Digital Comprehensive Summaries of Uppsala Dissertations
from the Faculty of Science and Technology 558
Evolution and taxonomy of
Cambrian arthropods from
Greenland and Sweden
MARTIN STEIN
ACTA
UNIVERSITATIS
UPSALIENSIS
UPPSALA
2008
ISSN 1651-6214
ISBN 978-91-554-7302-0
urn: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 of
North-East Greenland. Bulletin of the Geological Society of Denmark
56:1–10.
II. Stein, M. & Peel, J. S. 2008. Perissopyge (Trilobita) from the lower
Cambrian (Series 2, Stage 4) of North America and Greenland. GFF
130:71–78.
III. Stein, M., Waloszek, D., Maas, A., Haug, J. T. & Müller, K. J. 2008.
The stem crustacean Oelandocaris oelandica re-visited. Acta Palaeontologica Polonica 53:461–484.
IV. Stein, M. (manuscript) A new arthropod from the Early Cambrian of
North Greenland with a ‘great appendage’ like antennula. Submitted to
Zoological 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 Passet
Lagerstätte, lower Cambrian of North Greenland.
VI. Lagebro, L., Stein, M. & Peel, J. S. (manuscript) A new arthropod
from the early Cambrian Sirius Passet Fauna of North Greenland.
Reproduction of the published papers is made with kind permission from the
copyright holders:
Paper I © Bulletin of the Geological Society of Denmark, Dansk Geologisk
Forening
Paper II © GFF, Geologiska Föreningen
Paper 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 on
systematic palaeontology. J. S. Peel provided the introduction and the sections on material and stratigraphy.
Paper III: M. Stein and D. Waloszek carried out most of the studies of the
material. Initial reconstructions were created by M. Stein, D. Waloszek, and
A. Maas. M. Stein created an early version of the three dimensional model of
the largest stage and the animated sequence; J. T. Haug created the final version of the model of the largest stage and all models of the ontogenetic sequence. K. J. Müller collected the material. Manuscript mainly by M. Stein
and D. Waloszek with contributions by the other authors.
Paper IV: M. Stein performed the studies, created the reconstructions and
wrote the manuscript.
Paper V: Initial studies were carried out by M. Stein and J. S. Peel. Detailed
studies 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 performed 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 a
doctoral thesis only. They are not considered publications according to the
ICZN. Accordingly, all new taxonomic names and emendations in papers
IV–VI are void. Authority for taxonomic work in papers I–III is by the original publications.
Front cover
Reconstruction of Kiisortoqia avannaarsuensis from the early Cambrian Sirius Passet Lagerstätte of Greenland.
Contents
Introduction ................................................................................................... 7
Trilobite biostratigraphy ................................................................................ 9
Paper I ..................................................................................................... 10
Paper II .................................................................................................... 12
Arthropod phylogeny ................................................................................... 14
The current status of euarthropod phylogeny from a neontological perspective ................................................................................................... 15
The ground pattern and the importance of fossils ................................... 16
The position of Trilobita ......................................................................... 19
Paper III .................................................................................................. 21
Paper IV .................................................................................................. 22
Paper V .................................................................................................... 24
Paper VI .................................................................................................. 25
Svensk sammanfattning ............................................................................... 27
Trilobit-stratigrafi på Grönland ............................................................... 28
De tidiga leddjurens evolution ................................................................ 29
Acknowledgments ....................................................................................... 30
References ................................................................................................... 31
Introduction
Arthropods are a diverse group of multicellular animals (metazoans), estimated to account for more than three quarters of the present-day metazoan
diversity. They constitute an important element of many ecosystems, both
marine and terrestrial. Four major groups are recognized among extant
arthropods. The chelicerates include the familiar spiders, scorpions, mites,
and ticks, and less familiar forms such as the horseshoe crabs. Crabs and lobsters are grouped together as crustaceans, along with woodlice and krill, but
also with smaller and less familiar forms such as brine-shrimps and the truly
ubiquitous ostracodes and copepods, and even sessile forms such as barnacles. Millipedes and centipedes comprise the myriapods, but the all but too
familiar insects form the largest of all living arthropod groups. These four
groups sharing a common ancestor constitute the Euarthropoda and are characterized by a unique body structure and movement apparatus where muscles do not act as antagonists to bones or fluid-filled pressure vessels lodged
inside a ‘soft’ body. Rather, a firm cuticle, which is produced by the epidermal cells surrounding the body, forms an ‘exoskeleton’ consisting of hardened (sclerotized) plates conjoined by soft membranous regions that allow
for flexibility. Muscles, attached to the sclerotized parts, act against this ‘exoskeleton’. How the euarthropod groups are related to each other remains the
subject of sometimes contentious debate. The nature of several small groups
considered 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 minute
tardigrades (water bears) belong to the group or are more closely related to
roundworms. The pentastomids (tongue worms), a group of worm-like parasites infesting the respiratory organs of land-living vertebrates (amphibians,
‘reptiles’, birds, and mammals) are widely accepted as arthropods. But are
they the sister group to euarthropods or highly derived crustaceans, and thus
euarthropods?
Given their tremendous diversity, it is not surprising that arthropods have
had a long evolutionary history, evident not least in a rich fossil record spanning the whole Phanerozoic. Most common in the fossil record are forms
with well biomineralized (calcified) ‘exoskeletons’, such as trilobites or certain crustaceans, including ostracodes and malacostracans. Trilobites appeared early in the Cambrian and flourished through the whole Palaeozoic
7
prior to disappearing in the terminal Permian extinction. They are common
and diverse components of Cambrian faunas and have been of paramount
importance in biostratigraphic and palaeogeographic studies of that period
for more than a century and a half.
But the fossil record of arthropods is not just limited to forms with calcified ‘exoskeletons’. Throughout the Phanerozoic, examples of exceptional
preservation of soft parts offer unique glimpses into past ecosystems. Arthropods form a surprisingly common element of these so-called lagerstätten, although taphonomic bias against various organism groups means that even
these remarkable deposits do not necessarily mirror the true composition of
the original ecosystems. In strata of the Cambrian system (542–488 Ma BP),
the oldest period of the Phanerozoic, researchers have unearthed a number of
fossil lagerstätten that have received particular public attention, notably the
Burgess Shale of Canada, the Chengjiang Lagerstätte of China, or assemblages of ‘Orsten’-type fossils from several places in the world. This attention is certainly largely due to our fascination with the apparent abrupt appearance of fossil traces of multicellular life around the base of the Phanerozoic. Many investigators believe that the earliest major diversification of
metazoans 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 oldest
available direct evidence of the early diversity of many metazoan groups.
