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YARALOIDEA (MARSUPIALIA, PERAMELEMORPHIA), A NEWSUPERFAMILY OF MARSUPIAL AND A DESCRIPTION AND ANALYSIS OFTHE CRANIUM OF THE MIOCENE YARALA BURCHFIELDIAuthor(s): JEANETTE MUIRHEADSource: Journal of Paleontology, 74(3):512-523. 2000.Published By: The Paleontological SocietyDOI: http://dx.doi.org/10.1666/0022-3360(2000)074<0512:YMPANS>2.0.CO;2URL: http://www.bioone.org/doi/full/10.1666/0022-3360%282000%29074%3C0512%3AYMPANS%3E2.0.CO%3B2

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J. Paleont., 74(3), 2000, pp. 512–523Copyright q 2000, The Paleontological Society0022-3360/00/0074-0512$03.00

YARALOIDEA (MARSUPIALIA, PERAMELEMORPHIA), A NEWSUPERFAMILY OF MARSUPIAL AND A DESCRIPTION AND ANALYSIS OF

THE CRANIUM OF THE MIOCENE YARALA BURCHFIELDIJEANETTE MUIRHEAD

School of Biological Science, University of New South Wales, NSW 2052 Australia

ABSTRACT—The skull of the Miocene Yarala burchfieldi Muirhead and Filan, 1995, is described. Analysis of skull morphology supportsphylogenetic conclusions based on dental morphology. Y. burchfieldi shares a number of synapomorphies with other peramelemorphians,some of which are unique and help to define this order of marsupials. Y. burchfieldi is the most plesiomorphic peramelemorphianknown. Although sharing some derived characters with a number of extant taxa, Y. burchfieldi lacks synapomorphies that unite all otherperamelemorphian taxa as the Superfamily Perameloidea. The Yaraloidea and Yaralidae, a new superfamily and family of peramele-morphians, is proposed and diagnosed on the basis of Y. burchfieldi. Fossil evidence supports the late divergence of perameloids, whileperamelemorphian diversity in the Tertiary indicates an ancient derivation for the order.

INTRODUCTION

RELATIONSHIPS AMONG peramelemorphians (commonlycalled bandicoots) and their relationships to other marsu-

pials are contentious. While many morphological investigationscentered on their relationship to either dasyuromorphians or di-prototontians (e.g., Bensley, 1903; Marshall, 1972; Szalay,1982), recent biochemical work (e.g., Kirsch et al., 1991;Springer et al., 1994; Kirsch et al., 1997) is providing increasingevidence to suggest that they may lie outside the australidelphianclade (comprising dasyuromorphians, notoryctimorphians, di-protodontians, and microbiotherians; Szalay, 1982). These bio-chemical results have not only broadened views on the originsof the group, but have forced re-evaluation of current under-standing of their intercontinental dispersal and divergence dates(e.g., Kirsch et al., 1991; Woodburne and Case, 1996; Kirsch etal., 1997).

Similarly unresolved are the intergeneric and interfamilial re-lationships of peramelemorphians. Classification based on mor-phological features by Groves and Flannery (1990) distinguishthe families Peramelidae, comprising predominantly all Austra-lian taxa, and Peroryctidae, comprising almost exclusively NewGuinean taxa. While molecular studies generally support themonophyly of each of these two groups, they offer differentconclusions for the genus Macrotis. In serological studies byKirsch (1968, 1977), Macrotis was found to be distinct from allother peramelemorphians. Microcomplement fixation of albu-men (Baverstock et al., 1990a) found Macrotis equally distinctfrom peroryctids and a clade comprising all other peramelids,while a subsequent study (Baverstock et al., 1990b) found Ma-crotis to be monophyletic with peroryctids. DNA/DNA hybrid-ization indicates that Macrotis lies outside all other peramele-morphians as their sister-group (Kirsch et al., 1990; Kirsch etal., 1997). These studies support the morphological distinctionof Macrotis originally noted by Bensley (1903) as the subfamilyThylacomyinae, who remarked on the unique morphology ofthis group among marsupials and its rarity within Mammalia.This ranking was subsequently raised to the family Thylaco-myidae by Archer and Kirsch (1977) based on morphological,serological, and karyological evidence.

The peramelemorphian Yarala burchfieldi was described onthe basis of dental material from Tertiary deposits of Riversleigh,northwestern Queensland (Muirhead and Filan, 1995). It is theoldest peramelemorphian yet known, occurring in a number ofsites with an estimated time range of early to middle Miocene(Archer et al., 1997). The skull material of Y. burchfieldi de-scribed here is the first for a peramelemorphian older than the

Pleistocene. The inability to allocate this taxon to any currentlyrecognized peramelemorphian family based on dentition (Muir-head and Filan, 1995) suggests that it may predate the diver-gence date for their radiation. Cranial material described hereprovides a further testing ground for these conclusions.

Taxonomy follows Aplin and Archer (1987) unless otherwisestated. Cranial morphology follows Archer (1976a) and Marshalland Muizon (1995) unless otherwise stated, tooth morphologyfollows Archer (1976b), and tooth homology follows Flower(1867) and Luckett (1993) where the adult (unreduced) postca-nine cheektooth formula of marsupials is P1–3, M1–4. All Y.burchfieldi specimens used in the description are registered inthe fossil vertebrate collection of the Queensland Museum(QMF).

SYSTEMATIC PALEONTOLOGY

Supercohort MARSUPIALIA (Illiger, 1811) Cuvier, 1817Cohort AUSTRALIDELPHIA Szalay, 1982

Order PERAMELEMORPHIA (Kirsch, 1968) Aplin and Archer,1987

Superfamily YARALOIDEA new superfamily

Type species.—Yarala burchfieldi Muirhead and Filan, 1995,p. 127–134, figures 1–2.

Superfamily diagnosis.—Species of this superfamily differfrom all other peramelemorphians in the following features:postparacrista and premetacrista join on all molars rather thanbreach the ectoloph or connect to stylar cusps; vestigial meta-conule present on M3 while on M1 and M2 postprotocristastraight and connects to lingual base of metacone; cristid obliquaterminates against trigonid on the labial side of the metacristidcarnassial notch on M1 to M2 and at the base of the notch onM3; premaxilla-nasal suture (appears to be) shorter than the max-illa-nasal suture; nasals extend posteriorly past the anterior rimof orbital fossa; alisphenoid large and contacts parietal; presenceof a primary foramen ovale lying between the alisphenoid andperiotic without the presence of additional secondary foramenovale; infraorbital canal located dorsal to P3.

Family YARALIDAE new family

Diagnosis.—As for superfamily diagnosis until further taxaare known.

YARALA Muirhead and Filan, 1995

Diagnosis.—As for the family until further taxa are known.

