3Department of Geosciences, Texas Tech University, P.O. Box 4109, Lubbock, TX 79409

ABSTRACT

We describe the Terlingua local fauna, a rich assemblage of predominantly terrestrial microvertebrates from a locality in the Late Cretaceous Aguja Formation of Trans-Pecos Texas. Marine invertebrates (which include elements of both Cretaceous Western Interior and Gulf Coast zoogeographic provinces) from conformably underlying strata suggest that the fauna is of late Campanian age, probably correlative with Judithian assemblages of the Western Interior. A Judithian "age" for the fauna is further supported by its mammal and theropod assemblages, and by the faunas of overlying deposits. The previously reported diversity of the Aguja Formation, which we also summarize, is significantly enriched by this new fauna. The fauna also fills a major gap in the biogeography of Campanian terrestrial vertebrates.

Notable occurrences in the Terlingua local fauna include the therian mammal Gallolestes, previously known only from Baja California, and a hitherto unrecorded type of primitive 'tribothere.' At least 4 marsupial and 6 multituberculate taxa are present, several of which represent new taxa. Squamates comprise at least 10 taxa, including xenosaurs, necrosaurs, glyptosaurines, scincids, teiids, and a snake, several of which represent new taxa. In addition, the fauna includes at least 7 dinosaurs, 1 pterosaur, 2 crocodylomorphs, 3 turtles, 3 lissamphibians, 3 actinopterygians, and 8 chondrichthyans. Wood, amber, leaves, seeds, pollen, molluscs, and dinoflagellates are also preserved. The fauna is not strictly comparable to others from the Western Interior. It includes taxa that are either endemic or otherwise known only from relatively low latitudes, indicating an appreciable degree of latitudinal differentiation among Campanian terrestrial faunas bordering the Western Interior seaway.

INTRODUCTION

The North American record of Late Cretaceous vertebrates derives mainly from the central and northern Rocky Mountain region, where a relatively complete sequence is represented for Campanian and Maastrichtian time (see Clemens et al., 1979). Thus, much of the data fueling current controversies on diversity patterns approaching the Cretaceous-Tertiary boundary is based on collections from north of what is now roughly 44^o North latitude. Exceptions to this include Turonian through Campanian assemblages from southern Utah (Cifelli, 1990a-e; Cifelli and Eaton, 1987; Eaton, 1987; Eaton and Cifelli, 1988), Campanian and Maastrichtian faunas from the San Juan Basin, New Mexico (Armstrong-Ziegler, 1978, 1980; Clemens, 1973; Flynn, 1986; Rigby and Wolberg, 1987; Sullivan, 1981; Wolberg and LeMone, 1979), a small Campanian fauna from Baja California (Lillegraven, 1972, 1976; Morris, 1973), and several isolated occurrences (e.g., Krause and Baird, 1979).

We report here on a predominantly terrestrial fauna from a rich microvertebrate locality in the Late Cretaceous Aguja Formation of the Big Bend region of Trans-Pecos Texas (Fig. 1). This area lies at approximately 29^o North latitude and includes terrestrial sediments which record the Cretaceous-Tertiary boundary, and which have produced a diverse but as yet poorly known fauna. The Big Bend region is uniquely situated between the Cretaceous Western Interior and Gulf Coast zoogeographic provinces, and contains elements of both. The faunas of the Aguja and stratigraphically adjacent formations occupy a pivotal position in correlating biostratigraphic zonations developed for each province, and may eventually document Late Cretaceous faunal interchange between North and South America.

The microvertebrate assemblage discussed below rivals in diversity the well known Late Cretaceous faunas of Alberta, Wyoming, and Montana, and it adds substantially to the documented biotic diversity of the Aguja Formation as a whole. The locality has yielded the first Campanian therian mammals and squamates from Texas, and when more thoroughly studied we expect to name new species in these groups, and perhaps in others. The locality also produced wood, leaves, pollens, spores, molluscs, and dinoflagellates. The fauna is considerably richer than other southern faunas, such as that of the "El Gallo Formation" of Baja California (e.g., Lillegraven, 1976; Morris, 1973), and it exceeds the diversity of the 'Hunter Wash,' 'Fossil Forest,' and other local faunas of the Fruitland-Kirtland Formations of the San Juan Basin (Clemens, 1973; Armstrong-Ziegler, 1978, 1980; Flynn, 1986; Rigby and Wolberg, 1987). Because investigations are continuing and we expect that representation of most taxa will be greatly improved in the near future, we have refrained from naming new taxa, although several could be diagnosed on the basis of materials now in hand.

We refer to the fauna from the microvertebrate locality informally as the 'Terlingua local fauna.' The Aguja Formation includes both marine and terrestrial deposits, and it encompasses a considerable duration of Late Cretaceous time. Designating a local fauna avoids mixing references to non-comparable environments and significantly different time planes, and may aid the development of a biostratigraphic framework offering fine levels of stratigraphic resolution within the Aguja Formation. Because little has been mentioned in the literature of the many other important vertebrate fossils previously recovered from the Aguja Formation, we also present an uncritical summary of the approximately 70 vertebrate taxa from the Aguja Formation that have been reported in unpublished dissertations and theses (Davies, 1983; Lawson, 1972; Lehman, 1982, 1985a; Standhardt 1986), and elsewhere (e.g., Russell, 1988; Langston et al., 1989; Weishampel, 1990).

Data summarized below indicate that the Terlingua local fauna is temporally correlative with Judithian assemblages of the Western Interior. Though not yet adequately studied, it is evident that the Terlingua local fauna is not merely a southern extension of a more northerly Campanian fauna. It contains a number of distinctive elements that are either endemic or are restricted to this and other southern faunas.

Abbreviations for institutions cited in the text: OMNH, Oklahoma Museum of Natural History, Norman, Oklahoma; TMM, Texas Memorial Museum Vertebrate Paleontology and Radiocarbon Laboratory, University of Texas, Austin, Texas.

Geological Overview

The Aguja Formation comprises an eastward-thinning series of paralic and marine sandstones interbedded with shale and lignite deposited in a coastal plain environment. Two progradational cycles are represented in these deposits, the lower of which is present only in the western Big Bend region. Lehman (1985a,b, 1986, 1989) informally subdivided the Aguja Formation into several members (Fig. 2). The basal sandstone member is a progradational deltaic sandstone. It is overlain by the lower shale member, which consists of interbedded carbonaceous shale and lignite that accumulated in coastal marshes and swamps landward of the shoreline. A thin transgressive marine sandstone, the Rattlesnake Mountain sandstone member, overlies the lower shale and was deposited in a shallow marine shelf environment. A thin marine shale overlies the Rattlesnake Mountain sandstone. This shale is a westward-thinning tongue of the Pen Formation interposed within the Aguja, and is informally referred to as the McKinney Springs tongue. Overlying this marine shale is a second progradational deltaic sandstone, the Terlingua Creek sandstone member. This unit is extensive over the entire Big Bend region. Paleocurrent data indicate that the deltas which deposited the Terlingua Creek sandstone prograded eastward across this region, and that the paleo-shoreline trended roughly north-south (Lehman, 1985a,b, 1986). Overlying the Terlingua Creek sandstone is the upper shale member of the Aguja Formation. This unit consists of mudstone, carbonaceous shale, lignite, and lenticular sandstone deposited in coastal swamps and marshes, and in succeeding fluvial environments, landward of the advancing shoreline. Fluvial deposits of the Late Cretaceous Javelina Formation gradationally overlie the upper shale member of the Aguja Formation, followed by a thick sequence of terrestrial Tertiary sediments (e.g., Maxwell et al. 1967; Schiebout, 1974; Stevens, et al., 1969, 1984; Wilson, 1977, 1984, 1986; Wilson et al., 1968).

Fossil Recovery Methods

Recovery methods used at the site yielding the Terlingua local fauna are standard for microvertebrate collecting. Rock matrix was quarried at the locality, broken into golfball- to baseball-sized chunks, and removed from the site by hand. The matrix was processed by underwater sieving (e.g., McKenna, 1965), using fine-meshed (0.30 mm) screens. Although use of fine screens prolongs processing, the recovery of extremely small teeth, which otherwise would have been lost, justifies the extra investment in labor. Because the washed concentrate contained abundant residual carbon, the samples were subsequently separated with the heavy liquid zinc bromide (e.g., Reynolds, 1983), adjusted to a specific gravity of between 2.3 and 2.4, prior to picking concentrate under a dissecting microscope. Processing of samples collected in 1988 yielded approximately 17 mammal specimens per 100 kg of original rock matrix, a high level of productivity by Cretaceous standards.