Arthropods are a very prominent element also in these ancient Cambrian
biota, showing a striking diversity of form and life habits. At least to some
extent, this dominance may be due to the relatively good preservation potential of their cuticle encasing the whole body. This not only yields large quantities of arthropod fossils, but also allows preservation of morphological detail in extremely high fidelity, including details of limb structure. Thus, a
way has been opened for a more direct comparison of the anatomy of fossils
with extant taxa than is possible for many other fossils. Not surprisingly, this
wealth of detail has stimulated greater attention to fossil arthropods in phylogenetic studies of the group than fossils of most other invertebrate animal
groups.
This dissertation was originally conceived as a stratigraphic study of early
to middle Cambrian trilobite faunas of North-East and North Greenland but
the focus shifted gradually to more palaeobiological questions, inspired by
emerging views on arthropod evolution and phylogeny, including also the
question of the phylogenetic placement and significance of Trilobita. Thus,
attention turned to exceptionally preserved arthropod fossils, with material
derived from two renowned Cambrian lagerstätten: the early Cambrian Sirius Passet biota of North Greenland, and the late Cambrian (Furongian)
‘Orsten’ of Sweden, the type for a kind of exceptional preservation that is
now found in strata spanning the early Cambrian to the early Ordovician in
many localities world wide.
8
Trilobite biostratigraphy
Cambrian stratigraphy is currently undergoing a major revision aiming at a
standardized 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 four
globally recognizable series, of which the first two very loosely correspond
to the earliest traditional subdivision.
Figure 1: Subdivision of the Cambrian System according to the new chronostratigraphic framework. Series and stages with a grey background are yet to be defined.
9
So far, only the Furongian Series (Peng et al. 2004), which roughly correlates with the traditional Late Cambrian, and the Terreneuvian Series, which
replaces the provisional series 1 (Landing et al. 2007) have been defined by
Global Standard Stratotype-sections (GSSPs) (Figure 1). An informative example of some of the difficulties associated with the definition of these new
series is provided by ongoing discussions concerning the boundary between
series 2 and 3, corresponding to the traditional Early-Middle Cambrian
boundary. The former Middle Cambrian was traditionally defined by the occurrence of paradoxidid trilobites in Baltica, Avalonia, and West Gondwana,
but it has been shown that this first occurrence is diachronous. In the EarlyMiddle Cambrian boundary interval of Morocco paradoxidid trilobites occur
together with holmiid trilobites which, as part of Olenelloidea, traditionally
define the Early Cambrian (Geyer & Palmer, 1995; see Geyer 2005 for an
extensive review). Geyer (2005) listed five trilobite horizons being discussed
currently as a potential new boundary marker between the provisional series
2 and 3, but progress is hampered by the endemism of early Cambrian trilobite faunas, different litho- and biofacies distributions, and not least by the
different state of knowledge and taxonomic practice in relevant geographical
areas (Geyer 2005).
Studies of Cambrian faunas in North Greenland have recognized rich
trilobite faunas with considerable potential for intercontinental correlation
(e.g. Robison 1988, 1994, Babcock 1994a, b, Blaker & Peel 1997). The
stratigraphic sequence succession in North Greenland is relatively complete
and encompasses inner craton and outer shelf facies in close juxtaposition.
Thus, boreal faunas of Baltic aspect have been described from outer shelf facies in close proximity to warm water faunas of Laurentian aspect (Babcock
1994a, b). Recently, the diverse trilobite faunas of North Greenland described by Blaker and Peel (1997) has been recognized as particularly significant for resolution of the problem of the boundary interval between provisional series 2 and 3 (Fletcher 2003, Geyer 2005).
As of yet, the trilobite faunas of North-East Greenland are not well known
and correlation of the Cambrian provisional series 2 of this region with that
of other areas of Laurentia has been advanced by the study of small shelly
faunas (Skovsted 2006). Correlation with North Greenland remains imprecise but the present studies offer the first prospect for elucidating similarities
in the trilobite faunas of the two areas.
Paper I
Stein, M. 2008. Fritzolenellus lapworthi (Peach and Horne, 1892) from the
lower Cambrian (Cambrian Series 2) Bastion Formation of North-East
Greenland. Bulletin of the Geological Society of Denmark 56:1–10.
10
Figure 2: A, Fritzolenellus lapworthi from the Bastion Formation of North-East
Greenland; B, Reconstruction of F. lapworthi; C, Olenellus cf. hanseni from the Ella
Island Formation of North-East Greenland.
This paper deals with olenellid trilobites from the Bastion and Ella Island
formations (provisional Stage 4) of North-East Greenland, collected in the
early 1950s. The focus is on material from the older Bastion Formation,
from which the bulk of the material is derived. Collections from the overlying Ella Island Formation only yielded two immature cephala. The specimens of the material from the Bastion Formation are identified as Fritzolenellus lapworthi (Peach & Horne, 1892), a species originally described
from the 'Fucoid' Beds of the An t'Sron Formation of north-west Scotland.
The Greenland material is considerably better preserved than that of the type
region. This allowed a more detailed description, in particular of the postcephalic morphology. The material also includes immature specimens, permitting a first, limited account of the ontogeny of the species. The occurrence of F. lapworthi in Greenland offers a first opportunity of a direct corre11
lation of the Cambrian Stage 4 strata of north-west Scotland with this part of
Laurentia. So far, correlation between the areas rested largely on the occurrence of the problematic fossil Salterella, which, in Scotland, occurs in the
‘Fucoid’ Beds and the overlying ‘Salterella Grit’, in North-East Greenland in
the Bastion, Ella Island, and Hyolithus Creek formations (Skovsted 2006,
Poulsen 1932, Cowie & Adams 1957). Salterella also occurs in the Paralleldal Formation of North-Greenland and in lower Cambrian strata of Inglefield
Land, North-West Greenland (Peel & Yochelson 1892).
