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A skull of Proargyrolagus , the oldest argyrolagid (Late Oligocene Salla Beds, Bolivia), with brief comments concerning its paleobiology

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A skull of Proargyrolagus , the oldest argyrolagid (Late Oligocene Salla Beds, Bolivia), with brief comments concerning its paleobiology
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  This article was downloaded by: [Duke University Libraries]On: 13 August 2015, At: 12:23Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place,London, SW1P 1WG Journal of Vertebrate Paleontology Publication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/ujvp20 A skull of Proargyrolagus, the oldest argyrolagid (LateOligocene Salla Beds, Bolivia), with brief commentsconcerning its paleobiology Marcelo R. Sánchez-Villagra a  & Richard F. Kay aa  Department of Biological Anthropology and Anatomy Duke University Medical Center , Box3170, Durham, North Carolina, 27710Published online: 24 Aug 2010. To cite this article:  Marcelo R. Sánchez-Villagra & Richard F. Kay (1997) A skull of Proargyrolagus, the oldest argyrolagid (LateOligocene Salla Beds, Bolivia), with brief comments concerning its paleobiology, Journal of Vertebrate Paleontology, 17:4,717-724, DOI: 10.1080/02724634.1997.10011019 To link to this article: http://dx.doi.org/10.1080/02724634.1997.10011019 PLEASE SCROLL DOWN FOR ARTICLETaylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions  JournalofVertebratePaleontology17(4):717-724,December1997 © 1997bytheSocietyofVertebratePaleontology A SKULLOF PROARGYROlAGUS THE OLDESTARGYROLAGID (LATE OLIGOCENE SALLA BEDS,BOLIVIA), WITH BRIEFCOMMENTSCONCERNING ITS PALEOBIOLOGY MARCELO R. SANCHEZ-VILLAGRA and RICHARD F. KAY Department of Biological Anthropologyand Anatomy DukeUniversityMedical Center, Box 3170,Durham,NorthCarolina 27710 ABSTRACT-A skull of theoldestdescribedargyrolagid, Proargyrolagusbolivianus (late Oligocene of the SallaBeds ,Bolivia) allows an assessment of the phylogeny of this group andareconstruction of itspaleobiology.Severalaspects of thecranial anatomy of the Monodelphis-sizedProargyrolagus serve tosupportthehypothesisthat argyro lagidsaremarsupials:thepalatehaslargevacuities,theangularprocessismediallyinflected,the auditory bullais composed of awing of thealisphenoid,andthedentalformulaincludesfourmolars.LikeArgentine Plio-Pleistocene argyrolagids, Proargyrolagus hasanunfusedsymphysis,andaphanericand almost vertically orientedectotympanic intheshape of aflattenedring. Proargyrolagus ismoreprimitivein having aless globular braincase, smallerorbits ,anda shallower mandible. The dentalformulais4/3.1/1.3/2.4/4or4/4.1/0.3/2.4/4(vs.2/2 .0/0 .1/1.4/4for Argyrolagus . Largeinfraorbitalforaminatransmitted nerves andvessels of thesnout. The snout is long withthenasalsprojectingwell forward of theincisors and withastrongantorbitalfossaforfacial muscles .Itcanbeinferredthatthis animal hadwell-developedvibrissaeandamobileproboscis,andreliedheavilyonitsrostrumto gather tactileinformation. Food manipulationwasassistedbyanelongate, procumbentlower incisorthat occluded withthreesharply-edged upper