a reevaluation of the early history of the frogs. part ii

20 SYSTEMATIC ZOOLOGY

A Reevaluation or trie Early History

of the Frods. Part II.MAX K. HECHT

In the first part of this article (SystematicZoology, 11:39-44) the pre-frog level ofevolution was discussed. In that article Icast doubt on the classical interpretationof frog evolution. In effect this state in theevolution of the frogs is again unknown.

The earliest known true frog is Vieraellaherbstii, described by Reig (1961) from thelower Jurassic of Argentina. Recently I ex-amined the type material and in a forth-coming study will discuss in detail my owninterpretations. As a result of my investiga-tion it is clear that the imprint described asVieraella is that of an essentially modernfrog which bears only one primitive charac-ter, the presence of ribs. The presence ofribs on the third and fourth vertebra is ageneralized character found throughout so-called primitive frogs. It does not neces-sarily indicate discoglossid affinities. If thestratigraphic position of the locality is cor-rectly determined, then Vieraella is theearliest frog known.

The most complete Jurassic frog knownis Notobatrachus deguistoi (Reig, 1957)from the upper Middle or lower UpperJurassic of Argentina. I have recently ex-amined the entire series of Notobatrachusand have come to different interpretationsfrom those expressed by Reig (1957) andCasamiquela (1961a). The details of myown studies and interpretation will be leftto the study mentioned above. Briefly, itmight be stated that Notobatrachus repre-sents a generalized frog with a primitivecarpus, a primitive number of vertebraebearing ribs, and with procoelous centra,a postsacral complete vertebra, and lackof teeth on the lower jaw. The conditionof the pectoral girdle does not clearly indi-cate the presence of firmisternal or arciferal

girdles. I have clearly found evidence forthe presence of the plectral portion of thecolumella auris. The significance of thisdiscovery will be discussed under Eodisco-glossus. Briefly, Notobatrachus representsa generalized form with certain primitivefeatures but which does not clearly alignitself with any of the generally consideredprimitive living frogs, such as Liopelma,Ascaphus, or the discoglossids.

Nopsca (1930) described in a single sen-tence Stremmia scaber from the Jurassic ofAfrica. Stremme (1916) thought these tobe reptile forearm remains, whereas thecollections in Berlin have these labeled asbird forearm bones. My examination of thetype reveals that Stremme was right andthat these bones are not frog remains.

Vidal (1902) described Palaeobatrachusgaudryi from the Upper Jurassic of Spain.Fejervary (1921) proposed the genus Mont-sechobatrachus for this form, primarily be-cause of its great age. In truth, there is littlereason for a generic or specific designationfor this poorly preserved cast. No detailscan be made out of the specimen. All thatcan be determined from my examinationof the type (see Piveteau, 1955, Fig. 28) isthat a type of frog existed in the UpperJurassic of Spain.

Marsh (1887) described in a single sen-tence the genus Eobatrachus from theUpper Jurassic of Wyoming. Hecht andEstes (1960) have shown that the materialsallocated by Marsh (1887) and Moodie(1912, 1914) were really the remains ofseveral different organisms. Of the originalmaterial that has been found in the Yalecollections two were certainly frog humeribut of two clearly different types of frogs.One of these humeri Hecht and Estes (1960)

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EARLY HISTORY OF FROGS 21

refer to the genus Eobatrachus. PreviouslyEobatrachus had been assigned to themeaningless Montsechobatrachidae (Romer,1945) or to the Discoglossidae (Reig, 1957).If Eobatrachus must be assigned to anyfamily, it is most likely closer to the Pipidae.Comobatrachus is represented by a ratherordinary frog humerus of one of the moreadvanced families. It is therefore clear thatthe frog remains from the Jurassic of NorthAmerica, as of now, only indicate that twodivergent forms had already evolved.

In 1955 Piveteau published a photographof a magnificent fossil frog from the UpperJurassic of Spain. In 1957, Melendez pub-lished in the Spanish edition of Leonardi'sLa Evolucion Biologica (p. 146) the samephotograph with a brief discussion and thename Eodiscoglossus santonjae Vill. Thisspecimen and counterpart were collected inthe Upper Jurassic of northern Spain by Dr.L. Ferrer and have never been adequatelydescribed (Hecht, 1962b). A detailed studyis now under way by the author. Examina-tion of the above-mentioned photographimmediately reveals the preservation of theskin imprint and the pigments of the eye.More important, examination of the fossilreveals the condition of the carpus andmanus. The main features of this form area strong resemblance in the vertebrae andtheir ribs to both the Discoglossidae andAscaphidae. A detailed study and interpre-tation of the carpus reveals closer relation-ship to the Ascaphidae than to the Disco-glossidae, although this is not certain.There are eight presacral vertebrae and nopostsacral vertebra. The sacrum is simpleand not expanded. The clavicle is large andstraight and the coracoid is very discoglos-soid in shape. Most interesting, as is evenevident from the published photographs,are the clear skin imprints including an en-larged thumb print. Upon detailed analysisof the region of the manus, it is evident thatthere are five digits. The first appearsslightly hypertrophied and is made of twoelements with possibly an enlarged meta-carpal beneath it. Overlying this entireregion is the imprint of a pigmented nuptial

pad which would probably indicate by itslarge size that the specimen was a male.The nuptial pad completely encloses thefirst digit and is apparently supported onone side by the second digit. This is clearlyindicated by the close association of thesecond digit with the pad on both sides ofthe specimen. The third digit also bears asimilar integumentary imprint on the distalportion of the digit. The fact that the entirestructure encloses the first digit suggestswhy in primitive Ascaphus the pad islocated alongside the first functional digit(the second digit primitively) and over thevestiges of the first digit.

