Primitive Marsupials

From the late Early Cretaceous (Albian, about 98 Ma) of Utah comes another primitive metatherian, Kokopellia (Fig. 5.6A), known from a well-preserved lower jaw (Cifelli, 1993a; Cifelli and Muizon, 1997). It resembles marsupials in dental formula (three premolars, four molars) and various

Fig. 5.5. Skeleton and right upper and lower postcanine dentition of the primitive metatherian Asiatherium. (From Szalay and Trofimov, 1996.)

dental traits (simple premolars, P3 tall, first molar smaller than the others) but lacks a twinned hypoconulid-entoconid, often regarded as a key synapomorphy of marsupials. Here again, as in the transition to Mammalia, anatomical traits that have been widely used to identify a clade—in this case, Metatheria—evidently were acquired in mosaic fashion. Ironically, this staggered acquisition of defining characters has resulted in a blurring of the taxonomic boundary as the record has improved. The somewhat younger (Campanian) Anchistodelphys (Fig. 5.7) and Iugomortiferum are also very similar to marsupials but differ slightly in stylar cusp development (weaker cusps, with cusp D absent or variable; Cifelli, 1990b). These taxa either represent stem marsupials or the closest sister taxa to Marsupialia.

A diversity of mouse-sized Late Cretaceous forms that appear to be true marsupials, at least based on teeth, has been described since the late 1980s. These dental taxa, most known only from isolated teeth, have diagnostic marsupial traits, such as twinned entoconid-hypoconulid and several stylar cusps including the consistent presence of stylar cusp D (Cifelli, 1993a; Eaton, 1993). Among them, Aenigmadel-phys, Protalphadon, and especially Iqualadelphis (Fig. 5.7), dating from the Aquilan and Judithian land-mammal ages (=Campanian), have been suggested to be the most primitive known marsupial genera, based partly on the absence or variable presence of stylar cusp C (e.g., Clemens, 1979; Cifelli, 1990a; Marshall et al., 1990; Cifelli and Johanson, 1994). Protalphadon (Fig. 5.8D) is based on species previously included in Alphadon but later separated from it on the basis of the absence or weakness of stylar cusp C. Other rela tively large species formerly assigned to Alphadon are now placed in Turgidodon. These Late Cretaceous genera are variously allocated to Didelphidae, Peradectidae, or basal positions in Didelphimorphia or Marsupialia.

Despite the multiplicity of generic names and the instability of their higher taxonomic positions, all these early marsupial genera are dentally very similar and are characterized by primitive "didelphoid" or "didelphimorph" molars: the uppers are triangular with the paracone usually larger than the metacone, distinct conules, and a wide sty-lar shelf with several stylar cusps (usually A, B, and D, or A-D); the lowers have tall protoconids, paraconids only slightly lower than metaconids, and basined talonids with closely approximated hypoconulid and entoconid. Generic distinctions are based on minor variations in expression of the stylar cusps, presence or absence of cusp C, relative height of the trigonid and width of the talonid, and overall dental proportions.

Alphadon or a closely allied form (Figs. 5.7, 5.8B,C) has been considered to be the source of all post-Cretaceous marsupials (e.g., Clemens, 1966, 1979; Fox, 1987; Marshall

5 mm

Fig. 5.6. Cretaceous basal metatherian dentitions: (A) Kokopellia, left lower jaw; (B) stagodontid Didelphodon, right lower jaw; (C) Pediomys, right lower jaw with M . (A from Cifelli, 1993a; B from Fox and Naylor, 1986; C from Clemens, 1966.)

5 mm

Fig. 5.6. Cretaceous basal metatherian dentitions: (A) Kokopellia, left lower jaw; (B) stagodontid Didelphodon, right lower jaw; (C) Pediomys, right lower jaw with M . (A from Cifelli, 1993a; B from Fox and Naylor, 1986; C from Clemens, 1966.)




Fig. 5.7. Evolution of metatherian molars. (From Cifelli, 1993b.)

et al., 1990). It has conventionally been allocated to the Didelphidae (e.g., Clemens, 1966; Fox, 1979a, 1987; McKenna and Bell, 1997), an attribution that reinforced the view of didelphids as the most primitive marsupials and the ultimate source of all later forms. The classification of Alphadon has been controversial—other authors assign it to the Peradec-tidae (e.g., Cifelli, 1990a; Marshall et al., 1990) or to an expanded Pediomyidae (Szalay, 1994)—but there is little question that it represents one of the most primitive marsupials. Although forms with slightly more primitive teeth are now known, Alphadon is still a good model for the ancestral molar structure of most post-Cretaceous marsupials. Consequently, Peradectidae (or Didelphidae) is probably para-phyletic. During the Early Tertiary, marsupials potentially derived from these Cretaceous forms dispersed to all continents, including Antarctica.

