I Thc Bc,t.qlt,Rt,,'rJs,'f tht N,'nhrrn It,trtttr'\' llluscuntof Artsanr,/.Sr'icat r,.r. 199{) ?(:):51-80 l ALKWERTATHERIUMWEBBI. A NEW ZYGOMATURINE GENUSAND SPECIESFROM THE LATE MIOCENE ALCOOTA LOCAL FAUNA. NORTHERNTERRITORY (MARSUPIALIA:DIPROTODONTIDAE). PETERMURRAY NorthernTerritory Museum of Artsand Sciences, GPOBox 2109, Alice Springs NT 0871,Australia. ABSTRACT Alkv'ertatheriumw,ebbi, gen. et sp. nov., possessescharacteristics structurally intermediatebetween the diprotodontinegenus Pyramios and the zygomaturine gents Plaisiodon of the family Diprotodontidae.In its expressionof a clearly differentiatedand well-developedparastyle on the Pr, Alky,'ertatheriumis aligned with the SubfamilyZygomaturinae. In its absenceof a hypoconeon the P3and its markedlyconstricted diastemal palate. Alkv,ertatherltrm resembles the membersof the Subfamily Diprotodontinae.Alkwertatherium was derivedfrom diprotodonti- nes at a structuralgrade similar to that of P)-romios.lt may haveshared ancestry with the primitive zygomaturinesPlaisiodon andNimbadon. P,-ramios, Alkuer- tatheriumand Plaisiodon exemplify a gradualstructural succession in cheektooth and cranial morphologyfrom the Diprotodontinaeto the Zygomaturinae. KEywoRDS:Late Miocenemarsupials, Alcoota Local fauna,Diprotodontidae, zygo- maturinae,zygomaturine origins, Diprotodontinae,nototheriines, P '-ramios, Plaisiodon. INTRODUCTION commonand therefore the inclusion of theodd molar of thesespecies would not seriously Primarilya descriptionof a new taxon,this influencethe statisticaldefinition of Plaisio- study also attemptsto addressthe character don. states of the Diprotodontidae in relation to The upper third premolarsof three of the Alkwertatherium,which is comparedwith four Alcoota generaare readilydistinguish- other Chehenhamian-Mitchellianequivalent able. forms in considerabledetail, incorporating a The P3of Pyramiosalcootense is character- brief review of the mid- to late Miocenedipro- ized in havinga weakly expressedor absent todontid genera. parastyle,absence of a hypoconeand presence At leastthree large generaand one smaller of a faint posterobuccalcingulum. Its broadly diprotodontidgenus are present in the Alcoota trilobateocclusal profile, large, conical proto- Local Fauna.The molar dentitionsof the three cone and equally large triangular,undivided largest genera: Pyramios a lcoot e nse parametaconewith a short,steeply descending Woodburne,P laisiodon centralisWoodburne postparametacristaare associatedwith dipro- and Alkwertatheriumwebbi, gen. et sp. nov., todontine(Diprotodontinae) diprotodorrtids. overlap in size and morphology. Isolated The large size of the Pi relativeto the molars molars of these genera are difficult to dis- is morein proportionwith thecondition in zy- criminate without the associatedthird premo- gomaturines.In Plio- Pleistocenediproto- lar. It is possible,judging from thesize distri- dontines,(eg. Eury:ygoma, Nototherium and butions, that the tablesof measurementspro- Diprotodon) the Pr is reducedrelative to the videdby Woodburne( 1967a)may containan molars. admixture of these forms. Sampling of the Plaisiodon centralis permanentupper pre- Alcootasite between 1985 and 1989indicates molarsare elongated. with a prominent.pos- that Pyramios and Alkwertatherium are un- teriorly curved parastyleand a long, blade- 53 P.F. l\lunay like postparametacrista.The posterobuccal Diagnosis.Crrnium narro\\!'r :.' .,r:rll,, cingulumis abscnt.The hypoconeis at least Iongerin proportionto rr'idth) thrn l' . .; '. incipiently prescnt.somctinlcs budding off Pluisiotlott.Kolopis or Ncolir'/r''. .: ..:. the posteriorside of the protocone.In some profile forming a distinct vertc\. .(.::- individual s the parametacone is weaklydiffer- weak frontal and low sagittalcrc'\l' :r.:::: entiatedinto two conjoinedcusps. but the bfing the type Plttisiodon t't'rttrulit (-?r' conspicuousvertical buccal sulcusthat ac- 6784). Constricted and elon,r.r:e,: companies such a division in the postsquamosallamina, similar to, but mtrrc zygomaturinesKolopsis torusY,l oodburne and extreme than in Neohelosor P/alsiorloa:oc- Zrgomaturusspecies is lessdistinct. Kolopsis ciput lower and narrowerthan any similar- lrrrs has a broad,Iow molariform,5-cusped sizeddiprotodontid. Rostrum narrower than in Pr with a divided parametacone.well-devel- Pyramios, Plaisiodon and Neoftelos.con- opedanterobuccal and posterobuccal cingulae stricted at the level of the infraorbital fora- anda low, but distinctmesostyle. men; diastemalpalate similar to. though While thesethree generacannot be mis- longerand morenarrowly constricted imme- takenfor any otherdiprotodontid, Alkv,ertath- diately anteriorto P: than in P_vranrios;later- erium v'ebbi upper third premolarshave a ally expandedpremaxillary palate, as in