origin and evolution of the cenozoic vertebrate fauna of middle america || late cenozoic vertebrates...

Late Cenozoic Vertebrates from Honduras and El SalvadorAuthor(s): S. David Webb and Stephen C. PerrigoSource: Journal of Vertebrate Paleontology, Vol. 4, No. 2, Origin and Evolution of theCenozoic Vertebrate Fauna of Middle America (Oct., 1984), pp. 237-254Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4522986 .

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Santa Rosa EROCONTE de

CopnN-

0

N

Siguatepeque Gracias

1430 ORILASComayagua del

Esperanza. HUMUYA

EL o 25

km Tegucigalpa SALVADOR 8830' 880 87030'

FIGURE 1. Principal fossil vertebrate localities in Hon- duras. The late Miocene Gracias Formation is detailed in Figure 3. Yeroconte and Orillas del Humuya are late Pleis- tocene localities.

areas and regularly reach thicknesses of 500 to 1,000 m. Subsequent block-faulting and basin-filling provide extensive accumulations of tuffaceous sediments. The present subhumid climate and high topographic relief in this region tend to provide favorable exposures of fossiliferous sediments within the late Cenozoic ignimbrite complex. The distribution of six principal Ce- nozoic vertebrate sites in these countries is indicated in Figures 1 and 2.

It may be useful to note the circumstances of our paleontological involvement in this region. The senior author worked in Honduras for the Florida State Mu- seum two months a year in 1968, 1969, 1971, and 1977, mainly in the Gracias Formation and in the Comayagua Valley. The work was supported by the American Philosophical Society, the U.S. National Geographic Society, and the U.S. National Science Foundation. His collaborators on these trips included Professor Howell Williams of the University of Cali- fornia, Berkeley; Dr. John E. Mawby, now of the Cal- ifornia State University at Fresno; Dr. Jon A. Baskin and Mr. Howard H. Converse of the Florida State Museum, and his wife Barbara Webb. We were helped immeasurably by the faculty and staff of the Escuela Agricola Panamericana in Zamorano, Honduras.

The junior author worked in El Salvador from Feb- ruary 1977 through June 1979 as a Curator of Paleon- tology at the Museo de Historia Natural de El Salvador on assignment with the U.S. Peace Corps. His main goal was to collect fossil sites in order to develop edu- cational and display material for the museum. The Ministries of Education and Agriculture provided the necessary support, and the assistance of Sr. Victor Hellebuyck, Lic. Francisco Serrano, and Dr. Stanley Boggs was especially valuable.

The gracious hospitality of the late Dedicacion Be- netiz, Sr. Francisco Fuentes, and numerous other Sal- vadoran landowners is sincerely appreciated.

Prefixes to specimen numbers refer to the catalogs of the following institutions: P, Paleontology, Field

GUATEMALA HONDURAS

N-

Santa Ana BARRANCA del CORINTOO

San. SISIMICO

Salvador San HORMIGUER Vicente 0O

13030' San Miguel PACIFIC OCEAN

0 25km I• aIt

900 89030' 890 88030' 880

FIGURE 2. Principal vertebrate localities in El Salvador. Corinto is a late Miocene locality, Barranca del Sisimico is an early Pleistocene set of localities, and Hormiguero is a late Pleistocene locality.

Museum of Natural History, Chicago; WM, Walker Museum, University of Chicago, now a part of the Field Museum Collection; F:AM, Frick Collection at the American Museum of Natural History, New York; and UF, Florida State Museum, Gainesville. Virtually all specimens in the Museo de Historia Natural de El Salvador discussed are also represented by UF casts, and their catalog numbers are cited here.

This paper is University of Florida Contribution to Vertebrate Paleontology number 223.

PREVIOUS STUDIES

The earliest scientific paper on a fossil vertebrate from Honduras was Leidy's (1859) description of a proboscidean tooth from Tambla. In 1969, the senior author visited that remote village in the hope of finding further fossils, but no leads nor even any promising sediments within the ignimbrite mass were discovered. Presumably the "Tambla Mastodon" came from a lim- ited Pleistocene deposit in that area.

The next important discoveries of fossil vertebrates came from near the town of Gracias in central Hon- duras. Frick (1933:527) introduced the Gracias fauna as follows: "The occurrence of Tertiary mammal remains in the vicinity of Tepusuma, Honduras, was first reported to the American Museum by Mr. A. W. An- thony. The latter had visited this area in 1927 during the course of a collecting trip for the Department of Mammals, and had secured four teeth of a small Hip- parion-like form and a mastodont premolar." Childs Frick then sent John Blick back for more extensive work during the dry season (winter) in early 1929. The principal results were the two proboscidean mandibles described by Frick (1933) as Blickotherium blicki and Aybelodon hondurensis. In addition, Blick obtained a small collection listed by Frick as follows: "Hyaenog- nathus, Pliohippus, Hipparion, Merychippus, Proto- hippus, a teleocerine rhinoceros, and a procameline camelid."

