Polydolopimorphia

Fig. 5.10. Dentitions of South American Paleocene marsupials: (A) Proto-didelphine Protodidelphis, right M1-4; (B) protodidelphine Guggenheimia, left PrM4; (C) didelphine Marmosopsis, right M1-2; (D) eobrasiliine Gaylordia, right P3-M4; (E, F) caenolestoid Carolopaulacoutoia, right upper and lower dentitions; (G, H) microbiothere Mirandatherium, right upper and lower molars. A-D are didelphids. All scale bars = 1 mm. (From Marshall, 1987.)

zoic: Paucituberculata and Sparassodonta. Both are believed to have originated from didelphoid marsupials, and the oldest known members of both groups are also found at Tiu-pampa. It should be realized that assignment of many of these Paleocene marsupials to nondidelphoid groups is generally based on hindsight, citing rather subtle features. All of the Tiupampan marsupials are anatomically very similar.

Paucituberculata

This order is often used to encompass caenolestoids (including the living rat opossum Caenolestes and its extinct relatives) and the extinct argyrolagoids, caroloameghinioids, and polydolopoids (e.g., Aplin and Archer, 1987; McKenna

Fig. 5.11. Marsupials from the early Paleocene of Tiupampa, Bolivia: (A-C) Pucadelphys skull and left dentition; (D) Mayulestes skeleton. Key: FR, frontal; JU, jugal; LA, lacrimal; lc, lambdoid crest; lf, lacrimal foramen; MX, maxilla; NA, nasal; PA, parietal; PM (pm), mastoid part of petrosal; PMX, premaxilla; PP, postparietal; ptf, post-temporal foramen; pzf, postzygomatic foramen; SQ, squamosal. (A-C from Marshall et al., 1995; D courtesy of Christian de Muizon.)

Fig. 5.11. Marsupials from the early Paleocene of Tiupampa, Bolivia: (A-C) Pucadelphys skull and left dentition; (D) Mayulestes skeleton. Key: FR, frontal; JU, jugal; LA, lacrimal; lc, lambdoid crest; lf, lacrimal foramen; MX, maxilla; NA, nasal; PA, parietal; PM (pm), mastoid part of petrosal; PMX, premaxilla; PP, postparietal; ptf, post-temporal foramen; pzf, postzygomatic foramen; SQ, squamosal. (A-C from Marshall et al., 1995; D courtesy of Christian de Muizon.)

and Bell, 1997). Marshall (1987) united the same assemblage under the name Polydolopimorphia. The precise relationships of these clades to each other, as well as to other marsupials, is problematic, and it is far from certain that all belong to a monophyletic group. For example, Marshall et al. (1990) and Szalay (1994) classified caenolestoids and argy-rolagoids in the same clade (Marshall's Paucituberculata, Szalay's Glirimetatheria), but allocated polydolopoids to a separate order, Polydolopimorphia, and caroloameghinioids to either Peradectidae (Marshall et al., 1990) or suborder Sudameridelphia (together with polydolopoids and borhyae-noids; Szalay, 1994). Molecular studies have reached different conclusions about the relationships of extant forms (e.g., Kirsch et al., 1997) but, of course, cannot place the extinct groups.

The most primitive marsupials included in Paucituber-culata are the caroloameghinioids, a group of small mammals characterized by low-crowned, bunodont, "didelphoid" teeth. North American Cretaceous Glasbius (Fig. 5.12A) is usually considered the oldest known caroloameghinioid, whereas Roberthoffstetteria, from the early Paleocene at Tiu-pampa, Bolivia, is the oldest caroloameghiniid (Fig. 5.12B-D). Roberthoffstetteria has broad molars with swollen, rounded cusps (including very prominent stylar cusps B, C, and D), suggesting a more omnivorous diet than in other contemporary marsupials.

Goin et al. (2003) suggested that Roberthoffstetteria is the sister taxon of polydolopoids, rather than a caroloameghiniid, based on such features as a thick dentary; thick enamel; and a very large, lingual metaconule positioned like a hypo-cone. On this basis, they proposed that Roberthoffstetteria represents a structural stage in the evolution of polydolo-poid molars from a form like Glasbius. If this hypothesis is corroborated, it would constitute another lineage in addition to Alphadon that had Paleocene descendants.

Szalay (1994) used an expanded concept of Carolo-ameghiniidae, which included several taxa that other authors usually include in the Didelphidae (highlighting the primitive dental structure in this group), as well as the Proto-didelphinae, an assemblage of primitive Paleocene genera whose affinities are uncertain. Protodidelphines have been considered to be basal members of Ameridelphia, Didel-phimorphia, or Polydolopimorphia by other authors.

Polydolopoids include four families and a dozen genera of dentally derived marsupials primarily from the Paleocene and Eocene of South America. At least two genera are known from Antarctica. They are generally characterized by "pla-giaulacoid" lower dentition (Simpson, 1933), a complex including enlarged incisors, an anterior diastema (or greatly reduced anterior teeth behind the incisor) followed by an enlarged shearing tooth (in this case, P3), and small, broad, brachydont molars. This dental pattern is already present in the oldest polydolopid, Itaboraian Epidolops (Paula Couto, 1952a). Epidolops (Fig. 5.13) retains four lower molars, whereas Casamayoran forms typically have lost M4. Prepi-dolops is the sole representative of the family Prepidolopidae. Although known so far only from the Eocene (Casamayoran-

Metaconule

1 mm

Fig. 5.12. Marsupial dentitions: (A) Late Cretaceous Glasbius, right P3-M3; (B-C) early Eocene Caroloameghinia, right M1-4 and P2-M4; (D) early Paleocene Roberthoffstetteria, right P1-M4 and M1-4. (A from Clemens, 1966; B and C from Reig et al., 1987; D from Marshall et al., 1983.)

1 mm

Fig. 5.12. Marsupial dentitions: (A) Late Cretaceous Glasbius, right P3-M3; (B-C) early Eocene Caroloameghinia, right M1-4 and P2-M4; (D) early Paleocene Roberthoffstetteria, right P1-M4 and M1-4. (A from Clemens, 1966; B and C from Reig et al., 1987; D from Marshall et al., 1983.)

Mustersan), it seems to represent the most plesiomorphic branch of polydolopoids, or perhaps a transitional stage between didelphoids and polydolopoids (Pascual, 1980a,b). Its combination of didelphoid-like molars, tall and pointed P3, and compressed anterior teeth result in a striking convergence toward certain omomyid primates. Casamayoran Bonapartherium (Bonapartheriidae) has enlarged premolars and unusual quadrate, bunoselenodont molars, the uppers lacking a stylar shelf (Pascual, 1980a). Sillustania, a putative polydolopoid based on isolated teeth from Chulpas, Peru, was initially thought to date from Cretaceous/Tertiary (K/T) boundary strata (Crochet and Sige, 1996); but it is now believed to be of late Paleocene or earliest Eocene age (Sige et al., 2004).

Caenolestoids are not well known before the Deseadan SALMA (late Oligocene). Nevertheless, one Paleocene form, the Itaboraian sternbergiid Carolopaulacoutoia, is assigned to this clade and is its oldest known representative (Paula Couto,

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