So far, F. lapworthi has not been identified in any material from other regions of Greenland. The trilobites of the overlying Ella Island Formation
may prove to be of utility for correlation with North Greenland, but further
work is required. The two olenellid cephala from that formation are similar
to Olenellus hanseni (Poulsen, 1932) described from this formation. This
species was unfortunately based on immature cephala and fragmentary thoracic material only and may require revision. Similar material has been described from the Henson Gletscher and Kap Troedsson formations of North
Greenland, described as Olenellus cf. truemani Walcott, 1913 (Blaker & Peel
1997). This material also includes immature specimens which lend themselves for comparison with the type material of O. hanseni. If this material
could be positively identified as O. hanseni, it could serve as a basis for an
emendation of the species and would further provide a direct tie point between North Greenland and North-East Greenland.
Paper II
Stein, M. & Peel, J. S. 2008. Perissopyge (Trilobita) from the lower Cambrian (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 described
previously, 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 reported from the Sekwi Formation of Yukon Territory, Canada (Blaker et al. 1997,
Blaker & Peel 1997). This paper documents Canadian material for the first
time and adds a further occurrence in the Paralleldal Formation of North
Greenland, which is overlain by strata that can be correlated with Siberia
(Debrenne & Peel 1986). The occurrence of P. phenax in the Paralleldal and
Henson Gletscher formations thus further supports correlation of the Henson
Gletscher Formation with Siberia and China, while the occurrence in Yukon
Territory offers a tie point into that area of Laurentia.
12
Figure 3: A, Perissopyge triangulata, Harkless Formation, Nevada, USA; B, C, P.
phenax, Sekwi Formation, Yukon Territory, Canada, D, E, Perissopyge sp. Ella Island Formation, North-East Greenland.
An indeterminate species of Perissopyge is reported also from the Ella Island Formation of North-East Greenland. This co-occurrence with Olenellus
hanseni, which may also occur in the Henson Gletscher Formation of North
Greenland, may hold potential for further correlation between North and
North-East Greenland.
The distinctive pygidial morphology of Perissopyge makes it also attractive as a potential biostratigraphic marker, as it is easily identified among
early Cambrian trilobite faunas. Unfortunately, it seems that the limited distribution as yet only grants regional utility.
13
Arthropod phylogeny
As noted above, it is not clear which taxon is the closest relative (sister taxon) to the Euarthropoda. The living Onychophora are representatives of the
non-sclerotized forms among Arthropoda in the wider sense (Arthropoda
sensu lato), but there is a considerable morphological gap between these and
the euarthropods. Accordingly, there must have been a long evolutionary history between the last common ancestor (stem species) of both onychophorans and euarthropods and the stem species of Euarthropoda. Species
do change over time, i.e. along their phylogenetic lineage, but, given the
large morphological gap, it is less likely that all the morphological changes
—from dorsal cuticular sclerotization to eye, limb, and head formation to
name but a few—all occurred in the (anagenetic) lineage of a single species
of Euarthropoda. More likely, there were several branching points along the
way. All species that branched off along the evolutionary lineage toward Euarthropoda but became extinct, are stem lineage representatives or derivatives of the stem lineage of the Euarthropoda. The only way we may know of
them directly is if they are preserved as fossils. Identified correctly, they can
tell us in which order morphological changes occurred along the stem lineage.
It is clear that the cuticular ‘exoskeleton’ of euarthropods must have been
acquired 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 areas
between the tergites to allow for flexibility; dissolution of the subepidermal
muscle tube and formation of isolated muscle strands that develop tendonlike connectives into the cuticle; development of appendages that are subdivided into sclerotic rings and membranous connections; development of pivot joints on these limbs, limiting the flexure of the sclerotic rings to one axis
of movement. A number of fossil taxa mainly known from Cambrian lagerstätten have these features, but lack other euarthropods autapomorphies.
They are likely representatives of the euarthropod stem lineage and, together
with euarthropds, constitute the Arthropoda sensu stricto. The ground pattern
of 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 anteroventral compound eyes, a short head comprising two segments carrying the
eyes and one pair of appendages (the antennulae) respectively, and the
mouth in a ventral position, opening toward the posterior. All these changes
14
must have accumulated before the stem species of Arthropoda s. str. Additional changes occurred later in the euarthropod stem lineage. Thus the evolutionary lineage toward Euarthropoda can be divided into at least two sections, divided by the node of Arthropoda s. str. Examples of the first section
are the so-called lobopods, small to moderately long worm-shaped animals
with long, tubular segmental appendages. The possibility that also the
Tardigrada and the Pentastomida belong there is still debated. One hypothesis considers Pentastomida to be the closest living relatives to Euarthropoda,
having pivot jointed limbs but lacking segmental subdivision of the cuticle
in tergites and soft membranous areas (e.g. Maas et al. 2004). While this is
supported by further morphological and developmental data, an alternative
hypothesis based on sperm ultrastructure and molecular data suggests that
pentastomids are highly derived crustaceans (Møller er al. 2008 and references therein; see also Waloszek et al. 2006 for a recent review of the competing hypotheses).
The current status of euarthropod phylogeny from a
neontological perspective
Euarthropod phylogeny has been in a state of flux for a long time, and the
advent of molecular systematics during the last decade has added new controversial hypotheses about relationships rather than helping settle standing
issues. One ongoing debate concerns the phylogenetic position of Pycnogonida; they are currently placed either within Chelicerata as the sister taxon of
Euchelicerata (Xiphosura and Arachnida) or as the sister taxon of all other
euarthropods (see Dunlop & Arrango 2005 for a review). The phylogenetic
position of Myriapoda is another remaining problem. The traditional concept
of Mandibulata comprising Myriapoda, Insecta, and Crustacea has recently
been challenged by molecular studies that place myriapods as the sister taxon 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 than
some aspects of development of the nervous system. Analyses of morphological or combined data usually resolve myriapods together with Crustacea and
Insecta as part of Mandibulata (e.g. Giribet et al. 2001, 2005, Edgecombe
2004). Finally, the phylogenetic position of insects remains disputed. Traditionally 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) and
neuro-anatomical (e.g. Fanenbruck et al. 2004, Harzsch 2006) studies often
resolve them as more closely related to or even nested within Crustacea.