THE TERLINGUA LOCAL FAUNA Locality

The microvertebrate locality (OMNH V58; TMM 43057) lies approximately 8 km NNE of Study Butte, Brewster County, Texas (Figs. 1, 2). Detailed locality information is on file at the Oklahoma Museum of Natural History and the Vertebrate Paleontology and Radiocarbon Laboratory of the University of Texas.

The locality is stratigraphically within the lower part of the upper shale member of the Aguja Formation, within several meters of the top of the underlying marine Terlingua Creek sandstone member (Fig. 2). The bone-producing horizon rests upon a distinctive, laterally continuous lignite that contains twigs and branches of carbonized wood, leaf material, amber, and angiosperm seeds. The microvertebrate-bearing bed is 13 to 18 cm thick, and consists of discontinuous stringers of fine (granule to small pebble) conglomerate separated by thin lenses of fine sandstone and clayey siltstone. The conglomerate stringers are the principal source of the microvertebrate fossils. Clasts in the conglomerate range up to 2 cm in diameter, although most are less than 1 cm. The clasts comprise clay balls, wood, coal, amber, coprolites, and abundant, disarticulated microvertebrate bones and teeth. The site also produces isolated larger bones, most notably isolated limb elements and vertebrae of a hadrosaur, and shell fragments of a huge trionychid turtle. Many bones in all size classes are broken. Some bones exhibit varying degrees of abrasion and rounded edges indicative of transport before burial. Branches of carbonized wood lacking preferred orientation are common at the top of the bone-producing horizon. Most of this wood contains mud filled Teredolites (shipworm) borings.

The bone-bearing deposit ranges up to 10 m wide, and represents deposition in a shallow channel with a sluggish current. Downcutting into the underlying lignite was not observed at the locality itself, but it does occur in similar, adjacent deposits at the same stratigraphic level. Overlying the channel are overbank deposits of clay-rich siltstone, typical of Lehman's (1985a) coastal floodplain facies.

Both the geology and faunal composition (see below) indicate a predominantly terrestrial source for the sediment and fossils preserved at the site. However, there is also evidence of significant marine influence on the faunal composition, which is not surprising given the stratigraphic proximity of marine sediments. Teredolites indicates a brackish or saline environment, and relatively direct communication to an open marine environment. There are eight sharks in the fauna (see below) that further support this interpretation, in that all eight taxa have also been recovered from marine sediments. Lepisosteids and amiids, also well represented in the fauna, are primarily freshwater taxa although both have high salinity tolerances and are consistent with an estuarine environment (Hoese and Moore, 1977; Bryant, 1987).

Vertebrate Collections from the Aguja Formation

The first significant collections from the Aguja Formation were made between 1938 and 1940 under the direction of William Strain (University of Texas at El Paso), who directed a Works Progress Administration paleontological project in west Texas. At nearly the same time, collections were made by Barnum Brown for the American Museum of Natural History, and by William McAnulty, Donald Savage, and Wann Langston, Jr. for the University of Oklahoma (for details see Lehman, 1985a, 1989; Langston et al., 1989; Davies and Lehman, 1989). From the 1960s through the 1980s, Dr. Langston assembled a large collection of Aguja vertebrates for the Vertebrate Paleontology Laboratory of the University of Texas, an effort carried on by some of his students (e.g., Davies, 1983; Lehman, 1982, 1985a, 1989). Most of this work focused on the megafauna, but a recent collaborative effort by the University of Oklahoma and University of Texas collected a large sample of Aguja microvertebrates that significantly increased the documented diversity of Aguja vertebrates (Cifelli and Rowe, 1990). Judith Schiebout and Barbara Standhardt of Louisiana State University made additional microvertebrate collections in the 1970s and early 1980s. The net result of these 50 years of vertebrate collecting in the Aguja is more than 1000 cataloged specimens which represent roughly 70 vertebrate species.

Although Aguja vertebrates have been known for more than half a century, few Aguja taxa have received comprehensive treatment in the literature. The first descriptions of these materials were by Brown (1942), followed by Colbert and Bird (1954) on the giant crocodilian Deinosuchus (= Phobosuchus) riograndensis. Apart from the description of a shark by McNulty and Slaughter (1972), only some of the Aguja archosaurs were subsequently described (Coombs, 1978; Davies, 1983; Lehman, 1982, 1985a, 1989). Much of the work carried out by Louisiana State University in the Aguja Formation was summarized in a dissertation (Standhardt, 1986), but apart from a faunal summary tabulated by Standhardt (in Langston et al., 1989), none of this work is published.

The Terlingua Local Fauna

We present below a list of taxa comprising the Terlingua local fauna, followed by a brief discussion of its major components. We adopted a conservative approach in taxon identification, taking names only to the levels permitted by material recovered from this locality. For example, although isolated ceratopsian teeth recovered from the locality probably represent Chasmosaurus mariscalensis, named from material collected elsewhere in the Aguja (Lehman, 1982, 1989, 1990a), we prefer to assign them to Ceratopsidae until more specific diagnostic material is identified in collections from the site. We also note that many identifications are based more on phenetic resemblances than on the presence of demonstrable synapomorphies, because phylogenetic analyses at alpha levels are not yet available for most of the taxa at issue. Thus, many of the names used below could be thought of as 'form taxa' rather than monophyletic entities. The major exception to this is in our discussion of the squamates, which benefits from the phylogenetic hypotheses developed by Estes (1983b), Estes et al. (1988), Gauthier (1982) and Gauthier et al. (1988). Taxa marked with an asterisk (*) represent first occurrences for the Aguja Formation. Voucher specimens are listed in Appendix 1 for all taxa discussed below.

Chondrichthyes
    Euselachii
        Hybodontoidea
            Hybodontidae
                *Hybodus sp.
            Hybodontidae, indet.
        Polyacrodontidae
                Lissodus (= Lonchidion) selachos
      Lamniformes
            Mitsukurinidae
                Scapanorhynchys sp.
    Batomorphii
        Rajiformes
            Sclerorhynchidae
                Onchopristis dunklei
            *Ischyrhiza avonicola
      Rajiformes incertae sedis
                Squatirhina americana
                Ptychotrygon sp.
    Batomorphii indet.
        Myliobatiformes
            Dasyatidae
Chondrichthyes indet.

Actinopterygii
    Lepisosteidae
        Amiidae
    Teleostei
        *Phyllodontidae
    Teleostei indet.

Lissamphibia
    Urodela
        Prosirenidae
            Albanerpeton sp.
        Scapherpetontidae
    Urodela indet.
        *Anura

Reptilia
    Chelonia
        Cryptodira
            Trionychidae
                "Aspideretes"
            Baenidae
                "Baena"
    Chelonia indet.

    Squamata
        *Anguimorpha
            *Anguidae
                *Glyptosaurinae
                    * Odaxosaurus sp.
                    * Proxestops sp.
                Glyptosaurinae indet.
            Anguidae indet.
            *Xenosauridae
                    *Restes sp.
        Varanoidea
            *Necrosauridae

    *Scincomorpha
        *Scincidae
            *Sauriscus sp.
        *Scincidae sp. nov. A.
        *Teiidae
        *Teiidae sp. nov. B
    *Serpentes
    Squamata indet.
    *Squamata sp. nov. X
    *Squamata sp. nov. Y

    Crocodylia
        Mesosuchia
            Goniopholidae
       Eusuchia
            Deinosuchus riograndensis

    *Pterosauria

    Dinosauria
        Ornithischia
            Ankylosauria
            Ornithopoda
            Hadrosauridae
            Ceratopsia
                Ceratopsidae
        Saurischia
            Theropoda
                Tyrannosauridae
                Maniraptora
                    Troodontinae
                        *cf. Troodon
                    Velociraptorinae
                        *cf. Saurornitholestes
                    Dromaeosaurinae
                        *cf. Dromaeosaurus
                Maniraptora incertae sedis.
                        *cf. Ricardoestesia

Mammalia
    Multituberculata
        *Neoplagiaulacidae
            Cimolomyidae
                *Cimolomys clarki
                Meniscoessus
                *Meniscoessus sp. nov.
            *Cimolodontidae
                    *Cimolodon cf. electus
                    *cf. Cimolodon
    Multituberculata incertae sedis
                    gen. et. sp. nov.
    Marsupialia
        *"Peradectidae"
                *Alphadon cf. A. wilsoni
                *Alphadon sp. nov.
                *Turgidodon sp.
        Pediomyidae
                *Pediomys cf. P. krejcii
    ?Eutheria
                *Gallolestes sp.
    Mammalia, incertae sedis ("metatherian-eutherian grade")
                *sp. nov.