On the two fossils, Eodiscoglossus andNotobatrachus, the presence of the plec-trum portion of the columella is clearlyindicated. Stephenson (1951), Eiselt (1944),and many others have pointed out that themiddle ear bones are lacking in the Ascaph-idae, Bombina, and some other more ad-vanced frogs. Stephenson (1951) impliesthat this condition in the ascaphid frogsand in urodeles is perhaps indicative of theprimitive condition. This interpretation iscontested by Paterson (1960) and Eiselt(1944) who state that this condition is notnecessarily primitive, but probably the re-sult of convergent evolution due to adapta-tion to specialized modes of life. The factthat the plectrum is present in Eodiscoglos-sus and Notobatrachus would appear tobear out Paterson's interpretation.

Later Mesozoic FrogsCretaceous frogs are now known from

four localities: Texas, Israel, Argentina, andSouth Africa. The material from the TrinitySands (Zangerl and Denison, 1950) isunder study by the author. These fossilremains are single elements, not articulated,and badly worn. There are abundant re-mains of dorsal skull roofing elementswhich bear ornamentation similar to thatfound in leptodactylid genera. Quite similarornamentation is also known in some spe-cies of the Pelobatidae, Hylidae, Ranidae,and fossil Discoglossidae. All elements areso badly worn that at the present time it is

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best to consider the leptodactylid identifi-cation as tentative. There are several frag-ments of humeri which are quite distinctfrom one another. Although one type is themost abundant, there are at least one or twoother types preserved. Procoelous and opis-thocoelous vertebrae have been identified.Reig (1957) reported a personal communi-cation stating the presence of a frog lowerjaw with teeth in this deposit. This wasdue to an erroneous report based on anamphibian lower jaw fragment which wastentatively associated with the abundantfrog remains. It is clearly known to be uro-delan in origin and will be reported by Dr.R. Estes.

Due to the kindness of Mr. A. Nevo Ihave been able to examine his interestingfrogs from the early Cretaceous of Israel.Although his specimens are individuallypoorly preserved, they are abundant andclearly indicate through some unique ad-vanced morphological features that by thistime the major radiation of the frogs hadtaken place. There are at least two verydistinct types (one of which is aglossan inaffinity) represented here and we mustawait the publication of these discoveriesto weigh their phylogenetic implications.Nevo (1956), reporting on the same deposit,illustrates a preserved anuran tadpole withdistinctive features similar to certain pelo-batid tadpoles. If the interpretation is cor-rect, this indicates that by the early Creta-ceous, tadpoles of the type IV of Orton(1953) had evolved. Reig (1960) has re-ported on the frog Saltenia which he be-lieves is essentially a procoelous aglossanor pipid relative (living pipids being opis-thocoelous). This form, Saltenia and Eoxe-nopoides (Haughton, 1931), of the lateCretaceous or early Tertiary of South Africaand possibly one of the Asiatic forms showsthat the Aglossa and their relatives had aworldwide distribution during the Creta-ceous. Dr. R. Estes will soon report on alate Cretaceous fauna from Wyoming whichwill indicate the presence of a discoglossid,possibly of a pelobatid and with somedoubt hylids and leptodactylid frogs.

Early Tertiary Anura

The earliest described Tertiary anurans,the genera Shelania and Eophractus, areknown from Patagonia. Shelania pascualiof the Paleocene or early Eocene has beenadequately described and figured magnifi-cently by Casamiquela (1961). Casami-quela has correctly allocated this form asan aglossan or pipoid but has interpreted itas being most closely related to Eoxenop-oides of South Africa and has placed thetwo forms in another new family, Eoxeno-poididae. The diagnosis of both his newgenus and family is based on interpretationswith which I am not in accord after examin-ing the material. For example I see littledefinite evidence in the fossil to state thatShelania really has opisthocoelous verte-brae. In fact as can be clearly seen fromCasamiquela's plate the specimens of Shela-nia are not well ossified as shown by theform of the different bones. For examplethe humeral ball, characteristically solidlyfused to the humerus in all adult frogs, hasbeen disarticulated but not broken off fromthe main shaft of the humerus. The en-larged otic capsule is clearly another featurewhich is usually associated with fossilizedadvanced tadpoles and metamorphosing in-dividuals. It is my belief that Shelaniaactually represents a metamorphosing orrecently metamorphosed individual or pos-sibly a slightly neotenic species. On thebasis of the enlarged otic capsule, the poorlyossified limb bones, incompletely formedurostyle, possibly poorly ossified vertebrae,and the small size of the specimens, I be-lieve that it is most likely a recently meta-morphosed individual.

