Early Arikareean Research Articles

Phylogeny and paleobiogeography of the enigmatic North American primate Ekgmowechashala illuminated by new fossils from Nebraska (USA) and Guangxi Zhuang Autonomous Region (China)

Ekgmowechashala is a poorly documented but very distinctive primate known only from the late early Oligocene (early Arikareean) of western North America. Because of its highly autapomorphous dentition and spatiotemporal isolation, the phylogenetic and biogeographic affinities of Ekgmowechashala have long been debated. Here, we describe the oldest known fossils of Ekgmowechashala from the Brown Siltstone Beds of the Brule Formation, White River Group of western Nebraska. We also describe a new ekgmowechashaline taxon from the Nadu Formation (late Eocene) in the Baise Basin of Guangxi Zhuang Autonomous Region in southern China. Phylogenetic analysis suggests that North American Ekgmowechashala and the new Chinese taxon are sister taxa that are nested within a radiation of southern Asian adapiforms that also includes Gatanthropus, Muangthanhinius, and Bugtilemur. The new Chinese ekgmowechashaline helps fill the considerable disparity in dental morphology between Ekgmowechashala and more primitive ekgmowechashalids known from southern Asia. Our study underscores the fundamental role of southern Asia as a refugium for multiple primate clades during the cooler and drier climatic regime that prevailed after the Eocene–Oligocene transition. The colonization of North America by Ekgmowechashala helps define the beginning of the Arikareean Land Mammal Age and corresponds to an example of the Lazarus effect, whereby a taxon (in this case, the order Primates) reappears suddenly in the fossil record after a lengthy hiatus.

New bats in the tropical family Emballonuridae (Mammalia: Chiroptera) from the Oligocene and early Miocene of Florida

Three new genera and four new species in the tropical bat family Emballonuridae (Mammalia: Chiroptera) are described from the Oligocene and early Miocene of peninsular Florida. These are: a new genus Oligopteryx and two new species, O. floridanus and O. hamaxitos, from the late Oligocene (early Arikareean) Brooksville 2 Local Fauna in Hernando County, with referred specimens of each species from the early Oligocene (Whitneyan) I-75 Local Fauna in Alachua County; a new genus and species, Karstopteryx gunnelli, from the latest Oligocene (late Arikareean) Buda Local Fauna in Alachua County; and a new genus and species, Floridopteryx poyeri, from the early Miocene (early Hemingfordian) Thomas Farm Local Fauna in Gilchrist County. Characters that distinguish Oligopteryx from other emballonurid genera are: M1 with a much reduced parastylar region and a large talon with a triangular posterolingual extension; M1 and M2 with a prominent hypocone separated from the protocone by a deep, V-shaped notch in the postprotocrista, and deeply concave talon basin; small, single-rooted p3; m1 and m2 with the trigonid narrower and shorter than the talonid, the paraconid and metaconid located close together along the lingual margin, and the entocristid blade-like, V-shaped, and oriented labially. O. hamaxitos is distinguished from O. floridanus by its smaller size and well-developed paraloph and metaloph on M1 and M2. Karstopteryx gunnelli is separated from the two species of Oligopteryx by the prominent parastyle, anteriorly oriented preparacrista, and rounded talon. Floridopteryx poyeri is characterized by: M1 with a highly reduced parastylar region but with a well-developed parastyle, short preparacrista, small hypocone not separated from the protocone by a notch in the postprotocrista, and a squarish talon; m1/m2 with the talonid much broader than the trigonid, trigonid compressed, shorter than talonid, and with the paraconid and metaconid close together along the lingual margin, large hypoconulid, and anterior root flattened and compressed anteroposteriorly (m1 only). Oligopteryx and Karstopteryx are considered members of the subfamily Emballonurinae based on the reduced parastylar region of the M1 but are left unassigned at the tribal level. Oligopteryx differs from all living emballonurids in the presence of a p3. Floridopteryx has a reduced parastylar region on M1 and is also considered an emballonurine, but can be separated from Oligopteryx by the presence of a prominent parastyle and lack of a p3. Floridopteryx is placed in the monophyletic New World emballonurine tribe Diclidurini. Taphonomy indicates Oligopteryx was a colonial cave dweller, whereas Floridopteryx more likely roosted in trees, as do most living Neotropical emballonurids. A change from a tropical or subtropical climate in Florida in the Oligocene and early Miocene to a warm temperate climate from the middle Miocene to the present is reflected in the disappearance of emballonurids from Florida after the early Miocene. Molecular studies indicate the Emballonuridae dispersed from Africa to South America in the Oligocene, despite the oldest Western Hemisphere record of this family from the early Oligocene (~30 Ma) of Florida (Oligopteryx). We hypothesize emballonurids may have reached North America from Eurasia in the Eocene using an overland route and then dispersed overwater to the then-island continent of South America, with the oldest South American record of that family from the late Oligocene (~25 Ma) of Peru.

