grönland ............................................................... 28De tidiga leddjurens evolution ................................................................ 29

Acknowledgments ....................................................................................... 30

References ................................................................................................... 31

Introduction

Arthropods are a diverse group of multicellular animals (metazoans), esti-mated to account for more than three quarters of the present-day metazoandiversity. They constitute an important element of many ecosystems, bothmarine and terrestrial. Four major groups are recognized among extantarthropods. The chelicerates include the familiar spiders, scorpions, mites,and ticks, and less familiar forms such as the horseshoe crabs. Crabs and lob-sters are grouped together as crustaceans, along with woodlice and krill, butalso with smaller and less familiar forms such as brine-shrimps and the trulyubiquitous ostracodes and copepods, and even sessile forms such as barna-cles. Millipedes and centipedes comprise the myriapods, but the all but toofamiliar insects form the largest of all living arthropod groups. These fourgroups sharing a common ancestor constitute the Euarthropoda and are char-acterized by a unique body structure and movement apparatus where mus-cles do not act as antagonists to bones or fluid-filled pressure vessels lodgedinside a ‘soft’ body. Rather, a firm cuticle, which is produced by the epider-mal cells surrounding the body, forms an ‘exoskeleton’ consisting of hard-ened (sclerotized) plates conjoined by soft membranous regions that allowfor flexibility. Muscles, attached to the sclerotized parts, act against this ‘ex-oskeleton’. How the euarthropod groups are related to each other remains thesubject of sometimes contentious debate. The nature of several small groupsconsidered to be the closest relatives to the euarthropods is also debated.While there is consensus that the little known onychophorans (velvet worms)are arthropods in the wider sense, researchers still argue about if the minutetardigrades (water bears) belong to the group or are more closely related toroundworms. The pentastomids (tongue worms), a group of worm-like para-sites infesting the respiratory organs of land-living vertebrates (amphibians,‘reptiles’, birds, and mammals) are widely accepted as arthropods. But arethey the sister group to euarthropods or highly derived crustaceans, and thuseuarthropods?

Given their tremendous diversity, it is not surprising that arthropods havehad a long evolutionary history, evident not least in a rich fossil record span-ning the whole Phanerozoic. Most common in the fossil record are formswith well biomineralized (calcified) ‘exoskeletons’, such as trilobites or cer-tain crustaceans, including ostracodes and malacostracans. Trilobites ap-peared early in the Cambrian and flourished through the whole Palaeozoic

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prior to disappearing in the terminal Permian extinction. They are commonand diverse components of Cambrian faunas and have been of paramountimportance in biostratigraphic and palaeogeographic studies of that periodfor more than a century and a half.

But the fossil record of arthropods is not just limited to forms with calci-fied ‘exoskeletons’. Throughout the Phanerozoic, examples of exceptionalpreservation of soft parts offer unique glimpses into past ecosystems. Arthro-pods form a surprisingly common element of these so-called lagerstätten, al-though taphonomic bias against various organism groups means that eventhese remarkable deposits do not necessarily mirror the true composition ofthe original ecosystems. In strata of the Cambrian system (542–488 Ma BP),the oldest period of the Phanerozoic, researchers have unearthed a number offossil lagerstätten that have received particular public attention, notably theBurgess Shale of Canada, the Chengjiang Lagerstätte of China, or assem-blages of ‘Orsten’-type fossils from several places in the world. This atten-tion is certainly largely due to our fascination with the apparent abrupt ap-pearance of fossil traces of multicellular life around the base of the Phanero-zoic. Many investigators believe that the earliest major diversification ofmetazoans took place as this era unfolded, a key event in the history of life.Whether this be true or not, Cambrian lagerstätten do provide the oldestavailable direct evidence of the early diversity of many metazoan groups.

Arthropods are a very prominent element also in these ancient Cambrianbiota, showing a striking diversity of form and life habits. At least to someextent, this dominance may be due to the relatively good preservation poten-tial of their cuticle encasing the whole body. This not only yields large quan-tities of arthropod fossils, but also allows preservation of morphological de-tail in extremely high fidelity, including details of limb structure. Thus, away has been opened for a more direct comparison of the anatomy of fossilswith extant taxa than is possible for many other fossils. Not surprisingly, thiswealth of detail has stimulated greater attention to fossil arthropods in phylo-genetic studies of the group than fossils of most other invertebrate animalgroups.

This dissertation was originally conceived as a stratigraphic study of earlyto middle Cambrian trilobite faunas of North-East and North Greenland butthe focus shifted gradually to more palaeobiological questions, inspired byemerging views on arthropod evolution and phylogeny, including also thequestion of the phylogenetic placement and significance of Trilobita. Thus,attention turned to exceptionally preserved arthropod fossils, with materialderived from two renowned Cambrian lagerstätten: the early Cambrian Sir-ius Passet biota of North Greenland, and the late Cambrian (Furongian)‘Orsten’ of Sweden, the type for a kind of exceptional preservation that isnow found in strata spanning the early Cambrian to the early Ordovician inmany localities world wide.

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Trilobite biostratigraphy

Cambrian stratigraphy is currently undergoing a major revision aiming at astandardized global chronostratigraphic framework (Babcock et al. 2005,Babcock & Peng 2007). The traditional Early, Middle, and Late Cambrian,variously defined on different paleocontinents, are being replaced by fourglobally recognizable series, of which the first two very loosely correspondto the earliest traditional subdivision.

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Figure 1: Subdivision of the Cambrian System according to the new chronostrati-graphic framework. Series and stages with a grey background are yet to be defined.

So far, only the Furongian Series (Peng et al. 2004), which roughly corre-lates with the traditional Late Cambrian, and the Terreneuvian Series, whichreplaces the provisional series 1 (Landing et al. 2007) have been defined byGlobal Standard Stratotype-sections (GSSPs) (Figure 1). An informative ex-ample of some of the difficulties associated with the definition of these newseries is provided by ongoing discussions concerning the boundary betweenseries 2 and 3, corresponding to the traditional Early-Middle Cambrianboundary. The former Middle Cambrian was traditionally defined by the oc-currence of paradoxidid trilobites in Baltica, Avalonia, and West Gondwana,but it has been shown that this first occurrence is diachronous. In the Early-Middle Cambrian boundary interval of Morocco paradoxidid trilobites occurtogether with holmiid trilobites which, as part of Olenelloidea, traditionallydefine the Early Cambrian (Geyer & Palmer, 1995; see Geyer 2005 for anextensive review). Geyer (2005) listed five trilobite horizons being discussedcurrently as a potential new boundary marker between the provisional series2 and 3, but progress is hampered by the endemism of early Cambrian trilo-bite faunas, different litho- and biofacies distributions, and not least by thedifferent state of knowledge and taxonomic practice in relevant geographicalareas (Geyer 2005).