The second Patagonian genus, Eophrac-tus of Schaeffer (1949), is based on veryfragmentary remains but appears to clearlyrepresent a leptodactylid form. A compari-son of this fragment with new and moreabundant material from the Oligocene andMiocene of Argentina indicates that thisform represents a large and early form ofthe genus Calyptocephalella (probablyproperly called Caudiverbera, Myers, 1962).New fossil and osteological material indi-

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EAELY HISTORY OF FROGS 23

cate that the giant forms of the Eocene(Eophractus) and Miocene (Gigantoba-trachus of Casamiquela, 1958) are part ofa single phylogenetic line of the livinggenus Calyptocephalella. A study of theallometric growth pattern among a smallcollection of the living genus indicates thatthese giant forms and the Oligocene form,Calyptocephalella canqueli Schaeffer, fallwithin the possible variation of a singlespecies. Therefore this fossil series mayrepresent sections of a single phylogenetictrend differing primarily in the maximumsize attained.

There are several important collectionsfrom the Middle Eocene. The most impor-tant collection was described by Kuhn(1941) from the Geiselthale of easternGermany. In this report Kuhn describedone new family, 12 new genera, and 16 newspecies. A detailed examination of this col-lection by myself reveals this work to beone of the most erratic and unreliable mod-ern studies known. The description ofQuinquevertebron, which Kuhn placed inthe family Brachycephalidae, is character-istic of his work. It is described as havinga shortened vertebral column composed offive vertebra. Examination showed thatthere were at least eight presacral vertebraeof which three were missing but present asimprints. Although a small part of the col-lection was lost during the last war, it canbe stated that the Geiselthale anuran faunais composed of at least two distinct typesof pelobatids, two types of discoglossid,one ranid-like form, at least one type ofbufonid-like form, and another form poorlypreserved and of dubious relationships.Weitzel (1938) described from the MiddleEocene of western Germany a supposedpelodytid, Propelodytes. Examination of thetype failed to reveal any of the supposedfamilial characteristics and its general poorpreservation indicates that it is probablyunidentifiable. This interpretation meansthat the earliest fossil remains of the RecentOld World Pelodytidae are from the Mio-cene of Nevada (Taylor, 1941). Noble(1931) described Indobatrachus from the

early Eocene of India and ascribed it to thebufonid-leptodactylid complex. This fossilis also poorly preserved and overinter-preted. Hecht (1959) has presented a syn-opsis of some anurans from the Middle Eo-cene of Wyoming. In this study he hasidentified a single atlas which he allocatedto the new genus Eorhinophrynus whichsupposedly indicates the presence of thefamily Rhinophrynidae. Also preserved withEorhinophrynus were the remains of ilia,probably pelobatid, and diplasiocoelouscentra. Hecht (1960a) described a newfrog, Eorubeta, from the Middle Eocene ofNevada with supposed Australian leptodac-tylid affinities.

By Oligocene times the major featuresof modern world anuran fauna had devel-oped. Schaeffer (1949) reports the livinggenera Calyptocephalella and Eupsophusfrom Argentina. He also reports the pres-ence of the problematic Neoprocoela, aform of leptodactylid or possibly bufonidanuran. Tihen (1962b) has reexamined therelationships of Neoprocoela and in thelight of his osteological studies has tenta-tively placed this species in the genus Bufobut in the Old World Bufo calamita group.Noble (1924) described a new pelobatid,Macropelobates, from the late Oligocene ofMongolia. Zweifel (1956) states that thisform could be ancestral to either the OldWorld Pelobates line or the New WorldScaphiopus line. In the same paper Zweifel(1956) describes a new species of the genusEopelobates from the early Oligocene ofSouth Dakota. Two species had alreadybeen assigned to this genus; they are Eope-lobates anthracinus (Parker, 1929) andEopelobates bayeri (Spinar, 1952). Afterexamining the type, it seems to me (withthe concurrence of Dr. Z. Spinar) thatEopelobates bayeri is actually a form of theliving genus Pelobates. The American fossilform assigned to this genus probably rep-resents still another genus distinct from theEuropean form. From the early Oligoceneof Belgium, Hecht and Hoffstetter (1962)report an aquatic discoglossid related to thePhilippine genus, Barbourula, a pelobatid,

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and a distinctive bufonid. Hoffstetter (1945)reported a pelobatid from the Quercy ofFrance and corrects two misidentified ele-ments. Other anuran remains are as yet notproperly studied from the Quercy. Friant(1944, 1960) described a new genus of dis-coglossid frog from the Upper Oligocene ofFrance, but it is difficult to determine howdistinct the fossil form really is.

The later history of the Anura is primarilyinvolved in generic and infrageneric differ-entiation. Only one new major categoryhas been described as appearing in the laterTertiary, the Palaeobatrachidae. As Schaef-fer (1949) predicted this group is an unnat-ural assemblage. This assemblage of forms,including the type of the type species, is acollection of poorly preserved imprintsusually in lignite or similar material. Themany forms often have little in commonexcept their poor preservation, their dis-tortion due to compression, and their diffi-culty of interpretation. Many poorly pre-served juvenile stages have been assignedto the genus Palaeobatrachus primarilythrough the efforts of Wolterstorff (1886,1928). In general this family has been usedas a waste basket for difficult specimensand includes forms of several families.

The Relationship Between the Morphologyof Living and Fossil Anura

The paleobatrachiologist has interests incommon with the student of modern formsbut his methods of study are obviouslymore restricted and by necessity confinedto osteology. In the paleontology of theAnura one can hope for the remains of com-plete skeletons but one is more often re-quired to find diagnostic characters insingle bony elements. As a result such ex-cellent recent morphological studies asthose of Baldauf (1959), De Villiers (1931,1932, 1934) and students (Slabbert andMaree, 1945), Griffiths (1959a, 1959b),Ramaswami (1936, 1942), and many othersare for all practical purposes nearly uselessto the student of anuran fossils. Thesestudies, using microscopic techniques, de-scribe details which are usually not pre-

served in the fossil record. Studies on mus-culature and circulatory system have alsolimited value to the paleontologist. Theirprimary interest is their discussion of re-lationships within the higher categories.Osteological studies like the nearly for-gotten work of Bolkay (1919) are of imme-diate use to the paleobatrachiologist. Thework of Tihen (1962a) is a more recentvaluable addition to the osteology of asingle genus.