incisors,asinlivingphalangeriformmarsupials. LikePlio-Pleistocene argyrolagidsand extant elephant shrews , Proargyrolagus hasprismaticandhigh crownedcheek teethsuggestinganabrasive diet consisting perhaps of seeds . INTRODUCTIONThe Argyrolagidae are one of the most enigmaticextinctgroups of SouthAmerican mammals . Most of what is known about thisgroupis based on thedescription by Simpson (1970a,b) of skulls, jaws,and postcrania of severalhighly specialized Plio-PleistocenespeciesfromArgentina .Other, poorlyknown,argyrolagidshavebeenreportedfrom Neogene depositsinAr gentinaandBolivia (Hoffstetter and Villaroel ,1974; Villaroeland Marshall,1988), and thegroup is reported in Paleogenerocks of Chile (Wyssetal.,1994).In1984, Wolffdescribed Proargyrolagusbolivianus fromthe late Oligocene beds of Salla,Bolivia, based on two mandibularfragmentswith teeth. Here we offer a description of an almostcomplete skull andother cranial material of P.bolivianus. Thetwo specimensdiscussed inthis paper are housed at the Museo Nacionalde Historia Natural,La Paz,Bolivia (MNHN Bol-V). Other abbreviationsused inthe text areforthe Museo Municipal deCiencias Naturales yTradicionalde Mar del Plata, Mar delPlata, Argentina (MMMP)and theFlorida State Museum at theUniversity of Florida, Gainesville(UF) . SYSTEMATICPALEONTOLOGY Supercohort MARSUPIALIA Illiger, 1811 Order  PAUCITUBERCULATA Ameghino, 1897 Family ARGYROLAGIDAE Ameghino, 1904 Genus PROARGYROLAGUS Wolff,1984 Diagnosis-As for thetype and onlyspecies . PROARGYROLAGUSBOLlVlANUS Wolff, 1984 Emended Diagnosis--Proargyrolagus differsfrom Argyrolagus,Microtragulus, and Hondalagus inthatthemesial and distallobes of m2-m3 aresimilarinwidth,molarsarenotevergrowing,smallparaconid and hypoconulid can bedistinguishedon unworn teeth,andfrom Hondalagus and Microtragulus inhaving lower molarswith marked verticalgroovesrunning both labiallyandlingually. Proargyrolagus differsfrom Argyrolagus initssmaller, more convergentorbits,presence of apostorbitalprocess,less globular braincase,dentalformula4/3.1/1.3/2.4/4or 4/4.1/0.312.4/4 (vs. 212.0/0.1/1.4/4 .Proargyrolagus differs from Argyrolagus and Microtragulus (cf . Microtragulusbolivianus: Hoffstetter and Villaroel,1974)inhavinga shallowermandible ,and from Microtragulusbolivianus inlackinga canalretroden taire (seeHoffstetterandVillaroel,1974:1950). New Material-MNHN-Bol-V-003454 ,an incomplete skull withattached lowerjaws (Fig . I); andMNHN-Bol-V-003453, an incompletemaxilla and thecorresponding lower jaw . The Monodelphis-sized skull possess all of its replacement teeth but belonged toa young individual, given thelittle wear onits teeth and its open cranialsutures . Age andProvenance-The two specimensreported herecome from theSalla Beds,formed asan alluvial fill of a basin cut into deformed Paleozoicstrata at the edge of theeastern Andean Cordillera of northern Bolivia(McBride etal.,1987; Sempere etal.,1990). The Salla Beds contain a rich mammalian Deseadanfauna (Hoffstetter,1968). The argyrolagidspecimensdescribed herecome from stratigraphiclevels5 or 6atTapial Pampa (Villaroel and Marshall, 1982). These unitsare among the richestfaunallevelsinthe SallaBeds. All stratigraphically documented primates come fromUnit 5.Kayetal.