Barstow Formation Research Articles

Middle Miocene fire activity and C4 vegetation expansion in the Barstow Formation, California, USA

Abstract C4 grassland ecosystems expanded across North America between ca. 8 and 3 Ma. Studies of ungulate enamel and environmental indicators from the middle Miocene Barstow Formation of southern California (USA) have demonstrated the presence of C4 vegetation prior to the late Miocene expansion of C4 grasslands. Fire promotes the growth of modern C4 grasslands and may have contributed to the Miocene expansion of C4 vegetation. We analyzed the concentration and accumulation rate (CHAR) of microscopic charred particles from sediment samples spanning the Barstow Formation in order to investigate the relationship between fire activity, canopy cover, and the presence of C4 vegetation. Concentration and CHAR were low throughout the formation then increased dramatically at 13.5 Ma. Enriched values of δ13C from soil organic matter and phytolith counts indicate the presence of C4 grasses and seasonally dry, open-canopy habitats at this time. The spike in concentration coincides with climatic cooling and drying in southern California after the Miocene Climatic Optimum. Increased fire activity may have contributed to habitat opening from forest to woodland and promoted the spread of C4 plants. This is the first charcoal record of fire activity from the middle Miocene of southwestern North America.

Taphonomic bias in collections of horse phalanges from the Barstow Formation (Miocene) and Rancho La Brea (Pleistocene) of California, USA

Isolated equid phalanges are relatively common finds in the Barstow Formation (Miocene, 19 to 13 million years ago, southern California), but anecdotal observations suggested that not all positions (proximal, middle, and distal/ungual) of the primary digit (digit III) are recovered with equal frequency. Our sample includes primarily surface-collected phalanges from the Barstow Formation, which we compare with phalanges of Pleistocene horses from Pit 3 and Pit 77 from Rancho La Brea (Los Angeles, California). The null hypothesis is that the three positions of phalanges should be equally common. Our Barstow sample includes 228 proximal, 151 middle, and 36 distal phalanges. A chi-square test (p<0.001) is consistent with preservation bias in phalangeal frequency for the full Barstow Formation sample, and this pattern generally holds within sub-samples by locality or depositional environment. Pit 3 of La Brea produced 163 proximal, 144 middle, and 103 distal phalanges. A chi-square test with correction for multiple comparisons suggests that proximal and middle phalanges are preserved with statistically equal frequency (p=0.278) whereas distal phalanges are less common (p<0.001). For Pit 77, there are 54 proximal, 55 middle, and 51 distal phalanges; the chi-square test finds that all three types are equally common (p=0.922). Overall, differences in physical properties between phalangeal positions, such as surface area, density, shape, and size, could influence preservation within each environment. The observed differences between Barstow and La Brea might be caused by variations in depositional environment that influence the surface exposure time of fossils and disarticulation pre-burial, as well as by differences in the size of the horses at each locality. We suggest that when permitted by sample size, it is desirable to distinguish unguals from other phalanges when analyzing taphonomic patterns in the fossil record.

New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA

New material of three equids is described from the middle Miocene Cajon Valley Formation in San Bernardino National Forest, San Bernardino County, California. The material includes teeth of Archaeohippus mourningi, Scaphohippus sumani, and Parahippus brevidens. Scaphohippus intermontanus is considered a junior subjective synonym of S. sumani. Parahippus brevidens is identified from an upper molar that closely resembles the morphology of the holotype as well as referred specimens of Pa. brevidens from the Mascall Formation in Oregon and the Temblor Formation in California. The presence of Pa. brevidens in the Cajon Valley Formation represents a geographic range extension for the taxon of over 400 km. Interesting ecological implications emerge for the Cajon Valley Formation when compared to the nearby Barstow Formation, including the presence of chalicotheres and apparent lack of Hypohippus affinis and Megahippus mckennai.

