The Evolution of Mammalian Molars to and from the Tritubercular Type

Evolutionof Mammalian Molar8. 1067 THE EVOLUTION OF MAMMALIAN MOLARS TO AND FROM THE TRITUBERICULAR TYPE.' BY HENRY FA IRFIELD OSBORN. THE dentitionin the recentMammalia is so diversethat the most sanguineevolutionist of fifteenyears ago could not have anti- cipatedthe discoveryof a commontype of molar,in both jaws, as universal among the Mmamalia of an earlyperiod as the pentadactylefoot, and as centralin its capacityfor development into the widelyspecialized recent types. The tritubercularmolar, discovered by ProfessorCope in the Puerco,is exactlysuch a type,and may be consideredwith the pentadactylefoot as playinga somewhatanalogous r6le in mammalian history,with this importantdifference-the unmodified pentadactylefoot was probablyinherited direct from the reptiles,and its subsequentevolution, with a few exceptions,has been in the direc- tion of the greateror less reductionof primitiveelements towards special adaptation,as, to borrow an extreme illustration,in the transition.from Phenacodus with 26 elementsin the manus to Eqmucswith only 12 such elements. On the other hand, the tritu- berculartooth was not inherited,but in all probabilitydeveloped withinthe mammalianstock, from a hypotheticalform with almost, if not quite simple conical molars,implanted by single fangs,in a nearlyhoinodont series.2 No such primitivetype of mammalian dentitionis actuallyknown, although Dromotheriurnapproximates it; but the apparentreversion to this typeamong the C'etacea,and apparentretention of it in theEdentcta,3 support all theindependent evidence upon this point derived from the Mesozoic Mamimals. The principleof growthwas the regular addition of new parts to the simple cone,not at random,but accordingto a certaindefinite 1 Read in the geological section of the British Association at Bath, September,1888. Read in abstractby Prof. Cope, National Academy is Sciences, at New Haven, Nov., 1888. 2 See Autbor " Structureand Classification of the Mesozoic Mamma- Ila." Jour.Phila. Academy, 1888,p. 240. 3 See OldfieldThomas, " The Homologies and Succession of the Teeth in the Dasyuridm." Phil. Trans., 1887,p. 458. This content downloaded from 128.135.012.127 on November 15, 2016 14:32:18 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 1068 Evolutionof MiammalianMiolars. orderwhich apparentlyprogressed independently in differentphyla, througha seriesof sub-tritubercularstages until trituberculy1 was attained. The tritubercularmolar consistsessentially of three cusps,form- ing what may be called the primitivetriangles, so disposed that the upper and lowermolars alternate. This,when attained, formed a centralstage fromwhich the greatmajority of recentmolar types have divergedby the addition,modification and reductionof cusps; we mustexcept the Monotremes,the Edentates,and possibly the Cetaceans,although there is considerableevidence that the cetacean molarswere once of the triconodonttype.2 Among extinctorders, the Multituberculata (Plagiaulax, Tritylodon,etc.) must also be exceptedfrom this seriesand discussion. The almostuniversal predominance of trituberculyin the early geological periods,is verysignificant of the uniformityof molar origin. Of twentyknown Mesozoic genera,3all exceptthree 4 show trituberculyin someof its stages. As to the Lower Eocene, eighty- two Puerco species,representing twenty-six genera and five orders (Creodonta,Tillodontia, Lemuroidea, Condylarthra,Amblypoda), only fourspecies have quadritubercularteeth, all theremainder are tritubercular.5Prof. Riitimeyerhas recentlypointed out thepre- dominance of this type in the nearlyparallel Egerkingen beds The contemporaryCernaysien fauna in the collectionof Dr. Le- moine at Rheims, recentlyexamined by the writer,shows exclu- sively tritubercularmolars or their derivatives. By the Middle Eocene the linesof divergencetowards the existingtypes of molars were well advanced,but trituberculypersisted in the dentitionof several orders,in whichit is foundto-day (Lemuroidea, Insectivora, Carnivora,and manyMarsupialia). 1 First employed by Rfitimeyer, "Ueber Einige Bezi ehungen zwischen den SaugethierstdmmenAlter und Neuer Welt." Abh. d. .schweiz. pal. gesellsch., Vol. XV., 1888,'p. 54. 2 See Brandt, "Die Fossilen u. Subfoss. Cetacean Europas." Taf. XXXII., figs. 4-9. -3The list given by the writer (op. cit., p. 247) is foundto contain several synonyms. See " Additional observationsupon the Structureand Classificationof the Mesozoic Mammalia." Proc. Phila. Acad., Nov., 1888,p. 292 4Dicrocynodon (Diplocynodon), Docodon, Enneodon, Marsh. 5 Cope, " Synopsis of the Vertebrate Fauna of the Puerco Series," Am. Phil. Soc., 1888.p. 298. This content downloaded from 128.135.012.127 on November 15, 2016 14:32:18 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). Evolutionof MamtmalianMolars. 