238 JVP 4(2), October 1984



streams deposit boulders and gravel far coarser than the Gracias Formation sediments on its margins. Uplift of this mountain, composed of crystal-rich welded rhy- olite ignimbrites, probably accompanied faulting of the Gracias graben and deformation of the Gracias For- mation strata (Williams and McBirney, 1969).

The Gracias Formation consists predominantly of sandstones interbedded with occasional marl and clay lenses. The coarse sands and gravels are rich in quartz, feldspar, and biotite, indicating derivation from rapid erosion of nearby ignimbrites. Unfortunately, none of the tuffs or minerals from Gracias have produced dat- able samples. North of Tepusuna, near the northern end of the Gracias Formation, abundant red pumice fragments exactly resemble those exposed in the old ignimbrite hills just north of Las Flores. In general, the siliceous volcanic fragments become coarser in the northern part of the formation; lenses of clay and marl are much more common in the southern half of the formation near Gracias.

The maximum exposed thickness of the Gracias For- mation is about 150 m, measured at Cerro Lobo just south of Locality 9. Section 1 of Olson and McGrew (1941) taken at Rancho Lobo (Locality 8) represents the lower 60% of the section at this hill. The uppermost white ash bed in that section can be traced nearly con- tinuously northward to Locality 6 (the Saddle Locality) and then northward to Tepusuna Hill (Locality 3), where it crops out about 15 m above the base of the section (Olson and McGrew, 1941). Thus, the northern half of the basin represents in part younger sediments than the southern half. The caption of Figure 3 indicates the principal localities from which Florida State Museum parties obtained fossil vertebrate collections. Tepusuna Hill is the center of a very large and important collecting area; it was the original area worked by Frick parties and was later visited also by McGrew and his associates. The latter parties, however, con- centrated their efforts at Rancho Lobo to the south. It is noteworthy that productive sites seem randomly dis- tributed throughout the length of the Gracias graben. Localities are poorly developed on the margins, but this is largely attributable to the shallow erosion on the east flank and the boulder fans covering the west flank.

There are apparent paleoecological and sedimento- logical differences between the northern sites (1 through 3) around Tepusuna Hill and the southern sites around Rancho Lobo (6 through 9). Sites 4 and 5 may be regarded as transitional. As noted above, coarse sands and gravel lenses are more prevalent in the north end of the basin, whereas marls and clays are more frequent and persistent in exposures to the south. It is common to find recognizable plant remains, notably palm stems and leaves, in the latter deposits, especially at sites 6 and 8. Olson and McGrew (1941) gave the following brief floral listing:

Family Sapindaceae: Schmidelia sp. Family Cellastraceae: Gyminda sp. Family Leguminaceae: genus indeterminate

Family Palmae: genus indeterminate Family Gramineaceae: genus indeterminate

We found no recognizable plant remains in the generally coarser northern sediments, even though at Las Culebras we quarried extensively in preparation for screen-washing. Striking differences also occur in the relative abundances of the vertebrate fossils. In the northern localities Hipparion teeth are about as common as Pliohippus teeth, whereas they are exceedingly rare in the southern localities. Gomphotheriidae, Rhi- nocerotidae, and Gelocidae are also more common to the north. It appears that during deposition of the Gra- cias Formation the sediment source was generally from the north, where coarser floodplain deposits predomi- nate, whereas finer-grained sediments representing ephemeral ponds were more abundant in the southern part of the Gracias basin. This may explain the greater frequency of lowland or wetland species to the south and a more diversified, savanna woodland biota to the north. Although the northern sections occur in part stratigraphically higher than the southern sections, the principal differences are probably environmental and paleoecological.

Gracias Fauna--The following is a revised faunal list based on all known collections from the Gracias Formation. It excludes several taxa incorrectly listed in earlier studies:

Osteichthyes Teleostei indet.

Amphibia Ranidae

Rana sp. Reptilia

Emydidae Rhinoclemmys sp.

Testudinidae Geochelone sp.

Mammalia Canidae

Osteoborus cynoides (Martin) Gomphotheriidae

Rhynchotherium blicki (Frick) Equidae

Hipparion plicatile (Leidy) Pliohippus hondurensis Olson and McGrew

Rhinocerotidae Teleoceras cf. fossiger (Cope)

Tayassuidae Prosthennops cf. serus Gidley

Camelidae Protolabis cf. heterodontus (Cope) Procamelus cf. grandis Gregory

Gelocidae Pseudoceras skinneri Frick

The following discussion briefly considers the present status of each taxon.

