new primitive paromomyid from the clarkforkian of ...

Journal of Vertebrate Paleontology 22(2):366–379, June 2002 q 2002 by the Society of Vertebrate Paleontology

NEW PRIMITIVE PAROMOMYID FROM THE CLARKFORKIAN OF WYOMING AND DENTAL ERUPTION IN PLESIADAPIFORMES JONATHAN I. BLOCH, DOUG M. BOYER, PHILIP D. GINGERICH, and GREGG F. GUNNELL Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, Michigan 48109-1079, [email protected]

ABSTRACT—Clarkforkian late Paleocene freshwater limestone from the Clarks Fork Basin, Wyoming, has yielded four specimens of the plesiadapiform paromomyid, Acidomomys hebeticus gen. et sp. nov. A. hebeticus has a strong metaconid on p4, a small double-rooted p3, and a very diminutive i2. Presence of p3 and i2 is primitive, and their retention in A. hebeticus suggests that this taxon is part of a previously unknown lineage that diverged from other paromomyids by the early or middle Paleocene. Acidomomys hebeticus is represented by virtually complete upper and lower dentitions, all from less than fully mature individuals, with teeth at one of three stages of development. The dental eruption sequence differs from that of other plesiadapiforms in having p3 erupt before p4 (p3 erupts after p4 in microsyopids, and p3 and p4 erupt simultaneously in plesiadapids). A. hebeticus differs from Plesiadapis and primitive primates, in which the premolars erupt after m3 is in place, in having p3/P3 erupt before or at the same time as that of m3/M3. It is similar to Plesiadapis in having p4 erupt after the molars are in place, a condition considered primitive in primates. While these differences in dental eruption sequence between representatives of Paromomyidae, Microsyopidae, and Plesiadapidae could be phylogenetically significant, they may instead reflect adaptive differences in dental morphology, facial architecture, or life history.

INTRODUCTION Paromomyidae are plesiadapiformes known from the early Paleocene to middle Eocene of North America (e.g., Simpson, 1955; Bown and Rose, 1976; Rose and Bown, 1982; Robinson and Ivy, 1994), the early to middle Eocene of Europe (Russell et al., 1967; Godinot, 1984), and the early Eocene or late Paleocene of Asia (Tong and Wang, 1998; Beard and Dawson, 1999). They are characterized by: a long, slender, and procumbent i1; a specialized p4 with a tall protoconid, no paraconid, usually no metaconid, and a basined talonid; low-crowned lower molars with blunt cusps and shallow basins (m3 large, with an expanded third lobe); and upper molars with a well-developed postprotocingulum (Simpson, 1940; Bown and Rose, 1976; Gunnell, 1989). While paromomyids were once thought to be allied with primates (e.g., Szalay, 1972), more recent studies of cranial morphology (Kay et al., 1990, 1992) demonstrated that Ignacius graybullianus (and by inference all paromomyids) did not have the petrosal bulla characteristic of euprimates (an observation corroborated by a new specimen; Bloch and Silcox, 2001). Instead, it has been suggested that paromomyids were gliders and most closely related to extant Dermoptera (Kay et al., 1990, 1992; Beard, 1990, 1993a, b; although see Bloch and Silcox, 2001). Affinities of Paromomyidae are still the focus of debate, and aspects of paromomyid functional morphology are contested based on different interpretations of the same fossil material (Krause, 1991; Szalay and Lucas, 1993; Van Valen, 1994; Runestad and Ruff, 1995; Stafford and Thorington, 1998; Hamrick et al., 1999). Freshwater limestones from the late Paleocene and early Eocene of the Clarks Fork Basin, Wyoming, have been the source for a diversity of new mammals previously undocumented from traditionally collected floodplain mudstones and channel sandstones (Gingerich, 1987; Rose and Gingerich, 1987; Beard and Houde, 1989; Thewissen and Gingerich, 1989; Bloch et al., 1998; Bloch and Gingerich, 1998). The limestones were deposited in low-energy ponded water at the surface of hydromorphic paleosols (Bowen and Bloch, 2002), and their fossil

assemblages are likely derived from unique, rarely sampled, floodplain microenvironments (Bloch and Bowen, 2001). We recently discovered new paromomyid specimens in a freshwater limestone of Clarkforkian age that represent a new genus and species. The specimens include virtually complete dentitions, a fragmentary cranium, and associated postcrania (Bloch and Boyer, 2001). More detailed descriptions and discussion of postcranial anatomy will be presented elsewhere when preparation is finished (although see Boyer et al., 2001). We follow Silcox and Gunnell (in press) in recognizing five genera of North American Paromomyidae (contra Robinson and Ivy, 1994; McKenna and Bell, 1997): Paromomys Gidley, 1923, represented by two species from the Torrejonian; Ignacius Matthew and Granger, 1921, represented by four species from the Torrejonian through Uintan; Phenacolemur Matthew, 1915, represented by seven species from the Tiffanian through Uintan; Acidomomys gen. nov., represented by one species from the Clarkforkian of Wyoming, and Elwynella Rose and Bown, 1982, represented by one species from the Bridgerian of Wyoming. A single genus, Arcius Godinot, 1984, is represented in Europe by three species from the early Eocene. At least two undescribed paromomyids are known from the Eocene Canadian Arctic fauna of Ellesmere Island (West and Dawson, 1977; McKenna, 1980) and one from the early Eocene or late Paleocene Wutu fauna of China (Tong and Wang, 1998). Dental terminology follows that of Szalay (1969) in general, and Gingerich (1976) for cusp and crest nomenclature used to describe the incisors. Dimensions of the teeth and mandible were measured using calipers and a binocular microscope. Institutional Abbreviations AMNH, American Museum of Natural History, New York; LACM-CIT, Los Angeles County Museum (California Institute of Technology collection); UM, University of Michigan Museum of Paleontology; USNM, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D. C.; YPM-PU, Princeton University collection at Yale Peabody Museum, New Haven.