P.r'r- large parastylecombined with a P,-ramios- amios,but with a shallow.wide, as opposed to like absenceoithe hypocone;steep. triangular a narrow, deep,interincisival fossa. Nasals undivided parametaconeand large conical elevatedslightly and expanded anteriorly as in protocone.A weakposterobuccal cingulum is Plaisiodon andNeohe los. not downturnedand present.An indistinctmesostyle is confluent pointedas in P1-ramios. Zygomatic archlong, at its apex with the parametacone,as in P,r'r- as deepanteriorly as posteriorly,contrasting amios.The premolaris similarto thatof Py- with theshort. anteriorly narrowing zygomatic amios. but larger relative to the molars, and archesof Pyramios.Squamosal-jugal suture distinctive in its possessionof a massive. long, squamosalprocess extends to beneath stronglydifferentiated parasty'le. the orbit as in P_vramros:contour of archrela- The apparentabsence of unambiguousde- tively flat as in Plaisiodonand in contrastto scriminativefeatures in theupper molar denti- the strongly-bowedzygomatics of Ptrantios; tionr and the diprotodontine-z1gom;lturinemassetericprocesses broad and short as in continuityin thepremolar morphology result- Neohelos,in contrastto long.robust processes ing from the discovery of Alku'ertat heri unt in Pyramiosldeep, well-defined nasolabialis raisessome issuesin diprotodontidsys- fossasimilar to Plaisiodonand Neohelos. but tematics.On the basisof cranialcharacters. differing from Pyramios in which the fossais Alkv'ertatherium is very similarto Plaisiodon, poorlydeveloped. Trilobate, posteriorly broad but otherwiseexpresses affinities with Pyr- Pr composedof a large,slightly buccallyoff- amtos. setparastyle, pyramidal, undivided parameta- Abbreviationsof museumaccession num- coneand conical protocone; hypocone absent, bers: NTM, Northern Territory Museum; weakposterobuccal cingulum present, differ- NMV, Museumof Victoria(formerly National ingfrom Pyramlosin havinga well-developed Museumof Victoria): CPC, Commonwealth parastyleas Iarge or largerthan that of Plai sio- Palaeontological Collection (BMR); SGM, don. Upper molars more similar to P,-rantios Spencerand Gillen Museum (NTM Alice thanto Plaisiodonwith gradualprogression in Springs);VCMP, University of California molar width from Mr'ti wide interproximal Museumof Paleontology. contactbetween P3 and Mr, but differing from Pyramiosin having a more distinct,thicker SYSTEMATICS lingualcingulum, wider mouthof the lingual side of the interloph sulcus,more concave Family DiprotodontidaeGill posterior surfaceof the hypoloph, more dis- Subfamily ZygomaturinaeStirton, tinct "midlink", large metastyleon M1 and Woodburne and Plane much nanower hypoloph on M5. A conspicu- Gents Alkwertatherium gen. nov. ous obliquity of the lophs of M:'r resembles the condition of later diprotodontines,but Type species.Alkwertatherium webbi sp. Plaisiodon among the zygomaturines,has nov. similarly obliquity of the lophs.Dentary hori- 54 A new zygomalurlnegenus / Fig. l. Alkwerratheriumwebbi gen. et sp. nov. (holotype,SGM 888).A, lateralaspect of craniumiB, ventralaspect of cranium: C, dorsalaspect of cranium.Abbreviations: LC, lambdoidcrest; OC, occipitalcoodyle: GF, glenoid fossa;MP, massetericprocess; In, incisors;BOC, basioccipital;EG, entoglenoidswelling; PG. postglenoidprocess; NLF, nasolabialfossa: MPP, mastoid/paroccipitalprocess: MPS, maxillo-palatinesuture; FC, frontal crest;NFS, nasofrontalsuture; SC, sagittalcrest; SS, squamosalsulcus; SJS, squamosal-jugalsuture. 55 P.F.Murray : :,-/:l -+ $\- ;!', r / \-'3 t3_ t? PP Lrl: Fig.2. Plaisiodoncertralis Woodburne,1967 (SGM 871) for comparisonwith Alk*'ertatheriumt+ebbi. A,lateftl aspectof cranium;B, ventralaspect of cranium;C, dorsalaspect of cranium.Abbreviations not includedin Fig. l: FM. foramenmagnum; PP, paroccipitalprocess; PT, pterygoid;HTS, breachedhyporympanic sinus; PER, peiotic; PBS, paroccipital-basioccipitalsuture; FLP, foramer lacerumposterior; TW, vestigialsquamosal tympanlc wing; IPF. interpterygoidfossa; PLF, posterolateralpalatine foramina; PAL, palatine: IF, incisive foramina: SQ, squamosai:JU, jugal. 56 r\ nc$ 1\g()llllllurlnc llcnu\ zonlal ramusdccp but re'lativclythin in sc'c- Prruntictsalcrnttens( - SGM 871:palate u'ith tion, intcrior borderstraight and proportion- completeright cheekdentition. Nt,ohclos ti- / ally sintilarto Pluisiodon:differing l'romthe rurensis- SGM 891:right and left dentaries short.deep. thick-sectioned ramus with con- from craniumof NTM P8695-38.Kolopsis vex "rocker" inl'criorborder profile of Plr- torrrs- SGM 889: cranium.Supplementary .rrlio.iistrai-qht. strong. high diastemalcrests specimensreferred to arenoted in thetext and expandedinto wide,flattened surfaces anteri- figures. orly and elongateddiastema resembling Diagnosis.As for the