Cynocephalus (Novacek, 1986)

MENG, JIN, HU, YAOMING & LI, CHUANKUI, 2003, The Osteology Of Rhombomylus (Mammalia, Glires): Implications For Phylogeny And Evolution Of Glires, Bulletin of the American Museum of Natural History 2003 (275), pp. 1-247 : 141-144

acodus, Cynocephalus , Tupaia , and Notharctus .

Character 203: Posterior process on distal end of tibia—(0) absent, or (1) present. A distinctive posterior process or flange on the distal end of the tibia is typical in rodents (Szalay, 1985; Luckett and Hartenberger, 1993). A similar condition, although less distinctive than in Paramys , was present in Rhombomylus (Li and Ting, 1993) . A modest posterior process of the tibia is present in Leptictis (Szalay, 1985) . A similar process appears present in Mimolagus (Bohlin, 1951: figs. 26–34). Because it is difficult to judge the degree of the height of the process, we code ‘‘0/1’’ for Leptictis and Mimolagus .

Character 204: Astragalar trochlea—(0) absent, or (1) present. Absence of the astragalar trochlea is present in Ukhaatherium and Asioryctes (Horovitz, 2000) . The astragalar trochlea evolved independently in placentals and some marsupials (Szalay, 1984, 1994). The trochlea is present in selected placentals, in which the bone is known, except in Cynocephalus where it is absent.

Character 205: Lateral astragalar trochlear border—(0) higher than medial border, or (1) as high as the medial border (modified from Rose and Lucas, 2000; Hooker, 2001). The asymmetrical astragalar trochlea is the primitive condition for eutherians and is widely distributed in various groups, including Zalambdalestes , Hyopsodus , Phenacodus , Anagale , Leptictis , Pseudictops , lagomorphs, Mimolagus , Rhombomylus , Matutinia , Paramys , Reithroparamys , Florentiamys , Sciuravus , Paraphiomys , Sciurus , Marmota , Rattus , and Mus . A relatively symmetrical trochlea is found in primates, Tupaia , Petrodromus , Tribosphenomys , Neoreomys , Myocastor , Cavia , and Dasyprocta .

Character 206: Posterior trochlear shelf of astragalus—(0) absent or weakly developed, or (1) well developed. The posterior trochlear shelf of the astragalus limits the posterior extension of the bone and therefore plantar flexion of the foot. It is present in Plesiadapis , Notharctus , and Adapis .

Character 207: Ventral limit of astragalar trochlea—(0) not exceeding midpoint on posterior surface of body, or (1) more ventral (Hooker, 2001). The development of the posterior trochlear shelf in primates limits the astragalar trochlea to a relatively high point so that the ventral limit of the astragalar trochlea does not exceed the midpoint on the posterior surface of the astragalar body. Although Cynocephalus and Tupaia lack the posterior trochlear shelf, the ventral limit of the trochlea is similar to that of primates.

Character 208: Lateral process of astragalus—(0) present, or (1) absent (Geisler, 2001). Presence of the lateral process of the astragalus is probably a primitive eutherian condition and is widely distributed in fossil taxa, such as Zalambdalestes , Hyopsodus , Phenacodus , Anagale , Leptictis , Plesiadapis , Notharctus , Adapis , Rhombomylus , and Matutinia . It is absent in lagomorphs and most rodents.

Character 209: Astragalofibular facet— (0) sloping obliquely and laterally, or (1) flat (vertical) (modified from Ross et al., 1998). An oblique astragalofibular facet probably facilitates the transverse movement of the upper ankle joint. It is present in Asioryctes , Plesiadapis , Notharctus , and Adapis . It probably represents the primitive condition of eutherians as it is also present in Ukhaatherium (Horovits, 2000: fig 5) and various marsupials (Szalay, 1994). Most eutherians have a vertical and somewhat flat astragalofibular facet, which, together with the trochlea, limits the transverse mobility of the upper ankle joint (Szalay, 1984).

Character 210: Dorsal astragalar foramen—(0) present, or (1) absent. Presence of the dorsal astragalar foramen, indicating the development of the astragalar canal, is the primitive condition of therians. The foramen and canal are lost in many placentals (Horovitz, 2000). In taxa compared here, the foramen is present only in Petrodromus , Anagale , Pseudictops , Plesiadapis , Notharctus , Adapis , and Mimolagus .

Character 211: Sustentacular facet on astragalus—(0) on ventral side of the bone, or (1) shifted to ventromedial side of the bone. In Petrodromus , Elephantulus , and Rhynchocyon (unknown in Macroscelides ), as well as in Tupaia , the sustentacular facet of the astragalus is shifted to the ventromedial side of the bone. In other taxa in which the astragalus is known, the sustentacular facet is on the ventral side of the bone.