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BLOCH ET AL.—NEW PALEOCENE PAROMOMYID TABLE 1. Summary of maxillary dental measurements of Acidomomys hebeticus, from middle Clarkforkian University of Michigan locality SC-62. Total sample includes 2 specimens. Abbreviations: N, sample size; x¯, mean; s, standard deviation; V, coefficient of variation; L, crown length; W, crown width; and MD, mandibular depth. Measurements in mm. Tooth position

N

Deciduous dentition P3L 1 P3W 1 P4L 2 P4W 2 Adult dentition P3L 1 P3W 1 M1L 1 M1W 1 M2L 1 M2W 1

Range

x¯

s

V

1.57 1.13 2.43–2.53 2.47–2.57

— — 2.48 2.52

— — 0.07 0.07

— — 2.85 2.81

1.53 1.03 2.40 3.10 2.27 2.13

— — — — — —

— — — — — —

— — — — — —

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TABLE 2. Summary of mandibular dental measurements of Acidomomys hebeticus, from Cf-2 University of Michigan locality SC-62. Total sample includes 4 specimens. Abbreviations as in Table 1. Measurements in mm. Range

x¯

s

V

Deciduous dentition p3L 1 p3B 1 p4L 2 p4W 2

Tooth position

N

1.03 0.73 1.97–2.28 1.37–1.62

— — 2.13 1.50

— — 0.22 0.18

— — 10.32 12.00

Adult dentition p3L p3W p4L p4W m1L m1W m2L m2W m3L m3W MD (juvenile)

1.13–1.17 0.63–0.77 2.17 1.60 2.20–2.35 1.87–2.20 2.33–2.53 2.20–2.32 3.60 2.07 5.60–6.33

1.15 0.70 — — 2.28 2.04 2.50 2.27 — — 5.97

0.02 0.10 — — 0.11 0.23 0.12 0.06 — — 0.37

1.74 14.29 — — 4.82 11.28 4.80 2.64 — — 6.20

2 2 1 1 2 2 4 3 1 1 3

SYSTEMATIC PALEONTOLOGY Order PROPRIMATES Gingerich, 1989 Infraorder PLESIADAPIFORMES Simons, 1972 Superfamily PLESIADAPOIDEA Trouessart, 1897 Family PAROMOMYIDAE Simpson, 1940 ACIDOMOMYS, gen. nov. Type Species Acidomomys hebeticus, sp. nov., only known species of genus. Diagnosis Differs from all other known paromomyids in having a metaconid on p4. Differs from all but the earliest paromomyids (Paromomys and Ignacius fremontensis) in having a small, double-rooted p3. Differs from all other known paromomyids, except Paromomys, in retaining a very diminutive i2. Differs from Paromomys in lacking c1. Etymology Akis, akidos, Greek, point, pointed; omomys, Greek (masculine), ‘shoulder-mouse’, parallels use in Paromomys. Named in allusion to the acutely pointed incisors, and double-pointed premolars characteristic of the genus. ACIDOMOMYS HEBETICUS, sp. nov. (Figs. 1–5, Tables 1, 2) Holotype UM 108206: left dentary with p4–m3, right dentary with p3–m2, isolated left i1 (broken), isolated right I1 (Figs. 1–2). Juvenile with p4 partially erupted in both dentaries. Hypodigm UM 108207: left dentary with di1, i1, di2 (associated), p3, dp4, m1, and m3 (in the crypt); right dentary with di1 (associated), i1, di2, i2 (in the crypt), dp4, p4 (partial crown in the crypt), m1, m2, m3 (in the crypt); left rostrum with DI1, I1 (erupting), DI2, DC1, C1 (erupting), P3, DP4, M1, M2, M3 (fragment); and right rostrum with I1 (associated), C1 (erupting), P3, DP4, M1. UM 108208: right dentary with roots of di1–di2 and crowns of i1 (erupting), dp3, dp4, m2 (in the crypt), m3 (in the crypt); and right rostrum with right DP3 and DP4. UM 108209: isolated left m2. All specimens are from UM locality SC-62, Willwood Formation, Clark Quadrangle, northern Bighorn Basin, Wyoming (detailed locality information is archived at the University of Michigan Museum of Paleontology). Age and Distribution Acidomomys hebeticus is known only from Clarkforkian faunal zone Cf-2 (late Paleocene, ca. 55.3 Ma; Gingerich, 2000) in northwestern Wyoming. Diagnosis As for genus. Etymology hebetikos, Greek, youthful. Named in recognition of the immature stage of ontogenetic development of all known specimens.

DESCRIPTION Four specimens, including several complete, or nearly complete, lower dentitions and a nearly complete upper dentition, are known for the new species. Measurements of these specimens are summarized in Tables 1 and 2. Deciduous Dentition The crown of DI1 is present in UM 108207 (Figs. 2G, H, 3). It is short and straight and not particularly hook-shaped. There is a strong anterocone, a relatively strong and apical mediocone, a basal lateroconule, and no posterocone or laterocone. The crown of DI2 is present in UM 108207 (Figs. 2I, J, 3). It is simpler, and slightly smaller, than DI1, with a somewhat caniniform crown. It has a strong anterocone, a tiny mediocone that is less apically situated than that of DI1, and a small bulge of enamel at the position of a basal lateroconule. The crown of DC1 is present in UM 108208 (Fig. 5C). It is situated in the front of the maxilla and is strongly anteriorly procumbent. It is a simple, single-rooted, peg-like tooth with a single blunt, pointed, cusp. Its crown is smaller than that of DI1 and DI2. The DP3 is present in UM 108208 (Fig. 5C). The buccal edge of DP3 is made up of a strong anterior cusp and a smaller, but very distinct, posterior cusp. The DP3 has no lingual cusp, but does have a lingual extension with a concave basin sloping down from the buccal cusps to the lingual edge of the tooth. The DP3 has three distinct roots. The DP4 is present in UM 108207–8 (Figs. 4C, 5C). The DP4 is fully molariform and differs from M1 and M2 in being longer than wide. The paracone and metacone are both sharp and high and separated by a V-shaped centrocrista notch. The buccal side of the tooth is bordered by a low cingulum that is not completely continuous across a shallow ectoflexus. A small parastyle is present on the anterior extreme of the external cingulum and is joined to the paracone by a low crest. The small trigon basin is bordered by very distinct, strongly oblique preprotocone and premetaconule cristae. The posterolingual basin is greatly expanded and is about twice the area of the trigon basin. A weak paraconule is present midway between the paracone and protocone. The DP4 has three distinct roots. The crown of di1 is preserved in the left mandible of UM 108207 (Figs. 2E, F, 3, 4B) and the root in UM 108208 (Fig. 5A). It is very long, slender, procumbent, and comes to a sharp