Chondrichthyes

For convenience, we use the nomenclature of Cappetta (1987) for chondrichthyans, though we note that the monophyly of most of categories recognized below is not demonstrated. Eight chondrichthyan species were identified in the Terlingua local fauna (Fig. 3). Five are batoids and are represented by abundant isolated teeth, occasional denticles, and rostral spine fragments. These include two sclerorhynchid rajiforms, Onchopristis dunklei McNulty and Slaughter (1962) and Ischyrhiza avonicola Estes (1964), and two rajiforms of uncertain affinities, Squatirhina americana Estes (1964) and what may be a new species of Ptychotrygon (Fig. 3). The occurrence of Onchopristis dunklei is a range extension, being previously known no higher in stratigraphic position than the Cenomanian Woodbine Formation (McNulty & Slaughter, 1962; Slaughter and Steiner, 1968). Cappetta (1987) noted with skepticism reports of Onchopristis dunklei from the Upper Cretaceous (Campanian or Maastrichtian) of New Zealand and India.

The last batoid is an indeterminate dasyatid, represented by a single, large square tooth. Several fragmentary teeth in our collection may represent additional batoid taxa, but none is assignable to Myledaphus, a taxon common in the Fruitland-Kirtland faunas of the San Juan Basin (Armstrong-Ziegler, 1978, 1980; Wolberg and LeMone, 1979) and more northern faunas (e.g., Estes, 1964; Langston 1975, 1976; Brinkman, 1990).

Two of the remaining chondrichthyans are hybodontid sharks. Hybodus sp. (Fig. 3) is represented by abundant broken teeth, and probably also by the common fragments of hybodontoid cephalic and fin spines present at the site. The second hybodontoid is Lissodus (= Lonchidion) selachos (Estes, 1964), which is represented by several hundred isolated teeth (Fig. 3), many of which retain their bony bases. The final shark is a lamniform, Scapanorhynchus sp., represented by rare, broken but unabraded teeth. A number of unidentifiable chondrichthyan vertebrae and a few badly abraded teeth were also recovered. None of the chondrichthyans is sufficiently constrained temporally to provide useful information regarding the age of the Terlingua local fauna.

Actinopterygii

Actinopterygian remains are abundant in the Terlingua local fauna. All are disarticulated and most are difficult to identify. The distinctive scales and teeth of an indeterminate lepisosteid are common, and isolated vertebrae and skull fragments are present as well. Also present are vertebrae and teeth of an indeterminate amiid. The amiid Melvius is reported elsewhere in the Aguja (Boreske, 1974; Bryant, 1987). Numerous small, spheroidal teeth of phyllodontid teleosts are also recognizable components of the Terlingua material. It is likely that a greater diversity of teleosts is represented in the material now at hand, which includes hundreds of tiny isolated bones. The actinopterygians, though potentially interesting, offer no useful information on the correlation and age of the Terlingua local fauna.

Lissamphibia

Both frogs and salamanders are present in the Terlingua local fauna. There are at least two salamander species, of which the prosirenian Albanerpeton sp. (= Prodesmodon copei, in part, of Estes, 1964; see Estes, 1981) is the most abundant, being represented by more than 100 maxilla and dentary fragments. Albanerpeton is easily recognized by its non-pedicillate teeth and by a complex, interdigitating joint at the dentary symphysis (Estes, 1981). A wide size range of these elements is present in the large sample of jaws from Terlingua. The largest specimens are more than twice the size of the smallest specimens. Whether this is the manifestation of ontogenetic or taxonomic diversity is not yet known. A second salamander, probably a scapherpetonid, is represented by isolated vertebral elements. This may be the taxon that Standhardt (1986) identified from elsewhere in the Aguja as Scapherpeton tectum. Anurans, rare elements in the fauna, are represented by jaw fragments, distal ends of humeri, and partial ilia. The number and identity of the anuran taxa remain to be determined.

Albanherpeton, which ranges from the Middle Jurassic to the Middle Miocene (Estes, 1981), is uninformative regarding the age of the Terlingua local fauna. Without more specific identification, none of the lissamphibians is helpful in this respect.

Chelonia

Seven turtle taxa are reported from the Aguja Formation (Table 1), but only two were identified in the Terlingua local fauna. A large trionychid referable to the form genus "Aspideretes" is represented by numerous large shell fragments, and the rear half of a skull and possibly other shell fragments indicate the presence of a large baenid, which Hutchison (pers. comm.) assigns the form genus "Baena." We also recovered several isolated turtle dentaries that are from much smaller individuals and that may represent two or possibly three taxa. Although turtles provided significant stratigraphic resolution elsewhere in Late Cretaceous and Early Tertiary terrestrial sediments (e.g., Hutchison and Archibald, 1986), they are as yet too poorly known in the Terlingua local fauna to be informative in this regard.

Squamata

Until now, squamates were poorly represented in southerly Late Cretaceous faunas of North America (Estes, 1983a,b). Apart from a passing mention of indeterminate mosasaurs in marine facies (Maxwell et al., 1967), no squamates were previously reported in the Aguja Formation. Only four species are known in the Fruitland Formation (Armstrong- Ziegler, 1978, 1980; Sullivan, 1981), and only one is reported for the "El Gallo" Formation (Estes, 1983a). The Terlingua local fauna fills this important geographic gap with a diverse assemblage of squamates (Fig. 4). All of the material at this site is fragmentary, consisting of isolated cranial elements, jaw fragments, isolated teeth, and isolated scutes. Nevertheless, the material permits recognition of 10 taxa.

Anguimorpha - As in other Late Cretaceous and early Tertiary faunas from the Western Interior, the most common squamate elements are isolated osteoderms of glyptosaurine anguids. Also preserved are jaw fragments and isolated teeth with the characteristic square profile of Odaxosaurus sp. (= Pancelosaurus piger, in part, of Meszoely, 1970; = Peltosaurus piger of Estes, 1964; see Gauthier, 1982). These specimens are among the oldest known glyptosaurines; other Campanian occurrences are reported in the correlative Fruitland, Judith River, and "Mesaverde" Formations (Estes, 1964, 1983a; Sullivan, 1987), and from the eastern seaboard (Denton et al., 1991). Glyptosaurines are also well known from early Tertiary deposits of Wyoming and Montana (Meszoely, 1970; Gauthier, 1982; Sullivan 1982, 1987); the group became extinct near the end of the Oligocene. Scutes referable to the glyptosaurine Proxestops (see Gauthier, 1982) are also present in the Terlingua local fauna, though comparatively rare.

An additional anguid is represented by three jaw fragments. The teeth in these specimens are strongly recurved or bowed posteriorly, and the crowns are striated lingually but not laterally. The crowns are bicusped, with a small anterior lobe separated from the larger posterior lobe by a groove on both the lingual and lateral surfaces. Specimens such as these were commonly assigned to Gerrhonotinae in earlier literature (e.g., Armstrong-Ziegler 1978, 1980; Sullivan, 1981). However, Good (1988) recently argued that this tooth form is plesiomorphic for Anguidae, and that high variability among anguids in the others characters preserved in the fragmentary Terlingua material leave the specimens assignable to Anguidae, but not to a more specific level.

In addition to the three anguids, two other anguimorphs are present, though comparatively rare. One is a xenosaur, represented by a maxilla with teeth that have a weak anterior cusp that is set off from the large posterior cusp by a weak groove on the lateral surface. We refer this specimen to Restes (= Exostinus; Gauthier, 1982) sp. The second non-anguid anguimorph is provisionally referred to Necrosauridae. It is known from a few jaw fragments with pleurodont teeth that are laterally compressed and that have the characteristic infolded enamel of varanoids. All teeth are broken above their infolded bases. Several osteoderms in our collection may also represent this taxon. Both xenosaurs and necrosaurids are reported from Maastrichtian deposits elsewhere (Estes, 1983a; Sullivan, 1987); their occurrence in the Terlingua local fauna is the first report from Campanian sediments. Older anguimorphs were described by Winkler et al. (1990) from several localities in Early Cretaceous (Commanchean) sediments of Texas. None of the Terlingua anguimorphs is sufficiently constrained temporally to offer useful information on the age of the fauna.