513MUIRHEAD—NEW MIOCENE MARSUPIAL

YARALA BURCHFIELDI Muirhead and Filan, 1995Figures 1–3

Revised specific diagnosis.—That of the superfamily diagno-sis until further species are known.

Supplement to the original description based on topotype.—No nasal or premaxilla preserved; nasal-frontal-maxilla junctionlocated directly dorsal to M2; infraorbital foramen located abovealveoli of P3; deep area for attachment of maxillo-naso-labialispresent at anterior base of zygomatic arch; lacrimal canal locatedin maxilla-lacrimal suture; maxilla-lacrimal suture lines anteriorrim of orbit from anterodorsal corner to lacrimal foramen; lac-rimal extends further outside of orbit ventral to lacrimal foramenand onto facial region, rims anterior of zygomatic arch and lim-ited ventrally by palatine and infraorbital foramen; palatineforms small portion of posteromedial boundary of infraorbitalforamen; palatine irregular in shape with flanges extending ontofrontal and posterior of maxilla; sphenopalatine foramen large;no posteropalatine foramen evident.

Cranium narrowest at posterior region of orbital fossa; orbi-tosphenoid small; ethmoidal foramen located on dorsal tip oforbitosphenoid, directly anterior to base of frontal-alisphenoidsuture; palate floors ventral rim of orbitosphenoid; sphenorbitalfissure slightly narrower than foramen rotundum, both tube-like;orbitosphenoid forms anteromedial wall of sphenorbital fissure;foramen rotundum located immediately posteroventral to sphen-orbital fissure.

Alisphenoid-parietal suture horizontal; posterior base of zy-gomatic arch made of equal portions of alisphenoid and squa-mosal; cranial region dorsally flat in lateral view; small sagittalcrest continuous from frontal through parietal to posterior cranialwall; parietal flairs forming temporal crests along dorsal rim ofposterior cranial wall.

Squamosal separated from frontal by alisphenoid; squamosal-alisphenoid suture dorsoventrally orientated; squamosal-parietalsuture oblique from alisphenoid to mastoid; subsquamosal fossalarge, triangular, completely bound by squamosal.

Two palatal vacuities in palate; transverse palatine processlow; maxilla-palatine suture convex around anterolateral regionof palatine.

Glenoid fossa strongly arched; squamosal and small part ofalisphenoid (entoglenoid) contribute to articular surface for ar-ticular condyle of dentary; postglenoid foramen mostly sur-rounded by squamosal, mastoid defines medial foramen wall;transverse foramen large and ovoid; low alisphenoid arch liesimmediately lateral to transverse foramen and at anterior bound-ary of alisphenoid tympanic wing; small anterolateral and pos-teromedial arch openings do not open into cranium; alisphenoidwing not bulbous in shape, departs from cranium at a shallowangle, posterior and medial extent of alisphenoid wing unknowndue to breakage; alisphenoid hypotympanic sinus small andpoorly defined; alisphenoid-mastoid suture lies level with ante-rior rim of postglenoid foramen; primary foramen ovale large,extending anteriorly almost to alisphenoid arch; basisphenoidand basioccipital missing in this region; groove representingopening of entocarotid canal present in alisphenoid on medialside of primary foramen ovale.

Petrosal part of periotic (pars petrosa) is bulbous posteriorlywith flat, triangular shaped flange anteriorly, flange extendingslightly anteriorly past level of mastoid-alisphenoid suture; sharpdistinction between bulbous and flat anterior regions of pars pe-trosal, suggesting former location of alisphenoid tympanic wingtermination; fenestra cochleae (5ovalis) on lateral side of petro-sal is ovoid and large; fenestra vestibuli (5rotunda) is widerthan deep, ovoid, located posterolaterally on bulbous petrosal;anterior lamina of pars petrosa flat immediately posterior to pars

petrosa-alisphenoid suture with much of lateral side of pars pe-trosa occupied by epitympanic recess; recess is not rimmed onanterior side but with high ridge present on medial side; ridgeat medial side of epitympanic recess increases depth of incudalfossa located at posteriormost extremity of epitympanic recess,and lines channel of foramen faciale; no squamosal epitympanicsinus present.

Posterior cranial wall much wider than high; temporal crestcapped by sesamoid; supraoccipital wide with distinct notch atdorsal tip of foramen magnum; paroccipitals and exoccipitals notpreserved; obliquely orientated mastoid-supraoccipital contact;mastoid has a small contribution to lateral side of cranium; oc-cipital crest continues through dorsal half of mastoid.

Material examined.—Topotype QMF23385 skull with maxil-la, palate, lacrimal, frontal, parietal, alisphenoid, squamosal, ba-sisphenoid, mastoid and petrosal and dentition in skull (Figs. 1–3).

Occurrence.—As for holotype, Upper Site, Godthelp Hill,Riversleigh, northwestern Queensland (Upper Site Local Fauna).Upper Site is part of the System B deposits of Archer et al.(1997) and considered to be early Miocene in age.

PHYLOGENETIC DISCUSSION

Yarala burchfieldi is considered to be a peramelemorphianbecause it exhibits a combination of apomorphic dental char-acters otherwise only known to this group of marsupials (Muir-head and Filan, 1995). Because the sister-group of peramele-morphians is still under debate, the polarity of skull charactersfor this analysis were determined using a number of (possible)outgroup taxa, including plesiomorphic dasyuromorphians (e.g.,Badjcinus turnbulli Muirhead and Wroe, 1998), a variety of di-delphids (e.g., species of Didelphis, Metachirus, Philander, Chi-ronectes, Lutreolina, Monodelphis, Glironia, Lestodelphys), al-phadontines sensu Johanson (1996) (i.e., Alphadon and Turgi-dodon) and the early Paleocene didelphimorphian Pucadelphysandinus Marshall and Muizon, 1988. Because microbiotheriansare considered to be part of, and possibly basal, to the Austral-idelphia based on tarsal morphology and molecular studies (Sza-lay, 1982; Westerman and Edwards, 1991; Kirsch et al., 1991;Springer et al., 1994; Kirsch et al., 1997), they are an obviousgroup with which to compare peramelemorphians. However, inmany respects, particularly in the auditory region, the micro-biotherian skull is considered derived (Segall, 1969; Archer,1976a; Aplin and Archer, 1987; Reig et al., 1987). These fea-tures of microbiotherians were, therefore, not used in this anal-ysis. This work aims to consider the phylogenetic relationshipof Y. burchfieldi among other peramelemorphians, rather thanthe relationships of peramelemorphians within Marsupialia. Thephylogenetic relationships of peramelemorphians among mar-supials is currently being prepared as a numerical cladistic anal-ysis.