The present state of our knowledge of theclassification of the so-called higher frogsis debatable. For example we can point tothe debate concerning the ranoid or micro-hyloid affinities of certain African andMadagascan frogs (Parker, 1932, 1934; DeVilliers, 1931, 1932; Guibe, 1956, etc.) andthe ever-changing arrangements in the NewWorld leptodactylid-bufonid classification(Laurent, 1942; Griffiths, 1959a). Anotherexample is Griffiths' study (1959b) of theSeychelles genus Sooglossus. In this studyhe removes this group from the Pelobatidaeand suggests a closer relationship to theranoid group of frogs. These and manyother studies all seem to indicate that thepresent higher classification of the frogs,which is based mostly on adult morphology,suffers from our inability to distinguish truephylogenetic trends from the results of con-vergent evolution.

In general, part of the life cycle of thefrog has been ignored as a useful indicatorof relationships. The major adaptation ofthe adult is still the primitive unique methodof locomotion but it is in the tenuous tem-porary life of a tadpole that major adapta-tions have evolved. The study of larvae isnot of great importance to anuran paleon-tology because, except in rare cases (Nevo,1956) tadpoles are rarely well preservedenough for study. Orton (1953, 1957) sug-gested that there are four distinct types offrog tadpoles and that these could be usedas a basis of a new higher classification ofthe living Aniira. Orton regarded Type I,the tadpole of the Pipidae and Rhinophryn-idae, as the most primitive because of itsbilateral spiracles, separate gill chambers,

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TABLE 1—COMPARISON OF LARVAL TYPES AND SKELETAL TYPES (AFTER MERTENS, 1959).

GIRDLE TYPE VERTEBRAL TYPE TADPOLE TYPE

ArciferaArciferaArciferaArciferaArciferaArciferaArciferaArciferaArciferaFirmisterniaFirmisterniaFirmisterniaFirmisternia

AmphicoelaOpisthocoelaOpisthocoelaOpisthocoelaAnomocoelaAnomocoelaProcoelaProcoelaProcoelaProcoelaDiplasicoelaDiplasicoelaDiplasicoela

LiopelmidaeDiscoglossidaePipidaeRhinophrynidaePelobatidaePelodytidaeLeptodactylidaeBufonidaeHylidaeBrachycephalidae fRanidaeHyperolidaeMicrohylidae|

III*III

II

IVIVIVIVIVIVIVIVII

* The tadpole type given here is for the genus Ascaphus. The genus Liopelma has eliminated thefree tadpole and modified the position and structure of the spiracle and gills for direct development.

t The Brachycephalidae, as here defined, is an artificial category and is characterized by both typesof pectoral girdle (see Griffiths, 1959a).

t The condition of the vertebral type in the Microhylidae is oversimplified because procoelous typesare found in some groups (Parker, 1934).

and primitive mouthparts. All of thesefeatures indicate close relationship to a gen-eralized amphibian larva. Type II larvaeare characteristic of the Microhylidae andhave simple mouthparts (but more complexthan the pipids) and a single midventralspiracular opening. Type III larvae arecharacteristic of the Discoglossidae andLiopelmidae which have more elaboratemouthparts and a midventral spiracularopening. Type IV larvae are found in allthe remaining groups of frogs and are char-acterized by a single sinistral spiracle andhighly differentiated mouthparts. Mertens(1960) in a single chart, by necessityslightly artificial in division, shows the con-flicting arrangements that can result whenthe relationships between families are de-termined by sharply defined categories ofvertebrae, pectoral girdle, or larvae.

It is clear from Table 1 that the tadpoletypes apparently cut across the categoriesestablished for the skeletal system. It isapparent that the modifications of the pec-toral girdle and vertebral column are re-lated to the major function of locomotion.It is only the larval types which representclearly a new type of adaptation to a totallydifferent phase of the life cycle.

The larvae of the Microhylidae indicate

the isolated position of this family. Theapparent resemblances between the skele-ton of the Ranidae and the Microhylidaeare undoubtedly due to an ancient conver-gence. This is well shown by the study ofDunlap (1960). Although Dunlap used theclassification of Noble, which tends to sub-merge the differences between the Ranidaeand Microhylidae, he still noted the dis-tinctness of the Microhylidae. In a similarmanner, the monotypic Rhinophrynidae hasbeen misinterpreted. This family is charac-terized by a mixture of features as indicatedby the condition of the tadpole (Orton,1953), the pectoral girdle (Walker, 1938),the pelvic girdle and the hyoid (Stadt-muller, 1936). These characters show thatthe family stands intermediate between theprimitive assemblage of ascaphid-disco-glossoid frogs and the pipids. Dunlap(1960), although following Noble's classifi-cation, notes the peculiar position of thisfamily. It is unfortunate so little is knownof the details of the anatomy of this family,since it could represent the most primitiveof living anurans.

