new remains of prothoatherium columbianus (litopterna, mammalia) from the miocene of colombia

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New remains of Prothoatherium columbianus(Litopterna, Mammalia) from the Miocene of ColombiaRichard L. Cifelli a c & Javier Guerrero Diaz b ca Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma,Norman, Oklahoma, 73019b Insituto Nacional de Investigaciones Geológico Mineras, Diagonal 53 no. 34-53, Apt. Aéro4865, Bogotá, Colombiac Department of Geology, Duke University, Durham, North Carolina, 27710

Version of record first published: 24 Aug 2010

To cite this article: Richard L. Cifelli & Javier Guerrero Diaz (1989): New remains of Prothoatherium columbianus (Litopterna,Mammalia) from the Miocene of Colombia, Journal of Vertebrate Paleontology, 9:2, 222-231

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CIFELLI AND GUERRERO DIAZ-MIOCENE LITOPTERN FROM COLOMBIA 223

tion is called for in the description of worn, fragmentary dental specimens.

Abbreviations -AMNH, American Museum of Natural History, New York; DNPM, Divisao Nacio- nal de ProduSqio Mineral, Rio de Janeiro, Brazil; Duke- ING, collection made by Duke-INGEOMINAS (ING) expeditions and stored at the Instituto Nacional de Investigaciones Geol6gico Mineras, Bogota, Colom- bia; MNHN, Mus6um National d'Histoire Naturelle, Paris, France; UCMP, University of California Mu- seum of Paleontology, Berkeley; YPM-PU, Yale Pea- body Museum, Princeton University Collection, New Haven.

SYSTEMATIC PALEONTOLOGY

Order LITOPTERNA Ameghino, 1889 Family PROTEROTHERIIDAE Ameghino, 1887

PROTHOATHERIUM Ameghino, 1902:88 (including Neodolodus Hoffstetter and

Soria, 1986:1620; other synonymies are given by Soria, 1981:36)

PROTHOATHERIUM COLOMBIANUS (Hoffstetter and Soria, 1986), new combination

Neodolodus colombianus Hoffstetter and Soria, 1986: 1620.

Type Specimen - MNHN VIV 9, part of right mandibular ramus with well-worn P3-M3.

Hypodigm -The type specimen, and Duke-ING 86- 093, part of left mandibular ramus with unworn M1_3 (M3, exposed by preparation, is unerupted); UCMP 38910, right maxilla with root of P' and with P2-4, M2-3 present but in advanced wear; Duke-ING 86-274, part of left mandible with worn P4-M1 and anterior half of M2, and various postcranial fragments; Duke-ING 86- 288, fragment of left mandible with well worn M2-3; UCMP 37961, associated partial dentition and skeleton, including the following: left mandibular ramus with part of P3 and with P4-M3 complete, all in ex- ceedingly advanced wear; distal right humerus; proximal right and left radii; ?right metacarpal III, and part of adjacent metacarpal (IV if it is from the right side of the body; II if not), the former lacking its proximal end and the latter both ends; ?right phalanx 1 of manual digit III; left acetabular region of pelvis; distal right tibia; right astragalus; proximal left metatarsal III; phalanx 1 of pedal digit III; ungual phalanx of pedal digit III; and miscellaneous dental and bone fragments (UCMP 37961 also includes an upper molar of Mio- cochilius anomopodus and several fragmentary bones probably also belonging to an individual of that species); Duke-ING 88-332, associated dental and postcranial remains including broken right M2-3, fragment of ?left premaxilla with part of I2, mandibular rami with left P3-M3 and right MI_3, shaft and distal end of one femur, proximal and distal ends of one tibia, right calcaneus lacking the tuber, and various fragments; Duke- ING 88-374, associated maxilla with heavily worn, badly shattered right ?M'-2, right astragalus, and pha-

lanx 2 of manual or pedal digit III; Duke-ING 88-373, partial skeleton including right femur, tibia, astragalus, and calcaneus; and ING 182568, including associated right P2, P4, and M1, and fragments of both maxillae including left p3-M2 and right P2_M3.

