Evolution of river dolphins - DigitalCommons@University of
Transcrição
Evolution of river dolphins - DigitalCommons@University of
University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Publications, Agencies and Staff of the U.S. Department of Commerce US Department of Commerce 3-7-2001 Evolution of river dolphins Healy Hamilton Susana Caballero Allen G. Collins Robert Brownell, Jr. Follow this and additional works at: http://digitalcommons.unl.edu/usdeptcommercepub Part of the Environmental Sciences Commons Hamilton, Healy; Caballero, Susana; Collins, Allen G.; and Brownell, Jr., Robert, "Evolution of river dolphins" (2001). Publications, Agencies and Staff of the U.S. Department of Commerce. Paper 111. http://digitalcommons.unl.edu/usdeptcommercepub/111 This Article is brought to you for free and open access by the US Department of Commerce at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications, Agencies and Staff of the U.S. Department of Commerce by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Proceedings: Biological Sciences, Vol. 268, No. 1466 (Mar. 7, 2001), pp. 549-556 B 92 doi THE ROYAL SOCIETY 10.1098/rspb.2000.1385 Evolution of river dolphins Healy E{amiltonltsSusana Caballero2,Allen G. Coll;nsl and Robert L. BrownellJr3 CA94720, USA of California,Berkeley, Biology,University of Integrative andDepartment lMuseumof Paleontology 24Foeste,no3-110,Cali,Colombia Subarta,Carrara 2Fundacion POBox 271,Lajrolla,CA92038, USA FisheriesScienceCenter, 3Southwest Lipotesand Platanista)are among the least known and most The world's river dolphins (Inia,Pontoporia, endangered of all cetaceans. The four extant genera inhabit geographically disjunct river systems and exhibit highly modified morphologies, leading many cetologists to regard river dolphins as an unnatural group. Numerous arrangements have been proposed for their phylogenetic relationships to one another and to other odontocete cetaceans. These alternative views strongly aXect the biogeographical and evolutionary implications raised by the important, although limited, fossil record of river dolphins.We present a hypothesis of river dolphin relationshipsbased on phylogenetic analysis of three mitochondrial genes for 29 cetacean species, concluding that the four genera representthree separate, ancient branchesin odontocete evolution. Our molecular phylogeny correspondswell with the first fossil appearancesof the primary lineages of modern odontocetes. Integrating relevant events inTertiary palaeoceanography,we develop a scenario for river dolphin evolution during the globally high sea levels of the Middle Miocene. We suggest that ancestorsof the four extant river dolphin lineages colonized the shallow epicontinental seas that inundated the Amazon, Parana, Yangtze and Indo-Gangetic river basins, subsequently remaining in these extensive waterways during their transition to freshwater with the Late Neogene trend of sea-level lowering. Keywords: Cetacea; fossil; phylogeny; Odontoceti; Miocene; epicontinental seas 1. INTRODUCTION Four genera of toothed cetaceans comprise the peculiar and poorly known 'river dolphins'. Although several marine delphinids are commonly found in rivers quite far upstream, river dolphins are morphologically and phylogenetically distinct from marine dolphins and most are restricted to freshwaterecosystems. Since the first suggestions of their affinities were advanced in the l9th century (Gray 1863; Flower 1867), the evolutionary relationship of river dolphins to one another and to other odontocetes has remained controversial (Simpson 1945; Kasuya 1973; Zhou 1982; Muizon 1984, 1988a; Fordyce & Barnes 1994; Messenger 1994; Rice 1998). Despite diXering in detail, recent morphological systematic studies of modern and fossil taxa (Muizon 1988a,c, 1994; Heyning 1989; Messenger & McGuire 1998) largely corroboratedearlier views that each extant lineage is relatively ancient and that river dolphins comprise an unnatural group. Nonmonophyly of river dolphins is consistent with their highly disjunct geographical distributions (figure 1): the Amazon river dolphin, Inia geoffrensis,and the La Plata river dolphin, Pontoporiablainvillei,are found in South America; the Yangtze river dolphin, Lipotesvexillifer,and Indian river dolphin, Platanistagangetica,inhabit rivers on opposite sides of continental Asia. Placing the four river dolphin lineages within the evolutionary tree of cetaceans can help resolve the confused state of odontocete beta taxonomy (Heyning 1989; Fordyce et al. 1985; Fordyce & Barnes 1994; figure 2) and refine our understanding of odontocete evolution. The difficulties of confronting river dolphin systematics using morphological analyses may relate directly to their (heals(socrates.berkeley.edu). *Authorforcorrespondence Proc.R. Soc.Lond.B (2001)268, 54>556 24 October2000 4 October2000 Accepted Received long, independent evolutionary histories. River dolphins are highly modified taxa that have more autapomorphies than shared characters useful for determining their affiliations (Messenger 1994). Furthermore,river dolphin classifications have often assumed monophyly (Simpson 1945; Kasuya 1973;Zhou 