Class OSTEICHTHYES Subclass TELEOSTEI

240 JVP 4(2), October 1984



One small vertebra represents this class at the Las Culebras site. It is remarkable that, despite intensive screen-washing, no other fish scales, teeth, or bones have been observed in the Gracias Formation.

Class AMPHIBIA Family RANIDAE

RANA sp.

The posterolateral portion of a skull (UF 45895) from Las Culebras represents a large ranid frog. It reveals many details of the occipital and temporal re- gions.

Class REPTILIA Family EMYDIDAE

RHINOCLEMMYS sp. A nuchal bone from a small turtle (UF 46671) is

closely comparable to the genus Rhinoclemmys presently living in Central America. A number of distinctive features, including the narrow anteromedian scute, the strong midsagittal keel, and the strong postero- median concavity, combine to make this specimen diagnostic.

Family TESTUDINIDAE

GEOCHELONE sp.

One large shell and a number of shell fragments represent at least one species of tortoise. These samples are presently under study by Dr. Walter Auffenberg at the University of Florida.

Class MAMMALIA Family CANIDAE

OSTEOBORUS CYNOIDES (Martin)

McGrew (1944) described and figured an excellently preserved palate from Rancho Lobo that he referred to the well known Hemphillian species Osteoborus cynoides. Additional specimens belonging to this species are: UF 17771, RM' from the New Year locality; UF 45879, trigonid of RMI from Las Culebras; F:AM 27020, a lower jaw fragment; and F:AM 27021, a left M1-2 from Tepusuna.

Other specimens that represent this taxon were originally recognized by Olson and McGrew (1941) as the genus Amphicyon, while Frick (1933) listed the genus "Hyaenognathus." The broken M1 from Rancho Lobo, WM 1767, closely resembles the other first upper molars of Osteoborus from the Gracias Formation. The record of Amphicyon was based mainly on a poorly preserved left mandibular ramus, WM 1766 (Olson and McGrew, 1941). The jaw length is about 90 mm from canine to last molar alveolus. None of the post- canine teeth are present and the shattered alveolar bor- der is enveloped in plaster. The number and position of the teeth are discernible only from radiographs; these reveal the following features of the purported Am-

phicyon jaw that are characteristic of Osteoborus: (a) There is only a small space near the ascending ramus for M2, whereas a mature amphicyonid jaw would be much longer and would support a much larger M2; (b) there are two deeply divergent alveoli for a substantial P3 immediately anterior to P4, a characteristic of Epi- cyon and Osteoborus, but not of Amphicyon in which P3 has closely appressed shallow roots and is separated from P, by a diastema; and (c) if these alveoli are correctly assigned, then there is room for only one more small premolar between P3 and the canine, a normal feature of a "crowded" Osteoborus jaw, but not of a long Amphicyon jaw. This jaw is of size comparable to that of Gracias upper dentition that is definitely assignable to Osteoborus cynoides. Several postcranial remains, including those regarded in previous studies as amphicyonids, are referable to Osteoborus.

Family GOMPHOTHERIIDAE

RHYNCHOTHERIUM BLICKI (Frick)

The rich sample of proboscideans from the Gracias Formation provides the oldest evidence of the genus Rhynchotherium. Frick (1933) described two new genera and species from Honduras; however, continued collecting has provided new morphological and statis- tical evidence that there is but one species of Rhyn- chotherium, a synonymy proposed previously by Webb and Tessman (1968). It is possible, as Tobien (1973) has suggested, that Rhynchotherium species represent the antecedents of the highly successful Pleistocene neotropical proboscideans such as Haplomastodon.

A left mandible, P 27129, from the Rio Campuca locality (Fig. 4) embodies many of the characteristics of Rhynchotherium, including the deep, downturned symphysis with large, transversely flattened lower incisors (here represented by alveoli), and a stout hori- zontal ramus bearing a broad last molar with four and a half simple lophids. The angle of symphysial deflec- tion is about 250. This specimen is very close to the same late ontogenetic stage as the type of "Aybelodon hondurensis" of Frick (1933). Another mandible, UF 17797, represents an ontogenetic stage even earlier than the type of "Blickotherium." It now seems quite clear that Frick's generic distinctions resulted from the great ontogenetic differences between "Blickotherium" and

"Aybelodon.'" The measurements of all available cheek teeth (Table lA) indicate a single-size sample. The width/length index of M3 averages about 43, as compared to about 50 in many later samples (Webb and Tessman, 1968).

