Vertebrate Palaeodistributional Patterns and Continental Drift Author(s): C. Barry Cox Source: Journal of Biogeography, Vol. 1, No. 2 (Jun., 1974), pp. 75-94 Published by: Blackwell Publishing Stable URL: http://www.jstor.org/stable/3037956 Accessed: 01/09/2010 19:51

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http://www.jstor.org Journalof Biogeography(1974) 1, 75-94

Vertebratepalaeodistributional patterns and continentaldrift

C. BARRY COX merica' (Asia plus westernNorth America) and Zoology Department,King's College,Strand, London 'Euramerica' (easternNorth Americaplus Europe). A barrierappears to have formedbetween Asia and westernNorth America in the UppermostCretaceous. Bothmarsupials and condylarthplacental mammals Abstract appear to have crossedbetween Laurasia and Gond- wanaland in the Upper Cretaceous; their route is The patternsof distributionof vertebrates(primarily uncertain,but a trans-Caribbeanisland sweepstakes terrestrialforms) are analysed from the Silurian route seems the most likely. Marsupials probably onwards,using palaeogeographical maps whichshow enteredAustralia from South America via Antarctica, epicontinentalseas as well as intercontinentaloceans. placentalsevolving later and being unable to follow Silurian vertebrates(fish) are known almost thembecause theSouth America-Antarcticalink had exclusivelyfrom Euramerica. Many Devonian fish by thenbroken. whichare normallyfound in fresh waters seem to have The apparent areas of origin of the different been able to cross interveningseas between one mammalianorders are shownin Venndiagrams. The continentand another.It is suggestedthat this ability historyof the faunal relationshipsof the mammal maybe physiologicallyrelated to theircapacity to use faunas of thedifferent land masses during the Tertiary aerial respiration.There is some evidencefor a is outlined. separate Devonian osteostracanfish fauna in China. Amphibiansare first knownfrom the uppermost Devonian of the Euramericancontinent, and land Introduction vertebratesare knownalmost exclusivelyfrom that continentuntil the Mid Permian.It is suggestedthat Much of biogeographyis at presentin a state of tetrapodsmay have evolvedin Euramerica,and were adjustmentto the new frameworkof plate tectonics. only able to colonize Asia and Gondwanalandafter Geophysicistshave almostliterally taken the ground continentaldrift had linked these areas with Eur- from under old theories. Much biogeographical america, causing the Uralian and Alleghanian interpretationin even comparativelyrecent literature orogenies. has tended to accept the old hypotheses,based on The tetrapodfauna of the Upper Permian and static geography,and to seek new justificationsfor Triassic appears to have been cosmopolitan.Though them in the shiftingpatterns of plate tectonics.A Pangaea started to break up in the Jurassic,land fresh start should instead be made, plotting the vertebrateswere still able to dispersebetween Eur- distributionalpatterns on palaeogeographicalassem- americaand Asia (probablyvia theBering region) and blages and tryingto see whatthe implications are for between the Northernand Southern Hemispheres intercontinentalbiological connections.These new (probablyvia southernEurope). ideas formthe rationalebehind thispaper. In many Thereis littleevidence that dinosaurs were able to cases, palaeogeographicaldata are now morefirmly cross the Cretaceous Tethyansea barrierbetween establishedthan the data of palaeodistributions,and Laurasia and Gondwanaland. Lower Cretaceous should be used to generate-questions to ask the dinosaurs were able to disperse throughoutthe palaeobiological world. In some cases, however, NorthernHemisphere, presumably by a continuation distributionaldata are still the betterknown, and of theBering link. In the UpperCretaceous, the Mid- may help to indicatethe most likelysolution for the ContinentalSeaway of NorthAmerica, together with palaeogeographers.Of course, where there is an theTurgai Straits between Europe and Asia, separated apparentinconsistency between the implicationsof two land areas withdistinct dinosaur faunas: 'Asia- the two sets of data, both must be regarded as 6 75 76 C. BarryCox meriting reconsideration until an interpretation data of thisperiod indicatethe approximatepalaeo- acceptableto both disciplineshas been reached. latitudeof each fragment,it should be notedthat the There are many factors which can bias our palaeolongitudinal(east-west) distances between the knowledgeof the fossil record. Obviously enough, fragmentsare unknown.For a moredetailed discuss- fossilscan only be foundin a particulararea if they ion ofthese points, see Smith,Briden & Drewry,1973. were deposited in sediments laid down in the The maps presentedherein extend over 270? of appropriate environment,which are themselves longitude. Where they lie within a reassembled revealed by erosion or earth movement,in an area supercontinent,or at some distancefrom the edge of thatis physicallyaccessible and is visitedby a suffi- the continentalshelf, present day coastlines are cientlycurious or trainedobserver or collector.The shown as interruptedlines. Details of the shallow, absence of fossilsfrom a particularstretch of geo- epicontinentalseas have been added, mainly after logical time,from a particulargeographic area or Kummel (1970), as thesewere almost as effectiveas fromdeposits of a particularcharacter, may be due oceans in actingas barriersto thespread of terrestrial to the absence of any of thesenecessary conditions. organisms.Though the extentof theseseas, and the Such an explanation is probably often,even nor- width of the oceans, will have varied somewhat mally, correct.Nevertheless, this should not blind duringthe period of timecovered by each map, the us to the possibilitythat a taxon may not have been basic biogeographicalpatterns will not have been found in a particulararea because, quite simply,it affected.The locations of the major sites at which neverexisted there. vertebrateshave been found have also been added, Our new understandingof palaeogeographyalso to givesome idea of theextent of thefactual basis for reveals new possible anomalies. As will be seen statementswhich relate to biogeographicalpatterns later, the presence of a new taxon in a particular of distribution. continenthas in some cases been suggestedon the basis of somewhat unconvincingevidence, which does not includethe critical diagnostic features of the Silurian-Devonianpatterns (Fig. 1) taxon. One's doubts of thesesuggestions are streng- thenedif it now appears thatthey are the only basis In the Palaeozoic period, Gondwanaland seems to forbelieving the taxon to be presentnot merelyin a have had a historyquite differentfrom that of the single modern continent,but in an entire super- restof the world (Briden,1973). It seems alreadyto continentsuch as Gondwanaland. have been complete in the Ordovician, as it is Though ideally one should take into account all traversedby no orogenic belts youngerthan 500 the organicdistributions of any givenperiod of time million years. In contrast,North America,Europe when tryingto evaluate patternsof life,the record and Siberia were all separatefrom one another,and of the vertebratesis comparativelywell known,and the Palaeozoic whereabouts of China are still in particularthe normallyterrestrial mode of life uncertain(Smith, Briden & Drewry,1973). of tetrapodsprovides a good test of land connec- That NorthAmerica, Europe and Asia wereall in tions; accordingly,it is the palaeodistributional relativemotion duringthe Palaeozoic is suggested patternsof these groups from the Silurian period not only by the fact that theydid meet in the late onwards which are to be examined in the present Palaeozoic, but also by the orogenicactivity along study. Maps prepared for a 1971 Symposium in their margins. The fusion of the Canadian and Cambridge, England (Smith, Briden & Drewry, Baltic Shields took place in the Upper Silurian to 1973) have been used as a palaeogeographicbasis for Mid Devonian,judging by thedate of theCaledonian the study,with a few minormodifications noted in and Acadian orogenies which signal that meeting thelegends. For the Mesozoic period,these maps are (McKerrow & Ziegler,1972). Partlybecause of this based upon data from three independentlines of erosional-sedimentaryactivity in theNorth American enquiry-palaeomagnetism,sea floorspreading and and European plates,partly because of theirpalaeo- computer fittingof continentalshelves. For the equatorial position, and partly because of the Palaeozoic, beforePangaea had formed,the maps intensityof palaeontological investigationin these were produced by dividing Pangaea along pre- areas, our knowledge of Ordovician and Silurian Mesozoic orogenic belts and using palaeomagnetic vertebrateevolution, in the form of fish groups, is data alone to place each resultingfragment in an almost entirelyconfined to these two plates. The appropriateorientation. Though the palaeomagnetic acanthodian Nostolepis and undescribed hetero- Vertebratepalaeodistributions and continentaldrift 77 . '' ' " ------"". ....,.:-,.,-,.,.,-"...... '' ' ...... 1- ...... I...... 1.1.1.1.1. -.-.-.....-.-.-.....-...-.-...... 70,'N :.--'.-'..."-----""''--.''--"--'--'"--'-.... ' ' -...... : I.I.I.I... .-.11.. . _-1'-1-_-I_-i: , .... . "-l'-,-'-.I"..,.,..,.,.".,-...... ::-..::____--_-__----_I- ...... :-...... '.....,x...... 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stracansare knownfrom northwest Africa, but this groupswhich appear to be limitedto one environ- area may have been partof the NorthAmerican mentor the other,and whichmight therefore be plateuntil after closure of the proto-Atlantic and its expectedto have quitedifferent patterns of distri- Jurassicreopening. The onlyother exceptions are bution.But in practiceit is difficultto distinguish specimensof the marinethelodont Logania from preciselysuch different patterns, and thisis equally Siberia(Halstead & Turner,1973), and thepossibly truefor living fish. For example,Myers (1938) found UpperSilurian Kiangsuaspis from China. itnecessary to recognize three different 'divisions' of A morecomplete record of fishevolution com- freshwater fish: a primarydivision which includes mencesin theLower Devonian. Though Devonian thosestrictly confined to freshwater; a secondary fishare still known in greaternumbers and variety in divisionwith those that have a littlesalt water Euramericathan anywhere else, they are also known tolerance,and a peripheraldivision, with those that from Siberia, China and Gondwanaland.Most arevery tolerant of salt water. majorgroups (dipnoans, coelacanths, rhipidistians, It is thusclearly impossible to be entirelycertain heterostracans,arthrodires and antiarchs)are found ofthe environmental tolerances of fossil fishes. Many in bothmarine and freshwaterdeposits. It is often of theapparently fresh water forms may have been possibleto distinguish,at lowertaxonomic levels, tolerantof sea water,and thereforeable to colonize 78 C. BarryCox otherriver systems, either on theiroriginal tectonic Euramericawas stillisolated. Its fusionwith Siberia, plate or on anotherplate. In the case of the great marked by the Uralian orogeny,took place in Mid continentof Gondwanaland in particular,extension Permiantimes (McKerrow & Ziegler,1972). As will of theranges of such fishmight otherwise have been be seen later, the fusion betweenEuramerica and very slow. Some known examples support this Gondwanaland may also have taken place at this generalization. The Upper Devonian antiarch time. Figure 2 is thereforeintended to show the Bothriolepisis foundin fluviatilefresh water deposits Palaeozoic world afterland vertebrateshad evolved in Euramerica, Siberia, China, North Africa, but beforePangaea had formed. Antarcticaand centralAustralia. The inferencethat The existenceof these separatecontinents is fully it could cross interveningstretches of sea is con- supportedby referenceto the Upper Carboniferous firmedby its presencein the marineinter reef facies and Lower Permianplant distributions(Chaloner & Gogo Formationof northwesternAustralia (Gardi- Lacey, 1973), in whichthe restriction of the Glossop- ner & Miles, 1974). This fauna also providesanother terisflora to Gondwanaland,and of theEuramerican example, for it includes specimens of rhyncho- florato thatcontinent, is well known.*However, it dipteridlungfish, which are otherwiseknown only does not appear to have been noted thusfar that the fromfresh water sedimentsin Euramerica.It may Angaranflora is restrictedto Asia untilthe Uralian be relevantto note herethat, as Thomson (1969) has orogeny.It is also likelythat the differentiationof pointedout, livingfish which at any timeuse aerial the Cathaysianflora of China reflectsan isolation respiration exclusively must excrete their nitro- from Asia imposed by interveningoceans, or epi- genous waste in the formof urea. Since Devonian continentalseas. lungfishaestivation burrows are known, these fish Furtherevidence for this may be derivedfrom a were probably(like theirliving descendants Polyp- studyof the contemporarydistributions of amphi- terus and Lepidosiren)able to retain urea at least bians, which are firstknown from the Devonian/ seasonally. By its retentionin the body fluid to Carboniferous boundary. Since even partially increase the osmotic pressureof these fluids,urea terrestrialforms of lifeare much more reliable than can also be used forosmoregulation in the sea, as in purelyaquatic formsas indicatorsof separate land living chondrichthyans,the coelacanth Latimeria masses-potentiallyseparate faunal regions-fish are and the anuran Rana cancrivora.Retention of urea thereforeexcluded from further consideration in this forexcretory purposes may thereforehave provided study. the startingpoint for an evolutionarychange which The biogeographical standards for judging the allowed the Devonian lung bearingfish (for Both- typeof physicaland biologicalrelationships existing riolepis,too, had lungs) to survivein the sea and betweentwo faunal regionswas explainedlong ago even perhapsto colonize it. by Simpson (1940), who distinguishedthree types Currentinformation also indicatesthat thereare of relationship.First, two areas may share a con- a few cases in which contemporarydistributional siderableproportion of theirfaunal elements,which patternsdo seem to conformto the patternof late appear to representa wide range of environmental Palaeozoic continents.Though the earliestanaspid, preferences.This would imply that the two areas Jamoytius,appears to have been marine (Ritchie, were joined by an ecologically wide 'corridor' 1968), all later anaspids were freshwaterand have incorporatinga commensuratevariety of environ- been foundonly on the Euramericanplate. There is ments,such as the latitudinallywide link between also a hint of the existenceof a distinctendemic Europe and Asia today. Secondly,the faunas of the faunain China, as the galeaspidosteostracans which two areas may share some taxa, but each may lack Liu (1965, p. 131) describesas 'very peculiar' have taxa foundin a particularrange of environmentsin been found only in that area. It seems significant the otherarea. This would implythat the two areas that these distinctiveforms have not as yet been were joined by an ecologicallyrestricted 'filter', so foundin themuch better known faunas of Euramerica that animals unable to toleratethis restrictedrange or Australia. of habitatswould be unable to pass throughfrom one area to theother, just as thetropical Panamanian land bridge today denies passage to the temperate Carboniferous-LowerPermian patterns (Fig. 2) faunal elementsof the two continentsthat it links. * In the Devonian and Lower Carboniferousthere seems, At the beginning of the Carboniferous period, puzzingly,to be a single,world-wide flora. Vertebratepalaeodistributions and continentaldrift 79