Features shared by Y. burchfieldi and all other peramele-morphians (synapomorphies).—The dentition of Y. burchfieldishares many synapomorphies with other peramelemorphians,supporting its inclusion in Peramelemorphia (Muirhead and Fi-lan, 1995). These synapomorphies include: 1) the posterolabiallyorientated preparacrista on M1 terminating at stylar cusp B; 2)the labially positioned centrocrista with a narrow (relative todasyuromorphians) angle between the postparacrista and pre-metacrista; 3) the lingually positioned hypoconulid located al-most directly posterior to the entoconid and its overlapping ar-rangement with the anterior cingulum of the following molar;4) the low, indistinct hypoconulid; 5) loss of the posterior cin-gulid of the lower molars; 6) the cristid obliqua terminatesagainst the trigonid in a more lingual position from M1 to M4

(size of hypoflexid increases posteriorly); and 7) the third lowerincisor is bilobed.

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515MUIRHEAD—NEW MIOCENE MARSUPIAL

FIGURE 2—Yarala burchfieldi paratype QMF23385 partial skull from the early Miocene Upper Site, Riversleigh, northwestern Queensland. 1, Dorsalview. 2, right lateral view. Abbreviations AL, alisphenoid; ef, ethmoidal foramen; fno, fenestra ovalus; fnr, fenestra rotundum; FR, frontal; fr,foramen rotundum; if, infraorbital foramen; JU, jugal; LA, lacrimal; ld, lacrimal duct; MP, mastoid part of periotic; MX, maxilla; OS, orbitosphenoid;PA, parietal; PL, palatine; PP, petrosal part of periotic; sf, sphenorbital fissure; SO, supraoccipital; SQ, squamosal; ssf, subsquamosal fossa; tf,transverse foramen.

←

FIGURE 1—Yarala burchfieldi paratype QMF23385 partial skull from the early Miocene Upper Site, Riversleigh, northwestern Queensland. 1, Ventralview; 2, dorsal view; 3, right lateral view; 49–4 stereo views of basicranium. Scale bar 5 1 cm.

In addition to these dental features, the skull of Y. burchfieldihas two features that appear to be unique to peramelemorphians.These are: 1) the jugal is bifid at its contact with the maxilla;and 2) the nasals are continuously slender. Regarding these:

1) In all American marsupials examined, including the earlyPaleocene Pucadelphys andinus, the anterior portion of the jugalforms a single flange, posteriorly descending from an antero-dorsal contact with the lacrimal near the anterior edge of theorbital fossa. This is also the condition shown by all dasyuro-morphians, including fossil forms. In contrast, all peramelemor-phians, including Y. burchfieldi, have a Y-shaped jugal that bi-furcates at its contact with the maxilla at the base of the zygo-matic arch. The bifid processes surround the nasolabial fossa, adepression for the attachment of the maxillo-naso-labialis (Filan,1990). Novacek (1986) described a similar condition in Leptictis

as apomorphic for eutherians. The lack of the bifid jugal in per-amelemorphian outgroups indicates the bifid condition is aut-apomorphic for peramelemorphians (Muirhead, 1994). Also,

2) Broad nasals are considered to be plesiomorphic for mam-mals (Novacek, 1986). In all didelphimorphians (including theearly Paleocene P. andinus) and dasyuromorphians, as well asDromiciops gliroides, the nasals broaden in the region posteriorto the premaxilla-nasal suture relative to the more anterior por-tions. Peramelemorphians, in contrast, have narrow nasals alongtheir entire length. The distribution of these character states in-dicates the peramelemorphian condition is apomorphic (Muir-head, 1994). This is also the condition of the nasals in Y. burch-fieldi, where the nasal/maxilla suture indicates that the nasalswere consistently narrow posteriorly.

Features of Y. burchfieldi more plesiomorphic than any

516 JOURNAL OF PALEONTOLOGY, V. 74, NO. 3, 2000

FIGURE 3—Yarala burchfieldi paratype QMF23385 partial skull from the early Miocene Upper Site, Riversleigh, northwestern Queensland, ventralview of basicranium.

known peramelemorphian.—Although Y. burchfieldi shares apo-morphies with Recent peramelemorphians, it also exhibits manyfeatures that are more plesiomorphic than any previously knownperamelemorphian. In the dentition, these features include thatthe postparacrista and premetacrista join and do not breech theectoloph and the cristid obliqua does not terminate lingual to themetacristid notch (midpoint of the crown) on M1–3 (Muirheadand Filan, 1995). Yarala burchfieldi also displays features of theskull that are more plesiomorphic than any known peramele-morphian. These features are as follows:

1) The snout is shorter than in any other peramelemorphian.Although the premaxilla is absent, snout length is indicated bytwo features. In all Recent peramelemorphian taxa, the premax-illa tends to be enlarged, compared to ameridelphians and das-yurids. As a result, the premaxilla-nasal suture is longer than themaxilla-nasal suture. This is considered an apomorphy for per-amelemorphians. This does not appear to be the case for Y.burchfieldi, making it unique among peramelemorphians. An-other indicator of snout length is the degree of posterior exten-sion of the nasals. In all dasyuromorphians and didelphids ex-amined, including P. andinus, as well as D. gliroides, the nasalbones extend posteriorly past the anterior rim of the orbital fos-sa. This is therefore considered to be the plesiomorphic state. Inall extant peramelemorphians, the nasals do not extend posteri-orly past this region. Yarala burchfieldi displays the plesio-morphic condition, and is again unique among peramelemor-phians in the state of this character.

2) The alisphenoid in Y. burchfieldi is large and contacts theparietal. In Y. burchfieldi the posterior region of the zygomaticarch is made up of equal proportions of alisphenoid and squa-mosal. In contrast, extant peramelemorphians show no contactbetween the alisphenoid and the parietal, the zygomatic archbeing mostly made up of squamosal (Muirhead, 1994). In allextant peramelemorphians, the alisphenoid is smaller in lateralview than the corresponding bone in Y. burchfieldi, is lower inheight, and is less posteriorly extended.

An alisphenoid-parietal contact occurs in most didelphids,myrmecobiids, most dasyurids (Archer, 1976a) and P. andinus(Marshall and Muizon, 1995). There is no contact between thesebones in extant peramelemorphians, microbiotherians (personalcommun., Muizon), thylacinids, vombatids, some didelphids(personal commun., Muizon), and some dasyurids (Archer,1976a). This contact varies in species of borhyaenids, with thestructurally more generalized forms having an alisphenoid-pa-rietal contact, and the more specialized forms without (Archer,1976a). Murray et al. (1987) considered that alisphenoid-parietalcontact is plesiomorphic for diprotodontoid groups and that ex-pansion of squamosal (anteriorly) or frontal (posteriorly) ex-plains the frontal-squamosal contact in some specialized dipro-todontian genera. A squamosal-frontal contact is therefore con-sidered to be apomorphic for peramelemorphians.

Yarala burchfieldi is unique among peramelemorphians be-cause of its plesiomorphic alisphenoid-parietal suture. The lat-eral height of the alisphenoid in Y. burchfieldi is equal to that

517MUIRHEAD—NEW MIOCENE MARSUPIAL

of the lacrimal and can be seen in dorsal view. In other pera-melemorphian taxa, the alisphenoid is low and cannot be seenwhen viewed dorsally.