The correct generic assignment of the Gracias proboscidean depends not only on accurate description of this sample, but also on the phylogenetic framework of New World Gomphotheriidae. Tobien (1973) has done much to bring order to this complex subject. In a careful analysis of the genus Rhynchotherium he found most species of that genus to be transitional to the New World brevirostrine group that includes Haplomas- todon. However, he regarded the enamel-banded lower

JVP 4(2), October 1984 241



irj*

10 cm

10 c m

FIGURE 4. Rhynchotherium blicki (Frick), P 27129, left mandibular ramus in lateral view (a) showing downturned symphysis, and dorsal views of the symphysis (b) and the third molar (c).

incisors found in the type mandibles of R. edensis and R. blicki as "enigmatic," and to this degree tended to disunite the group.

The distinctive features of Rhynchotherium lower tusks are lateral compression with lingual flattening and development of a broad enamel band on the labial side, presumably to facilitate "scissor-like" occlusions with the upper enamel edge. These features are evident in the adolescent type mandible of R. blicki (F:AM 27062), and were well described in the "neotype" of R. edensis (Frick, 1933). They are further confirmed in the Gracias sample by isolated tusks of similar char- acter, such as UF 18033, which differ in their straight, compressed form from other specimens that are surely upper tusks. Webb and Tessman (1968) correctly sur- mised that the absence of enamel on the lower tusks of "Aybelodon hondurensis" is due to old age and breakage.

May and Repenning (1982) offer another interpre- tation to explain the reduced condition of lower-tusk enamel bands in some Rhynchotherium specimens. Comparing the Mt. Eden fauna of very late Hemphil- lian age and other California late Hemphillian faunas, they found that the Warren local fauna Rhynchothe- rium has a short, narrow band of enamel on its lower tusk in contrast to the longer and wider band described by Frick (1933) in R. edensis. Having already shown

that the Warren fauna is at least 0.5 million years older than the Mt. Eden, they interpet the narrow band in the former as a relatively primitive condition. The broad enamel bands on much older Rhynchotherium lower tusks from Honduras, however, render this view unlikely. Is is more probable that the narrowness of the enamel band in available specimens from Warren is due to either poor preservation or old ontogenetic age; indeed, such variation is common in the larger samples from the Gracias Formation.

Savage (1955), Simpson and Paula Couto (1957), and Tobien (1973) have made successively stronger cases for allying some progressive species of Rhyn- chotherium with such New World brevirostrine genera as Cuvieronius, Notiomastodon, and Haplomostodon, representing Osborn's subfamily Notiomastodontinae. Most recently, May and Repenning (1982) have trans- ferred R. edensis to the genus Cuvieronius. Without denying this phylogenetic connection, which is indicated by such shared features as spiral upper tusks, we believe that it does not adequately take into account the closer connections between the Mt. Eden species and other earlier Rhynchotherium species. We prefer to confine the definition of Cuvieronius to species ex- hibiting an abbreviated symphysis and the absence of lower tusks. Members of the genus Rhynchotherium, including R. edensis, can be distinguished by their (1)

242 JVP 4(2). October 1984



A B C

0 1 2cm I I I

D E

FIGURE 5. Cheek teeth of Hipparion plicatile (Leidy) from the Gracias Formation. Occlusal views of right upper teeth: a, UF 45897, crown height of M2; b, same tooth sectioned at 31 mm crown height; c, UF 18034, crown height of P4 = 41 mm, early wear stage. Occlusal views of left lower teeth: d, UF 45900, slightly worn M3; e, UF 45898, moderately worn M2.

In virtually all features the hipparionine horse teeth from the Gracias Formation appear to fall within the range of Hipparion plicatile (Leidy) originally described from Mixson's Bone Bed in Florida. Since only one or two upper teeth and no lower teeth have been

described or figured, these comparisons rely on un- described material from such sites as Mixson's and McGehee Farm (MacFadden, in press). Comparison with Nannippus montezuma (Leidy), suggested by Ol- son and McGrew (1941), is restricted to the only known

TABLE 2. Measurements (in mm) of Hipparion plicatile cheek teeth. *Cut at 30 mm where crown dimensions were measured; **cut at 36 mm where crown dimensions were measured.

