Richard H. Tedford

Calibration of the Great American Interchange

From radioisotopic (potassium-argon) age determinations of tuffs and magnetostratigraphy of Late Tertiary mammal-bearing beds in Catamarca Province, northwest Argentina, refined estimates have been obtained for the durations and boundaries of beds of Chasicoan (Middle Miocene) through Chapadmalalan (Pliocene) age. An age of 9.0 million years is tentatively accepted for the Chasicoan-Huayquerian boundary, 5.0 million years for the Huayquerian-Montehermosan boundary, and 3.0 million years for the Montehermosan-Chapadmalalan boundary. Procyonids (raccoons and their allies), a group of North American origin, are first recorded in South America in a level immediately below a unit dated at 6.0 million years. Cricetine rodents of the tribe Sigmodontini are first recorded in South America in beds of Montehermosan age in Argentina. Ground sloths, a group of South American origin, first appear in North America in Early Hemphillian time in beds dated between 9.5 and 9.0 million years. The Panamanian land bridge was established by 3.0 million years ago, and an interchange of the terrestrial faunas was well under way by Late Blancan time (around 2.5 million years before present) in North America and by Chapadmalalan time in South America.

Phylogenetic Systematics of the North American Fossil Caninae (Carnivora: Canidae)

Abstract The canid subfamily Caninae includes all the living canids and their most recent fossil relatives. Their sister taxon is the Borophaginae with which they share an important modification of the lower carnassial, namely the presence of a bicuspid talonid, which gives this tooth an additional function in mastication. Contributing to this function is the enlargement of the posterolingual cingulum of M1 and development of a hypocone. The Caninae diverged from the Borophaginae in the narrowing and elongation of the premolars separated by diastemata and placed in a shallow ramus and narrow muzzle. These latter features allow the Caninae to be recognized in the fossil record as early as the beginning of the Oligocene (34 Ma) and constitute evidence that they represent a monophyletic group. In striking contrast to the history of the Borophaginae, the Caninae remain confined to a closely similar group of fox-sized species (Leptocyon spp.) throughout the Oligocene and showing very limited cladogenesis into the end of the medial Miocene (12 Ma), a span that saw marked adaptive divergence in the Borophaginae and the origin of all its major clades. By 12 Ma (beginning of the Clarendonian Land Mammal age) few fox-sized borophagines remained and most of those held hypocarnivorus adaptations. At that point the Vulpini appear both as mesocarnivores (Vulpes spp.) and hypocarnivores (Metalopex spp.) reproducing, on a much smaller scale, the range of adaptations shown in the initial radiation of the Borophaginae. By the end of the Clarendonian (9 Ma) the first members of the tribe Canini appear. Initially this group was represented by the genus Eucyon, largely by a single widespread North American species E. davisi. Our cladistic analysis predicts that the roots of the South American clade subtribe Cerdocyonina, sister taxon to E. davisi and Canis species (together, subtribe Canina), must also have been present, but taxa representing this group do not appear in the North America record until the earliest Pliocene (latest Hemphillian, 5 Ma). Species of three genera (Cerdocyon, Chrysocyon, and possibly Theriodictis), now confined to South America, appear in the fossil record of the southern United States and northern Mexico prior to and just after the opening of the Panamanian Isthmus (ca. 3 Ma), indicating that important cladogenesis within the South American clade took place in North America. Species of Eucyon make their appearance in the Old World in the late Miocene, and E. davisi has a Pliocene record in Asia. Species of this genus undergo a modest adaptive radiation in Eurasia during the Pliocene. In the late Miocene and early Pliocene two species of Canis appear in North America (C. ferox and C. lepophagus), representing the initial cladogenesis within the genus. These animals are all coyote-sized and represent a broadening of body size range within a mesocarnivorous dental adaptation. Toward the end of the Pliocene and into the Pleistocene in North America a curious and rare group of jackal-like species (C. thöoides, C. feneus, and C. cedazoensis) seem to form an endemic clade arising near C. lepophagus. These taxa are dentally similar to jackals, especially C. aureus, but share no synapomorphies with them. The early cladogenesis of Canis in the Pliocene of North America produced a somewhat larger