THE INFLUENCE OF DEPOSITIONAL ENVIRONMENT AND BASIN HISTORY ON THE TAPHONOMY OF MAMMALIAN ASSEMBLAGES FROM THE BARSTOW FORMATION (MIDDLE MIOCENE), CALIFORNIA

ABSTRACTThe Barstow Formation in the Mojave region of California was deposited in an extensional-basin setting of the Basin and Range province and preserves diverse middle Miocene mammalian assemblages. Six facies associations represent the dominant depositional environments in the basin, which changed through time from alluvial-fan and playa-dominated settings to floodplains and spring-fed wetlands. The majority of fossil localities and specimens occur in later-forming facies associations. We analyzed the taphonomic characteristics of fossil assemblages to test whether basin-scale facies associations or locality-scale facies exert more control on the preservational features of mammalian assemblages through the formation. We documented the facies settings of 47 vertebrate localities in the field in order to interpret depositional setting and the mode of accumulation for fossil assemblages. We evaluated skeletal material in museum collections for taphonomic indicators, including weathering stage, original bone-damage patterns, hydraulic equivalence, and skeletal-element composition. We evaluated four alternative modes of accumulation, including attritional accumulation on the land surface, accumulation by fluvial processes, carnivore or scavenger accumulations, and mass-death events. The majority of localities represent attritional accumulations at sites of long-term mortality in channel-margin, abandoned-channel, poorly drained floodplain, and ephemeral-wetland settings. Skeletal-element composition and taphonomic characteristics varied among facies, indicating an important role for depositional setting and landscape position on fossil-assemblage preservation. We find that locality-scale facies have a greater influence on the taphonomic characteristics of fossil assemblages; the taphonomy of each facies association is influenced by the facies that compose it. The facies composition and distribution within facies associations change through the formation, with a greater variety of depositional settings forming later in the history of the basin. Heterogeneous landscapes present more settings for fossil accumulation, contributing to the increase in fossil occurrence through the depositional history of the formation.

Vegetation and habitat change in southern California through the Middle Miocene Climatic Optimum: Paleoenvironmental records from the Barstow Formation, Mojave Desert, USA

AbstractThe Barstow Formation (ca. 19–13 Ma) of southern California, USA, is a terrestrial sequence known for its diverse Miocene mammalian faunas. The formation spans the Middle Miocene Climatic Optimum (MMCO; 17–14 Ma) and offers the opportunity to study environmental change during the last major interval of global warming of the Cenozoic. We combined isotopic analyses of carbon and hydrogen from sedimentary n-alkanes and bulk soil organic matter with analysis of phytoliths (plant silica) and diatoms to reconstruct vegetation composition, habitat structure, and moisture dynamics through the formation. The δ13C and δD of long-carbon-chain n-alkanes derived from terrestrial plants and preserved in sediments of the Barstow Formation record drying prior to the MMCO and through the Middle Miocene Climatic Transition (MMCT; beginning ca. 14 Ma), punctuated by increased moisture in fluvial environments during the peak of the MMCO. Variable isotopic results relate to changes in moisture, driven partly by facies, vegetation composition, tectonic activity, and climatic variability coinciding with intervals of high pCO2. Phytolith assemblages are dominated by forest indicators in riparian habitats during the MMCO, and grass morphotypes constitute significant components of phytolith assemblages after the MMCO, indicating a shift to drier, more open-canopy habitats. The establishment of dry, wooded grasslands in the Barstow Basin coincides with the beginning of the MMCT and cooling sea-surface temperatures in the Pacific Ocean. Our results indicate that moist, closed-canopy habitats formed in southern California during climatic warming, followed by savannah during climatic cooling and the shift to seasonal precipitation regimes.

FACIES, ENVIRONMENTS, AND FOSSIL PRESERVATION IN THE BARSTOW FORMATION, MOJAVE DESERT, CALIFORNIA

Continental deposits of the middle Miocene Barstow Formation formed in an extensional basin of the Mojave region. Mammalian faunas of the type section in the Mud Hills, California, are the basis for defining the Barstovian North American Land Mammal Age. Prior mapping, lithostratigraphy, geochronology, and fossil collecting have established a structural, chronostratigraphic, and biostratigraphic framework for the Barstow Formation. Here, we analyze the sedimentary facies of the lithostratigraphic sequence, infer the major depositional environments, and assess controls on fossil preservation in relation to major facies. We recognize six major facies associations that represent alluvial fans, shallow or seasonal lakes, and channels and floodplains. The transition from older, fan-dominated to younger, floodplain-dominated environments reflects changes in tectonics, accommodation, drainage, and depositional settings during the formation and filling of the basin. Fossil-mammal concentrations are common in four facies associations. Body fossils are abundant in two facies associations that represent well-drained floodplains and spring-fed wetlands. Fossil concentrations are moderately abundant in facies associations that represent floodplains and channel deposits, and fossils are rare in facies associations that represent alluvial fans and lake margins. The highest frequency of fossil localities occurs in the facies associations representing both suitable life habitats and high preservation potential for terrestrial mammals.