Biomechanical adaptations for burrowing in the incisor enamel microstructure of Geomyidae and Heteromyidae (Rodentia: Geomyoidea).

The enamel microstructure of fossil and extant Geomyoidea (Geomyidae, Heteromyidae) lower incisors incorporates three‐ or two‐layered schmelzmusters with uniserial, transverse Hunter‐Schreger bands having parallel and perpendicular or exclusively perpendicular oriented interprismatic matrix. Phylogenetically, these schmelzmusters are regarded as moderately (enamel type 2) to highly derived (enamel type 3). Our analysis detected a zone of modified radial enamel close to the enamel–dentine junction. Modified radial enamel shows a strong phylogenetic signal within the clade Geomorpha as it is restricted to fossil and extant Geomyoidea and absent in Heliscomyidae, Florentiamyidae, and Eomyidae. This character dates back to at least the early Oligocene (early Arikareean, 29 Ma), where it occurs in entoptychine gophers. We contend that this specialized incisor enamel architecture developed as a biomechanical adaptation to regular burrowing activities including chisel‐tooth digging and a fiber‐rich diet and was probably present in the common ancestor of the clade. We regard the occurrence of modified radial enamel in lower incisors of scratch‐digging Geomyidae and Heteromyidae as the retention of a plesiomorphic character that is selectively neutral. The shared occurrence of modified radial enamel is a strong, genetically anchored argument for the close phylogenetic relationship of Geomyidae and Heteromyidae on the dental microstructure level.

A New Mormoopid Bat from the Oligocene (Whitneyan and Early Arikareean) of Florida, and Phylogenetic Relationships of the Major Clades of Mormoopidae (Mammalia: Chiroptera)

