4,773 research outputs found
The Sterkfontein Valley australopithecine succession
Abstract for paper presented at the 5th SASQUA Conference, July 1979If we knew the kinds and relative frequencies of animal species belonging to a natural living
community, we would be able to predict the supporting environment with some accuracy.
Unfortunately for the palaeoecologist the equivalent parameters of a fossil assemblage usually
differ substantially from those of the ancient living parent community. This distortion results
from the action of a number of taphonomic factors during the passage of remains "from the
biosphere to the lithosphere". The major steps of palaeoenvironmental reconstruction from
fossils follow a circuitous route of erecting hypotheses upon hypotheses:
1. Analyses of taxonomy and relative frequency.
2. Recognition of environmental indicators (El): Which fossil groups are environmentally specialized
(i.e. good Els); and precisely what kind of environments do they indicate? (estimated
from modern analogy).
3. Recognition of taphonomic biases: Have the proportions of Els in the original community
been distorted by preferential inclusion and survival in the assemblage? Such bias or distortion
may be caused by many factors, for example seasonality and duration of deposition,
geographic area sampled, mode of death, transport and accumulation, species
death rate, and so forth.
4. Estimation of El proportions in the original community by correcting where necessary for
taphonomic biases.
5. Interpretation of taxonomic and morphologic change: Let us assume that estimates of original
EI proportions, resulting from steps 1-4, can be seen to change significantly in chronologically
successive strata in one area like the Sterkfontein Valley. Must such morphologic/
taxonomic change necessarily imply a change in the ecosystem, or may it imply no more
than the passage of time?
A particular palaeoenvironmental study on fossil assemblages from Sterkfontein, Swartkrans
and Kromdraai is followed through steps 1-5 to its conclusion.Non
The Bak-Sneppen Model on Scale-Free Networks
We investigate by numerical simulations and analytical calculations the
Bak-Sneppen model for biological evolution in scale-free networks. By using
large scale numerical simulations, we study the avalanche size distribution and
the activity time behavior at nodes with different connectivities. We argue the
absence of a critical barrier and its associated critical behavior for infinite
size systems. These findings are supported by a single site mean-field analytic
treatment of the model.Comment: 5 pages and 3 eps figures. Final version appeared in Europhys. Let
New fossils of Alcelaphini and Caprinae (Bovidae: Mammalia) from Awash, Ethiopia, and phylogenetic analysis of Alcelaphini
Main articleAlcelaphine antelopes comprise one of the most species-rich groups among the mammalian
assemblages from the Middle Awash, Ethiopia, and in Africa as a whole. I describe a new genus and
species Awashia suwai from Matabaietu 3, and other new alcelaphine species, Damaliscus ademassui
from Gamedah I and Beatragus whitei from Matabaietu 3-5, all dated ca. 2.5 m.y. (millions of years).
Other new a lcelaphine fossils from Middle Awash include an Early Pliocene species allied to
Damalops, Late Pliocene records of Parmularius c f.pandatus and Beat rag us amiquus, and Middle
Pleistocene records of Megalotragus kattwinkeli, P. angusticornis, Damaliscus niro, Connochaetes
taurinus olduvaiensis, Numidocapra crassicornis, and Alcelaphus buselaphus. My comparisons of
these fossils with all other known fossil and Recent Alcelaphini inc ludes a cladistic analysis . The
results suggest that during or before the Miocene-Pliocene transition two alcelaphine subtribes
dive rged for which I suggest the names Alcelaphina and Damaliscina. Alcelaphina consists of two
ancient subclades: ( I) the s is te r-group of Damalacra neanica and Beatragus known since 5.0-4.5
m.y. ago, and (2) a large c lade first recorded 4.4 m.y. ago (genera Damalops, Numidocapra,
Alce/aphus, Rabaticeras, Megalotragus, Oreonagor, and Connochaetes) that had a high diversification
rate since 3 m.y. ago. The earliest record of Damaliscina is the form that Gentry ( 1980) named
Damalacra acalla, which emerges as the hypothetical direct ancestor of the Early-Middle Plioce ne
split into Parmularius and the Damaliscus group. The placement of the new genus Awashia remains
problematic. A new ov ibovine genus and spec ies, Nitidarcus asfawi, and a new caprine genu and
species, Bouria anngettyae, both from Bouri I, are also described. I discuss some evolutionary and
biogeographic implications of the new fossils from Middle Awash.Non
On the âDuelâ Nature of History: Revisiting Contingency versus Determinism
Are we âhistorical accidentsâ of an undirected evolutionary history? In his recent book, Islands in the Cosmos, Dale Russell addresses this question, and Brian Swartz reviews his synthesis of this âcosmicâ evolutionary debate
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
GTC Osiris spectroscopic identification of a faint L subdwarf in the UKIRT Infrared Deep Sky Survey
We present the discovery of an L subdwarf in 234 square degrees common to the
UK Infrared Telescope (UKIRT) Infrared Deep Sky Survey Large Area Survey Data
Release 2 and the Sloan Digital Sky Survey Data Release 3. This is the fifth L
subdwarf announced to date, the first one identified in the UKIRT Infrared Deep
Sky Survey, and the faintest known. The blue optical and near-infrared colors
of ULAS J135058.86+081506.8 and its overall spectra energy distribution are
similar to the known mid-L subdwarfs. Low-resolution optical (700-1000 nm)
spectroscopy with the Optical System for Imaging and low Resolution Integrated
Spectroscopy spectrograph on the 10.4 m Gran Telescopio de Canarias reveals
that ULAS J135058.86+081506.8 exhibits a strong KI pressure-broadened line at
770 nm and a red slope longward of 800 nm, features characteristics of L-type
dwarfs. From direct comparison with the four known L subdwarfs, we estimate its
spectral type to be sdL4-sdL6 and derive a distance in the interval 94-170 pc.