Studies of Cambrian faunas in North Greenland have recognized richtrilobite faunas with considerable potential for intercontinental correlation(e.g. Robison 1988, 1994, Babcock 1994a, b, Blaker & Peel 1997). Thestratigraphic sequence succession in North Greenland is relatively completeand encompasses inner craton and outer shelf facies in close juxtaposition.Thus, boreal faunas of Baltic aspect have been described from outer shelf fa-cies in close proximity to warm water faunas of Laurentian aspect (Babcock1994a, b). Recently, the diverse trilobite faunas of North Greenland de-scribed by Blaker and Peel (1997) has been recognized as particularly signif-icant for resolution of the problem of the boundary interval between provi-sional series 2 and 3 (Fletcher 2003, Geyer 2005).

As of yet, the trilobite faunas of North-East Greenland are not well knownand correlation of the Cambrian provisional series 2 of this region with thatof other areas of Laurentia has been advanced by the study of small shellyfaunas (Skovsted 2006). Correlation with North Greenland remains impre-cise but the present studies offer the first prospect for elucidating similaritiesin the trilobite faunas of the two areas.

Paper IStein, M. 2008. Fritzolenellus lapworthi (Peach and Horne, 1892) from thelower Cambrian (Cambrian Series 2) Bastion Formation of North-EastGreenland. Bulletin of the Geological Society of Denmark 56:1–10.

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This paper deals with olenellid trilobites from the Bastion and Ella Islandformations (provisional Stage 4) of North-East Greenland, collected in theearly 1950s. The focus is on material from the older Bastion Formation,from which the bulk of the material is derived. Collections from the overly-ing Ella Island Formation only yielded two immature cephala. The speci-mens of the material from the Bastion Formation are identified as Frit-zolenellus lapworthi (Peach & Horne, 1892), a species originally describedfrom the 'Fucoid' Beds of the An t'Sron Formation of north-west Scotland.The Greenland material is considerably better preserved than that of the typeregion. This allowed a more detailed description, in particular of the post-cephalic morphology. The material also includes immature specimens, per-mitting a first, limited account of the ontogeny of the species. The occur-rence of F. lapworthi in Greenland offers a first opportunity of a direct corre-

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Figure 2: A, Fritzolenellus lapworthi from the Bastion Formation of North-EastGreenland; B, Reconstruction of F. lapworthi; C, Olenellus cf. hanseni from the EllaIsland Formation of North-East Greenland.

lation of the Cambrian Stage 4 strata of north-west Scotland with this part ofLaurentia. So far, correlation between the areas rested largely on the occur-rence of the problematic fossil Salterella, which, in Scotland, occurs in the‘Fucoid’ Beds and the overlying ‘Salterella Grit’, in North-East Greenland inthe Bastion, Ella Island, and Hyolithus Creek formations (Skovsted 2006,Poulsen 1932, Cowie & Adams 1957). Salterella also occurs in the Parallel-dal Formation of North-Greenland and in lower Cambrian strata of InglefieldLand, North-West Greenland (Peel & Yochelson 1892).

So far, F. lapworthi has not been identified in any material from other re-gions of Greenland. The trilobites of the overlying Ella Island Formationmay prove to be of utility for correlation with North Greenland, but furtherwork is required. The two olenellid cephala from that formation are similarto Olenellus hanseni (Poulsen, 1932) described from this formation. Thisspecies was unfortunately based on immature cephala and fragmentary tho-racic material only and may require revision. Similar material has been de-scribed from the Henson Gletscher and Kap Troedsson formations of NorthGreenland, described as Olenellus cf. truemani Walcott, 1913 (Blaker & Peel1997). This material also includes immature specimens which lend them-selves for comparison with the type material of O. hanseni. If this materialcould be positively identified as O. hanseni, it could serve as a basis for anemendation of the species and would further provide a direct tie point be-tween North Greenland and North-East Greenland.

Paper IIStein, M. & Peel, J. S. 2008. Perissopyge (Trilobita) from the lower Cambri-an (Series 2, Stage 4) of North America and Greenland. GFF 130:71–78.

Paper II deals with the distinctive trilobite Perissopyge Blaker, Nelson &Peel, 1997 and its occurrence in Laurentia. Two species have been describedpreviously, both from the Cambrian Stage 4; P. triangulata Blaker, Nelson &Peel, 1997, occurring in the Harkless Formation of Nevada, and P. phenax,occurring in the Henson Gletscher Formation of North Greenland and report-ed from the Sekwi Formation of Yukon Territory, Canada (Blaker et al. 1997,Blaker & Peel 1997). This paper documents Canadian material for the firsttime and adds a further occurrence in the Paralleldal Formation of NorthGreenland, which is overlain by strata that can be correlated with Siberia(Debrenne & Peel 1986). The occurrence of P. phenax in the Paralleldal andHenson Gletscher formations thus further supports correlation of the HensonGletscher Formation with Siberia and China, while the occurrence in YukonTerritory offers a tie point into that area of Laurentia.

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An indeterminate species of Perissopyge is reported also from the Ella Is-land Formation of North-East Greenland. This co-occurrence with Olenellushanseni, which may also occur in the Henson Gletscher Formation of NorthGreenland, may hold potential for further correlation between North andNorth-East Greenland.

The distinctive pygidial morphology of Perissopyge makes it also attrac-tive as a potential biostratigraphic marker, as it is easily identified amongearly Cambrian trilobite faunas. Unfortunately, it seems that the limited dis-tribution as yet only grants regional utility.

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Figure 3: A, Perissopyge triangulata, Harkless Formation, Nevada, USA; B, C, P.phenax, Sekwi Formation, Yukon Territory, Canada, D, E, Perissopyge sp. Ella Is-land Formation, North-East Greenland.

