Fossil Evidence, Terrestrialization and Arachnid Phylogeny

8 Pages·1999·8.2 MB·English

by  Jason A Dunlop

1999. The Journal of Arachnology 27:86-93
FOSSIL EVIDENCE, TERRESTRIALIZATION AND
ARACHNID PHYLOGENY
Jason A. Dunlop: Institute fiir Systematische Zoologie, Museum fur Naturkunde,
D-10115 Berlin, Germany
Mark Webster: Department of Earth Sciences, University of California, Riverside,
California 92521 USA
ABSTRACT. Geological and morphological evidence suggests that the earliest scorpions were at least
partially aquatic and that terrestrialization occurred within the scorpion clade. Scorpions and one or more
other arachnid lineages are therefore likely to have come onto land independently. The phylogenetic
position of scorpions remains controversial and we question Dromopoda, in which scorpions are placed
derived within Arachnida, as this is not supported by scorpions’ lateral eye rhabdomes, embryology and
sperm morphology. We propose a synapomorphy for scorpions + eurypterids, a postabdomen of five
segments as part of an opisthosoma of 13 segments. Scorpions and tetrapulmonates must have evolved
their book lungs convergently while fossil evidence indicates that a stomotheca, synapomorphic forDrom-
opoda, is probably convergent too. ‘Arachnid’ characters such as Malpighian tubules, the absence of a
carapace pleural margin, and an anteriorly directed mouth may also be convergent, although their status
as synapomorphies can be defended using parsimony. Convergence is difficult to prove unequivocally, but
when there are strong grounds for suspecting it, such characters are questionable evidence for arachnid
monophyly.
Living arachnids are predominantly terres- least two arachnid lineages moved onto land
trial, with those groups found in aquatic hab- independently it would be surprising to find
itats, such as the water spider Argyroneta or terrestrial adaptations in the common ancestor
halacaroid mites, assumed to have returned to of all arachnids, irrespective ofwhether or not
the water secondarily. It is also reasonable to they were monophyletic, as this was an aquat-
assume that the earliest chelicerates were ic animal. These arguments are expanded be-
aquatic, including the ancestors of arachnids low.
(e.g., Kraus 1976). Terrestrialization was a
THE AQUATIC NATURE OF PRIMITIVE
key event in arachnid evolution and as there
SCORPIONS
is good evidence for aquatic fossil scorpions
(Jeram 1998), terrestrialization probably oc- A number of authors have suggested that
curred independently within at least two some of the oldest fossil scorpions were par-
arachnid lineages. The position of the scorpi- tially or wholly aquatic (e.g., Pocock 1901;
ons within Chelicerata has proved to be con- Kjellesvig-Waering 1986). This proposal has
troversial (e.g., Weygoldt 1998) and we do not not gone unquestioned. Petrunkevitch (1953)
accept that scorpions are a derived group found no evidence for scorpion gills and ar-
within arachnids, nor that Arachnida is mono- gued that all other arachnids have book lungs
phyletic. A number of characters which have or tracheae. More recently Weygoldt & Paulus
been used to support arachnid monophyly (1979) again questioned whether scorpions re-
could also be interpreted as adaptations for ally were aquatic, with Weygoldt (1998) find-
life on land (e.g., Kraus 1976). We consider ing Kjellesvig-Waering’s (1986) evidence for
these to include book lungs, Malpighian tu- scorpion gills unconvincing. Stqrmer (1970)
bules, absence ofcarapace pleural margin, and and Brauckmann (1987) also described scor-
an anteriorly directed mouth. Parsimony ana- pion gills, although these structures projecting
lyses (e.g., Wheeler & Hayashi 1998) suggest beyond the body margin are equivocal as re-
that these characters should be treated as ho- spiratory organs and do not resemble xipho-
mologous and apomorphic. However, if at suran book gills. Gills in scorpions have not,
86
DUNLOP & WEBSTER—ARACHNID PHYTOGENY 87
therefore, been proven, but in his recent phy- group to all three, forming the taxon Dromo-
logeny of Silurian and Devonian scorpions poda. Several authors have suggested that
Jeram (1998) presented both sedimentological scorpions are most closely related to euryp-
and morphological evidence that at least some terids (e.g., Versluys & Demoll 1920; Bris-
of these early scorpions were aquatic. His towe 1971; Kjellesvig-Waering 1986). These
mostbasal scorpion taxon comes from the De- studies generally relied on overall similarities
vonian Hunsriickschiefer, a fully marine se- rather than discrete synapomorphies, and were
quence (Bartels et al 1998), while other Si- justifiably criticized for this by Shultz (1990).