Horizons and Localities--The type specimen was collected near Villavieja in the Monkey Unit (Fields, 1959) of the Villavieja Formation (Wellman, 1970), according to Hoffstetter and Soria (1986). UCMP 38910 was collected from the unit above the Fish Bed (Fields, 1959) at UCMP locality V4524; UCMP 37961 from the Monkey Unit at UCMP locality V452 1; and Duke- ING 86-093 from the Monkey Unit at Duke-ING locality 031. All of these localities lie within the Baraya Volcanic Member of the Villavieja Formation (Well- man, 1970). Duke-ING 86-274 and 88-332 were collected from below the El Libano Sands (Fields, 1959), Duke-ING locality 040; and Duke-ING 88-374 and Duke-ING 86-288 from that unit at Duke-ING localities 092 and 041, respectively. Stratigraphically, these fall within the Perico Member of the La Dorado For- mation, which underlies the Villavieja Formation (Wellman, 1970). The stratigraphic distribution of specimens and localities is shown in Figure 1. The rock units in question are of Miocene age; precise correla- tion remains to be demonstrated (see above and Kay et al., 1987). All localities are in the upper Magdalena Valley, Huila Department, Colombia.

Revised Diagnosis - Smallest known species of genus and of family Proterotheriidae. Cheek teeth similar in most respects to those of P. lacerum and P. scamnatum, but P3-4 mesostyle generally not as well developed as in those species. Distinct from P. lacerum in lacking P3 entoconid and from P. scamnatum in having small hypocone and metaconule on M3; distinct from these and all other species of Proterotheriinae in having lower-crowned cheekteeth.

Description

All remains of Prothoatherium colombianus are no- table for their small size, and are more diminutive than those of any other known member of the Protero- theriidae; the species is, in fact, comparable in size to several pygmy litopterns of the family Adianthidae. The upper dentition (Fig. 2) is known from UCMP 38910, Duke-ING 88-332, 88-374, and ING 182568. These collectively include almost all of the upper dentition. Advanced proterotheres bear a single, tusk-like upper incisor, identified by Scott (1937) as 12. Duke- ING 88-332 preserves a fragment of the ?left premaxilla enclosing part of one upper incisor. Both ends of the tooth are missing, but it was trihedral in cross section, very deeply rooted, and curved. P2 is subtrian- gular in outline, with a faint bulge in the middle of the ectoloph representing the position of the paracone. A faint labial cingulum is visible at the base of the ectoloph, as it is on some but not all of the succeeding teeth; similar cingula are lacking on known teeth of the Argentine species Prothoatherium lacerum and most

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A

Bs

. '

CC

A

D T ......... . ,

cm

FIGURE 3. Lower dentitions of Prothoatherium colombianus. A, B: Occlusal (A) and labial (B) views of Duke- ING 86-274, showing P4, M, and partial M2, C, D: Occlusal (C) and labial (D) views of Duke-ING 86-093, showing M,_3

molariform but differs from succeeding teeth mainly in having a distinct paraconid (Hoffstetter and Soria, 1986) and a more open trigonid. An entoconid, as seen in Prothoatherium lacerum, is lacking. P4-M3 are mor- phologically similar to each other and are bicrescentic; their appearance of being less lophate than the corre- sponding teeth of other Prothoatherium species is due to lower crown height. Labial and anterolingual cingula are variable but generally well developed on P4, unlike P. lacerum, in which they are poorly developed or

lacking. The paraconid is lacking, and the paracristid terminates in a median position. The cristid obliqua is strong and achieves anterior attachment to the base of the metaconid; all three talonid cusps are present, but the entoconid remains separate from the hypoconulid until advanced wear. The molars (Fig. 3C, D) increase in length from first to third; M2 is the broadest lower molar (Table 2). M3 differs from that of P. lacerum in having a lesser development of the labial sul- cus separating hypoconulid from hypoconid; nonetheless, the tooth has a characteristic trilobed appearance, as in the Argentine species (Soria, 198 1). Lower dental measurements are given in Table 2.