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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 2, 2002

FIGURE 1. Acidomomys hebeticus, gen. et sp. nov., UM 108206 (holotype). A–C, left dentary in buccal, lingual, and occlusal views. D–F, right dentary in lingual, buccal, and occlusal views. Occlusal views of both dentaries are stereophotographs.

point at its apex. No grooves are apparent on this tooth, although a strong internal ridge of enamel, running from the base of the crown to its apex, is present. The crown of di2 is present in UM 108207 (Fig. 4A, D). Its alveolus is preserved in UM 108208 (Fig. 5A). The crown of di2, is small, slightly procumbent, single-rooted, and peglike. This tooth, whose crown height is about two-thirds that of p3, is rooted in a small alveolus just posterior to that of i1. The crown of dp3 is preserved in UM 108208 (Fig. 5A, B). It is diminutive and single-rooted. The crown of dp3 has a distinct anterior cusp followed by a small talonid heel. The prominent anterior cusp has a strong posteroapical wear facet.

The crown of dp4 is preserved in the left and right dentaries of UM 108207 and in UM 108208 (Figs. 3, 4A, B, 5A, B). It is molariform and almost equal in length to m1, with the talonid almost equal in width and the trigonid significantly narrower than that of m1. The talonid basin is broad with a strong hypoconid and entoconid of nearly equal size, and no hypoconulid. The cristid obliqua intersects the posterior wall of the trigonid at about the midline of the tooth, forming a fairly deep hypoflexid. The cristid obliqua only extends up about one-third of the height of the trigonid. The paraconid is positioned just anterior to the metaconid, is lower than the other trigonid cusps, and is connected to the protoconid by a deeply notched paracristid that extends anteriorly, effectively rendering the para-

BLOCH ET AL.—NEW PALEOCENE PAROMOMYID

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FIGURE 2. Incisors of Acidomomys hebeticus, gen. et sp. nov. UM 108206 (holotype): Left i1 (broken) in lateral, A, and occlusal, B, views; right isolated I1 in occlusal, C, and lateral, D, views. UM 108207: left di1 in lateral, E, and occlusal, F, views; right dI1 in occlusal, G, and lateral, H, views; right dI2 in occlusal, I, and lateral, J, views. Note the different scales for adult and deciduous incisors.

conid an isolated anterolingual cusp. The dp4 has no trace of a precingulid. Adult Dentition The crown of I1 is present in UM 108206–7 (Figs. 2C, D, 3). The crown is ‘‘mitten-shaped’’ with three distinct cusps at its apex. The central cusp (anterocone) is the most prominent cusp. A strong laterocone and a fairly large and distally projecting mediocone make up the other two cusps. The ‘‘thumb’’ of the mitten-shaped crown is a small posterocone that appears to be doubled in its unworn state. A short but very distinct mediocrista curves proximolaterally from the mediocone. The crown of I2 is not preserved in any of the specimens. The crown of C1 is present in UM 108207 (Figs. 3, 4A). This single-rooted tooth is similar in morphology to DC1, but

about twice the diameter. The crown of C1 is about twice the size of P2. The crown of P2 is preserved in UM 108207 (Fig. 4A). It is peg-like, smaller than P3 (about half the length), and singlerooted. The crown of P3 is present in UM 108207 (Fig. 4A, C). This double-rooted tooth is nearly identical in morphology to DP3, but about one-third smaller in size. The crown of M1 is preserved in both the right and left maxillae of UM 108207 (Fig. 4A, C). The paracone is somewhat larger than the metacone. A small paraconule is present with a preparaconular crista that wraps around the external face of the paracone to form an external cingulum that terminates on the anterior face of the metacone. A short postparaconular crista extends up much of the lingual face of the paracone. A small trigon basin is bordered by very distinct, strongly oblique

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FIGURE 3.

Acidomomys hebeticus, gen. et sp. nov., UM 108207: Skull and left dentary in lateral view.

preprotocone and premetaconule cristae. The posterolingual basin is expanded and is significantly larger than the trigon basin. No distinct metaconule or hypocone is present. The crown of M2 is present in UM 108207 (Fig. 4C). It is similar in most respects to the M1. It differs in being relatively shorter and in having a larger paracone relative to the metacone than those of the M1. The crown of the M3 is partly preserved in UM 108207 (Fig. 4C). A fragment of the lingual side of the tooth is preserved, demonstrating that it had a low-cusped protocone. The crown of i1 is preserved in UM 108206–8 (Figs. 2A, B, 3, 4A, B, D, 5A). Overall, this tooth appears to be very similar in morphology and size to di1 although it is slightly broader towards its apex. The most notable feature is a distinct anterolingual ridge of enamel that runs down the length of this slender, strongly procumbent tooth. The crown of i2 is partially formed in UM 108207 (Fig. 4D). This small, single-rooted, and peg-like tooth appears to have been identical in all observable morphology to di2. The crown of p3 is preserved in UM 108206–7 (Figs. 1D– F, 3, 4A, B). It is a diminutive, double-rooted tooth. The crown has a distinct anterior cusp followed by a small talonid heel. The roots are anteroposteriorly splayed and planted in two distinct alveoli just anterior to p4. The crown of p4 is present in both dentaries of UM 108206 (Fig. 1). The crown of p4 is about equal in height to that of m1. The talonid of p4, while as broad as the trigonid, is very anteroposteriorly compressed with a notably short basin. The talonid has a small, yet distinct, entoconid and a less distinct