3) Yarala burchfieldi has a large primary foramen ovalebound between the alisphenoid and periotic, without the con-current presence of additional secondary foramen ovale. Thepresence of a primary foramen ovale (sensu Gaudin et al., 1996)lying between the alisphenoid and periotic (when in this loca-tion, termed a foramen pseudovale by Archer, 1976a) is ple-siomorphic for Australian marsupials (Muirhead, 1994; Muir-head and Wroe, 1998; and see review by Wroe, 1997a). Yaralaburchfieldi has a large primary foramen ovale in this position,but although this is not unique among peramelemorphians(Muirhead, 1994), Y. burchfieldi is unique in that it does nothave an additional secondary foramen ovale (sensu Gaudin etal., 1996). For all other peramelemorphians, when a primaryforamen ovale lies between the alisphenoid and periotic, addi-tional secondary foramen ovale are present (Muirhead, 1994).Yarala burchfieldi does, however, have a small bridge formedin the alisphenoid in the area often occupied by a secondaryforamen ovale in other peramelemorphians. This bridge mayfunction in the separation of the mandibular branch of the tri-geminal nerve outside the cranium, but no perforation exits inthis area through the alisphenoid into the cranium on Y. burch-fieldi.

4) The anterior position of the infraorbital foramen. The in-fraorbital canal has its anterior foramen located dorsal to P3 inY. burchfieldi, while in all other peramelemorphians it is locatedbetween P3 and M1. Although the variation of the position ofthis foramen across families could be related to snout length,within families its location tends to be relatively consistent. Inother peramelemorphians there is variation in snout length, whilethe position of the infraorbital foramen remains stable. Its lo-cation in these taxa cannot, therefore, be considered part of acharacter complex with snout length. Many didelphids (e.g., spe-cies of Glironia, Lestodelphys, Lutreolina, Chironectes, Didel-phis) have the infraorbital foramen located above P2 or P3, moreanterior than in any peramelemorphian. Dasyurids and notoryc-tids generally have this foramen located above M1 while in thy-lacines it is usually (but not exclusively) even more posteriorlylocated (Muirhead, 1997; Muirhead and Wroe, 1998). The an-teriorly placed infraorbital foramen (as in didelphids) is hereconsidered plesiomorphic. The anterior condition of this fora-men on Y. burchfieldi is therefore considered to be plesio-morphic relative to other peramelemorphians.

Symplesiomorphies of Y. burchfieldi and other peramelemor-phians.—The distribution of the features listed above for Y.burchfieldi supports the conclusions drawn from dental charac-ters alone (Muirhead and Filan, 1995): Yarala burchfieldi is aperamelemorphian, but it is more plesiomorphic than others andlies outside a clade comprised of the families of extant taxa.Yarala burchfieldi is symplesiomorphic with some other pera-melemorphian taxa where these character states have been ple-siomorphically retained. These plesiomorphic features includethe following:

1) The lack of a distinct lacrimal rim. There is very littledevelopment of a rim or crest around the orbit in Y. burchfieldi;the orbit has no distinct rim separating it from the anterior facialregion (lacrimal rim). This is similar to the condition in Micro-peroryctes longicauda, Peroryctes raffrayana, Echymipera ka-lubu, and E. clara. Isoodon obesulus and I. macrourus have asharp change in the angle of the lacrimal over this region, whileall species of Perameles, Chaeropus, and Macrotis have a strongrim developed on the lacrimal, anterior to the orbital fossa. Thisrim development is considered by Groves and Flannery (1990)to be a synapomorphy of the Peramelidae.

2) The morphology of the foramen rotundum. The plesio-morphic condition of the foramen rotundum is that of a tubelikecanal with an opening facing anterolaterally. This is the condi-tion found in P. andinus, many didelphids (Archer, 1976a) andplesiomorphic dasyurids. Among peramelemorphians, this ple-siomorphic state is seen in all species except species of Pera-meles, where there is a direct lateral opening of the foramenrotundum into the basisphenoid and no tubelike canal, a condi-tion considered here to be the apomorphic state. Yarala burch-fieldi shows the plesiomorphic condition. This polarity decisionis in contrast to that of Groves and Flannery (1990), who usethe tube-like condition of the foramen rotundum as a synapo-morphy for the Peroryctidae.

3) Size of the orbitosphenoid. The large and conspicuous sizeof this bone in Y. burchfieldi is only otherwise seen in Macrotislagotis among peramelemorphians. The large contribution of thisbone to the orbital region of the skull is considered to be ple-siomorphic because of the similar condition found in P. andinusand plesiomorphic didelphids and dasyurids. In all other speciesof peramelemorphians, this bone is very small or completelyabsent from the lateral surface of the skull.

4) Lack of a distinct squamosal epitympanic sinus. Plesio-morphically (as in most didelphids examined) the squamosalforms no distinct epitympanic sinus. It is considered to be ru-dimentary, absent or small in didelphids and most borhyaenids,but always present in dasyurids (Archer, 1976a). Yarala burch-fieldi has no distinct squamosal epitympanic sinus. This is alsothe case for species of Microperoryctes, Peroryctes, and Echy-mipera, while all species of Perameles, Chaeropus, Isoodon, andMacrotis have various degrees of enlargement of this sinus.

5) Shape of the supraoccipital. In Y. burchfieldi, the supra-occipital is wider than tall, and there is no development of thesupraoccipital onto the dorsal surface of the skull. The dorsalsurface of the posterior cranial wall is defined by a well devel-oped crest. Species of Perameles, Isoodon, Macrotis, Chaero-pus, and Echymipera tend to have supraoccipitals that are tallerthan wide. Species of Microperoryctes and Peroryctes have su-praoccipitals with more equal dimensions. The flatter shape, asseen in didelphids, appears to be plesiomorphic.