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Thirdly,two areas may have faunas which are in both the North American and the European parts mostrespects different from one another,but do also of Euramerica.However, there are differencesin the share a fewtaxa. This suggeststhat there is a major fresh water aquatic tetrapods in which, although barrierto dispersalbetween the two regionsbut that, sevenfamilies are foundin both areas, anotherseven overlong periodsof geologicaltime, the passage of a are confinedto NorthAmerica and threeto Europe. few organismsdoes occur by chance. This type of Milner & Panchen (1973) commentthat theremust faunal relationship,known as a 'sweepstakes'route have been a partial barrierto the spread of fresh or 'waifdispersal', is exemplifiedtoday by thefaunal water forms,but not to that of terrestrialforms. compositionof such isolatedislands as theHawaiian They suggestthat this mightwell have been caused group. by the Caledonian chain of mountainsthat marks An excellentexample of the consequencesof the the Devonian meetingof the North American and existenceof an ecological filteris provided in the European plates. distributionof patternsof Upper Carboniferousand Studies of the Carboniferoustetrapods also serve Lower Permian tetrapods, studied by Milner & to illustrateanother aspect of the approach to Panchen (1973). Terrestrialtetrapods are similarin palaeodistributionalpatterns, as based -on palaeo- 80 C. BarryCox