A. UPPER CHEEK TEETH

M3P4 -2

UF UF UF F:AM F:AM F:AM F:AM F:AM 45897 18034 18034 46323 27041 27042 27043 31111

length* 16.6 19.0 18.8 23.0 - - - -

width* 14.4 19.9 18.2 17.0 - - - -

height* 40.8 41.1 38.2 36.0 -

length** - - - - 17.0 19.0 20.3 17.6 width** 16.9 18.8 19.4 17.2 height** - - - - 46.8 40.0 42.0 41.5

B. LOWER CHEEK TEETH-associated series, UF 18036 P2 P3 P4 M2 M2 M3

length 19.5 18.0 19.4 18.7 17.5 18.1 width 10.3 10.7 11.2 10.9 11.3 8.7 height 21.5 31.9 40.7 34.5 35.0 37.3

244 JVP 4(2), October 1984



j$

I "I 2'

II 'I I

0 11 2 c

FIGURE 6. Occlusal and anterior views of a slightly worn right upper premolar (UF 46319) of Teleoceras cf. fossiger (Cope) from the New Year locality.

specimen, the worn type molar from Lacualtipan near Vera Cruz, Mexico. It seems preferable to leave that taxon as a nomen dubium (MacFadden, pers. comm.). Thus, it is appropriate to refer this sample to Hipparion plicatile on the basis of the following features: crown height in upper molars about 48 mm, oblique elliptical protocone, markedly plicated metaloph borders, short labial P2 parastyle, and, in lower teeth, moderately to deeply penetrating ectoflexid, subcircular metaconid and metastylid, lingually placed circular entoconid, and strong pli-caballinid and isthmian plications.

PLIoHIPPUS HONDURENSIS Olson and McGrew

This is the most common species in the Gracias Formation. Olson and McGrew (1941) presented an excellent description of the material and a discussion of its affinities. Although extensive new collections have been added, they do not equal the quality of those taken by McGrew's party at Rancho Lobo, which were described and figured in their earlier paper.

Family RHINOCEROTIDAE

TELEOCERAS cf. FOSSIGER

The Gracias Formation provides the southernmost records of Rhinocerotidae in the New World. Frick (1933) listed a "teleocerine rhinoceros" on the basis

of F:AM 27023, a jaw fragment with several broken teeth, and Olson and McGrew (1941:1790) assigned an edentulous jaw fragment and a cervical vertebra from Tepusuna to "the general teleocerine group."

Figure 6 shows a nearly unworn right P2 (UF 46319) of a rhinocerotid from the New Year locality. The maximum height of the ectoloph is 26.5 mm, the anteroposterior length is 27.1 mm, and the maximum width is 23.9 mm. The reduced dimensions and degree of hypsodonty mark this as an advanced species of Teleoceras. With respect to such features as incomplete protoloph and the weak anticrochet, this tooth is closely comparable to P2 in the large Teleoceras samples from the Love Bone Bed and Mixson's Bone Bed in Florida. It is lower-crowned than P2 in late Hemphillian Tele- oceras samples from the Bone Valley.

Other molars, including an M' (UF 18103) and several upper molar fragments, UF 18015, from the lower part of the El Carrizal locality, also indicate the presence of a progressive species of Teleoceras. In UF 18015 the tallest crown measures 35 mm, but this is a well-worn tooth. A series of four vertebrae and ribs, the distal end of a tibia (UF 18039), and the fragment of a juvenile phalanx (UF 18040) were collected about 20 m below the peak of Tepusuna Hill. These rhinos are one of the taxa that are much more common in the northern sites of the Gracias graben than in the southern sites.

JVP 4(2), October 1984 245



0 1 2cm I I I

FIGURE 7. Occlusal view of a left third upper molar (UF 45881) of Prosthennops cf. serus Gidley from the Las Cu- lebras locality.

Family TAYASSUIDAE

PROSTHENNOPS cf. SERUS Gidley

This is the first record of the family in the Tertiary south of Mexico.

A left M3 from Las Culebras in the Gracias For- mation (UF 45881, Fig. 7) represents a large Pros- thennops species. It is a fairly well worn molar 20.6 mm long and 16.9 mm wide across the anterior loph, but only 11.7 mm across the posterior loph. The enamel is thick and the pattern includes four major cusps, several stout accessory cusps, and abbreviated anterior and posterior cingula. The measurements and cusp pattern compare well with P. serus and P. graffhami described by Schultz and Martin (1975) from the Ame- belodon fricki Quarry in Nebraska.

Family CAMELIDAE

Olson and McGrew (1941) compared the three postcranial specimens then available to "large specimens of Procamelus from the Great Plains of North Amer- ica." Substantial additions to the camelid sample indicate the presence of a second smaller camelid genus in addition to the large Procamelus.

PROTOLABIS cf. HETERODONTUS (Cope)

Several specimens in the new collections indicate a moderately large camelid. Figure 8 shows a mandibular fragment (P 27127) containing a slightly worn M2 from the Rancho Lobo locality. This lower molar is 28.0 mm long and 13.2 mm wide and has an unworn crown about 18.0 mm in height. It is much narrower and less hypsodont than in Procamelus or Hemiauchenia, but

falls within the range of Protolabis heterodontus (e.g. Webb 1969:150) known from Barstovian and Claren- donian deposits of the western United States.