Scincomorpha - Three scincomorphs are now known in the Terlingua local fauna. Two are referable to Scincidae (Fig. 4), extending the distribution of Scincidae into the Campanian for the first time in North America. As Estes (1983a) and Sullivan (1987) argued, earlier reports of Campanian scincids (Russell, 1975, 1982) were in error. The most common squamate in the Terlingua local fauna, next to the glyptosaurines, is a large scincid with a highly distinctive dentition, which we refer to informally as species A. It is represented by a number of jaw fragments and isolated teeth. A peculiar feature of the dentary is that the Meckelian groove is enclosed to form a tube by fusion of the dentary along its ventral edge, from very near the symphysis posteriorly to near the level of the coronoid. The splenial is absent from the tooth-bearing portion of the mandible. The most comparable dentaries are in other scincids, although similar features also occur in dibamids, pygopodids, gekkonids, xantusiids, in some gymnophthalmids, iguanines, and amphisbaenians, evidently arising independently in each case (Estes et al. 1988; Ethridge and de Queiroz, 1988). The teeth in the Terlingua material are similar to the large, bulbous teeth of the extant scincid, Tiliqua scincoides. However, the Terlingua taxon is seemingly unique in that the teeth occluded to produce on each tooth a pair of crescentic wear facets, separated by a transverse ridge. Teeth lying near the front of the mouth are compressed antero-posteriorly and are considerably smaller than the more posterior, 'molariform' teeth. They also developed wear facets. In their functional arrangement and degree of heterodonty, these teeth are reminiscent of the massive dentitions of the Late Cretaceous teiid Peneteius (Estes, 1969), and to a lesser degree the polyglyphanodontines (Estes, 1983a), both of which are absent from the Terlingua local fauna.

The second scincid is referable to Sauriscus sp., and is represented in the fauna by several broken jaws. It has tooth crowns that bear weak striations lingually and marked striations laterally. The teeth appear somewhat different from previously described species in having crowns that are only incipiently divided into three weakly defined lobes; in other species the crowns are markedly tricusped (e.g., Estes, 1964). Both of the Terlingua scincids represent new taxa.

In addition to the two scincids, at least one teiid occurs in the Terlingua local fauna, which we refer to informally as species B. It is represented by broken jaw fragments and isolated teeth in which there is a widely open Meckelian fossa (indicating a large splenial), extensive deposition of cementum around the bases of the teeth, and a well developed subdental shelf and sulcus dentalis (Estes, et al. 1988). The Terlingua taxon differs from other Cretaceous teiids in having teeth that are weakly tricusped. The crowns are smooth laterally, weakly striated lingually, and faintly defined anterior and posterior cusps are set off from the main crown by a shallow lingual groove. Teiids have an extensive Cretaceous record in North America. Winkler et al. (1990) tentatively referred material to Teiidae from the Commanchean (Early Cretaceous) of Texas, and there is a diversity of named teiids from the Late Cretaceous. Chamops segnis and Leptochamops denticulatus are known from the correlative Fruitland Formation (Armstrong-Ziegler 1978, 1980; Estes, 1983a; Sullivan, 1987). Kequin and Fox (1991) recently described five new teiid species in the correlative Oldman Formation, and they list eight additional Lancian teiids from North America. Like the anguimorphs, the Terlingua scincomorphs are currently of little use in refining the age of the Terlingua local fauna.

Serpentes - A snake is present in the Terlingua local fauna. One tiny jaw fragment preserves one recurved tooth that is set into a shallow socket, and on either side is an additional shallow socket. The tooth has a circular base, is strongly recurved, and lacks serrations. Armstrong-Ziegler (1978, 1980) described a snake, which she identified as an aniliid, from the Campanian Fruitland-Kirtland Formations (Armstrong-Ziegler, 1978, 1980). The specimen was later identified as a boid (Rage, 1984; Estes and Baez, 1985; Sullivan, 1987). Apart from its being a snake, little can be said of the identity of the Terlingua specimen at present.

Squamata indet. -- Two additional lizards, which we refer to informally as species X and species Y, are assignable at present only as indeterminate members of Squamata. In the former, the teeth are solidly ankylosed to the jaws and the crowns are blunt and rounded. The inframandibular septum was evidently long and relatively slender. Species Y has distinctive teeth that have smooth, unicusped crowns, and there is a very narrow dental gutter. Few other details of morphology are available for either taxon at present.

Archosauria

Crocodylia - At least two crocodilians occur in the Terlingua local fauna. These are Deinosuchus (= Phobosuchus) riograndensis and an indeterminate goniopholid. The Terlingua goniopholid may prove to be Goniopholis cf. G. kirtlandicus, which is known elsewhere in the Aguja (Lehman, 1985a), but available material is too fragmentary to permit more specific identification. Standhardt (in Langston et al. 1989) referred isolated teeth from the Aguja to Leidyosuchus and Brachychampsa, but we found no record of either in the Terlingua local fauna. We also note that Dr. Langston (pers. comm.) doubts that either taxon can be diagnosed by any of the fragmentary material currently known from the Aguja.

Dinosauria: Ornithischia - Three ornithischians occur in the Terlingua local fauna, the most common of which is an indeterminate hadrosaur represented by isolated teeth, ossified tendons, isolated limb elements, and vertebrae. This material includes very young and adult individuals. Both Kritosaurus (= ?Gryposaurus of Weishampel and Horner, 1990) cf. K. navajovius (Lawson, 1972, Davies, 1983) and an indeterminate lambeosaurine (Davies, 1983; Weishampel and Horner, 1990) are reported from elsewhere in the Aguja. Ceratopsians are also present, but in less relative abundance than the hadrosaurs. Chasmosaurus mariscalensis (Lehman, 1989), described from material discovered elsewhere in the Aguja, is in all likelihood the Terlingua ceratopsian, but diagnostic material has yet to be recovered from the locality. The last ornithischian recorded is an ankylosaur, known only from osteoderms. None of the ornithischian remains recovered from the Terlingua locality offers any refinement of the age of the Terlingua local fauna, although specimens from elsewhere in the Aguja support our Judithian assignment (see below).

Theropoda - Theropods are also present, although only isolated teeth and a few isolated bones and bone fragments have yet been recovered. We followed the criteria described by Currie et al. (1990) in identifying isolated teeth from the Judith River Formation. The Terlingua local fauna includes at least one large tyrannosaurid, and four smaller maniraptorans (Fig. 5) which we provisionally refer to cf. Dromaeosaurus, cf. Saurornitholestes, cf. Ricardoestesia, and cf. Troodon. Standhardt (1986) identified Paronychodon lacustris in the upper Aguja, which may be what we refer to as cf. Saurornitholestes (see Currie et al., 1990). There are several other teeth that do not easily fit into any of those categories but which generally resemble teeth of the small, bird-like theropods.

The general similarity of the Judith River theropod assemblage to that of the Terlingua local fauna may offer further corroboration of a Judithian assignment for the latter. As Currie et al. (1990) point out, however, small theropod teeth are common in most Late Cretaceous faunas, and the distributions of these recently recognized taxa are not yet well documented.

Pterosaurs - Several crushed, incomplete bones indicate the presence of pterosaurs in the Terlingua local fauna, although specimens now in hand are too fragmentary for more specific identification.

Mammalia

Order and Family Uncertain, gen. and sp. indet. - A presumably new taxon is represented by several lower molars. These teeth (Fig. 6) bear fully developed, tricuspid, basined talonids, and thus belonged to a tribosphenic therian in the sense of Crompton (1971). Beyond this, the affinities of the species are uncertain. Distinctive marsupial or eutherian characters are lacking, and for convenience we refer to the taxon informally as a therian of "metatherian-eutherian grade" (Patterson, 1956; Kielan-Jaworowska et al., 1979) or "tribothere" (Clemens and Lillegraven, 1986), though we recognize that neither term refers to a monophyletic unit. The species is small, with teeth slightly smaller than those of Alphadon wilsoni Lillegraven, 1969. The position(s) of the referred teeth are unknown. All specimens have rounded, posteriorly narrowing talonids, suggesting that they may be ultimate lower molars, but this may simply be a characteristic of all lower molars of the taxon. The trigonid is broadly open lingually, with the paraconid, subequal to the metaconid in size, anteriorly placed, but not so lingual as is generally seen in marsupials (Clemens, 1979). The cristid obliqua attaches anteriorly to the base of the trigonid in a median position; a distal metacristid is lacking. The hypoconulid is in a median position, not shifted lingually as is generally true of marsupials (Clemens, 1979; Clemens and Lillegraven, 1986; Cifelli and Eaton, 1987). A labial postcingulid, also characteristic of early marsupials but seen in some Cretaceous eutherians as well (Cifelli, 1990b), is lacking.

The affinities of this taxon are puzzling. Differences in height and width of the trigonid and talonid are less pronounced than in presumably primitive Tribosphenida such as Kermackia texana (cf. Butler, 1978). The lack of a distal metacristid is also a derived condition within Tribosphenida (Fox, 1975), perhaps suggesting a relationship to certain "advanced" Theria of metatherian-eutherian grade, marsupials, and eutherians (Cifelli, 1990e). However, the unreduced, anteriorly placed paraconid is unlike that typical of Cretaceous Eutheria. Likewise, the median position of the paraconid, lack of a labial postcingulid, and lack of entoconid to hypoconulid twinning differ from conditions seen in early presumed marsupials or marsupial-like taxa (Cifelli, 1990c,d). An unusual tribosphenic upper molar, almost certainly too small for this species, is tentatively referred below to Gallolestes.