form, C. edwardii, that appears in the late Blancan at ca. 3 Ma. It also seems to have a sister relationship with C. lepophagus and with the coyote C. latrans, which appears much later in the record (late Irvingtonian) and quickly becomes distributed across the United States. The golden jackal (C. aureus) shares synapomorphies with the coyote and C. edwardii but does not appear in the fossil record until the early Pleistocene of North Africa. Canis edwardii is extinct by the end of the Irvingtonian. Large wolflike species of Canis seem to be the products of evolution in Eurasia. They appear early in the North American record as immigrants of the crown group of Canis that augment the essentially stem group native species of the New World. The first of these is Canis armbrusteri, which appears early in the Irvingtonian, initially in the Southwest but later in the eastern United States where it survived into the early Rancholabrean of Florida. This is a large wolf, a sister taxon of C. lupus, whose appearance early in the Pleistocene predates the earliest midcontinent occurrence of C. lupus by nearly 1 m.y. In the New World C. armbrusteri gave rise to the native dire-wolf (C. dirus), as evidenced by intergrading morphologies of late Irvingtonian examples that show the transformation to the more hypercarnivorous giant form. The earliest evidence of C. dirus is in the midcontinent and it appears to have displaced C. armbrusteri into the eastern part of the continent while it expanded westward and particularly southward into South America during the late Pleistocene. Canis lupus itself does not appear in midlatitude North America until the late Rancholabrean (0.1 Ma, last glacial cycle), although it was a resident of Arctic North America since the mid-Pleistocene (ca. 0.8 Ma). North America has a limited record of canine diversity during the Pleistocene. Most clades of vulpines and canines that reached the Old World during that span underwent significant cladogenesis so that the canid fauna of Eurasia was always more diverse than that of the New World. From time to time waifs from the Old World centers of origin wandered south into midcontinent North America to briefly enrich the fossil record: Xenocyon spp. in the late Irvingtonian; Cuon alpinus and Canis lupus in the late Rancholabrean, along with the living fox species Vulpes vulpes (late Rancholabrean) and perhaps the swift-fox earlier in the Pleistocene. The center of evolution of the Caninae thus shifted to Eurasia and to South America when avenues of dispersal to those continents were available at the end of the Cenozoic. Because of the diversity of Old World forms that became resident in North America, our analysis of the New World fauna has been expanded to include relevant Old World taxa and to present a broader phylogenetic reconstruction than could be offered only on the basis of strictly New World evidence. This expanded view brings in a greater diversity of morphology, which allows us to better separate homoplasy from true homology. We have considered only Old World taxa that are represented by the most complete material so that missing data are kept to a minimum in our analysis. This still affords sufficient taxa so that the major structure of the phylogeny of Canis can be discerned. Our cladistic analysis found two robustly recognized crown clades within Canis: the mesocarnivorous lupus clade, and the hypercarnivorous Xenocyon clade. The first contains the wolf, C. lupus, and its sister taxon C. armbrusteri + C. dirus, with the latter showing some dental features related to hypercarnivory as an autapomorphy. The Chinese late Pliocene wolf C. chihliensis appears to be a stem group in the Lupus clade and may be closely allied to C. armbrusteri. The Xenocyon clade is also Eurasian in origin and is marked by character reversals to states primitive within Canis. Its earliest record is in the medial Pliocene of eastern Asia (Xenocyon dubius), after which it dispersed to western Eurasia in the early Pleistocene (X. lycaonoides). In the Pleistocene Xenocyon achieved a Holarctic distribution including midlatitide North America (X. texanus and X. lycaonoides). During this episode of expansion the sister taxa Cuon and Lycaon arose in Eurasia and Africa, respectively, most likely from isolated Xenocyon populations. In the latest Pleistocene Cuon alpinus expanded its range into the middle latitudes of the New World. A number of Pliocene and Pleistocene wolf and coyote-sized Eurasian Canis species (C. arnensis, C. etruscus, C. palmidens, C. mosbachensis, and C. variabilis) were included in our cladistic analysis, but the relationships of those forms were difficult to resolve beyond their paraphyletic relationship to the crown clade.