A new genus and species of fossil bat, Koopmanycteris palaeomormoops, representing an ancient member of the endemic Neotropical family Mormoopidae, is described from the Oligocene of Florida. This new species is known from two paleokarst deposits in northern peninsular Florida, the early Oligocene (Whitneyan, 30–32 Ma) I-75 Local Fauna in Alachua County and the late Oligocene (early Arikareean, 26–28 Ma) Brooksville 2 Local Fauna in Hernando County. The discovery of Koopmanycteris extends the fossil record of the Mormoopidae back in time almost 30 million years. Compared to most other described taxa of North American Tertiary bats, the fossil sample of Koopmanycteris palaeomormoops is substantial, consisting of 50 specimens including: P4, all lower teeth from p3 through m3, dentary, petrosal, humerus, proximal radius, and femur. Koopmanycteris is morphologically intermediate between the two living genera of mormoopids, Mormoops and Pteronotus, but is most similar to the former taxon. Characters shared by Koopmanycteris and Mormoops include: large anterolabial basin on P4; large, double-rooted p3; secondary process ventral to angular process on dentary; absence of groove separating capitulum into medial and lateral portions on distal humerus; presence of prominent ridge on posterolateral edge of distal humeral shaft; rounded extremity on proximal radius; and presence of a prominent ridge or tubercle on posterodistal shaft of femur. All of these features appear to be relatively derived, with the exception of the large, double-rooted p3. In several mandibular characters, Koopmanycteris more closely resembles Pteronotus, including: pronounced posteroventral process on the symphysis; lack of curvature of ventral edge of horizontal ramus between posterior edge of symphysis and p4; and less pronounced dorsal upturning of ascending ramus compared to Mormoops. Koopmanycteris retains the primitive dental morphology of Mormoops, with a large double-rooted p3, and the primitive posterior mandibular morphology of Pteronotus, with the lesser degree of dorsal flexion of the ascending ramus. A phylogenetic analysis including all known extant mormoopid lineages as well as representative outgroups from other noctilionoid clades confirms that Koopmanycteris and Mormoops are sister taxa. These findings indicate that the lineages leading to Pteronotus and Mormoops were distinct by ∼30 Ma. The fossil record tentatively indicates that the Mormoopidae originated in North America in the early Oligocene or earlier, and may not have reached South America until the Pliocene following the onset of the Great American Biotic Interchange. Several lines of evidence support this hypothesis: (1) the earliest fossil record of the Mormoopidae is in the early Oligocene of Florida; (2) the greatest diversity and endemism of modern mormoopid species is in the West Indies; and (3) the absence of a pre–late Pleistocene record, lack of endemism, and marginal recent distribution of mormoopids in South America. Most species of living mormoopids are obligate cave dwellers, suggesting that the occurrence of caves and paleokarst deposits has affected both their modern distribution and fossil record.

Review of the marsupials (Mammalia: Metatheria) from the late Paleogene (Chadronian–Arikareean: late Eocene–late Oligocene) of North America

The taxonomy of marsupials from the late Paleogene of North America (Chadronian to Arikareean North American land mammal ages: late Eocene–late Oligocene) is reviewed based on new and previously undescribed fossil material as well as reevaluation of previously described material. Two families are recognized, the Herpetotheriidae and Peradectidae. Nine species of herpetotheriids are recognized within two genera: Herpetotherium Cope, 1873a and Copedelphys Korth, 1994, including one new species, H. tabrumi. The greatest diversity of herpetotheriids was in the Chadronian (four species). By the late Arikareean, only a single species is recognized. The range of Copedelphys is extended into the early Whitneyan (previously limited to Chadronian–Orellan). Among species of Herpetotherium, the ranges of two species have been extended: H. valens (Lambe, 1908) from the Chadronian is reported from the Orellan, and H. marsupium (Troxell, 1923) from the Uintan and Duchesnean is reported from the early Chadronian. The range of H. merriami (Stock and Furlong, 1922), previously only known from the Arikareean of Oregon, is expanded geographically eastward to Montana. Within the Peradectidae only three species are recognized: Peradectes cf. californicus (Stock, 1936) and Didelphidectes pumilis Hough, 1961 from the Chadronian, and Nanodelphys hunti (Cope, 1873b) from the Orellan to early Arikareean. Specimens previously identified as an unnamed new species of Nanodelphys from the Whitneyan and Arikareean are referred here to N. hunti.

A late-surviving apatemyid (Mammalia: Apatotheria) from the latest Oligocene of Florida, USA.