The changing position of the Microhyli-dae and the Rhinophrynidae demonstratesthat the higher categories of frog classifica-tion are in a state of flux. This state makes

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the problem of the paleobatrachiologistseven more difficult.

Zoogeography and the Fossil Record

In the latest review of anuran distributionDarlington (1957) attempted to erect ahypothetical history of the Anura based onthe distribution of the modern families, al-though he indicated that the classificationof these families was artificial. For Dar-lington, the origin of the frog families beganin the Cretaceous and in the Old Worldtropics. For example he states that "Prob-ably late in the Cretaceous, aquatic pipidsgot from the Old World tropics to SouthAmerica." From this base he further de-velops a succession of invasions eventuallyresulting in the present distribution of theAnura.

It is quite clear from the fossil evidencethat there is no reason to assume that theAnura evolved in the Old World tropics.There are probably at least two basalgroups of living frogs, each evolved inde-pendently from an as yet unknown ancestor.The discoglossid-ascaphid line may be rep-resented in the Upper Jurassic of Spainand the Middle and Lower Jurassic of Ar-gentina. As noted by Reig (1961) this isquite distant in space and time from theCretaceous Old World tropics. The secondbasal group, characterized by tadpole TypeI (Orton, 1953), had in fact already devel-oped in the late Jurassic, which of courseis ample time to explain its present disjunctdistribution. A secondary radiation tookplace in the early Cretaceous or late Jurassicwhich involved primarily frogs of the tad-pole Type IV or the higher modern frogs.It therefore appears that origin of the frogtaxa took place long before Darlington hadthought possible and his picture is furtherobscured by the artificiality of his classifi-cation and the complexity of the origin ofthe frog phyla in the time dimension. Ittherefore appears to me that further anal-ysis of the distributional patterns in themanner of Darlington (1957) is useless untilthese criticisms can be answered.

Reig (1961b) and Casamiquela (1961b)

have independently come to conclusions op-posite to those of Darlington. The similarityof their conclusions is not surprising sincethey are based on the same data. Their es-sentially Gondwana hypothesis with a centerof evolution of the Anura in the SouthernHemisphere, probably South America, isbased on the relationships and geographicposition of Protobatrachus, the supposedlyprimitive fossil frogs of Argentina, and therelict distribution of the living Liopelmidae.Based on these facts it was inevitable thatthey recognize the inherent weakness in theinterpretation of Darlington. Both of themthen accepted the simplest hypothesis, thatthe distribution and primitive status of theirfossils were real and not merely accidents ofdiscovery and preservation. Furthermorethe several pipid fossils from South Americaand Africa and the present similar distribu-tion of the group seem to indicate for Casa-miquela (1961) further proof for his posi-tion. Unknown to him at that time was thereidentification of a single Upper JurassicWyoming anuran humerus clearly aglossanin affinity. On first appearance it wouldseem that the distribution of early fossilfrogs and their primitive living relativeswould support a Gondwana distributionpattern, but this is not the only possibleinterpretation. As I have stated previouslythe paleogeography of Protobatrachus is nolonger relevant to this problem. The dis-tribution of Jurassic frogs is directly relatedto future discoveries and explorations, andit is certain to include new finds in othergeographic areas. Thirdly, the position ofthe Liopelmidae as the most primitive frogsis not necessarily definitive. Their lack ofthe middle ear and its presence in twoJurassic frogs clearly demonstrate that theyare specialized and perhaps secondarilyprimitive and simplified "discoglossid" stock.The above conclusions are of course basedon my own interpretations of the crucialJurassic fossils and are not necessarily final.

In conclusion there is little in the presentdistribution of living frogs that providesany clues as to the center of origin of thefrogs or of the different frog groups. The

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position of Darlington is untenable in thelight of the fossil record. Likewise, thepositions of Reig and Casamiquela are un-tenable because their basic assumptions aretoo simple and in part are based on somemisinterpretations of phylogeny and mor-phology. It is important to realize that theaforementioned zoogeographers have madeinteresting contributions but I do not be-lieve that their hypotheses can stand up toall the facts.

Paleontological Record and the HigherClassification

The fossil record clearly indicates thata reevaluation of our criteria for determina-tion of what is primitive and what is ad-vanced is impending. The contribution ofthe fossil record is also dependent uponthese pending interpretations. Reig's studieson Vieraella and Notobatrachus create forus almost archetypal ancestors. My owninterpretation of the fossils demonstrates tome that Vieraella does not have the primi-tive forearm but a true modern radio-ulnaof the typical Anura and that furthermorethere is little evidence in this fossil to indi-cate greater primitiveness than is alreadyknown in living and fossil frogs. Likewise,I feel that Notobatrachus has been misinter-preted, because of our preconceived notions,as a very primitive form. I certainly cannot agree that Notobatrachus is an amphi-coelous anuran with teeth on the lower jaw,but instead I find it has a melange of primi-tive and so-called modern features. Reig(1957) has interpreted the girdle of thisform as of the firmisternal type and thusimplied that this characteristic, previouslyconsidered an advanced feature, is reallyprimitive. I have found no definitive evi-dence for this condition in the fossil, buton the other hand I cannot determine whichcondition was present in Notobatrachus.Griffiths (1959a), Reig, and others do callattention to the fact that the artificial sepa-ration into primitive arciferal and advancedfirmisternal girdles is naive. If my interpre-tation is correct that the types and most ofthe other material associated with Noto-

batrachus are really procoelous, althoughperhaps weakly so, then we are faced withseveral possibilities: 1) either procoely hasevolved more than once independently; 2)that procoely is possibly primitive; 3) thatamphicoely (primitive by definition forfrogs) was primitive and abandoned earlyby Notobatrachus. It seems to me that itis impossible at this time to determinewhether the living liopelmids (Ascaphusand Liopelma) are secondarily amphicoe-lous or primitively so. The key to our prob-lem lies in the fossil record and its correla-tion with the morphology of primitive frogs.