1982), although some characters used to unite river dolphins, such as an elongate rostrum and mandibular symphysis, may be primitive for odontocete cetaceans. When exisiting taxa are few and so distinctly modified that homologous characters are difficult to detect, the fossil record of the group should play an important role in resolving taxonomic relationships (Gauthier etal. 1988). There are various fossil taxa related to extant genera, Unfortunately,the record is with the exception of Lipotes. not yet complete enough to determine key character polarities at intermediate stages. The fossil history of river dolphins has a long and confusing treatment in the literature, with many fossils described as members of taxonomic groups no longer recognized; a comprehensive re-examination is needed. A robust hypothesis of the relationships among extant lineages is critical for exploring the biogeographical and evolutionary implications of river dolphin fossils. Higher-level molecular phylogenetic studies of cetaceans have primarily focused on the relationshipbetween cetaceans and artiodactyls (Graur & Higgins 1994; Montelgard et al. 1997) and on the hypothesis of odontocete paraphyly (Milinkovitch et al. 1993; Hasegawa et al. 1997; Messenger & McGuire 1998). River dolphins were discussed in Arnason & Gullberg's (1996) cytochrome b phylogeny of cetaceans, which provided additional evidence for a distinct, though unresolved, position for Two recent studies have specifically addressed Platanista. river dolphin phylogeny using DNA sequence analysis. Yang & Zhou (1999) were the first to include all four :549 This article is a U.S. government work, and is not subject to copyright in the United States. <) 2001 The Royal Society broadly approach river from dolphins haswithin been in every the to epicontinental sample primary both lineageextensively seas of ofodontocete. the Middle and 8 550 H. Hamilton and others Evolution ofriverdolphins With problematicphylogenies,for which odontocetes certainlyqualify,it maybe moreusefulto add taxa rather {mboldtlana than to add characters(Hillis 1996;Graybeal1998).Our :) F I [ < t } l r ( ensis Inia geoffrensisJ geoffrensif z^/ | : ,/- Miocene. / / Our objective is to reconstruct the evolutionary history of river dolphins.We begin by presentinga hypothesisof the phylogeneticrelationshipsof extant river dolphinsbased on a multiplemitochondrialgene phylogeny of 29 species of cetaceans. We consider biogeographicaland stratigraphicalaspectsof the fossil recordof river dolphinsin relationto our phylogenetic hypothesis.Integratingthe palaeontologicaldata with knowneventsinTertiarypalaeoceanography, we conclude with a detailedscenariofor the evolutionof the world's Pontoporiablainvillei - 2. MATERIAL ANDMETHODS Our data set is comprisedof the completecytochromeb (1140bp),partial 12S (385bp), and partial16S (530bp) mitochondrialgenes, for 29 speciesbroadlyrepresentative of each primarylineageof odontocete.In additionto sequencesavailable from previousstudiesof cetacean molecularsystematics (Milinkovitchet al. 1994;Arnason& Gullberg1996;LeDuc et al. 1999),we sequencedeitherthe ribosomalgene fragments and/orthe completecytochromeb for non-overlapping taxa. In all, we generated 44 new sequences (GenBank accession numbersAF334482-AF334525). We analysedsequencesof Inia of knownprovenancefromBrazil,Peruand Bolivia,as well as Inia from GenBank(accessionnumberX92534; Arnason & Gullberg1996), in order to evaluatethe suggestionthat the Bolivian form, Inia geoXrensis boliviensis, is distinct from Inia geoXrensis geoXrensis (da Silva 1994; Pilleri & Gihr 1977).The partial12SsequenceforLipotes vexillifer was not availableforthis analysis.The mysticeteoutgroupconsistsof four speciesfrom three families.The taxa in this study, with tissue source, scientificand commonnames,are listed at the archivedweb pagesof the Universityof CaliforniaMuseumof Paleontology (www.ucmp. berkeley.edu/archdata/ HamiltonetalOl/river. html), as are the primersequences,gene sequences,and dataset alignments. Figure1. Geographicaldistributionof extantriverdolphins. Samples were obtained either by biopsy darting, from (a) IniageoXrensis humboldtiana inhabitsthe OrinocoRiver museumspecimens,or fromthe GeneticsTissueArchive,Southsystem.I.g.geoXrensis is foundthroughoutthe mainstem west FisheriesScienceCenter,LaJolla, CA, USA. DNA was AmazonRiverand its tributaries.I.g.boliviensis occursin the extractedby standardphenol-chloroform/ethanol precipitation Amazontributariesof easternBolivia,geographicallyisolated or with the QIAamp DNA extraction kit (Qiagen, Inc., by severalhundredkilometresof rapids.Pontoporia blainvillei is restrictedto coastalSouthAtlanticwaters.(b) Lipotesvexillifer Valencia,CA, USA). After an initial 2 min denaturationat 94 °C, PCRconsistedof 35 cycles,30 s at 94 °C, 45 s at 48-52°C is an extremelyendangeredriverdolphinthat occursonly in the lowerand middlereachesof the YangtzeRiver.Platanista and 90s at 72°C. The productswere visualized,cleanedand minorinhabitsthe IndusRiversystem.P. gangetica is foundin directlysequencedin both directionson an ABI 377 automated the Ganges-Brahmaputra Riversystem. DNA sequencer(AppliedBiosystems,FosterCity, CA, USA). Sequenceswereeditedwith Sequencherv. 