lurian taxa come from marginal marine While Shultz (1990) found scorpions to be de-
deposits. Morphological evidence for an rived arachnids, he excluded from his analysis
aquatic habitat includes the presence of gna- Weygoldt & Paulus’ (1979) character 21 (star-
thobases in some taxa, lack ofan oral tube for shaped lateral eye rhabdomes in scorpions and
liquid feeding, single-clawed, digitigradetarsi, xiphosurans, quadratic in all other arachnids
and abdominal plates lacking book lung spi- bearing lateral eyes). Wheeler & Hayashi
racles. Jeram (1998) further commented that (1998) did include this character. Both Wheel-
&
two or more scorpion lineages might have er Hayashi (1998) and Shultz ignored An-
come onto land independently, but that this derson’s (1973) observation that the embryo-
would be difficult to detect as many of the logical development of scorpions and
characters he analyzed were likely to have al- xiphosurans is similar in possession of a
tered state during terrestrialization. growth zone giving rise to both the prosoma
Scorpions have obvious autapomorphies and opisthosoma, while in all other arachnids
(pectines, sting, pedipalpal claws) and there is this growth zone gives rise to the opisthosoma
no evidence to derive any other arachnid order only. The distribution ofthese characters does
directly from the scorpion clade, e.g., Ander- not favor placement of scorpions as derived
son (1973); something which would require arachnids with Xiphosura as an outgroup. Fur-
autapomorphy reversal. It is conceivable that thermore, the analysis of Shultz (1990) found
scorpions and other arachnids evolved on land spermatozoa with a coiled flagellum axoneme
from a common terrestrial ancestor and that to be synapomorphic in Arachnida, despite re-
some scorpions then re-entered the water and tention of a free flagellum in scorpion sper-
lost their terrestrial adaptations. However, as matozoa (the presumed plesiomorphic state).
the fossil record shows an accumulation of Weygoldt & Paulus (1979) proposed a coiled
terrestrial-related features through the Palaeo- flagellum as synapomorphic only for non-
&
zoic (e.g., Selden Jeram 1989, fig. 4), this scorpion arachnids. Alternatively, the eye
remains an unlikely scenario. We therefore rhabdome, growth zone character and sperm
suggest that if the oldest scorpions were flagellumcould all be reversals in the scorpion
aquatic (Jeram 1998), they must have di- clade.
verged from the other arachnids while still in Dunlop (1998) stated that the clearest po-
the water and moved onto land independently. tential synapomorphy for scorpions and eu-
SCORPIONES + EURYPTERIDA rypterids is the five-segmented postabdomen,
an obvious character (Fig. 1) ignored by cla-
The phylogenetic position of scorpions re- distic studies. Shultz (1990) and Weygoldt
mains controversial with the three principal (1998) argued that similarities between euryp-
&
cladistic analyses (Weygoldt Paulus 1979; terids and scorpions were probably symple-
Shultz 1990; Wheeler & Hayashi 1998) in- siomorphic, as they occurred in xiphosurans,
cluding themin a monophyletic Arachnida, al- too. Outgroup comparison with trilobites and
though differing in placement of the order. other arachnates indicates that lack of a post-
Weygoldt & Paulus (1979) and Weygoldt abdomen is the plesiomorphic state with re-
(1998) placed scorpions as a sister group to spect to Chelicerata, while in synziphosurines
&
all other arachnids. However, the analyses of (primitive xiphosurans; Anderson Selden
Shultz (1990) and Wheeler & Hayashi (1998) 1997) and some arachnids there is a postab-
(primarily using Shultz’s morphological data) domen of three segments (the pygidium of
placed scorpions higher in the Arachnida as a Shultz 1990). In support of the homology of
sister group to Haplocnemata (Pseudoscorpi- the postabdomen we argue that scorpions and
ones + Solifugae) with Opiliones as sister eurypterids have a groundplan of 13 opistho-
88 THE JOURNAL OF ARACHNOLOGY
—
Figure 1. Proposed homology of the ventral opisthosoma of a Paleozoic scorpion (left) a scorpion-
like eurypterid (center) and a synziphosurine (right). This model differs in its placement of the pectines
from Dunlop (1998, fig. 4) and it should be added that the chilaria in at least some synziphosurines may
have been fully pediform (L. Anderson pers. comm.). Our model lines up four morphological reference
points: the genital segment (A), the last opisthosomal appendage pair (B), the firstpostabdominal segment
(C) and the posteriormost, 13th segment (D). Points A-C, i.e., the preabdomen, are seen in Scorpiones,
Eurypterida and Xiphosura; C-D, i.e., the 5-segmented postabdomen, is synapomorphic for Scorpiones +
Eurypterida. Opisthosomal segments numbered. Abbreviations: T = telson, st = sternum, emb = embry-
onic appendages, mt = metastoma, go = genital operculum, ego = eurypterid genital operculum formed
from two fused appendage pairs, pt = pectines, ga = genital appendage, ch = chilaria, op = appendage-
derived operculum, a structure usually termed Blatfuss (literally ‘sheet foot’) in eurypterids.