hypoconid. No hypoconulid is present. The protoconid makes up most of the anterior part of the tooth, with no distinct hypoflexid present. A strong metaconid is present fairly low on the lingual edge of the posterior trigonid wall. There is no paraconid. The crown of m1 is preserved in both dentaries of UM 108206 and in UM 108207 (Figs. 1, 3, 4A, B). It is low crowned, with the trigonid notably low relative to the talonid. The postvallid is strongly inclined anteriorly. The talonid is broad, with a shallow talonid basin bordered by a sloping entoconid and hypoconid, and no hypoconulid. The trigonid consists of a very shallow, rectangular basin with small, but distinct, metaconid and paraconid. The metaconid is higher than the protoconid, with no distinct protoconid–metaconid notch present. The paracristid extends anteriorly from a weak protoconid and curves around to enclose the small, rectangular, basin. A distinct, angular hypoflexid has a cristid obliqua terminating one-half the distance up the posterior trigonid wall under the protoconid. The crown of m2 is preserved in UM 108207–8 (Figs. 1, 4A, B, 5B). It is similar in morphology to that of m1, but with a slightly wider trigonid relative to the talonid and a weaker paraconid. In crown size, m2 is slightly larger than that of m1. The crown of m3 is preserved in UM 108206–8 (Figs. 1A– C, 4B). The m3 is a large tooth with a greatly expanded third lobe covered by fairly rough, crenulated enamel. The trigonid of m3 is slightly more anteroposteriorly compressed than that of m1 and m2. It lacks a distinct paraconid. Otherwise, the morphology of the trigonid is very similar to that of m2.


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FIGURE 4. Acidomomys hebeticus, gen. et sp. nov., UM 108207: A, maxilla and right dentary in lateral view, B, left dentary in occlusal view; C, left maxilla in occlusal view, D, anterior close-up of right dentary in lingual view. Note the i2 erupting under the di2 in Fig. 4D.

COMPARISONS

Dentary Dental Formula Aspects of the morphology of the dentary are preserved in UM 108206–8 (Figs. 1, 3, 4A, B, 5A, B). A moderate diastema is present between i2 and p3. The depth of the jaw is greatest beneath the anterior root of m1. Two mental foramina are present below the anterior roots of p4 and m1 respectively. The symphysis is unfused. The coronoid process is high and broad. The angular process is recurved and tapers posteriorly in a hooklike process. The condyle is situated noticeably higher than the tooth row and the articular surface covers its entire dorsal aspect. The transverse diameter of the condyle is approximately 2.6 mm.

The lower dental formula of Acidomomys differs from that of Paromomys in lacking a p2 and c1 and Phenacolemur in retaining p3. All species of Ignacius have lost p3 except Torrejonian Ignacius fremontensis Gazin, 1971, which, like Acidomomys, had a small double-rooted p3. Bown and Rose (1976) noted that a specimen of Ignacius frugivorus (PU 14790) from Cedar Point Quarry retained a small, double-rooted p3. However, PU 14790 is actually a specimen of Ignacius fremontensis from Rock Bench Quarry that was ‘‘. . . incorrectly identified as Ignacius frugivorus, and its locality erroneously given as

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FIGURE 5. Acidomomys hebeticus, gen. et sp. nov., UM 108208: A, right dentary in anterobuccal view, and B, occlusal view; C, right maxilla in occlusal view. Note that i1 is replacing di1 and that dp3 is single-rooted in Fig. 5A.

Cedar Point Quarry by Bown and Rose, 1976:113.’’ (Rose, 1981:147). Thus, no specimens of I. frugivorus are known to have a p3. Bridgerian Elwynella also retained a p3, but in this taxon the tooth is single-rooted. Acidomomys is also primitive in retaining a diminutive i2, a condition shared only with Torrejonian species of Paromomys, and unlike all other paromomyids, which lack this tooth. The upper dental formula of Acidomomys is the same as that of all other paromomyids for which it is known, except early Eocene Ignacius graybullianus Bown and Rose, 1976 and Arcius rougieri Godinot, 1984 which have lost P2. The maxilla of Acidomomys hebeticus is similar to that of Phenacolemur pagei Jepsen, 1930 and Ignacius frugivorus Matthew and Granger, 1921 in having no diastema between P2 and P3, and differs from Phenacolemur jepseni Simpson, 1955, which has a short diastema between P2 and P3. The mandible of Acidomomys hebeticus is similar to that of later occurring Ignacius and Phenacolemur in having a long diastema between the posterior incisor and anterior premolar, and unlike Paromomys (Simpson, 1955; Rigby, 1980), Ignacius fremontensis Gazin, 1971 (Szalay and Delson, 1979), and Elwynella (Rose and Bown, 1982), in which this diastema is either not present or very short.

Deciduous Dental Morphology The DI1 of Acidomomys is the first to be described as such for the Paromomyidae. Surprisingly, it is nearly identical to that described as ‘‘Unidentified left incisor. . . possibly Palaechthon alticuspis’’ (Rose et al., 1993:fig. 3B) from the Torrejonian Gidley Quarry (also identified by Gidley, 1923 in plate 3, figs. 8– 9 as ‘‘cf. Pronothodectes species’’) in having a short and not particularly hook-shaped crown, a strong anterocone, a relatively strong and apical mediocone, a basal lateroconule, and no posterocone or laterocone. Rose et al. (1993) reasoned that because this tooth did not look like other known paromomyid I1s (Paromomys depressidens is also known from Gidley Quarry), that it must either belong to Palaechthon alticuspis or to a non-plesiadapiform. They concluded that if the assignment were correct, similarity to the I1 of Tinimomys might be an indication that micromomyids and palaechthonids were closely related. Based on new knowledge of the morphology of the DI1 for Acidomomys, we propose that these incisors (USNM 10010 and 10090) are more likely to be the DI1 of Paromomys depressidens. However, if these incisors are the adult I1 of either Palaechthon alticuspis or Paromomys depressidens, it is of some significance that they are identical to that of the deciduous I1 of Acidomomys. It follows that the morphology of the Aci-

