Tingamarra Tooth

1970a; McKenna and Bell, 1997). Marshall (1980) suggested that caenolestoids evolved from didelphoid ancestors, which is consistent with the dentition of Carolopaulacoutoia (Fig. 5.10E,F).

Argyrolagids, best known from the Neogene, were highly specialized jerboa-like, bipedal marsupials with enormous orbits (Simpson, 1970a). Were it not for possession of several diagnostic marsupial traits—presence of four molars, a medially inflected mandibular angle, palatal vacuities, and an alisphenoid bulla (Sanchez-Villagra and Kay, 1997)— argyrolagids might not be recognized as marsupials at all. Their origin and relationships remain uncertain, although they are usually allied with groeberiids, based mainly on the enlarged incisors. A mandibular fragment from the Eocene/ Oligocene boundary strata (Tinguirirican) of Chile could represent the only pre-Deseadan record of the family (Wyss et al., 1994), but it has since been tentatively referred to the groeberiid genus Klohnia (Flynn and Wyss, 1999).

Groeberia (Fig. 5.14), from the middle Eocene (Muster-san? and Divisaderan SALMAs) of Argentina and Chile, is the oldest groeberiid and the oldest argyrolagoid (if groeberiids are indeed related to argyrolagids). Its affinities have long been enigmatic. It resembles rodents in having a pair of enlarged, ever-growing incisors with enamel limited to the anterior surface (Simpson, 1970c). The enamel has two layers, as in rodents, but unlike rodents, the inner layer is much thinner and consists of tangential enamel that lacks Hunter-Schreger bands (Koenigswald and Pascual, 1990). Moreover, the skull has palatal openings, an inflected mandibular angle, and four molars. Based on these last three features, most current authorities classify groeberiids as marsupials (Pascual et al., 1994; Flynn and Wyss, 1999). Most recent accounts suggest that groeberiids are closely related to Argyrolagidae, but the evidence is not conclusive, and some authors believe their similarities are convergent.

Sparassodonta (Borhyaenoids)

Borhyaenoids are an extinct clade of South American carnivorous marsupials that existed from the Paleocene through the Pliocene (Marshall, 1978). They were dentally convergent toward placental creodonts and carnivorans, and are the only South American marsupials that evolved Hunter-Schreger bands in the enamel (Koenigswald and Goin, 2000).

Fig. 5.13. Late Paleocene polydolopoid Epidolops: (A) skull; (B) mandible; (C) left lower dentition. (From Paula Couto, 1952a.)

Pictures Lateral Animal Skull View
Fig. 5.14. Snout and mandible of Eocene Groeberia. (A) lateral view; (B) anterior view. (From Pascual et al., 1994.)

Mayulestes (Fig. 5.11D), a hedgehog-sized animal known from a skull and skeleton from the early Paleocene at Tiupampa, Bolivia, is considered to be the oldest borhyaenoid (Muizon, 1994, 1998). Compared to didelphids, its upper molars have a reduced paracone, and its lower molars have a paracristid with a carnassial notch and narrow and elongate talonids with a reduced entoconid—features suggestive of incipient carnivorous adaptation. The postcranial skeleton was adapted for an agile, arboreal life, as inferred from the prehensile tail, reduced humeral tuberosities and prominent supinator crest, contour of the ulna, shape of the radial head, and many other features (Muizon, 1998; Argot, 2001). Like Pucadelphys, Mayulestes appears to be primitive in lacking a tympanic process of the alisphenoid, which is usually considered diagnostic of marsupials (Muizon, 1994).

Andinodelphys, the largest didelphid from Tiupampa, has a reduced paracone and entoconid, suggesting that it could represent a transitional stage between didelphids and bor-hyaenoids (Muizon et al., 1997). If so, it implies that resemblances between stagodontids and borhyaenoids are convergent. Marshall et al. (1990), however, placed Andinodelphys as the sister taxon to Australian australidelphians. In a recent study, Muizon and Argot (2003) found that Pucadelphys, Andinodelphys, and Mayulestes show a gradient of increasing size and arboreal adaptation, although all were agile forms adept both on the ground and in the trees.

In addition to Mayulestidae, borhyaenoids comprise four or five families (sometimes regarded as subfamilies of Bor-hyaenidae), three of which are present in the early Tertiary Hathliacynidae, exemplified by Itaboraian-Casamayoran

Patene (Fig. 5.15A), were small to medium-sized borhyaenoids (Marshall, 1981). Early Paleocene Allqokirus, based on isolated teeth from Tiupampa, has been considered to be either the oldest hathliacynid or a mayulestid. Although dentally more primitive than other borhyaenoids, these genera show the first stages of carnivorous specialization: incipient reduction of the upper molar protocone and the lower molar metaconid and talonid, together with elongation of the postmetacone crista. The auditory bulla was large and was composed of the alisphenoid and periotic. Borhyae-nids (sensu stricto) include a number of more specialized, dog-sized Eocene genera (e.g., Angelocabrerus [Fig. 5.15D], Plesiofelis) with reduced protocones and paracones on the upper molars, and sectorial lower molars with enormous roots and reduced metaconids and talonids (Simpson 1948, 1970b; Marshall, 1978).