A new species of Apatemyidae, Sinclairella simplicidens, is based on four isolated teeth that were screenwashed from fissure fillings at the late Oligocene Buda locality, Alachua County, Florida. Compared to its only congener Sinclairella dakotensis, the new species is characterized by upper molars with more simplified crowns, with the near absence of labial shelves and stylar cusps except for a strong parastyle on M1, loss of paracrista and paraconule on M2 (paraconule retained but weak on M1), lack of anterior cingulum on M1–M3, straighter centrocristae, smaller hypocone on M1 and M2, larger hypocone on M3, distal edge of M2 continuous from hypocone to postmetacrista supporting a large posterior basin, and with different tooth proportions in which M2 is the smallest rather than the largest molar in the toothrow. The relatively rare and poorly-known family Apatemyidae has a long temporal range in North America from the late Paleocene (early Tiffanian) to early Oligocene (early Arikareean). The new species from Florida significantly extends this temporal range by roughly 5 Ma to the end of the Paleogene near the Oligocene-Miocene boundary (from early Arikareean, Ar1, to late Arikareean, Ar3), and greatly extends the geographic range of the family into eastern North America some 10° of latitude farther south and 20° of longitude farther east (about 2,200 km farther southeast) than previously known. This late occurrence probably represents a retreat of this subtropically adapted family into the Gulf Coastal Plain subtropical province at the end of the Paleogene and perhaps the end of the apatemyid lineage in North America.

Geology of the Early Arikareean Sharps Formation on the Pine Ridge Indian Reservation and Surrounding Areas of South Dakota and Nebraska

Based on geologic mapping, measured sections, and lithologic correlations, the local features of the upper and lower type areas of the Early Arikareean (30.8–20.6 million years ago) Sharps Formation are revised and correlated. The Sharps Formation above the basal Rockyford Member is divided into two members of distinct lithotypes. The upper 233 feet of massive siltstones and sandy siltstones is named the Gooseneck Road Member. The middle member, 161 feet of eolian volcaniclastic siltstones with fluvially reworked volcaniclastic lenses and sandy siltstone sheets, is named the Wolff Camp Member. An ashey zone at the base of the Sharps Formation is described and defined as the Rockyford Ash Zone (RAZ) in the same stratigraphic position as the Nonpareil Ash Zone (NPAZ) in Nebraska. Widespread marker beds of fresh water limestones at 130 feet above the base of the Sharps Formation and a widespread reddish-brown clayey siltstone at 165 feet above the base of the Sharps Formation are described. The Brown Siltstone Beds of Nebraska are shown to be a southern correlative of the Wolff Camp Member and the Rockyford Member of the Sharps Formation. Early attempts to correlate strata in the Great Plains were slow in developing. Recognition of the implications of the paleomagnetic and lithologic correlations of this paper will provide an added datum assisting researchers in future biostratigraphic studies. Based on similar lithologies, the Sharps Formation, currently assigned to the Arikaree Group, should be reassigned to the White River Group.

Reassessment ofScaphiopus neuterKluge, 1966 (Anura: Pelobatoidea: Pelobatidae), Based on New Material from Anceney, Montana (Early Barstovian)

Anuran fossils from the middle Miocene, early Barstovian Anceney Locality in Gallatin County, Montana, are described and identified as Scaphiopus neuter Kluge, 1966. The material consists of many isolated, complete to partial bones that represent most elements of the skeleton. Scaphiopus neuter was previously known by the holotype, a partial skeleton, from the late Oligocene, early Arikareean of South Dakota and a partial ilium from the Arikareean of Nebraska, and thus the new material increases the geologic age and geographic distribution of the species. Results from a phylogenetic analysis conducted to test the relationship of Sc. neuter to other pelobatids places it in a clade containing four extant species of Spea Cope, 1866. The new combination, Spea neuter (Kluge, 1966), is proposed for the oldest and only extinct member of this genus currently known. Within Spea, Sp. neuter is the sister taxon to the clade comprised of Spea intermontana (Cope, 1863) Spea bombifrons (Cope, 1863), a relationship supported by one synapomorphy: the septum nasi of the sphenethmoid forms a laterally expanded, vertical plate at its anterior end. Relationships of other members of Spea remain unresolved. A stratocladogram based on the pelobatid phylogeny presented here suggests an approximately 28 Ma ghost lineage for Spea.