(1995)re ported Ar40/Ar39 dates bracketing 5 and 6 levels at between 24.93 ±/-0.50 and 24.79 ±/-0.27 Ma (late Oligocene),con firming previousageestimates of MacFadden etal. (1985) andNaeser etal.(1987). DESCRIPTION OF NEWMATERIAL Skull Table I givesdimensions of theskull. Snout-The long nasalsproject well forward of the incisors asin Argyrolagus . They gradually widen posteriorly , lapping onto the frontalsat thelevel of themaxillo-frontal suture,ap proximatelydorsal to M3. The suture between thepremaxilla andthe maxilla makes a jaggedcourse dorsoventrally onthe side of the snout.It begins dorsallyat thelevel of the most anteriorpart of 13. From thereittakesa craniad and ventral course to thealveolarregion just717    D  o  w  n   l  o  a   d  e   d   b  y   [   D  u   k  e   U  n   i  v  e  r  s   i   t  y   L   i   b  r  a  r   i  e  s   ]  a   t   1   2  :   2   3   1   3   A  u  g  u  s   t   2   0   1   5   ...I -   Cs   r- :;;   ::d   r- CJ C C§ t .: -J   \ \ FIGURE I MNHN -Bol-V-003454  Proargyrolagusbolivi anu s , anincompleteskullwithattachedrightl ower jaw.A.rightlateralview.B.leftlateralview.C,palatalview.D,dorsalview.   Subd ivisionsonscalerepresentImm.    D  o  w  n   l  o  a   d  e   d   b  y   [   D  u   k  e   U  n   i  v  e  r  s   i   t  y   L   i   b  r  a  r   i  e  s   ]  a   t   1   2  :   2   3   1   3   A  u  g  u  s   t   2   0   1   5  SANCHEZ VILLAGRA AND KAY THE OLDEST ARGYROLAGIDSKULL 719 TABLE I.Measurements(mm) of theskull of Proargyrolagusboli  vianus (MNHN-Bol- V-003454).SeeCartmill(1970)andKayandCartmill(1977)fordefinitionandexplanation of measurements.Rhinionandnasionrefertotheanteriorandposteriorpointontheinternasalsuturerespectively,andprosthionisthemidpoint of thelineconnectingthemostanteriorpointsonthemargins of thesockets of thetwoanteriorupperteeth. posteriorto14. It thenturnssharplycaudallytothealveolarmargin.Itappearsthatthereisaverynarrowcontactbetweenthepremaxillaandthefrontal,consequentlyexcludingamaxillary-nasalcontact.Anarrowposteriorprojection of thepremaxillabetweenthemaxillaandthenasalisalsopresentin Argyrolagus (Simpson,1970a).Asinmostmarsupials(exceptborhyaenoids:Marshalland Kielan-J aworowska , 1992;seeArcher,1982),thereisafrontal-maxillarycontact(Fig.2).Alargeinfraorbitalforamen,whichtransmittednervesandvessels of thesnout,issituatedabovethesecondpremolar.Inlateralview,themaxillaanteriortotheinfraorbitalforamenisdistinctlyconcave.Craniadfromtheinfraorbitalforamenisawell-developedantorbitalfossa. The antorbitalfossaservedastheoriginforsnoutmusclesandmuscles of thesnoutvibrissae(Novacek,1993). Zygoma and Orbits-The zygomaticarchisbroken.Atitsrootitisbroaddorsoventrally,asisillustratedfor Argyrolagus bySimpson(1970a:72). The orbitalregionisnotwellpreserved. From theposition of apostorbitalprocessonthefrontal(thedorsalattachmentforthepostorbitalmembrane),however,itcanbeinferredthat Proargyrolagus hadsmall,convergentlyorientedorbits,suggestingithadsmalleyes,roughlyequivalentinproportionstothose of Caenolestes (Fig.3).Thisisamajordeparturefromtheconditionin Argyrolagus, inwhichtheorbitsreportedlyarelargeandfacemorelaterally(Simpson,1970a). The frontaloccupiesmost of theskull roof