BLOCH ET AL.—NEW PALEOCENE PAROMOMYID domomys DI1 might represent a primitive form similar to that from which all later plesiadapiform adult I1s evolved. While this hypothesis is of interest, it remains to be tested as the I1 of Purgatorius has never been described. The DI2 of Acidomomys differs in morphology from the adult I2 of Ignacius graybullianus and Arcius rougieri, which are high and caniniform (Rose and Gingerich, 1976; Godinot, 1984), in having a strong anterocone, a tiny mediocone that is less apically situated than that of DI1, and a small bulge of enamel at the position of a basal lateroconule (although Arcius does seem to have a small basal lateroconule). Absence of any known specimens of DI2 or DC1 for other plesiadapiform taxa precludes any statement of direct comparison for these teeth. The adult C1 of Acidomomys is of similar size and morphology as that described for I. graybullianus (Rose and Gingerich, 1976). The crown of DC1, as evidenced by small size and extreme procumbency, was most likely not a functional tooth. The crown of DP3 is similar in morphology to that of P3, although it is somewhat larger. It does not differ significantly from the DP3 of Nannodectes gidleyi (Simpson, 1935). The DP4 is very similar to those described for Phenacolemur (Rose, 1981) in being molariform and relatively longer and less transverse than M1 and M2. It differs subtly from those of Phenacolemur, however, in having a more pronounced stylar shelf, a stronger parastyle, and a shorter posterointernal basin. The DP4 of Plesiadapis (Rose, 1981) and Nannodectes (Simpson, 1935) appears to differ from those of other known paromomyids in having a small mesostyle, small para- and metaconules, and a very reduced posterointernal basin. The DP4 of microsyopid Craseops (Gunnell, 1989) also has a mesostyle, but no conules and no posterointernal basin. Presence of a large and seemingly functional di1 contrasts with the monophyodont condition in other mammals with large and procumbent central incisors. For example, phalangeroid marsupials, which have striking parallel dental adaptations to carpolestid and paromomyid plesiadapiformes (Simpson, 1933; Gingerich, 1974), have a functional i1 with di1 only forming as a tooth germ that never erupts (Wilson and Hill, 1897; Luckett, 1993). This modified anterior dentition is thought to be related to the prolonged lactation period of marsupials and the developmental constraints related to a period of nipple fixation (Leche, 1895; Wilson and Hill, 1897; Luckett, 1993), and is probably primitive for Marsupialia (Luckett, 1977, 1993; Tyndale-Biscoe and Renfree, 1987; Cifelli et al., 1996). The incisor morphology of rodents is also highly convergent on that of some plesiadapiformes, leading some investigators to suggest a sister-group relationship (Wood, 1962, 1977; McKenna, 1969; Van Valen, 1971), a notion that is not generally accepted because the enlarged incisors of the two groups are not thought to be homologous (Simons, 1972; Gingerich, 1976; Szalay and Delson, 1979; Butler, 1980; Luckett, 1985). Rodents and lagomorphs also have a single generation of incisors, but in these groups the development of an adult generation is suppressed with a deciduous tooth (di2) maintained throughout life as the enlarged and ever-growing incisor (Luckett, 1985). Among extant primates, the aye-aye (Daubentonia madagascariensis) is unique in having a very large, procumbent, and ever-growing il. Daubentonia is similar to Acidomomys in having a small dil present in its juvenile dentition (Ankel-Simons, 1996). However, the small size of di1 in Daubentonia clearly contrasts with that of Acidomomys which has a di1 nearly as large as i1. Multituberculates are also very similar to some plesiadapiformes in having an enlarged and procumbent incisor as well as parallel modifications of the premolars (Simpson, 1933). Interestingly, the diphyodont condition of the i1 of Multituberculata most closely approximates the condition in Acidomomys

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among mammals. Juvenile multituberculates are known to have a fairly large dil being replaced by a larger i1 early in ontogeny (e.g., Szalay, 1965; Greenwald, 1988). It is likely that the di1/ i1 of Acidomomys functioned in a similar way to those of ptilodontid multituberculates, which may have used their anterior incisors for grasping, holding, and piercing functions (Clemens, 1963) or some other, possibly non-ingestive, related activity (Krause, 1982). The diminutive, single-rooted, peg-like crown of di2 is nearly identical in size and morphology to that of i2. It also similar in size to the i2 of Paromomys maturus (Rigby, 1980). The crown of dp3 is similar in size and morphology to that of p3, but it is single-rooted and has a strong posteroapical wear facet. Presence of a wear facet indicates that this tooth was functional early in ontogeny of Acidomomys. The single-rooted condition of the dp3 of Acidomomys is similar to the p3 of Bridgerian Elwynella, which also has a single root (Rose and Bown, 1982). The morphology of the crown of the dp3/p3 of Elwynella is unknown. The crown of dp4 is generally similar to those described for other plesiadapids. In the dp4 of Plesiadapis rex (UM 81127, 81178) the paracristid is much less deeply notched, the paraconid is more centrally positioned on the front of the trigonid, a small precingulid is present, the hypoconid is much larger than the entoconid, and the cristid obliqua runs up the apex of the metaconid. Adult Dental Morphology The morphology of the I1 of Acidomomys hebeticus is most similar, among known paromomyids, to that of Phenacolemur simonsi Bown and Rose, 1976, in having a mediocone nearly as big and as distal as the anterocone (Rose et al., 1993). The I1 of Acidomomys differs from other paromomyids in having stronger mediocrista (terminating in a small cuspule in an unworn state), laterocone, and posterocone. In this way, the I1 of Acidomomys is very similar to that described for Saxonella crepaturae (Russell, 1964; Rose et al., 1993), possibly supporting the idea that the Saxonellidae and Paromomyidae are closely related (Gingerich, 1976; Rose et al., 1993), a hypothesis thought not to be supported in other features of the dentition (Rose, 1975; Fox, 1991). It differs from Saxonella in having a double posterocone, a feature thought to be characteristic of the Carpolestidae (Rose et al., 1993; Bloch and Gingerich, 1998). The P2 of Acidomomys hebeticus differs from that of all other paromomyids for which it is known in being single-rooted. It is similar to the P2 of Phenacolemur and Ignacius (when known or present) in having a tiny posterior cuspule (Simpson, 1955). The P2 of Acidomomys is most similar in size, among known paromomyids, to that of Ignacius frugivorus, and notably smaller than that of Phenacolemur. The small size of P3 relative to the M1 of Acidomomys is most like that of Paromomys maturus. In morphology, the P3 of Acidomomys is closest to that of Phenacolemur jepseni, among known paromomyid taxa, in having a small metacone and a weak posterolingual basin. The posterointernal basins of M1 and M2 are more expanded in Acidomomys, Ignacius, and Phenacolemur than those of Paromomys. The M1 and M2 of Acidomomys are similar to those of Ignacius in having a paracone only somewhat larger than the metacone, versus the much more reduced condition of the metacone in Phenacolemur. The M1 and M2 are different from those of Ignacius, however, in having a slightly stronger cingulum along the lingual border of the stylar shelf, and in having less inflated crowns (especially on the lingual face of the tooth). As for all paromomyids for which it is known, the i1 of A. hebeticus is long and slender for its entire length. It differs from that of Phenacolemur in not broadening at the base and from