The molar has a simple pattern with angular cres- cents and styles, and no development of a "llamine buttress," as might be expected in Procamelus or Hemiauchenia. The jaw is remarkably shallow (29 mm below M2) even for Protolabis, but this may be caused by its immature stage of development. The angular region, shown by Honey and Taylor (1978) to be so diagnostic of Protolabis, is missing. Even so, the anterior insertion area of the masseter muscle shows con- siderable rugosity, as is especially common in Proto- labis.

A maxillary fragment (UF 45882) from the New

_

9 2 3 4 5cm

9 3cm

\ \,\

..

0 1 2 3cm

FIGURE 8. Protolabis cf. heterodontus (Cope), P 27127, from the Rancho Lobo locality. Above, lateral view of a left mandibular fragment with M2; below, enlarged occlusal view of M2.

246 JVP 4(2), October 1984



0 1 2 3cm

FIGURE 9. Pseudoceras skinneri Frick, UF 18041, from the Tepusuna Locality, occlusal view of a right mandibular fragment with well-preserved second and third molars.

Year locality bears a heavy-rooted canine tooth about 5 mm in diameter. Anteriorly, the fragment reveals a premaxillary suture; posteriorly, there is a strong lateral concavity or "pinched" rostrum characteristic of Protolabis.

PROCAMELUS cf. GRANDIS Gregory

To the record of the large Procamelus from the Gra- cias Formation, we add a few more fragmentary specimens. Olson and McGrew (1941) noted two distal radio-ulnae, one of which was figured, and the proxi- mal end of a radius. The most helpful new specimen is a broken cannon bone (fused metacarpals III and IV), UF 18042, from the Tepusuna locality. Near mid- length it measures 49.8 mm in transverse diameter and 35.4 mm in anteroposterior diameter. This compares exactly with large specimens of Procamelus grandis (e.g. Webb, 1969:162). A radio-ulnae, presumably the basis for Frick's (1933) recognition of a "procameline" (F:AM 27022), is also comparable in size to the largest species of Procamelus. Such large Procamelus species occur in late Clarendonian and early Hemphillian deposits of the United States.

Family GELOCIDAE

PSEUDOCERAS SKINNERI Frick

Olson and McGrew (1941) figured and discussed at some length a heavily worn M3 of "a small pecoran, probably a cervid, which was found near Tepusuna." Despite its advanced wear, they noted that it was much more hypsodont than molars of Mazama. Fortunately, better and more extensive material now indicates the true affinities of this small ruminant.

Figure 9 shows a right mandible with a moderately worn molar series, UF 18041, from Tepusuna. It strongly resembles the type mandible of Pseudoceras skinneri (Frick, 1937). The narrow molars, strong protostylids and entostylids, and especially the simple narrow heel of M3 are diagnostic. This specimen is slightly larger than the type which is among the largest representatives of the genus. Two other molars, an LM2 (P 27090) and an RM3 (P 27130), also represent this genus.

Pseudoceras cannot belong to the Camelidae as Frick

(1937) suggested, but it is a distinctive New World representative of the Gelocidae (Webb, ms.). The limb elements, unknown to Frick, generally resemble those of moschids. Seven postcranial elements can now be recognized from the Gracias Formation, including UF 18132-34, 24997, 45880, and 458933. Evidently, these small hornless ruminants were rather common elements of the Gracias fauna, in contrast to their extreme rarity in temperate North America where they occur in late Barstovian through mid-Hemphillian faunas.

Corinto, El Salvador Geological Setting-During 1977-1978 the junior

author made numerous trips to a set of late Miocene localities roughly 5 km west of the town of Corinto, Department of Morazan, in northeastern El Salvador (Fig. 2). The localities are in an area known as Canton Calavera on the east side of the Torolla River Valley. The primary locality investigated is on the land owned by the family of the late Dedicacion Benetiz, who ex- tended the courtesy of allowing several investigations at the site. In this area the Torolla River has exposed a sequence of lava flows and ignimbrites interrupted by alluvial sediments. The ignimbrites are white and form resistant outcrops. They correspond to Miocene- and Pliocene-age ignimbrites described by Williams and McBirney (1969) in adjacent parts of Honduras.

The fossil vertebrates from Corinto occur in alluvial sediments between the lower and upper ignimbrite units. Exposures are not common except in incised valleys. The primary locality occurs as an exposure produced by erosion of agricultural land. Stratigraphic continuity has not been established between the four fossil-bearing localities at Corinto, but they occur within a radius of 1 km and faunally appear to be of about the same age.