Arthropod phylogeny

As noted above, it is not clear which taxon is the closest relative (sister tax-on) to the Euarthropoda. The living Onychophora are representatives of thenon-sclerotized forms among Arthropoda in the wider sense (Arthropodasensu lato), but there is a considerable morphological gap between these andthe euarthropods. Accordingly, there must have been a long evolutionary his-tory between the last common ancestor (stem species) of both ony-chophorans and euarthropods and the stem species of Euarthropoda. Speciesdo change over time, i.e. along their phylogenetic lineage, but, given thelarge morphological gap, it is less likely that all the morphological changes—from dorsal cuticular sclerotization to eye, limb, and head formation toname but a few—all occurred in the (anagenetic) lineage of a single speciesof Euarthropoda. More likely, there were several branching points along theway. All species that branched off along the evolutionary lineage toward Eu-arthropoda but became extinct, are stem lineage representatives or deriva-tives of the stem lineage of the Euarthropoda. The only way we may know ofthem directly is if they are preserved as fossils. Identified correctly, they cantell us in which order morphological changes occurred along the stem lin-eage.

It is clear that the cuticular ‘exoskeleton’ of euarthropods must have beenacquired as a novel feature somewhere along the euarthropod stem lineage.This must have occurred in several steps as it includes different novelties:formation of sclerotized dorsal areas (tergites) and soft membranous areasbetween the tergites to allow for flexibility; dissolution of the subepidermalmuscle tube and formation of isolated muscle strands that develop tendon-like connectives into the cuticle; development of appendages that are subdi-vided into sclerotic rings and membranous connections; development of piv-ot joints on these limbs, limiting the flexure of the sclerotic rings to one axisof movement. A number of fossil taxa mainly known from Cambrian lager-stätten have these features, but lack other euarthropods autapomorphies.They are likely representatives of the euarthropod stem lineage and, togetherwith euarthropds, constitute the Arthropoda sensu stricto. The ground patternof the stem species of Arthropoda s. str. as promoted by Maas et al. (2004)and Waloszek et al. (2007) includes further features, e.g. a pair of antero-ventral compound eyes, a short head comprising two segments carrying theeyes and one pair of appendages (the antennulae) respectively, and themouth in a ventral position, opening toward the posterior. All these changes

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must have accumulated before the stem species of Arthropoda s. str. Addi-tional changes occurred later in the euarthropod stem lineage. Thus the evo-lutionary lineage toward Euarthropoda can be divided into at least two sec-tions, divided by the node of Arthropoda s. str. Examples of the first sectionare the so-called lobopods, small to moderately long worm-shaped animalswith long, tubular segmental appendages. The possibility that also theTardigrada and the Pentastomida belong there is still debated. One hypothe-sis considers Pentastomida to be the closest living relatives to Euarthropoda,having pivot jointed limbs but lacking segmental subdivision of the cuticlein tergites and soft membranous areas (e.g. Maas et al. 2004). While this issupported by further morphological and developmental data, an alternativehypothesis based on sperm ultrastructure and molecular data suggests thatpentastomids are highly derived crustaceans (Møller er al. 2008 and refer-ences therein; see also Waloszek et al. 2006 for a recent review of the com-peting hypotheses).

The current status of euarthropod phylogeny from aneontological perspectiveEuarthropod phylogeny has been in a state of flux for a long time, and theadvent of molecular systematics during the last decade has added new con-troversial hypotheses about relationships rather than helping settle standingissues. One ongoing debate concerns the phylogenetic position of Pycnogo-nida; they are currently placed either within Chelicerata as the sister taxon ofEuchelicerata (Xiphosura and Arachnida) or as the sister taxon of all othereuarthropods (see Dunlop & Arrango 2005 for a review). The phylogeneticposition of Myriapoda is another remaining problem. The traditional conceptof Mandibulata comprising Myriapoda, Insecta, and Crustacea has recentlybeen challenged by molecular studies that place myriapods as the sister tax-on to Chelicerata (e.g. Hwang et al. 2001, Mallatt et al. 2004, Dunn et al.2008). There is little morphological support for this hypothesis, other thansome aspects of development of the nervous system. Analyses of morpholog-ical or combined data usually resolve myriapods together with Crustacea andInsecta as part of Mandibulata (e.g. Giribet et al. 2001, 2005, Edgecombe2004). Finally, the phylogenetic position of insects remains disputed. Tradi-tionally considered to be the sister taxon of Myriapoda in the Atelocerata (=Tracheata) hypothesis, molecular or combined molecular and morphological(e.g. Giribet et al. 2001, 2005, Hwang et al. 2001, Mallatt et al. 2004) andneuro-anatomical (e.g. Fanenbruck et al. 2004, Harzsch 2006) studies oftenresolve them as more closely related to or even nested within Crustacea.

The controversies about euarthropod phylogeny are largely beyond thescope of the present studies. Further, most of the competing hypotheses out-

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lined above do not involve any fossil data. Nevertheless, the topology of theeuarthropod phylogenetic tree has implications for the ground pattern of Eu-arthropoda and therefore needs to be considered.

The ground pattern and the importance of fossilsCharacterizing the ground pattern of Euarthropoda based on neontologicaldata alone is hampered not least by the uncertainties of in-group relation-ships. Here, fossils provide an invaluable source of information, as the onlydirect evidence of morphology in the stem lineages of taxa which maybridge significant morphological gaps between terminal taxa.

It is clear that an advanced cephalization with a head tagma incorporatingseveral segments was part of the euarthropod ground pattern. The most ante-rior, protocerebral segment carries the eyes, and in certain crustacean taxapaired, so called frontal organs (Vilpoux & Waloszek 2003, Fanenbruck &Harzsch 2005, Semmler et al. 2008). The following, deutocerebral, segmentcarries a pre-oral appendage, the chelicera in Chelicerata or antennula inCrustacea, Insecta, and Myriapoda (traditionally called antenna in the lattertwo taxa). The third, tritocerebral segment carries the first postantennularlimb, called pedipalp in Chelicerata or (second) antenna in (Eu-)Crustacea.In Myriapoda and Insecta this limb is absent, most likely reduced. The as-sumption of a tripartite brain incorporating neuromeres of the three most an-terior segments as part of the euarthropod ground pattern is problematic(Walossek & Müller 1998), as even derived euarthropod taxa such as bran-chiopods and isopods still have separate ganglial knots.