somal segments: A preabdomen (segments 1- segment 1 despite evidence for their presence
8) and a postabdomen (segments 9-13). embryologically in the less derived, apoiko-
THE QUESTION OF OPISTHOSOMAL genic scorpions (Anderson 1973). Scorpion
SEGMENTATION opisthosomal segmentation has therefore been
— proposed as: (1) embryonic, (2) genital oper-
In scorpions. The groundplan of arach- cula, (3) pectines, (4-7) book-lungs, (8) last
nids has widely been quoted as comprising an preabdominal segment and (9-13) postabdo-
opisthosoma of 12 segments (e.g., Kraus men, plus a telson (e.g., Dunlop 1998).
1976; Shultz 1990). However, there is evi- In some ofthe earliest scorpions there is an
dence for a transitory pair of pregenital limb additional abdominal plate (Jeram 1998) ap-
buds described in scorpion embryos (Brauer parently representing an additional segment.
1895) representing an ‘extra’ segment, the While modern scorpions have four append-
nerve ganglion ofwhich is retained (Anderson age-derived book lungs on the preabdomen,
1973). A scorpion with 13 segments, includ- some early derivative scorpions have five ap-
ing this embryonic one, was figured by Millot pendage-derived opercula (Fig. 1) posterior to
(1949, figs. 52, 53). Shultz (1990) coded scor- the genital operculae and the pectines. This
pions as lacking appendages on opisthosomal could be interpreted as evidence that scorpi-
DUNLOP & WEBSTER—ARACHNID PHYLOGENY 89
ons have a groundplan of 14 opisthosomal of eurypterids does not tuck under the poste-
segments, e.g., (1) embryonic, (2) genital rior margin ofthe carapace, as the tergite does
opercula (3) pectines, (4-8) book=lungs, (9) in xiphosurans. Instead, the membrane behind
last preabdominal segment and (10-14) post- the eurypterid carapace is reflexed forward
abdomen, plus a telson. However, all known and then doubles back on itself in a manner
fossil and Recent scorpions express only 12 consistent with it being a reduced and poorly
tergites, the last five of which are fused with sclerotized tergite (Fig. 2). One ofHolm’s eu-
the sternites into the postabdomen (Fig. 1). It rypteridpreparations (BritishMuseumNatural
should be possible to match all tergal and ster- History specimen I. 3406 (6)) was examined
nal elements. and shows these reflexed membranes (Fig. 3).
Weygoldt & Paulus (1979, fig. 2) accepted What has traditionally been called tergite 1 in
thatthere are 13 ventrally expressed structures eurypterids is in contact with the carapace at
in Recent scorpions, but cautioned that there its lateral margins, but has a slightly concave
is no musculature evidence for an additional anterior margin forming a narrow gap on the
tergal element; i.e., they interpreted scorpions midline in which there is a membrane con-
as having only 12 segments. They suggested taining thin fragments of sclerotized cuticle
that the ‘extra’ segment in scorpions is the (Fig. 3). This, we believe, is consistent with it
result of the division of the ventral elements being a highly reduced tergite; our tergite 1.