The controversies about euarthropod phylogeny are largely beyond the
scope of the present studies. Further, most of the competing hypotheses out15
lined above do not involve any fossil data. Nevertheless, the topology of the
euarthropod phylogenetic tree has implications for the ground pattern of Euarthropoda and therefore needs to be considered.
The ground pattern and the importance of fossils
Characterizing the ground pattern of Euarthropoda based on neontological
data alone is hampered not least by the uncertainties of in-group relationships. Here, fossils provide an invaluable source of information, as the only
direct evidence of morphology in the stem lineages of taxa which may
bridge significant morphological gaps between terminal taxa.
It is clear that an advanced cephalization with a head tagma incorporating
several segments was part of the euarthropod ground pattern. The most anterior, protocerebral segment carries the eyes, and in certain crustacean taxa
paired, so called frontal organs (Vilpoux & Waloszek 2003, Fanenbruck &
Harzsch 2005, Semmler et al. 2008). The following, deutocerebral, segment
carries a pre-oral appendage, the chelicera in Chelicerata or antennula in
Crustacea, Insecta, and Myriapoda (traditionally called antenna in the latter
two taxa). The third, tritocerebral segment carries the first postantennular
limb, called pedipalp in Chelicerata or (second) antenna in (Eu-)Crustacea.
In Myriapoda and Insecta this limb is absent, most likely reduced. The assumption of a tripartite brain incorporating neuromeres of the three most anterior segments as part of the euarthropod ground pattern is problematic
(Walossek & Müller 1998), as even derived euarthropod taxa such as branchiopods and isopods still have separate ganglial knots.
There is ample fossil evidence that such a head tagma, incorporating the
protocerebral, deutocerebral, and three additional segments was present in
the earliest representatives of the crustacean (and possibly mandibulatan)
evolutionary lineage (Walossek & Müller, 1990, 1998). There is further evidence, that these early representatives hatched with a ‘head larva’, i.e. a larva with only the segmental composition of this head tagma (Walossek &
Müller, 1990, 1998). The ‘short-head larva’ of Eucrustacea incorporates one
segment less and is interpreted as an autapomorphy of that taxon. It is the
body tagmosis of chelicerates that poses a pertinent problem.
The anterior tagma in Euchelicerata, the prosoma, incorporates the protoand deutocerebral segments plus five additional limb-bearing segments.
Adult Pycnogonida have a cephalosoma that corresponds in segmental composition to the euarthropod head, but this is first acquired during ontogeny in
extant representatives. The protonymph larvae appear to be ‘short-head larvae’ with only the proto-, deutocerebral, and two additional limb-bearing
segments (Vilpoux & Waloszek 2003). The caudal outgrowths of the larva of
the only Cambrian representative of Pycnogonida (Waloszek & Dunlop
2002) were subsequently interpreted as a fourth pair of limbs (Vilpoux &
16
Waloszek 2003). This would mean that the larva of the Cambrian species has
one more pair than the protonymph larvae of extant species.
Some investigators have taken the problem of chelicerate body tagmosis
as support for rejecting a head with the eye segment and four limb-bearing
segments as a ground pattern feature of Euarthropoda. They rather regard
such a head tagma as a feature that evolved first in the mandibulatan stem
lineage (Scholtz & Edgecombe 2005). The alternative hypothesis assumes
that the euarthropod head became modified within Chelicerata (Waloszek &
Maas 2005). This may gain support from so called ‘great appendage’ arthropods, that are considered to be chelicerate stem lineage representatives
(Chen et al. 2004, Cotton & Braddy 2004) and have such a head tagma. This
contrasts with reports of a head comprising only three appendage-bearing
segments for some of the taxa (García-Bellido & Collins 2007), but recent
detailed re-investigations of one species cast doubt on that assessment. They
rather confirm a head incorporating no less than four appendage-bearing segments (Liu et al. 2007), as assumed already for the euarthropod ground pattern.
Assignment of these taxa to the chelicerate stem lineage rests on the specific morphology of the ‘great appendage’, a stout grasping appendage
which is considered to be the first (deutocerebral) limb and homologous to
the chelicera of Chelicerata (Chen et al. 2004, Cotton & Braddy 2004). This
homology statement is particularly based on one species, Haikoucaris ercaiensis Chen, Waloszek & Maas, 2004, which is intermediate in the morphology of its grasping leg between that of the previously described taxa and
the chelicerate chelicera. These new views on the chelicerate stem lineage
mark a radical departure from previous assumptions of the evolution of chelicerae, which were traditionally considered to be the limbs of the tritocerebral segment. The deutocerebral segment, carrying the antennula of crustaceans, myriapods and insects, was considered to have been lost along the
chelicerate stem lineage (e.g. Bitsch & Bitsch 2007). This view, first challenged 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 trilobites and closely related fossil arthropods that were considered to be representatives of the chelicerate stem lineage in the ‘arachnomorph’ concept
(Scholtz & Edgecombe 2005 for a review).
Accepting homology between the chelicerae of Chelicerata and the deutocerebral antennula of the other euarthropod taxa forced reconsideration of
the morphology of the deutocerebral limb in the ground pattern of Euarthropoda. The traditional view assumed a sensorial flagelliform, i.e. multiannulated, limb, as was developed in trilobites and related fossil arthropods. This
morphology was readily homologized with the sensorial antennulae of insects, myriapods and certain crustaceans. The limb was considered to be lost
17
together with the deutocerebral segment in the upper chelicerate stem lineage. The new view required that the chelicerae were derived from a flagelliform antennula, or, more likely, from a more limb-like deutocerebral appendage that must have had a feeding function already in the stem species of
Euarthropoda. From a purely neontological perspective, the latter idea was
nurtured by the potential placement of Pycnogonida as the sister taxon to the
remaining Euarthropoda. In the case of a sensory antennula in the euarthropod ground pattern, this placement would require convergent transformation
into a feeding limb in two taxa (Dunlop & Arrango 2005). Fossils provided
additional support for a feeding function of the deutocerebral limb in the euarthropod ground pattern, independent of the position of Pycnogonida.