Character 212: Sustentacular facet on as­ tragalus—(0) separate from the astragalonavicular facet, or (1) confluent. In hystricognathous rodents examined, the sustentacular and astragalonavicular facets are confluent. A similar condition exists in Tupaia and Cynocephalus . In macroscelideans (unknown in Macroscelides ) the two facets are separated by an angled border, which is different from the smoothly confluent surfaces in Tupaia and Cynocephalus .

Character 213: Long axes of calcaneoastragalar and sustentacular facets of astragalus—(0) forming an angle, or (1) roughly parallel (modified from Geisler, 2001). The parallel condition occurs in lagomorphs, Mimolagus , Rhombomylus , Paramys , Neoreomys , Dasyprocta , Marmota , Rattus , and Mus .

Character 214: Astragalar neck—(0) short (neck ratio: length divided by body width <1), or (1) moderate (neck ratio subequal to 1) or long (neck ratio> 1) (modified from Rose and Lucas, 2000; Ross et al., 1998). A short astragalar neck is present in various eutherians, including Asioryctes, Uhkaatherium , Zalambdalestes , Phenacodus , Anagale , Pseudictops , Plesiadapis , Mimolagus , Rhombomylus , Matutinia , and various rodents examined here. Hyopsodus , Cynocephalus , Tupaia , macroscelideans, primates, lagomorphs (unknown in Ochotona ), and Sciuravus have a relatively long astragalar neck.

Character 215: Astragalonavicular facet on astragalus—(0) transverse, or (1) dorsoventrally extended. The derived condition occurs in lagomorphs (Bleefeld and McKenna, 1985). Although the gross morphology of the astragalus in Pedetes is similar to that of lagomorphs, the astragalonavicular facet on the astragalus is transversely oriented.

Character 216: Cuboid facet on astragalus—(0) absent, or (1) present. The head of the astragalus articulates with the navicular, but in Asioryctes (Kielan­Jaworowska, 1977) , anagalids (Simpson, 1931; Bohlin, 1951), and macroscelideans (Szalay, 1985) it also has a small contact with the cuboid. The contact is absent in lagomorphs (Szalay, 1985: p. 125, fig. 15). Although Mimolagus was considered a lagomorph (Bleefeld and McKenna, 1985; Szalay, 1985), an astragalar cuboid facet is present in the taxon (Bohlin, 1951). An articular facet for the astragalus exists on the cuboid of Mimolagus (Bohlin, 1951) . The astragalus does not articulate with the cuboid in other taxa examined.

Character 217: Calcaneus—(0) relatively broad, or (1) narrow and proportionally long. In dorsal or plantar view, the sustentacular and peroneal processes are transversely extended in primitive forms (Szalay, 1985, 1994). This condition exists in most eutherians in which the calcaneal fibular fac­ et is commonly absent. In mimotonids, Mimolagus , and lagomorphs, these processes are compressed and the body of the calcaneus appears relatively long and slim. A similar condition appears to be in Zalambdalestes (Kielan­Jaworowska, 1978) , Pseudictops (Sulimski, 1968) , macroscelideans, and primates (Szalay and Drawhorn, 1980).

Character 218: Calcaneofibular facet (CaFi) on calcaneus—(0) present as an oblique surface, (1) absent, (2) modified as facing dorsally and broad, or (3) restricted and convex. The upper ankle joint has three contacts: the astragalus­tibia, astragalus­fibula, and calcaneus­fibula in metatherians (Szalay, 1985, 1994). This condition is present in Asioryctes (Kielan­Jaworowska, 1977) . The condition in Rhombomylus and Heomys looks similar to that of Asioryctes (contra to Li and Ting, 1993). The derived condition for eutherian mammals is that the calcaneus is shifted more ventral to the astragalus and the calcaneus­fibula contact is lost (bi­contact of Szalay, 1985). The bi­contact state is present in Mimolagus (Bohlin, 1951; Szalay, 1985) and Tribosphenomys (Meng and Wyss, 2001) . Li and Ting (1985) also indicated lack of CaFi in mimotonids, Anagalidae , Leptictidae , and Zalambdalestidae . The CaFi is present in lagomorphs, macroscelideans, and Pseudictops (Szalay, 1985; Li and Ting, 1985). The question then is whether the CaFi in these forms is a primitive retention or secondarily acquired. The lagomorph condition is considered to be primitive by several authors (Li and Ting, 1985; Luckett and Hartenberger, 1993), but Szalay (1985: 120) thought it a secondary acquisition, which is followed by Li et al. (1987). Szalay (1985: p. 120, fig. 14) also pointed out that Pseudictops has the calcaneus­fibula contact and stated that ‘‘ Pseudic­ tops shares this feature with the Leporoidea, almost certainly convergently, or possibly parallel with them.’’ We found that the CaFi in lagomorphs is different from the condition of Asioryctes and Rhombomylus in facing more dorsally and is broad. Averianov (1991) described some isolated calcanea, probably belonging to the mimotonid Anatolmylus and Aktashmys , which are similar to those of Mimolagus in general morphology and in lacking the calcaneum­fibula facet. The CaFi is more restricted and somewhat convex in macroscelideans.