There is ample fossil evidence that such a head tagma, incorporating theprotocerebral, deutocerebral, and three additional segments was present inthe earliest representatives of the crustacean (and possibly mandibulatan)evolutionary lineage (Walossek & Müller, 1990, 1998). There is further evi-dence, that these early representatives hatched with a ‘head larva’, i.e. a lar-va with only the segmental composition of this head tagma (Walossek &Müller, 1990, 1998). The ‘short-head larva’ of Eucrustacea incorporates onesegment less and is interpreted as an autapomorphy of that taxon. It is thebody tagmosis of chelicerates that poses a pertinent problem.

The anterior tagma in Euchelicerata, the prosoma, incorporates the proto-and deutocerebral segments plus five additional limb-bearing segments.Adult Pycnogonida have a cephalosoma that corresponds in segmental com-position to the euarthropod head, but this is first acquired during ontogeny inextant representatives. The protonymph larvae appear to be ‘short-head lar-vae’ with only the proto-, deutocerebral, and two additional limb-bearingsegments (Vilpoux & Waloszek 2003). The caudal outgrowths of the larva ofthe only Cambrian representative of Pycnogonida (Waloszek & Dunlop2002) were subsequently interpreted as a fourth pair of limbs (Vilpoux &

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Waloszek 2003). This would mean that the larva of the Cambrian species hasone more pair than the protonymph larvae of extant species.

Some investigators have taken the problem of chelicerate body tagmosisas support for rejecting a head with the eye segment and four limb-bearingsegments as a ground pattern feature of Euarthropoda. They rather regardsuch a head tagma as a feature that evolved first in the mandibulatan stemlineage (Scholtz & Edgecombe 2005). The alternative hypothesis assumesthat the euarthropod head became modified within Chelicerata (Waloszek &Maas 2005). This may gain support from so called ‘great appendage’ arthro-pods, that are considered to be chelicerate stem lineage representatives(Chen et al. 2004, Cotton & Braddy 2004) and have such a head tagma. Thiscontrasts with reports of a head comprising only three appendage-bearingsegments for some of the taxa (García-Bellido & Collins 2007), but recentdetailed re-investigations of one species cast doubt on that assessment. Theyrather confirm a head incorporating no less than four appendage-bearing seg-ments (Liu et al. 2007), as assumed already for the euarthropod ground pat-tern.

Assignment of these taxa to the chelicerate stem lineage rests on the spe-cific morphology of the ‘great appendage’, a stout grasping appendagewhich is considered to be the first (deutocerebral) limb and homologous tothe chelicera of Chelicerata (Chen et al. 2004, Cotton & Braddy 2004). Thishomology statement is particularly based on one species, Haikoucaris er-caiensis Chen, Waloszek & Maas, 2004, which is intermediate in the mor-phology of its grasping leg between that of the previously described taxa andthe chelicerate chelicera. These new views on the chelicerate stem lineagemark a radical departure from previous assumptions of the evolution of che-licerae, which were traditionally considered to be the limbs of the tritocere-bral segment. The deutocerebral segment, carrying the antennula of crus-taceans, myriapods and insects, was considered to have been lost along thechelicerate stem lineage (e.g. Bitsch & Bitsch 2007). This view, first chal-lenged by molecular data (e.g. Damen et al. 1998, Telford & Thomas 1998)was mainly based on a misinterpretation of the chelicerate nervous system(Mittmann & Scholtz 2003). According to the traditional view, it was trilo-bites and closely related fossil arthropods that were considered to be repre-sentatives of the chelicerate stem lineage in the ‘arachnomorph’ concept(Scholtz & Edgecombe 2005 for a review).

Accepting homology between the chelicerae of Chelicerata and the deuto-cerebral antennula of the other euarthropod taxa forced reconsideration ofthe morphology of the deutocerebral limb in the ground pattern of Euarthro-poda. The traditional view assumed a sensorial flagelliform, i.e. multiannu-lated, limb, as was developed in trilobites and related fossil arthropods. Thismorphology was readily homologized with the sensorial antennulae of in-sects, myriapods and certain crustaceans. The limb was considered to be lost

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together with the deutocerebral segment in the upper chelicerate stem lin-eage. The new view required that the chelicerae were derived from a flagelli-form antennula, or, more likely, from a more limb-like deutocerebral ap-pendage that must have had a feeding function already in the stem species ofEuarthropoda. From a purely neontological perspective, the latter idea wasnurtured by the potential placement of Pycnogonida as the sister taxon to theremaining Euarthropoda. In the case of a sensory antennula in the euarthro-pod ground pattern, this placement would require convergent transformationinto a feeding limb in two taxa (Dunlop & Arrango 2005). Fossils providedadditional support for a feeding function of the deutocerebral limb in the eu-arthropod ground pattern, independent of the position of Pycnogonida.

Studies of crustacean (or possibly mandibulatan) stem lineage representa-tives have demonstrated that the stem species of Crustacea had an antennulaadapted for feeding and locomotion rather than sensorial purposes (seeWalossek 1999 for a summary). This was originally considered to be an au-tapomorphy, adopted from the traditional view of such an antennula in theground pattern of Euarthropoda, but had to be reevaluated in the light of thenew data. The flagellate antennula, often considered characteristic of thegroup is an autapomorphy of certain in-group taxa, foremost Malacostraca(e.g. Waloszek 2003). In the light of homology between the feeding and lo-comotory crustacean antennula and the feeding chelicera of chelicerates, it isapparent now that the adaptation of the antennula for feeding in the crus-tacean stem species was not an autapomorphy but a symplesiomorphy, re-tained from the last common ancestor of chelicerates and crustaceans (possi-bly mandibulates; e.g. Waloszek et al. 2007). A novelty developed in thecrustacean (or mandibulate) stem lineage may be that the antennula works inconcert with the postantennular limbs as part of the cephalic feeding appara-tus.