of opisthosomal segment 2; for them a scor- Reduced first opisthosomal tergites are
pion autapomorphy. Essentially they argued common in chelicerates as shown by xipho-
that both the genitalia and pectines belonged suran microtergites (Anderson & Selden
to opisthosomal segment 2, a proposal which 1997) and the pedicels of some arachnids
we support (Fig. 1). However, these authors (Shultz 1990). These eurypterid cuticle frag-
were not aware of the additional abdominal ments could be dismissed as having simply
plate in fossil scorpions, a fully expressed sutured off from the carapace or adjacent ter-
structure which appears to bring us back to a gite, and we have noted Weygoldt & Paulus’
body plan of 13 opisthosomal segments: (1) (1979) evidence against an extra tergite in
embryonic, (2) genital opercula + pectines, scorpions above. However, eurypterid ventral
(3-7) abdominal plates, (8) last preabdominal anatomy can also be homologized with our
segment, (9-13) postabdomen, plus a telson scorpion model (Fig. 1). The eurypterid me-
(Fig. 1). tastoma is interpreted as opisthosomal seg-
Our model is tentative, but we believe it fits ment 1, though it is unclear whether it is a
the available data and means that, like xiph- sternal or appendicular structure. Jeram
osurans, the scorpion groundplan consists of (1998) suggested that what is traditionally
eightpreabdominal segments with appendages called the scorpion sternum is homologous
on segments 1-7 (Fig. 1). We are still ‘miss- with the eurypterid metastoma, and as such
ing’ a tergite in scorpions, but the first tergite both may be appendage-derived elements, ho-
in chelicerates is often reduced and has been mologous with xiphosuran chilaria (Fig. 1). It
overlooked in eurypterids (below). This mod- is conceivable that the embryonic limb buds
el requires loss of one abdominal plate in in scorpions actually become the scorpion
more derived scorpion clades. This is reflected sternum. Both are in approximately the same
in the cladogram of Jeram (1998, fig. 2, node position and the fate of the limb buds and or-
F). — igins of the sternum are equivocal. This limb
In eurypterids. Eurypterids are typically bud/sternum question merits investigation.
reconstructed with 12 opisthosomal segments. Assuming homology (segment 1), the next
However, Raw (1957) cited evidence foundby segment in both scorpions and eurypterids
Holm (1898, pp. 8-9, fig. 15) for an ‘extra’ bears the gonopore (segment 2). This fits the
segment in eurypterids, a proposal overlooked general chelicerate pattern of a gonopore on
by recent authors. Raw (1957, pp. 160-161, opisthosomal segment 2, which appears to be
fig. 8c) proposed that the membranous fold a valuable marker for homologizing segments
between the carapace and the opisthosoma (e.g., Millot 1949).
formed a discrete but highly reduced segment The eurypterid gonopore on opisthosomal
in eurypterids. Raw (1957) argued that what segment 2 lies at the base of the genital ap-
was traditionally interpreted as the first tergite pendage. The appendage is part of the large
90 THE JOURNAL OF ARACHNOLOGY
—
Figures 2-3. Eurypterid dorsal segmentation. 2. Schematic adapatation ofHolm’s (1898) observation
that the dorsal membrane between the prosoma and opisthosoma in eurypterids folds back on itself. This
differs from the way that the othertergites connectto eachother. Raw (1957, fig. 8C) citedthis asevidence
that this membrane represents a highly reduced first opisthosomal tergite giving eurypterids 13, not 12,
opisthosomal segments; 3. Camera lucida drawing ofBMNH I. 3406 (6) a small, translucent specimen of
Baltoeurypterus acid-etched from the matrix. This specimen supports Raw’s interpretation by showing the
folding of the membranes at the prosoma-opisthosoma junction (fl) and the slight sclerotization within
this membrane (sc), which we interpret as opisthosomal tergite 1. Scale = 5 mm.
genital operculum which is divided by a trans- men. No other chelicerates show an 8-seg-
verse suture, suggesting it is formed from the mented preabdomen.
fused appendages of segments 2 and 3. That Thirteen opisthosomal segments could be
scorpion pectines possibly belong to the gen- interpreted as a synapomorphy for Scorpiones
ital segment is interesting in this context. Si- + Eurypterida, though in reality this and the
mon Braddy (pers. comm.) suggested that the postabdomen are probably best treated as ex-
paired pectines of scorpions could be homol- pressions of the same character. We have ar-
ogous with the paired furcae at the end ofthe gued that the postabdominal segments (9-13)
eurypterid genital appendage; a structure are homologous in these taxa and that the
which also appears to belong to opisthosomal character state is derived. No outgroup shows
segment 2 (see also Braddy & Dunlop 1997). a 5-segmented postabdomen. The three-seg-
In eurypterids, segments 3-7 bear gill tracts mented postabdomen of some arachnids
(probably non-homologous with book gills (Shultz 1990) is unlikely to be derived from
[Manning & Dunlop 1995]), and the last this condition by loss oftwo segments, as this
preabdominal segment is segment 8. There is would reduce the total number of opisthoso-
a five-segmented postabdomen (segments 9- mal segments to only 11 (arachnids such as
13) plus a telson (Fig. 1). This segmentation thelyphonids have 12 segments).
