Eupantotheres

The eupantotheres (Dryolestoidea and Peramura of McKenna and Bell, 1997; Figs. 4.15, 4.16) occupy a structural and phylogenetic position essentially between symmetro-donts, on the one hand, and aegialodonts + therian mammals on the other. Among eupantotheres, dryolestoids (dryolestids and paurodontids), amphitheriids, and pera-murans are successively more closely related to crown the-rians. This conclusion is founded on the anatomy of the teeth and jaws, which constitute almost all known fossils. Eupantotheres are derived compared to symmetrodonts in having wider upper than lower teeth (although they still lack the protocone of tribosphenic forms), larger talonids on the lower molars, and a well-developed angular process on the dentary (Figs. 4.16, 4.17; Simpson, 1928; Kraus, 1979). The coronoid process was high. The dentary was long and slender in most types, but usually shorter and deeper in pauro-dontids. The trigonid cusps of the lower molars and, to a lesser extent, the cusps of the upper molars are arranged in reversed acute triangles. Where the dental formula is known, most forms have four incisors, a canine, four simple premolariform teeth, and four to nine molariform teeth. Eupantotheres had a dentary-squamosal jaw joint. Although small accessory (postdentary) bones were still present in the Upper Jurassic paurodontid Henkelotherium, they did not participate in the jaw articulation (Krebs, 1991). A vestigial coronoid bone was present in the lower jaw of primitive dryolestids (Martin, 1999b), but postdentary bones were probably absent in the Early Cretaceous dryolestid Crusa-fontia (Kraus, 1979; Krebs, 1993).

Eupantotheres have been found on all continents except Antarctica and range from the Middle Jurassic through Late Cretaceous and possibly early Paleocene. Again, some of the best preserved eupantothere fossils come from the Late Jurassic Guimarota Mine in Portugal (Martin and Krebs, 2000). Most eupantotheres were small shrew- to mouse-sized mammals, although the South American Mesungula-tum and Vincelestes were somewhat larger. Body size and dental morphology indicate that eupantotheres were insectivorous or carnivorous.

Eupantotheres have traditionally been assigned to the families Amphitheriidae, Dryolestidae, Paurodontidae, and Peramuridae (Kraus, 1979). Like triconodonts and symmetrodonts, eupantotheres now appear to be a paraphyletic structural grade. Middle Jurassic Amphitherium is the oldest well-known eupantothere (but still known only from the lower dentition). Although once considered the most primitive eupantothere, Amphitherium is now thought to be more derived than dryolestids in having larger talonids. It had 11 postcanines, interpreted as five premolars and six molars (Butler and Clemens, 2001). The molars had a well-formed trigonid followed by a much lower, bladelike talonid with one cusp. Amphitheriids are an important structural stage in the evolution of tribosphenic molars (Crompton, 1971). The Middle Jurassic amphitheriid Palaeoxonodon has

Triconodont Tribosphenic
Fig. 4.15. Eupantothere skeletons: (A) Henkelotherium; (B) Vincelestes. (A from Krebs, 1991; B from Rougier, 1993.)

a somewhat better developed talonid on the lower molars. Nonetheless, a protocone is not yet present on the upper molars; they are triangular, with lingual paracone, less lingual metacone, and buccal parastyle, stylocone, and meta-style (Sigogneau-Russell, 2003a). Palaeoxonodon is sometimes considered a peramuran.

Dryolestidae were the most diverse eupantotheres, and have been found in the Late Jurassic of North America and Europe, the Early Cretaceous of Europe, and the Late Cretaceous and possibly early Paleocene of South America. The dryolestid Leonardus and several taxa assigned to closely allied separate families from the Los Alamitos Formation of Argentina are the latest known eupantotheres (Bonaparte, 1990, 1994), unless the early Paleocene Peligrotherium is actually a dryolestoid, as recently suggested by Gelfo and Pascual (2001) and Rougier, Novacek, et al. (2003). Dryolestids, such as Krebsotherium from Guimarota, usually have 12 postcanines in both upper and lower jaws, four premolars and eight molars (Fig. 4.17). The trigonids are tall and anteroposteriorly compressed ("closed"), with the metaconid almost as high as the protoconid, and the talonids are smaller than in other eupantotheres. The upper molars are transversely very wide. Dryolestids replaced all of their antemolar teeth, like placental mammals but unlike marsupials, which replace only dP3 (Martin, 1999b). Martin believes this trait implies that their reproduction was unlike that of marsupials.

Paurodontidae are known primarily from the Late Jurassic Morrison Formation of North America. Most have a short, robust mandible containing eight postcanines, and the molars have a shorter talonid than in Amphitherium. The best-known paurodontid, Henkelotherium, is not from North

America, however, but from the Guimarota Mine in Portugal (the same site that produced the docodont Haldanodon). Henkelotherium is represented by a nearly complete skeleton (the only one known for eupantotheres), which has a more advanced pectoral girdle than in monotremes and morganu-codonts (Fig. 4.15A). As in therians, only the scapula and clavicle are present, and there is a large supraspinous fossa. The pelvic girdle has a long iliac blade and retains epipubic bones. A long tail and sharp, curved claws suggest arboreal habits or at least climbing capability. Henkelotherium differs from the Morrison paurodontids in having more postcanines (dental formula is 4-5.1.4.5/4.1.4.7; Krebs, 1991).