Character 219: Calcaneoastragalar facet (CaA) on calcaneus—(0) diagonal to the long axis of the bone, (1) narrow facet roughly parallel to the long axis, or (2) facet short showing no obvious orientation. In most eutherians, the calcaneoastragalar facet either shows no apparent directions in its dimensions or it is elongate but oriented diagonally to the long axis of the calcaneus body. In mimotonids (Averianov, 1991), Mimolagus (Bohlin, 1951; Szalay, 1985), and lagomorphs the CaA is oriented such that the major axis is nearly parallel to the long axis of the calcanear body. This condition can be inferred from the articular facet on the astragalus, in which the CaA has a similar orientation. In macroscelideans, the CaA on the calcaneus is short and its orientation is either unrecognizable or perpendicular to the long axis of the bone.

Character 220: Calcanealoastragalar facet—(0) near the distal end of the bone (the tuber being relatively long), or (1) at the midpoint or near the proximal end (tuber short) (modified from Hooker, 2001). The long tuber is the condition for primitive eutherians, such as Uhkaatherium and Zalambdalestes , Hyopsodus , Phenacodus , macroscelideans, and basal gliroid mammals including Mimolagus , Rhombomylus , Heomys , and Tribosphenomys . Lagomorphs and many rodents exhibit state 1. Marmota , Rattus and Mus , however, display state 0.

Character 221: Sustentaculum on the calcaneus—(0) anteromedial to and separated by a distinctive sulcus calcanei from the CaA, or (1) immediately medial to CaA. The sustentaculum on the calcaneus is anteromedial to the calcaneus­astragalus (CaA) fac­ et and both are separated by a distinctive sul­ cus calcanei in most eutherian taxa we selected here. In contrast, the sustentaculum in lagomorphs is immediately medial to CaA facet, while in macroscelids, the sustentaculum is anterior to the CaA facet (Szalay, 1985: p. 125, fig. 14). The condition in Mimolagus is similar to that of lagomorphs (see Szalay, 1985: fig. 18).

Character 222: Calcanear­navicular contact (CaN)—(0) absent, or (1) present. Primitively, the calcaneus does not extend more distally than the astragalus; therefore, the astragalar­navicular articulation (ANa) and the calcaneum­cuboid articulation (CaCu) are at the same level and the ankle is more flexible in rotation. In some lagomorphs, the calcaneus is anteriorly extended to contact the navicular. The foot bones are more firmly interlocked so that sideward movement of the foot is limited but fore­and­aft movement of the foot is emphasized (Dawson, 1958). Dawson (1958: 15, 69) pointed out that in Palaeolagus haydeni and Megalagus turgidus , ‘‘the astragalus reaches approximately the same distance distally as does the calcaneum, but in Hypolagus and the recent leporids the calcaneum extends more distally than the astragalus, making possible a calcaneonavicualr contact.’’ The calcaneonavicualr contact is not present in ochotonids (McKenna, 1982). The calcaneonavicualr facet also occurs on the calcaneus of Mimolagus (Bohlin, 1951; Szalay, 1985), suggesting that the calcaneus extends more distally than does the astragalus.

Character 223: Cuboid—(0) short, or (1) elongate. A relatively long and narrow cuboid is present in Asioryctes , macroscelideans, and Plesiadapis .

Character 224: Medial tarsal sesamoid bone—(0) absent, or (1) present. Szalay (1985) considered presence of a medial tarsal sesamoid bone as unique for rodents, including early forms such as Paramys . His view was followed by Luckett and Hartenberger (1993). A medial tarsal sesamoid bone was probably present in Matutinia ( Ting et al., 2002) . Whether this minute bone occurs in other early gliroid mammals is unknown. In Rhombomylus the articular surface of the astragalar head extends proximally to the medial side of the astragalar neck, which may have articulated with a medial tarsal sesamoid bone.

Character 225: Plantar process of navicular—(0) short, or (1) elongate. The derived condition is seen in leporids, Mimolagus , Rhombomylus , Matutinia (Ting et al., in press) and in hystricognathous rodents.

Character 226: Metatarsal IV—(0) present, or (1) absent. The metatarsals are reduced to three in Cavioidea (Landry, 1999: 303). According to Cooper and Schiller (1975), digits IV and V are lost in Cavia .

Character 227: Metatarsal V—(0) present, or (1) absent. The fifth digit including the metatarsal is lost in Myocastor , Cavia , and Dasyprocta .