pattern merits further investigation. Our mod-
PROBABLE CONVERGENT
el (Fig. 1) suggests homology of several ref- CHARACTERS
erence points: The genital segment (A), the
posteriormost opisthosomal appendages (B), We have proposed one morphological syn-
the start of the postabdomen (C), and the five apomorphy shared by scorpions and eurypter-
segments to the 13th, posteriormost segment ids. Other characters have been used to sup-
(D). Points A, B and C also match the body port a monophyletic Arachnida and/or a
plan of xiphosurans (Fig. 1). Scorpions, eu- derived position for scorpions within arach-
rypterids, and xiphosurans share an 8-seg- nids. Some of these characters appear to be
mented preabdomen with appendages on seg- adaptations for, or associated with, life on
ments 1-7, while scorpions + eurypterids land. If arachnids terrestrialized more than
show an apomorphic 5-segmented postabdo- once, then terrestrial adaptations are likely to
DUNLOP & WEBSTER—ARACHNID PHYLOGENY 91
be convergent. It would be surprising to find ing process is less likely (although possible)
terrestrial characters in the aquatic common in an aquatic animal. Weygoldt (1998) reject-
ancestor predicted by arachnid monophyly ed Dromopoda, arguing that a stomotheca is
and the models ofWeygoldt & Paulus (1979), clearly absent in many fossil scorpions (see
Shultz (1990) a—nd Wheeler & Hayashi (1998). also Jeram 1998, character 23), in solifuges,
Book lungs. Book lungs are a Textbook’ and in pseudoscorpions. Weygoldt (1998)
arachnid character. However, their distribution concluded that either the stomotheca is con-
indicates that although book lungs in scorpi- vergent (supported here) or that scorpions
ons and other arachnids are clearly homolo- must be paraphyletic. This example illustrates
gous with respect to the pre-existing abdom- the dangers of ignoring fossil data when cod-
inal appendages, the book lungs of scorpions ing characters. —
are not directly homologous with those of te- Malpighian tubules. Malpighian tubules
&
trapulmonates (contra Wheeler Hayashi are endodermal extensions ofthe gut found in
1998). Weygoldt (1998) and Dunlop (1998) most arachnids, but not in palpigrades, opi-
independently noted that only tetrapulmonate lionids and pseudoscorpions (Shultz 1990).
arachnids retain a respiratory organ on the They also occur convergently in insects. Their
genital segment. It is absent in xiphosurans, function is to remove excretory products such
scorpions and probably eurypterids. Weygoldt as guanine and uric acid from the body (e.g.,
(1998) regarded loss of a respiratory organ on Seitz 1987). They are not present in xipho-
the genital segment as most likely being con- surans and their presence or absence is un-
vergent. Dunlop (1998) noted that outgroups known in eurypterids. The tubules could be
such as trilobites have a respiratory organ on convergent terrestrial adaptations for remov-
all opisthosomal segments, and proposed that ing dry, low-toxicity excretory products (e.g.,
the most parsimonious interpretation of this guanine), given the importance of water con-
character is to treat it as plesiomorphically re- servation for animals on land (Kraus 1976).
tained in tetrapulmonates and synapomorphi- Po—orly developed carapace pleural mar-
cally lost in a xiphosuran, scorpion and eu- gin. The carapaces of xiphosurans, and to a
rypterid clade. lesser extent eurypterids, project laterally and
In either case, book lungs in spiders and form a cavity around the coxosternal region
scorpions are not directly homologous, be- (Shultz 1990). This projection is not seen in
longing to opisthosomal segments 2-3 in te- arachnids. Its association primarily with taxa
trapulmonates and 4-7 in scorpions (Dunlop that masticate food using gnathobases may be
1998; Kraus 1998). Probable independent ter- significant; it is also seen in outgroups such
restrialization of scorpions and other arach- as trilobites, although it is not apparent in fos-
nids indicates that their book lungs evolved sil or Recent scorpions. Shuster (1950) noted
independently from gills inresponse to the de- that xiphosurans feed by burying themselves
mands ofbreathing on land. Differences inde- into the substrate in pursuit of worms and
tailed lung anatomy might be predicted be- mollusks. The carapace pleural margin, and
tween scorpions and other arachnids. This the associated cavity it creates, forms a semi-
represents an interesting line of research enclosed chamber within the substrate in
which could be applied to other morphologi- which the gnathobases are free to masticate
cal structures which may be terrestrial adap- food. With a move towards terrestrialization
tations, e.g., trichobothria, and details of limb and away from feeding in the substrate, a car-