Several new monotypic "eupantothere" families thought to be closely related to therian mammals have been described from the Cretaceous of South America, Asia, and Africa over the past decade or so, indicating that mammals dentally approaching therians were nearly cosmopolitan and rather diverse. Among the most important of these is Vincelestes (Fig. 4.15B) from the Early Cretaceous of Argentina. It has been considered to be related to tribosphenic therians because its upper molars have a lingual expansion

Amphitherium

Fig. 4.16. Eupantothere dentitions: (A) Peramus (left upper and right lower); (B) Amphitherium; (C) Kielantherium (right molar in crown and lingual views); (D) Arguimus (right lower teeth). Key: alc, anterolingual cuspule; entd, entoconid; hyd, hypoconid; hyld, hypoconulid; me, metacone; med, metaconid; pa, paracone; pad, paraconid; prd, protoconid; sty, stylocone. (A from Clemens, 1971; B from Clemens, 1970; C from Crompton and Kielan-Jaworowska, 1978; D from Dashzeveg, 1994.)

Fig. 4.16. Eupantothere dentitions: (A) Peramus (left upper and right lower); (B) Amphitherium; (C) Kielantherium (right molar in crown and lingual views); (D) Arguimus (right lower teeth). Key: alc, anterolingual cuspule; entd, entoconid; hyd, hypoconid; hyld, hypoconulid; me, metacone; med, metaconid; pa, paracone; pad, paraconid; prd, protoconid; sty, stylocone. (A from Clemens, 1971; B from Clemens, 1970; C from Crompton and Kielan-Jaworowska, 1978; D from Dashzeveg, 1994.)

Fig. 4.17. Dentition of the dryolestid Krebsotherium: upper teeth in (A) crown and (B) lingual views; lower teeth in (C) lingual, (D) crown, and (E) buccal views. (From Martin, 1999b.)

Tribosphenic Dentition

Fig. 4.17. Dentition of the dryolestid Krebsotherium: upper teeth in (A) crown and (B) lingual views; lower teeth in (C) lingual, (D) crown, and (E) buccal views. (From Martin, 1999b.)

where a protocone would be expected to form, and it has a partly coiled cochlea (about 270°) and several other derived conditions of the ear region found in marsupials and pla-centals (Hopson and Rougier, 1993; Rowe, 1993; Rougier et al., 1996a). But there is disagreement over whether Vince-lestes really had an incipient protocone (Sigogneau-Russell, 1999). Moreover, it is dentally too derived (dental formula 4.1.2.3/2.1.2.3, with a small talonid present only on M2, and the first premolar and last molar reduced), and probably too late in time (Neocomian), to have been directly ancestral to marsupials and placentals. Vincelestes may be a basal member of Zatheria, the higher taxon that also includes peramurid eupantotheres (McKenna and Bell, 1997; Martin, 2002).

Peramuridae are best known from the Late Jurassic Per-amus of England (Sigogneau-Russell, 1999). Peramus is generally similar to Amphitherium (Fig. 4.16A,B), but differs in several respects that suggest that it represents a more derived stage in the evolution of tribosphenic molars. Peramus has only eight postcanines—both lower and upper—variously interpreted as four premolars and four molars or, now usually as five premolars and three molars. Significantly, the talonids of M1-2 (assuming three molars) have an incipient basin bordered by a second cusp in addition to the one present in Amphitherium. The last premolars are higher crowned than the adjacent teeth. The upper molariform teeth are dominated by a large paracone followed by a much lower metacone, flanked by stylar cusps. The lingual border is somewhat inflated but there is still no protocone (Clemens and Mills, 1971; Sigogneau-Russell, 1999). Although these conditions suggest an approach toward the tribosphenic molars of marsupials and placentals, Dashzeveg and Kielan-Jaworowska (1984) concluded that peramurids may already be too derived to be directly ancestral to modern therians. Sigogneau-Russell (1999), however, considered permaurids to be structurally intermediate between symmetrodonts and tribosphenic mammals.

The Arguitheriidae and Arguimuridae, from the Early Cretaceous of Mongolia (Dashzeveg, 1994), have been considered to have more progressive molars than those of am-phitheriids and peramurids. Arguitherium has a relatively open trigonid and an incipient talonid basin, whereas Arguimus (Fig. 4.16D) has well-developed trigonids and unbasined talonids bearing three cusps. Together with the recently described Nanolestes from the Late Jurassic-Early Cretaceous of Portugal, these forms are perhaps best regarded as peramurid-grade stem zatherians (Sigogneau-Russell, 1999; Martin, 2002).

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