intheorbitalregion. Palate-The floor of thepalateisperforatedbylarge posteriorpalatalvacuities (sensuOsgood,1921)atthelevel of themolars,asin Caenolestes. It isnotpossibletoseeiftherearelargeanteriorvacuities. Basicranium- The leftside of thebasicraniumispreservedanddisplacedmedially;therightsideismissing(Figs. 4,5). The ossifiedauditorybullaisovoidwiththelongaxisdirected posterolatero-anterornedially, asin Argyrolagus (Simpson, 1970a).Itiscomposed of anexpandedtympanicprocess of the FIGURE 3.Dorsalview of theskull of Proargyrolagusbolivianus (MNHN-Bol-V-003454).Arrowindicatesavisualestimate of orientation of opticalaxis.Scale bar = 5mm.30.010.713.912.813.59.213.9Length of skullInterorbitalwidthLength of preorbitalrostrumLength prosthion-nasion Length of internasalsuture(na sion-rhinion) Width of dentalarcadeLength of dentalarcade FIGURE 2.Rightlateralview of theskulland jaw of Proargyrolagus bolivianus (MNHN-Bol-V-003454).Reconstructedanteriorupperpremolars,uppercanine,andupperdistalincisorarebasedonrootsockets,asareallantemolarlowerteeth.Scalebar = 10mm. FIGURE 4.Drawing of leftbasicranium of Proargyrolagusbolivianus (MNHN-Bol- V-003454)viewedventrally.Sketch of a cranium indicatestheorientation of thedrawing.Scalebar = 5mm. Abbreviations: acf ,anteriorcarotidforamen; as, alisphenoid;bo,basioccipital;bs,basisphenoid;e.ectotympanic; hff ,hypoglossalforamina;pe,petrosal; sq, squamosal.    D  o  w  n   l  o  a   d  e   d   b  y   [   D  u   k  e   U  n   i  v  e  r  s   i   t  y   L   i   b  r  a  r   i  e  s   ]  a   t   1   2  :   2   3   1   3   A  u  g  u  s   t   2   0   1   5  720 JOURNAL OF VERTEBRATEPALEONTOLOGY VOL. 17, NO  4 1997 FIGURE 5.Scanningelectronmicrographshowsportion of basicraniumillustratedinFigure4.Anterioristotop.Scalebar = 2mm.AbbreviationssameasinFigure4. alisphenoid.Wearecertain of theidentity of thisbonebecauseabonybridgewithoutsuturesjoinsthealisphenoidwiththebulla.Inaddition:I)asuturebetweenthealisphenoidandthesquamosalisseenlateraltotheauditorybulla,rulingoutthepossibility of asquamosalcontributiontothebulla;2)theectotympanicisasimple,verticallyoriented,flattenedringwithaminormedialflangeonitscaudo-ventralend;itispressedagainstthelateralside of thebulla,butnotfusedtoit,inavisibleor phaneric position(MacPhee,1981);3)thebasioccipitalandbasisphenoidareseparatedfromthebullabyamarkedfissure;and,4)acrest of thepetrosalismedialtothebulla.Thebasioccipital/basisphenoidcomplexisflat(thereisnomedialkeel).Alarge,anteriorlyfacinganteriorcarotidforamenisvisible just anteriortothesuture of thebasisphenoidwiththebasioccipital,whichisthemostcommonpositionamongmammals(Wible,1986).Twohypoglossalforaminaareseenadjacenttotheoccipitalcondyle.Simpson(1970a)emphasizedthatin Argyrolagus theforamenmagnumwasdirectedventrally,supportingtheideathatthisanimalwasbipedal,withanerectposture. The conditionin Proargyrolagus isunclearsincetheborder of theforamenismostlydestroyedandthepreservedregion of thebasicraniumissomewhatdisplaced. Mandible Proargyrolagus hasashallowermandibularramusthan Argyrolagus (Figs. 1,2; Wolff,1984).Bothtaxahaveanunfusedsymphysis.Alargementalforamenispresentbelowthealveolusforthelast of