It appears to me that the types of tad-poles (Orton, 1953, 1957) are the onlyclear-cut key to the phylogeny of the frogs.The mosaic of characters and the recombi-nation of characters that make difficult theprocess of distinguishing phyletic lines byutilization of adult features does not seemto be true of the types of tadpoles. Thefour types of tadpoles demonstrate thatthere have been at least three levels oforganization and adaptation in the frogs,all of which involved much parallelism andconvergence.

The earliest appearing level of organiza-tion is characterized by the most primitiveType I larvae. Although morphologicallythese tadpoles are undoubtedly the mostprimitive type, there is as yet no earlierfossil representation of this group than fromthe Upper Jurassic. This group is repre-sented by the living rhinophrynids and thefossil and living Aglossa or pipids. Bothof these groups are undoubtedly remnantsof this earliest radiation of the Anura whichprobably took place in the late Triassic orearly Jurassic. The Aglossa with their pe-culiar suction-type of feeding tadpoles haveas adults reinvaded and exploited theaquatic medium. The living rhinophrynidshave evolved their own specialized niche,burrowing as adults. These two groups arenot the core of the first level and radiationof the frogs but specialized remnants ofwhat was probably a widespread group ofAnura. The opisthocoely of the living formsand the procoely (?) of some of the fossil

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forms may have been secondary develop-ments. The second level of frog evolutionmust be represented by larval Types II andIII. This second radiation most probablytook place during the middle Jurassic, prob-ably replacing and displacing the earlierradiation of Type I frogs. This radiationis represented today by the ascaphid-dis-coglossid line and the microhylid line. Forthe ascaphid-discoglossid line there areavailable several fossils at the proper time,Eodiscoglossus, Notobatrachus, and possi-bly Vieraella. Here again in this radiationof the Anura can be found independentevolution of opisthocoely, procoely, andamphicoely. Furthermore it is possible toextrapolate from the fossil record that thebreeding patterns characteristic of the mod-ern frogs were already well-established bythis time. While there is no fossil evidencefor the early origin of the Microhylidae, itstadpole implies its early origin. The thirdradiation of the frogs is represented by theType IV tadpole and it is clear from thefossil record that this type had already ap-peared by the Upper Jurassic or at thelatest the early Cretaceous. It seems obvi-ous that this last radiation of the frogs hastaken place rapidly with a mosaic of char-acters representing the major phyletic linesor families. As a result these phyletic linesare not clear. This last radiation has alsoreplaced and displaced the second and firstradiation with the exception of the Micro-hylidae. The success of this last family canbe easily determined by examination of theclassic study of Parker (1934). The familyhas paralleled in adult morphology one ofthe most successful phyletic lines, the ranidsand their relatives, of the Type IV radia-tion, and for this reason has remained suc-cessful in the tropics. In the adaptiveradiation of the Type IV frogs and themicrohylid frogs there have developed agreat number of adaptive adult types allexploiting (with one exception) the basicsaltatorial adaptation of the frog and theadaptive advantages of their respective tad-poles. In this modern fauna parallelism andconvergence are rampant and every phy-

letic line present in a given zoogeographicrealm has developed arboreal types, aquatictypes, burrowing types, etc. in the adult.One group, the Bufonidae, particularly thehuge genus Bufo, can be described as par-tially abandoning the saltatorial mecha-nism and developing a walking mechanism.As adaptive radiation has taken in place inthe adult of any phyletic line, so have simi-lar processes operated on the tadpole phaseof the life history (Orton, 1953). She hasclearly shown that by adaptive radiationfrom her "generalized central type" of tad-pole there can develop arboreal, surface-feeding, mountain-stream, carnivorous, andnektonic types. Lastly and most importantis the general trend in many phyletic linesto eliminate the hazardous tadpole exist-ence by direct development. Actually thereis direct evidence that the adaptive radia-tion of the ascaphid-discoglossid tadpolemust have been similar to the radiation ofthe modern frog fauna. In Leiopelma ofNew Zealand there is direct developmentand in the supposedly closely related liopel-mid, Ascaphus of western North America,there is a mountain-stream type of tadpole.The Microhylidae have also paralleled theType IV tadpole evolution in its own adap-tive radiation, and in fact as far as knownin the Australasian zoogeographical regionmicrohylids have only direct development.

In summary it can be stated that eachof the major three levels in anuran evolu-tion has had its own adaptive radiation andreinvasion of the old adaptive zones dis-placing and replacing parts of the olderfaunas. These processes have resulted inconvergence and parallelisms which nowconfuse the phylogenetic and taxonomicpicture.