Synapomorphies of Y. burchfieldi and other peramelemorphi-ans.—Not all characters found in Y. burchfieldi support the con-clusion that this species lies phylogenetically outside of all otherperamelemorphians. If this position is accepted, some charactersmust have undergone reversal or convergence. Synapomorphicstates found in Y. burchfieldi and other species of peramele-morphians are as follows:

1) The antorbital fossa. This is formed by the depression ofthe maxilla and lateral enlargement of the jugal at its contactwith the maxilla. The maxillo-naso-labialis attaches to the dor-sally enlarged jugal and the depression of the maxilla and func-tions in lateral movement of the nose, while the deep masseterand superficial masseter (that function in mastication) attach tothe lower portion of the depression of the maxilla and the ventralarm of the bifid jugal (Filan, 1990). Peramelemorphians show arange of antorbital fossa development from none (e.g., Isoodonauratus) to extreme (e.g., Macrotis lagotis and Chaeropus ecau-datus). Pucadelphys andinus and all didelphids examined in thisstudy have very little or no fossa development on the anteriorregion of the zygomatic arch. The jugal and the region of themaxilla immediately anterior to the jugal, lacks any form ofdepression. There is also a lack of bone thickening around thisregion for attachment of the deep masseter or superficial mas-seter. The condition of this character in P. andinus and didel-phids is interpreted to represent the plesiomorphic state, and adeep fossa (such as in M. lagotis) the apomorphic state. Thedegree of enlargement of this fossa in Y. burchfieldi is greater

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than that seen in Microperoryctes longicauda and Isoodon au-ratus, but smaller than that seen in some species of Perameles,Macrotis, and Chaeropus. One would expect this feature to varydepending on diet, with greater muscle mass correlated withconsumption of harder food types. The size of the maxillo-naso-labialis may also depend on the length of the snout, or require-ments for accessing food. The larger size of this fossa in Y.burchfieldi indicates a diet comprising tough food type, but thedentition required to masticate tough plant matter is absent. Per-haps this species used its large masticatory muscles in foragingto obtain access to its prey.

2) Palatal vacuities. The palate of Y. burchfieldi has two va-cuities. The anterior one is of variable size; it is large in speci-men QMF23385, extending from P1 to medial of the center ofP3, while in specimen QMF16862, it extends from P1 to posteriorof P2. The posterior foramen is larger and extends from M1 toM4. Possible fusion of the vacuities over the midline of the pal-ate is unknown because of breakage. The size and number ofpalatal vacuities in peramelemorphians varies considerably.Some species (e.g., Isoodon macrourus) have the usual set ofvacuities in the molar region, and one set of very small vacuitiesposterior to these. Other species (e.g., Perameles bougainville)have very long vacuities in the premolar region, with two setsof large vacuities in the molar region (the posterior set withoutmedium septum), and an additional number of irregular vacuitiesposterior to these. This palatal condition would be described asvery open. Archer (1976a) considers palatal vacuities to be apoor indicator of relationship, being highly correlated with hab-itat. It is likely that this character is subject to some degree ofconvergence; however, polarity may be assigned because of thelack of large vacuities in ameridelphians (e.g., P. andinus, bor-hyaenoids). In terms of size and number of vacuities, Y. burch-fieldi appears to be more similar to M. longicauda than all otherperamelemorphians, showing a tendency to form a second (an-terior) set of vacuities in the premolar region, while retainingthe septum between the vacuities in the molar region. This openform of palate is more apomorphic than seen in many otherperamelemorphians including species of Echymipera, Isoodon,and Peroryctes.

3) Postglenoid foramen. The plesiomorphic condition for thisforamen is to be completely surrounded by squamosal at its ex-ternal opening, as is the case in P. andinus (Marshall and Mui-zon, 1995) and many didelphids (Muirhead, 1994). Some pera-melemorphians are apomorphic in having the periotic contrib-uting to the medial wall of the external opening of this foramen,together with a concave surface of the anterior end of the ec-totympanic, a condition considered by Archer (1976a) to beunique. Yarala burchfieldi shows the apomorphic condition asexhibited in species of Perameles, Chaeropus, and Isoodon,where the periotic forms the medial wall of the foramen’s open-ing. This character appears to be relatively stable within genera.

4) Periotic hypotympanic sinus. The lack of, or a shallow andpoorly defined, sinus is considered to be the plesiomorphic con-dition in peramelemorphians because of its absence in most di-delphids (Archer, 1976a). The presence of a periotic hypotym-panic sinus is generally restricted to dasyurids (Archer, 1976a)and is considered here to be an apomorphic specialization indasyurids and some specialized peramelemorphians (e.g., spe-cies of Isoodon and Macrotis). In Y. burchfieldi, this sinus isdeveloped to a greater degree than that seen in Peroryctes raf-frayana. However, all other peramelemorphians have the periotichypotympanic sinus developed to a similar or greater degreethan Y. burchfieldi.

5) Mastoid epitympanic sinus. As for the periotic hypotym-panic sinus, a small to absent mastoid epitympanic sinus is theplesiomorphic state, this being absent to very small in didelphids

and large in specialized dasyurids (Archer, 1976a). Yaralaburchfieldi shows a greater development of this sinus than inspecies of Microperoryctes, Peroryctes, and Echymipera, and asimilar degree of enlargement to species of Perameles. It issmaller than in species of Isoodon, while the shape of this sinusin species of Macrotis is unique within Peramelemorphia. Thischaracter would suggest some affinity between Y. burchfieldi andspecies of Perameles.

6) Lacrimal duct. The lacrimal duct in Y. burchfieldi lies in-side the lacrimal-maxilla suture. This is also the case in Macrotislagotis. However, in some specimens of M. lagotis, a thin regionof the lacrimal extends around the duct within the region of themaxilla. The lacrimal duct consistently lies within the lacrimalin all other peramelemorphians. In all dasyurids and didelphidsexamined, this duct lies wholly within the lacrimal, which hasa more extensive facial development than in Y. burchfieldi. Thisdistribution suggests that the condition in Y. burchfieldi and M.lagotis is derived.

7) Lacrimal shape. The lacrimal of Y. burchfieldi does notextend dorsally outside of the orbit. It is wider than it is tall. Itsposterior extent terminates at approximately one third the widthof the orbit. This contrasts with most peramelemorphians wherethe lacrimal is taller than wide. Peroryctids and M. lagotis showa greater similarity in lacrimal shape to Y. burchfieldi than dospecies of Perameles, Isoodon and Chaeropus, in that they havevery little dorsal development of the lacrimal. However, none ofthese Recent species has such a large posterior development ofthe lacrimal as does Y. burchfieldi. The greater degree of pos-terior development of the lacrimal in Y. burchfieldi may be re-lated to the smaller anterior development of this bone outsidethe orbit compared to other peramelemorphians. Many didel-phids and dasyurids have the same lacrimal shape as most per-amelemorphians. In these taxa, the lacrimal has both dorsal andanterior development outside the orbital fossa with very littleposterior development. The extent of anterior development ofthe lacrimal is possibly related to the position of the lacrimalduct relative to the lacrimal and maxilla. Species with greateranterior development of the lacrimal are more likely to have thelacrimal duct within the lacrimal. The unique posterior devel-opment of this bone in Y. burchfieldi, and the position of thelacrimal duct in the maxilla-lacrimal suture, cannot simply beconsidered plesiomorphic because of the widespread occurrenceof the alternate condition in both the sister group and perame-lemorphian outgroup taxa. The condition of this feature in Y.burchfieldi may, therefore, be an autapomorphy.