Studies of crustacean (or possibly mandibulatan) stem lineage representatives have demonstrated that the stem species of Crustacea had an antennula
adapted for feeding and locomotion rather than sensorial purposes (see
Walossek 1999 for a summary). This was originally considered to be an autapomorphy, adopted from the traditional view of such an antennula in the
ground pattern of Euarthropoda, but had to be reevaluated in the light of the
new data. The flagellate antennula, often considered characteristic of the
group is an autapomorphy of certain in-group taxa, foremost Malacostraca
(e.g. Waloszek 2003). In the light of homology between the feeding and locomotory crustacean antennula and the feeding chelicera of chelicerates, it is
apparent now that the adaptation of the antennula for feeding in the crustacean stem species was not an autapomorphy but a symplesiomorphy, retained from the last common ancestor of chelicerates and crustaceans (possibly mandibulates; e.g. Waloszek et al. 2007). A novelty developed in the
crustacean (or mandibulate) stem lineage may be that the antennula works in
concert with the postantennular limbs as part of the cephalic feeding apparatus.
Studies of early members of Arthropoda s. str. yielded further support for
this hypothesis, as they have revealed that these forms had a limb-like feeding antennula while lacking any other obvious structures for feeding (Maas
et al. 2004, Waloszek et al. 2005, 2007). It should be noted that there are alternative interpretations of the head structure in at least one of these fossils,
Fuxianhuia protensa Hou, 1987, which consider the most anterior limb to be
protocerebral rather than deutocerebral (Scholtz & Edgecombe 2005, 2006).
In this interpretation, another structure under a large, shield-like tergite is
considered to be a jaw-like deutocerebral limb. The supposed protocerebral
limb is considered to be a ‘primary antenna’ homologous to the onychophoran palp, either lost in the euarthropod stem lineage or retained in a
vestigial form as the protocerebral frontal organs of certain eucrustacean
taxa (Semmler et al. 2008). This interpretation suffers from several problems; firstly, the supposed jaw-like deutocerebral limb in F. protensa could
be demonstrated to be an internal structure, most likely gut diverticula, and
in other early Arthropoda s. str. the structure is clearly missing (Waloszek et
18
al. 2005). Further, the notion that Chelicerata rely on a ‘whole limb mouthpart’ for feeding, whereas Mandibulata rely on the gnathobasic parts of their
postantennular limbs (Scholtz & Edgecombe 2005) is an over simplification.
As discussed, there is ample evidence that the antennula was part of the
cephalic feeding apparatus in the crustacean stem species, while Xiphosura
as euchelicerates use the median edges of the limb bases (basipods) of their
postcheliceral limbs for mastication. Finally, the assumption of a specialized
deutocerebral jaw in the ground pattern of Euarthropoda is problematic because a sensorial antennula is more likely to be derived from a generalized
limb-like structure. The latter point needs to be considered in particular with
respect to the various competing hypotheses of euarthropod relationships
which may require sensory antennulae to be evolved convergently more than
once. A further independent origin may be required to accommodate a group
of fossil taxa, as will be discussed in the next section.
The position of Trilobita
Trilobita is an entirely extinct group of euarthropods. Putative autapomorphies are the calcification of the cuticle and dorsal eyes with circumocular
sutures. The trilobation of the trunk tergites with a raised axis and lowered
tergopleurae often considered characteristic for the group is a symplesiomorphy retained from the ground pattern of Arthropoda s. str.
In contrast to many other euathropod taxa, Trilobites and closely related
fossil taxa have long flagelliform, probably sensory antennulae composed of
a large number of short annulae. Traditionally regarded to be related to the
Chelicerata, more recent work affiliates them with the mandibulate stem lineage based on their antennular morphology, which is considered to represent
the ‘secondary antenna’ evolved in the stem lineage of Mandibulata according to Scholtz & Edgecombe (2005). This interpretation faces the dilemma
that there is reason to assume a feeding antennula as part of the crustacean
ground pattern, which would require multiple character reversals (once for
the stem species of Crustacea, a second time for the stem species of Malacostraca). Thus, caution should be exercised against using the mere presence
of sensory antennulae as an argument for placement of fossil taxa along the
stem lineage of Mandibulata. Even more so, in the light of a possibly convergent evolution of sensory antennulae in various euarthropod taxa discussed
above. Based on details of limb morphology and development of the cephalic feeding apparatus, trilobites and related fossil arthropods with a flagelliform antennula must have branched off prior to early representatives of the
crustacean (or possibly mandibulatan) stem lineage. Rather, a flagelliform
antennula in these taxa may represent a possible synapomorphy of a group of
these ‘trilobitomorphs’ (the classical ‘Trilobitomorpha’ originally comprised
19
also the ‘great appendage’ arthropods now regarded as chelicerate stem lineage representatives).
Monophyly of a group of Trilobita and e.g. naraoiids, hemeltiids,
tegopeltids, and xandarellids, all with a flagelliform antennula, might also be
supported by aspects of limb morphology. All have a bipartite exopod with a
proximal portion carrying lamellar setae and a distal lobe-like portion with a
setal fringe (Edgecombe & Ramsköld 1999, Bergström & Hou 2003, Cotton
& Braddy 2004, Scholtz & Edgecombe 2005) which is a now widely noted
autapomorphy. Inside this grouping, xandarellids are often regarded as the
sister taxon to the remainder, due to the absence of a pygidium, defined as a
point at which tergites ceased being released anteriorly in ontogeny (Ramsköld 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 of
Emeraldella brocki Walcott, 1912 has been debated (Bruton & Whittington
1983, Hou & Bergström 1997, Edgecombe & Ramsköld 1999), with the latter authors arguing for a bipartite exopod as in the other taxa. Retifacies abnormalis Hou, Chen & Lu, 1989 and Sidneya inexpectans Walcott, 1911
have lamellar setae but apparently no subdivision of the exopod. The latter
species is also considered to have had a ‘head’ that only incorporates the protocerebral and deutocerebral segments (Bruton 1981). A putatively closely
related species, Sidneya sinica Zhang, Han & Shu, 2002 has a relatively
longer head shield than S. inexpectans with setae protruding from underneath
it, indicating incorporation of postantennular limbs. However, unless Bruton’s (1981) assessment of the head in S. inexpectans can be disproved, an
ad 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 no
clearly defined synapomorphy with any of the major extant groups is currently recognized. Placement in the mandibulatan stem lineage has been justified by the sensory antennulae or the presence of a head incorporating no
less than four appendage-bearing segments (Scholtz & Edgecombe 2005,
2006). Both arguments are problematic; the presence of flagelliform antennulae alone may not suffice, as outlined above, and the head with four limbbearing segments is likely a ground pattern feature of Euarthropoda. Other
characters, such as the lamellar exopods, may represent autapomorphies of
the group, while an endopod with seven podomeres intermediates between
nine or ten in the euarthropod ground pattern and five to six in Crustacea and
is difficult to evaluate. It becomes clear that the problem needs thorough
reinvestigation, also in the light of problematic taxa such as Sidneya inexpectans or another problematic taxon that may be allied based on antennular
morphology, the Marellomorpha.
