Gomphos

MENG, JIN, BOWEN, GABRIEL J., JIE, YE, KOCH, PAUL L., TING, SUYIN, QIAN, LI & JIN, XUN, 2004, Gomphos elkema (Glires, Mammalia) from the Erlian Basin: Evidence for the Early Tertiary Bumbanian Land Mammal Age in Nei-Mongol, China, American Museum Novitates 3425, pp. 1-24 : 15-17

es between Gomphos and Rhombomylus are applicable to Matutinia .

The presence of two pairs of lower incisors readily distinguishes Gomphos elkema from species frequently referred to as eurymylids, rodents, Mimolagus , and lagomorphs. Double lower incisors of Gomphos are shared with Mimotona and Anatolmylus . Mimotona ( Li, 1977; Li and Ting, 1985, 1993) differs from Gomphos in having small­ er body size, less developed mesoconid on the lower molars, shorter trigonid, a more elongated talonid on m3, a less molariform p4, absence of the mesostyle on upper molars, upper cheekteeth anteroposteriorly shorter, and lower degree of hypsodonty.

Averianov (1994: 401) recognized that Anatolmylus ‘‘Differs from Mimotona and Gomphos by extremely deep and relatively short horizontal ramus of mandible with distinctly more curved tooth row and incisor (i2). Diastema is short, shorter than in Mimotona . The lower and upper cheek teeth with unilateral hypsodonty; p4 molariform. Paraconid on the lower molars virtually absent. On the upper molars paraconule and metaconule absent, the mesostyl[e] small.’’ Except for depth of the mandible, these differ­

ences between Gomphos and Anatolmylus are not fully demonstrated because of the fragmentary specimens (Zhegallo and Shevyreva, 1976; Dashzeveg and Russell, 1988; Averianov, 1994).

The astragalus and calcaneus of Gomphos are very similar to those of Mimolagus ( Bohlin, 1951; Bleefeld and McKenna, 1985; Szalay, 1985) in several aspects: the tuber of the calcaneus gradually expanding distally; the calcaneoastragalar and the sustentacular facets being aligned at the same level; the calcaneoastragalar facet being a narrow convex band in a distoproximal orientation, nearly parallel to the long axis of the bone; a distinctive pit being present on the plantar side of the process bearing the calcaneoastragalar facet; astragalonavicular facet on the head of the calcaneus with similar orientation; and the astragalus being ventrodorsally (or anteroposteriorly) narrow. They differ in that in Mimolagus a longitudinal ridge is present on the plantar surface of the calcaneus, the head of the astragalus is transversely narrower, the astragalonavicular facet is less medially extended, and the trochlear rims are more sharply delineated. This combination of features makes the ankle bones distinctive from other gliroids, such as Rhombomylus ( Li and Ting, 1993; Meng et al., 2003), Tribosphenomys ( Meng and Wyss, 2001) , and early rodents ( Wood, 1962). The astragalus and calcaneus of Gomphos are also different from those that were referred to ‘‘ Mixodontia sp.’’ from the early Eocene of Kirgizia ( Averianov, 1991). The astragalus and calcaneus of ‘‘ Mixodontia sp.’’ described by Averianov (1991) are more elongate. The general shape and the condition of calcaneoastragalar facet and sustentacular facet aligned at the same level, with the calcaneoastragalar facet being a narrow convex band in a distoproximal orientation, are similar to those of lagomorphs. However, in both Gomphos and Mimolagus a calcaneofibular facet is absent on the calcaneus and the distal portion of the calcaneus is not elongated. In addition, the astragalus is transversely broader than that of lagomorphs ( Dawson, 1958; Szalay, 1985). Nonetheless, the foot bones of Gomphos are similar to Mimolagus in being more lagomorphlike, not rodentlike.

The head of the astragalus articulates ex­ clusively with the navicular in most eutherians, but in anagalids ( Simpson, 1931; Bohlin, 1951) and macroscelideans ( Szalay, 1985) it also has a small contact with the cuboid. As in Gomphos , the contact is absent in lagomorphs ( Szalay, 1985) but is present in Mimolagus ( Bleefeld and McKenna, 1985; Szalay, 1985).