Relative terms describing postcranial anatomy are based on comparison with primitive ungulates (Mat- thew, 1937; Gazin, 1965, 1968) and with Santacrucian proterotheres (Scott, 1910). The distal humerus (Fig. 4A), highly similar to that of other advanced proterotheres (Fig. 4B), is mediolaterally compressed. As preserved, the ulnar fossa is a fenestra, as is common in cursorial and saltatorial mammals. Supinator and pro- nator crests are poorly developed; an entepicondylar foramen is lacking. The radial condyle is little rounded and the ulnar notch deep. The proximal radius is mediolaterally expanded, with a broad, shallow depres- sion for articulation with the humerus. The ulnar facet faces posteromedially. The third metacarpal (Fig. 4C, E) lacks its proximal end; nonetheless, even as preserved it is relatively longer and more slender than that of any other known South American ungulate except Thoatherium. This element is in other respects similar to those of Santacrucian proterotheres (Fig. 4D, F). Judged from the proximal end of the first phalanx that belongs with it, it is likely that the third metacarpal of UCMP 37961 was from the right side of the body, in which case the partial metacarpal adhering to it is the fourth. Regardless, this lateral (or medial) metacarpal is more slender, particularly anteroposteriorly, than is either element of "Proterotherium" or Diadiaphorus (Fig. 4C-F), but not as reduced as in Thoatherium. The lengths of metacarpals III and IV (?or II) are not known.

The femur is poorly preserved but in observable

TABLE 1. Upper dental measurements (mm) of Prothoatherium colombianus.

UCMP ING 38910 182568

P3 length 6.8 7.0 width 8.2 8.3

p4 length 6.4 7.3 width 8.9 8.9

M' length - 7.2 width - 9.0

M2 length 7.0 7.9 width 9.4 10.3

M3 length 7.0 - width 8.2 -

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ent on the lateral surface of the femur slightly less than half the distance from head to condyles.

The proximal tibia bears a salient cnemial crest. This crest terminates proximally in a well-marked, grooved protuberance, the site of insertion for the patellar tendon, as in Santacrucian proterotheres and many other cursorial mammals. The distal end of the tibia bears a strong median ridge separating two grooves for articulation with the raised edges of the astragalus. An- teromedian and posteromedian processes are well developed. The astragalus (Fig. 5C, F) generally follows the litoptern pattern (Cifelli, 1983b). The body is spool- like, with the medial and lateral edges of the tibial trochlea forming raised lips; as in Thoatherium (Fig. 5B, E), these are of nearly equal development, whereas in Protheosodon and the Paleocene litoptern taxa de- scribed by Cifelli (1983b) the medial is notably the shorter of the two. As in Thoatherium, the tibial trochlea extends far onto the posteroplantar surface of the bone, forming a sharp process there; the tibial trochlea is strongly developed in all litopterns (Cifelli, 1983b), but does not primitvely extend so far as it does in Thoatherium and Prothoatherium colombianus. The medial and lateral walls of the astragalus are vertical; a fibular shelf, weakly developed in Protheosodon and other primitive taxa, is lacking, as in Santacrucian proterotheres. The astragalar neck is relatively longer than in any of the Santacrucian forms. The head, comparable in relative size to that of Santacrucian proterotheres but smaller than in Protheosodon and the Pa- leocene litoptern taxa, bears a subcylindrical facet for the navicular. On the inferior side of the bone, the sustentacular, navicular, and "digital ligament" (Sza- lay and Decker, 1974) facets are merged, as they are in Thoatherium. In Protheosodon, these facets are somewhat more distinct entities. The ectal facet forms a deep, laterally-facing trough, another point of contrast with primitive forms such as Protheosodon and a similarity to other advanced proterotheres such as Diadiaphorus. No complete calcanea are known, but those preserved with Duke-ING 88-332 and 88-373 are highly similar to the homologous element in San- tacrucian proterotheres. The neck (that part anterior to the astragalocalcaneal facets) is long and bears an obliquely-oriented facet at its end for the cuboid. The ectal facet is broadly developed on both faces of the ectal prominence, a point of similarity to litopterns in general. The fibular facet is a small, convex, obliquely- oriented surface immediately posterolateral to the ectal facet.