Evolution of the Anura and theAssociated Biotas

There is no indication of the relativeabundance of the Anura through time.From the sparse Jurassic representation andthe non-existent late Triassic fossil record,it could be assumed that they are relativelyminor elements in these faunas. The rapid

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development of the modern frog fauna(Type IV tadpole) during the Jurassic-Cre-taceous transition would seem in part cor-related with the coincident rise, develop-ment, and dominance of the angiospermflora. It is obvious that there are no herbiv-orous adult frogs, but correlated with theevolution of the angiosperm flora are theassociated radiation of the holometabolousinsect phyletic lines which likewise undergorapid expansion during this time. In fact,with the exception of one phyletic line ofAnura, they all can be described as primarilyinsectivorous. Like the lizards, and many ofthe primitive mammals, the frogs early ex-ploited the insectivorous adaptive zone. Tofurther substantiate the hypothesis that theadaptive radiation of the later frogs is asso-ciated with the developing angiospermfloras and their related insect and othersmall invertebrate faunas, I would like topoint out that the only aberrant anuranphyletic line (which can be considered inthe terrestrial sense) evolved early in theJurassic, probably before the modern insectfaunas became dominant. This phyleticline, the Aglossa, developed early in theJurassic, becoming aquatic, probably feed-ing on the aquatic immature stages of themore primitive insects and aquatic Arthrop-

FIG. 1. Modified from Noble's (1922) schemaentitled "Genetic Relations of the Salientia."

ON

o

CO

a>O

FIG. 2. Modified from Noble's (1931) "dia-gram illustrating the phylogeny of the Salientia."

oda, as its modern representatives do today(Inger and Marx, 1961).

The Phylogenetic Tree

Sooner or later all phylogeneticists at-tempt to diagram the relationships resultingfrom their systematic or morphologicalstudies. It is a diagrammatic method usedto express a complex method of evaluation,presenting a simplified picture (perhapsoversimplified). In the following series ofdiagrams (Figs. 1-7) can be traced the his-tory of our recent changing views on frogphylogeny. It is interesting to note thatmost of our changes are not due to errorsbut to reevaluation of old data and additionof some new data. The data on frog tad-poles have been available for a long timebut their significance was only pointed outin 1953 by Orton.

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G,C,U

co

o

a.CO

Q>

Q

Dc

FIG. 3. Diagram from Slabbert and Maree(1945) illustrating relationships of primitive livingfrogs.

Figure 1 shows Noble's early attempt ata phylogenetic arrangement before he fullyappreciated the significance of Ascaphusor what he later called the suborder Am-phicoela. Figure 2 shows Noble's newinterpretation under the impact of the re-evaluation of the anatomy of Ascaphus andits relationship to Liopelma. Figure 3 isthe result of detailed histological studieson cranial anatomy of primitive frogs byworkers of the South African school of anat-omy, but who were strongly influenced bythe previous work on the Amphicoela. Fig-ure 4 is a composite by Terentiev (1950)in his Russian compendium on the biologyof the Ranidae. His phylogeny is stronglyinfluenced by Watson (1941), Piveteau(1937), and Noble (1931). Figure 5 is asummary of the fossil and osteological databy Brattstrom (1957) attempting to synthe-size these data within the classification ofRomer (1945). Figure 6 is Casamiquela's(1961b) phylogenetic interpretation basedon the classification of Reig (1958), super-

imposed on the schema proposed by Slab-bert and Maree (1945). This diagram isalso strongly influenced by the fossil dis-coveries in Argentina. Figure 7 representsthe point of view of the present study. Themajor changes expressed in this diagramare the result of the increased weight givento the tadpole studies of Orton, the fossilrecord and its relation to the time dimen-sion, and the increased significance of con-vergence and parallelism in comparativemorphology of the Anura. It is not ex-pected that this proposed phylogeny willstand the test of time any longer than theother proposals, but it seems to me the bestpossible arrangement at this time. For ex-ample the time of divergence of Ascaphus(Z) and Liopelma (Za) is unknown butwe can approximate the time unless ofcourse their similarities are due to conver-gence (which seems unlikely). The samecan be said of the Hyperolidae (F) and theranid (Y) divergence. I have approachedthe problem of classification with a "lump-ing" point of view, merely for simplicity'ssake. Several groups of genera or familiesare lumped under the symbols of Micro-hylidae and Leptodactylidae. Phrynomerusis considered a microhylid on the basis ofthe allocated tadpole. The entire Neotrop-ical radiation of the leptodactyloid complexis under the symbol L which includes the

rc

FIG. 4. Diagram from Terentiev's (1950)study on the Ranidae.

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Pseudidae (Savage and Carvalho, 1953),Atelepodidae, and Rhinodermatinae but ex-cludes Dendrobates and its allies, whichare referred to the ranid phyletic line byGriffiths (1959a). For comparison withthe phylogenetic diagrams of older authorsthe Brachycephalidae may be includedunder this symbol. The Australian lepto-dactylid radiation may represent a segmentof the formerly widespread Leptodactylidaebut on the basis of zoogeography and thefossils I would guess the divergence of thetwo groups must have been Paleocene orearlier. This arrangement in no way im-plies that I believe that the Australiangroup must be a single natural group nomore than the South American leptodac-tyloid melange (Parker, 1940). I have fol-lowed Taylor (1941) and Zweifel (1956)in recognizing the Pelodytidae. I haveplaced the leptodactyloid group near thecenter of the Type IV radiation, because Ifeel that they represent the most general-ized members of the group and somethingnear the ancestral type from which themodern frogs must have arisen. If the Mi-crohylidae are removed from the Neoba-trachia (Reig, 1958) and the Pelobatidaeand Pelodytidae added to the same, thenReig's term could be applied to the groupcharacterized by Type IV tadpoles and

QXa

ARCIFERAL FIRMISTERNAL

FIG. 5. Diagram from Brattstrom (1957) illus-trating the relationship between osteology, paleon-tology, and anuran phylogeny. In this figure thesymbols Z, Za represent the Liopelmidae orAscaphidae and Zb represents the Montsecho-batrachidae.

