Mesozoic Mammals Of Uncertain Affinity

A few Mesozoic mammals are so unusual that they cannot be placed with confidence in any of the established higher taxa, and their broader relationships remain uncertain. One

Pappotherium
Fig. 4.19. "Tribothere" teeth: upper molars of (A) Pappotherium; (B) Holocle-mensia. Left lower molars of (C) Pappotherium; (D) Holoclemensia; (E) Kermackia; (F) Trinititherium. (G) Lower teeth of Slaughteria. C-G in crown and lingual views. (From Butler, 1978.)

such form is Shuotherium, from the Middle and Upper Jurassic of China and England, whose teeth are superficially similar to the tribosphenic teeth of therians. Its lower molars have a trigonid and a "talonid," but the talonid-like structure is attached to the front of the trigonid (Chow and Rich, 1982). Such a "pseudotalonid" is present also in the derived docodont Simpsonodon (Kermack et al., 1987). Upper molars from the Middle Jurassic of England and the Late Jurassic of China that possibly represent Shuotherium are tricuspid, superficially like those of tribosphenic mammals. But they differ in wear pattern and crown morphology (the cusps are paracone, metacone, and a lingual "pseudoprotocone") from those of advanced tribosphenic therians (Sigogneau-Russell, 1998; Wang et al., 1998). Furthermore, they differ from each other in overall shape as well as size of the stylar shelf, suggesting that both may not belong to the same genus. Various authors have proposed that Shuotherium is a highly specialized docodont, an aberrant symmetrodont, or (most recently) the sister taxon of the Australosphenida (Kermack et al., 1987; Kielan-Jaworowska, 1992; Luo et al., 2002). Kielan-Jaworowska et al. (2002) postulated that the anterolingual cingulid characteristic of australosphenidans was the precursor of the pseudotalonid in Shuotherium.

Even stranger are the Gondwanatheria, a rare and bizarre group apparently restricted to the Southern Hemisphere. Gondwanatheres are known from the Upper Cretaceous-Paleocene of South America, the middle Eocene of Antarctica, and the Upper Cretaceous of Madagascar and India

Gondwanathere Teeth

Fig. 4.20. Gondwanathere dentitions: (A) Gondwanatherium molar; (B) Sud-america molar; (C) Sudamerica jaw (i indicates posterior end of evergrowing incisor); (D) jaw initially referred to Ferugliotherium, now considered to belong to an undetermined multi-tuberculate. (A and B from Bonaparte, 1990; C from Pascual et al., 1999; D from Kielan-Jaworowska and Bonaparte, 1996.)

Fig. 4.20. Gondwanathere dentitions: (A) Gondwanatherium molar; (B) Sud-america molar; (C) Sudamerica jaw (i indicates posterior end of evergrowing incisor); (D) jaw initially referred to Ferugliotherium, now considered to belong to an undetermined multi-tuberculate. (A and B from Bonaparte, 1990; C from Pascual et al., 1999; D from Kielan-Jaworowska and Bonaparte, 1996.)

(Bonaparte, 1990; Krause et al., 1992, 1997; Krause and Bonaparte, 1993; Reguero et al., 2002). Four genera have been described: Gondwanatherium and Ferugliotherium (Upper Cretaceous, Patagonia), Lavanify (Upper Cretaceous, Madagascar), and Sudamerica (Paleocene, Patagonia; Fig. 4.20). A dentary with a large, procumbent incisor and five single-rooted, hypsodont cheek teeth from the Cretaceous of Tanzania might represent the first African gondwanathere (Krause et al., 2003). Gondwanatheres include the oldest known hypsodont mammals, and at least one, Sudamerica, had an ever-growing incisor. This trait suggests an abrasive diet, and perhaps fossorial or semiaquatic habits like those of beavers (Koenigswald et al., 1999). The recent discovery of silicified phytoliths representing several kinds of grasses in the Late Cretaceous of India suggests the possibility that gondwanatheres were the earliest mammalian grazers (Prasad et al., 2005).

Gondwanatheres were first identified from isolated molars that are so different from those of contemporaneous mammals that their broader attribution was (and is) uncertain. The molars are weakly bi- or trilobate, with three transverse ridges separated by furrows—a pattern superficially suggestive of some derived rodent teeth. Additional isolated teeth and a dentary fragment containing P4 (questionably belonging to Ferugliotherium; Fig. 4.19D) suggested affinity with multituberculates (Krause et al., 1992; Kielan-Jaworowska and Bonaparte, 1996): the incisors, like those of multituberculates, have a limited band of enamel, and P is bladelike, with oblique ridges, as in multituberculates. In addition, microwear on the occlusal surface of the molars indicates that the lower jaw moved palinally (posteriorly), as in multituberculates generally. Ferugliotherium has brachy-dont molars, whereas other known gondwanatheres have very hypsodont molars. The enamel microstructure, consisting mainly of radial enamel and interprismatic matrix, provides little insight on the relationships of gondwanatheres (Koenigswald et al., 1999). Sudamerica, from the early Paleocene ("Peligran") of Argentina, is the only gondwanathere known to have survived into the Cenozoic. A newly found lowerjaw of Sudamerica (Fig. 4.19C), however, has four hypsodont molariform teeth but no bladelike tooth, which casts doubt on the multituberculate affinities of gondwanatheres and the attribution of the supposed Ferugliotherium P4 (Pascual et al., 1999). Indeed, Kielan-Jaworowska et al. (2004) now regard the dentary with the bladelike P4 to be a multituberculate of uncertain affinity rather than a gond-wanathere. A possible sudamericid has recently been reported from the middle Eocene La Meseta Formation of Seymour Island, Antarctica (Reguero et al., 2002). The unusual southern distribution of gondwanatheres suggests that they represent a very ancient mammalian clade. Additional evidence that will resolve gondwanathere relationships is eagerly awaited.

Finally, there is the newly described Fruitafossor, a mouse-sized mammal from the Late Jurassic Morrison Formation of Colorado (Luo and Wible, 2005). Fruitafossor is unique among Mesozoic mammals in having simple, tubular teeth, the molars apparently open-rooted (ever-growing), thus superficially resembling the dentition of armadillos. The dental formula is ?.1.3.3/3.1.3.3. The skeleton is robust, the forelimbs converging toward those of living monotremes and moles (Talpidae). The humerus is short and very wide, the olecranon is elongate and medially inflected, and the manus is broad, with four digits bearing wide, flat terminal phalanges. Fruitafossor is reported to have xenarthrous articulations between the lumbar vertebrae, a specialization otherwise known only in the placental order Xenarthra; but this trait would clearly have evolved convergently, as there is otherwise no evidence of relationship to Xenarthra. The anatomy of Fruitafossor suggests that it was a specialized fossorial mammal that fed on insects and other small invertebrates, but its relationships remain uncertain. Although comparable dental reduction in extant mammals is often associated with myrmecophagy, ants and termites have not been reported from before the Cretaceous, strongly implying that social insects were not the diet of this Jurassic mammal. Fruitafossor offers a glimpse of even greater diversity than we have come to expect among Mesozoic mammals.

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