Studies of early members of Arthropoda s. str. yielded further support forthis hypothesis, as they have revealed that these forms had a limb-like feed-ing antennula while lacking any other obvious structures for feeding (Maaset al. 2004, Waloszek et al. 2005, 2007). It should be noted that there are al-ternative interpretations of the head structure in at least one of these fossils,Fuxianhuia protensa Hou, 1987, which consider the most anterior limb to beprotocerebral rather than deutocerebral (Scholtz & Edgecombe 2005, 2006).In this interpretation, another structure under a large, shield-like tergite isconsidered to be a jaw-like deutocerebral limb. The supposed protocerebrallimb is considered to be a ‘primary antenna’ homologous to the ony-chophoran palp, either lost in the euarthropod stem lineage or retained in avestigial form as the protocerebral frontal organs of certain eucrustaceantaxa (Semmler et al. 2008). This interpretation suffers from several prob-lems; firstly, the supposed jaw-like deutocerebral limb in F. protensa couldbe demonstrated to be an internal structure, most likely gut diverticula, andin other early Arthropoda s. str. the structure is clearly missing (Waloszek et

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al. 2005). Further, the notion that Chelicerata rely on a ‘whole limb mouth-part’ for feeding, whereas Mandibulata rely on the gnathobasic parts of theirpostantennular limbs (Scholtz & Edgecombe 2005) is an over simplification.As discussed, there is ample evidence that the antennula was part of thecephalic feeding apparatus in the crustacean stem species, while Xiphosuraas euchelicerates use the median edges of the limb bases (basipods) of theirpostcheliceral limbs for mastication. Finally, the assumption of a specializeddeutocerebral jaw in the ground pattern of Euarthropoda is problematic be-cause a sensorial antennula is more likely to be derived from a generalizedlimb-like structure. The latter point needs to be considered in particular withrespect to the various competing hypotheses of euarthropod relationshipswhich may require sensory antennulae to be evolved convergently more thanonce. A further independent origin may be required to accommodate a groupof fossil taxa, as will be discussed in the next section.

The position of TrilobitaTrilobita is an entirely extinct group of euarthropods. Putative autapomor-phies are the calcification of the cuticle and dorsal eyes with circumocularsutures. The trilobation of the trunk tergites with a raised axis and loweredtergopleurae often considered characteristic for the group is a symplesiomor-phy retained from the ground pattern of Arthropoda s. str.

In contrast to many other euathropod taxa, Trilobites and closely relatedfossil taxa have long flagelliform, probably sensory antennulae composed ofa large number of short annulae. Traditionally regarded to be related to theChelicerata, more recent work affiliates them with the mandibulate stem lin-eage based on their antennular morphology, which is considered to representthe ‘secondary antenna’ evolved in the stem lineage of Mandibulata accord-ing to Scholtz & Edgecombe (2005). This interpretation faces the dilemmathat there is reason to assume a feeding antennula as part of the crustaceanground pattern, which would require multiple character reversals (once forthe stem species of Crustacea, a second time for the stem species of Mala-costraca). Thus, caution should be exercised against using the mere presenceof sensory antennulae as an argument for placement of fossil taxa along thestem lineage of Mandibulata. Even more so, in the light of a possibly conver-gent evolution of sensory antennulae in various euarthropod taxa discussedabove. Based on details of limb morphology and development of the cephal-ic feeding apparatus, trilobites and related fossil arthropods with a flagelli-form antennula must have branched off prior to early representatives of thecrustacean (or possibly mandibulatan) stem lineage. Rather, a flagelliformantennula in these taxa may represent a possible synapomorphy of a group ofthese ‘trilobitomorphs’ (the classical ‘Trilobitomorpha’ originally comprised

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also the ‘great appendage’ arthropods now regarded as chelicerate stem lin-eage representatives).

Monophyly of a group of Trilobita and e.g. naraoiids, hemeltiids,tegopeltids, and xandarellids, all with a flagelliform antennula, might also besupported by aspects of limb morphology. All have a bipartite exopod with aproximal portion carrying lamellar setae and a distal lobe-like portion with asetal fringe (Edgecombe & Ramsköld 1999, Bergström & Hou 2003, Cotton& Braddy 2004, Scholtz & Edgecombe 2005) which is a now widely notedautapomorphy. Inside this grouping, xandarellids are often regarded as thesister taxon to the remainder, due to the absence of a pygidium, defined as apoint at which tergites ceased being released anteriorly in ontogeny (Ram-sköld et al. 1997, ‘subterminal generative zone’ of Hughes et al. 2008).There are additional ‘trilobitomorph’ taxa with flagelliform antennualae;apart from the undisputed presence of lamellar setae, the exopod structure ofEmeraldella brocki Walcott, 1912 has been debated (Bruton & Whittington1983, Hou & Bergström 1997, Edgecombe & Ramsköld 1999), with the lat-ter authors arguing for a bipartite exopod as in the other taxa. Retifacies ab-normalis Hou, Chen & Lu, 1989 and Sidneya inexpectans Walcott, 1911have lamellar setae but apparently no subdivision of the exopod. The latterspecies is also considered to have had a ‘head’ that only incorporates the pro-tocerebral and deutocerebral segments (Bruton 1981). A putatively closelyrelated species, Sidneya sinica Zhang, Han & Shu, 2002 has a relativelylonger head shield than S. inexpectans with setae protruding from underneathit, indicating incorporation of postantennular limbs. However, unless Bru-ton’s (1981) assessment of the head in S. inexpectans can be disproved, anad hoc ‘loss’ of the euarthropod head in this taxon would have to be invoked.

A major dilemma that besets any attempt at placing Trilobita and these'trilobitomorph' taxa in the phylogenetic system of arthropods, is that noclearly defined synapomorphy with any of the major extant groups is cur-rently recognized. Placement in the mandibulatan stem lineage has been jus-tified by the sensory antennulae or the presence of a head incorporating noless than four appendage-bearing segments (Scholtz & Edgecombe 2005,2006). Both arguments are problematic; the presence of flagelliform anten-nulae alone may not suffice, as outlined above, and the head with four limb-bearing segments is likely a ground pattern feature of Euarthropoda. Othercharacters, such as the lamellar exopods, may represent autapomorphies ofthe group, while an endopod with seven podomeres intermediates betweennine or ten in the euarthropod ground pattern and five to six in Crustacea andis difficult to evaluate. It becomes clear that the problem needs thoroughreinvestigation, also in the light of problematic taxa such as Sidneya inex-pectans or another problematic taxon that may be allied based on antennularmorphology, the Marellomorpha.