DISCUSSION

Intraordinal relationships.—Analysis of dental characters ofY. burchfieldi concluded that this plesiomorphic marsupial wasa peramelemorphian, sharing a combination of apomorphiesonly otherwise found within this order (Muirhead and Filan,1995). Analysis of the skull of Y. burchfieldi strengthens thisassessment. Yarala burchfieldi has two apomorphic characterstates of the skull that are unique to peramelemorphians amongmarsupials and found in every member of the order. These arethe bifid jugal at its contact with the maxilla and the continu-ously slender nasals.

Yarala burchfieldi shares a number of apomorphies with var-ious extant peramelemorphian taxa, but, these character statesdo not indicate a close relationship to one particular group ortaxon. New Guinean taxa (Microperoryctes, Peroryctes, Echy-mipera) tend to show fewer derived character states than doAustralian taxa (Perameles, Chaeropus, Isoodon, Macrotis)(Muirhead, 1994). Yarala burchfieldi shares many plesiomor-phies with the New Guinean taxa, and while phenetically similar,

519MUIRHEAD—NEW MIOCENE MARSUPIAL

no stronger ties can be drawn to these based on synapomorphiesof the skull.

The most parsimonious decision regarding the relationship ofY. burchfieldi to other peramelemorphians, is that it lies outsideof a clade comprising all other taxa. This relationship, however,implies some character conflicts: apomorphic character states ofthe antorbital fossa, palatal vacuities, postglenoid foramen, peri-otic hypotympanic sinus, mastoid epitympanic sinus, and lacri-mal duct and shape have either been derived convergently in Y.burchfieldi and various other taxa, or have been reversed in taxain which the plesiomorphic state exists. For instance, if Y. burch-fieldi is correctly interpreted to lie outside a clade comprisingall other known peramelemorphian taxa, the synapomorphic dis-tribution of the postglenoid foramen and mastoid epitympanicsinus in Y. burchfieldi and the Australian taxa suggests that thesecharacters have either been secondarily lost in all peroryctids(thereby uniting these as a clade), or have evolved independentlyin Y. burchfieldi and the Australian clade. There are far fewersynapomorphies tying Y. burchfieldi to any particular taxon thanthere are synapomorphies excluding Y. burchfieldi from a cladeuniting all remaining taxa.

Skull morphology indicates that Y. burchfieldi lies outside allother peramelemorphians in lacking apomorphies that unitethese taxa as a clade. These uniting apomorphies are: a longsnout defined by a premaxilla-nasal suture that is longer thanthe maxilla-nasal suture and nasals that do not extend posteriorlypast the level of the anterior rim of the orbit; contact of frontaland squamosal; posteriorly located infraorbital foramen; and pri-mary foramen ovale located wholly within the alisphenoid, or iflocated in the alisphenoid-periotic suture, with concurrent sec-ondary foramen ovale.

Classification within the Order Peramelemorphia.—Becauseextant peramelemorphians are united by a number of synapo-morphies to the exclusion of Yarala burchfieldi, this taxon can-not be classified within any of the currently recognised families.It is proposed that Y. burchfieldi should be recognized in a sep-arate family, the Yaralidae within a new superfamily, the Yara-loidea, exclusive of all other taxa united as a clade comprisingthe superfamily Perameloidea. Thus the order Peramelemorphiacontains two superfamilies; the Yaraloidea containing the singlefamily Yaralidae, and the Perameloidea containing the familiesPeramelidae (modified from that of Groves and Flannery, 1990to exclude Macrotis), Peroyctidae Groves and Flannery, 1990,and Thylacomyidae Archer and Kirsch, 1977—a distinction sup-ported by the work of Kirsch (1968, 1977), Baverstock et al.(1990a), and Kirsch et al. (1990, 1997).

Order PERAMELEMORPHIA (Kirsch, 1968)Aplin and Archer, 1987

Superfamily PERAMELOIDEA (Waterhouse, 1838)Family PERAMELIDAE (Gray, 1825) Archer and Kirsch, 1977

Family PERORYCTIDAE Groves and Flannery, 1990Family THYLACOMYIDAE (Bensley, 1903) Archer

and Kirsch, 1977New Superfamily YARALOIDEA

New Family YARALIDAE

Interordinal relationships.—Marsupials were originally sep-arated into the cohorts Australidelphia and the Ameridelphia bySzalay (1982) on the basis of tarsal morphology. Although morerecent classifications of marsupials vary in detail from Szalay’soriginal, the basic concepts of these two major divisions of mar-supials have been retained. The Australidelphia is a monophy-letic clade comprising all Australian marsupials plus microbio-theres. The Ameridelphia is variously considered to comprise all

of the remaining marsupials (Szalay, 1982) or a subset of these(e.g., Aplin and Archer, 1987; Marshall et al., 1990).

Serological and immunological studies such as those byKirsch (1968, 1977), Lowenstein et al. (1981), Baverstock et al.(1987, 1990a, 1990b) found peramelemorphians to be the sistergroup to dasyuromorphians rather than other Australian marsu-pials, didelphoids, or Caenolestes. These studies, however, havetended to include non-Australian marsupials as outgroups ratherthan serious contenders for possible close peramelemorphian re-lationship. In contrast, DNA hybridization (Kirsch et al., 1991;Kirsch et al., 1997), 12S ribosomal DNA (Springer et al., 1994),protamine P1 genes (Retief et al., 1995), and combinations ofthese data (Krajewski et al., 1997) have all suggested that per-amelemorphians lie outside a monophyletic group includingDromiciops, diprotodontians, dasyurids and, when included, No-toryctes (see Luckett, 1994; Woodburne and Case, 1996 for areview of these relationships). The inclusion of Peramelemor-phia within the cohort Australidelphia has also been questioned(Springer et al., 1994; Woodburne and Case, 1996; Kirsch et al.,1997).

The major morphological evidence used for the inclusion ofPeramelemorphia within Australidelphia is tarsal morphology(Szalay, 1982, 1994). Additional data supporting an Australi-delphian placement for Peramelemorphia includes teat arrange-ment (Reig et al., 1987), dental synapomorphies (Marshall et al.,1990), and the case for syndactyly supporting a monophyly ofPeramelemorphia with Diprotodontia (see Aplin and Archer,1987 for a review of this evidence).

Although the inclusion of Peramelemorphia within the aus-tralidelphian clade is currently under question, if included withinAustralidelphia, it is likely to be one of the earliest branches(Springer et al., 1994) and the possible sister-taxon to all or themajority of other australidelphians (Woodburne and Case, 1996).Being the most plesiomorphic known peramelemorphian, Y.burchfieldi would, therefore, be expected to display features thatare plesiomorphic for australidelphians.

The present work intends to clarify the relationships of Y.burchfieldi to other marsupials, particularly other peramelemor-phians, rather than the relationships of peramelemorphians with-in Marsupialia; however, Marshall et al. (1990) provide a listingof morphological character states that can be used to assess theserelationships. Wroe (1999) and Godthelp et al. (1999) discussthe distribution of some of these characters states within Mar-supialia. Yarala burchfieldi allows for further discussion of someof these features.