geographic reconstructions.Like the bulk of the position of the two continents.From Northern Devonian fish,Carboniferous tetrapods have been Brazil,Price in 1945 foundthe remains of an aquatic foundonly in Euramerica(Panchen, 1973). Of course, amphibian, a long-snoutedfish-eating archegosaur this area also includes the Greenland sites of the (Price, 1948). The best evidence for tetrapods in earliestknown amphibians (from the Upper Devon- Gondwanaland before the Upper Permian comes ian/Carboniferousboundary as previouslynoted) fromthe Permo-Carboniferousbeds of the Kashmir and Panchen has suggestedthat the Amphibia may region of northernIndia. Several amphibianshave actuallyhave evolved in this area. This picturehas been foundhere: aquatic archegosaurs(Woodward, sincebecome slightlymore complicated with the dis- 1905; Tewari, 1962), an actinodont (Wadia & covery of apparent tetrapod tracks in the Upper Swinton, 1928; Branson, 1935) and an eryopoid Devonian of Australia(Warren & Wakefield,1972). (Verma, 1962). Finally,though Romer (1973) refers Nevertheless,Panchen's inference is supportedby the to a captorhinidbone fromTasmania, the identifi- Permian patternsof distiibution,as we shall see. cation of this is uncertainand it is in any case of Furthermore, terrestrialamphibians may have Lower Triassicage (Cox, 1973d). evolved in response to a combinationof predator It has been usual to ascribe the lack of tetrapods pressurein thewater and of unexploitedinvertebrate fromAsia and Gondwanalandmerely to thefact that food on land (Schmalhausen,1957; Cox, 1967). If so, less work has been carried out in these areas, and the near equatorial positionof Euramerica,and the fewer continentaldeposits have been found. This resulting lush Carboniferous coal swamps of would be an acceptable explanation if there were southern Euramerica, would have provided an only a differenceof degree,the tetrapodfaunas of environmentin whichthere could well have been an these areas being merely poorer than those of adequate food supply for the emergenceof these Euramerica. But this is not the case. With the amphibians. exceptionsnoted above, Carboniferousand Lower Like the Carboniferoustetrapods, the amphibians Permiantetrapods are unknownfrom three-quarters and reptiles of the Lower Permian are almost of the land surfaceof the world. One cannot even entirely restricted to Euramerica. This Lower say thatthese areas have lacked competentpalaeon- Permianfauna is known best fromthe Red Beds of tological collectors. Such skilled workers as Texas, from which thirteenfamilies and forty-five Llewellyn Price in Brazil and James Kitching in generaof reptileshave been recorded(Romer, 1973). South Africa have spent their lifetimescollecting The European recordis certainlypoorer, containing fossil vertebrates. Romer has taken at least one onlyfive families and eightgenera. But theseinclude major expeditionto Argentinaspecifically to look a fair cross section of the North Americanforms, forvertebrates in the PermianPaganzo Beds, but to from little captorhinidsand araeoscelids to large no avail. edaphosaurs(the genusEdaphosaurus is knownfrom This explanationwas moreovercertainly credible both areas). It thereforeseems likely that the in the days before continentaldrift was accepted. differencebetween the two faunas is only due to the Since Europe is adjacent to Asia and has had inter- fact that the North American strata are unusually mittentcontacts with Africa during the Cenozoic fossiliferousand well prospected. and Mesozoic, and theserelationships were thought A surprisingfact is that these amphibians and to have been permanent,it was also thoughtto be reptiles of the Upper Carboniferousand Lower 'obvious' that the fauna of Europe must have been Permian are almost unknownfrom other parts of able to penetratethese adjacent continentsover the the world, such as the continentof Asia, then millions of years of the Permo-Carboniferous- approachingEurope fromthe northeast, or the vast especiallyas it seemed as thoughit had managed to southernsupercontinent of Gondwanaland.There is cross an ocean to or fromNorth America. But there in fact one lone recordfrom Asia, and a few from now seems littledoubt that Asia was separatefrom Gondwanaland. The Asian record is of a small, Euramericauntil the mid-Permian (Hamilton, 1970). apparently larval aquatic amphibian, Tungusso- It is surelynot merelycoincidence that it was only gyrinus(wrongly regarded as Lower Triassic in my afterthat event that tetrapods first appeared in Asia 1973a paper), of uncertain affinities.The small -therocephalians in centralAsia and pareiasaursin Upper Carboniferousaquatic reptile Mesosaurus, China. fromboth Brazil and South Africa,was for long a The timeof themeeting of Euramericaand Gond- tantalizinglyinconclusive suggestion of a juxta- wanaland is moredifficult to establish.Certainly, the Vertebratepalaeodistributions and continentaldrift 81

date of theAlleghanian orogeny, which resulted from stakes or filterroute connectionbetween the two the approach and meetingof thesetwo continentsis continents,one would expect that the immigrants the least well established of all the Appalachian to Gondwanaland would, in more or less complete orogenies(Rodgers, 1967, p. 419). It is knownto have isolationthere, have evolvedinto an endemicfauna, begun in the Upper Carboniferous(McKerrow & differentin manyways from that found in theNorth. Ziegler,1972; Burrett,1972), and to have continued What we in factfind, in the Upper Permianof South until some time afterthe Lower Permian(Rodgers, Africa,is quite different.It is a fauna whose roots 1967). But the early date of the beginningof this can be traced,with few gaps of lineage,through the orogenydoes not implythat the two continentsmet earlierMid Permianand Lower Permianfaunas of at this time,for a contractingocean betweenthem Euramerica.Romer says (1973, p. 164) 'As between must have contained a subduction trench.If this (European) Russia and South Africa,however, the trenchwas adjacent to Euramerica,an Andean type situationis clear. The two areas show faunas closely orogenywould have commencedthere long before related to one another (the Africanone somewhat collisionof the continentscaused a Himalayan type later). . .'. The index of faunal similarityof Simpson orogeny. (C/C1x 100, where C is the number of taxa in It is indeed temptingto suggestthat there is a common, and Ci is the number of taxa in the connectionbetween the date of thiscollision and the smallerof the two faunas) betweenthe two areas is factthat tetrapods do not appear in Gondwanaland 70 %, despitetheir great difference in palaeolatitude. untilthe Upper Permian,or in otherwords that,as Even the differencesbetween the two areas are not in the case of Asia, land tetrapodswere unable to due to the presenceof quite differentgroups in the cross fromEuramerica to Gondwanaland until the two areas. Thus the reptile fauna of European process of continentaldrift had united these two Russia containsseven suborders,and that of South continents.Following their arrival in Gondwanaland, Africaeight, but six of theseare commonto the two theythen appeared in vast numbersand variety,for areas. The two unique South Africanforms (mille- Romer (1973) listssixteen families and seventy-seven rettidsand bauriamorphs)are merelylines which genera of reptile from the early Upper Permian evolved within the Upper Permian of that TapinocephalusZone of South Africa.These reptiles continent. are mainly mammal like reptiles. Their sudden Finally,as we shall see, the picturein thesucceed- evolutionaryradiation might,under this suggested ing Triassic Period is completelydifferent, with hypothesis,be the resultof the sudden accessibility extensivesimilarities between the faunas of all of to themof new and extensiveareas forcolonization, today's continents.The Gondwanaland record is involvingmuch greaterranges of palaeolatitudeand richer,not poorer, than that of Laurasia: out of new floral regions, for the Euramerican plate fifty-sevenfamilies of wholly terrestrialtetrapod, extended to no more than c4O0N palaeolatitude, thirty-nineare presentin Laurasia and forty-sixin whereasAsia and Gondwanaland extendedto their Gondwanaland. Are we reallyto believe that these respectivegeographical poles. The newlyaccessible differencesbetween the Permian and the Triassic regionsof Asia and Gondwanaland also contained recordsin Gondwanaland are simplybecause there the distinctiveAngaran, Cathaysian and Glossopteris was, by chance, so great a differencebetween the floras,which must also have increased the possi- areas of depositionof fossiliferousrocks of appro- bilitiesof ecological diversificationopen to the early priateage, or of thediscovery of fossils in theserocks, land vertebrates.If my hypothesisis correct,then in the 50 million years of the Lower and Middle Irving & Brown's (1964) discoverythat the distri- Permianas comparedwith the 35 millionyears of the butionof Upper Carboniferousand Lower Permian Triassic? amphibians is closely related to the palaeoe- Though, as suggestedabove, it is possible that quator may be merelya resultof theirgeographical continentaldrift was thefactor that eventually made opportunities rather than the provision of it possible for tetrapodsto reach Gondwanaland,it an insightinto their physiologicalpreferences, or is also possible that they were excluded by other tolerances. factors, such as unknown epicontinentalseas or There seems,at least, littledoubt that the Upper desertregions. Whatever the reason may be, there Permiantetrapods of Gondwanaland are the results seems at least a primafacie case for the possibility of a full meetingbetween that continentand Eur- that tetrapodsdid not colonize the major part of america. If, instead,there had only been a sweep- Gondwanaland until the Upper Permian. 82 C. BarryCox