The most carnivorously adapted early Tertiary borhyae-noids, referable to the Proborhyaenidae, were also the largest. Casamayoran Callistoe (Fig. 5.15C), known from a nearly complete skull and most of the skeleton, was a terrestrial, wolf-sized animal with massive canines and molars developed as bladelike carnassials (Babot et al., 2002). The last premolars (Pp were also enlarged, as might be expected in a bone-crushing form. In Callistoe and all other probor-hyaenids, the canines were ever-growing. Casamayoran Arminiheringia (Fig. 5.15B) was dentally one of the most specialized borhyaenoids (Marshall, 1978). It was even larger than Callistoe, with a more robust skull with widely flaring zygomae and a long, fused mandibular symphysis (Simpson, 1948). The dentition of these large proborhyaenids

Tingamarra Fossil

Fig. 5.15. Borhyaenoids: (A) Patene, right P3-M4 (B) Arminiheringia; (C) Callistoe mandible in lateral and dorsal views; (D) Angelocabrerus, left P3-M2 in lingual and crown views. (A from Marshall, 1981; B from Simpson, 1948; C from Babot et al., 2002; D from Simpson, 1970b.)

bears a superficial resemblance to that in some placental carnivores, such as hyaenids and oxyaenid creodonts.

Australidelphia

Current evidence suggests that the Australian radiation of marsupials emerged from South American Microbio-theriidae, a relict of which, the monito del monte (Dromi-ciops australis), survives in southern Chile and Argentina to the present day. Together, these marsupials are classified as the clade Australidelphia, which is supported by tarsal specializations indicative of a grasping hind foot typically associated with arboreal habits (Szalay, 1982). Molecular data also generally support the monophyly of microbiotheres

Tingamarra Images

1 mm

Fig. 5.16. Early Eocene(?) marsupials from Australia: (A) Djarthia, right upper and lower dentition; (B-C) Thylacotinga, fragmentary upper and lower teeth; (D) Tingamarra, right lower molar. (A from Godthelp et al., 1999; B-C from Archer et al., 1993a; D from Godthelp et al., 1992.)

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Fig. 5.16. Early Eocene(?) marsupials from Australia: (A) Djarthia, right upper and lower dentition; (B-C) Thylacotinga, fragmentary upper and lower teeth; (D) Tingamarra, right lower molar. (A from Godthelp et al., 1999; B-C from Archer et al., 1993a; D from Godthelp et al., 1992.)

and Australian marsupials (e.g., Burk et al., 1999). Microbiotheres were small, opossum-like marsupials represented by several early Tertiary genera, including Tiupampan Khasia and Itaboraian Mirandatherium (Fig. 5.10G,H). The Paleogene genera differ from didelphids and resemble later microbiotheres in several details of dental morphology, including reduction of the stylar shelf and stylar cusps on the upper molars, similar P3 structure, talonids wider than trigo-nids on lower molars, and a reduced last lower molar (Marshall, 1987).

Teeth of several marsupials, including a microbiothere and opossum-like didelphimorphs and polydolopids, have recently been found in the middle Eocene La Meseta Formation of Seymour Island, Antarctica (Goin and Carlini, 1995; Goin et al., 1999). Although their strongest affinities are with Itaboraian and Patagonian faunas, their presence in Antarctica—the presumed route for dispersal to Australia— suggests that the Australian marsupial fauna could have reached that continent earlier than previously thought (perhaps by the K/T boundary, according to Goin et al., 1999).

The oldest and most primitive Australian marsupial is the recently named Djarthia murgonensis (Fig. 5.16A), known from upper and lower jaw fragments from the Tingamarra Local Fauna. The formation has been dated at 54.6 Ma, just above the Paleocene/Eocene boundary (Godthelp et al., 1992, 1999). Nevertheless, Woodburne and Case (1996) suggested that the fossil could actually be as young as Oligocene. The cheek teeth of Djarthia are very reminiscent of those of New World didelphids; they have a wider stylar shelf and larger stylar cusps than in microbiotheres. Djarthia is suitably ancient and unspecialized to lie near the ancestry of all other Australian marsupials and suggests an ultimate didelphoid origin for Australidelphia. A few other early Eocene marsupial teeth were previously described from the Tingamarra Local Fauna (Thylacotinga, Fig. 5.16B,C), but they were too incomplete to indicate clear affinities (Archer, Godthelp, and Hand, 1993). Woodburne and Case (1996) postulated that Tingamarra (Fig. 5.16D), a genus based on a lower molar originally attributed to a placental (condylarth), is more favorably interpreted as a marsupial close to protodidelphines.

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