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Paper IIIStein, M., Waloszek, D., Maas, A., Haug, J. T. & Müller, K. J. 2008. Thestem crustacean Oelandocaris oelandica re-visited. Acta PalaeontologicaPolonica 53:461–484.

The third paper provides a detailed study of the stem lineage crustacean Oe-landocaris oelandica Müller, 1983 from the ‘Orsten’ lagerstätte of Sweden.The ‘Orsten’ is not a fossil lagerstätte in the sense of a geographically andstratigraphically confined unit, as the fossils occur in bituminous concretionsthat occur in the Alum Shale successions that crops out in several regions of

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Figure 4: Oelandocaris oelandica; A, Holotype from Öland, Sweden; B, reconstruc-tion of the large stage present in the material; C, larval specimen from Västergöt-land, Sweden; D, reconstruction of different stages.

Sweden and span the Cambrian series 3 and Furongian boundary interval.Fossils of this ‘Orsten’ type preservation have also been reported from otherdeposits in several areas of the world and different geological times (seeMaas et al. 2006 for a review). The fossils are usually in the submillimetricrange and, in contrast to many other lagerstätten, preserved in full three-di-mensional detail. ‘Orsten’ type preservation is also notable for being one ofthe few lagerstätten that has yielded largely complete ontogenetic series ofarthropod fossils.

Oelandocaris oelandica was originally described on the basis of a singlefragmentary specimen from the Cambrian Stage 7 (Series 3) of the island ofÖland, Sweden (Müller 1983). Subsequently identified material from thePaibian Stage (Furongian) of Västergötland in mainland Sweden permitted amore complete description of the species and a discussion of its phylogeneticsignificance, which was presented in an initial report by Stein et al. (2005).In the present paper, additional information, including the recognition of dif-ferent ontogenetic stages in the material is provided, the life habits are dis-cussed, and the phylogenetic position suggested by Stein et al. (2005) is fur-ther confirmed. The species, together with other representatives of the crus-tacean stem lineage from the ‘Orsten’, provides important evidence for thestructure of the cephalic feeding apparatus in the ground pattern of Crus-tacea. By comparison to other fossil euarthropods, the morphology of thepostantennular limbs in this early representative of the crustacean stem lin-eage also provides important insight into some aspects of limb morphologyin the euarthropod ground pattern, in particular of the exopods

Paper IVStein, M. A new arthropod from the Early Cambrian of North Greenlandwith a ‘great appendage’ like antennula. Submitted to Zoological Journal ofthe Linnean Society.

The fourth paper is the first in a series of three on fossils from the Sirius Pas-set Lagerstätte of the Cambrian Stage 3 (Series 2) of North Greenland. Thislagerstätte, roughly coeval with the Chengjiang Lagerstätte (MaotianshanShale) of China, is one of the oldest Cambrian lagerstätten yielding excep-tionally preserved arthropod macrofossils.

The paper describes a new fossil arthropod species, Kiisortoqia avannaar-suensis, that has a large and robust antennula remarkably similar to the an-tero-ventral raptorial appendage of the problematic anomalocaridids. Whilethe phylogenetic scope, position, and significance of the latter is contentious-ly debated (e.g. Budd 2002, Chen et al. 2004, Hou & Bergström 2006) withsome authors questioning euarthropod or even arthropod affinities, K. avan-

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naarsuensis shares features with euarthropods that justify placement in thistaxon or in the stem lineage in direct proximity to the euarthropod node.While this does not resolve the question of anomalocaridid affinities, it doesprovide strong support for homology between the antero-ventral appendageof anomalocaridids with the deutocerebral antennula of euarthropods andfurther for the antennula as a primary feeding limb in the euarthropod stemspecies.

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Figure 5: Kiisortoqia avannaarsuensis from the Sirius Passet Lagerstätte, NorthGreenland; A, holotype; B, detail of the most complete antennulae in the material; C,D, reconstructions in ventral and lateral view.

Paper VStein, M., Peel, J. S., Siveter, D. J. & Williams, M. Isoxys (Arthropoda) withpreserved soft anatomy from the Sirius Passet Lagerstätte, lower Cambrianof North Greenland. Manuscript.

In this paper an account is given of the soft anatomy of Isoxys volucrisWilliams, Siveter & Peel, 1996, from the Sirius Passet Lagerstätte. Thisspecies has a large, more or less bivalved shield encasing most of the body.There are numerous fossil and extant arthropods with such a shield, and it iswidely accepted that this feature arose independently in several groups. Inthe fossil record, they are known often by the shield alone, which is whyphylogenetic or taxonomic assignments of a species to a certain group oftenrests on the morphology of the shield alone. In some cases it has been shownthat the assignment by shield morphology was misleading, with the Phos-phatocopina originally considered to be ostracods, now recognized as the sis-ter taxon to Eucrustacea (Siveter et al. 2003) as a prime example.

There are numerous species assigned to Isoxys Walcott, 1890, mostly knownby the shield morphology alone. Ventral morphology is known only in a fewinstances and there are indications that it may differ between species (e.g.Vannier & Chen 2000). The present paper gives a review of what is knownand makes comparisons with the new data from the Sirius Passet lagerstätte.

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Figure 6: Isoxys volucris from the Sirius Passet Lagerstätte, North Greenland; A,specimen preserving limbs; B, specimen showing shield in dorsal view; C, specimenpreserving possible eye.

There are sufficient similarities between I. volucris and one species from theChengjiang fauna of China to justify close relationships, but relationshipswith other species remain problematic. This cautions against composite re-constructions and palaeoecological considerations based on multiple species.

Paper VILagebro, L., Stein, M. & Peel, J. S. A new arthropod from the early Cambri-an Sirius Passet Fauna of North Greenland. Manuscript.