The alluvial deposits include interbedded sandstone, shale, and occasional pebbly conglomerate lenses. Pumice fragments up to 40 mm in diameter are found in the sandstone and sandy shale. The formation ex- hibits a high degree of lateral variability, and individ- ual beds cannot be correlated over moderate distances. At the primary locality, the fossil-bearing sediments include about 10 m of interbedded, buff, tuffaceous sandstone and shale. The fossils occur mainly as large, isolated dental elements in scattered areas. No rich concentrations have been found.

Corinto Fauna--The fauna from the Corinto sites resembles that from the Gracias sites in most respects. The five mammalian taxa include:

Canidae Osteoborus cynoides, UF 57481

Gomphotheriidae Rhynchotherium blicki, UF 57482-5

Equidae Cormohipparion cf. occidentale, UF 57487 Pliohippus hondurensis, UF 57488-91

Camelidae Procamelus cf. grandis, UF 57486

JVP 4(2), October 1984 247



c0.

V~

01/2 2. 3 4 8 9 1

FIGURE 14. Partial skeleton of Pteronotus cf. parnelli, MHNES 70-64, from the diatomite beds at Barranca del Sisimico, San Salvador. A lateral view of the left mandibular ramus and the folded left forelimb may be seen near the left side; the cranium is exposed ventrally, facing right with its large bullae near the mandible; the rib cage lies at upper right. Scale in cm.

ciliid fish (Alvarez de Villar and Aguilar, 1957; Rosen and Bailey, 1963). It is also reported that bird remains have been found in the diatomaceous shales; however, this has not been confirmed.

The diatomaceous shales have produced an impressive invertebrate fauna, including ostracods (Triebel, 1963), molluscs (Schmidt-Thome, 1975), insects including Hymenoptera and Diptera, and fresh-water sponges (Demospongia: Spongillidae). The flora (Lot- schert and Madler, 1975) is equally impressive, with 42 genera of vascular plants. Federovich (pers. comm., 1980) identified 21 varieties of diatoms from the Bar- ranca del Sisimico diatomite. The dominant form is Melosira granulata, which is indicative of mesotrophic to eutrophic lake conditions. Most of the attendant species are epiphytes, which grow on submerged vege- tation. The diatom flora indicates deposition in a deep alkaline lake. A similar diatomite from Palancia, Gua- temala, containing primarily Melosira granulata and sponge spicules, was reported by Williams (1960). Another useful comparison is with the Mexican diatomite described by Maldonado-Koerdell (1953).

The age of the vertebrate fauna from the upper unit is certainly Pleistocene, possibly early Pleistocene. The tuffaceous beds in this section are too impure to yield radiometric dates. At present, the only carefully stud- ied taxa are the megalonychid sloths. Because the new species of Megalonyx was smaller and less progressive than M. jeffersoni, Webb and Perrigo (1984) suggested that it probably represented an early Pleistocene stage of evolution. Clearly, further study of this fauna is needed.

I I

FIGURE 15. Left M, of Equus sp., UF 57497, from late Pleistocene of Hormiguero, El Salvador.

Hormiguero Local Fauna, El Salvador Late Pleistocene vertebrate occurrences are rather

common in many parts of Central America. We have encountered and recorded at least two-dozen vertebrate fossil sites in Honduras and El Salvador which produced abundant elements of Cuvieronius, the short- jawed gomphotheriid, and/or the giant megatheriid ground sloth Eremotherium. Larde y Larin (1950) cited over 40 such sites for El Salvador alone, and Rodriguez (1942) recognized a number of similar occurrences in Costa Rica. Many of these localities, however, yield few if any samples of other, smaller vertebrates. We briefly review here three late Pleistocene faunal samples that are more diverse than most.

The junior author relocated and reopened the Hor- miguero Quarry (Stirton and Gealey, 1949) some 35 years after Stirton had developed it, and obtained extensive new collections for the Museum in San Sal- vador. The most abundant remains were those of Ere- motherium, the giant ground sloth. Additional genera not previously reported by Stirton and Gealey (1949) are Geochelone, Neochoerus, Glyptotherium, Cuviero- nius, and Equus. Figure 15 indicates a characteristic Equus lower molar. A revised list of the Hormiguero local fauna includes the following taxa:

Reptilia Testudinidae

Geochelone sp. Mammalia

Hydrochoeridae Neochoerus sp., UF 57578

Glyptodontidae Glyptotherium sp., UF 57579

Megatheriidae Eremotherium sp., UF 57495

250 JVP 4(2), October 1984



Felidae Smilodon sp.

Toxodontidae Mixotoxodon cf. larensis UF 57494

Gomphotheriidae Cuvieronius sp.

Equidae Equus sp., UF 57496-7

Camelidae Bovidae

Bison sp.