Magnetostratigraphy and paleontology of Wagner Quarry, (late Oligocene, early Arikareean) basal Arikareean Group of the Pine Ridge Region, Dawes County, Nebraska

Mammalian fossils (the Wagner Quarry local fauna) from the basal Arikaree Group (Late:Oligocene) near Chadron, Dawes County, Nebraska, are described. It is the first large mammal concentration described from the Pine Ridge Arikaree Group. Twenty-seven species of mammals are present: 1 marsupial (Herpetotherium), 2 insectivores (Proscalops, Ocajila), 9 rodents (Downsimus; Aywoodia, Cedromus savannae n. sp., Nototamias, Agnotocastor, Palaeocastor, Proheteromys, Leidymys, Geringia), 3 lagomorphs (Palaeolagus {2 sp.], Megalagus), 3 carnivores (Paradaphoenus, Canidae, Nimravus), 3 perissodaciyls (Miohippus, Diceratherium [2 sp.]), and, 6 artiodaciyls (Entelodontidae, Desmatochoerinae, Leptauchenia, Anthracotheriidae, Pseudolabis, Nanotragulus). Faunal correlation, with the Ridgeview local fauna and the radibisotopically constrained faunas of the Wildcat Ridge indicates an early Arikareean (Arl, 30-28 Ma, Tedford et al. 2004) North American Land Mammal age for the fauna. The first paleomagnetic study of the Pine Ridge basal Arikaree Group (Wagner Quatry section) shows that polarity signals are not the same, as basal Arikaree Group sediments (Gering Formation) in the Wildcat Ridge region. Correlation of the Wagner Quarry section with Chron 10n (28.25- 28.9 Ma) places it older than the Gering Formation (Chron 9r, 27.95-28.25 Ma) and younger than the lowermost White River Group "Brown Silistone" (Chron 10r-11n, 28.9-30.08 Ma) of Nebraska. These sedimbnts could represent time not recorded in the Wildcat Ridge. Cedromus (Sciuridae), Oligospermophilus (Sciuridae), and Alwoodia (A lodontidae) are newly recognized in the earliest Arikareean (Arl) of the central Great Plains.

The first Paleogene mammal record of Middle America: Simojovelhyus pocitosense (Helohyidae, Artiodactyla)

ABSTRACT Simojovelhyus pocitosense, gen. et sp. nov., is based on a mandibular fragment with m1-3 collected from an unnamed Late Oligocene formation, in an amber mine near Simojovel, in northwestern Chiapas. The source strata consist of black, very fossiliferous, shaley phyllarenitic sandstone and mudstone that were deposited in an estuarine to swampy lagoonal environment, dated in Chrons C10–C9 (~29.4–26.8 Ma), or early Late Oligocene. Simojovelhyus pocitosense possesses small, rectangular, bunodont molars that increase in size posteriorly. This combination of characters sets it apart from the Oligocene bunodont artiodactyls (entelodontids, tayassuids and leptochoerids), and bring it within the Helohyidae. It differs from recognized genera in having a tiny paraconid, and a unique combination of cristids and cuspulids as follows. Cristids: paracristid, anteromesocristid (neomorphic), and anterolingualcristid (neomorphic) on the trigonid anterior slope, cristid obliqua (short to remnant), and a robust postcristid in the talonid. Cuspulids (small, accessory): mesoconid, hypoconulid (large and blunt in m3), postmesoconulid (neomorphic), and pseudohypoconulid (neomorphic). This record extends the geochronologic range and geographic distribution of the Helohyidae from the Late Eocene of North America and Asia to the early Late Oligocene (Early Arikareean) of Middle America. Simojovelhyus pocitosense is also the southernmost record of Paleogene mammals, not only of Mexico, but of all North America. This record significantly adds to the meager, but important, record of Paleogene vertebrates in the tropics.