20
Paper III
Stein, M., Waloszek, D., Maas, A., Haug, J. T. & Müller, K. J. 2008. The
stem crustacean Oelandocaris oelandica re-visited. Acta Palaeontologica
Polonica 53:461–484.
Figure 4: Oelandocaris oelandica; A, Holotype from Öland, Sweden; B, reconstruction of the large stage present in the material; C, larval specimen from Västergötland, Sweden; D, reconstruction of different stages.
The third paper provides a detailed study of the stem lineage crustacean Oelandocaris oelandica Müller, 1983 from the ‘Orsten’ lagerstätte of Sweden.
The ‘Orsten’ is not a fossil lagerstätte in the sense of a geographically and
stratigraphically confined unit, as the fossils occur in bituminous concretions
that occur in the Alum Shale successions that crops out in several regions of
21
Sweden and span the Cambrian series 3 and Furongian boundary interval.
Fossils of this ‘Orsten’ type preservation have also been reported from other
deposits in several areas of the world and different geological times (see
Maas et al. 2006 for a review). The fossils are usually in the submillimetric
range and, in contrast to many other lagerstätten, preserved in full three-dimensional detail. ‘Orsten’ type preservation is also notable for being one of
the few lagerstätten that has yielded largely complete ontogenetic series of
arthropod fossils.
Oelandocaris oelandica was originally described on the basis of a single
fragmentary specimen from the Cambrian Stage 7 (Series 3) of the island of
Öland, Sweden (Müller 1983). Subsequently identified material from the
Paibian Stage (Furongian) of Västergötland in mainland Sweden permitted a
more complete description of the species and a discussion of its phylogenetic
significance, which was presented in an initial report by Stein et al. (2005).
In the present paper, additional information, including the recognition of different ontogenetic stages in the material is provided, the life habits are discussed, and the phylogenetic position suggested by Stein et al. (2005) is further confirmed. The species, together with other representatives of the crustacean stem lineage from the ‘Orsten’, provides important evidence for the
structure of the cephalic feeding apparatus in the ground pattern of Crustacea. By comparison to other fossil euarthropods, the morphology of the
postantennular limbs in this early representative of the crustacean stem lineage also provides important insight into some aspects of limb morphology
in the euarthropod ground pattern, in particular of the exopods
Paper IV
Stein, M. A new arthropod from the Early Cambrian of North Greenland
with a ‘great appendage’ like antennula. Submitted to Zoological Journal of
the Linnean Society.
The fourth paper is the first in a series of three on fossils from the Sirius Passet Lagerstätte of the Cambrian Stage 3 (Series 2) of North Greenland. This
lagerstätte, roughly coeval with the Chengjiang Lagerstätte (Maotianshan
Shale) of China, is one of the oldest Cambrian lagerstätten yielding exceptionally preserved arthropod macrofossils.
The paper describes a new fossil arthropod species, Kiisortoqia avannaarsuensis, that has a large and robust antennula remarkably similar to the antero-ventral raptorial appendage of the problematic anomalocaridids. While
the phylogenetic scope, position, and significance of the latter is contentiously debated (e.g. Budd 2002, Chen et al. 2004, Hou & Bergström 2006) with
some authors questioning euarthropod or even arthropod affinities, K. avan22
naarsuensis shares features with euarthropods that justify placement in this
taxon or in the stem lineage in direct proximity to the euarthropod node.
While this does not resolve the question of anomalocaridid affinities, it does
provide strong support for homology between the antero-ventral appendage
of anomalocaridids with the deutocerebral antennula of euarthropods and
further for the antennula as a primary feeding limb in the euarthropod stem
species.
Figure 5: Kiisortoqia avannaarsuensis from the Sirius Passet Lagerstätte, North
Greenland; A, holotype; B, detail of the most complete antennulae in the material; C,
D, reconstructions in ventral and lateral view.
23
Paper V
Stein, M., Peel, J. S., Siveter, D. J. & Williams, M. Isoxys (Arthropoda) with
preserved soft anatomy from the Sirius Passet Lagerstätte, lower Cambrian
of North Greenland. Manuscript.
In this paper an account is given of the soft anatomy of Isoxys volucris
Williams, Siveter & Peel, 1996, from the Sirius Passet Lagerstätte. This
species 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 is
widely accepted that this feature arose independently in several groups. In
the fossil record, they are known often by the shield alone, which is why
phylogenetic or taxonomic assignments of a species to a certain group often
rests on the morphology of the shield alone. In some cases it has been shown
that the assignment by shield morphology was misleading, with the Phosphatocopina originally considered to be ostracods, now recognized as the sister taxon to Eucrustacea (Siveter et al. 2003) as a prime example.
Figure 6: Isoxys volucris from the Sirius Passet Lagerstätte, North Greenland; A,
specimen preserving limbs; B, specimen showing shield in dorsal view; C, specimen
preserving possible eye.
There are numerous species assigned to Isoxys Walcott, 1890, mostly known
by the shield morphology alone. Ventral morphology is known only in a few
instances and there are indications that it may differ between species (e.g.