Yeroconte Local Fauna, Honduras The Yeroconte Quarry in northwestern Honduras,

briefly reported by McGrew (1942), yielded another late Pleistocene fauna remarkably similar to the Hor- miguero local fauna (Fig. 1). It represents a swampy setting, with bones and some pebbles accumulated at the bottom of about a meter of black organic mud, which overlies a clean gray clay stratum. We have not been able to relocate this site, which has not been car- bon-dated. The following faunal list is slightly ampli- fied from that of McGrew (1942) on the basis of the collection at the Field Museum:

Glyptodontidae Glyptotherium sp.

Megatheriidae Eremotherium sp.

Felidae Felis concolor

Toxodontidae Mixotoxodon cf. larensis

Gomphotheriidae Equidae

Equus sp. Camelidae

Camelops sp. Cervidae

Odocoileus sp.

Orillas del Humuya Local Fauna, Honduras Geological Setting--The late Pleistocene Orillas del

Humuya locality is exposed at an elevation of about 600 m in the cutbank of the Rio Humuya, 3 km south of the bridge for the Carretera Nacional north of the city of Comayagua, in the central graben of Honduras (Fig. 1). Physiographically, this lower (northern) end of the great Comayagua Valley is narrow and immature. Here the river course is relatively straight and cuts through high banks, dissecting extensive tuffaceous sands from an earlier, presumably late Pleisto- cene, episode of valley filling. The exposed sediments attain a thickness of about 15 m, the upper two-thirds consisting of massive reddish-brown, fine to medium tuffaceous sands. The lower third is capped by a well- cemented gray, fine tuffaceous sandstone unit about 0.5 m thick. The quarries that produced concentrations of fossils were dug into the lower 3 m near the dry- season river level in cross-bedded, light gray, pebbly, tuffaceous channel sands that rest unconformably on

A

0 .

5cm

B

-,p %'1

FIGURE 16. Left maxilla of Mixotoxodon larensis, UF 12256, from Orillas del Humuya: A, occlusal view of P2_M3; B, lingual view.

Tertiary ignimbrites. In the vicinity, Cretaceous con- glomeratic red beds of the Valle de Angeles Formation are also exposed unconformably below the Pleistocene alluvium.

Orillas del Humuya Fauna--Terrestrial and fresh- water vertebrate fossils were collected in pockets ex- tending about 60 m along the riverbank. The most abundant elements of the fauna were large toxodontid ungulates (Fig. 16) and megatherid ground sloths. An excellent mandibular ramus represents an essentially modern capybara (Fig. 17). The smaller elements of the fauna were extremely rare even though we at- tempted to concentrate them by screen-washing. The faunal list includes the following taxa:

Reptilia Emydidae

Rhinoclemmys sp., UF 17722 Mammalia

Hydrochoeridae Hydrochoerus sp., UF 17723

Leporidae Sylvilagus sp., UF 17724

Megalonychidae Megalonyx sp., UF 32100

JVP 4(2), October 1984 251



ican taxa prior to the Great American Interchange. Clearly, much more paleontological work in this criti- cal area is needed.

There is no clear faunal evidence of a climatic trend toward progressively more aridity on the Pacific slopes of Honduras and El Salvador between late Miocene and late Pleistocene. One clue to broad climatic changes might be changes in ungulate species diversity. Al- though the number of browsing and grazing ungulate species did decline from late Miocene (eight species known in the Gracias local fauna) to late Pleistocene (five species known at the Hormiguero local fauna), the ungulates were joined by non-ungulate herbivores from South America such as the capybaras (Neochoe- rus), giant ground sloths (Eremotherium) and glypto- donts (Glyptotherium). At Hormiguero, if these non- ungulate herbivore immigrants are included, the number of late Pleistocene large-herbivore taxa rises to eight, thus equalling the number in the Gracias fauna. Furthermore, so many of the lesser late Pleistocene localities, for example Yeroconte and Hormiguero, were deposited under paludal conditions that it becomes difficult to imagine that they represent an interval of drier climates. The present more arid regimes have apparently come to the Pacific slopes of Honduras and El Salvador since the late Pleistocene.

Once the Great Faunal Interchange had begun, Cen- tral America must have been the focus of many unique biological interactions between previously separated species. When giant ground sloths and toxodonts crossed paths and, more importantly, shared food resources with mastodons and mammoths, what really happened? We now have direct evidence that such encounters probably occurred at such late Pleistocene localities as Orillas del Humuya and Yeroconte. But the ecological subtleties of how they interacted are still too complex to solve on the basis of the available samples. In the Pleistocene, as now, Central America was the heartland of faunal mingling. We firmly believe that further paleontological investments in this area will pay valuable dividends toward our understanding of the history of the Great American Faunal Inter- change.

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origin and evolution of the cenozoic vertebrate fauna of middle america || late cenozoic vertebrates...

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