3.0 sequenceanalysis riverdolphintaxa in a molecularphylogeneticanalysis, software(GeneCodesCorporation,Ann Arbor,MI, USA) and but their limiteddata set of only 307 base pairs (bp) of aligned manuallyin BioEdit4.7.8 (Tom Hall). Four sites of the cytochromeb gene is insufficientto addressthe phylo- ambiguousalignmentin the 16Sgenewereexcluded. All phylogenetic analyseswerecarriedout usingPAUP4.0b3a geny of deeplydivergingtaxa. In contrast,the molecular phylogenyof Cassenset al. (2000) analysesfive genesfor (Swofford2000).Treesearcheswereconductedwith optimality 19 cetaceanspecies,both nuclearand mitochondrial,yet criteriaof parsimonyandmaximumlikelihood.Twentyreplicate even this largedata set resultsin low bootstrapvaluesfor searchesweremadefor the maximum-likelihood tree,assuming key nodesin riverdolphinphylogeny,particularlyunder the HKY85modelof nucleotideevolution(Hasegawaetal. 1985) (Ti:Tv)ratioof 6.0 anda gamma the maximum-likelihood criterionof molecularevolution. witha transitionto transversion - Proc.R. Soc.Lond.B (2001) - _ Pontoporiidae Delphinidae | _ | Monodontidae * Delphinidae Monodontidae Pontoporiidae H. Hamiltonandothers 551 ofriverdolphins Evolution morphologyof extanttaxa morphologywithfossil taxa (e) (c) (a) - [ molecularsequences | Physeteridae Platanistidae Physeteridae Ziphiidae Physeteridae Ziphiidae Squalodontidaet LX r-{platanistid Squalodelphidaet t Eurhinodelphidae Lipotidae Iniidae _ _ _ _ Platanistidae | _ Physeteridae SZiphudae r _ E . _ g 7 .. Imldae Pontoporiidae r | te Phocoenidae Phocoenidae Delphinidae Delphinidae Physeteridae (f) Ziphiidae l l g Pontoporiidae Monodontidae | Kentnodontidaet l Phocoenidae _ I = Mysticeti L Physeteridae Ziphiidae Platanistidae . Lipotidae Iniidae | Squalodelphidaet | '1 _ Platanistidae Pontoporiidae _ Iniidae Lipotidae _ Monodontidae Phocoenidae - Pontoporiidae -Monodontidae -Phocoenidae Delphinidae .b} Ziphiidae | | Lipotidae Iniidae Platanistidae Delphinoidea e u H g 1 -r Llpotldae Iniidae Figure2. Alternativehypothesesof odontocetephylogeny.Someendingshave been emendedto standardizetaxonomic comparisons.(a) Muizon(1988a,1991), (b) Barnes(1990);(c) Heyning(1989), (d) Messenger& McGuire(1998); (e) Arnason & Gullberg(1996), (G)Yang & Zhou (1999). shapeparameterof 0.2.The assumedratioof Ti:Tvandthe shape porpoises,monodontidsand moderndolphins,essentially rateswereestimatedunderthe Muizon'sconceptof the InfraorderDelphinida(Muizon of the distributionof substitution criterionof likelihoodusing trees obtainedby both neighbour 1988a, 1991).In both analyses,beakedwhales compose joining and unweightedparsimony.Parsimonysearches(with the sistergroupto Delphinida(Heyning1989). The data riverdolphinsare the extant 1000 replicates)were carriedout with a range of differential indicatethat non-platanistid of early lineagesthat divergedfrom the weightingto assess the impact of these correctionson tree representatives topology.Twobootstrapanalyseswereperformed,one with trees stem leading to Delphinoidea(porpoises,monodontids found by neighbourjoining (with Jukes-Cantor corrected and dolphins), supporting their ranking as separate distances)andonewithtreesobtainedusingweightedparsimony families. Our analysis suggestsInia and Pontoporiaare Finally, monophyleticand together form the sister group of countingsix timesas muchas transitions). (transversions Delphinoidea(Muizon 1984), and suggestsa distinction supportindiceswere calculatedfor each node presentin the weightedparsimonyanalysis(Bremer1988). betweenthe Bolivianand Amazonformsof Inia. The two 3. RESULTS The maximum-likelihoodtree and the consensusof three most parsimonioustrees are largely congruent (figure3). The Physeteridae,representedby Physeterand Kogia,are basalodontocetesand do not forma cladewith Ziphiidae,the beakedwhales,contradictingsomeclassifications (Fordyce1994;Muizon1991).The long-suspected polyphylyof riverdolphinsis supportedby the mitochondrial sequencedata. In both trees,Platanistagangeticaand Platanistaminor,representingPlatanistidae,are sister to the remainingodontocetes,althoughbootstrapsupport for this node is low. The remainingriver dolphintaxa arranged are paraphyletically (Litotes,Inia and Pontoporia) at the base of a well-supportedclade that also includes Proc.R. Soc.Lond.B (2001) analysesyield contradictinghypothesesfor the relationtree shipswithinDelphinoidea.The maximum-likelihood indicates that porpoises and marine dolphins form a clade, while the weighted parsimony tree groups porpoiseswith monodontids,a view recentlyadvanced (Waddellet al. 2000). 