Vannier & Chen 2000). The present paper gives a review of what is known
and makes comparisons with the new data from the Sirius Passet lagerstätte.
24
There are sufficient similarities between I. volucris and one species from the
Chengjiang fauna of China to justify close relationships, but relationships
with other species remain problematic. This cautions against composite reconstructions and palaeoecological considerations based on multiple species.
Paper VI
Lagebro, L., Stein, M. & Peel, J. S. A new arthropod from the early Cambrian Sirius Passet Fauna of North Greenland. Manuscript.
Figure 7: Siriocaris trolla from the Sirius Passet Lagerstätte, North Greenland; A,
holotype; B, specimen showing the morphology of the tergites.
The sixth and final paper describes Siriocaris trolla, a new species of fossil
arthropod from the Sirius Passet Lagerstätte. It shares some features of certain ‘trilobitomorph’ arthropods, including flagelliform antennulae and articulation devices of the thoracic tergites, such as flanges on the tergopleurae
and an edge-to-edge articulation with larger overlap in the axis and a horizontal inner portion of the tergopleurae. It is intriguing to consider ‘trilobitomorph’ affinities based on these features, as at least the tergal morphology
may imply particularly close affinities with trilobites and helmetiids (Ramsköld et al. 1997, Edgecombe & Ramsköld 1999). The prominent tergopleural spines and the small tail shield are even reminiscent of olenelline trilo25
bites, commonly considered to be ‘basal’ trilobites (e.g. Lieberman 2001 and
references therein). An indication of trilobation in the head shield and an axial furrow at least in the posterior part of the thorax are other features that invite comparison with trilobites. Unfortunately, with the limited material at
hand, alignment with trilobites and helmetiids can at present not be substantiated, as some characters in S. trolla remain unclear. While there are indications that the exopods may be bipartite as in helmetiids and trilobites, compelling evidence for lamellar setae on a putative proximal element is not
available, and there is no direct evidence of fringing setae on the putative
outer 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 trilobites have a similarly small pygidium that in some species even contains
only a single axial ring (Paterson & Edgecombe 2006).
26
Svensk sammanfattning
Leddjur (Arthropoda) är den artrikaste gruppen av flercelliga djur och utgör
uppskattningsvis tre fjärdedelar av alla nu levande arter. De bebor många habitat, både i havet och på land. De nu levande leddjuren är fördelade på fyra stora grupper: spindeldjur (Chelicerata), kräftdjur (Crustacea), mångfotingar
(Myriapoda) och insekter (Insecta), som utgör den största gruppen. Alla dessa
djur har ett unikt yttre skelett uppbyggt av härdade (sklerotiserade) plattor som
är ledade mot varandra. Tillsammans utgör de gruppen Euarthropoda. Hur de
fyra grupperna är besläktade med varandra är mycket omdiskuterat. Dessutom
finns det några mindre grupper som t. ex. havsspindlar (Pycnogonida), som av
vissa forskare betraktas som spindeldjur, men som av andra anses vara systergrupp till Euarthropoda, dvs. euarthropodernas närmaste släktingar. Det är inte
heller helt klarlagt, vilka de övriga arthropoderna är, alltså de som inte hör
hemma i Euarthropoda. Medan det är en allmänt utbredd uppfattning att klomaskar (Onychophora) är arthropoder, finns det fortfarande oenighet om huruvida trögkrypare (Tardigrada) tillhör gruppen eller i stället är närmare besläktade med rundmaskar. Ytterligare en omdiskuterad grupp är tungmaskar (Pentastomida), som är parasiter på bl. a. ren. De betraktas allmänt som athropoder,
men det råder delade meningar om huruvida de är euarthropodernas systergrupp eller starkt specialiserade kräftdjur.
Med den stora diversiteten och spridningen som arthropoder har idag är det
föga förvånande att gruppen har haft en lång utvecklingshistoria, rikligt belagd
med fossil. De tidigaste leddjursfossilen uppträder redan i nedre kambrium,
bland de äldsta fossilen av flercelliga djur. Oftast är det arter med kutikula
förstärkt av mineralinlagringar som bevarats, som t. ex. musselkräftor och trilobiter. Dessa är vanliga i havsavlagringar och viktiga verktyg för relativa åldersdateringar (stratigrafi). Trilobiter är särskilt viktiga i detta sammanhang, då de
uppträder 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 är
tydligt präglad av spår av flercelliga djur) och hade stor spridning genom hela
paleozoikum tills de dog ut i slutet av permtiden för ca. 250 miljoner år sedan.
Under särskilda förhållanden kan dock även andra djur utan mineraliserade
kroppsdelar bevaras. Även i sådana avlagringar är leddjur bland de vanligaste
fossilen. I kambrium har man hittat många avlagringar som bevarar mer än bara
mineraliserade delar av organismer. Dessa avlagringar, särskild de berömda fossila faunorna från Burgess Shale i Kanada och Chengjiang i Kina har väckt stort
27
intresse, inte minst bland forskarna. Detta intresse kan nog förklaras med fascinationen av det till synes plötsliga uppträdandet av många grupper av flercelliga
djur i början av fanerozoikum (även om det finns spår av flercelligt liv i tidigare
avsnitt av jordens historia). Många forskare tror att det var en kritisk fas i de
flercelliga djurens tidiga evolution. Även om det finns argument för att flercelliga djur, inklusive arthropoder, uppstod tidigare, så ger oss de kambriska avlagringarna de äldsta direkta bevisen för hur de tidiga djuren såg ut, vilket i sin tur
har stor betydelse för teorier om djurgruppers släktskap.
Mina studier behandlar såväl frågor om leddjurens tidiga evolution som
relativa dateringar av avlagringar med hjälp av trilobiter.