Upper Permian-Triassicpatterns (Fig. 3) major geographical barriersbetween these areas, but also thatthere were no effectiveclimatic barriers These are the periods of radiation of (mainly separatingthem. mammal like) reptileswithin the single world wide It is difficulteven to substantiatea faunaldifference continentalmass of Pangaea, undivided by epi- betweenLaurasia and Gondwanaland. Taking the continentalseas, major mountainchains or deserts. Triassicas a whole,thirty-three of the sixtyfamilies The Upper Permiantetrapod faunas, known best of terrestrialtetrapod are knownin both areas, and fromSouth Africa,have been recordedalso in Asia, theremainder are mostlyeither rare forms, found in China, India and Euramerica(Romer, 1973). only one or two adjacent localities,or formsof an For the Triassic, I have already published(Cox, age representedby fossiliferousstrata in only one of 1973a, b) a detailed analysis of the distributionof the two super continents.If this latter effectis terrestrialvertebrate faunas, and shown that the reduced by taking a shortertime span, then for fauna is a world wide one (Fig. 4). These Triassic examplein the Upper Triassicthere are twenty-eight coefficientsare very similar to those given by families of terrestrialtetrapod in Laurasia, and McKenna (1973) for familiesof livingmammal in twenty-fivein Gondwanaland,of whichnineteen are the states of Oregon and New York. The surprising common to the two areas-a coefficientof faunal aspect of the Triassic figuresis thereforehow high similarityof 76 %. they are, even for areas then thousands of miles The Triassic is probably the simplest of all apart,such as Europe and India, or Asia and Africa. geologicalperiods to interpretbiogeographically, for The clear implicationis that not onlywere thereno thereis positiveevidence linking all of today's con-

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Vertebratepalaeodistributions and continentaldrift 83

Europe (41) Asia (18-5) South Africa (42 5) India (16) America(19)

North 87 5 % 44% 56% 75% 59% America (16)

Europe 80% 74 % 59% 81%

Asia 51% 89% 41%

South America 74% 56%

Africa 7 5 %

Fig. 4. Coefficientsof faunalsimilarity at familylevel between the Triassicfaunas of today'scontinents, after Cox (1973a, b), amendedto includethe existence of traversodontcynodonts in EuropeanRussia (Tatarinov,1973) and Asia (Young, 1974).The numbershown in bracketsafter each continentshows the number of terrestrialTriassic families found in thatcontinent; where thepresence of a familyis not yetfully confirmed, a score of 0 5 has been added.

tinents. The distributionpatterns of vertebrates faunas. This resemblancedoes not end merelyat become more difficultto understandin theJurassic familylevel, but extendsas far as genera: Bothrio- and Cretaceous,for threereasons. First,the super- spondylus,Brachiosaurus and Barosaurusare genera continentsbegan to breakup withthe appearanceof commonto both faunas (Charig, 1973). This degree spreadingridges; it is not always possible,however, of similarityimplies a land route round at least one to be sure exactly when each of these spreading end of the expandingCentral Atlantic.This expan- movements commenced. Secondly, there remain sion was by movementof Gondwanalandaway from uncertaintiesas to the exact relationshipsbetween NorthAmerica, which must therefore have separated land masses in particular,crucial areas, as we shall from both Africa and South America. The route see-e.g. North America in respect of South from North America to Africa was therefore America, South America in respect of Antarctica, probablyvia Europe. The movementsof the differ- and NorthAmerica in respectof Asia. Thirdly,even ent small areas which caused the Alpine System when two areas lay on the same tectonicplate, the orogenyare so complex(see Dewey et al., 1973) that spread of shallow epicontinentalseas in the Jurassic it is impossibleto establishcoastlines in this area, and Cretaceous may have made it impossiblefor and one can only presume that a land bridge animals to travel between them. For example, a existedfor a briefperiod or periodsin the Jurassic, shallow Turgai Sea separated Europe from Asia as shown in Fig. 5, permittingthe passage of these fromthe Mid Jurassicuntil the Middle Eocene. dinosaurs. The relationshipsbetween the dinosaurfaunas of these differentareas are shown in Fig. 6, the data Jurassicpatterns (Fig. 5) being derivedfrom Charig (1973), omittingmono- generic groups known from only one area. Only The Central Atlantic,between North America and three Jurassicfamilies (the coelurids,megalosaurs Africa,began to open in the Lower Jurassic(Dewey and stegosaurs)are foundin all threeareas. Though et al., 1973). Though this part of the connection most of the Euramerican groups are known in between Laurasia and Gondwanaland became Gondwanaland, only half are known in Asia, impassable,there must have been some connection mainly because the four Euramerican sauropod forland vertebratesbetween the two. This is clearly subfamiliesare unknownthere. Much of the poverty shownby thegreat similarity of theirdinosaur faunas of the Asian Jurassicdinosaur fauna is probably in theUpper Jurassic,best known from the Morrison spurious, being due to lack of knowledge, but Formation of the U.S.A. and from Tendaguru in Jurassicpalaeogeography does suggestthat any route Tanzania (Charig, 1971; Colbert, 1973). Hypsilo- betweenAsia and Euramericawould have involveda phodonts and the large sauropods (cetiosaurs, filterregion. An occasional trans-Turgaiconnection brachiosaursand diplodocines) are found in both would have provided one such route. The other 84 C. Barry Cox