4. DISCUSSION The phylogenetic relationships of river dolphins suggestedbyouranalysisallowsfora refinedunderstanding of odontocetesystematicsand evolution,a long-elusive goal. Just as the extensiveadaptationsinvolvedin the transitionfrom land mammalto aquaticmammalhave obscured cetacean origins, each primary odontocete lineage exhibits a suite of highly derived characters 552 H. Hamiltonandothers Evolution ofriverdolphins (a) (b) Balaena mysticetus Eschrichtiusrobustus Balaenopteraphysalus Megapteranovaeangliae Physeter catodon Kogia breviceps Kogia simus Plal;anistagangetica Platanista minor Berardiusbairdii Tasmacetusshepardii Ziphiuscavirostris Mesoplodonbidens Mesoplodoneuropaeus Lipotes vexillifer Pontopona blvinvillei lnia geoffrensis boliviensis Inia geoffrensis-GenBanl Inia geoffrensis-Brazfl Inia geoffrensis-Peru Delphinapterusleucas Monodonmonoceros Neophocoenaphocoenoides Phocoena phocoena Lagenorhynchusalbirostris Sousa chinensis Steno bredanensis Lagenorhynchusobscurus Lissodelphisborealis Orca orca Pseudorca crassidens Figure3. Optimaltreesunderthe criteriaof (a) maximumlikelihoodand (b) parsimony.The maximum-likelihood treewas obtainedby carryingout 20 replicateheuristicsearches,assumingthe HKY85modelof nucleotideevolutionwith a transitionto transversionratioof 6.0 and a gammashapeparameterof 0.2. Bootstrapvalues (derivedfrom 1000replicatesof neighbourjoiningsearchesusingJukesWantorcorrecteddistances)are shownat the nodes.Valuesless than 50 are denotedby ' < '. The tree to the rightis the consensusof threemostparsimonioustreesof length5416 foundwith 1000replicateheuristicsearches. Transversions wereweightedsix timesas heavilyas transitions.Aboveeach node are parsimonybootstrapvalues ( 1000 replicates)and Bremersupportindices,separatedby a verticalbar. The rangeof transitionto transversionweighting(fromequal to ten times,as well as transversions only, denotedby an asterisk)that yieldseach cladeis reportedbeloweach corresponding node. The GenBankaccessionnumberfor 'Inia-GenBank' is X92534 (Arnason& Gullberg1996). without clear evidence of sequential forms. Thus alpha taxonomic assignments are considerablyless controversial than higher-level systematics. River dolphins provide an extreme example. Although the generic designations are not disputed, their taxonomic ranks are undecided, and many possible combinations of their interrelationship have been proposed (figure 2). Similarly, the phylogenetic affinities of the remaining odontocete lineages are also unresolved (Heyning 1989; Rice 1998). The placement of the river dolphins among these lineages, as indicated by our molecular analysis, suggests a resolution that is notably concordant with the first appearance of these groups in the fossil record (figure 4). (a) Thefossil record oSriver dolphins The fossil record of pelagic animals is understandably limited. Fossil cetaceans are primarily recovered from rocks that formed in nearshore and continental-shelf depositional environments, and only rarely from deep-sea Proc.R. Soc.Lond.B (2001) settings.Duringepisodesof low sea level, nearshoresediments are eroded, abridgingthe record.Archaicforms disappearand more advancedgroupsemergein successive waves with no clear origins. Many fossil cetaceans are known from single specimens,numeroustaxa have beenerectedon the basisof undiagnostic,isolatedor fragmentarybones, and the classificationhistoryof extinct cetaceansis long and bewildering.A confidentgrasp of modernphylogenywill help clarify the relationshipsof pastto presenttaxa. Extincttaxa assignedto the Platanistidae arewell documented, particularlyZarhachisand Pomatodelphis, longbeaked Middle to Late Miocene cetaceans recovered primarilyfromshallowepicontinentalsea depositsof the Atlanticcoastof NorthAmerica(Kellogg1959;Gottfried et al. 1994; Morgan 1994; table 1). Possibleplatanistid relativesareSqualodelphinidae andat leastsomemembers of Squalodontidae (Muizon1994;Fordyce1994),two wellknown, extinct families of archaic, medium-sized H. Hamilton and others 553 ofriverdolphins Evolution I ,1 111 111111111 fL Early | Late | Miocene Early | Middle I 1111111111 111111111 l Oligocene y L 0 5 10 15 20 25 30 35 Late | | Pliocene | g | |Early|Late | O | _ _ _ _ _ _ _ _ _ _ _ _ _ Mysticeti _ _ _ _ _ _ _ _ _ _ _ _ Physeteridae _____________Platanistidae _____________Ziphiidae Lipotidae ,, ,,,, _ _ _ _ _ _ _ _ Iniidae _ _ _ _ _ _ _ _ _ _ Pontoporiidae ,,,,,,,_ _ _ _ _ _ Delphiriidae I - - - - - - - 4 s it _________. * s nono ontloae _ _ _ _ _ _ _ _ _ Phocoenidae betweenthe hypothesizedphylogenyand fossilrecordof Odontoceti.Finerdottingindicates Figure4. Generalcorrespondence the uncertaindatesfor someearliestfossiloccurrences.Lipotidaeis the only cladefor whichfossilsare not yet definitivelyknown. heterodonts.Other fossil relativesof the Platanistidae include membersof the Dalpiaziniidae(Muizon 1994) and Waipatiidae(Fordyce1994, p. 147).If these lineages are monophyletic,then Platanista is the sole extant memberof a once-abundantand diversecladeof archaic odontocetes.The side-swimming,blindand highlyendangered Indian riverdolphinhas long been recognizedas 'thegenus. . . presentingthe greatesttotalof modifications known in any cetacean' (Miller 1923,p.41). Both fossil and extant platanistidswarrantfurtherinvestigationfor potentialinsightsinto cetaceanevolution. The assignmentof fossil taxa within non-platanistid river dolphins has been misdirected by inaccurate conceptsof the systematicrelationshipof extanttaxa. In most earlierclassifications,Inia and Lipoteswere placed in earlier togetherin Iniidae,whilePontoporia(Stenodelphis works)was sometimesclassifiedwithin Delphinidae,the marine dolphins(Miller 1923).For over a century,this conceptof