This small but highly distinctive group is known from the Paleocene and Eocene of Europe and North America; a single rare genus, Sinclairella, survived into the Oligocene in North America. The half-dozen known genera are all included in the family Apatemyidae. Based on dental morphology, their relationships have been variously considered to lie with "proteutherians," Plesiadapiformes, or Cimolesta, but none of these proposals has been convincingly demonstrated. Apatemyids were small to medium-sized animals, mostly mouse- to squirrel-sized, the largest species reaching the size of a beaver (Castor). The full range of sizes is known among species of the European genus Heterohyus (Koenigs-wald, 1990).
Apatemyids are known primarily from jaws and teeth (Fig. 7.11), which are characterized by an enlarged front tooth in each quadrant, a reduced number of mostly small teeth between this front tooth and the first molar, and rather low-crowned molars (West, 1973a,b; Gingerich and Rose, 1982). The large front tooth is usually considered to be the first incisor; I1 is procumbent, whereas I1 is more nearly vertical. The dental formula is interpreted as 126.96.36.199/188.8.131.52 in most Paleocene members (Jepsenella and Labidolemur).
Torrejonian-Tiffanian Unuchinia, however, is more primitive than other apatemyids in retaining two lower incisors (Gun-nell, 1988). Later apatemyids (Apatemys, Heterohyus, and Sinclairella) have only two lower premolars. The lower pre-molars of apatemyids contrast sharply with those of other early eutherians. The anterior lower premolar (P2) enlarges, becoming an elongate, bladelike tooth that overhangs the back of the large incisor. The middle lower premolar (P3) of early apatemyids was a small, one-rooted tooth and is therefore presumed to be the one that was lost in later forms. These later apatemyids retain a large, bladelike P2, but P4 is reduced to a one-rooted vestige. Unlike in many cimolestans, the lower molar trigonids are only a little higher than the talonids. The unique addition of a fourth, anterobuccal cusp makes the trigonid trapezoidal instead of triangular, as in other placentals. The talonids are broad, basined, and rounded in back, but usually narrower than the trigonids except on M1. In some of the more derived apatemyids M3 is elongate, as in some plesiadapiforms. The upper molars are relatively narrow transversely and have three main cusps (conules very small or absent), together with a small hypocone and a moderate stylar shelf with a pronounced parastyle.
Details of cranial anatomy of apatemyids are best seen in the unique skull of Sinclairella (Scott and Jepsen, 1936; Fig. 7.12), unfortunately now lost. Like many insectivores, it apparently lacked an ossified auditory bulla. The upper incisors of Sinclairella were enormously enlarged and canini-form; together with the relatively short face and narrow rostrum they give the skull a superficial resemblance to those of rodents, the phalangeroid marsupial Dactylopsila (striped possum), and the lemur Daubentonia (aye-aye), all of which also have enlarged incisors used in food procurement.
Several complete articulated skeletons of the European genus Heterohyus are known from the middle Eocene Messel site in Germany (Koenigswald and Schierning, 1987; Koenigswald, 1990; Kalthoff et al., 2004; Fig. 7.13, Plate 3.1). In addition, a partial skeleton of Labidolemur is known from the Clarkforkian of Wyoming (Bloch and Boyer, 2001), and a complete, articulated skeleton of Apatemys (Plate 3.2) was recently described from the late early Eocene Green River Formation of Wyoming (Koenigswald et al., 2005). These skeletons provide an unusually detailed view of apatemyid anatomy and insight into their paleobiology. They have numerous arboreal specializations, such as flexible ankles; terminal phalanges that are short, deep, and laterally compressed; and a long, bushy tail (an outline of which is preserved in one of the Messel specimens). Most remarkable, however, is the modification of the second and third digits of the hand into elongate probes, comparable to the third and fourth digits of Daubentonia and the fourth digit of Dactylopsila (Fig. 7.14). In analogy with these two extant mammals, it is inferred that apatemyids used their enlarged incisors to gouge into bark and rotten wood in search of wood-boring grubs and other insects, which they retrieved with their long, slender fingers. They may even have used these fingers, like Daubentonia, to tap on the wood to deter-
mine where to excavate. Like species of Dactylopsila today (Flannery, 1994), the apatemyid genera all share this specialized adaptation but differ in proportions of the fingers. Thus apatemyids exhibit remarkable convergence toward these two unrelated extant mammals and were the first of the three to adopt one of the most bizarre foraging behaviors known in mammals.
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