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possibilityis dispersalvia Alaska and Siberia. The latitudes. It would thereforeseem that both the boundary between the Euramericanplate and the climatesand the florasof theseregions would have Asian plate appears to have lain in the Cherskiy provideda migrationroute for dinosaurs, which were regionof northeastern Siberia (Churkin,1972), and themselvesapparently warm blooded (Russell, 1965). the northwestward movement of the Euramerican Finally,as we shall see, thereis also some evidence plate appears to have caused both thecollision of the thatdinosaurs used thisAlaska-Siberia route during adjacent partsof thesetwo plates in the late Jurassic the Cretaceous. (Churkin, 1972), and also the appearance of new land in westernNorth America in the Nevadan orogeny (Coney, 1973). The Jurassic palaeogeo- Cretaceouspatterns (Figs 7 and 9) graphic map suggests that the resultingAlaska- Siberialink would have lain at a highpalaeolatitude. In general, the patterns of palaeogeographical However,in theabsence of polar icesheetsduring the change seen in the Jurassic continued into the Mesozoic, mild climatesseem to have extendedinto Cretaceous. Epicontinentalseas across Asia and quite high palaeolatitudes. The Early Cretaceous North America were still extensiveenough in time florasof North America and Siberia show similari- and space that they mighthave caused detectable ties (Barnard, 1973) suggestinga Jurassicbiological effects on patterns of faunal distribution.The connectionbetween these two areas, and Cretaceous Central Atlanticcontinued to widen, but the Eur- florasare certainlyknown from equally highpalaeo- american plate remained whole. The continuing Vertebratepalaeodistributions and continentaldrift 85

cited, the Early Cretaceous floras of Siberia and BritishColumbia are very similar(Barnard, 1973). In addition to this link within the Northern Hemisphere,it is commonlystated that the dinosaur Euramerica Asia faunas of the Cretaceous world require land con- \0 5 nection between Laurasia and Gondwanaland (Charig,1971; Colbert,1974). As alreadyshown, the Upper Jurassicdinosaur faunas of the two areas are similar. It follows, therefore,that both areas in- heriteda similardinosaur fauna in the Cretaceous, 8 and that the bulk of their Cretaceous dinosaur faunas could be merelythe descendantsof common Gondwanaland Jurassic stock. Evidence as to Cretaceous inter- continentalconnections can thereforeonly be pro- vided by a considerationof those new formswhich evolved solely withinthe Cretaceous period. There are sevensuch new Cretaceousdinosaur families: the Fig. 6. Venndiagram of the distribution of Jurassicdinosaur saurischiantyrannosaurs, dromaeosaurs and orni- faunas. Figuresrefer to familiesexcept in the case of the sauropods,where subfamilies have been used instead. thomimids,and the ornithischianhadrosaurs, proto- ceratopians,ceratopians and pachycephalosaurs.All westwardmovement of NorthAmerica led to further are knownin bothEuramerica and Asia. But though theliterature contains some reports of thesegroups in mountain building (the Sevier and Laramide Gondwanaland,few of these appear to be reliable. orogenies) in the western part of the continent The two supposed ornithomimidsfrom India have (Coney, 1973), and thismay have increasedthe land recentlybeen statedto be nominavana, of doubtful linkage to Asia. Though South America finally nature (Russell, 1972). The supposed tyrannosaur separatedfrom Africa in the Upper Cretaceous,this and ceratopian specimens from Argentina (von took place too late to affect the distributional Huene 1929) are all incompletespecimens which patternof dinosaurs though,as will be seen, it is show none of thediagnostic features of thesegroups. relevantwhen consideringthose of mammals. The Though a dromaeosauris reportedfrom the Upper distributions of dinosaurs and mammals will Cretaceous of South America (Ostrom, personal accordinglybe consideredin turn. communication),this group is known from the earliest Lower Cretaceous in North America and mayhave spreadto Gondwanalandbefore that super- Dinosaurs continentbecame isolated. The onlyreliable evidence The patterns of distributionof the Cretaceous for faunal connection between Northern and dinosaur faunas of Euramerica, Asia and Gond- Southern Hemispheres during the Cretaceous is wanaland are shown in Fig. 8. Withinthe northern thereforethe presenceof a hadrosaurinehadrosaur continents, there was considerable interchange in the Upper Cretaceous of Argentina (Casami- betweenEuramerica and Asia. Since the Turgai Sea quela, 1964; Brett-Surman,personal communica- was still in existence,and in view of the geological tion). Like that of the Upper Cretaceous mammals evidence for land linkage between Alaska and of South America,the access routeof thishadrosaur Siberia,the latterroute was probablystill the path- group is uncertain,but was probably via a filter way of intercontinentaldispersal. The climate of route fromNorth America (see p. 89). such a route certainlyseems to have been quite At generic level, the only Cretaceous dinosaur warm. Smiley's (1972) studies on the Cretaceous reportedin both Laurasia and Gondwanaland is florasof northernAlaska show a peak of warmthin the sauropod Titanosaurus,but the evidencefor this the late Lower Cretaceous (Albian), when the is unreliable(Colbert, 1974). climatethere was frostfree and subtropical,and a Accordingly it seems clear that the dinosaur gradual cooling till the end of the Cretaceous,when faunas of the Cretaceous provide no reliable evi- it had become cool-temperate.Certainly, plants dence to contradict the diverse evidence from seem to have dispersedby this routefor, as already marine invertebratestudies (Hallam, 1973) that a 86 C. B

- ::::::::::::::::...... :: ::: I' 7 0 N :::::::::::::::I: ...... '.'.'...... '...... : ...... :::.:.:.:,':::::. - ,.,.,.,.....- - ... . - ...... I...... - I...... '- ...... _....II...... '...... -... , j.j_:_:-:.:_ ...... -...... -.. ::::::':':::":::::- ...... ------'...... _...... ':"I.. -:-:.:-:-:,',.------' ...... -,.-,-,.-:::::::::::::::::::::::::P '::'-'-'-'-'-"'-'.'-'-'-'-'.... . - ...... I...... , ...... -:-:-:-:'._:_:_:_:_:-:-:-:- ...... '.,I I__ --:L;4-:-:-:- ...... I...... '.."::: .1511 .1.1. .1 : ...... :.:.-*. - .1.1-.:.: . , .... I - - .4.. . .:-:SL:L::^:_:_.. --- ,-,-,.,-..,.,--.-.-.,.--'-'-""-" -:-:.:,:.I.:-: ... .'.'...... -1 ' ' ...... ,...... -:::::...... _...... 1.1.1 ...I...... ,...... ,...... -...... -...... ,...,...... -.-...... -...... I...... ` .,.,.,.,.-.-_...... '...... -...... '...... ,...... -....,...... _ ...... '...... I...... I.,...... -...... '' ...... -.....,...... ".-,.,.,...... - - ... -:-: ..'...... - : -:-:_:_:_:_:_X_:_:-:-: ...... _X-:-:-:-:-:-:-'.-'.-:-: ...... ,...... :.:.:.:.:-:-:-:-:-:.:.:-:- .I...... :.:.:.:.:.:.:.:.:.:.:.:--...... I.I.I...... - ...... -:-:-:-:-:-:.:-: ...... ,...... : . , , ...... --..-...... :.:.:.:.:,:.:,:.:-:-:...... :.:.:.:-:-:-:-:-:-:.:.:i ...... :.:.:-:.:.:.:.:.:.:.:.I...... ,:.:.:.:.:-:-:-:-:-:.:...... ''I,- ...... I...... ,...... :-:-:-:-:-:-:-:.:.:.:.:.\-...... :...... :_:-:-:-:-:-:-:-:-:_:_T _ ...... : -:-:-:_:_:-:_:-:-:_:-X-%.....'.-I ...... ,...... :.:-:.:.:.:.:.:.:-:.:-:-:-N:..%...... :.-:-:_:_:-:-:-:-:-:.:.:.:.'...... I...... '' . "" .'A ...... ' ,.-.-.-.-.-.-...... :...... '...... '..'.'..:::::::::::.:.:.:.:]::.:j::.'-. I...... 'iNN ':,::, ""'.' ...... --- ' ...... -...... 11:1 ...... '' ...... '-'.'.'.'.'.'-'-,-,." ' - ...... '...... '.'...... : " ..'.',...... '...... I...... ',', .." . .-.-.-:-A:-: " ...... " ...... I . ...:.:.-.:.:.:.:.:.:.I.:.:.:.:. " ...I...... 1...... :...... '...... : ....I...... _...... ,...... -:-:-:-"5. "C -:.:-:-:-X -:-:-:-: ' ...... '...... ::.:.:,:,:.:.:.:-:-:-:-:-:-.-Ik;.: ...... -.1 ...... " - -.-..:.:--..-..... I.I.I...... '...... ' ...... - ...... _...... : ,:.:.:.:.:. . .,:,:-.\.,,.---...... '...... ' ...... I.I.I...... ' ..I.I...... '...... :...... , . . . . . -...... ,...... , ...... - ...... "'...... , ...... '...... -...... ,...... ,, _...... '...... -...-....:.:.:-:.:-:-:-:-:-:.:.:.:.:.:.....--.-.-....-...:-:-:-:-:_:_:_'-'-....,.,.,-,-'-- .1 "-;...... -- - I I...... _...... '...... : .,.-.-.-.-.-.-.-...-...... -,I...... ,I...... :-:-:-:.:...... ::::::::::: ....- .,.-.-.-.-.-.-.,...... ,...... ,.-.-.,.-.,.%,...... "A .. .I...... '.,.-.-.-.-.-.-.,...... ' ...... '