Iniidaewas a repositoryfor earlydolphin-like fossilodontocetes(Kellogg1944;Rensberger1969;Wilson (Barnes1984, 1935).Withthe descriptionof Parapontoporia 1985),an extinctgenus consideredintermediatebetween Lipotes and Pontoporia,subsequentclassificationssometimes placed Lipotes in the Pontoporiidae(Fordyce& Barnes 1994). Systematicrevision and more rigorous diagnosisof fossiltaxa leave the majorityof generalized small odontocetes outside of Lipotidae, Iniidae and Pontoporiidae.The Lipotidaehave essentiallyno fossil record. A single mandibularfragmentfrom freshwater sedimentsin southernChina, known as Prolipotesand tentativelydatedas Miocene(Zhouet al. 1984),cannotbe confirmedas a Lipotid.Both Iniidaeand Pontoporiidae are representedby South American fossil relatives Proc.R. Soc.Lond.B (2001) (table 1).With the placement of most previously described 'iniids' in other extinct groups (Muizon 1988b; Cozzuol 1996), the family may be regarded as a freshwaterSouth American endemic. The partial skull, rostral and from the mandibular fragments known as Goniodelphis, Early Pliocene Palmetto Fauna of central Florida, are the only fossil remains outside South America that can be considered plausibly as Iniidae (Morgan 1994). However, Muizon (1988b) regarded this material as too incomplete for a confident determination. Significantly, both fossil and genera clearly assigned to Iniidae, Ischyrorhynchus Saurocetes,are found far south of Inids present range, occurring only in the fluvial Late Miocene Ituzaingo formation of the Parana basin, Argentina (with the possible exception of fragmentary mandibular remains reported from Brasil; Rancy et al. 1989). The Pontoporiidae have a broader geographical and geological have been described range. Three species of Parapontoporia from nearshore shallow water deposits of California and Baja California (Barnes 1985). The members in this Northern Hemisphere genus have been placed in their own subfamily, Parapontoporiinae,based on their asymmetrical cranial vertices. The subfamily Pontoporiinae, identified by symmetrical cranial vertices, is restricted to the Southern Hemisphere. Two fossil genera have been described from the Pisco formation of southern coastal Peru, the Pliocene Pliopontos,very similar to Pontoporia, and the geologically youngest occurrence of the family, (Muizon 1983, 1988c). the Middle Miocene Brachydelphis Another fossil, the Late Miocene Pontistes,is found in the Parana formation, marine sediments of the Parana basin, Argentina, underlying and adjacent to those with fossil iniids (Cozzuol 1985). 554 H. Hamilton and others Evolution ofriverdolphins Table 1. IdentiWication andstratigraphy offossil riverdolphins taxon location stratigraphy:formation/age Maryland Florida Calvert Formation/Middle Miocene Kellogg (1924); Gottfried etal. (1994) Agricola Fauna, Bone Valley/ Kellogg (1959); Morgan (1994) Middle Miocene Southern China Miocene (?) Zhou etal. (1984) coastal Peru coastal Peru Argentina California, Mexico Pisco Formation/Middle Miocene Pisco Formation/Early Pliocene Parana Formation/Late Miocene SanOiego/LatePliocene;Almejas/ Late Miocene Muizon (1988c) Muizon (1983), (1984) Cozzuol (1985), (1996) Barnes (1984), (1985) Ituzaingo Formation/Late Miocene ItuzaingoFormation/LateMiocene Palmetto Fauna, Bone Valley/ Late Miocene Cozzuol (1985), (1996) Cozzuol (1988), (1996) Morgan (1994) reference family Platanistidae Zarhachis Pomatodelphis family Lipotidae Prolipotes (?) family Pontoporiidae Brachydelphis Pliopontos Pontistes Parapontoporia family Iniidae Ischyrhorhynchus Argentina Saurocetes Argentina Goniodelphis (?) Florida (b) The evolution of river dolphins The Middle Miocene was a time of globally high sea levels, with three significantmarine trangressive-regressive cycles recorded worldwide (Haq et al. 1987). With the resulting large-scale marine transgressions on to lowlying regions of the continents, shallow epicontinental seas became prominent marine ecosystems. The IndoGangetic plain of the Indian subcontinent, the Amazon and Parana river basins of South America, and the Yangtze river basin of China are vast geomorphic systems whose fluvio-deltaic regions were penetrated deeply by marine waters during high sea-level stands. The shallow estuarine regions created by the mixing of riverine and marine waters probably supported diverse food resources, particularly for aquatic animals able to tolerate osmotic diffierences between fresh and saltwater systems. We propose that the ancestorsof the four extant river dolphin taxa were inhabitants of Miocene epicontinental seas. Draining of the epicontinental seas and reduction of the nearshore marine ecosystem occurred with a Late Miocene trend of sea-level regression, which continued throughout the Pliocene, interrupted by only moderate and relatively brief events of sea-level rise (Hallam 1992). As sea levels fell, these archaic odontocetes survived in river systems, while their marine relatives were superceeded by the radiation of Delphinoidea. Cassens et al. (2000) also noted the persistence of river dolphins during the radiation of delphinoids. They suggest that extant river dolphin lineages 