Unworn teeth of Yarala burchfieldi have a conical-shaped en-toconid with a weak carnassial notch on the anterior cristid ofthe entoconid (preentocristid sensu Muirhead, 1992). This con-dition is considered by Marshall et al. (1990) to be synapo-morphic for Australadelphia. Muirhead and Filan (1995) consid-ered that a blade on the entoconid with a carnassial notch, typ-ical of peroryctids, to be the plesiomorphic state forperamelemorphians, while the absence of a blade and notch,typical of Australian taxa, to be the derived condition for pera-melemorphians. The distribution of this feature varies consid-erably both in and outside of Australidelphia. Kokopellia juddihas a small entoconid with a weak preentocristid. Specimensreferred to Alphadon marshi have a strong blade on the ento-conid with a notch. Peradectes elegans has a spirelike entoconid(Fox, 1983) with a weak cristid. The dasyuromorphians Keeunawoodburnei and Ankotarinja tirarensis have a blade on the en-toconid with a carnassial notch. The variation in this character,even within peramelemorphians, places doubt on the polarity ofthis character. In peramelemorphians, the weak notch on thepreentocristid appears to be an intermediate state between the

520 JOURNAL OF PALEONTOLOGY, V. 74, NO. 3, 2000

strong preentocristid blade with a notch, and the (apparently)apomorphic conical-shaped entoconid with loss of the carnassial.

The medial termination of the cristid obliqua below the notchof the metacristid is characteristic of basal marsupials (Clemens,1979; Clemens and Lilligraven, 1986; Johanson, 1996; Cifelliand Muizon, 1997) and is therefore the plesiomorphic conditionfor less basal marsupial groups. A cristid obliqua terminatingagainst the trigonid labial to the metacristid carnassial notch isinterpreted as a synapomorphy for australidelphians by Marshallet al. (1990). The occurrence of labial termination of the cristidobliqua, however, is also common to taxa outside Australidel-phia (e.g., species of Didelphis, Peradectes, and Peratherium).Wroe (1997b) concludes that both labial and lingual terminationcan be considered derived within Dasyuromorphia.

In extant peramelemorphians, the cristid obliqua terminates atthe base of the metacristid notch on M1–2 and against the trigonidlingual to the metacristid notch on M3–4. The lingual terminationof the cristid obliqua enlarges the hypoflexid and narrows thetalonid basin (to provide more efficient shear of tougher food)posteriorly down the molar row. The angle formed by crests atthe hypoconid (hypocristid and cristid obliqua) is reduced. Thisreaches its most extreme condition in the extinct herbivoreChaeropus ecaudatus (Wright et al., 1991), where the cristidobliqua on the M3–4 terminates at the lingual edge of the toothrather than against the posterior wall of the trigonid. In manytaxa (e.g., species of Isoodon), the cristid obliqua is also con-cave, lying even more perpendicular to the long axis of the toothalong most of its length, before turning strongly to meet thetrigonid. This further narrows the talonid basin and enlarges thehypoflexid. If the labial termination of the cristid obliqua is apo-morphic for australidelphians, extant peramelemorphians may beinterpreted to lie outside this clade (as suggested by Kirsch etal., 1991; Springer et al., 1994; Kirsch et al., 1997), or havereversed this condition.

Muirhead and Filan (1995) interpreted the labial terminationof the cristid obliqua in Y. burchfieldi to be plesiomorphic forperamelemorphians. On basal dasyurids and Y. burchfieldi, thecristid obliqua terminates on the labial side of the metacristidnotch on M1–2, on M3 it terminates at the base of the notch, andon M4 of Y. burchfieldi, it terminates lingual to the notch. If seenin exclusion of Y. burchfieldi, the condition of this character onextant peramelemorphians supports their exclusion from Aus-tralidelphia. The distribution of this character within Peramele-morphia, including Y. burchfieldi, however, indicates that rever-sal of this character state has occurred, with C. ecaudatus ex-hibiting the derived state and Y. burchfieldi exhibiting thecondition not far removed from dasyurids.

Woodburne and Case (1996) provide a list of dental featurescharacterizing a possible common ancestor of dasyuromorphiansand peramelemorphians. While the morphology of Y. burchfieldidoes not conflict with this possibility, many of these featureswould be considered plesiomorphic for the entire Australidelphia(e.g., trigonid and talonid unspecialized, M4 preparacrista un-specialized) and therefore do not support close relationship be-tween these two groups.

Although the morphological information provided by Y.burchfieldi clearly places it at the base of a clade comprising allother peramelemorphians, the information it sheds on the rela-tionships of peramelemorphians to other marsupials is currentlylimited without broader cranial comparisons. The extent of den-tal homoplasy between groups of marsupials has made the test-ing of hypotheses formulated by other morphological and mo-lecular characters difficult using dental characters alone. This isparticularly unfortunate because fossils should provide a vitaltesting ground for hypotheses of phylogeny based on moleculardata, which are generally unable to utilize these taxa. New fossil

finds provide an increased opportunity for detailed phylogeneticcomparisons using both dental and cranial morphology of lessderived representatives of marsupial groups, without restrictingthese comparisons to dentitions.

Age considerations.—No material from the Tertiary depositsof Riversleigh has yet provided an absolute date for the deposits.Their age has been estimated by biocorrelation. This method ofdating is acknowledged to be limited to the accuracy of the datesof other deposits, as well as to knowledge of the temporal rangesof taxa (Archer et al., 1989). The type locality for Y. burchfieldiis part of the System B faunas, estimated to be early Miocenebased on biocorrelation with the mammal-bearing deposits infaunas of South Australia (Archer et al., 1997). Specimens of Y.burchfieldi have been recovered from a number of System Band C sites in the Riversleigh deposits, estimated as spanningthe early to middle Miocene (Archer et al., 1997).

Only four fossil peramelemorphians have been described todate. Ischnodon australis Stirton, 1955, from the early to middlePliocene Palankarinna Fauna (Stirton, 1955), Perameles allingh-amensis Archer, 1976, from the early Pliocene Bluff Downs Lo-cal Fauna (Archer, 1976c), Perameles bowensis Muirhead, Daw-son, and Archer, 1997, from the early Pliocene Bow Local Fauna(Muirhead et al., 1997), and Y. burchfieldi. While other fossilperamelemorphians are known, including taxa from the earlyEocene Tingamarra Local Fauna, late Oligocene and Mioceneassemblages of Riversleigh and South Australia, and early Pli-ocene deposits in Queensland (Rich et al., 1991; Archer et al.,1993; Mackness et al., 1993; Muirhead, 1994; Woodburne andCase, 1996), none of these has yet been formally described.