Tethyan marine belt (presumablya shallow epi- As Colbert(1974) has pointedout, thisgeographical continentalsea) separatedGondwanaland fromthe situation explains the restrictionof the tyran- northernland areas duringthe Cretaceous. nosaursand protoceratopidsto the firstof thesetwo Even withinthe NorthernHemisphere, epiconti- land masses-and it should be noted that both nental seas stronglyaffected the distributionof groups evolved withinthe Upper Cretaceous,after Cretaceous terrestrialvertebrates. The new typesof the seaway had formed. However, Colbert also dinosaur which evolved in the Lower Cretaceous suggeststhat the presence of hadrosaursin both land (comithomimids, pachycephalosaurs and hadro- masses shows that it was possible for some dino- saurinehadrosaurs) were able to spread throughout saurs, at least, to cross the Seaway. In fact,closer the NorthernHemisphere. But the formationof the analysis shows that the only hadrosaursknown in Mid-ContinentalSeaway across North America in Euramerica are the most primitiveforms, the the Upper Cretaceous (Fig. 9) createdtwo separate hadrosaurines.As already noted, these evolved in land masses in the NorthernHemisphere: 'Asia- the Lower Cretaceous, before the Seaway formed. merica' (Asia plus westernNorth America) and The otherthree hadrosaur subfamilies (the Saurolo- 'Euramerica' (easternNorth America plus Europe). phinae, Cheneosaurinaeand Lambeosaurinae)evol- Vertebratepalaeodistributions and continentaldrift 87

doubtless became widely distributedduring the Jurassicand Lower Cretaceous, when Gondwana- land was still whole. Their presencein the separate parts of Gondwanaland in the Upper Cretaceous Euramerica Asia does not, therefore,imply land routesbetween these 18 112 12 areas at that time, as Colbert (1974) suggests,but merelythe presenceof descendantsof the dinosaur fauna that was common to all of Gondwanaland in the Lower Cretaceous.

9 Gondwanaland Mammals The distributionof Cretaceousmammals presents a varietyof problems,mainly because of the imperfec- tion of our knowledgeboth of theearly evolution of marsupialsand placentalsand also of the finaldates Fig. 8. Venn diagram of the distributionof Cretaceous of separationbetween the differentland masses. The dinosaurfaunas. Figures refer to familiesexcept in the case palaeogeographyof the Upper Cretaceous is shown wheresubfamilies have been used instead. of thesauropods, in Fig. 9, and threemajor biogeographicproblems involving the differentialdistributions of Upper ved in the Upper Cretaceous,after the Seaway had Cretaceous mammals will be considered in turn: formed,and are foundonly in Asiamerica. these relate to distributionsin North America as The survivalof iguanodontsand primitiveanky- against Asia; Laurasia as against Gondwanaland; losaurs (the acanthopholids) in the Upper Creta- and withinGondwanaland itself. ceous of Euramericamay also have been due to the The distributionof UppermostCretaceous mam- isolation of that area at that time. These groups mals providesfurther evidence of a barrierbetween became extinctin the Upper Cretaceous of Asia- Asia and westernNorth America. North America merica,where they would have been in competition appears to have been invaded by severalfamilies of with such new groups as the protoceratopidsand Asian placentals (Lillegraven, 1969) and multi- advanced hadrosaurs. tuberculates(Kielan-Jaworowska, 1974). However, Although,if one takes the Upper Cretaceousas a the multituberculatesand varied marsupials of whole, it is thus possible to identifya single Asia- North America do not appear to have been able to merican faunal region, this itself had apparently colonize Asia.* Kielan-Jaworowska(1974) points become subdivided by the Uppermost Cretaceous out that this situation is not easily explicable by (Kielan-Jaworowska,1974). This factis not apparent referenceto a possiblefilter connection between Asia at familylevel, as thefamilies found in bothAsia and and North America, as this should have permitted westernNorth America in the early Upper Creta- passage equally well in each direction.Instead, she ceous survive in both areas until the end of the believesthat it suggestsa sweepstakesroute between Cretaceous. Kielan-Jaworowska(pers. comm.) has the two areas, possiblyacross marinestraits within pointed out, however, that only two genera are whichthe currentsfavoured eastward crossings but commonto both areas, and also thatthe Uppermost not westwardcrossings. Cretaceous North American family Ceratopidae Relationshipsbetween mammals in SouthAmerica may well be absent from Asia, since its supposed on the one hand, and North Americaand Africaon presencethere is based only on a singleskull bone the other,pose difficultproblems in both the Upper (Charig, 1973) of doubtful diagnosticvalue. This Cretaceousand the Lower Tertiary,and neitherthe suggeststhat a barrierhad formedbetween Asia and * North America,and a spread of epicontinentalseas Butler& Kielan-Jaworowska(1973) have suggestedthat the Asian familyDeltatheridiidae may have some marsupial betweenthem seems the most likelyexplanation. As dentalcharacters, as well as placentalcharacters; they prefer will be seen, the distributionof UppermostCreta- to classifythe group as early mammals of metatherian- ceous mammalssupports this hypothesis. eutheriangrade, and theirpresence in Asia does not neces- sarilyimply the presencethere of fullymarsupial mammals, Within the Southern Hemisphere, dinosaurs closelyrelated to themarsupials of NorthAmerica. 88 C. BarryCox