Multituberculata - Multituberculates are by far the most common mammals at the Terlingua locality, with more than 400 specimens recovered to date. The majority of these are incisors and premolars, and many of the larger teeth are fragmented. With three exceptions, the specimens are isolated teeth.

Meniscoessus is the most common multituberculate. Nine lower fourth (ultimate) premolars are now recovered, but none is intact. These show size and structural diversity that suggest the possibility of more than one taxon, though the small sample size leaves doubt on this point. Complicating identification of this material are upper and lower first molars, clearly referable to Meniscoessus, but which do not show divergence like that of the premolars. The Terlingua specimens most closely resemble M. major, but are smaller than known specimens of that taxon. We refer to these as Meniscoessus sp. nov. elsewhere in this paper.

Cimolodon is known from four entire and several fragmentary lower fourth premolars. These are nearly identical in size and structure to those of C. electus from the Milk River Formation (Fox, 1971). Specific identification of the Terlingua Cimolodon is complicated, however, by the presence of upper fourth premolars that are also assignable to Cimolodon (Fig. 6). In most aspects of morphology they more closely resemble C. nitidus than C. electus, but they are smaller than any known specimens of C. nitidus. We provisionally refer to all of these teeth as Cimolodon cf. electus.

Also provisionally referred to Cimolodon are several molars showing an unusual, complex pattern of pitting that creates reticulated ridges on the occlusal surfaces. Teeth of Essonodon also share this feature, but Essonodon has a more regular pattern of occlusal pits and ridges, and the upper second molars have a much lower length to width ratio than is seen in the Terlingua material. Eaton (1987) described similar teeth from the Kaiparowits plateau of southern Utah, arguing that, with the exception of the pitting, the teeth most closely resemble Cimolodon. We follow Eaton's conclusions by provisionally referring to the Terlingua material as cf. Cimolodon.

Cimolomys clarki Sahni, 1972, also occurs in the Terlingua local fauna (Fig. 6), as indicated by two upper first molars, one of which is fragmentary. This occurrence is of significance in evaluating the age and correlation of the Terlingua local fauna in that C. clarki is known exclusively from Judithian faunas (Lillegraven and McKenna, 1986). This supports the more compelling marine invertebrate data (see below) which indicate the Terlingua local fauna to be of Judithian "age."

Two additional multituberculate taxa occur in the Terlingua local fauna. One is based on two upper first molars that are small, with low length to width ratios, and that have distinctive, square-based pyramidal cusps. These features indicate a new taxon endemic to the Terlingua local fauna, but one whose more general affinities are uncertain. For the moment, we refer it to Multituberculata incertae cedis, gen. et sp. nov.

The last multituberculate is based on a number of tiny second molars. These teeth closely resemble Mesodma, Cimexomys, and Paracimexomys, but second molars alone do not permit discrimination among these taxa. We refer them to an indeterminate position within Neoplagiaulacidae.

Marsupialia: "Peradectidae" - We follow the usage of Reig et al. (1987) in referring generally primitive Cretaceous marsupials to "Peradectidae," thereby permitting Didelphidae, formerly used for this purpose, to be diagnosed on the basis of presumed synapomorphy (Cifelli, 1990a,d). We acknowledge, however, that this practice perpetuates, rather than eliminates, usage of "wastebasket" taxa. Remains of peradectid marsupials are abundant in the assemblage, but sample sizes are currently insufficient to identify or circumscribe any species reliably. Present evidence suggests the presence of two small species, herein placed in the genus Alphadon, and at least one large species, probably referable to Turgidodon (genera as defined and diagnosed by Cifelli, 1990a).

Alphadon sp. - This taxon, the smaller of the two Aguja species referred to Alphadon, is represented by several upper molars (Fig. 6); lower molars have not been positively identified. Morphologically, this species is typical of the genus Alphadon. It is distinct in having small, sharp cusps C and D on the stylar shelf and, especially, in being noticeably smaller than other known Cretaceous marsupials. For example, the average antero-posterior length (A-P length as defined by Lillegraven, 1969) for M2 is 1.39 mm; averages for M2 of other small species are: Iqualadelphis lactea, 1.70; Alphadon wilsoni, 2.1; and A. attaragos, 1.83 (data from Cifelli, 1990a and Lillegraven, 1969).

Alphadon cf. A. wilsoni - This species is the most abundant therian in the fauna. It is closely comparable to Alphadon wilsoni Lillegraven, 1969, and especially resembles that species in that stylar cusp C is well-defined and conical (Fig. 6). However, the Aguja material is slightly more than one standard deviation smaller than Alphadon wilsoni (cf. Lillegraven, 1969, pp. 36-37), suggesting that it may prove to be a distinct taxon as our sample improves.

Turgidodon sp. - A third peradectid taxon, currently known only from molar fragments, indicates an unidentified species of Turgidodon, identified by large size and by robust, inflated tooth cusps and heavy wear (Cifelli, 1990a). It is approximately the size of the Judithian Turgidodon russelli and T. madseni, but is insufficiently known to make detailed comparisons.

Pediomyidae: Pediomys cf. P. krejcii - One small fragment of an upper molar records a tribosphenic therian with a reduced anterior stylar shelf. Recognizing the difficulties involved in interpreting the affinities of such taxa (see Fox, 1987), we refer the species tentatively to Pediomys. The species is small, comparable in size to Lancian P. krejcii Clemens, 1966; as in that species, cusp D is small and C is apparently lacking.

?Eutheria incertae sedis: Gallolestes sp. This small species (average A-P length for lower molars = 1.44) resembles Gallolestes pachymandibularis, from the Campanian "El Gallo Formation," Baja California (Lillegraven, 1972, 1976). The Aguja species is known by lower molars (Fig. 6); a poorly preserved upper molar may belong to it, but cannot be assigned to G. pachymandibularis with confidence. As in G. pachymandibularis, the entoconid is placed lingually and the paraconid is low and placed well anterior and labial to the metaconid. In addition to its smaller size, the Aguja species is distinct in having less robust molars, less height differential between protoconid and metaconid, and a deeper labial ectoflexid dividing the trigonid and talonid. An upper molar lacking most of the enamel possibly belongs to this species, based on size (upper molars of Gallolestes are otherwise unknown) and its dissimilarity to other tribosphenic therians from the fauna (i.e., it does not belong to a marsupial and appears to be too small for the metatherian-eutherian grade taxon noted above). The stylar shelf, which appears to have lacked cusps in the C to D positions, is modestly developed, except that an enormous parastyle is present, giving the tooth the appearance of an ultimate upper molar. The metacone is well-developed, a point of difference from ultimate upper molars of marsupials and certain Late Cretaceous eutherians, such as Cimolestes. The tooth most resembles ultimate upper molars of other Eutheria, such as Procerberus formicarum; however, it may be a penultimate molar of a more primitive taxon (cf. Fox, 1975).

The affinities of Gallolestes are problematic. Lillegraven (1976) referred the genus to Eutheria, but Clemens (1980), more conservatively, considered it simply as Theria incertae sedis (see also Clemens and Lillegraven, 1986). More recently, Nessov (1985) returned Gallolestes to Eutheria, placing it in the otherwise Asiatic suborder Myxotheridia. However, the trigonid configuration does not resemble that of any other Cretaceous mammal, although the reduction of the paraconid is suggestive of Eutheria. Evidence provided by the Aguja materials is inconclusive; if the upper molar is correctly referred and if it represents M3, there is some suggestion of relationship to Late Cretaceous Eutheria.

In summary, the Terlingua mammals include at least one strictly Judithian element, Cimolomys clarki (Fox 1971; Lillegraven and McKenna, 1986), and the remaining taxa are all consistent with a Judithian assignment.

Summary of Aguja Vertebrates

We present in Table 1 an uncritical listing of all taxa reported from the Aguja Formation by previous researchers, with citations of the original reports, and we have included the Terlingua local fauna. The Aguja Formation includes marine, terrestrial, and intermediate deposits. It appears to span a considerable length of time, including most of the Campanian and perhaps the early part of the Maastrichtian (see below). This list mixes taxa that may have been separated by several million years and that lived in different environments. Table 1 should thus be viewed more as an entrant into the literature on Aguja vertebrate paleontology than as a verified tabulation of the Aguja biota.

DISCUSSION

Age of the Terlingua Local Fauna

Because radiometric dates have not been reported from the Upper Cretaceous section in the Big Bend region, dating of these deposits rests entirely on correlation of the vertebrate and invertebrate faunas, and on limited paleomagnetic stratigraphy. The Big Bend region is uniquely situated between the Cretaceous Western Interior and Gulf Coast marine zoogeographic provinces, and allows correlation of the biostratigraphic zonations developed for each province. Marine invertebrates from the intertonguing Pen and Aguja formations provide a lower limit on the age of the Terlingua local fauna assemblage; continental vertebrates from the overlying Javelina Formation provide an upper limit.