Miocene cyclopid copepod from a saline paleolake in Mojave, California

There are remarkably few direct fossil records of Copepoda, which implies that current estimates of the lineage divergence times and inferences on the historical biogeography remain highly dubious for these small-sized crustaceans. The Cyclopidae, a predominantly freshwater copepod family with 1000+ species and distributed worldwide, has no fossil record at all. Recent collections from the middle Miocene Barstow Formation in Southern California resulted in ample material of finely preserved cyclopid fossils, including both adult and larval stages. To document the antennulary setation pattern in the adult and copepodid instars we used a coding system that is coherent between sexes and developmental stages. The majority of the cyclopid fossils, coming from saline lake environment, represent the modern genus Apocyclops, a euryhaline, thermophilic group occurring both in the New World and Old World. A new species Apocyclops californicus is described, based on the short medial spine and spiny ornamentation of ...

Stable carbon isotope evidence for the abundance of C4 plants in the middle Miocene of southern California

The abrupt spread of grasslands using C4 photosynthesis, sometimes referred to as the rapid increase in C4 ecosystems (RICE), occurred in the late Miocene in North America. While fossil plant specimens from the Miocene Dove Spring Formation of California as well as data from phylogenetic studies and molecular clocks show that C4 grasses evolved prior to the RICE, most isotopic data from paleosols and mammal tooth enamel suggest that its abundance on the landscape was minimal. However, a few recent studies from the Great Plains suggest that C4 grasslands may have been more prominent prior to the RICE event. Here we examine stable carbon isotope values from ungulate tooth enamel from the Barstow Formation of southern California, which is geographically and temporally close to the Dove Spring Formation, and records a diverse and abundant paleofauna of medial Miocene age. Tooth enamel δ13C values were examined in seven ungulate genera including the hypsodont equids; Acritohippus sp. and Scaphohippus sp.; the camelids, Aepycamelus sp., Hesperocamelus sp., and Procamelus sp.; the antilocaprid, Merycodus sp.; and the proboscidean, Gomphotherium sp. More positive δ13C values observed within the equids, camelids, and antilocaprids are suggestive of C4 grasses being included in the diets of these taxa. The equids exhibited the most positive mean δ13C values, which indicate a higher component of dietary C4 grasses (up to 18%) when compared to the other sampled ungulate taxa. The tooth enamel isotope values presented in this study show the presence of C4 grasses as a significant component of ungulate diets millions of years before the RICE. The abundance of C4 plants earlier in the Miocene may imply a more significant role in the major ungulate diversification events than previously suspected. The few pre-RICE localities showing evidence of C4 abundance suggests that these grasslands were geographically restricted which limits, but may not exclude, the possibility that a world-wide mechanism controlled its spread.

Partial skull of Zygolophodon (Mammalia, Proboscidea) from the Barstow Formation of California

Click to increase image sizeClick to decrease image size ACKNOWLEDGMENTS We thank R. Tedford, M. Woodburne, and D. Pagnac for helpful discussions, A. Farke, G.-S. Kim, and J. Taylor for help with figures, J. Meng and J. Galkin from the AMNH for access to specimens, G. Jorgenson, D. Lawler, and J. Shearer of the California Bureau of Land Management for field work permits, the Mary Stuart Rogers Foundation and the David B. Jones Foundation for financial support, and J. J. Saunders and D. Pagnac for their helpful suggestions. Handling editor: Jessica Theodor

Surface ages and rates of erosion at the Calico Archaeological Site in the Mojave Desert, Southern California