Archer et al. (1993) note that the early Eocene Thylocotingabartholomaii Archer, Hand, and Godthelp, 1993, shares somesimilarities with peramelemorphians. Woodburne and Case(1996) argue for this allocation, stating that a strong metaconule,hypertrophied entoconid, tiny hypoconulid and similar thegoticwear patterns are shared by T. bartholomaii and more plesio-morphic undescribed peramelemorphian taxa from the late Oli-gocene to early Miocene. Archer et al. (1993), however, argueagainst this allocation, noting that no peramelemorphian isknown to have mesially positioned stylar cusps or lingual cin-gula on the flanks of the protocone. Woodburne and Case (1996)also note that the reduced parastyle and metastyle is not knownto any peramelemorphian.

Further to this reasoning, the interlocking of molars is alto-gether different in peramelemorphians. Thylocotinga bartholo-maii has a notch formed between the parastyle and the anteriorcingulum, a plesiomorphic character state also found in Alpha-don, Pediomys, Pucadelphys, Andinodelphys and Mayulestes(Muizon, personal commun.). In marsupial taxa with this feature,the notch holds the metastylar corner of the preceding tooth. Itis absent in M2–4 of all peramelemorphians. The M1 of all per-amelemorphians has a posterolabially directed preparacrista thatconnects to the stylar cusp B, a plesiomorphic state found in allbasal taxa (e.g., Kokopellia, Alphadon, Pucadelphys, Andino-delphys, Mizquedelphys, Tiulordia, Incodelphys, Muizon, per-sonal commun.). The preparacristas of T. bartholomaii connectto the anterobuccal corner of the crown (parastylar cusp), a de-rived character state relative to peramelemorphians, thereby ex-cluding T. bartholomaii molars from being considered perame-lemorphian M1s. In all peramelemorphians with a hypoconulid,it is positioned posterior to the entoconid, lies relatively low,and projects posteriorly. Regardless of the presence or absenceof a hypoconulid, the posterolingual corner of the peramele-morphian crown lingually overlaps the projecting and unnotched

521MUIRHEAD—NEW MIOCENE MARSUPIAL

anterior cingulum of the following molar. This overlapping, rath-er than interlocking, connection of molars appears to be auta-pomorphic to peramelemorphians, along with the loss of poste-rior cingulum on the lower molars. Similarly unknown to pera-melemorphians is the inflated surface on the basins labial to theparacone, metacone, protocone, and lingual to the protoconidand hypoconid; these surfaces on all known peramelemorphiansare concave or flat.

The similarities noted by Archer et al. (1993) and Woodburneand Case (1996) as shared by T. bartholomai and peramele-morphians are mostly derived features shared with some, but notall peramelemorphians, and are probably convergent. One fea-ture shared by T. bartholomai and all peramelemorphians forwhich it appears to be apomorphic, is the lack of posterior cin-gulum on the lower molars. This condition also occurs on mi-crobiotherians (Muizon, personal commun.), supporting the casefor inclusion of T. bartholomai within either order.

If T. bartholomai is considered to be a (albeit unique) pera-melemorphian as argued by Woodburne and Case (1996), thelack of some key diagnostic peramelemorphian apomorphies inT. bartholomai supports its antiquity. The unusual combinationof features considered to be both extremely derived and ple-siomorphic for peramelemorphians, while sharing a greater num-ber of similarities to other marsupial groups (Archer et al.,1993), however, argues against this allocation.

Yarala burchfieldi is, to date, the geologically oldest and mostcomplete fossil peramelemorphian formally described. WhileIschnodon australis, P. allinghamensis and P. bowensis have allbeen placed in perameloid families (i.e., Thylacomyidae for I.australis and Peramelidae for P. allinghamensis and P. bowen-sis), the older Y. burchfieldi is considered to lie outside of theperameloid clade and in its own superfamily (Yaraloidea).

Microcomplement albumen fixation has provided dates of 32(Baverstock et al., 1990a) and 24 million years (Baverstock etal., 1990b) for the primary divergence of all previously knownperamelemorphians (i.e., perameloids), while Kirsch et al.(1997) proposed the divergence of the Peroryctidae at 25 millionyears ago. The Riversleigh fossil taxa, however, suggest a morerecent divergence date for the radiation of perameloids. Pera-melemorphians are the most numerous and one of the most di-verse marsupial groups recovered from the Tertiary deposits atRiversleigh. Among these specimens are a number of well pre-served skulls of taxa yet to be formally described (Muirhead,1994). Like Y. burchfieldi, none of the Oligo-Miocene taxa canbe placed within the perameloid clade (Muirhead, 1994). Furtherundescribed taxa recovered from deposits in central Australia(Woodburne and Case, 1996) similarly support a relatively lateradiation of living peramelemorphians. In contrast, fossils youn-ger than the Miocene, although few in number, can be assignedto the Perameloidea and, as yet, no evidence exists in thesedeposits of the archaic taxa typical of older deposits. This dis-tribution suggests the possibility of a post middle Miocene–prePliocene bottleneck for peramelemorphians, followed by the di-vergence of perameloids. This scenario is consistent with vicar-iance events between New Guinea and the Australian mainlandbetween 10–12 million years ago (Aplin et al., 1993), but con-trasts with older divergence dates of the ‘modern’ taxa proposedby biochemical analyses.

At the same time, the diversity of the unassigned Tertiary taxaand the presence of possible undescribed peramelemorphians inthe early Eocene Tingamarra Local Fauna (Archer et al., 1993;Godthelp et al., 1992) supports a considerably older derivationfor Peramelemorphia as an order, supporting the hypothesis ofKirsch et al. (1997), and not in conflict with the 65 million yearold date proposed by Woodburne and Case (1996).

ACKNOWLEDGMENTS

This study was undertaken with the assistance from the QueenElizabeth II Silver Jubilee Trust for Young Australians, the Aus-tralian Commonwealth Department of Employment and Educa-tion, and the University of New South Wales. Support for theresearch project has been provided by the Australian ResearchCouncil (to M. Archer); National Estate Grants Scheme(Queensland) (to M. Archer and A. Bartholomai); World Heri-tage Unit, Environment Australia; Queensland National Parksand Wildlife Service; University of New South Wales; Austra-lian Geographic Society; Queensland Museum; Australian Mu-seum; Surrey Beatty & Sons Pty Ltd; Riversleigh Society Inc.;Royal Zoological Society of New South Wales; Linnean Societyof New South Wales; and many private supporters.

Constructive criticism on this work was provided by M. Ar-cher, W. Turnbull, S. Wroe, A. Bartholomai, S. Hand, H. God-thelp. Comments on drafts of this paper were provided by S.Hand and S. Wroe. Reviewers J. Kirsch and C. de Muizon pro-vided valuable improvements. Preparation of specimens was un-dertaken by A. Gillespie and R. Murphy printed photographs.Many thanks to all.

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ACCEPTED 2 NOVEMBER 1999