Trilobit-stratigrafi på Grönland
Kambrium sträcker sig från 542 till 488 miljoner år sedan. Framförallt i detta
äldsta avsnitt av paleozoikum är trilobiter ett viktigt verktyg för stratigrafi. Det
finns dock många problem med relativ datering av avlagringar med hjälp av
dessa fossil. Dels uppträder många arter bara i vissa regioner av jorden, dels
var de som alla organismer beroende av miljöförhållanden vilket gör att vissa
arter bara uppträder i vissa sorters avlagringer. Dessutom finns det många områden på jorden, där fossilen inte har studerats tillräckligt för att möjliggöra
jämförelser med andra regioner. Under de senaste årtionden har man upptäckt
att vissa relativa dateringar av avlagringar jorden över inte håller, och en revidering av den kambriska tidsskalan är just nu på gång. Ett särskilt problem rör
gränsen mellan de två mellersta avsnitten av kambrium. Här tycks trilobitfaunan från norra Grönland ha en stor potential för att förbättra korrelationen mellan olika delar av världen. Det finns dock även avlagringar på nord östra Grönland som inte är ordentligt studerade. De första två publikationerna handlar om
enstaka trilobit-arter som kan bidra till att förbättra korrelationen av de två regionerna och därmed också bidra till att förbättra den relativa dateringen av de
viktiga avlagringarna i Norra Grönland.
Den första publikationen handlar om Fritzolenellus lapworthi, en art som
uppträder i tidiga kambriska lager både i nord östra Grönland och i nord västra Skottland och som kan bidra till att korrelera dessa delar av Skottland
med resten av den kambriska kontinenten Laurentia. Lite högre i lagerföljden på nord östra Grönland uppträder Olenellus hanseni, en som också kan
finnas i norra Grönland, och därmed hjälpa till att korrelera nord östra med
norra Grönland.
Den andra publikationen diskuterar förekomster av flera arter av den distinkta trilobiten Perissopyge på olika ställen i Nordamerika och Grönland,
och deras möjliga betydelse för stratigrafin. Nya förekomster rapporteras
från norra och nord östra Grönland som möjligtvis kan förbätta korrelationen
av dessa regioner.
28
De tidiga leddjurens evolution
Som tidigare nämnts råder stor oenighet om hur de olika euarthropod-grupperna är besläktade med varandra. Förhoppningar att gamla kontroverser
skulle kunna lösas med hjälp av molekylära data har än så länge inte uppfyllts. Den rådande osäkerheten om släktskap innebär att det är svårt att
kartlägga vilka av de olika morfologiska särdrag som finns i nu levande
grupper redan fanns i leddjurens gemensamma anfader. Detta är ett dilemma,
då kunskap om anfadern tvärtom skulle kunna bidra till att utvärdera hypoteser om släktskap. Chansen att hitta själva anfadern som fossil är realistiskt sett obefintlig. Däremot hittar vi fossil som kan vara nära besläktade
med anfadern; arter som hör hemma i den evolutionära linje som ledde till
anfadern. De är i princip de enda direkta belägg för vilka särdrag anfadern
kan ha haft. Det är dock klart att vi till viss mån även här konfronteras med
ett cirkulär problem, då vi behöver en viss uppfattning av släktskapen av de
levande djuren för att kunna hänvisa ett fossil till ett stamlinje. Fossil, precis
som morfologiska eller molekylära data från nu levande djur, är alltså bara
pusselbitar och måste diskuteras i sammanhang med helheten. De resterande
publikationerna i avhandlingen beskriver nya data från fossila arthropoder,
dels från senkambriska avlagringar i Sverige, dels från tidiga kambriska
avlagringar i Grönland. Där det är möjligt diskuteras dessa fossils innebörd
för evolutionen av morfologiska särdrag i vissa arthropod-grupper.
Den tredje publikationen är ett detaljstudie av en tidig släkting till kräftdjuren, Oelandocaris oelandica från orstensavlagringar på Öland och i
Västergötland. Tillsammans med andra arter från orsten och liknande avlagringar i hela världen ger Oelandocaris oelandica och dess morfologi och
möjliga levnadssätt viktig kunskap om kräftdjurens anfader.
De resterande publikationerna handlar om fossil från tidiga kambriska
avlagringar på norra Grönland. I den fjärde publikationen beskrivs Kiisortoqia avannaarsuensis, en ny art som med stor sannolikhet antingen var en
tidig euarthropod eller en nära släkting till euarthropodernas anfader. Arten
har vissa likheter med några mycket svårtolkade fossil kända från kambriska
avlagringar 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 kroppen. Det finns många arter som tycks tillhöra Isoxys, men bara av några få
finns lämningar av andra delar än just skölden. Publikationen diskuterar
likheter och olikheter mellan de bättre kända arterna och granskar tidigare
publicerade idéer om möjliga levnadssätt.
I den sjätte och sista publikationen beskrivs Siriocaris trolla, en ny art
som har vissa likheter med trilobiter och deras nära släktingar.
29
Acknowledgments
First and foremost I want to thank my supervisors without whom I would not
have been able to complete these studies: John S. Peel provided most of the
material studied and spent immeasurable time and effort on advice and discussions as well as reviewing countless iterations of manuscripts. Financial
support from Vetenskapsrådet (Swedish Research Council) through Grants to
J. S. Peel is gratefully acknowledged. Dieter Waloszek was an invaluable
source of inspiration, advice, and criticism. He, too, took the time to review
manuscripts at various stages and is also thanked for his hospitality during
various visits to his department in Ulm. In his efforts, he was often assisted
by Andreas Maas, whom I also wish to thank. David J. Siveter helped with
advice and reviews of manuscripts and invited me to visit Leicester University. Even he had his backup, Mark Williams. Reinhardt M. Kristensen
helped with numerous discussions and kindly hosted a brief visit to Copenhagen. I also wish to thank Jan Bergström who supervised the first half of
my 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 I
wish to thank Jan Ove Ebbestad and Pär Eriksson who have been of great
help. Learning the modeling software used for the reconstructions could be a
long-winded process at times. I was fortunate to be accompanied by Joachim
T. Haug on the way. On the theoretical side, Uwe Balthasar has to be
thanked for trying to raise my awareness of the convoluted problems of the
peculiar preservation of the fossils of the Sirius Passet Lagerstätte.
One of the greater challenges in final completion of the thesis was to write
a Swedish summary. Anders Nordin is thanked for his help with that. Getting
a thesis into printed form involves a surprising amount of paperwork and errands. 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 for
bearing with me and keeping my spirits up during this venture.
30
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