Erosion rates and surface exposure ages were determined at the Calico Archaeological Site in the Calico Hills of the Mojave Desert, California, using 10Be terrestrial cosmogenic nuclides (TCN) methods. The Calico Hills are composed of Miocene lacustrine deposits of the Barstow Formation and fanglomerates/debris flows of the Pleistocene Yermo Deposits. These deposits are highly denuded and dissected by arroyos that have surfaces armored with chert. Surface erosion rates based on cosmogenic 10Be concentrations in stream sediments range from 19 to 39 m/Ma, with an average of 30.5 ± 6.2 m/Ma. Surface boulders have 10Be TCN ages that range from 27 ka to 198 ka, reflecting significant erosion of the Calico Hills. The oldest boulder age (197 ± 20 ka) places a minimum limit on the age of Yermo deposits. Depth profile ages at four locations within the study area have minimum ages that range from 31 to 84 ka and erosion rate-corrected surface exposure ages ranging from 43 to 139 ka. These surface exposure ages support the view that the surfaces in Yermo deposits formed during the Late Pleistocene to latest Middle Pleistocene. This chronology has important implications for interpreting the context of possible artifacts/geofacts at the site that might provide evidence for early human occupation of North America, and for reconstructing paleoenvironment change and landscape evolution in the region.

Exceptionally preserved ostracodes from a Middle Miocene palaeolake, California, USA

Abstract: Exceptionally well-preserved specimens of a new cypridid ostracode (Crustacea), Raepula ira sp. nov., are described from palaeolake sediments of the Middle Miocene Barstow Formation of the Mud Hills, southern California. This is only the second occurrence of exceptionally preserved ostracodes from the Miocene. Based on ages obtained from associated volcanic tuffs the palaeolake sediments were deposited between 16.3 and 15.8 Ma. The ostracodes form one element of a diverse lake community dominated by fairy shrimps, copepods, diatoms, larvae of diving beetles, flies and mosquitoes, and body fossils and ephippia of branchiopods and anomopods. The ostracodes are preserved three dimensionally with their soft anatomy replicated in microcrystalline silica. Submicron-scale details such as sensory setae are preserved, surpassing the resolution of most other ostracode-bearing lagerstätten and allowing their biology to be compared with extant taxa. Supplementary material: Red–blue anaglyphs of the exceptionally preserved ostracodes are available at http://www.geolsoc.org.uk/SUP18407 .

Revised large mammal biostratigraphy and biochronology of the Barstow Formation (Middle Miocene), California

A new biostratigraphic zonation for the middle Miocene Barstow Formation based on the mammalian megafauna is presented. Four biostratigraphic zones are outlined, two containing fossil assemblages of early Barstovian (Ba1) age, and two with late Barstovian (Ba2) assemblages. Recommendations are made for defining and characterizing the Barstovian North American Land Mammal Age (NALMA). The Ba1 biochron is defined based on the first occurrence of Plithocyon, and the base of the Ba2 biochron is revised based on the first appearance of the antilocaprid Ramoceros. The ursid Plithocyon remains a valid defining taxon for the base of the Barstovian NALMA. The use of both gomphotheriid and mammutid proboscideans to define the base of the Ba1 or Ba2 is abandoned due to the diachrony of first appearances across North America. The Ba1 biochron is additionally characterized based on the first appearance of the equid Scaphohippus. The base of the Ba2 biochron is revised based on the first appearance of Ramoceros. The Ba2 can also be characterized by the first appearance of the borophagine canid Protepicyon and the anchitherine equid Megahippus.

Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression in the central Mojave Desert, California

New geologic mapping, structural data, and 40 Ar/ 39 Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Mountains, located in a complexly deformed region of California9s central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fine-grained lacustrine beds (here referred to as the Calico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region. In the southern Calico Mountains, the Calico fault (part of the Eastern California shear zone) forms a west-northwest–striking, transpressional restraining bend with ∼3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging-wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and directly overlies metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member was coeval with rapid unroofing of the central Mojave metamorphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression. Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ∼0.5 km) north-south shortening. Folds are detached along the base of the Calico Member and thrust over the Pickhandle Formation, which dips homoclinally ∼15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpression between the Calico Member and the Pickhandle Formation within a positive flower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