The main body of the Pen Formation lies within the lower Campanian Delawarella delawarensis zone, based on the occurrence of this Gulf Coast and Tethyan zonal ammonite, as well as Menabites belli, Submortoniceras vanuxemi, S. mariscalense, and S. chicosense (Young, 1963). The upper part of the Pen Formation may extend into the overlying Delawarella sabinalensis zone, based on a single occurrence of this ammonite (Young, 1963). However, the most common ammonite in the Pen Formation, and in the paralic basal sandstone member of the Aguja Formation, is Placenticeras syrtale, known elsewhere from the lower Campanian of both the Gulf Coast and the Western Interior. Placenticeras meeki also occurs in the Pen Formation, the McKinney Springs tongue of the Pen Formation, and all sandstone members of the Aguja Formation, but this is a wide-ranging species known elsewhere from throughout the Campanian and Maastrichtian (e.g., Reeside, 1927). The lower Campanian Gulf Coast ammonite Parapuzosia paulsoni occurs in the Rattlesnake Mountain sandstone member of the Aguja Formation. Several fragments of Hoplitoplacenticeras sp. are now known from the Terlingua Creek sandstone and, more importantly, poorly preserved specimens of Baculites maclearni are known from all sandstone members of the Aguja Formation. Baculites maclearni is a zonal index for the base of the upper Campanian in the Western Interior (Kauffman, 1977), and is the highest ammonite zone recorded in the Aguja. Lillegraven and Ostresh (1990) recently placed the Aguja above the Baculites gregoryensis zone (their Zone 16), whereas the data available to us indicate it to lie no higher than the Baculites maclearni zone (zone 11). Collectively, the ammonites indicate that the Pen Formation and the lower part of the Aguja Formation, up to and including the Terlingua Creek sandstone member, are Campanian in age. The lower/upper Campanian boundary probably lies at or above the level of the Rattlesnake Mountain sandstone member.

Gryphaeid, ostreid, and inoceramid bivalves provide a useful supplement to the correlation based on ammonite zonation. Exogyra ponderosa ponderosa is abundant in the main body of the Pen Formation and in the lower part of the McKinney Springs tongue of the Pen Formation. In the Gulf Coast, E. ponderosa ponderosa is known from lower and upper Campanian strata, up to the zone of Hoplitoplacenticeras vari (Young, 1963). Exogyra ponderosa upatoiensis is also known from the main body of the Pen Formation; it is not known to occur elsewhere above the zone of Delawarella delawarensis (Young, 1963). E. ponderosa erraticostata also occurs throughout the Pen Formation, but it is a temporally wide-ranging subspecies. The oysters, Flemingostrea pratti and F. subspatulata, are abundant in the Rattlesnake Mountain sandstone member of the Aguja Formation, and also occur in the Terlingua Creek sandstone member. Flemingostrea pratti is known from upper Campanian, and F. subspatulata from upper Campanian and lower Maastrichtian, strata of the Gulf and Atlantic coasts (Wolleben, 1977; Sohl and Christopher, 1983). The oyster Crassostrea cusseta is locally abundant in the basal sandstone and Rattlesnake Mountain sandstone members; this species is known elsewhere only from middle Campanian strata of the Gulf Coast (Sohl and Kauffman, 1964). Inoceramus vanuxemi, I. barabini, and I. oblongus occur abundantly in the McKinney Springs tongue of the Pen Formation, suggesting an early late Campanian age for these strata. Hence, oyster and inoceramid faunas also indicate a Campanian age for the Pen and for the lower, marine part of the Aguja formation, with the lower/upper Campanian boundary falling perhaps at the level of the Rattlesnake Mountain Sandstone member.

Continental Cretaceous strata above the Terlingua Creek sandstone member of the Aguja Formation must at present be correlated on the basis of their vertebrate faunas. The theropod and mammal assemblages in the Terlingua local fauna suggest a Judithian assignment, and all of the remaining Terlingua taxa are consistent with this position. The dinosaurs Chasmosaurus, Kritosaurus, Panoplosaurus, and ?Stegoceras are known by specimens from higher levels in the upper shale member, and also suggest a Judithian or "Edmontonian" age for these beds (Lehman, 1989). The overlying Javelina Formation bears dinosaurs (Alamosaurus, Torosaurus, Tyrannosaurus) and mammals considered to be Lancian (middle to late Maastrichtian) in age (Lawson, 1976; Standhardt, 1986; Lehman, 1990b). Limited paleomagnetic stratigraphy suggests that the base of the Javelina Formation must lie at least as low as polarity chron C30N (mid-Maastrichtian; see Standhardt, 1986; Lehman, 1990b).

In sum, the combined evidence now available suggests that the Terlingua local fauna, which lies at the base of the upper shale member of the Aguja Formation, is late Campanian in age. This fauna appears to be correlative with Judithian Land Mammal "age" faunas known from more northerly parts of the Western Interior, although its composition is considerably different from the typical Judithian faunas upon which the Land Mammal "age" was established. It is also likely that the Aguja Formation as a whole records a considerable expanse of time, and that the Aguja section lying stratigraphically above the Terlingua locality may span Judithian through the beginning of Lancian time.

Relationship with Other Southern Late Cretaceous Terrestrial Faunas

The Terlingua local fauna is not strictly comparable to any other known Cretaceous local fauna, though it does include elements known elsewhere from the Western Interior. Comparison of the Aguja microvertebrate locality with faunas of the Fruitland-Kirtland Formations of the San Juan Basin are problematic, in part because the age of the Fruitland-Kirtland Formations has been controversial, and in part because their fauna is different in many regards from the classic northern faunas. Like the Aguja Formation, the Fruitland-Kirtland Formations probably span a considerable expanse of time. Most recent authors concluded that the Fruitland-Kirtland faunas are of "Edmontonian" age (Lillegraven and Ostresh, 1990; see also Clemens, 1973; Clemens et al., 1979; and Lillegraven and McKenna, 1986; Kequin and Fox, 1991), but data from ammonite stratigraphy and radiometric dates suggest that the best-known mammal faunas, those from the Hunter Wash and Fossil Forest localities, are Judithian, even though they do not resemble the typical northern Judithian assemblages. These localities lie along the southwestern edge of the San Juan Basin, in the stratigraphically lowest part of the Fruitland-Kirtland Formations. Overlying the localities is a well-dated ash bed that yields an age of 72 to 75 Ma (Brookins and Rigby, 1987), which is about the age of the Didymoceras cheyennense zone. The highest ammonite beneath the localities is Baculites scotti, known from the underlying Pictured Cliffs Sandstone. Hence, the Hunter Wash and Fossil Forest mammal faunas are bracketed between the B. scotti and D. cheyennense zones, which are equivalent in age to the Judithian faunas of Montana and Canada (e.g., Goodwin and Deino, 1989); stratigraphically higher parts of the Fruitland-Kirtland Formations probably extend into the Edmontonian and perhaps also Lancian. These conclusions are further supported by Flynn's (1986) comparison of the Hunter Wash mammals with northern Late Cretaceous assemblages. Flynn noted that no Lancian species is present in the Hunter Wash fauna, and that the latter's closest resemblance is to the Pre-Lancian Judith River and Milk River assemblages that define the Judithian "age."

If our preliminary identifications of mammals bear up under more detailed comparisons, the Terlingua and Hunter Wash/Fossil Forest local faunas share Alphadon cf. wilsoni and Cimolodon cf. electus (Flynn, 1986); the former taxon is also reported in the Fossil Forest local fauna (Rigby and Wolberg, 1987). Both taxa occur in northern faunas (Sahni, 1972; Fox 1971); their occurrence in the Terlingua local fauna is southern range extension.

Apart from ceratopsians, which are different in the two faunas, the archosaurian components are not well known in either case. Several ceratopsians are tentatively reported in older literature from the Fruitland-Kirtland Formations, but all diagnosable specimens recovered from the Hunter Wash Fossil-Forest stratigraphic interval are referable to Pentaceratops sternbergii (Rowe et al., 1981; Lehman, 1990a). Its close relative, Chasmosaurus mariscalensis, is the only diagnosable ceratopsian yet recognized in the Aguja (Lehman, 1982, 1989, 1990a) and is probably the ceratopsian represented in the Terlingua local fauna. The remaining vertebrates of the New Mexican faunas are too poorly known to afford informative comparisons.