Arikareean (Oligocene Miocene) Herpetotherium (Marsupialia, Didelphidae) from Nebraska and Florida

North American Herpetotherium (Marsupialia, Didelphidae) is a small mouse-sized (<80g) opossum that ranged from the middle Eocene into the early Miocene (Uintan-Hemingfordian) of the Great Plains, Pacific Northwest, and Atlantic Southeast. New Arikareean (30-18.8Ma) Herpetotherium samples are described from the lower Arikaree Group of Nebraska (Dw-121 or Ridgeview Local Fauna, a locality south of Chadron, from localities of the Wildcat Ridge region, and the McCann Canyon Local Fauna) as well as from 4 localities in Florida (I-75, Brooksville 2, SB-1A, and White Springs). The samples from Dw-121 and Brooksville 2 are the first large (>25) populations of Herpetotherium described from the Arikareean age. Three species are recognized in the Arikareean: H. fugax, H. youngi, and H. merriami. Most early Arikareean samples are referred to H. fugax and are slightly smaller on average than older H. fugax. Later Arikareean Herpetotherium samples are referable to H. youngi on the basis of possessing a single central stylar cusp. A large species, H. merriami, from the early Arikareean John Day Formation in Oregon is similar to Herpetotherium molars described from the early Arikareean or late Whitneyan Florida I-75 locality. Taxonomy of Herpetotherium has previously been based on tooth size and upper molar stylar cusp morphology. H. fugax exhibits several configurations of stylar cusp morphology, separate to fused central stylar cusps, and a wide range of dental size variation. These ranges of variation are similar to those exhibited by the Chadronian species, H. valens.

Ekgmowechashala (Mammalia, ?Primates) from the Gulf Coastal Plain

A single, small, water-worn tooth from the “middle” Arikareean Toledo Bend Local Fauna of the Gulf Coastal Plain closely resembles the lower fourth premolar of the questionable primate Ekgmowechashala. The only known species of the genus, Ekgmowechashala philotau Macdonald (1963), was originally recovered from the early Arikareean Sharps Formation of South Dakota, but is also known from similar aged strata of the John Day Formation, Oregon. Although the Toledo Bend specimen differs somewhat in morphology from the p4 of E. philotau, a new species is not named in this report because of such limited material and because the specimen is incomplete. Unfortunately, the specimen does not provide information that helps clarify current arguments regarding the affinities of Ekgmowechashala with primates or plagiomenids. It does, however, provide (1) a temporal range extension for the genus to the early late Arikareean, or about four million years younger than previously known, and (2) a geographic extension east and considerably south of its prior distribution. If Ekgmowechashala is ultimately determined to belong to the Primates, then the Toledo Bend species would become the last known North American representative of the order.

Chapter 15

Exceptionally well-preserved partial skeletal remains representing a minimum of six individuals of an oreodont are described from the White Springs Local Fauna, Columbia County, northern Florida. Although oreodonts are very common from classic Oligocene and early Miocene deposits in the western United States, this group is poorly represented from Florida. The White Springs oreodont pertains to a new species, Mesoreodon floridensis, and is derived, particularly in the development of the auditory bulla, relative to more primitive, closely related species such as Merycoidodon culbertsoni, the latter of which is well known from the badlands of the western United States. Mesoreodon floridensis differs from most other species assigned to this genus in the relative development of the nasal region, possible presence of a facial vacuity, configuration of the preorbital fossa, relatively simple occipital and zygomatic morphology, and imbricated premolar morphology. Based on the associated faunal remains and age determinations, M. floridensis is late early Arikareean (Ar2) in age, ca. 25–24 million years old, and occurs in an interval not well represented in the classic Arikareean sequence of western Nebraska. A mounted skeleton of M. floridensis, on exhibition, is also described.

