Until recently, triconodonts were viewed as including three families: Morganucodontidae (then including Mega-zostrodontidae), Triconodontidae, and Amphilestidae ( Jenkins and Crompton, 1979). A fourth family, Austrotricono-dontidae, was based on very fragmentary fossils from the Late Cretaceous of South America (Bonaparte, 1994). Two additional genera, Dinnetherium and Jeholodens, significant because of their excellent state of preservation, have proven difficult to accommodate within these four families. Din-netherium has been considered an amphilestid ( Jenkins and Schaff, 1988), a morganucodontid (Luo, 1994; Rougier et al., 1996a), and most recently a megazostrodontid (Kielan-Jaworowska et al., 2004).

Recent studies indicate that this traditional concept of triconodonts represents a grade of primitive mammals rather than a monophyletic group. Morganucodonts, as noted earlier, are now widely considered to be basal mammals, whereas amphilestids, triconodontids, and Jeholodens (together comprising the Eutriconodonta) share a more recent common ancestry with advanced therians and appear to be monophyletic (Rougier et al., 1996a; Luo et al., 2002). Eutriconodonts are derived compared to morganucodonts in having a pterygoid fossa on the medial side of the dentary and in lacking an angular process and a postdentary trough (which is associated with retention of postdentary bones; Kielan-Jaworowska et al., 2004).

Eutriconodonts (Fig. 4.13) were a very successful Meso-zoic group, being known from Jurassic and Cretaceous strata and existing on all continents except Australia and Antarctica, but they left no Cenozoic descendants. Most taxa are known only from isolated teeth or jaw fragments, although several important skulls and skeletons have substantially improved our understanding of eutriconodonts over the past 20 years.

The triconodonts derive their name from their narrow molars with three principal longitudinally-aligned cusps, an arrangement similar to that in morganucodonts as well as the presumed cynodont ancestors of morganucodonts (and therefore considered primitive). A much smaller cin-gular cusp is present distally on both upper and lower molars. In most types the central cusp (designated 'A' on the upper teeth, "a" on the lowers; Jenkins and Crompton, 1979) is most prominent, but in triconodontids the three cusps are of about equal height, giving the molar series a saw-tooth appearance. Both eutriconodonts and morganucodonts are further distinguished by having precise molar occlusion, as reflected by consistently developed shearing facets. The way the teeth occlude varies, however. In morganucodontids and triconodontids upper and lower molars occlude essentially one on one, whereas in megazostrodontids and am-philestids the main upper cusp occludes between the high cusps of two adjacent lower molars ( Jenkins and Crompton, 1979). Where known, eutriconodonts have a straight cochlea, as in docodonts, multituberculates, and their cynodont ancestors, unlike the bent or coiled cochlea of higher therians (Rougier et al., 1996a).

Fig. 4.13. Eutriconodonts: (A) triconodontid Trioracodon, left dentition (lateral view); (B) amphilestid Gobiconodon, skeleton; (C, D)Jeholodens right dentition and skeleton. (A from Simpson, 1928; B from Jenkins and Schaff, 1988; C, D from Ji et al., 1999.)

Triconodontids are known from the Upper Jurassic through Upper Cretaceous. Where known, the incisors are reduced in number compared to morganucodonts and there are either three or four premolars and from three to five molars, depending on the genus (Jenkins and Crompton, 1979). Austrotriconodontids differ in having a larger central cusp on the lower molars, and a bladelike arrangement on the uppers, with the highest cusp in front and the next three cusps successively lower (Bonaparte, 1994).

Amphilestids have been recorded from Middle Jurassic through Lower Cretaceous strata. The dentition is best known in the lower jaws of such genera as Phascolotherium and Amphilestes, in which the lower dental formula is 3 or (Jenkins and Crompton, 1979). In these forms the cheek teeth are characterized by a large central cusp flanked by smaller cusps in front and back.

The most completely known amphilestid is Gobiconodon (Fig. 4.13B; now sometimes assigned to its own family Go-biconodontidae). It is among the most wide-ranging Cretaceous mammals, being known from the Early Cretaceous of Asia, western Europe, north Africa, and Montana (Jenkins and Schaff, 1988; Kielan-Jaworowska and Dashzeveg, 1998; Cifelli, 2000; Li et al., 2003; Sigogneau-Russell, 2003b). Gob-iconodon is unusual in having enlarged, caninelike incisors, reduced canines, and replacement of the molars. The scapula is distinctly therian-like, with a large supraspinous fossa, and the humerus has a grooved trochlea for the ulna, unlike the ulnar condyle of morganucodonts. The forelimb skeleton of Gobiconodon is relatively much more robust than that of morganucodonts. The humerus has a prominent delto-pectoral crest and is very broad distally, and as in morganucodonts it displays torsion (i.e., the articular ends are twisted relative to each other). The phalanges are relatively stout and the terminal phalanges are especially large, with prominent extensor and flexor processes. When found in therians these forelimb traits are typically associated with digging habits.

Gobiconodon and its close relative Repenomamus (Early Cretaceous of China) were large mammals for the Meso-zoic, reaching at least the size of the opossum Didelphis. One species of Repenomamus had a skull more than 15 cm long and was about the size of the wolverine Gulo. It is the largest known Mesozoic mammal—apparently large enough to consume small dinosaurs, based on one individual, whose stomach contents consisted of a juvenile ceratopsian Psittacosaurus (Hu et al., 2005). Y. Wang et al. (2001) reported the presence of an ossified Meckel's cartilage in the jaws of Repenomamus and Gobiconodon, which they interpreted as an intermediate stage in the evolution of the definitive (i.e., fully) mammalian middle ear.

A virtually complete skeleton of a previously unknown eutriconodont, Jeholodens, was recently reported from Early Cretaceous beds of China (Fig. 4.13C,D; Ji et al., 1999). It is autapomorphic in having a reduced number of premolars compared to other eutriconodonts (dental formula Jeholodens had generalized body proportions and a primitive sprawling posture. Surprisingly, however, it also had numerous derived anatomical features typical of ther-ian mammals (some of which also occur in Gobiconodon). For instance, the scapula has a large supraspinous fossa, the coracoid is fused to the scapula, the humeral epicondyles are reduced, and there is an incipient trochlea for the ulna. As the dentition of eutriconodonts would seem to preclude them from direct ancestry to therians, some or all of these therian-like traits could be homoplasies.

The conflicting characters of eutriconodonts have led to instability of their phylogenetic position with respect to other mammals. Although they are now usually placed within crown-group Mammalia (e.g., Luo et al., 2002), this position is far from certain.

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