Early descendants of these Cretaceous peradectids (or didelphids) include the mainly North American Cretaceous families Stagodontidae and Pediomyidae (sensu stricto), both characterized by somewhat derived teeth relative to other Cretaceous marsupials (e.g., Clemens, 1979; Fox, 1987). Pediomyids (Figs. 5.6C, 5.8A) were small marsupials characterized by upper molars with a narrower stylar shelf than in Alphadon and related forms and reduction or loss of sty-lar cusps B (stylocone) and C (Clemens, 1966, 1979; Fox, 1979b). Based on these dental features, Marshall et al. (1990) grouped pediomyids with Australidelphia (specifically, microbiotheres), a relationship that remains to be corroborated by other evidence.

Stagodontids were the first marsupials to modify the dentition for carnivory They include some of the largest Cretaceous mammals, Didelphodon (Fig. 5.6B), reaching the size of the opossum Didelphis. Like eutherian carnivores and borhyaenoid marsupials, stagodontids evolved shearing between the paracristid (prevallid) of the lower molars and the posterior crest (postvallum) of occluding upper molars. They also had large posterior premolars that may have been used to crack shellfish, bones, or other hard materials. Palatal vacuities, a distinctive feature of marsupial skulls, are present in both stagodontids and Alphadon, suggesting that they were primitively present in marsupials (Fox and Naylor, 1995). The putative stagodontid Pariadens, based on a jaw from the earliest Late Cretaceous (Cenomanian) of Utah, appears to be the oldest known "definitive" marsupial (Cifelli and Eaton, 1987). It was initially referred questionably to the Stagodontidae, based on its relatively large size compared to other Cretaceous marsupials, molars that increase in size posteriorly, and larger paraconid than meta-conid; but subsequent discoveries call into question its referral to Stagodontidae (Fox and Naylor, 1995).

Presumably because of their carnivorous specializations, Stagodontidae were classified by Marshall et al. (1990) as basal borhyaenoids in the order Sparassodonta (extinct South American carnivorous marsupials). However, other authors variously assign them to Didelphimorphia (or equivalent) or separate them from other marsupials, either in the higher taxon Archimetatheria (which unites Stagodontidae and Pediomyidae; Szalay, 1994) or in Alphadelphia (Case et al.,

Pediomys Mammal
Fig. 5.8. Primitive Cretaceous marsupial dentitions: (A) Pediomys, right P3M1-3; (B) Albertatherium (alphadontine), right upper molars; (C) Alphadon, right lower dentition; (D) Protalphadon, right lower dentition. All scale bars = 1 mm. (A, C, and D from Clemens, 1966; B from Fox, 1987.)

2004). The weight of the evidence indicates that borhyae-noids are more closely related to didelphoids than to stago-dontids (see below).

Further diversity in Cretaceous marsupials is indicated by Glasbius (see Fig. 5.12A), a mouse-sized form with low bunodont cusps; upper molars with enlarged cusps B and D (but all five stylar cusps present); and broad lower molars with a strong ectocingulid variably bearing distinct cuspules (Clemens, 1966). Glasbius is generally thought to be related to the South American early Tertiary Caroloameghiniidae (which is variously assigned to Peradectidae, Pediomyidae, or Paucituberculata); but Reig et al. (1987) thought Glasbius was more closely allied with microbiotheres, and Szalay (1994) grouped it with Peradectes and Alphadon in his expanded Pediomyidae. It remains possible that resemblances between Glasbius and either of the South American clades are convergent.

Krause (2001) attributed a partial molar from the Late Cretaceous of Madagascar to an indeterminate marsupial, but Averianov et al. (2003) suggested that it might instead belong to a eutherian, specifically, a zhelestid. Pending more conclusive remains, the presence of marsupials in Madagascar should be regarded as doubtful.

Although nearly all remains of Cretaceous marsupials consist of teeth and jaws, a few skull elements have been reported. Isolated Late Cretaceous petrosals, thought to belong to primitive marsupials, lack grooves for branches of the internal carotid artery but already had a fully coiled cochlea, which presumably allowed for a longer basilar membrane and relatively acute high-frequency hearing (Meng and Fox, 1995).

Paleontologists have long debated the place of origin of marsupials. Revised age estimates of key South American localities have shifted the balance in favor of North America, where the record of early marsupials is both older and more diversified than anywhere else (Cifelli, 1993a, 2000). However, the presence of a diversity of basal metatherians in Asia, at least one of which is even older than those from North America, has rekindled the debate about where meta-therians and marsupials first originated and diversified. In any case, the Cretaceous radiation of marsupials appears to have been predominantly (or exclusively) Holarctic. The major radiations of post-Cretaceous marsupials, however, took place in South America and, later, Australia.