The proximal end of the third metatarsal (Fig. 6C) resembles that of Thoatherium (Fig. 6B) in having less extensive facets for metatarsals II and IV than seen in "Proterotherium" or Diadiaphorus (Fig. 6A). Presum- ably this is related to a greater reduction of the lateral digits than in those forms, although direct evidence bearing on this is lacking. The first phalanx of digit III (Fig. 6F), longer than its counterpart in the manus, is slender for its length, as in Thoatherium (Fig. 6E), unlike three-toed Santacrucian proterotheres (Fig. 6D).

A B C

!':.: .: ..•

i

D EF

cm

FIGURE 5. Astragali of a didolodont, Paulacoutoia pro- tocenica (A, D); an advanced proterothere, Thoatherium minisculum, AMNH 9167 (B, E); and Prothoatherium colombianus, UCMP 37961 [reversed] (C, F) in plantar (A, B, C) and dorsoanterior (D, E, F) views. Scale applies only to P. colombianus; the other astragali have been standardized to a common width to facilitate comparison of proportions.

The terminal phalanx of digit III is developed into a hoof, as in other proterotheres.

DISCUSSION

Hoffstetter and Soria (1986:1620) placed Prothoa- therium colombianus in its own genus, "Neodolodus," referring it without question to the Didolodontidae. They excluded the species from the Proterotheriidae on the basis of its more bunodont and apparently less selenodont cheekteeth, although they noted that in proterotheres "la molarisation des premolaires est tres poussee, suivant un module similaire mais non 6gal a celui de Neodolodus. " Our referral of the species to the Litopterna raises two related issues: 1) the criteria for distinguishing didolodonts from litopterns, and 2) the basis for placing the Colombian form within the Proterotheriidae.

Although workers had long considered Didolodus and allied forms from the Early Tertiary of Patagonia as close relatives of the Litopterna (Ameghino, 1904; Scott, 1913), this view was first detailed by Simpson (1948). In its essential form, this hypothesis of rela- tionships has not been challenged by subsequent workers. Simpson (1948) used dental criteria to separate didolodonts from litopterns, recognizing the latter as distinct on the basis of their more lophodont cheekteeth, although he noted (p. 95) that, "the distinction between some of these [didolodonts] and the Litop- terna is sometimes difficult to draw." No really useful

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cm

'7 s.s .

cm

FIGURE 6. Pedal elements of a three-toed proterothere, Diadiaphorus majusculus (A, D); a monodactyl proterothere, Thoatherium minisculum (B, E); and Prothoatherium colombianus (C, F). A-C, proximal metatarsal III in fibular (lateral) view. A, YPM-PU 15799, after Scott (1910: pl. 13, fig. 9); B, YPM-PU 15719, after Scott (1910: pl. 13, fig. 11); C, UCMP 37961 (reversed). Scale applies only to P. colombianus. D-F, Phalanges of digit III in dorsoanterior view. D, AMNH 9196, after Scott (1910: pl. 5, fig. 2); E, YPM- PU 15719, after Scott (1910: pl. 13, fig. 13); F, UCMP 37961. Scale applies only to P. colombianus; the other elements have been standardized to proximal width of phalanx 1 to facilitate comparison of proportions.