morphology and—mouthpart structure. apace pleural margin could become non-func-
Stomotheca. Shultz’s (1990) taxon tional and lost. In contrast, many arachnids
Dromopoda is supported by a number of ap- show a trend towards developing a preoral
pendicular characters coded primarily from cavity (e.g., Selden & Jeram 1989) which sur-
Recentterrestrial forms. The strongest appears rounds the food during cheliceral mastication
to be presence of extensor muscles, but spe- and extraintestinal ingestion. —
cializations of the femorpatellar and patello- Anteroventrally directed mouth. In
tibial leg articulations, and a stomotheca xiphosurans the mouth is directed posteroven-
formed from coxal endites are included. The trally, towards the postoral gnathobases from
stomotheca creates a preoral cavity where ex- where they receive masticated food (Shultz
traintestinal digestion takes place. This feed- 1990). Eurypterid mouths have also been re-
92 THE JOURNAL OF ARACHNOLOGY
constructed with this orientation, and although ters should be assumed homologous unless
supportive fossil evidence is weak, the inter^ detailed arguments in favor oftheirhomology
pretation is probably correct on functional are presented (e.g., our postabdomen charac-
grounds. Mouth orientation offossil scorpions ter). We worry that in an attempt to compile
is unknown. Trilobites were also gnathobasic ever larger databases of characters for parsi-
feeders and had posteroventrally directed mony analysis, homologies are being assumed
mouths. Recent arachnid mouths are all di- at face value without assessment of character
rected anteroventrally, towards the preoral validity. Which is more important, the quan-
chelicerae. Terrestrialization could have tity or the quality of the data used?
caused a shift from gnathobasic masticationin ACKNOWLEDGMENTS
water to cheliceral mastication on land. Post-
oral gnathobases along the length of the pro- We thank Simon Braddy, Lyall Ander-
soma are of little use for mastication on land son, Otto Kraus, Andy Jeram, Roger Farley,
where food would drop frombetween the cox- Scott Stockwell, Paul Selden, David Walter,
ae. However, a similar function appears to PeterWeygoldt & Manfred Ade foruseful dis-
have been retained in the palpal coxae of spi- cussions and correspondence and Bill Shear
ders, adjacent to the mouth, which are used to for valuable criticisms of an earlier version of
manipulate food (e.g., Bristowe 1971). Unlike the manuscript.
xiphosurans, which trap their food in soft sed- LITERATURE CITED
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and generally catch their food in front ofthem York.
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and/or anterior appendages. An anteriorly di- fusion and phylogeny of Palaeozoic Xiphosura.
rected mouth can be interpreted as an adap- Lethia, 30:19-31.
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a terrestrial animal. Fossils ofthe Hunsrack Slate. MarineLife inthe
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PROOF morphology ofmating in the Silurian eurypterid,
Baltoeurypterus tetragonopthalamus (Fischer,
Arachnid monophyly is supported by a 1839). Zool. J. Linn. Soc., 121:435-561.
number of characters which may represent Brauckmann, C. 1987. Neue Arachniden-Funde
convergence in response to terrestrialization. (Scorpionida, Trigonotarbida) aus dem west-
These characters can be defended by parsi- deutschen Unter-Devon. Geol. Palaeont., 21:73-
mony and assumed to be homologous and 85.
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parsimony analysis. Kraus (1998) discussed twicklungsgeschicte des Skorpiones, IL Z. Wiss.
ZooL, 59:351-433.
the limitations of parsimony analysis and ar-
Bristowe, W.S. 1971. The World of Spiders. 2nd
gued that characters for phylogeny should be
ed, Collins, London.
selected and weighted a priori based on struc- Dunlop, J.A. 1998. The origins of tetrapulmonate
tural and functional considerations. We have book lungs and their significance for chelicerate
presented functional arguments to question phylogeny. Pp. 9-16. In Proc. 17th Europ. Coll.
some of the characters supporting arachnid ArachnoL, Edinburgh 1997.
monophyly. Unfortunately, unequivocal proof Holm, G. 1898. Uber die Organisation von Euryp-
for convergent evolution of characters can be termfischeri Eichw. Acad. Imp. Sci. St. Peters-
difficult to establish, especially when they in- burg, Mem., ser. 8, voL 8, no, 2.
volve characters in fossil taxa where function- Jeram, A.J. 1998. Phylogeny and classification of
al morphology often has to be inferred (e.g., Palaeozoic scorpions. Pp. 17-31. In Proc. 17th
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