In most eutherians, the calcaneoastragalar facet is either somewhat rounded or is elongate but oriented diagonal to the long axis of the calcaneus body. In mimotonids ( Averianov, 1991), Mimolagus ( Bohlin, 1951; Szalay, 1985), and lagomorphs the calcaneoastragalar facet is oriented such that the major axis is nearly parallel to the long axis of the calcanear body. A similar condition is present in Gomphos .

The sustentaculum on the calcaneus is anteromedial to the calcaneoastragalar facet and in most eutherian taxa the two facets are separated by a distinctive sulcus calcanei. In contrast, the sustentaculum in lagomorphs is immediately medial to the calcaneoastragalar facet. Mimolagus and Gomphos are similar to each other and comparable to lagomorphs in this condition.

Primitively, the calcaneus does not extend farther distally than does the astragalus; therefore, the astragalar­navicular articulation and the calcaneus­cuboid articulation are roughly at the same level and the ankle is more flexible in rotation. In some lagomorphs, the calcaneus is distally extended to contact the navicular. The foot bones are more firmly interlocked such that sideward movement of the foot is limited but fore­andaft movement of the foot is emphasized ( Dawson, 1958). In some early lagomorphs, such as Palaeolagus haydeni and Megalagus turgidus , the astragalus reaches approximately the same distance distally as does the calcaneum; in others, such as Hypolagus and the recent leporids, the calcaneus extends farther distally than does the astragalus, making possible a calcaneonavicular contact ( Dawson, 1958). The calcaneonavicular contact is not present in ochotonids ( McKenna, 1982). The calcaneonavicular facet also occurs on the calcaneus of Mimolagus ( Bohlin, 1951; Szalay, 1985), suggesting that the calcaneus extended farther distally than did the astragalus.

The calcaneal canal is a perforation present in the mammalian calcaneus to provide channels for small blood vessels. In lagomorphs, the canal starts from the lateral side of the calcaneal tuber as a circular foramen, traverses the calcaneal body diagonally, and exits the calcaneus via an aperture situated between the sustentaculum and the cuboid facet ( Bleefeld and Bock, 2002). According to Bleefeld and Bock (2002), the calcaneal canal is a feature that appeared early in the history of this order, and it occurs in all recognized Recent and fossil lagomorph calcanea. The ancientness of the calcaneal canal and its ubiquity among lagomorphs emphasize the monophyly of the order. The canal is greatly reduced, or lost, in extant leporids, which is regarded as a derived lagomorph feature ( Bleefeld and Bock, 2002). Because the calcaneal canal is unknown in the calcaneus of any rodent, or other suggested Recent or fossil lagomorph relatives (e.g., macroscelidids, anagalids), Bleefeld and Bock (2002) concluded that the morphological distinctiveness of the earliest recognized pedes of lagomorphs from those of macroscelidids and anagalids may indicate a long evolutionary separation of those mammalian orders.

Mimolagus and Gomphos are stem taxa to Lagomorpha and share many derived similarities with lagomorphs ( Meng and Wyss, 2001; Meng et al., 2003). The calcaneus does not have the calcaneal canal in either taxa. This indicates that the calcaneal canal characterizes only Lagomorpha , but not Duplicidentata that includes Lagomorpha and its stem taxa ( Meng and Wyss, 2001). The oldest lagomorph calcanei observed by Bleefeld and Bock (2002) are from the early Oligocene Hsanda Gol Formation of Mongolia. It is probable that in earlier lagomorphs the calcaneus may have had the calcaneal canal. Nonetheless, because the age of Mimolagus is roughly early Oligocene ( Bohlin, 1951), it seems that the morphological distinctiveness in the calcaneal structure may not adequately indicate the relative timing of the evolutionary separation between Lagomorpha and its stem taxa.

Because several recent phylogenetic studies of Glires have shown that Gomphos is a stem taxon to Lagomorpha ( Meng and Wyss, 2001; Meng et al., 2003; Meng, in press), we will not provide additional phylogenetic analyses concerning this taxon in the present study. However, we note that the foot bones of Gomphos are apparently similar to those of lagomorphs in several aspects, which undoubtedly strengthens the position of Gomphos as a stem taxon to lagomorphs. Our discussion will focus on the implications for faunal correlation of the presence of Gomphos in Nei Mongol.