Ameridelphia Peradectidae and Didelphidae

Although some of the Cretaceous marsupials in the foregoing discussion (those referred variously to Peradectidae or Didelphidae, as well as stagodontids and pediomyids) are sometimes included in Ameridelphia, they may well belong to a more plesiomorphic paraphyletic assemblage that includes the ancestors or sister taxa of other ameridelphians as well as australidelphians. By the Paleocene and Eocene, however, more than two dozen genera usually accepted as early members of the extant family Didelphidae (opossums: sensu lato, as used by McKenna and Bell, 1997) are known, mostly from North America and Europe, and especially South America. A few are now known from Asia and northern Africa as well, but they were clearly very rare on those continents and never diversified there. These animals were dentally conservative, showing only minor variations in dental anatomy, and they are central to most concepts of Ameridelphia.

Peradectids in the strict sense (Peradectinae of some authors) are a group of small marsupials known mainly from the Paleocene-Eocene of North and South America and Europe, and presumably derived from an Alphadon-like form. There is no dispute that they were didelphid-like, but whether they should be subsumed in Didelphidae is a contentious issue. Mouse-sized Peradectes was present on all three continents. Peradectid skeletons from the middle Eocene of Messel, Germany, show specializations for arboreal life, including a long, prehensile tail—preserved in coiled position—

Fig. 5.9. Marsupial skeletons from the middle Eocene of Messel, Germany: (A, C), Amphiperatherium; (B, D) Peradectes. Scale bar = 2 cm for (A), 1 cm for (B). (A and B courtesy of C. Kurz and the Hessisches Landesmuseum Darmstadt; C and D from Koenigswald and Storch, 1992.)

and greatly reduced lumbar transverse processes, which allowed more flexibility of the lower spine (Fig. 5.9B,D; Koenigswald and Storch, 1992; Kurz, 2001).

Uncontested didelphids were diverse and widespread in the Early Tertiary, being represented by herpetotheriines in

North America, Europe, northern Africa, and Asia, and three subfamilies (including Didelphinae) from the Paleo-cene (and possibly the latest Cretaceous) of South America. Although they persisted into the Miocene in North America and parts of the Old World, no other marsupials subse quently populated those regions (until the Pleistocene in North America). Amphiperatherium (Fig. 5.9A,C) and a tiny mouse-sized herpetotheriine are known from skeletons from Messel, which have shorter tails than peradectids and well-developed lumbar transverse processes. These features suggest that they inhabited the forest floor, or at least were less committed tree dwellers than were peradectids (Koenigswald and Storch, 1992; Kurz, 2001). Interestingly all the Messel marsupials have an opposable hallux, which apparently bore a nail. The other ungual phalanges in both terrestrial and arboreal types are short and dorsoventrally deep, similar to those of arboreal mammals generally (MacLeod and Rose,

1993), which suggests that the presumed terrestrial forms evolved from arboreal marsupials and retained the ability to climb.

The oldest marsupials from South America come from the early Paleocene Santa Lucía Formation of Tiupampa, Bolivia. Despite the antiquity of this site, even conservative estimates identify at least five families of marsupials already present there (Muizon, 1998), demonstrating that once they were established in South America, marsupials diversified rapidly. This cladistic diversity is inferred from relatively minor differences, however, and the dentitions of these early marsupials are generally quite similar and can be characterized as "didelphoid."

Didelphids were especially diverse in the Paleocene of South America, where at least a dozen genera representing this family are recognized from three main sites: the early Paleocene at Tiupampa, Bolivia; the late Paleocene of Itab-oraí, Brazil (Fig. 5.10A-D; see e.g., Paula Couto, 1952b, 1962, 1970a; Marshall and Muizon, 1988); and the late Paleocene of Laguna Umayo, Peru (formerly thought to be late Cretaceous or early Paleocene, but redated as late Paleo-cene or earliest Eocene; Sigé et al., 2004). Marshall (1987) recognized 16 genera of didelphids and didelphid-like forms from Itaboraí. Based on the anatomy of isolated postcranial bones from Itaboraí, both terrestrial and arboreal marsupials were present in the fauna (Szalay and Sargis, 2001).

The Tiupampan didelphid Pucadelphys (Fig. 5.11A-C, Plate 1.2) is one of the best-known marsupials, being represented by virtually complete skulls and articulated skeletons with a long, nonprehensile tail (Marshall et al., 1995). It is apparently primitive in lacking an alisphenoid bulla (Muizon,

1994), the presence of which is a hallmark of marsupials; but it must be noted that very few early marsupials are known from skulls preserved well enough to assess this feature. Pu-cadelphys was an agile generalist, probably mainly terrestrial and capable of digging (as suggested by the occurrence of the skeletons in burrows), but also capable of climbing, perhaps resembling present-day dasyurids more than didelphids in locomotor habits (Muizon, 1998; Argot, 2001; Muizon and Argot, 2003). The Tiupampan didelphoids provide a structural link between North American Cretaceous didel-phoids and later South American didelphoids, such as those from Itaboraí (Muizon and Cifelli, 2001).

Besides didelphids, at least two other clades of ameridel-phian marsupials thrived in South America during the Ceno-

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