Enrichment and stability in the Pliocene mammalian fauna of North China

The Late Neogene vertebrate fossil record from Yushe Basin presents multiple, superposed assemblages from a single area, spanning roughly the interval of 6-2 Ma. Both large and small mammals show peak species richness in the middle Pliocene but indicate relative faunal stability throughout the Pliocene. Large mammals show turnover, especially extinction, around 5 and 2.5 Ma. Small mammals indicate change (over half of the species and several genera), as well as turnover at the species level, between 4 and 3.4 Ma. The loosely controlled dating of these events does not disprove hypothetical correlation with events in North America and with global climatic shifts. Elements that lack Yushe antecedents, some being long-distance dispersers, appear throughout the section, but with little effect on the resident assemblage. First records of well-documented immi- grants (from North America, Europe, Africa, southern Asia, or high latitudes) generally do not coincide with ecomorph extinctions. Early Pliocene exchange between Asia and North America appears to have been balanced in both directions and involved a small proportion of the fauna. Immigration probably was opportunistic and contributed to faunal enrichment. We interpret the Yushe Pliocene mammalian assemblages as representing a fauna that was stable from ca. 5 to 2.5 Ma and changed mainly by additions and congeneric species substitutions.

A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia

Modern pinnipeds (seals, sea lions and the walrus) are semi-aquatic, generally marine carnivores the limbs of which have been modified into flippers. Recent phylogenetic studies using morphological and molecular evidence support pinniped monophyly, and suggest a sister relationship with ursoids (for example bears) or musteloids (the clade that includes skunks, badgers, weasels and otters). Although the position of pinnipeds within modern carnivores appears moderately well resolved, fossil evidence of the morphological steps leading from a terrestrial ancestor to the modern marine forms has been weak or contentious. The earliest well-represented fossil pinniped is Enaliarctos, a marine form with flippers, which had appeared on the northwestern shores of North America by the early Miocene epoch. Here we report the discovery of a nearly complete skeleton of a new semi-aquatic carnivore from an early Miocene lake deposit in Nunavut, Canada, that represents a morphological link in early pinniped evolution. The new taxon retains a long tail and the proportions of its fore- and hindlimbs are more similar to those of modern terrestrial carnivores than to modern pinnipeds. Morphological traits indicative of semi-aquatic adaptation include a forelimb with a prominent deltopectoral ridge on the humerus, a posterodorsally expanded scapula, a pelvis with relatively short ilium, a shortened femur and flattened phalanges, suggestive of webbing. The new fossil shows evidence of pinniped affinities and similarities to the early Oligocene Amphicticeps from Asia and the late Oligocene and Miocene Potamotherium from Europe. The discovery suggests that the evolution of pinnipeds included a freshwater transitional phase, and may support the hypothesis that the Arctic was an early centre of pinniped evolution.