The sixth and final paper describes Siriocaris trolla, a new species of fossilarthropod from the Sirius Passet Lagerstätte. It shares some features of cer-tain ‘trilobitomorph’ arthropods, including flagelliform antennulae and artic-ulation devices of the thoracic tergites, such as flanges on the tergopleuraeand an edge-to-edge articulation with larger overlap in the axis and a hori-zontal inner portion of the tergopleurae. It is intriguing to consider ‘trilobito-morph’ affinities based on these features, as at least the tergal morphologymay imply particularly close affinities with trilobites and helmetiids (Ram-sköld et al. 1997, Edgecombe & Ramsköld 1999). The prominent tergopleu-ral spines and the small tail shield are even reminiscent of olenelline trilo-

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Figure 7: Siriocaris trolla from the Sirius Passet Lagerstätte, North Greenland; A,holotype; B, specimen showing the morphology of the tergites.

bites, commonly considered to be ‘basal’ trilobites (e.g. Lieberman 2001 andreferences therein). An indication of trilobation in the head shield and an axi-al furrow at least in the posterior part of the thorax are other features that in-vite comparison with trilobites. Unfortunately, with the limited material athand, alignment with trilobites and helmetiids can at present not be substan-tiated, as some characters in S. trolla remain unclear. While there are indica-tions that the exopods may be bipartite as in helmetiids and trilobites, com-pelling evidence for lamellar setae on a putative proximal element is notavailable, and there is no direct evidence of fringing setae on the putativeouter lobe. It is not clear whether or not the tail of S. trolla is a pygidium.This character, however, is difficult to evaluate, as certain olenelline trilo-bites have a similarly small pygidium that in some species even containsonly a single axial ring (Paterson & Edgecombe 2006).

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Svensk sammanfattning

Leddjur (Arthropoda) är den artrikaste gruppen av flercelliga djur och utgöruppskattningsvis tre fjärdedelar av alla nu levande arter. De bebor många habi-tat, både i havet och på land. De nu levande leddjuren är fördelade på fyra sto-ra grupper: spindeldjur (Chelicerata), kräftdjur (Crustacea), mångfotingar(Myriapoda) och insekter (Insecta), som utgör den största gruppen. Alla dessadjur har ett unikt yttre skelett uppbyggt av härdade (sklerotiserade) plattor somär ledade mot varandra. Tillsammans utgör de gruppen Euarthropoda. Hur defyra grupperna är besläktade med varandra är mycket omdiskuterat. Dessutomfinns det några mindre grupper som t. ex. havsspindlar (Pycnogonida), som avvissa forskare betraktas som spindeldjur, men som av andra anses vara syster-grupp till Euarthropoda, dvs. euarthropodernas närmaste släktingar. Det är inteheller helt klarlagt, vilka de övriga arthropoderna är, alltså de som inte hörhemma i Euarthropoda. Medan det är en allmänt utbredd uppfattning att klo-maskar (Onychophora) är arthropoder, finns det fortfarande oenighet om huru-vida trögkrypare (Tardigrada) tillhör gruppen eller i stället är närmare besläk-tade med rundmaskar. Ytterligare en omdiskuterad grupp är tungmaskar (Pen-tastomida), som är parasiter på bl. a. ren. De betraktas allmänt som athropoder,men det råder delade meningar om huruvida de är euarthropodernas syster-grupp eller starkt specialiserade kräftdjur.

Med den stora diversiteten och spridningen som arthropoder har idag är detföga förvånande att gruppen har haft en lång utvecklingshistoria, rikligt belagdmed fossil. De tidigaste leddjursfossilen uppträder redan i nedre kambrium,bland de äldsta fossilen av flercelliga djur. Oftast är det arter med kutikulaförstärkt av mineralinlagringar som bevarats, som t. ex. musselkräftor och trilo-biter. Dessa är vanliga i havsavlagringar och viktiga verktyg för relativa ålders-dateringar (stratigrafi). Trilobiter är särskilt viktiga i detta sammanhang, då deuppträder tidigt i kambrium, det äldsta avsnittet av det så kallade fanerozoikum(den del av jordens historia som började för 542 miljoner år sedan och som ärtydligt präglad av spår av flercelliga djur) och hade stor spridning genom helapaleozoikum tills de dog ut i slutet av permtiden för ca. 250 miljoner år sedan.Under särskilda förhållanden kan dock även andra djur utan mineraliseradekroppsdelar bevaras. Även i sådana avlagringar är leddjur bland de vanligastefossilen. I kambrium har man hittat många avlagringar som bevarar mer än baramineraliserade delar av organismer. Dessa avlagringar, särskild de berömda fos-sila faunorna från Burgess Shale i Kanada och Chengjiang i Kina har väckt stort

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intresse, inte minst bland forskarna. Detta intresse kan nog förklaras med fasci-nationen av det till synes plötsliga uppträdandet av många grupper av flercelligadjur i början av fanerozoikum (även om det finns spår av flercelligt liv i tidigareavsnitt av jordens historia). Många forskare tror att det var en kritisk fas i deflercelliga djurens tidiga evolution. Även om det finns argument för att flercelli-ga djur, inklusive arthropoder, uppstod tidigare, så ger oss de kambriska avla-gringarna de äldsta direkta bevisen för hur de tidiga djuren såg ut, vilket i sin turhar stor betydelse för teorier om djurgruppers släktskap.

Mina studier behandlar såväl frågor om leddjurens tidiga evolution somrelativa dateringar av avlagringar med hjälp av trilobiter.

Trilobit-stratigrafi på GrönlandKambrium sträcker sig från 542 till 488 miljoner år sedan. Framförallt i dettaäldsta avsnitt av paleozoikum är trilobiter ett viktigt verktyg för stratigrafi. Detfinns dock många problem med relativ datering av avlagringar med hjälp avdessa fossil. Dels uppträder många arter bara i vissa regioner av jorden, delsvar de som alla organismer beroende av miljöförhållanden vilket gör att vissaarter bara uppträder i vissa sorters avlagringer. Dessutom finns det många om-råden på jorden, där fossilen inte har studerats tillräckligt för att möjliggörajämförelser med andra regioner. Under de senaste årtionden har man upptäcktatt vissa relativa dateringar av avlagringar jorden över inte håller, och en rev-idering av den kambriska tidsskalan är just nu på gång. Ett särskilt problem rörgränsen mellan de två mellersta avsnitten av kambrium. Här tycks trilobitfau-nan från norra Grönland ha en stor potential för att förbättra korrelationen mel-lan olika delar av världen. Det finns dock även avlagringar på nord östra Grön-land som inte är ordentligt studerade. De första två publikationerna handlar omenstaka trilobit-arter som kan bidra till att förbättra korrelationen av de två re-gionerna och därmed också bidra till att förbättra den relativa dateringen av deviktiga avlagringarna i Norra Grönland.