Extinction and replacement among predatory mammals in the North American late Eocene and Oligocene: Tracking a paleoguild over twelve million years

Changes in taxonomic and morphologic diversity within the paleoguild of predatory mammals were explored within a mammalian chronofauna spanning a twelve million year interval, from the latest Eocene to the end of the Oligocene of North America (36–24 ma). The timespan encompassed a modest extinction event among terrestrial mammals (circa 34 ma) followed by a period of relative stability. Morphological diversity was assessed with estimates of body mass, relative tooth size, and tooth shape. Principal component, nearest‐neighbor, and minimum‐spanning‐tree analyses were used to compare morphological diversity and species packing within predator paleoguilds in the mid‐Chadronian (37–34 ma), Orellan (34–32), Whitneyan (32–29.5), and early Arikareean (29.5–24) land mammal ages. Species richness of predators throughout the interval was relatively constant, fluctuating between 15 and 18 total taxa. Moreover, despite significant differences in taxonomic composition and a modest extinction event among terrestrial m...

The Skull of Hitonkala (Florentiamyidae, Rodentia) and Relationships within Geomyoidea

Morphology of the four known skulls of the early Arikareean (late Oligocene) Hitonkala andersontau Macdonald indicates that this species is a member of the Florentiamyidae. Hitonkala lacks some of the derived characters of the skull present in the most-advanced florentiamyids such as Sanctimus and Florentiamys , but is more derived than the Whitneyan Kirkomys . Florentiamyids share a number of derived cranial features with the Heliscomyidae and are the immediate sister group of that family.

Additional Specimens of Leptodontomys douglassi (Eomyidae, Rodentia) From the Arikareean (late Oligocene) of Nebraska

Additional specimens of the eomyid rodent Adjidaumo douglassi Burke from the early Arikareean (late Oligocene) Gering Formation in Nebraska imply an Arikareean (rather than Whitneyan) age for the Killdeer Formation of North Dakota, from which the holotype of this species was recovered. The new material also supports the allocation of this species to Leptodontomys, a genus previously known only from the Miocene. The Barstovian genus Pseudadjidaumo, and the North American Hemingfordian specimen referred to the otherwise European genus Eomys also are referred to Leptodontomys. These synonymies suggest a nearly continuous record for Leptodontomys from the Arikareean through the Hemphillian (late Oligocene to late Miocene). This genus belongs to the Adjidaumo-Paradjidaumo lineage of eomyids. The Eomyidae are a primitive family of generally small geomyoid rodents that range from the late-middle Eocene (Uintan) to late Miocene (Hemphillian) in North America, and as late as the Pleistocene in Europe. Their diversity was greatest in the early Oligocene (Fahlbusch, 1973, 1979), then diminished rapidly in North America, with only a few species existing through the Miocene. No lineages of eomyids previously have been traced from the Oligocene to the Miocene of North America; the majority of the Miocene species being morphologically quite distinct from the older species. The specimens discussed here provide the first record of one of the indigenous-North American Miocene genera of eomyids ex

Single-Crystal 40 Ar/ 39 Ar Dating of the Eocene-Oligocene Transition in North America

Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) (40)Ar/(39)Ar dates on tephra from key magnetostratigraphic and fossilbearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal "Age" boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian boundary.

