Dominican Amber - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 225

mines. Most mines are in the northern
mountain range, the Cordillera
Septentrional ( 250 ), but a couple of mines
are found in the eastern Cordillera
Oriental, north-east of the capital Santa
Domingo. The mountain ranges were
formed by severe crustal folding some
25,000-10,000 years ago, which is very
recent in geological terms, but the rocks
they are made of were laid down between
about 15 and 45 million years ago (Eocene
to Miocene). The few dating studies which
have been done on the strata, mainly using
marine microfossils in the matrix, indicate
that amber from different sites could be of
different ages, in which case the amber
forest lasted for some 30 million years. It is
quite likely, however, that some of the
dating studies, particularly the older ones,
are spurious. The amber-bearing rocks
region in the north occur in the upper 300
m (984 ft) of an Oligocene to Middle
Miocene suite called the La Toca
Formation, which consists mainly of
sandstones containing thin beds of lignite
and occasional conglomerates. The
eastern amber deposit occurs in lignite
and sandy clay of the 100 m (328 ft) thick
Yanigua Formation ( 251 ). These two
amber occurrences are thought to have
been deposited in the same sedimentary
basin. Iturralde-Vinent (2001) provided
an up-to-date review of the geology of the
amber-bearing strata of the Greater
Antilles, and he placed the amber-bearing
part of the La Toca Formation as late Early
to earliest Middle Miocene (16-19 Ma).
At first sight, insects in amber appear to
be preserved in three dimensions, with
their original cuticle and coloration, but as
empty husks lacking any internal organs.
This appearance was first shown to be only
part of the story as long ago as 1903, when
Kornilovich described striated muscles in
Baltic amber insects. Later, Petrunkevitch
(1950) recognized internal organs in Baltic
amber spiders, and the scanning electron
microscope studies of Mierzejewski
(1976a, b) showed that the preservation of
internal structure of spiders in Baltic
amber (including spinning glands, book

lungs, liver, muscles, and haemolymph
cells) was better than that shown by the
insects. Using the transmission electron
microscope, Poinar and Hess (1982)
revealed muscle fibres, cell nuclei,
ribosomes, endoplasmic reticulum, and
mitochondria in a Dominican amber gnat.
An excellent review of amber taphonomy
was provided by Martínez-Delclós et al .
(2004). Organisms are preserved in amber
by a process known as mummification. In
this process, dehydration of the tissues
results in their shrinkage to some 30% of
their original volume (thus giving the fossil
the appearance of an empty husk). The
organic material is not removed from
oxidation because amber does allow slow
gaseous diffusion (so cannot be used in the
study of ancient atmospheres as had been
hoped), but amber has fixative and
antibacterial properties, like many resins.
The ancient Egyptians used resins in the
preparation of their mummies, and the
antibacterial properties of many resins are
well known; the distinctive flavor of Greek
retsina wine comes from the use of resin to
prevent its oxidation to vinegar.
Because of the excellent preservation
of structures at the subcellular level there
has been considerable interest in the
possibility of recovering pieces of the
macromolecule deoxyribonucleic acid
(DNA) from amber-preserved organisms.
In the movie Jurassic Park , it was suggested
that fossilized dinosaur blood could be
extracted from the gut of a gnat preserved
in Mesozoic amber, and the DNA
sequence of the dinosaur thus revealed
could then be used to generate living
dinosaurs. Like all the best science fiction,
this idea is based on possibility. However,
while attempts have been made to extract
DNA from ambers, none has been
successful, and since it is now known that
amber is not as impermeable as was
thought, it is highly unlikely that the
molecule could have survived for millions
of years (or even for more than a few
hours after death; it is known that DNA
breaks down rapidly after cell death)
without degradation.