'escapedextinction' by adaptation to their current riverine habitats. All extant organisms have escaped extinction by being adequately adapted to their presentcircumstances.By integratingphylogenetic, palaeoceanographicand fossil data, we provide an explicit hypothesis for the evolution and modern distribution of riverdolphins. The Indo-Gangetic foreland basin is a broad, flat plain of sediment delivered throughout the Cenozoic by an intricate network of migrating rivers descending from the tectonically dynamic Himalayan mountains (Burbank et al. 1996). The increased sea levels of the Middle Proc.R. Soc.Lond.B (2001) Miocene would have inundated large areas of the foreland basin, creating a shallow marine habitat. Fossils have not yet been recovered from these regions, but platanistids are known to have inhabited Miocene epicontinental seas in North America (table 1; Morgan 1994; Gottfried et al. 1994). Platanistais the only surviving descendant of an archaic odontocete that ventured into the epicontinental seas of the Indo-Gangetic basin, and remained through its transition to an extensive freshwater ecosystem during the Late Neogene trend of sea-level regression. Although the palaeogeography of the two river systems would suggest a history of isolation, the genetic distance we observed in our small sample of E gangetica and E minor is surprisinglylow (figure 3). Several lines of evidence suggest Miocene marine incursions penetrated deeply into continental South America (Hoorn et al. 1995; Lovejoy et al. 1998). To the north, incursions were along the course of the Amazon river palaeodrainage (Hoorn 1994), and to the south, into the Parana river basin (Cozzuol 1996). During the highest global stand of Miocene sea levels, the Parana and Amazon river basins may have been connected, forming an interior seaway that divided the continent, termed the ParanenseSea (Von Ihering 1927).The largely ignored hypothesis of the Paranense Sea is supported by sedimentological data (Rasanen et al. 1995) and biogeographical data from foraminifera (Boltovskyl991) and molluscs (Nuttall 1990). The existence of the Paranense Sea is consistent with the distribution of both modern and fossil South American river dolphin taxa. We hypothesize that the dolphins entered the seaway from the north, diversified within its complex fluvialestuarine-marine system, and colonized its farthest reaches, to the south-west Atlantic Ocean. Lowering of global sea levels drained the inland sea, separating the northern and southern river basins, and isolating the taxa. Iniid ancestors remained in the immense Amazon basin, which was developing its modern transcontinental aspect with the uplift of the Venezuelan Andes and clockwise rotation of its palaeodrainage (Hoorn et al. 1995).Inia Evolution ofriverdolphins H. Hamilton and others 555 their guidanceregardingtechnicaladvice.We thank D. Lindevolved during the Amazon's transformation to a freshberg andJ. Lippsfor accessto the facilitiesof the Molecular water system of extraordinary size, diversity and abunPhylogeneticsLaboratory,U.C. Berkeley.The detailed comdance. The Parana river basin is a fraction of the size of its northerncounterpart.The iniid fossil genera Ischyrorhynchus ments of two anonymousreviewersgreatly improved the manuscript,whichalsobenefitedfromthe thoughtfulcomments and Saurocetes) found along the banks of the Rio Parana, ofJ. W.Vatentine. This is UCMPpublication1733. belong to genera that disappeared with the retreat of the continental sea ecosystem. Pontoporia followed the marine REFERENCES waters receding from the Parana basin to colonize the nearshore coastal zone north and south of the La Plata Arnason,U. & Gullberg,A. 1996 Cytochromeb nucleotide estuary. sequencesand the identificationof five primarylineagesof extantcetaceans.Mol.Biol.Evol.13,407-417. Parts of eastern and southern China are low-lying Barnes,L. G. 1984Fossilodontocetesfromthe Almejasformadeltaic regions formed of sediments deposited by the tion, IslaCedros,Mexico.Paleobios 42, 1-46. area's river systems, such as the Yangtze and the Yujiang. Barnes,L. G. 1985Fossilpontoporiiddolphinsfromthe Pacific Significant sea-level rise would transform these regions Coastof NorthAmerica.Contrih Sci.Aat.Hist.Mus.LACounty into shallow waterways of mixed fluvial and marine 363, 1-34. origin. Several fossil locales in nearbyJapan confirm the Barnes,L. G. 1990Thefossilrecordandevolutionary relationship presence of odontocetes in the western Pacific during the of thegenusSursiops. InThebottlenose dolphin (ed.S. Leatherwood Miocene (Ichishima et al. 1995), potential colonizers of & R. Reeves),pp. 3-26. SanOiego,CA:AcademicPress. the Asian epicontinental seas. Our scenario is consistent Boltovsky,E. 1991Ihering'shypothesisin the light of foraminiwith the geographical occurrence of the mandibular fragferologicaldata.Lethaia24, 191-198. ment known as Prolipotes, Bremer,K. 1988 The limits of amino acid sequencedata in inland of the Yujiangriver delta angiosperm phylogenetic reconstruction. Evolution 42, 795-803. in southern China. If our phylogenetic interpretation is correct, then non-platanistid river dolphins are