We provide a rough estimate of the space density for mid-L subdwarfs of
1.5x10^(-4) pc^(-3).Comment: 5 pages, 1 table, 3 figures, published in ApJ Letters (January 2010
issue
Low-extinction windows in the inner Galactic Bulge
We built K band extinction maps in the area of two candidate low-extinction
windows in the inner Bulge: W0.2-2.1 at (l,b) = (0.25o,-2.15o), and W359.4-3.1
at (l,b) = (359.40o,-3.10o). We employed JHKs photometry from the 2MASS Point
Source Catalog. Extinction values were determined by fitting the upper giant
branch found in the present 2MASS Ks x J-Ks diagrams to a de-reddened bulge
stellar population reference giant branch. We tested the method on the well
known Baade's and Sgr I windows: the 2MASS mean extinction values in these
fields agreed well with those of previous studies. We confirm the existence of
low-extinction windows in the regions studied, as local minima in the A_K maps
reaching A_K values about 2 standard deviations below the mean values found in
the neighbouring areas. Schlegel et al.'s (1998) FIR extinction maps, which
integrate dust contributions throughout the Galaxy, are structurally similar to
those derived with 2MASS photometry in the two studied windows. We thus
conclude that the dust clouds affecting the 2MASS and FIR maps in these
directions are basically the same and are located on foreground of the bulk of
bulge stars. However, the A_K absolute values differ significantly. In
particular, the FIR extinction values for W359.4-3.1 are a factor ~1.45 larger
than those derived from the 2MASS photometry. Possible explanations of this
effect are discussed. The lower Galactic latitudes of the low-extinction
windows W359.4-3.1 and W0.2-2.1, as compared to Baade's Window, make them
promising targets for detailed studies of more central bulge regions.Comment: 8 pages, 10 figures, LaTeX, aa.cls. To appear in Astron. & Astroph.,
in pres
Fine Structure in the Circumstellar Environment of a Young, Solar-like Star: the Unique Eclipses of KH 15D
Results of an international campaign to photometrically monitor the unique
pre-main sequence eclipsing object KH 15D are reported. An updated ephemeris
for the eclipse is derived that incorporates a slightly revised period of 48.36
d. There is some evidence that the orbital period is actually twice that value,
with two eclipses occurring per cycle. The extraordinary depth (~3.5 mag) and
duration (~18 days) of the eclipse indicate that it is caused by circumstellar
matter, presumably the inner portion of a disk. The eclipse has continued to
lengthen with time and the central brightness reversals are not as extreme as
they once were. V-R and V-I colors indicate that the system is slightly bluer
near minimum light. Ingress and egress are remarkably well modeled by the
passage of a knife-edge across a limb-darkened star. Possible models for the
system are briefly discussed.Comment: 19 pages, 5 figure
Infrared Excess in the Be Star Delta Scorpii
We present infrared photometric observations of the Be binary system delta
Scorpii obtained in 2006. The J,H and K magnitudes are the same within the
errors compared to observations taken 10 months earlier. We derive the infrared
excess from the observation and compare this to the color excess predicted by a
radiative equilibrium model of the primary star and its circumstellar disk. We
use a non-LTE computational code to model the gaseous envelope concentrated in
the star's equatorial plane and calculate the expected spectral energy
distribution and Halpha emission profile of the star with its circumstellar
disk. Using the observed infrared excess of delta Sco, as well as Halpha
spectroscopy bracketing the IR observations in time, we place constraints on
the radial density distribution in the circumstellar disk. Because the disk
exhibits variability in its density distribution, this work will be helpful in
understanding its dynamics.Comment: 12 pages, 14 figures, to be published in PASP May 200
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