The best represented Late Cretaceous fauna from the southern part of the Western Interior is that of the Kaiparowits Formation, southern Utah, whose mammals are now becoming well-known (Eaton, 1987; Eaton and Cifelli, 1988; Cifelli, 1990a-e). The Kaiparowits assemblages, distributed through 350 m of section, span a considerable period of time and, although quite distinct from more northerly local faunas, almost certainly include part or perhaps all of the Judithian. The Terlingua fauna shares with the Kaiparowits assemblages a new multituberculate taxon, which we referred to above as cf. Cimolodon, following Eaton's (1987) suggestion for the Kaiparowits material. No other mammalian species have yet been recognized that are shared by the Kaiparowits and Terlingua faunas, but this will bear reassessment as the faunas are more thoroughly studied. The non-mammalian vertebrates are not yet adequately studied to afford informative comparisons.

The "El Gallo Formation" of Baja California produced the only other Late Campanian non-marine fauna known at present (Morris, 1973; Lillegraven, 1972, 1976; Clemens et al., 1979). In the "El Gallo" fauna is the only other occurrence of Gallolestes. The "El Gallo" and Terlingua faunas also share the wide ranging taxa Pediomys and possibly Mesodma, but neither taxon is adequately known in these southern faunas to permit more specific comparisons. Absent from the Terlingua fauna are polyglyphanodontine squamates, which occur in the "El Gallo" and many northern localities (Estes, 1983a).

Campanian Faunal Differentiation

The composition of the Terlingua local fauna corroborates previous suggestions that latitudinal biotic differentiation developed along the Interior Seaway, and that a biogeographically distinct southern terrestrial faunal existed during the Campanian (Clemens, 1973; Clemens et al., 1979; Flynn, 1986; Eaton, 1987; Lehman, 1987; Wolfe and Upchurch, 1987; Lillegraven and Ostresh, 1990). This is not surprising in light of ample evidence of a distinct southern marine biota at this time in the Western Interior Seaway (e.g., Sohl, 1971; Kauffman, 1977, 1984; Nicholls and Russell, 1990).

Among mammals, the ?eutherian Gallolestes is known only in the two southern-most localities. The distinctive multituberculate that we refer to as cf. Cimolodon, which is present in the Terlingua and Kaiparowits faunas (Eaton 1987), has not been found anywhere north of the Kaiparowits Plateau. In this context, it is also notable that pediomyid marsupials are rare and stagodontids appear to be absent in the Terlingua fauna, as is the case with other southern faunas (e.g., Clemens, 1973; Eaton and Cifelli, 1988). Further demarcating southern from northern faunas are endemic multituberculate and marsupial species, which occur in all of the southern faunas except the poorly known "El Gallo."

The Terlingua reptiles also show differences from northern faunas. Four new, endemic squamate taxa occur in the Terlingua fauna, and the only known Campanian snakes occur in the Terlingua and Fruitland-Kirtland faunas. Among archosauromorphs, the ceratopsians of southern faunas are also distinct from northern faunas. Pentaceratops sternbergii (Rowe et al., 1981) and Chasmosaurus mariscalensis (Lehman, 1982, 1989, 1990a) are endemic to the Fruitland-Kirtland and Terlingua faunas, respectively. Notably absent from the archosauromorph component of the Terlingua fauna, and from the Aguja in general, are champsosaurs. They are reported in the San Juan Basin (Wolberg and LeMone, 1979) and are abundant in more northern faunas (Estes, 1964; Langston, 1975, 1976; Brinkman, 1990).

Collectively, the vertebrates of the Terlingua local fauna indicate an appreciable degree of latitudinal differentiation among contemporaneous faunas bordering the Cretaceous interior seaway. The full significance of Late Cretaceous faunal differentiation will become clear only after the faunas become known in greater detail.

ACKNOWLEDGMENTS

We are indebted to Dr. Tony Runkel, Mr. Ray Kaptijna, and Mr. Arnold Rogers for showing us the study area, and Mr. Wayne Swoboda for providing access to the site. We also thank Welton and Patty Fiedler for their warm hospitality during our fieldwork. Many colleagues helped us to identify the Aguja fossils and with the considerable labor that went into their recovery, including Bud Barnett, Gorden Bell, William Clemens, Lowell Dingus, Richard Estes, Jacques Gauthier, Elizabeth Gordon, Howard Hutchison, Wann Langston, Beth Larson, Jay Lillegraven, Ernest Lundelius, David Polly, Amy Sheldon, Rick Toomey, Hillary Tulley, and Jack Wilson. For generous access to collections, we thank Drs. Richard Fox, Jeff Eaton, Jay Lillegraven, Bill Clemens, and Howard Hutchison. This project was supported by the following: NSF BSR 8906992, Petroleum Research Fund 20311-G8 (to RLC), and NSF BSR 8958092 and Petroleum Research Fund 21470-G8 (to TBR). We also thank Bill R. Payne, the Owen Coates fund, and the Lowe fund of the Geology Foundation of the University of Texas for supporting this research.

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FIGURE CAPTIONS

FIGURE 1-- Map of western North America showing the approximate locations of Judithian terrestrial vertebrate faunas.

FIGURE 2-- Stratigraphic relationships of the Pen and Aguja formations through Big Bend National Park, including informal units designated by Lehman (1985a, 1989). The shaded area of the map, including the approximate location of the microsite, is Brewster County, Texas.

FIGURE 3-- Chondrichthyans of the Terlingua local fauna. A) Scapanorhynchus sp. (TMM 43057-301) in lingual and labial views.

B) Lissodus sp. (TMM 43057-302), tooth crown in labial, occlusal, and lingual views. C) Hybodus sp., (TMM 43057-321) in labial and lingual views. D) Onchopristis dunklei (TMM 43057-305) tooth in labial, mesial, and lingual views. E) Squatirhina americana (TMM 43057-297) in labial, mesial, and lingual views. F) Ptychotrygon sp. (TMM 43057-304) in labial, mesial, and lingual views. G) Ischyrhiza avonicola (TMM 43057-306) tooth in labial, mesial, and lingual views.

Figure 4-- Scincids of the Terlingua local fauna. Scincidae sp. nov. (TMM 43057-286), right dentary in lateral (A) and lingual (B) views, and details of posterior dentary tooth crowns in (C) occlusal and (D) lingual views. Sauriscus sp. (OMNH 25347), maxilla in lateral (E) and medial (F) views, and (G) details of tooth crowns in labial view.

Figure 5-- Small theropods of the Terlingua local fauna. Cf. Saurornitholestes, tooth in A) lateral and lingual views, B) detail of tip, and C) detail of serrations along posterior edge (TMM 43057:-316). Cf. Ricardoestesia, tooth in D) lateral and lingual views, E) detail of tip, and F) detail of serrations along posterior edge (TMM 43057-313). Cf. Dromaeosaurus, tooth in G) lateral and lingual views, H) detail of tip, and I) detail of serrations along posterior edge (TMM 43057-314). Cf. Troodon, tooth in J) lingual and lateral views, K-L) detail of tip in lateral and lingual views, and M) detail of serrations along posterior edge (TMM 43057-323).

FIGURE 6-- Mammals of the Terlingua local fauna. A) Tribosphenida, incertae sedis, right lower molar (OMNH 22789). B) Alphadon sp. indet., left upper molar (OMNH 22721). C) Alphadon sp. cf. A. wilsoni, left upper molar (OMNH 22724). D) Gallolestes sp. left lower molar (OMNH 22788). E-F) Cimolodon cf. electus, left fourth lower premolar in lingual (E) and labial (F) views (OMNH 25042). G) Cimolomys clarki, upper right first molar in occlusal view (OMNH 25014). A,B,C,D, and G in occlusal view; scale is 1 mm.

Appendix 1 -- Voucher specimens for taxa from the Terlingua local fauna mentioned in the text.

Chondrichthyes: Hybodus sp. TMM 43057-289, TMM 43057-293, TMM 43057-321; Hybodontidae, indet. TMM 43057-290, TMM 43057-291; Lissodus selachos TMM 43057-294, TMM 43057-302; Scapanorhynchys sp. TMM 43057-301; Onchopristis dunklei TMM 43057-295, TMM 43057-305; Ischyrhiza avonicola TMM 43057-292, TMM 43057-306; Squatirhina americana TMM 43057-297, TMM 43057-298; Ptychotrygon sp. TMM 43057-300, TMM 43057-304; Batomorphii indet. TMM 43057-307; Dasyatidae indet. TMM 43057-296; Chondrichthyes indet. TMM 43057-299, TMM 43057-303.

Actinopterygii: Lepisosteidae TMM 43057-34, TMM 43057-242, TMM 43057-243, TMM 43057-245; Amiidae TMM 43057-246; Phyllodontidae TMM 43057-244.

