Leptictida

Leptictida is an ordinal-level group that accommodates the Early Tertiary Leptictidae and the closely allied Late Cretaceous Gypsonictops (Novacek, 1986a). McKenna and Bell (1997) included two additional families in their superorder Leptictida: Kulbeckiidae (now considered a junior synonym of Zalambdalestidae) and Didymoconidae (here tentatively included in Cimolesta). They also united Gypsonictops with a half-dozen other Cretaceous genera, including Proken-

nalestes, Kennalestes, and Zhelestes, in the family Gypsonictop-idae. This association, however, is mainly based on primitive characters. Prokennalestes is indeed a very primitive euther-ian and its precise position is uncertain (see Chapter 6), whereas the last two genera are now generally transferred to Asioryctitheria and Ungulatomorpha (Zhelestidae), respectively. Consequently, Leptictida is used here in the more restricted sense of Novacek (1986a).

Late Cretaceous Gypsonictops, known from jaws and teeth from western North America, is the oldest known lepticti-dan and the only pre-Cenozoic representative. It is the sister taxon of Leptictidae (Novacek, 1977, 1986a). Like early Tertiary leptictids (but unlike many other insectivorans), Gypsonictops (Fig. 9.2A) has molariform posterior premolars; the lower molariform teeth have less elevated, more mesio-distally compressed trigonids and relatively larger, broader talonids than those of its contemporary Cimolestes (Clemens, 1973). The upper molars are less transverse than in Cimolestes, with a reduced stylar shelf and well-developed postcingula. Gypsonictops differs from leptictids and most other eutheri-ans in having five premolars (the primitive state for euthe-rians), the last one slightly less molariform than in leptictids, and in some other details of tooth structure.

Leptictidae includes 10 genera, mostly from North America. All were small, roughly the size of hedgehogs. The teeth, such as in Paleocene-early Eocene Prodiacodon (Fig. 9.2B), are similar to those of Gypsonictops except that there are only four premolars (presumably the central premolar, dP3, has been lost), the last lower premolar has an elongate trigonid with a stronger paraconid, and the upper molars have a small hypocone (Novacek, 1977). These trends were accentuated in Chadronian-Orellan Leptictis (=Ictops; Figs. 9.2C, 9.3, Plate 4.1), for which exquisitely preserved skulls and skeletons are known. The skull has been described in detail by Butler (1956) and Novacek (1986a). It has a long, tapered rostrum, a deep antorbital fossa for snout muscles, paired parasagittal (temporal) crests, an entotympanic bulla, and a reduced lacrimal bone restricted to the orbit (Fig. 9.3). The olfactory region is extensive and well developed, between and anterior to the orbits. The postcranial skeleton of leptictids has been described by Cavigelli (1997) and Rose (1999a, in press). The forelimbs are relatively short and moderately robust, suggesting they were adapted for digging. The hind limbs are exceptionally long and more slender, the femur with a narrow and elevated patellar groove, and the tibia and fibula extensively fused distally. The astragalar trochlea is deeply grooved; the tarsals and metatarsals moderately elongated. Overall, the skeleton suggests that leptictids were terrestrial animals, capable of occasional bouts of running or jumping and digging with the front legs. Less well-preserved skulls and skeletal remains of Paleocene and early Eocene leptictids, such as Palaeictops, do not differ appreciably from Leptictis, except that tibiofibular fusion is limited to the distal end in Torrejonian Prodiacodon.

Leptictids were long held to be closely related to lipo-typhlans, because of alleged craniodental similarities to

Gypsonictops

2 mm

Fig. 9.2. Leptictidan dentitions, right upper teeth (left column), left lower teeth (right): (A) Late Cretaceous Gypsonictops; (B) Paleocene Prodiacodon; (C) late Eocene-early Oligocene Leptictis. (A, C from Lillegraven, 1969; B from Novacek, 1977.)

2 mm

Fig. 9.2. Leptictidan dentitions, right upper teeth (left column), left lower teeth (right): (A) Late Cretaceous Gypsonictops; (B) Paleocene Prodiacodon; (C) late Eocene-early Oligocene Leptictis. (A, C from Lillegraven, 1969; B from Novacek, 1977.)

hedgehogs (e.g., Gregory, 1910; Butler, 1956). This alliance was questioned by later authors (McDowell, 1958; McKenna, 1975a) and was also subsequently rejected by Butler (1972), because of differences in the anatomy of the orbit and the presence of an entotympanic bulla in leptic-tids, unlike the basisphenoid or unossified bulla of lipo-typhlans. Following detailed cranial analysis, however, Novacek (1986a; MacPhee and Novacek, 1993) revived the hypothesis of leptictid-lipotyphlan ties, based on numerous shared derived cranial traits. Nevertheless, McKenna and Bell (1997) continued to separate these two groups at the superordinal level (Leptictida vs. Preptotheria for Lipotyphla and most other eutherians). Alternatively, it is possible that leptictids are more closely related to macroscelideans and Glires than to lipotyphlans (Rose, 1999a; Asher et al., 2003). Thus, in spite of detailed knowledge of the anatomy of lep-tictids, their relationships remain controversial.