1069 It followsthat it is quite as essentialfor the comparativeanato- mist to thoroughlygrasp the meaning and historyof each of the componentcusps of the tritubercularmolar and of theirderivatives, as it is to perfectlyunderstand the elements of the manus and pes. For, the homologiesof the cusps can now be determinedalmost as certainlyas thoseof the digits. Take a human molar,for example, ever componenttubercle has its pedigree,and it can be demon- strated,almost beyond a doubt,which of thesetubercles is homolo- gous with the single reptiliancone. The writer recently(op. cit., p. 242) proposed the adoption of a distinctnomenclature for the differentcusps of the tritubercularmolar, and offereda series of termsfor the primarycusps based as faras possibleupon the prim- itive positionand order of development,and in most instancesin accordwith their secondaryposition. This nomenclaturecan be extendedto the secondarycusps in the sextubercularsuperior, and quinquetubercularinferior molars. The termsnow in generaluse are based,for the most part,upon the secondaryor acquiredposi- tion,and in no instanceupon the homologiesof the cusps in the upper and lowermolars, or even in correspondingmolars of differ- ent genera, thus involving much confusion. For example, the Antero-internalcusp of the lowermolar of M1iiolcenusis not homol- ogous with the antero-internalcusp of Hyopsodus,nor with the antero-internalcusps of the upper molar of eithergenus. The presentcontribution is based principallyupon the writer's studies amongthe Mesozoic Mammalia, and, with some additions, upon Prof. Cope's numerousessays upon the trituberculartype in the TertiaryMammalia.l Four propositionsmay be laid down fordiscussion: (1.) That trituberculywas acquired duringthe Mesozoic period, in a seriesof stages beginningwith the single cone and attaining to the primitivesectorial type in the Jurassicperiod. 1 ProfessorCope's essays abound with discussions and notes upon the originand successionof the trituberculartype. (See collection,in " Ori- gin of the Fittest"). He has outlined the transitionfrom the single cone to the tritubercularcrown (p. 347); the tubercular sectorial (p. 246); the quadrituberculartype (p. 245 and p. 359); the Spalacotherium molars as a transitionto the trituberoular(p. 259). The acquisition of the superiorand inferiorquadritubercular molar (p. 361). The predic- tion of the discoveryof Carnivora with triconodontmolars (p. :365),and of the simple trituberculartype in both jaws (p. 362). This content downloaded from 128.135.012.127 on November 15, 2016 14:32:18 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 1070 Evolutionof Mammalian Molars. (2.) The majorityof Mesozoic mammals showedtrituberculy in some of its stages. Presentevidence goes to show thatthe remain- ing,or aberranttypes, if such existed,did not persist. The majority of the persistingforms of laterperiods were derived from the forms, withsimple tritubercular molars, of earlierperiods. It followsthat trituberculywas an importantfactor in survival. (3.) The definitehomologies of the primaryand to some degree of the secondarycusps in the upperand lower molarscan be estab- lished. (4.) The mode of successionof toothforms favors the kinetogen- esis theoryadvanced by Ryder and Coope. There are threegeneral observations to be made: First.-In attemptingto completethe historyof each of the cusps,we naturallyfind that the paleontologicalrecord is not suffi- cientlyperfect to admitof ourfollowing a certaintype along a single phylumback to the primitivetype. We must at the outsetpro- ceed upon the principleof similar effects,similar causes. For example,since the historyof the developmentof the intermediate tuberclesin the superiormolars of the Lemuroidea (Pseudolemu- roidea,Schlosser) is perfectlyclear during the Wasatch and Bridger epochs-it is safe to infer that the intermediatetubercles of the Ungulate molars, which are fully developed in the underlying Puerco,had the same history. Second.-There are in each period Aberranttypes which embrace either incomplete or degeneratetri- tubercularstages, i.e., a high specializationin which the past rec- ord is obliterated,or, finally,stages in non-tritubercular lines of development. Third.-In the parallel evolution of trituberculy in differentphyla we findthat the progressionis by no meansuni- form. In everygeological period in 'whichthe fauna is well known we observeprogressive genera which outstrip the othersin reaching a certainstage of molardevelopment, contrasted with persistent types which representarrested lower stages

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