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I...... ::::::::::::::::::::::::::: :...... "."'..""""'...... "..."'. """"""",..."...",""",., :::I_...... A...- ... .. E 0 U AT 0 ...... :-:-,.:-:.:.:.:.:-:.:.:-:-:-],:-i-ii ;.' ; "i., ...... -.:-:-:-:-:.:-:-:.:...... ,...... -- ...... -...... -...-...- ...... '...,...,..,.,.-.-.-.-...... ,.,.,.,.,.,...... "...',.,.,.,.,...... L_2-.-.-.-.-.-.%-.-...... "...... - ...... "'....., ...... '...... -.-...::::::::::::::::.::::::,:,::.-...... '-...... '.. _'--...... -.-.-.,.-.-.-.-.,.,.-.I.,.,.,.,...... ' ...... X...... '.....'...... -""...... :...... ::::::::..::-:::::.''...... '...... '...... :...... :...... ,_I_...... '.:-:...... 3...... :-:-:-:.. -..:.: ...... :::::::::::::::"'._I ...:..-:-"::::.-'.."'.:.'.. '....1111 -, '.'..

geologicalnor the biogeographicalevidence is as yet group had evolved in that area. But Cretaceous adequate to provide reliable answersto these. The marsupials and condylarthplacentals have now geological historyof the Caribbean area remains been found in South America (Sige, 1968, 1972); in complex and uncertain.Sea floor spreadingdata, particularthe diversity of thiscontinent's Cretaceous now givingreliable estimatesof the Upper Creta- marsupialfauna has alreadyprovided a considerable ceous position of Africarelative to North America surprise,leading Hoffstetter(1970) to suggestthat (Pitman & Talwani, 1972) and to South America marsupials might have evolved somewhere in (Le Pichon & Hayes, 1971; Sclater & McKenzie, Gondwanaland. More surprisesmay well be in store, 1973), show that the North Atlantic was already both from South America and also from Africa, comparativelywide, whereas northern South America whose Cretaceous and Early Tertiary mammal was stillnot fardistant from Africa (Fig. 9). faunas are completelyunknown. As Sige (1972) The Cretaceous biogeographical evidence is pointsout, it would be unwiseto attemptto erectan adequate onlyfor North America. Until recently, the elaboratebiogeographic hypothesis on existingdata. fact that it contained the most diverseCretaceous One can only suggesta few possibilities. marsupial fauna known led to the belief that the Since marsupials are known from both Gond- Vertebratepalaeodistributions and continentaldrift 89

wanaland and Euramericain the Upper Cretaceous, have excludedthe known Paleocene South American theymust have crossedthe sea barrierwhich separated placentalsfrom access to Australia. Though earlier the dinosaur faunas of the Northernand Southern workers had suggested that the break was not Hemispheres.Two routes are possible, and both untilEarly Tertiarytimes, the recentview of Dalziel involve difficulties.Though, as just discussed, it et al. (1973) that it took place beforethe Late Cre- seems likely that there was a considerable gap taceous is helpfulto my suggestedexplanation.* betweenNorth and South America,it is possiblethat thisgap containedvolcanic islands resulting from the complicated tectonic events taking place in the Tertiarypatterns Caribbean (Malfait & Dinkelman, 1972). These mighthave actedas a filterroute, allowing marsupials To understandproperly the distributionalpatterns and condylarthplacentals to cross betweenNorth of differentmammalian families,it is necessaryto and South America, but preventingthe passage of know where each of these originated.For some dinosaurs,except hadrosaurs. familiesit is impossibleto be sure of this,because of The alternativeis a more circuitousroute from gaps in the record in particularareas and times. westernNorth Americato Euramerica,and thento Many families,however, are endemic to one par- Africaand SouthAmerica. This would have involved ticular tectonic plate, and accordingly it seems crossingthe Mid-ContinentalSeaway, the Tethyan reasonableto assume provisionallythat this includes seaway betweenEurope and Africa,and the early, theirarea of origin. narrowSouth Atlantic.This last mighthave been no Figure 10 is a Venn diagramof the areas of first great problem, since the interveningMid-Atlantic known occurrenceof differentmammalian orders. Ridge may have surfacedas volcanic islands which Many of theseare firstknown from North America, would have providedpossible aids to the dispersalof because the recordfor that continent is knownfrom terrestrialanimals fromone continentto the other; theUpper Cretacousonwards, whereas the European the Cape Verde Islands and Canary Islands seem to record does not begin until the Mid-Upper Paleo- be relics of such a chain in the more northernpart cene. Late thoughthis European record is, it none ofthe Ridge (Klerkx& de Paepe, 1971).Nevertheless, the less contains representativesof most of the the threesea barriersinvolved in thisroute together 'North American' groups. As North America and form a formidable series of obstacles. Another Europe were stilljoined untilthe end of the Lower difficultyis that, if marsupialshad spread by this Eocene, one cannot localize the apparent area of route,it is surprisingto findno trace of themin the origin of these orders more preciselythan 'Eur- well known European Upper Paleocene mammal america' and theyare shownas such in Figs 10 and faunas. 11. Only thoseNorth American orders which do not The presentevidence, though inconclusive, there- appear in the European recordin the Upper Paleo- fore favours a trans-Caribbeanfilter route for the cene are shown as originatingin North America. passage of marsupials and condylarthsbetween As already mentioned,placentals appear to have Northand South America. originated in Asia. The earliest placentals are I have recently(Cox, 1973c) discussedthe problem normallyplaced in the order Insectivora but, as of the connectionbetween South America and the Butler (1972) has pointed out, theyare really only rest of Gondwanaland, and its relationshipto the primitive placentals, rather than being closely spread of marsupials to Australia. In brief, the related to the living types of insectivore.Butler simplestexplanation is thatmarsupials crossed from suggeststhat the earlyforms be placed in an order South Americato theAntarctica-Australia landmass Proteutheria,and the later, more modern types of beforethis broke away, and thatthis separation took insectivorebe placed in an order Lipotyphla.This place before placentals arrived in South America. suggestionhas been acceptedhere. As a result, the placentals were unable to reach Figure 11 shows the faunal relationshipsof the Australia. Australia later split from Antarctica; sea floor spreadingdata (Weissel & Hayes, 1971) * Though Raven & Axelrod(1972, 1974) referto the final show thatthis took place in theEocene. One crucial break of the link betweenAustralia and South Americaas factorin this theory is the timing of the break beingmiddle Eocene, theyare in factreferring to the date of the break betweenAustralia and Antarctica;they do also betweenSouth Americaand Antarctica,which must (1974) note the above-quoted data for the separationof have taken place by at least the Late Cretaceousto South Americafrom Antarctica. 90 C. BarryCox

ASIA ProteutheriatU Cret LagomorphaU Eo

Dlnocerata t U Pal

/esmostylia t Mu/ti- U Oligo tLubercutat

/NPrimataU Cret/L Pal U P |Carnivora L Pal Lipotyphla Mid Pal Taeniodontat L Pal MUd Chiroptera Pal Pantodonta t Mid Pal Rodentia U Pal Perissodactyla U Pal Creodonto t U Pal EUROPE Pholidota U Pal Tst llodontat L Eo EdentatP U i PalL EArtiodactylayrUcoidraLsligMid Eo \ NORTH \SU \ AMERICA I CA \ \ E~~~~~URAMERICA//

on y arthrat Li Cret \ /~\ >< Marsupiaba U Cret > / \ / \ / Sl~~~~~~~~~~reniaM id Eo \ Fig. 10. Venn diagram oftheareas,andtimes,offirstknownoccueProboscidea U.Eo o H order s nEdentata L.Obigo U Pal yrcolde | Xenutngul/ato tUPal\ E-mbrIlhopododt L Oligo0 Trlgonost/oodet U P IMscrosce/wd(es L Olilgo

Notoungulatat U Pal \uulett L/i L 11oplernott U Pal\/ \ Astropothertlot L Eo / \ A FRICA / \ PYrotherlo t L E o/