Mammalian biochronology of the late Cenozoic basins of New Mexico

Late Cenozoic mammalian remains have been collected from New Mexico for more than a century. The most important nineteenth-century collections came from the Espanola Basin. Work in this century has continued to focus mainly on the Miocene faunas in that basin and the northern part of the adjoining Albuquerque basin. The lithostratigraphy of these basins has recently been reviewed and revised, but the description of the large collections of fossil remains from the basinal deposits is largely incomplete. Pliocene and early Pleistocene faunas have been found in deposits that mark the development of external drainage of the major rivers of the state. Study of these assemblages has been cursory, although the lithostratigraphy of these deposits has received considerable attention in recent years. The Miocene basinal deposits accumulated under conditions of internal drainage in local structural environments that led to the development of partly synchronous depositional records. Biostratigraphies worked out in each basin overlap sufficiently so that the composite succession forms an important biochronological standard for southwestern North America. The Espanola and Albuquerque-Belen basins contain the only Miocene faunas of importance presently known in New Mexico. Their fossil records complement one another in that the northern Albuquerque basin has the only adequate representation of the early Miocene and the Espanola basin a more complete record of the medial and late Miocene. Synchronous faunas in these basins show a high degree of taxonomic resemblance extending to elements that represent endemic taxa or forms rare elsewhere. Compared with coevel assemblages in the Great Plains, there is considerable generic and specific similarity. This allows easy reference of the New Mexican faunal succession to the sequence of North American mammal ages that are essentially typified by the Great Plains biochronology. In addition to these taxonomic relationships, the New Mexican faunas retain throughout the Miocene characters that may typify a southwestern faunal province. The general features of the latter would include diverse and abundant artiodactyls, especially Camelidae, including a late occurrence of endemic stenomyline genera, and less diverse and abundant perissodactyls, especially horses, when compared with the Great Plains. The fossil vertebrate record of Pliocene and early Pleistocene time in New Mexico is largely confined to the basins of the Rio Grande and the Gila River and is derived from deposits of the axial rivers that were the percursors of the modern streams and from the piedmont slope facies of the basin margins. Sites scattered along the length of the Rio Grande rift from the Santo Domingo basin near Santa Fe south to the international border have yielded Blancan and early Irvington faunas. These assemblages can be arranged in temporal order from their position in local stratigraphic columns, from their biological affinities, and with respect to dated basalts and ashes interbedded in the sediments containing the fossil remains. Important reference sequences are available in adjacent Arizona and Texas, where mammalian faunas and calibrated biochronologies compare closely with those in New Mexico. The New Mexican record supports the southwestern biochronology for Pliocene and early Pleistocene time, including the following events and their calibration: the last appearance of hipparionine horses ( Nannippus ) and the first appearance of the South American immigrants, the glyptodont Glyptotherium and probably the mylodontid sloth genus Glossotherium , were nearly synchronous events close to the Gauss-Matuyama boundary (2.5 m.y. B.P.); at least three species Equus were present in the Southwest by the time of the Nannippus extinction; the first appearance of the Asiatic immigrant Mammuthus in the southwest between 1.2 and 1.4 m.y., postdating the Olduvai Event and, by definition, indicating that the Blancan-Irvingtonian boundary, at least as determined for the Southwest, lies close to the beginning of medial Pleistocene time.

An Arctic mammal fauna from the Early Pliocene of North America

A peat deposit on Ellesmere Island, Nunavut, Canada, allows a unique glimpse of the Early Pliocene terrestrial biota north of the Arctic Circle. The peat accumulated in a beaver pond surrounded by boreal larch forest near regional tree line in coastal hills close to the Arctic Ocean. The ecological affinities of the plant and beetle remains contained in the peat indicate that winter temperatures on Ellesmere Island were nearly 15 °C higher and summer temperatures 10 °C higher than they are today. Here we show that the mammalian remains buried in the peat represent mainly taxa of Eurasiatic zoogeographic and phyletic affinities, including the first North American occurrence of a meline badger (Arctomeles). This deposit contains direct evidence of the composition of an Early Pliocene (4–5 million years ago) arctic mammalian fauna during an active period of interchange between Asia and North America.

