Table Of Content1994. TheJournal ofArachnology 22:169-=172
RESEARCH NOTES
THE LIMITS OF STRATIGRAPHIC EVIDENCE
IN ASSESSING PHYLOGENETIC HYPOTHESES
OE RECENT ARACHNIDS
Inrecentreviewsofthearachnidfossilrecord, the order in which morphologically distinguish-
Selden (1990, 1993) suggests that alternative able taxa appear in the fossil record. According
phylogenetic hypotheses ofRecent arachnids be tothisreasoning,stratigraphictestsofphylogeny
evaluated with stratigraphic evidence. He rea- can be successful only to the extent that phylo-
sonsthat“successivedichotomiesincladesmust geneticdivergence is coupledwith the evolution
occur in ascending chronological order; thus a of diagnostic characters. But phylogenetic sys-
cladogram reflectingevolutionary events should tematicsandevolutionarytheoryrequirenosuch
concur with a complete fossil record in the se- coupling. In fact, the earliest members of two
quence ofevents” (Selden 1990). Based on this sisterlineagesmaybeindistinguishablefromone
premise, Selden proposes that the relative ac- another and from their immediate ancestors in
curacyofalternativephylogenetichypothesescan morphology, behavior, genetics, etc. This situ-
be evaluated by comparing discrepancies be- ation presents few problems for systematists
tween the hypothesized branching orders ofex- workingondeepdivergencesoflivingtaxa, since
tant lineages and the order in which they first independent lineages are differentiated by fea-
appear in the fossil record. Aspects ofthis pro- turesthatmayhaveevolvedlongaftertheirphy-
posal may appeal to common sense (e. g., Cod- logenetic origin. In contrast, sameness among
dington&Levi 1991)andhavebeenanticipated early members ofsister taxa and their ancestors
in a quantitative method called stratocladistics createsproblemsforconstructingaccuratestrati-
(Fisher 1988, 1991; Maddison & Maddison graphic ranges for clades, as one or both oftwo
1992). However, I argue here that stratigraphic sister lineages may exist for long periods before
tests ofphylogeny are unworkable, as they rest acquiring the characters that are used to recog-
uponthequestionableassumptionthattheorigin nize their living members. Because there is no
ofextant lineages and the origin oftheir diag- necessaryconnectionbetweenthe timealineage
nostic characters are coupled (Fig. 1). In fact, first appears and the time it first acquires a di-
diagnostic characters (unique autapomorphies) agnostic characterofits living members, the or-
mayevolvelongafteralineage divergesfrom its der ofstratigraphic occurrence and the order of
sister, and it is likely that the first recognizable phylogeneticdiversificationneednotcorrespond
fossilmembersofanextantlineagewouldappear (Fig. 1). Thus the order of stratigraphic occur-
in the stratigraphic record well after the lineage renceshouldnotbeusedtoevaluatephylogenetic
had originated. This would be the case even if hypotheses ofextant taxa.
thestratigraphicrecordwasessentiallycomplete. The assumed coupling ofphylogenetic diver-
The purpose ofthis essay is to develop this line gence and the origin ofmodem diagnostic char-
ofreasoningandtoargueagainsttheuseofstrati- acters has also inspired an expectation that a
graphic tests ofphylogenetic hypotheses. “tme”cladogramshouldpredictthestratigraph-
Phylogenetic systematics and biostratigraphy ic occurrence offossils that have yet to be dis-
attemptto estimatethe relativetimingofevents covered, such as Devonian palpigrades, Silurian
in evolutionary history, but these events differ opilionidsandDevoniansolifuges(Selden 1990;
in kind. Phylogenetic systematics estimates the Shear et al. 1989). For example, Selden (1990)
orderinwhichgeneticlineagesdivergefromone states that the van der Hammen and Shultz
another, and biostratigraphy is concerned with cladograms (Fig. 2B, C) predict the existence of
169
THEJOURNALOFARACHNOLOGY
170
Figure 1.—Trees illustrating the limits ofstratigraphic data forevaluating phylogenetic hypotheses ofliving
taxa.Darkregionsindicatetheoccurrenceofdiagnosticcharacters(uniqueautapomorphies)oflivingtaxawithin
the history ofeach lineage. Prior to the occurrence ofthe first modem diagnostic character, members ofone
lineageareessentiallyindistinguishablefromtheirimmediateancestorsandearlymembersoftheirsisterlineage.
Tree A shows coupling between the origin ofa lineage and the origin ofits modem diagnostic characters, the
situation required for stratigraphic assessmentofphylogeny. Trees B and C show otherpossibleconfigurations
in which stratigraphic tests would falsify the “tme” phylogeny.