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Elwynella in being more buccolingually compressed for its entire length. The il is most similar to that of Ignacius, although it appears to be even more slender and elongate than any known species of Ignacius. The il of A. hebeticus is similar to that of some Phenacolemur and Ignacius species in having a strong internal cingulum that runs from the base of the crown to its apex. The lower dentition of A. hebeticus differs from all other paromomyids, except Paromomys, in retaining an i2. The i2 of A. hebeticus is similar to that of Paromomys maturus Gidley, 1923 in being small, single-rooted, and positioned very close to i1 on the buccal rim of the mandible (Rigby, 1980). The p3 is very similar to that of Ignacius fremontensis in being small and double-rooted with a crown that has a distinct anterior cusp followed by a small talonid heel. As for I. fremontensis, the roots are anteroposteriorly splayed and implanted in two distinct alveoli just anterior to p4. The p3 in Paromomys is similar in morphology to that of Acidomomys, but is somewhat larger relative to p4. The p3 of Elwynella is distinctly different from that of Acidomomys in having a single root, a condition shared with the deciduous p3 of A. hebeticus. The p4 of Acidomomys is similar to that of Ignacius in being small relative to m1 and distinctly different from that of Phenacolemur and Elwynella, which have a larger p4 relative to m1. It is different from all other paromomyids (including the most primitive Paromomys species) in having a strong metaconid. Presence of a metaconid on the p4 of A. hebeticus is similar to the condition in some mixodectids, palaechthonids, microsyopids, and plagiomenids (Gunnell, 1989). This cusp is also present in carpolestids and Chronolestes (Rose, 1975; Beard, 1995; Bloch et al., 2001) as well as some euprimates such as the early Eocene adapid, Cantius. Absence of this cusp in Paromomys suggests that the metaconid on the p4 of A. hebeticus is likely to be neomorphic. The m1 of Acidomomys is similar to that of Ignacius, Phenacolemur, and Elwynella, in having the paraconid and metaconid closely appressed or connate. This differs from the condition in Paromomys and Arcius for which the paraconid is more distinct and anteriorly placed relative to the metaconid, approximating an equilateral triangle (Bown and Rose, 1976). The talonid of m1 is most similar to that of Paromomys maturus in having a more deeply invaginated hypoflexid than Paromomys depressidens, Ignacius, Phenacolemur, Elwynella, and Arcius. As for all paromomyids except Paromomys, the m1 of Acidomomys lacks a well-developed external cingulum. The m2 of Acidomomys is similar to all other paromomyid species in having the paraconid and metaconid closely appressed or connate. It differs from all other species, except Paromomys maturus, in having a cristid obliqua that terminates at the base of the posterior trigonid wall instead of continuing lingually up the face of the trigonid. As for all paromomyid species except Paromomys, the m2 of Acidomomys lacks a well-developed external cingulum. The m3 of Acidomomys appears to be distinct from other paromomyid genera in having the cristid obliqua intersect the posterior wall of the trigonid towards the midline of the tooth. In this way, the m3 of Acidomomys is most similar to that of Paromomys maturus and differs from both Phenacolemur and Ignacius, in which the m3 cristid obliqua intersects the trigonid wall near the buccal side of the tooth. The m3 of Acidomomys is similar to all paromomyid species, except Paromomys depressidens, in having an inflated posterior third lobe. PLESIADAPIFORM DENTAL ERUPTION A large number of deciduous teeth are known for plesiadapids from both North America and Europe (Gingerich, 1976), but few of these teeth have ever been described, and fewer still