Lissamphibia: Albanerpeton cf. A. nexosus TMM 43057-247, TMM 43057-248, TMM 43057-249, TMM 43057-250, TMM 43057-251, TMM 43057-252, TMM 43057-253, TMM 43057-254, TMM 43057-255, TMM 43057-257, TMM 43057-258; Prosirinidae sp. TMM 43057-259; Scapherpetontidae TMM 43057-248; Urodela indet. TMM 43057-237, TMM 43057-239; Anura indet. TMM 43057-238, TMM 43057-256, TMM 43057-310, TMM 43057-311, OMNH 25293, OMNH 25305.

Chelonia: Baenidae OMNH 23143; "Aspideretes" TMM 43057-324; isolated dentaries: TMM 43057-129; TMM 43057-130; TMM 43057-131; TMM 43057-132; TMM 43057-133.

Squamata: Odaxosaurus sp. TMM 43057-278, TMM 43057-279, TMM 43057-280, OMNH 25226; Proxestops sp. TMM 43057-137; TMM 43057-138; Glyptosaurinae indet. TMM 43057-281, TMM 43057-282, TMM 43057-283, OMNH 22981, OMNH 25232, OMNH 25251; Restes sp. OMNH 25337; Anguidae indet. OMNH 23035 (in part), OMNH 23038 (in part), OMNH 23035; Necrosauridae indet. TMM 43057-266, OMNH 25222, OMNH 25334; Sauriscus sp. TMM 43057-284, TMM 43057-285, OMNH 25336, OMNH 25339, OMNH 25341, OMNH 25347; Scincidae sp. A TMM 43057-286, TMM 43057-287, TMM 43057-288, OMNH 23034, OMNH 23039, OMNH 25223, OMNH 25225, OMNH 25227, OMNH 25242; Teiidae sp. B TMM 43057-274, TMM 43057-275, TMM 43057-276, OMNH 23037, OMNH 25339, OMNH 25342, OMNH 25343; Squamata indet. sp. X TMM 43057-261, TMM 43057-263, TMM 43057-264, TMM 43057-265, TMM 43057-273, OMNH 25224; Squamata indet. sp. Y TMM 43057-267, TMM 43057-268, TMM 43057-269, TMM 43057-270, TMM 43057-271, OMNH 22982, OMNH 23040; Serpentes TMM 43057-322.

ARCHOSAURIA: Goniopholidae indet. TMM 43057-6; Deinosuchus TMM 43057-20 TMM 43057-147; Pterosauria TMM 43057-13; Ankylosauria TMM 43057-136; Hadrosauridae TMM 43057-145; TMM 43057-146; TMM 43057-152; Ceratopsidae TMM 43057-32; cf. Troodon TMM 43057-323; cf. Ricardoestesia TMM 43057-313; cf. Dromaeosaurus TMM 43057-314; cf. Saurornitholestes TMM 43057:-316, 43057:-317;

MAMMALIA: Meniscoessus sp. nov. TMM 43057-62, 43057-63, 43057-64, OMNH 22667, 25089; Cimolodon cf. electus OMNH 22658, 22661, 22692, 25038 25041, 25005, 25006, 25042; cf. Cimolodon 22670, 22673, 23031, 23032, 25035, 25088; Cimolomys clarki OMNH 22682, 25014; Multituberculata, incertae sedis sp. nov. OMNH 22683, 25019. Alphadon cf. A. wilsoni OMNH 22724; Alphadon sp. nov. OMNH 22721; Turgidodon sp. TMM 43057-82; Pediomys sp. cf. P. krejcii OMNH 22748; Gallolestes sp. OMNH 22788; Mammalia, incertae sedis ("metatherian-eutherian grade") OMNH 22786.

Table 1 -- uncritical Tabulation of Vertebrates Reported from the Aguja Formation

Taxon references

CHONDRICHTHYES

Hybodus sp. this paper

Lissodus selachos this paper, Standhardt 1986

Cretorectolobus olsoni Standhardt 1986

Odontaspis sp. Langston et al., 1989

Anomotodon angustidens Lehman 1985

Scapanorhynchus texanus Lehman 1985

Scapanorhynchus raphidon Langston et al. 1989

Scapanorhynchus sp. this paper, Russell 1988

Squalicorax kaupi Lehman 1985

Squalicorax sp. Russell 1988

Onchopristis dunklei this paper

Onchopristis sp. Davies 1983, Russell 1988

Ischyrhiza avonicola this paper

Ischyrhiza mira Lehman 1985

Squatirhina americana this paper, Standhardt 1986

Ptychotrygon agujaensis McNulty and Slaughter 1972, Langston et al. 1989, Russell 1988

Ptychotrygon sp. this paper, Lehman 1985, Standhardt 1986

Myledaphus bipartitus Standhardt 1986

Dasyatidae sp. this paper

Pseudocorax (= Corax) sp. Wixson 1963

Isurus sp. Wixson 1963

Lamna appendiculata Langston et al. 1989

Cretolamna sp. Russell 1988

ACTINOPTERYGII

Melvius thomasi Boreske, 1974

cf. Melvius Bryant 1987, Russell 1988

Amiidae this paper, Lehman 1985, Standhardt 1986

Lepisosteidae this paper, Davies 1983, Lehman 1985, Standhardt 1986

Lepisosteus sp. Russell 1988

Atractosteus sp. Standhardt 1986

LISSAMPHIBIA

Albanerpeton this paper

Albanerpeton nexosus Standhardt 1986

Scapherpetontidae indet. this paper

Scapherpeton tectum Standhardt 1986

Anura sp. this paper

CHELONIA

Adocus sp. Lehman 1985, Langston et al. 1989

"Aspideretes" this paper, Lehman 1985, Langston et al. 1989

"Baena" cf. B. nodosa Lehman 1985, Langston et al. 1989

"Baena" this paper

cf. Basilemys sp. Davies 1983, Lehman 1985, Langston et al. 1989

Compsemys victa Standhardt 1986, Langston et al. 1989

? Taphrosphys sp. Lehman 1985, Langston et al. 1989

Thescelus cf. T. insiliens Lehman 1985, Langston et al. 1989

Thescelus sp. Lawson 1972, Langston et al. 1989

SQUAMATA

Odaxosaurus sp. this paper

Proxestops sp. this paper

Restes sp. this paper

Necrosauridae this paper

Scincidae, sp. A this paper

Sauriscus sp. this paper

Teiidae, sp. B this paper

Squamata indet., sp. X this paper

Squamata indet., sp. Y this paper

Serpentes this paper

Mosasauridae Maxwell et al. 1967, Lehman 1985, Standhardt 1986, Langston et al. 1989

CROCODYLIA

Goniopholidae this paper

Goniopholus cf. kirtlandicus Lehman 1985

Deinosuchus riograndensis this paper, Colbert and Bird 1954

cf. Leidyosuchus Langston et al. 1989

cf. Brachychampsa Standhardt 1986, Langston et al. 1989

PTEROSAURIA

Pterosauria this paper

DINOSAURIA

Kritosaurus

cf. K. navajovius Lawson 1972, Davies 1983

(= ?Gryposaurus) Weishampel and Horner 1990

Hadrosauridae this paper, Weishampel and Horner 1990, Weishampel 1990

Lambeosaurinae indet. Davies 1983, Weishampel 1990

Hypsilophodontidae Davies 1983

Stegoceras sp. Lehman 1985, Weishampel 1990

Panoplosaurus sp. Coombs 1978, Carpenter 1990

cf. Euoplocephalus Standhardt 1986

Nodosaurinae Lehman 1985

Edmontonia cf. rugosidens Coombs and Maryanska 1990, Weishampel 1990

cf. Ankylosauridae Standhardt 1986

Chasmosaurus mariscalensis Lehman 1989, Weishampel 1990

Ceratopsidae this paper, Standhardt 1986

Tyrannosauridae this paper, Lehman 1985, Weishampel 1990

Ornithomimidae Davies 1983, Weishampel 1990

Paronychodon lacustris Standhardt 1986, Langston et al. 1989

Troodon sp. Standhardt 1986

cf. Troodon this paper

cf. Saurornithoides this paper

cf. Dromaeosaurus this paper

cf. Ricardoestesia this paper

MAMMALIA

"tribothere" new sp. this paper

Ptilodontoidea Standhardt 1986

Eucosmodontidae Standhardt 1986

Cimolomyidae, new sp. Standhardt 1986

Meniscoessus sp. Standhardt 1986

Meniscoessus sp. nov. this paper

Cimolodon cf. electus this paper

cf. Cimolodon this paper

Cimolomys clarki this paper

Neoplagiaulacidae this paper

Multituberculata

incertae sedis, sp. nov. this paper

Alphadon cf. A. wilsoni this paper

Alphadon cf. A. marshi Standhardt 1986

Alphadon sp. this paper

Turgidodon sp. this paper

Pediomys cf. P. krejcii this paper

Gallolestes sp. this paper