thepremolars.Posteriortoittherearetwoothersmallmentalforamina.Thecoronoidprocessisnarrow,slender,andstronglyangledposteriorly.Theposteriorborder of themassetericfossaismarkedbyacrestrunninganteroventrallyfromthesideofthecondyle.Eventhoughitisnotfullypreserved,theangularprocess of thedentaryclearlyisinflectedmedially,amarsupialfeature(Sanchez-VillagraandSmith,1997). TABLE 2.Dentaldimensions(mm) of Proargyrolagusbolivianus (MNHN-Bol-V-003454).Tooth Mesio-di sta1length Bucco-lingual width111.840.64121.600.36 13 0.840.3614P41.080.76MI1.561.52M21.521.56M31.401.52M41.121.00Trigonid-talonidwidthml1.640 .84-1.44 m21.40 1.36-1.40 m31.40 1.36-1.24 m41.40 1.08-0 .76 Dentition-Table 2presentsdentaldimensions of thespecimensstudiedinthispaper. Dental Formula-Three teethandonealveolusarepresentinthepremaxilla. P4-M4 arealsopreservedalongwiththreealveolianteriortoP4.Ifoneassumesthateachemptyalveolusinthemaxillarepresentsasingle-rootedtooth,thentheupperdentalformulais4.1.3.4.Themandiblespreservetheroot of alargeprocumbentincisorandfouralveolianteriorto p4-m4 .Weinterpretthelowerdentalformulatobe3.1.2.4or 4.0 .2.4.Alternativeinterpretations of thetoothformulawouldbedictatedifanypremolarsweretwo-rooted,becauseinsuchacasethenumber of teethwouldbelessthanthenumberofalveoli. Incisors-Most ofthecrown of theanteriorlowerincisor of Proargyrolagus isbrokenaway. The toothhasalong,labiolinguallycompressedprocumbentrootthatpassesunderthecheekteeth. The rootisopen,implyingcontinuedgrowththroughoutlife(Wolff,1984).Enamelispresentonlyonthelabialsurface.Behindtheanteriorlowerincisor(?i2,seebelowfordiscussion of homologies)thereisasmallerrootsocketfor?i3.Theobliqueorientation of thissocketsuggeststhat?i3wasprocumbentandpressedalongtheside of ?i2;?i3isalsoprocumbentin Argyrolagus (Simpson,I970a).11-3arelabiolinguallycompressedandhighcrowned;theircrownsaretightlypackedagainstoneanotherformingasharp,ribbon-like,occlusaledge.Presumably,thelateraledge of theprocumbentlowerincisoroccludedlinguallywithinthesesharply-edgedupperincisorsinamannerreminiscent of livingphalangeriformmarsupials.14isknownonlybyitsalveolus.Thereisawidespacebetweenthe11s of thetwosides of thepalate,probablyallowingfreepropalinaltranslation of themandibles.However,thetemporomandibular joint isnotpreserved,sowecannotconfirmwhetheritsshapewouldhaveallowedsuchmovements. Cheek Teeth Proargyrolagus hasprismaticandhigh crownedcheekteeth(Figs.6,7).Wolff(1984)reportedthatinUF-27896  Proargyrolagusbolivianus themolarshaveclosedrootsexceptform4. They aredividedintotwolobesbyverticalgroovesonthelingualandlabialfaces.Mesialanddistallobesaresimilarinwidthon m2-m3 ,unliketheconditionin Arg y rolagus, inwhichtheanteriorlobeislarger. The upperandlowerM4saresmallerthanthemoreanteriormolars.Intheunwornstate,fourprincipalmolarcuspscanbeidentifiedasthemetaconid,protoconid,hypoconid,andentoconid.Asmallcolumnarparaconidisvisibleanteriorly,andasmalllinguallyplacedhypoconulidistobeseendistally.Thesesmallcuspsaresoonremovedbywear.Inindividualswithmoreheavily    D  o  w  n   l  o  a   d  e   d   b  y   [   D  u   k  e   U  n   i  v  e  r  s   i   t  y   L   i   b  r  a  r   i  e  s   ]  a   t   1   2  :   2   3   1   3   A  u  g  u  s   t   2   0   1   5
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