Silurian opilionids because Silurian scorpions by evidence or else they are reduced to ad hoc
have already been documented. But this is not assertions. He proposes an optimality criterion
a precise interpretation. Given the existence of termed stratigraphic parsimony which favors
Silurian scorpions, the cladograms predict that cladogramsthat minimizethe numberofadhoc
a lineage which eventually gave rise to modem hypotheses ofunreliable stratigraphic sampling,
opilionidswaspresentduringthe Silurian, butit justasphylogeneticparsimonyminimizesadhoc
does not claim that any diagnostic characters of assumptions ofhomoplasy. In Fisher’s method
modemopilionidswerepresentatthattime.Op- (stratocladistics)theorderinwhichcladesappear
ilionid ancestors may have occurred in the Si- in the stratigraphic record is treated as a special
lurian but may be indistinguishable from the kind of ordered multistate character, and the
common ancestors ofOpiliones and their sister method favors that distribution ofstratigraphic
grouporfromearlymembersofthesistergroup. appearances that minimizes the number of ad
Ifarachnologists agreed to include unobserved hocassumptionsthatacladewaspresentbutnot
andunrecognizableopilionidancestorsfromthe preservedinthefossilrecord. Inthisway, Fisher
Silurian within Opiliones, then Selden’s state- claims to give stratigraphic characters empirical
mentwouldtruebydefinition,butthisapproach equivalence with traditional phylogenetic char-
would impose severe problems on the develop- acters.
ment ofany phylogenetic system based on em- Like Selden’s more general approach, strato-
piricalevidence, as some “opilionids” wouldbe cladistics is based on the questionable assump-
indistinguishable from some non-opilionids. tion that the origin ofclades and the origin of
Thus,Iseenoimportantrolefortheuseofclado- their diagnostic characters are coupled. Accord-
gramsin forecastingthe occurrenceoflivinglin- ing to Fisher’s logic, uncertainty as to the first
eages within particular strata. stratigraphicoccurrence ofalivinglineageisde-
The phylogenetic significance ofstratigraphic rived solely from uncertainty about the com-
evidence has been questioned repeatedly, pri- pleteness ofthe fossil record; it is assumed that
marilyfrom the widelyheldperspectivethatthe ifan organism is preserved and found, its inclu-
fossilrecordisincompleteandunreliable. Fisher sion or exclusion from a clade will be obvious.
(1989, 1991; Maddison & Maddison 1992) has However, I have argued that stratigraphic tests
counteredthatifsuchcriticismsaretoconstitute ofphylogenetic hypotheses oflivingtaxa are in-
scientific arguments, they mustbe accompanied appropriateregardlessofthe qualityofthe fossil
RESEARCH NOTES 171
ABC
Figure2.—PhylogenetichypothesesofRecentChelicerata.A)Weygoldt&Paulus(1979);B)vanderHammen
(1989);C)Shultz(1990).Abbreviations:AC,Acariformes;AM,Amblypygi;AN,Anactinotrichida;AR,Araneae;
OP, Opiliones; PA, Palpigradi; PS, Pseudoscorpiones; SC, Scorpiones; SO, Solifugae; SZ, Schizomida; TH,
Thelyphonida(Uropygi); XI, Xiphosura.
record.Theearliestmembersofanextantlineage In summary, I have argued that stratigraphic
would not be recognized as such ifnone ofthe evidence is inappropriate for assessing phylo-
diagnosticcharactersofthatlineagehadevolved. genetic hypotheses of Recent arachnids or in
Earlymembersofaparticularclademaybewell forecasting the occurrence ofclades within par-
known as fossils, but their actual phylogenetic ticularstrata. Stratigraphicmethodsassumethat
affiliation will not be realized because the diag- the order in which diagnostic characters appear
nosticcharactersareabsent.Thisproblemwould in the fossil record reflects the order of phylo-
exist even ifearly members ofa clade were pre- geneticdivergence.However,thisassumptionhas
served in abundance and in ’perfect’ condition. nologicalorempiricaljustification.Paleontology
Asstratigraphicparsimonyfailstoaccommodate plays an importantrole in phylogenetic analysis
this possibility, stratigraphic ’characters’ are in- bydiscoveringanddescribingnewtaxaandchar-
appropriate forevaluating phylogenetic hypoth- acters, but fossil evidence has no empirical pri-
eses ofextant lineages. orityoverneontologicalevidenceinreconstruct-
Themostcompellingevidenceagainsttheuse ing phylogenetic history.
of stratigraphic occurrence in assessing phylo- I thank Jason Dunlop, Norman I. Platnick,
genetic hypotheses has emerged from a recent Paul A. Selden and William A. Shear for com-
empiricalstudy.Norell&Novacek(1992)tested ments on the manuscript.
for positive correlations between the order of
stratigraphic occurrence and the order of phy-
logeneticdivergenceinover20well-studiedver- LITERATURECITED
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