Biostratigraphy and magnetostratigraphy, late Pliocene rocks, 111 Ranch, Arizona

Fossiliferous strata exposed along the northwestern flank of Dry Mountain on the 111 Ranch, 27 km southeast of Safford, Graham County, Arizona, were dated by means of isotopic methods and the magnetic-reversal time scale. A 100-m fossiliferous interval contains the later part of the Gauss Chron and the early part (pre-Olduvai) of the Matuyama Chron. Zircon fission-track ages on an ash bed just below the Gauss-Matuyama boundary support this identification. Owing to these age constraints, the fossil mammals at the 111 Ranch locality are all constrained within the later part of the Blancan Mammal Age rather than the latest Blancan and early Irvingtonian, as previously stated in the literature. They are, therefore, synchronous with certain localities (Wolf Ranch and California Inst. Technology) in the Saint David Formation of the San Pedro Valley 130 km to the southwest of Dry Mountain. The horse Nannippus persisted in the 111 Ranch locality at least until the end of the Gauss Chron, in confirmation of data from the San Pedro Valley. A diverse group of equine and asinine equids is evident in the 111 Ranch fauna, which argues for an early divergence of these major branches of horse phylogeny. The 111 Ranch fauna records the first appearance of three neotropical immigrants in the southwest, including the sloth Glossotherium , the glyptodont Glyptotherium , and the capybara Neochoerus . A palearctic immigrant, the vole Synaptomys , is also present.

PLIONARCTOS, A TREMARCTINE BEAR (URSIDAE: CARNIVORA) FROM WESTERN NORTH AMERICA

Abstract The Pliocene Ringold Formation of eastern Washington has yielded important new materials of tremarctine bears of the anagenetic Plionarctos lineage. The genus is reviewed in light of this new material and observations made on other described specimens. One of these was previously described from the medial Hemphillian Rattlesnake Formation of Oregon and is recognized as a tremarctine bear, thus extending the earliest record of the group into the early part of the late Miocene. The late Hemphillian P. edensis, the genotypic species, is rediagnosed, although no new material is added to its hypodym. A new species from the early Blancan White Bluffs sites, P. harroldorum, appears to have been derived from P. edensis. Referred Plionarctos sp. from the medial Blancan Taunton Locality has some dental features that are more derived and approach those of the Pleistocene Tremarctos floridanus. Plionarctos forms a paraphyletic stem-group for the Tremarctinae. Species of this genus can be traced successively into the Pliocene where they form the stock from which the Pleistocene and Recent species of Tremarctos, Arctodus, and Pararctotherium arose.

Development and Application of Land Mammal Ages in North America and Europe, a Comparison

During the last century the geologic time scale was developed from a sequential array of fossiliferous marine deposits, primarily in Europe. Prior to the last century the concept of geologic time was addressed by numerous natural scientists, including Steno, Buffon, Hutton, and Smith. With subsequent refinements, this time scale is now widely accepted as the chronologic framework for all geologic and biologic events in earth history. However, with rare exceptions (e.g., the Paris Basin) the application of this framework to continental sediments has been difficult because continental deposits are usually less extensive, superposition is more difficult to demonstrate, and interdigitation of marine and nonmarine rocks are rare. Because of these factors, vertebrate paleontologists have made few contributions to the development of the geologic time scale, even though their contributions to the concept of organic evolution has been significant.

Miocene marine-nonmarine correlations, atlantic and Gulf Coastal Plains, North America

Abstract A survey of the records of the remains of Miocene land mammals from nearshore marine deposits on the Atlantic and Gulf Coastal Plains of the United States reveals a number of occurrences where the relationship between mammalian and invertebrate biochronologies can be directly compared. Vertebrate paleontologists have constructed and calibrated a time scale of geochrons (North American Mammal Ages and informal subages) based on the occurrence of certain mammalian genera. This scale can be compared directly with neritic molluscan, diatom and foraminiferal biostratigraphies that in turn can be calibrated with the Neogene planktonic foraminiferal zones in the warm-water provinces surrounding southeastern North America. Such cross-correlations allow the tracing of geochrons from the land to sea as well as across facies boundaries within these major environments. The demonstration of the wider utility of the mammalian time scale in correlation and the testing of correlations proposed by other disciplines is an important conclusion from this study.