paraphy- Burbank,O.W., Beck,R. A. & Mulder,T. 1996The Himalayan forelandbasin. In Thetectonic evolution of Asia (ed. A.Yin & letic, and Lipotes, like Platanista, is the sole surviving M. Harrison), pp. 149188. Cambridge University Press. taxon of a deeply divergent branch in cetacean evolution. Cassens,I. (and 12 others) 2000 Independentadaptationto The ancestry of non-platanistid river dolphins might be riverinehabitatsallowed survivalof ancient lineages.Proc. found in the progenitorsof one of two well-known groups AatlAcad.Sci.USA97, 11343-11347. of fossil cetaceans. Eurhinodelphinids were long-beaked, Cozzuol,M. A. 1985The Odontocetiof the 'Mesopotamiense' of medium-sized odontocetes, sometimes encountered as the the ParanaRiverravines.Invest. Cetacea 17,39-54. dominant vertebrates in Miocene marine fossil formaCozzuol, M. A. 1988 Una nueva especie de Saurodelphis tions. In the TarkaroolooBasin of the Lake Frome region Burmeister, 1891.Anaeghiniana 25, 39-45. of Southern Australia, eurhinodelphinid fossils from Cozzuol, M. A. 1996 The record of aquatic mammals in southern South America. Munchner-Geowissenschaftliche several distinct horizons of the Middle Miocene Namba Abhandlungen 30, 321-342. formation record the adaptation of at least one member of this group to a freshwater environment (Fordyce 1983). da Silva,V. M. F. 1994 Aspectsof the biologyof the Amazon dolphinsgenusIniaand SotaliaJ2uviatilis. Ph.O.dissertation,St Kentriodontids were small to medium-sized odontocetes John's College, Cambridge, UK. that are probably basal delphinoids (Barnes 1990). Both Flower,W. H. 1867Descriptionof the skeletonof Iniageof)=rensis. groups were widespread, and both have a fossil record Wrans. zZool. Soc.Lond.6, 87-116. extending from the late Oligocene to the Late Miocene. Fordyce,R. E. 1983 Rhabdosteiddolphinsfrom the Middle Significantly, some fossil specimens now classified as Miocene, Lake FromeArea, South Australia.Alcheringa 7, either kentriodontids or eurhinodelphinids were first 27-40. described as iniids (Kellogg 1955; Rensberger 1969). Fordyce,R. E. 1994 Waipatia maerewhenua, new genusand new Neither eurhinodelphinidsnor kentriodontidsare likely to species,an archaiclate Oligocenedolphin.Proc.SanDiegoSoc. Nat.Hist.29,147-176. have given rise to non-platanistid river dolphins, as each group is diagnosed based on their distinctivemorphologies. Fordyce,R. E. & Barnes,L. G. 1994The evolutionaryhistoryof whalesanddolphins.A. Rev.EarthPlanet.Sci.22,419-455. Nevertheless, a small, long-beaked, polydont Oligocene Fordyce, R. E., Barnes,L. G. & Miyazaki,N. 1995 General ancestor of either extinct group is a plausibleprogenitorof aspectsof the evolutionaryhistoryof whales and dolphins. extant I)elphinida SensuMuizon). A re-evaluation of both IslandArc3, 373-391(for1994). Kentriodontidae and Eurhinodelphinidaein light of our Gauthier,J., Kluge,A. G. & Rowe,T. 1988Amniotephylogeny revised understanding of river dolphin phylogeny should andthe importanceof fossils.Cladistics 4, 105-209. provide further insights into the evolution of marine and Gottfried,M. O., Bohaska,O. J. & WhitmoreJr,F. C. 1994 freshwaterodontocetes. Miocenecetaceansof the Chesapeakegroup.Proc.SanDiego Soc.;Nat.Hist.29, 229-238. Graur,O. & Higgins,O. G. 1994 Molecularevidencefor the This researchwas supportedby the RemingtonKelloggFundof inclusionof cetaceanswithin the orderArtiodactyla.Mol. the Museumof Paleontology, tJniversityof California,Berkeley, Biol.Evol.11,357-364. and the InternationalFundforAnimalWelfare(bothto H.H.). of the cetaceans.Proc.zZool. A.G.C. is supportedby NSF grantEAR-9814845.Forgenerous Gray,J. E. 1863On the arrangement Soc.Lond.pp. 197-202. facilitationof access to tissue samples, we are indebted to B Curryand K. Robertsonat the SouthwestFisheriesScience Graybeal,A. 1998 Is it better to add taxa or charactersto a difficult phylogenetic problem? Syst.Biol. 47, 9-17. Center,LaJolla,California,to N. Bernaland L Villalbaof the Hallam, A. 1992 Phanerozoic sea-levelchanges.NewYork:Columbia National Museumof Natural History,La Paz, Bolivia, and to J. Mead at the SmithsonianInstitution,Washington,DC. University Press. R. LeDuc and E. Archerof the SouthwestFisheriesScience Haq, B. H., Hardenbol,J. & Vail, P. R. 1987 Chronology of flucCenterare gratefullyacknowledged for cyt b PCR primersand tuating sea levels since theTriassic. Science235, 1156-1162. Proc. R. SOG. Lond.B (2001) ofriverdolphins 556 H. Hamiltonandothers Evolution Hasegawa,M., Kishino, H. & Yano,T. 1985 Dating of the human-apesplittingby a molecularclock of mitochondrial DNA. 7. Mol.Evol.22, 160-174. Hasegawa,M., Adachi,J. & Milinkovitch,M. C. 1997Novel phylogenyof whalessupportedby totalmolecularevidence.7 Mol.Evol.44(Suppl.1),117-120. Heyning,J. E. 1989 Comparativefacial anatomyof beaked whales (Ziphiidae)and a systematicrevision among the Sci.Aat.Hist.Mus.LA familiesof extantOdontoceti.Contrib. County. 405, 1-64. Hillis, D. M. 1996 Inferringcomplexphylogenies.Aature383, 130-131. reconstruction of the palaeoHoorn,C. 1994An environmental Amazon River system (Middle-Late Miocene, NW Palaeoclim. Palaeoecol. 