illustrated. Very little is known about the order of dental eruption in Paleogene proprimates. The first erupting tooth to be described for a plesiadapiform was a partially erupting incisor of Plesiadapis (Lemoine, 1887). This specimen was never illustrated and subsequently lost (Gingerich, 1976). Schlosser (1921) described some possible deciduous incisors of Plesiadapis tricuspidens that were later illustrated by Russell (1964), although he was less certain of their Plesiadapis affinities. Simpson (1935) figured and described dp3–4/DP3–4 of Nannodectes gidleyi from Mason Pocket. Stock (1938), in his description of a new species, described and illustrated Microsyops kratos. The type specimen is a juvenile with dp3–m3 (LACM-CIT 2232). The dp3 is in place, p4 is partially erupted and all the molars are fully erupted. Szalay (1969) published photographs of LACM-CIT 2232. In his monograph on the cranial anatomy and evolution of Plesiadapidae, Gingerich (1976) mentioned but did not illustrate deciduous incisors of Nannodectes gidleyi from Mason Pocket. He also noted that ‘‘About a dozen mandibles and maxillae of Plesiadapis and Nannodectes in North American collections show deciduous teeth being replaced’’ (Gingerich, 1976:77). Based on eight PU specimens and a UW specimen, Gingerich (1976) put together the following four-stage eruption sequence for plesiadapids: Upper Dentition: (1) dP2; (2) dP3; (3) dP4, M1, M2, P2; (4) M3, P3, P4. Lower Dentition: (2) dp3; (3) dp4, m1, m2, p2; (4) m3, p3, p4. Gingerich (1976) noted that Plesiadapis was similar to Adapis, Tupaia, Microcebus, and Lemur in having P3 and P4 erupt after M1–3 are in place (i.e., the primitive pattern in primates; in contrast to Tarsius, some Lemuriformes, and higher primates, which delay eruption of M3 as a result of shortening the jaws). He further noted that: C1 erupted before P2 (PU 17622: Plesiadapis fodinatus with alveolus for C1, erupting P2, DP3, P3 in crypt, DP4, P4 in crypt, erupted M1). Gingerich (1976) observed that DP2 was very small in Nannodectes (AMNH 17387: Nannodectes gidleyi with DP2, P2 erupting, DP3, DP4, erupted M1). Gingerich (1976) stated that p2 was more important functionally in early stages of tooth eruption than it was once the full permanent dentition was in place in those species of Nannodectes and Plesiadapis that retained p2 (PU 20799: Plesiadapis rex with alveolus for i1, p2, dp3, p3 in crypt, dp4, p4 in crypt, m1, m2). Rose (1981) described dP4s of Phenacolemur pagei and Phenacolemur praecox from the Clarkforkian (UM 71026, 71029, respectively). Rose gives measurements for these teeth (but no illustrations). Rose (1981:60) also described and illustrated (fig. 25) a possible DP4 of Plesiadapis cookei. Gunnell (1989) illustrated a possible dP4 of microsyopid Craseops sylvestris and provided measurements of the type specimen of Microsyops kratos, with dp3 (same specimen as Stock, 1938; Szalay, 1969). New Specimens One of the interesting taphonomic biases of the limestone assemblage yielding Acidomomys is that many of the mammals represent juveniles. The three specimens that preserve dentaries of A. hebeticus are all immature, with replacement of teeth in progress (Fig. 6). The youngest A. hebeticus individual is represented by UM 108206 (Fig. 6A, B). The dp3–dp4 are still in place, and m3 is still in the crypt with the alveolus through which the tooth would have erupted at least partially closed over by ossification (a state that would have presumably changed as the tooth formed in the mandible). The m2 is mostly erupted (m1 is missing from the specimen). The il is just beginning to erupt, with


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FIGURE 6. Composite drawings of the dentition of Acidomomys hebeticus, gen. et sp. nov. at three stages of dental eruption. All reconstructions are in left lateral view with the external surface cut away to expose tooth roots, and unerupted teeth. A–B, Stage 1 is reconstructed from UM 108208 and represents the youngest individual. C–D, Stage 2 is reconstructed from UM 108207, and represents a somewhat older individual. E– F, Stage 3 is reconstructed from UM 108206 and represents the most mature individual. Dashed lines represent material not present in the actual specimen. However, these reconstructed areas are based on other specimens and are believed to be an accurate reflection of the condition at that stage of dental eruption.

the di1 still in place. The di2 was also still in place. A fragment of the upper jaw shows that the DC1 and DP3–4 are still in place. A somewhat older individual is represented by UM 108207 (Fig. 6C, D). It has dp4 in place with p4 just beginning to form in the crypt (as evidenced by an enamel bud visible through a break in one of the jaws). The small double-rooted tooth just anterior to dp4 (the same tooth as in UM 108206) is probably p3, and not dp3, based on the fact that it does not appear to have a replacement tooth in either of the known specimens, and is different from that of UM 108208 in having a double, rather than single, root, and in having no evidence of wear (as opposed to that of UM 108208, which has strong postero-apical wear). The p3 is fully erupted in UM 108207. The di2 is in place, with i2 still forming in the crypt as evidenced by some breakage of the right dentary of UM 108207 in lingual view. The dil is in place in both dentaries of UM 108207, with il about halfway erupted and displacing its deciduous precursor. The m1–2 are both fully erupted, with the roots of m1 fully formed and the roots of m2 only partially formed (implying that m1 erupted before m2). The m3 of this specimen is mostly or completely formed, but is still in the crypt with no part of the tooth above the alveolar rim. The alveolus of m3 is fully open, however, with no bone covering the area through which the m3 would have erupted. In the upper jaw of UM 108207, DI1 is in place

and being replaced by the adult Il which is just beginning to erupt. The DI2 and C1 are in place, with I2 starting to erupt. The P3 and DP4 are both in place, with no evidence of replacement for DP4. The crowns of M1–2 are fully erupted, but their roots are still open and in the process of forming. Based on a fragment of the lingual part of M3 in the maxillary of UM 108207, M3 was at least partially, and possibly fully, erupted. The last specimen relevant to interpreting the dental eruption sequence of A. hebeticus is the type specimen, UM 108206 (Fig. 6E, F). It has the p4 mostly or fully formed and beginning to erupt (the tip of the protoconid is just beginning to emerge from the crypt). The dP4 has been lost in both the right and left dentaries of UM 108206. The p3 and m1–m3 are fully erupted at this stage of p4 development. In summary: UM 108208 represents the youngest individual with i1 just beginning to erupt, dp3–dp4 in place, m2 erupted, and m3 not yet erupted or formed in the crypt; UM 108207 represents a slightly older individual, with i1 more fully erupted and replacing di1, p3 erupted, dP4 in place, m1–m2 erupted, and m3 not yet erupted but fully formed in the crypt; and UM 108206 represents the most mature individual, with p3 erupted, dp4 lost with p4 partially above the alveolar rim, and m1–m3 fully erupted. The following five-stage sequence of dental eruption can be reconstructed for Acidomomys hebeticus:

