Symmetrodonts

Symmetrodonts were small shrew- to mouse-sized animals, known mainly from teeth and jaw fragments. They are considered to lie at or near the base of the therian radiation because they are the first mammals to show a nearly symmetrical triangular arrangement of the three main cusps on the upper and lower molariform teeth. The pattern varies from obtuse angled in primitive and some derived types to acute angled in the most derived. The resulting reversed triangles of the upper and lower molars form a series of oblique shearing edges. This configuration was a marked advance compared to the condition in eutriconodonts, and is regarded as an important step in the evolution of the tri-bosphenic molar. Mammals that possess a reversed-triangle molar pattern—symmetrodonts, eupantotheres, and therian mammals—are sometimes united in the higher taxon Holo-theria. The concept of Holotheria is problematic, however, because its contents are controversial (variously including or excluding Kuehneotherium, monotremes, and eutriconodonts) and because of recent arguments that its defining feature, the reversed-triangle molar pattern, has evolved more than once. For these reasons, the name Holotheria was rejected in the most recent treatise on Mesozoic mammals (Kielan-Jaworowska et al., 2004).

Where known, the lower incisors and canine are small, and there are 7-11 postcanines (Fig. 4.14). The premolari-form teeth are simple and have one main cusp with accessory cusps in front and behind. The lower molars primitively had a small talonid, which was lost in more derived forms. The dentary is long and slender and has no angular process, and the articular condyle is above the level of the toothrow. Postdentary bones were present in the most primitive forms (kuehneotheriids), but are absent in more derived types (Cassiliano and Clemens, 1979). Symmetrodonts are recorded from the latest Triassic (if kuehneotheriids are in cluded; otherwise Early Jurassic) to Late Cretaceous, and are known from all continents except Australia and Antarctica.

The 20 or so known genera of symmetrodonts are classified in at least three and as many as seven families. The disparity has arisen because many genera are known only from isolated teeth, the affinities of which are often unclear. Like the traditional concept of triconodonts, symmetrodonts appear to be a paraphyletic assemblage. Late Triassic or earliest Jurassic Kuehneotherium (see Fig. 4.5) has been regarded as the most primitive symmetrodont, but recent studies suggest that it is actually not very closely related to later symmetrodonts (Rougier et al., 1996a; Luo et al., 2002; Kielan-Jaworowska et al., 2004).

Fig. 4.14. Symmetrodont dentitions: (A) Tinodon, crown and buccal views of left lower molars; (B) Spalacolestes, left upper and right lower cheek teeth (crown and lingual views); (C) Zhangheotherium, left upper and lower molars and right lower jaw in medial view. (A from Crompton and Jenkins, 1967; B from Cifelli and Madsen, 1999; C from Hu et al., 1997.)

Fig. 4.14. Symmetrodont dentitions: (A) Tinodon, crown and buccal views of left lower molars; (B) Spalacolestes, left upper and right lower cheek teeth (crown and lingual views); (C) Zhangheotherium, left upper and lower molars and right lower jaw in medial view. (A from Crompton and Jenkins, 1967; B from Cifelli and Madsen, 1999; C from Hu et al., 1997.)

Most later symmetrodonts are classified in the families Amphidontidae, Tinodontidae, and Spalacotheriidae. The first two had obtuse-angled molars, whereas spalacotheriids had acute-angled molars in which the three main cusps form a tight triangle (superficially similar to the trigonid of tribo-sphenic therians) and the talonid has been greatly reduced or lost. Although these families have long been grouped in the Symmetrodonta based on their rotated cusps, Luo et al. (2002) recently suggested that obtuse-angled forms represent a grade of primitive mammals that bear no special relationship with acute angled forms. Most spalacotheriids had well-developed shearing surfaces on the front and back of the trigonid and on opposing surfaces of the upper molars (Cifelli and Madsen, 1999). Spalacotheriids had fewer premolars and more molars than Kuehneotherium; the dental formula was ?.1.3.6-7/3+.1.3.6-7; Cassiliano and Clemens, 1979; Sigogneau-Russell and Ensom, 1998). The specialized dentition of spalacotheriids suggests that they were not directly ancestral to advanced therians, although they are believed to be more closely related to them than are the obtuse-angled symmetrodonts.

Although most symmetrodonts are known only from teeth and jaws, a nearly complete skeleton of a new sym-metrodont, Zhangheotherium (Fig. 4.14C), was reported from the same Early Cretaceous site in China that yielded the eutriconodont Jeholodens (Hu et al., 1997). Zhangheo-therium had fewer postcanines than in most other sym-metrodonts: the dental formula is 3.1.2.5/3.1.2.6. It also had an uncoiled cochlea, in contrast to therians. The post-cranial skeleton shows many features intermediate between those of multituberculates or monotremes and those of therians, particularly in the shoulder girdle (e.g., presence of a supraspinous fossa, retention of a smaller inter-clavicle than in monotremes), elbow joint (incipient trochlea for ulna), pelvis, and femur. At the same time, these features indicate that Zhangheotherium had an abducted, sprawling forelimb posture more like that of monotremes than like the parasagittal posture of advanced therians. Cifelli and Madsen (1999) consider Zhangheotherium to be the most primitive known spalacotheriid symmetrodont. A closely allied new genus, Maotherium, was recently proposed, based on a skeleton preserving fur impressions from Jurassic/ Cretaceous boundary strata in China (Rougier, Ji, and No-vacek, 2003).

A variety of new symmetrodonts and eupantotheres has been found recently in the Late Cretaceous of Argentina, indicating both greater diversity and broader distribution of these groups than previously suspected (Bonaparte, 1990, 1994). At the same time, they have blurred the distinction between the two groups. Unfortunately, most are known only from isolated teeth or fragmentary dentitions, making interpretation tenuous. Other new genera of sym-metrodonts have been reported from North Africa and several parts of Asia, but their precise relationships with other symmetrodonts are also uncertain (Sigogneau-Russell and Ensom, 1998).

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