112,187-238. Amazonia).Palaeogeo. Hoorn, C., Guerrero,J., Sarmiento,G. & Lorente,M. 1995 Andeantectonicsas a causeforchangingdrainagepatternsin 23, 237-240. MiocenenorthernSouthAmerica.Geology Ichishima,H., Barnes,L. G., Fordyce,R. E., Kimura,M. & Bohaska,D. J. 1995 A review of kentriodontinedolphins: systematicsand biogeography.IslandArc 3, 486-492 (for 1994). Kasuya,T. 1973 Systematicconsiderationof Recent toothed whales based on the morphologyof the tympano-periotic Res.Inst.24, 87-108. bone.Sci.Rep.Whales Kellogg, A. R. 1924 A fossil porpoise from the Calvert Formation of Maryland.Proc.US. AatlMus.63, 1-14. Kellogg,A. R. 1928The historyof whales theiradaptationto life in the water.Q;Rev.Biol.3, 29-76, 174-208. Kellogg,A. R. 1944 Fossilcetaceansfromthe FloridaTeriary. Bull.Mus.Comp. Zool.94, 433-471. Kellogg,A. R. 1955ThreeMioceneporpoisesfromthe Calvert Cliffs,Maryland.Proc.US. AatlMus.105,101-154. inaeKellogg,A. R. 1959Descriptionof the skullof Pomatodelphis Z°°l121,1-26. qualis.Bull.Mus.Comp. LeDuc,R. G., Perrin,W. F. & Dizon, A. E. 1999Phylogenetic relationshipsamong the delphinidcetaceansbased on full cytochromeb sequences.Mar.Mamm.Sci.15,619-648. Lovejoy,N. R., Bermingham,E. & Martin,A. P. 1998 Marine 396, 421-422. incursionsinto SouthAmerica.Aature Messenger,S. 1994 Phylogeneticrelationshipsof platanistoid riverdolphins:assessingthe significanceof fossil taxa. Proc. SanDiegoSoc.Aat.Hist.29, 125-133. Messenger,S. & McGuire,J. 1998Morphology,molecules,and the phylogenetics of cetaceans.Syst.Biol.47,90-124. Milinkovitch,M. C., Orti, G. & Meyer,A. 1993Revisedphylogeny of whalessuggestedby mitochondrialribosomalDNA sequences.Aature 361,346-348. Milinkovitch,M. C., Meyer, A. & Powell, J. R. 1994 Phylogenyof all majorgroupsof cetaceansbased on DNA sequencesfrom 3 mitochondrialgenes. Mol. Biol. Evol.11, 939-948. Miller,G. S. 1923The telescopingof the cetaceanskull.Smith. Misc.Coll.76, 1-70. Montgelard, C., Catzeflis, F. M. & Douzery, E. 1997 Phylogeneticrelationshipsof artiodactylsand cetaceansas deducedfromthe comparisonof cytochromeb and 12SrRNA mitochondrial sequences.Mol.Biol.Evol.14,550-559. Morgan,G. S. 1994 Miocene and Pliocenemarinemammal faunasfrom the Bone Valley formationof central Florida. Proc.SanDiegoSoc.Aat.Hist.29, 239-268. Proc.R. Soc.Lond.B (2001) un noveauPlatanistidae littoralis, Muizon,C. de 1983Pliopontos Cetaceadu Pliocenede la cote peruvienne.C. R. Acad.Sci. Paris,296, 625-628. Muizon,C. de 1984Les vertebresfossilesde la FormationPisco (Perou)II. Les odontocetesdu Pliocene Inferieurde SudSacaco.Edit.Rech.SurlesCivil.Mem.50, 1-188. Muizon, C. de 1988a Les relations phylogenetiquesdes 74, 157-227. Delphinida.Anns.Paleon. Muizon, C. de 1988b Le polyphyletismedes Acrodelphidae, du Mioceneeuropeen.Bull.Mus.Aatl odontoceteslongirostres Hist.Aat.C 4 Ser.10,31-88. Muizon,C. de 1988cLesvertebresfossilesde la FormationPisco (Perou).III. Les odontocetesdu Miocene.Edit.Rech.Surles Civil.Mem.78, 1-244. Muizon,C. de 1991A new Ziphiidaefromthe EarlyMioceneof Washingtonstate and a phylogeneticanalysisof the major groupsof odontocetes.Bull. Mus.Aatl Hist. Aat. Paris34, 279-326. relatedto the plataMuizon,C. de 1994Are the squalodontids nistoids?Proc.SanDiegoSoc.Aat.Hist.29, 135-146. Nuttall, C. P. 1990 A review of the Tertiarynon-marine molluscanfaunasof the Pebasianand otherinlandbasinsof SouthAmerica.Bull.Br.Mus.Aat.Hist.Geol.45, northwestern 165-371. on theBolivian(IniaboliPilleri,G. & Gihr,M. 1977Observations viensisd'Orbigny,1834) and the AmazonianbuSeo (Inia de Blainville,1817)with descriptionof a new subgeofJrrensis Cetacea 8, 11-76. Invest. species(Iniageofjrrensishumboldtiana). J.,Pereirade SouzaFilho,J., Rancy,A., BoquetinVillanueva, Santos,J. c. R. & Negri, F. R. 1989 Lista preliminarda 26, 249. faunado Neogenoda regiaoriental.Ameghiniana Rasanen, M., Linna, A., Santos,J. & Negri, F. 1995 Late Miocene tidal deposits in the Amazonianforelandbasin. Science 269, 386-389. J.M. 1969A new iniidcetaceanfromthe Miocene Rensberger, Publ.Geol.Sci.82, 1-43. of California.Univ.Cal.it: oftheworld.SpecialPublication Rice, D.W. 1998Marinemammals no.4. Lawrence,KS: MarineMammalSociety. and a classiSimpson,G. G. 1945The principlesof classification ficationof mammals.Bull.Am.Mus.Aat.Hist.85, 1-350. analysisusingparsimony Phylogenetic Swofford,D. L. 2000 PAUP*: Sunderland,MA: SinauerAssociates. ( *andothermethods). Ozeans. Jena, desAtlantischen Von Ihering,H. 1927Die Geschichte Germany:GustavFisher. Waddell,V., Milinkovitch,M. C., Berube,M. & Stanhope, M. J. 2000 Molecular phylogeneticexaminationof the Evol.15,314-318. Mol.Phylogenet. Delphinoideatrichotomy. Wilson, L. E. 1935 Miocene marine mammals from the Mus.AatHistBull.4, BakersfieldRegion,California.Peabody 1-143. Yang,G. & Zhou,K. 1999A studyon the molecularphylogeny Sinica19,1-9. of riverdolphins.ActaTheriol. and phylogenyof the Superfamily Zhou, K. 1982Classification with notes on evidenceof the monophylyof Platanistoidea, 34, 93-108. Res.Inst.fiokyo the Cetacea.Sci.Rep.Whales Zhou, K., Zhou, M. & Zhao, Z. 1984 First discoveryof a Res.Inst. TertiaryplatanistoidfossilfromAsia. Sci.Rep.Whales 35, 173-181. As this paperexceedsthe maximumlengthnormallypermitted, the authorshaveagreedto contributeto productioncosts.