Den första publikationen handlar om Fritzolenellus lapworthi, en art somuppträder i tidiga kambriska lager både i nord östra Grönland och i nord väs-tra Skottland och som kan bidra till att korrelera dessa delar av Skottlandmed resten av den kambriska kontinenten Laurentia. Lite högre i lagerfölj-den på nord östra Grönland uppträder Olenellus hanseni, en som också kanfinnas i norra Grönland, och därmed hjälpa till att korrelera nord östra mednorra Grönland.

Den andra publikationen diskuterar förekomster av flera arter av den dis-tinkta trilobiten Perissopyge på olika ställen i Nordamerika och Grönland,och deras möjliga betydelse för stratigrafin. Nya förekomster rapporterasfrån norra och nord östra Grönland som möjligtvis kan förbätta korrelationenav dessa regioner.

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De tidiga leddjurens evolutionSom tidigare nämnts råder stor oenighet om hur de olika euarthropod-grup-perna är besläktade med varandra. Förhoppningar att gamla kontroverserskulle kunna lösas med hjälp av molekylära data har än så länge inte upp-fyllts. Den rådande osäkerheten om släktskap innebär att det är svårt attkartlägga vilka av de olika morfologiska särdrag som finns i nu levandegrupper redan fanns i leddjurens gemensamma anfader. Detta är ett dilemma,då kunskap om anfadern tvärtom skulle kunna bidra till att utvärdera hy-poteser om släktskap. Chansen att hitta själva anfadern som fossil är realis-tiskt sett obefintlig. Däremot hittar vi fossil som kan vara nära besläktademed anfadern; arter som hör hemma i den evolutionära linje som ledde tillanfadern. De är i princip de enda direkta belägg för vilka särdrag anfadernkan ha haft. Det är dock klart att vi till viss mån även här konfronteras medett cirkulär problem, då vi behöver en viss uppfattning av släktskapen av delevande djuren för att kunna hänvisa ett fossil till ett stamlinje. Fossil, precissom morfologiska eller molekylära data från nu levande djur, är alltså barapusselbitar och måste diskuteras i sammanhang med helheten. De resterandepublikationerna i avhandlingen beskriver nya data från fossila arthropoder,dels från senkambriska avlagringar i Sverige, dels från tidiga kambriskaavlagringar i Grönland. Där det är möjligt diskuteras dessa fossils innebördför evolutionen av morfologiska särdrag i vissa arthropod-grupper.

Den tredje publikationen är ett detaljstudie av en tidig släkting till kräftd-juren, Oelandocaris oelandica från orstensavlagringar på Öland och iVästergötland. Tillsammans med andra arter från orsten och liknande avla-gringar i hela världen ger Oelandocaris oelandica och dess morfologi ochmöjliga levnadssätt viktig kunskap om kräftdjurens anfader.

De resterande publikationerna handlar om fossil från tidiga kambriskaavlagringar på norra Grönland. I den fjärde publikationen beskrivs Kiisorto-qia avannaarsuensis, en ny art som med stor sannolikhet antingen var entidig euarthropod eller en nära släkting till euarthropodernas anfader. Artenhar vissa likheter med några mycket svårtolkade fossil kända från kambriskaavlagringar världen över och kan bidra till en bättre förståelse av dessa djur.

Den femte publikationen beskriver kroppsbyggnaden av Isoxys volucris,tidigare bara känd genom den stora sköld som täckte större delen av krop-pen. Det finns många arter som tycks tillhöra Isoxys, men bara av några fåfinns lämningar av andra delar än just skölden. Publikationen diskuterarlikheter och olikheter mellan de bättre kända arterna och granskar tidigarepublicerade idéer om möjliga levnadssätt.

I den sjätte och sista publikationen beskrivs Siriocaris trolla, en ny artsom har vissa likheter med trilobiter och deras nära släktingar.

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Acknowledgments

First and foremost I want to thank my supervisors without whom I would nothave been able to complete these studies: John S. Peel provided most of thematerial studied and spent immeasurable time and effort on advice and dis-cussions as well as reviewing countless iterations of manuscripts. Financialsupport from Vetenskapsrådet (Swedish Research Council) through Grants toJ. S. Peel is gratefully acknowledged. Dieter Waloszek was an invaluablesource of inspiration, advice, and criticism. He, too, took the time to reviewmanuscripts at various stages and is also thanked for his hospitality duringvarious visits to his department in Ulm. In his efforts, he was often assistedby Andreas Maas, whom I also wish to thank. David J. Siveter helped withadvice and reviews of manuscripts and invited me to visit Leicester Univer-sity. Even he had his backup, Mark Williams. Reinhardt M. Kristensenhelped with numerous discussions and kindly hosted a brief visit to Copen-hagen. I also wish to thank Jan Bergström who supervised the first half ofmy studies, but even continued to lend advice during the remaining years.

Working with fossils requires technical skills which have to be learned,not least when it comes to preparation and photography. In this context Iwish to thank Jan Ove Ebbestad and Pär Eriksson who have been of greathelp. Learning the modeling software used for the reconstructions could be along-winded process at times. I was fortunate to be accompanied by JoachimT. Haug on the way. On the theoretical side, Uwe Balthasar has to bethanked for trying to raise my awareness of the convoluted problems of thepeculiar preservation of the fossils of the Sirius Passet Lagerstätte.

One of the greater challenges in final completion of the thesis was to writea Swedish summary. Anders Nordin is thanked for his help with that. Gettinga thesis into printed form involves a surprising amount of paperwork and er-rands. I managed to unload the bulk of this onto my supervisor, John S. Peel.Sebastian Willman is thanked for helping me handle the remainder.

Finally, I wish to thank my colleagues at the Museum of Evolution forbearing with me and keeping my spirits up during this venture.

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Babcock, L. E. & Peng, S. 2007. Cambrian chronostratigraphy: current state and fu-ture plans. Palaeogeography, Palaeoclimatology, Palaeoecology 254:62–66.

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