Xb

Za

FIG. 6. Diagram from Casamiquela (1961b).

therefore be equivalent to my term, the"modern frogs."

ConclusionsThe fossil record of the frogs is as yet

poorly known, but better than is commonlybelieved. The earliest known form ascribedto the ancestral line of the frogs, Protoba-trachus, is not on the phylogenetic line ofthe frogs. If this is true, then no interme-diate types are known and as Williams(1959) indicates there are no clear ancestralgroups known for the frogs. If Protoba-trachus is on the ancestral line of the frogs,then various homologies of the tarsus mustbe reconsidered.

As indicated by the fossil record themajor adaptations of the frogs had alreadybeen completed by the early or middleJurassic. There are apparently two basalgroups, the aglossan-rhinophrynid groupand the discoglossid-ascaphid group. Bythe early Cretaceous the modern type orhigher groups of frogs evolved, probablyfrom the discoglossid-ascaphid basal group.The Microhylidae are probably derivedfrom the same basal complex and have

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R M P DZZaX

FIG. 7. Diagram illustrating relationships of the frogs as suggested by this study. The differentphyletic lines are placed along a time axis to indicate probable time of origin. Round dots representfossils allocated to living families. Roman numerals indicate presence of tadpole Types I, II, III, or IVin a given phyletic line. Fossil families in which tadpoles are unknown are encircled. Geological periodsare: Tr—Triassic, Ju—Jurassic, K—Cretaceous, Te—Tertiary.

independently paralleled the modern frogswith the Type-IV tadpole.

The origin of the frog families in the OldWorld Tropics as proposed by Darlington(1957) is not substantiated by the fossilrecord. The present distribution of frogs isthe result of an earlier evolution than postu-lated by Darlington. With our present stateof knowledge it is impossible to determinethe center of origin. Similarly, the oppositepoint of view of Reig and Casamiquela for

a southern continent center of origin isbased on simplification and an inadequatefossil record. Until a more complete fossilrecord becomes available zoogeography ofthe frogs will remain in the realm of my-thology.

Whereas the fossil record cannot contrib-ute much to the clarification of our system-atic problems in frog classification, it canplace the appearance of the frog categorieson the time scale. Briefly, the major frog

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adaptations were completed by the MiddleJurassic. The major frog families hadevolved by or during the early Cretaceousand by the early Tertiary the main genericlines of living frogs had already begun.

Acknowledgments

I would like to acknowledge the help ofmany colleagues who facilitated my studiesof the material in their respective institu-tions. They are: J. P. Lehman, R. Hoff-stetter, M. Lamotte, J. Guibe in France;H. Wermuth, H. Matthes, G. Krumbiegel,R. Dehm, W. Gross, H. Tobien, F. West-phal, K. Klemmer, E. Jorg in Germany;H. Zapfe, J. Eiselt in Austria; L. Ferrer-Condal, M. Crusafont-Paero, B. Melendezin Spain; E. Casier in Belgium; Z. Spinarin Czechoslovakia; E. Kuhn-Schnyder, J.Huerzler in Switzerland; W. Swinton, A.Grandison in England; G. Haas, A. Nevo inIsrael; N. Cattoi, O. Reig, R. Casamiquela,J. Bonaparte in Argentina; L. Gazin, D.Dunkle, R. Zangerl, A. Romer, P. McGrew,E. E. Williams, D. D. Davis, R. Wilson, J.Wilson, R. Estes, C. Camp, T. Downs in theUnited States.

I have had many fruitful discussions withB. Schaeffer, E. E. Williams, S. B. McDow-ell, R. Zweifel, R. Estes, I. Griffiths, O.Sokol, R. Inger, C. Gans, K. Koopman, O.Reig, and N. and E. Stephenson. This entirestudy has been supported by the NationalScience Foundation (G-7467, G-23981), theAmerican Philosophical Society, and theAmerican Academy of Arts and Sciences.

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KEY TO FIGURES 1-7

A—Amphicoela; B—Bufonidae; C—Procoela; D—Discoglossidae; Da—Discoglossus; Db—Alytes; Dc—Bombina; E—Pelodytidae; F—Hyperolidae (Polypedatidae, Rhacophoridae); G—Diplasicoela; H—Hylidae; L—Leptodactylid radiation in Neotropical region including Leptodactylidae, Atelopodidae,Rhinodermatinae, Pseudidae, etc.; La—Leptodactylidae (Australasian region); M—Microhylidae (Brevi-cipitidae); N—Notobatrachidae; O—Opisthocoela; P—Pelobatidae; Q—Palaeobatrachidae; R—Rhino-phrynidae; U—Anomocoela; V—Vieraellidae; W—Proanura, Protobatrachidae; X—Aglossa; Xa—Pipi-dae; Xb—Eoxenopoididae; Y—Ranidae; Z—Liopelmidae; Za—Liopelma; Zb—Ascaphus.

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a reevaluation of the early history of the frogs. part ii

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