cranial or postcranial remains are known in direct, articulated association with didolodontid or primitive litoptern dentitions. However, the large, well-repre- sented Itaborai Fauna, from Paleocene fissure fills at Sao Jos6 de Itaborai, Brazil, includes proximal ankle elements that may be confidently associated with several important didolodont and litoptern species (Ci- felli, 1983b). These associations are based on occurrence, relative abundance, relative size, and, where appropriate, conformation to predicted morphology as established by other means. As established by comparison of known articulated materials of Santacrucian proterotheres and macraucheniids, litoptern proximal ankle bones (astragalus, calcaneus) are highly modified

for terrestrial locomotion and are as characteristic of the order as are the "double pulley" astragali of artio- dactyls (Schaeffer, 1948). Based on these criteria, a proterothere (Paranisolambda prodromus) and two other Itaborai species previously considered to be didolodonts on the basis of their dentitions bear the di- agnostic litoptern ankle specializations, while two other didolodonts (e.g., Fig. 5A, D) do not. Because of this lack of a dental distinction between forms differing strikingly in their postcranial anatomy, Cifelli (1983b) proposed that the Litopterna be recognized on the basis of their specializations in the ankle (as are many other groups of ungulates) rather than the teeth, which are apparently nondiagnostic. The dentally archaic forms, not distinct on that basis from didolodonts but recog- nizably litoptern based on the ankle, were placed in the family Protolipternidae (Cifelli, 1983a). Hoffstetter and Soria (1986:1620) expressed some doubt regarding the ankle-to-dentition associations, arguing that perhaps all didolodonts (dentally defined) had a litoptern ankle and that the bones attributed by Cifelli (1983b) to at least one didolodont species, Paulacoutoia pro- tocenica, could correspond "a un Notoungulk, groupe dont le tarse est 6galement primitif." The morphotypic notoungulate ankle, although not strongly specialized, is readily recognized on the basis of synapomorphies, and cannot be confused with that of other Eutheria (Cifelli, 1983b). These modifications contrast with those seen in the astragali and calcanea attributed to the Didolodontidae by Cifelli (1983b). Furthermore, ankle bones of appropriate morphology, size, relative abundance, and occurrence may be confidently associated with the known notoungulate species of Itaborai; tarsals are known also for the remaining Eutheria of that fauna (Cifelli, 1983b). Accordingly, it is virtually im- possible that notoungulate (or other eutherian) astragali and calcanea have been mistaken for those of didolodonts, and that, "l'authentique tarse des Didolodontidae soit semblable a celui de Protolipterna [a dentally primitive litoptern]" (Hoffstetter and Soria, 1986:1620). Among Itaborai tarsals, only those belonging to a didolodont-like form are of appropriate size and relative abundance for Victorlemoinea pro- totypica, a taxon once regarded as belonging to the advanced litoptern family Macraucheniidae (Simpson, 1948; Paula Couto, 1952), but now regarded as an aberrant form not related to higher litopterns (Soria, 1980; Cifelli, 1983a, 1985). Because of the close tarsal similarity of Victorlemoinea to the didolodonts Di- dolodus, Lamegoia, and Paulacoutoia, including some presumed synapomorphic features, Cifelli (1983a) grouped them in a common superfamily, Didolodon- toidea. Hoffstetter and Soria (1986) objected to this on the basis of the strong dental specializations of the Sparnotheriodontidae (Sparnotheriodon is a probable synonym of Victorlemoinea; Cifelli, 1985). We feel that this interpretation underscores autapomorphy and begs the issue of synapomorphy; by this logic, Equus and Hyracotherium would be placed in separate families.

Although the precise circumscription of all groups

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in question must await discovery and description of more complete specimens, we refer Prothoatherium colombianus to the Litopterna on the following grounds:

1) The style and degree of posterior premolar (p3-4,

P3-) molarization is matched in no member of the

Didolodontidae; in fact, the morphology of these teeth specifically excludes the species from that family (see Cifelli, 1983a, and below).