\SOUTH/

Fig. 10. Venn diagram of the areas, and times, of firstknown occurrence of orders of mammals. Endemic orders italic. Extinct orders shown by t symbol. differentland masses fromthe Upper Cretaceousto the north,where northernGreenland was sliding the Middle Eocene. Arrows indicate the time of past Svalbard (Spitzbergen) along the de Geer appearanceof an orderin a newarea and itsprobable fracturezone. As Szalay & McKenna (1971) have geographical origin. The firstappearance in the pointed out, this connectionwas at a fairlyhigh palaeontologicalrecord is probablyoften some time palaeolatitude,and would have acted as a faunal later than its actual time of immigration,but it is filter.The closenessof the relationshipbetween the best to portrayonly what is at presentestablished. two areas is demonstratedby thefact that ten Upper During thisperiod, the Turgai Sea stillobstructed Paleocene mammal genera are found in both areas directland travelbetween Europe and Asia. Though (Kurten, 1973). As can be seen from Fig. 12, the the disappearanceof the Mid-ContinentalSeaway at NorthAmerican fauna is less similarto thatof Asia the beginning of the Tertiary allowed dispersal than to that of Europe. Szalay & McKenna (1971) betweenWestern North America and Euramerica, suggestthat this is because the Beringconnection to Greenland began to separate from Europe in the Asia would have lain at c 750 N palaeolatitude,8? Paleocene (Bott, 1973). Connectionsbetween North furthernorth than the Svalbard route to Europe. Americaand Europe were thereforepossible onlyin However, this differencein latitudeis small, and it Vertebratepalaeodistributions and continentaldrift 91

A S I A Multituberculata Dinocerata Proteutheria Lagontorpha U Pal U Pal L Eo UPa RUPHIU Cret La U Eo N _t JORTH lPantodonta L ipotyphla AMERICA "Perissodactyla L [o Creodonta U Pal Carnivora Rodentia Marsupialia Artiodactyla Taeniodonta Multituberculata U Pal Til aodonta Dinocerata Condylarthra I Chiroptera Pholidota Primata Mid Pal EURAMERICA

AFRICA IINDIA Notoungulata Condylarthra Xenungulata Edentata Astrapotheria Litopterna Pyrotheria Marsupialia Trigonostylopoidea 2

SOUTH AMERICA ? ANTARCTICA - AUSTRALIA

Fig. 11. Geographicaldistribution and dispersalof mammalianorders during the Upper Cretaceousto Lower Eocene.

tion, thisappears to have been a sweepstakesroute, again presumably along a Caribbean chain of islands. The faunas of Africa,India and Australia at this time are unknown. These patterns of possible faunal movements became radically altered in the Middle Eocene. Szalay & McKenna (1971) have suggestedthat the divergencebetween the faunas of North America and Europe,which begins at thistime, may have been due to the final breaking of the Svalbard link NorthAmerica Europe between Greenland and Europe. Due to the dis- appearance of the Turgai Sea at this same time, Europe's links withthe rest of the NorthernHemi- spherewere henceforthvia Asia and not via North America. Elsewhere,Tertiary climatic changes, and changes Fig. 12. Venn diagram of the distributionof mammalian familiesin the NorthernHemisphere during the Paleocene in sea levels, produced a complicated pattern of and LowerEocene. alternatingperiods of faunal isolation and linkage betweenAsia and North America fromthe Oligo- cene onwards,which has recentlybeen discussedby seemsmore likelythat the faunal differencewas due Colbert (1974). Recent work (Powell & Conaghan, to a continuationof the Late Cretaceous barrier 1973) has suggestedthat India had become attached betweenAsia and NorthAmerica. to Asia by the Middle Eocene, and thisis supported Notoungulateswere able to dispersefrom South by the presence of condylarths,artiodactyls and Americato NorthAmerica and thenceto Asia in the perissodactylsin the Upper Eocene Kalakot fauna Upper Paleocene. Since no other groups crossed of India (Ranga Rao, 1971, 1972; Ranga Rao & betweenNorth and South America in eitherdirec- Obergfell,1973). 7 92 C. BarryCox

The mammal fauna of Africais unknownbefore puzzles.This was primarilybecause thegeographical the Upper Eocene-Lower Oligocene Fayum faunas, unitsused in compilingfaunal listswere, as we can whichhave recentlybeen discussedby Coryndon& now see, ofteninvalid. The acceptanceof continental Savage (1973). Though theyare probablyonly very drifttheory provides a seriesof datesfor the union or limitedsamples of the total Africanfauna, so that separationof differenttectonic plates. It is therefore othergroups may have been present,the fact that the essentialfor zoogeographers now to use appropriate faunas are dominatedby groups which evolved in palaeogeographicmaps and the appropriatestretch Africaand did not reachother continents until much of time duringwhich a particularcontinental con- later, suggeststhat Africa was linked to Europe figurationwas in existence,when attemptingto solely by a sweepstakes route during this time, analyse the faunas of the past. Such maps show the probablydue to the presenceof epicontinentalseas appropriategeographic units for faunal analysis and overnorthern Africa. comparison,suggest possible routesof faunal inter- Monkey-likeprimates and hystricognathrodents change at specifictimes, and also aid in the identi- appeared in both Africaand South America in the ficationof specificissues of physiologicalor climatic Lower Oligocene.The sourceof the South American tolerance. membersof these two groups is stillhotly debated, Problemsof interpretationstill of course remain, Wood (1974) favouringa North American origin, but at leastzoogeographers can now attemptto solve and Lavocat (1974) and Hoffstetter(1973) an African the real problems(e.g. why didn't placentals enter origin.Whichever was theircontinental homeland, Australia fromthe rest of Gondwanaland?) rather the fact that other groups fromthat continentdid than spurious ones (why didn't placentals enter not accompany them suggestsagain that a sweep- Australia fromAsia ?). Finally, some new types of stakes route was involved.This could have been a queries-and answers-arise. Thus in the past, chain of islands, either across the Caribbean or palaeontologistshave tended to explain all cases along the Mid-Atlanticridge betweenWest Africa involvingthe absence of individual groups from and Brazil. It was not untilthe Upper Pliocene that particularareas as merelythe resultof inadequate thefinal establishment of a continuousland routeby explorationor sampling.It is now apparentthat, in way of the Panama isthmusresulted in a complex at leastsome cases, theseabsences are consistentwith patternof extinctionand faunal interchangewith palaeogeographicevidence that the area in question North America, which has been fullydocumented was isolatedat thattime, and withthe idea thatfossils by Simpson(1950). have not been found theresimply because the ani- The mammalfauna of Australiais firstknown in mals did not live there. the Upper Oligocene-LowerMiocene. The factthat it already contained ten differentfamilies of mar- supial stronglysuggests that the group had entered Acknowledgments Australiaconsiderably earlier. If thesuggestion made of this was at the earlier(page 89) is correct,marsupials would have A preliminaryversion paper given and Evolu- reached the Antarctica-Australiacontinent in the InternationalCongress of Systematic in Late Cretaceous. It was, however,not until much tionary Biology, Boulder, Colorado, U.S.A., to later that placental mammals were able to enter August 1973. I am pleased acknowledgehelpful with Drs M.K. Australia from Southeast Asia. Rats are the only discussions and correspondence A.J. E.H. terrestrialplacentals to have made this journey Brett-Surman,W.G. Chaloner, Charig, Z. unaided by man, but theirabsence from the Late Colbert, B.G. Gardiner, Kielan-Jaworowska, A.R. Milner and J.H. Pliocene H4amiltonfauna of Victoria, Australia M.C. McKenna, R.S. Miles, (Turnbull & Lundelius, 1970) suggests that their Ostrom. arrival was quite late. References

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