Leptictids have always been construed as very primitive eutherians. Consequently, ancestral-descendant relationships between leptictids and various other eutherians, including Primates, were considered likely through the 1960s and 1970s (e.g., Van Valen, 1965; McKenna, 1966; Bown and Gingerich,

1973; Clemens, 1973). The current consensus is that leptic-tids are among the most primitive known eutherians and offer important information on plesiomorphic eutherian features, but close relationship to other eutherians remains to be convincingly demonstrated.

Probably closely related to leptictids are two European genera with the ungainly family name Pseudorhynco-cyonidae—an allusion to their superficial resemblance to the living elephant shrews, particularly the genus Rhynchocyon. Leptictidium (Fig. 9.4, Plate 4.2) is the better-known genus, being represented by several complete skeletons from the middle Eocene of Messel, Germany (Storch and Lister, 1985; Maier et al., 1986; Koenigswald et al., 1992a). They were the largest leptictidans, ranging in length from just over a half meter to a little less than a meter. Leptictidium had a long, slender snout with antorbital muscle fossae (as in leptictids), suggesting a mobile snout, as in elephant shrews. The molars are generally similar to those of leptictids, although the upper molars are less transverse, and the fourth premolars are molariform. Leptictidium is more derived than leptictids in having wide diastemata separating the anterior teeth from the cheek teeth. Stomach contents, preserved in several

Homo Erectus With Condyloid Process

Fig. 9.3. Skull of early Oligocene Leptictis. Key: Ang. Pr., angular process; AS, alisphenoid; Cond. Pr., condyloid process; Cor. Pr., coronoid process; DEN, dentary; DEN (Asc. Ram.), ascending ramus; DEN (Hor. Ram.), horizontal ramus; ECT, ectotympanic; ENT, entotympanic; FR, frontal; Infra. Ca., infraorbital canal; JU, jugal; LA, lacrimal; MX, maxilla; NA, nasal; OS, orbitosphenoid; PA, parietal; PL, palatine; PMX, premaxilla; PR, petromastoid; PT, pterygoid; SQ, squamosal. (From Novacek, 1986a.)

Fig. 9.3. Skull of early Oligocene Leptictis. Key: Ang. Pr., angular process; AS, alisphenoid; Cond. Pr., condyloid process; Cor. Pr., coronoid process; DEN, dentary; DEN (Asc. Ram.), ascending ramus; DEN (Hor. Ram.), horizontal ramus; ECT, ectotympanic; ENT, entotympanic; FR, frontal; Infra. Ca., infraorbital canal; JU, jugal; LA, lacrimal; MX, maxilla; NA, nasal; OS, orbitosphenoid; PA, parietal; PL, palatine; PMX, premaxilla; PR, petromastoid; PT, pterygoid; SQ, squamosal. (From Novacek, 1986a.)

Leptictida

Fig. 9.4. Leptictidium from the middle Eocene of Messel: (A) skull; (B) right hind foot. Two hypotheses of locomotion in Leptictidium: (C) bipedal saltation; (D) bipedal running. (A, B from Storch and Lister, 1985; C from Frey et al., 1993; D from Maier et al., 1986.)

Fig. 9.4. Leptictidium from the middle Eocene of Messel: (A) skull; (B) right hind foot. Two hypotheses of locomotion in Leptictidium: (C) bipedal saltation; (D) bipedal running. (A, B from Storch and Lister, 1985; C from Frey et al., 1993; D from Maier et al., 1986.)

Messel skeletons, reveal (as the teeth suggest) that Leptictidium ate insects and small vertebrates.

The forelimbs of Leptictidium are extremely short and the hind limbs elongate. The tibia and fibula are separate, although apparently closely appressed and perhaps joined by strong ligaments for much of their length (Frey et al., 1993). The tail was twice as long as the body. These unusual body proportions indicate that Leptictidium must have been functionally bipedal, but its mode of progression is controversial. According to one hypothesis (Maier et al., 1986), it was a bipedal cursor but did not jump or hop. However, Frey et al. (1993) suggested that Leptictidium was more likely saltatorial, like extant mammals with similar limb proportions. The latter interpretation was supported by a recent bio-

mechanical analysis, which cited the flexible lumbar region and long, rodlike ilia of Leptictidium as particular characteristics of bipedal hoppers (Christian, 1999).

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