2) Although the cheekteeth of the Colombian species are very low-crowned, loph development (or selen- odonty) exceeds that seen in Didolodontidae and is compatible with that for the Lopholipterna, which includes the three advanced families of litopterns (Cifelli, 1983a).

3) Postcranial materials referred to Prothoatherium colombianus are strongly specialized, and in these respects are highly similar to those of litopterns. The astragalus bears all litoptern synapomorphies (Cifelli, 1983b). Comparison of the other skeletal elements with those of didolodonts is precluded, but close similarity to litopterns is noted in the degree and mode of digital reduction, in hoof development, and in modifications of the proximal tibia, distal humerus, and proximal radius.

Within the Litopterna, the affinities of the Colom- bian species clearly lie within the Proterotheriidae. Macrauchenioid (Macraucheniidae and Adianthidae) specializations of the dentition (including a strongly- developed M3 hypocone; columnar metaconid and entoconid on the lower molars) are lacking. Converse- ly, numerous proterothere synapomorphies are present: P3 molarized, with subequal and widely spaced paracone and metacone; P3 molarized, with crescentic talonid loph; protocone and hypocone of upper molars strongly joined by lingual crest (Cifelli, 1983a), and upper molar metaconule detached from adjacent cusps. Further evidence for proterothere affinities lies in the skeleton. The astragalus is distinctly specialized over the protolipternid condition, with complete reduction of the fibular shelf, hyperdevelopment of the inferior face of the tibial trochlea, deepening and reorienting of the ectal facet, and modification of the head into a more cylindrical shape. In these respects, Prothoatheri- um colombianus is more advanced than the primitive anisolambdine proterothere, Paranisolambda prodromus (Cifelli, 1983a, b) and is similar to proterotheriines. Macraucheniid astragali lack these features, having broad bodies, foreshortened necks, and somewhat flattened heads. Furthermore, the degree of lateral digit reduction in Prothoatherium colombianus is comparable only to that seen in advanced Proterotheriidae. Archaic proterotheriids, subfamily Anisolambdinae, are characterized by certain dental specializations, notably a strongly connate lower molar paraconid, which are lacking in Proterotheriinae and in Prothoatherium colombianus. The anterior dentition is not known in anisolambdines, but P. colombianus resembles proterotheriines in the derived condition of 12, which is developed into a tusk that is trihedral in cross section.

Within the Proterotheriinae, the Colombian species clearly belongs to an unnamed clade comprised of Thoatherium and the allied genera Prothoatherium and Deuterotherium (Simpson, 1932; Scott, 1937). In this group, size is small (perhaps a derived condition, judged from related taxa including other proterotheriines and anisolambdines), there is some tendency toward loss of the upper molar metaconule in advanced taxa (Pro- thoatherium colombianus is primitive in this respect), and the postcranial skeleton (hitherto known only for Thoatherium; see Scott, 1910) is more specialized than that of other proterotheres, specifically in the advanced degree of digital reduction and in the elongate, slender metapodials. Within this group, the known morphology of the Colombian species is so close to that of Colhuehuapian Prothoatherium species that we refer it to the genus with little question. In specific, the cheekteeth are lower-crowned and less selenodont that in Deuterotherium or Thoatherium; a metaconule, lacking in Deuterotherium, is present on all three upper molars; and the lower molar talonid contrasts with that of Thoatherium in having a distinct entoconid and in having the hypoconulid of M3 projecting as a distinct lobe. Within the genus, Prothoatherium colombianus is more primitive than either of the well-defined Col- huehuapian species, P. lacerum and P. scamnatum, in its somewhat lower-crowned cheekteeth. Further comparison with these species is rendered difficult because of non-comparable materials. P. colombianus resembles the type, P. lacerum, in its smaller size and differs from P. scamnatum in having an M3 metaconule and hypocone, but that tooth is unknown for the type. P3 of P. lacerum bears an entoconid, a cusp lacking on P3 of P. colombianus. The p3-4 mesostyle of P. lacerum and P. scamnatum tends to be somewhat better developed than in P. colombianus, but this is apparently a variable feature (see Soria, 1981 :fig. 5).