Mid-Cenozoic Tectonic Timing, Trans-Pecos Texas: ABSTRACT

This work attempts to establish the age and chronologic sequence of mid-Cenozoic tectonic events in Trans-Pecos Texas through the use of radiometric dates, and new or revised structural, lithostratigraphic, and vertebrate biostratigraphic information. Late Mesozoic to early Cenozoic Laramide faulting, folding, and jointing superimposed on older trends, established the fabric governing younger structures. Late Oligocene events, occurring about 28 to 26 m.y.B.P., include right-lateral divergent wrench faulting, local compression, and the last episodes of silicic intrusion-extrusion. The major period of basin-and-range faulting began about 19 m.y.B.P., accompanied by late and mafic minor intrusion. This paper follows the sequence of tectonic events established by Muehlberger in 1980 and DeCamp in 1981 for Trans-Pecos Texas. A widespread low relief surface was cut across Laramide structures as deformation decreased in the Eocene. Integrated, perennial streams flowing southeastward began extensive laterally continuous aggradation in the area south and southwest of the stable Diablo uplift. Mainly fine, volcaniclastic sediments accumulated on surfaces of continued low relief. Initially, sediment sources were distant but became progressively more local. Episodic ignimbrites and flows covered large areas with increasing frequency. Eocene climate in Trans-Pecos Texas was humid and subtropical, but an irregular trend toward increasing dryness was evident by 31 m.y.B.P. Sedimentary bodies younger than middle Oligocene have little lateral continuity. Deposited under semiarid conditions, as destructional volcanic sediment aprons, alluvial fans, or bolson fills, these units show progressive divergence from depositional styles of early Tertiary sediments. Early Arikareean (early late Oligocene) right-lateral divergent wrench faulting interrupted long-established drainage patterns. The faulting, dated by intrusions and biostratigraphy at about 28 to 26 m.y.B.P., closed the interval of laterally continuous, and preceded that of discontinuous, deposition. The irregular Terlingua monocline, long considered a Laramide structure, is re-interpreted as another example of Trans-Pecos Texas linear east-west tectonic elements discussed by Dickerson in 1981. The str cture is a large monocline cut by a set of an echelon-left normal faults and smaller monoclines, modified by compression. Formation of the large monocline involved rocks as young as late Eocene-early Oligocene. The Terlingua monocline provides clear evidence of the sequence of events and some indication of timing. Later Arikareean sediments lie on uneven eroded older rocks, disturbed by early stages of this wrench faulting. About 23 m.y.B.P. downfaulted basins began to retain bolson/alluvial fan sediments. Deposition may have resulted from progressive deformation of the change to mafic volcanism. Increasing aridity may also have been a factor. Basin-and-range faulting affected Trans-Pecos Texas during the Hemingfordian (early Miocene), and continues. This tectonism faulted the later Arikareean-Hemingfordian alluvium by reactivating old faults and creating new ones. Basin-and-range faulting shifted, deepened, and more completely restricted basins of deposition by forming a series of northwest oriented grabens which received great thickness of later Miocene and younger alluvium. (See Figure on page 615). End_of_Article - Last_Page 552------------

Potassium-Argon Dates of Some Cenozoic Volcanic Rocks of the Sierra Nevada, California

A date of 19.9 m. y. (early Hemingfordian) from a welded rhyolite tuff at the top of the type section of the Valley Springs Formation, and one of 25.7 m. y. (early Arikareean) from the basal welded rhyolite tuff in the Sonora Pass area, indicate that the Valley Springs Formation was deposited before late Miocene, and that rhyolitic volcanism in the Sierra Nevada began by earliest Miocene time. Three dates of 8.8–9.0 m. y. (early Hemphillian) on biotite-augite latite, which stratigraphically overlies the Table Mountain latite in the Stanislaus drainage, show that the Table Mountain latite is a member of the Mehrten Formation, which contains the late Hemphillian Oakdale fauna and the early Clarendonian Two Mile Bar fauna. A 3.5-million year date on basalt filling a canyon cut in the Chagoopa surface at Little Kern River, a 9.5-million year date on basalt from Jose Basin in Fresno County, and dates of 3.5–3.6 m. y. from basalts in the Upper San Joaquin drainage indicate that the Chagoopa surface and the Mountain Valley stage are at least as old as early Pliocene, and that the Sierra Nevada had a minimum relief of 4000 feet at that time. Basalt beneath McGee Till on McGee Mountain gives a date of 2.6 m. y., and basalt on Coyote Flat gives a date of 9.6 m. y. Basalt beneath Sherwin Till in Owens Gorge has an age of 3.2 m. y. A rhyolite tuff and an overlying basalt in Deep Springs Valley, White Mountains, give ages of 10.9 and 10.8 m. y., respectively.