Scaphohippus, A New Genus of Horse (Mammalia: Equidae) from the Barstow Formation of California

A new genus of hypsodont equid, Scaphohippus, is recognized from the Green Hills Division (Ba1), Second Division (Ba1), and Barstow Faunas (Ba2) of the medial Miocene Barstow Formation, as well as from Ba1 and Ba2 faunas in the Punchbowl Formation of California, the Tesuque Formation of New Mexico, the Pawnee Creek Formation of Colorado, and the Olcott Formation of Nebraska. The genus includes two species, Scaphohippus sumani n. sp. and Scaphohippus intermontanus n. sp. Scaphohippus is a member of the “protohippine” clade, but is differentiated from its more derived sister taxon, Protohippus, by an arc-shaped incisor battery, a shorter I3-P1 diastema, and a more rounded protocone. The upper cheek teeth of S. sumani n. sp. have an unworn crown height of 40 mm. The protocone remains isolated until as much as 60% of wear, and the enamel fossettes are more complex than those of S. intermontanus n. sp. S. intermontanus n. sp. has an unworn crown height of 48–50 mm, the protocone opens by 30% of wear, and the enamel fossettes are simpler than those of S. sumani n. sp. Phyletic analysis shows S. sumani n. sp. to be the more primitive species. Similar facial and dental characteristics reveal a likely common ancestor with Merychippus insignis. The wider geographic and temporal range of S. sumani n. sp. reported here allows for useful phyletic and biogeographic interpretations. S. sumani n. sp. and M. insignis diverged during the late Hemingfordian (He2) of the Great Plains and both taxa dispersed throughout the western United States. S. intermontanus n. sp. is only recognized from the Barstow Formation, where it was likely directly descended from S. sumani n. sp.

Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California

Research Article| March 01, 2005 Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California Jennifer M. Cole; Jennifer M. Cole 1Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794-4364, USA Search for other works by this author on: GSW Google Scholar E. Troy Rasbury; E. Troy Rasbury 2Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA Search for other works by this author on: GSW Google Scholar Gilbert N. Hanson; Gilbert N. Hanson 2Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA Search for other works by this author on: GSW Google Scholar Isabel P. Montañez; Isabel P. Montañez 3Department of Geology, University of California, Davis, California 95616-8605, USA Search for other works by this author on: GSW Google Scholar Vicki A. Pedone Vicki A. Pedone 4Department of Geological Sciences, California State University, Northridge, California 91330-8266, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jennifer M. Cole 1Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794-4364, USA E. Troy Rasbury 2Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA Gilbert N. Hanson 2Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA Isabel P. Montañez 3Department of Geology, University of California, Davis, California 95616-8605, USA Vicki A. Pedone 4Department of Geological Sciences, California State University, Northridge, California 91330-8266, USA Publisher: Geological Society of America Received: 31 Dec 2003 Revision Received: 23 Jul 2004 Accepted: 30 Jul 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (3-4): 276–287. https://doi.org/10.1130/B25553.1 Article history Received: 31 Dec 2003 Revision Received: 23 Jul 2004 Accepted: 30 Jul 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jennifer M. Cole, E. Troy Rasbury, Gilbert N. Hanson, Isabel P. Montañez, Vicki A. Pedone; Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California. GSA Bulletin 2005;; 117 (3-4): 276–287. doi: https://doi.org/10.1130/B25553.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sedimentary rocks and associated fossils are the direct record of past life and its environmental context. Few methods allow direct and precise dating of this important archive, making it difficult to correlate climate, biologic, and tectonic events preserved in terrestrial records. We sampled lacustrine tufa calcite from the middle member of the Miocene Barstow Formation to further test the feasibility of uraniumlead dating of terrestrial carbonates. Five samples yield U-Pb ages (2σ) of 14.81 ± 0.39 Ma, 15.30 ± 0.25 Ma, 15.39 ± 0.15 Ma, 16.14 ± 0.40 Ma, and 16.24 ± 0.23 Ma. These precise ages are consistent with existing ages from K-Ar and 40Ar/39Ar dating of intercalated volcanic ashes. Our most precise U-Pb age on tufa has an uncertainty of <1% (150 ka), which is equivalent to uncertainties for K-Ar ages from the same time period. Our results indicate that U-Pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrographic and trace element analysis of uranium‐rich tufa calcite, middle Miocene Barstow Formation, California, USA

AbstractAn integrated petrographic and spectroscopic (X‐ray diffraction, phosphor imaging and synchrotron X‐ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X‐ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X‐ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine‐scale inclusions.