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Upper Dentition: (1) DI1/DI2/DP3/DP4/M1; (2) M2; (3) P3; (4) I1/M3; (5) I2/P4. Lower Dentition: (1) di1/di2/dp3/dp4/m1; (2) m2; (3) p3; (4) i1/m3; (5) i2/p4. This differs from Plesiadapis and Nannodectes (Gingerich, 1976), which have m1 and m2 erupting simultaneously as well as p3, p4, and m3 erupting at the same time. It also differs from Microsyops (Microsyops elegans: UM 33452), which has p4 and m3 in place before p3 is fully erupted. It is possible that these differences in dental eruption could be extended to family level comparisons, but more juvenile specimens of plesiadapiform taxa need to be studied before any general statements can be made about the possible evolution of dental formulas in proprimates. CONCLUSIONS The new Clarkforkian paromomyid Acidomomys hebeticus differs from all previously described paromomyids in having a unique combination of characteristics of the upper and lower dentition. These include: (1), simple, single-rooted i2; (2), small, double-rooted p3; (3), distinct metaconid on p4; (4), m3 with the cristid obliqua intersecting the posterior trigonid wall towards the midline of the tooth; (5) I1 with a strong mediocrista, laterocone, and posterocone; (6), and distinctly small P3 relative to M1. The lower dental formula alone, 2.0.2.3, likely precludes derivation of A. hebeticus from any known species of Phenacolemur (which have all lost i2 and p3) or Ignacius (which have all lost i2). Torrejonian Paromomys depressidens and Paromomys maturus both have a lower dental formula, 2.1.3.3, from which that of Acidomomys could easily be derived. Among these species, the molar morphology of Acidomomys is most similar to that of Paromomys maturus. Acidomomys seems to be a sole representative of a ghost lineage of paromomyids that extends from the middle Clarkforkian to at least the late Torrejonian of North America (Fig. 7). It is possible that Bridgerian Elwynella, which is unique among Eocene paromomyids in retaining a single-rooted p3, represents a descendant of an Acidomomys-like form in the Clarkforkian– Wasatchian. However, the fact that the fossils described here are the first known for this apparently long-ranging taxon might imply that the diversity of unsampled paromomyid taxa, from which a form like Elwynella could have been derived, is much greater than generally imagined. The dental eruption sequence in Acidomomys differs from Plesiadapis and Microsyops in erupting p3 before p4. In Plesiadapis, p4 erupts at the same time as p3 and in Microsyops p4 erupts before p3. Acidomomys is similar to Plesiadapis in having p4 erupt after the molars are in place, a condition considered primitive in primates (Bennejeant, 1936; Schultz, 1956; Smith, 2000), although it differs from this condition in seemingly having p3 and P3 erupt before, and at the same time as, m3 and M3 respectively. It is possible that these differences in dental eruption sequence between representatives of Paromomyidae, Microsyopidae, and Plesiadapidae are phylogenetically significant (Tattersall and Schwartz, 1974; Schwartz, 1974; Byrd, 1981). However, it is more likely that they reflect adaptive differences in dental morphology, facial architecture, or life history (see Smith, 2000, for a discussion of this topic in primates and ungulates). A more detailed study of deciduous teeth and dental eruption promises to contribute a great deal to our understanding of the biology of the Plesiadapiformes and the earliest primates. ACKNOWLEDGMENTS We thank R. Secord, M. Silcox and an anonymous reviewer for improvements to the manuscript. We also thank B. Holly Smith for information about tooth eruption in primates and oth-

FIGURE 7. Hypothesis of phylogenetic relationship among select genera of North American paromomyids based on cladistic analysis of 12 dental characters (Appendices 1 and 2) with stratigraphic ranges indicated by solid black lines. Bridgerian Elwynella was excluded from the analysis because it is known only from fragmentary specimens from a restricted stratigraphic range. Cladistic analysis yielded a single mostparsimonious cladogram generated by an exhaustive search algorithm (Swofford, 1993) and rooted with Puercan Purgatorius: tree length 5 14, consistency index 5 0.93, retention index 5 0.94. All characters were unordered. The lower dental formulas are plotted here at the points on the cladogram at which they change. The lower dental formula alone, 2.0.2.3, likely precludes derivation of Acidomomys from any known species of Phenacolemur (which have all lost i2 and p3) or Ignacius (which have all lost i2). Torrejonian Paromomys has a lower dental formula, 2.1.3.3, from which that of Acidomomys could easily be derived. Thus, Acidomomys seems to be a sole representative of a ghost lineage of paromomyids that extends back from the middle Clarkforkian to at least the late Torrejonian of North America.

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APPENDIX 1 Description of characters used in phylogenetic analyses. Characters were polarized with respect to Purgatorius as the outgroup. Autapomorphic characters known to be present in only one of the taxa analyzed were excluded from the analysis. All characters were unordered. 1. Three or two lower incisors (0), or one lower incisor (1). 2. Canine present (0), or absent (1). 3. p3 present (0), or absent (1). 4. p4 strong and trenchant hypoflexid present (0), or hypoflexid weak (1). 5. p4 talonid height about one-third that of the trigonid (0), or about one-half that of the trigonid (1). 6. m1 paraconid distinct and anteriorly placed relative to the metaconid (0) or closely appressed to the metaconid (1).

BLOCH ET AL.—NEW PALEOCENE PAROMOMYID 7. m1–2 anterior external cingulum present (0), or absent (1). 8. Lower molar trigonid moderately anteriorly-inclined (0), or strongly anteriorly inclined (1). 9. m3 cristid obliqua intersects the posterior wall of the trigonid towards the midline of the crown (0), or near the buccal side of crown (1).

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10. m3 posterior third lobe not inflated (0), or inflated (1). 11. m3 paraconid present (0), or absent (1). 12. Upper molar postprotocingulum weak (0), moderate, with a small posterolingual basin (1), or strong, with a well-developed posterolingual basin (2).

APPENDIX 2 Character matrix used for phylogenetic analysis.

Purgatorius Paromomys depressidens Paromomys maturus Ignacius Ignacius fremontensis Phenacolemur Acidomomys

1

2

3

4

5

6

7

8

9

1 0

1 1

1 2

0 0 0 1 1 1 0

0 0 0 1 1 1 1

0 0 0 1 0 1 0

0 0 0 1 1 1 1

0 0 0 1 1 1 0

0 0 0 1 1 1 1

0 0 0 1 1 1 1

0 1 1 1 1 1 1

0 0 0 1 1 1 0

0 0 1 1 1 1 1

0 0 0 0 0 1 1

0 1 1 2 2 2 2