In linear dimensions of the teeth, Prothoatherium colombianus is somewhat smaller than P. lacerum, the smaller of the two well-defined Colhuehuapian species (see Soria, 1981), and thus is probably the smallest known proterothere. In terms of its postcranial anatomy, the Colombian species is similar to, although less specialized than, the monodactyl Santacrucian Thoatherium. Relative to a "condylarthran" morpho- type, the skeleton of P. colombianus is modified in having a mediolaterally compressed distal humerus, with the ulnar fossa developed as a fenestra; a very tall greater trochanter on the femur; a tibia with large cnemial crest and well-developed, grooved protuberance for the patellar tendon; an ankle with great capacity for flexion and extension but little mediolateral mo- bility in either the crurotarsal or midtarsal joints; and especially in having greatly elongated, gracile distal limb elements (metapodials, phalanges), with the lateral digits reduced. In this last respect, P. colombianus is more advanced than typical Santacrucian three-toed forms such as "Proterotherium," Diadiaphorus, or Li- caphrium, and approaches the monodactyly seen in Thoatherium. These skeletal specializations are gen-

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erally associated with advanced cursoriality that, in turn, is usually characteristic of mammals preferring open country habitat (Howell, 1965). This latter gen- eralization does not pertain to forms of small body size. Living ungulates most comparable to Prothoathe- rium colombianus in body size and degree of postcranial specialization include species placed in genera of tragulid (Tragulus, Hyemoschus), cervid (Moschus, Muntiacus, Mazama, Pudu), and bovid (Madoqua) ar- tiodactyls. With the exception of the dik-diks (Ma- doqua), which are found in arid regions with dense scrub cover (Nowak and Paradiso, 1983), these forms are restricted to forested habitats. Similarly-sized, cursorial rodents of the family Dasyproctidae (agutis and pacas) are apparently ecologic equivalents (Dubost, 1968; Eisenberg and McKay, 1974; Eisenberg, 1981). These Recent taxa vary in dietary preference from browsing (e.g., Madoqua) to a regime dominated by fruits and seeds (e.g., Dasyprocta). While the diet of Prothoatherium colombianus remains undetermined, the low-crowned, bunoselenodont cheekteeth accord with either browsing or frugivory/gramnivory and contrast with the specialized morphology found in grazing ungulates of many orders. It is worth of note that cheekteeth of P. colombianus were, apparently, rapidly reduced by wear; in several specimens, the tooth crowns are virtually or entirely worn off (Fig. 3A, B). Tooth surfaces are deeply pitted and scratched, perhaps by ingestion of considerable grit or by seeds.

In sum, the body size, cursorial specializations, and presumed diet of Prothoatherium colombianus suggest that it was most comparable to small, cursorial, forest- adapted Recent ungulates and rodents. This accords with other faunal evidence now available for the La Venta assemblage, which indicates a community dominated by forest to woodland-savanna forms (Webb, 1978).

ACKNOWLEDGMENTS

We thank Dr. Don Savage (UCMP) for loaning us materials, and Dr. Richard F. Kay, Dr. Carlos Villar- roel, and Mr. Richard H. Madden for various com- ments and advice. We gratefully acknowledge support for this research, provided by grants from the National Geographic Society and from the National Science Foundation (BSR 8614533) to Richard Kay. The il- lustrations were prepared by Ms. Coral D. McCallister.

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Received 5 May 1988; accepted 22 November 1988.

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new remains of prothoatherium columbianus (litopterna, mammalia) from the miocene of colombia

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