CALICOVATELLUS PETRODYTES, A NEW GENUS AND SPECIES OF PRIMITIVE VATELLINE DIVING BEETLE (COLEOPTERA: DYTISCIDAE: HYDROPORINAE: VATELLINI) FROM THE MIOCENE BARSTOW FORMATION, SOUTHERN CALIFORNIA, USA

Relatively few fossil dytiscids have been described. This is unfortunate since fossils can provide useful phylogenetic and evolutionary information including unique character combinations not present in extant taxa and minimum ages for divergences. However, even when fossils are found, important characters are often not visible since they may be poorly preserved or obscured. The fossil insects present in calcareous nodules from the Miocene Barstow Formation of the Calico Mountains in Southern California are exceptionally well preserved (Palmer, 1957). The original organisms are replaced by silica or other minerals and when the nodules are dissolved in formic acid, the three-dimensional fossil can be retrieved from the resulting residue. These nodules have yielded a wide variety of fossils, including larvae of the dytiscid species Schistomerus californense Palmer, 1957 and numerous other terrestrial and fresh-water arthropods (Palmer, 1957). The purposes of this paper are to describe a new dytiscid genus and species from an exceptionally well-preserved specimen from the Barstow Formation and to present a hypothesis of the phylogenetic placement of the new taxon.

Calicovatellus petrodytes, a new genus and species of primitive vatelline diving beetle (Coleoptera: Dytiscidae: Hydroporinae: Vatellini) from the Miocene Barstow Formation, southern California, USA

Relatively few fossil dytiscids have been described. This is unfortunate since fossils can provide useful phylogenetic and evolutionary information including unique character combinations not present in extant taxa and minimum ages for divergences. However, even when fossils are found, important characters are often not visible since they may be poorly preserved or obscured. The fossil insects present in calcareous nodules from the Miocene Barstow Formation of the Calico Mountains in Southern California are exceptionally well preserved (Palmer, 1957). The original organisms are replaced by silica or other minerals and when the nodules are dissolved in formic acid, the three-dimensional fossil can be retrieved from the resulting residue. These nodules have yielded a wide variety of fossils, including larvae of the dytiscid species Schistomerus californense Palmer, 1957 and numerous other terrestrial and fresh-water arthropods (Palmer, 1957). The purposes of this paper are to describe a new dytiscid genus and species from an exceptionally well-preserved specimen from the Barstow Formation and to present a hypothesis of the phylogenetic placement of the new taxon.

Paleoecology of an Exceptionally Preserved Arthropod Fauna from Lake Deposits of the Miocene Barstow Formation, Southern California, U.S.A

Abstract A unique aquatic arthropod Konservat Lagerstatte occurs in lacustrine-derived carbonate concretions within the Miocene Barstow Formation of southern California. Faunal elements are primarily aquatic, salinity-tolerant, autochthonous predaceous diving beetle larvae, fly larvae and pupae, mosquito and thrip larvae, and fairy shrimp. Three-dimensional preservation of soft tissues is via replacement by silica-based minerals as well as calcite, celestite, apatite, and gypsum. The fauna occurs in three beds (each 1–3 m thick) within an approximately 100 m sequence of predominantly microlaminated mudstone deposited in a saline-alkaline lacustrine environment. Faunal composition is consistent between sites and concretion beds, but faunal composition and diversity changed significantly during lake history, corresponding to differences in brine chemistry as well as preservational facies, resulting from the shift toward a more nearshore regime. The changes in lake chemistry and subsequent changes in fauna i...

Paleoecology of an Exceptionally Preserved Arthropod Fauna from Lake Deposits of the Miocene Barstow Formation, Southern California, U.S.A.

Paleoecology of an Exceptionally Preserved Arthropod Fauna from Lake Deposits of the Miocene Barstow Formation, Southern California, U.S.A.