Geology Reference
In-Depth Information
laminae of very fi ne detrital carbonate sediments with plant and other organic debris
transported during occasional fl oods characterized sedimentation during dry periods
when the water column was reduced drastically to probably just a few centime-
ters. The warm environment, prolonged periods of stagnation, and the high rate of
accumulation of organic debris carried by fl oods maintained anoxic to dysaerobic
conditions in sediments and bottom waters and prevented bioturbation.
Sediments of wet periods record the maximum productivity of Las Hoyas,
although the sediments of dry periods contain the best preserved and most abundant
fossils (about 7× the number in sediments of wet periods). This difference refl ects
greater environmental stress, mainly due to dehydration, and the greater prevalence
of microbial mats. The microbial mats facilitated rapid burial and the establishment
of conditions favourable for preservation as revealed by this and previous studies.
The Konservat-Lagerstätte of Las Hoyas is a product of the interaction of geo-
logical and biological conditions widespread at the margin of Tethys in the
Northern Hemisphere during the Lower Cretaceous (Buscalioni and Fregenal-
Martínez
2006
)
Fish, decapods and plants are the most abundant fossils at Las Hoyas. The sam-
ple LH-50.082 corresponds to
Montsechiavidali
, an aquatic freshwater small wedge
plant. LH-50.083 is a small decapod attributed to the
Austrapotamobius
(Family
Astacidae). Fish remains include an isolated 2 cm scale of an actinopterigianamii
form (LH-50.081) and two juveniles (LH-50.085 and LH-50.087) of primitive tele-
osteans of uncertain phylogenetic relationship that were formerly considered to be
“leptolepid-like” fi shes.
Analytical Protocol
The structure and mineralogy of fossil samples were analysed using a Philips XL 30
environmental scanning electron microscope with energy dispersive x-ray analysis
(Princeton Gamma-Tech).
Fossil samples and modern equivalents were pyrolysed using a CDS 5150
Pyroprobe by heating at 650 °C for 20s to fragment macromolecular organic com-
ponents. Compound detection and identifi cation was performed by on-line gas
chromatography-mass spectrometry (GC-MS) in full-scan mode on a Hewlett
Packard HP6890 gas chromatograph interfaced to a MicromassAutoSpecUltima
magnetic sector mass spectrometer. GC separation was performed on a J&W
Scientifi c DB-1MS capillary column (60 m length, 0.25 mm internal diameter,
0.25
m fi lm thickness) using He as the carrier gas. Samples were injected in
splitless mode at 300 °C. The oven was programmed from 60 (held for 2 min) to
150 °C at 10 °C min
−1
, then at 3 °C min
−1
to 315 °C and held isothermal for 24 min.
The source was operated in electron ionization (EI) mode at 70 eV ionization energy
at 250 °C. The AutoSpec full-scan rate was 0.80 s/decade over a mass range of
50-600 Da and an interscan delay of 0.20 s.
μ
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