|
|
|
CETACEAN EVOLUTION |
|
The fact that the Cetaceans are splendid
and efficient marine creatures does not mean that always they
have been it. Their evolutionary history is as interesting as
uncertain; the fossil rests are in fact insufficient and
fragmentary.
|
|
The supporters of the theories that try to answer
to the question if the evolution is the result of accidental
genetic modifications in the individual patrimony, such to allow to
the same individual, a better adaptation to the ambient in which lives (C.
Darwin ), or if it is the same ambient to influence directly genetic
modifications (J.B. Lamarck ), they interrogate themselves on which
have been the evolutionary mechanisms that have carried originally
land animals, to radically modify therefore their life style.
|
Approximately 65 million years ago (Paleocene) the uncontested dominion of the
Dinosaur on the earth
finished and those that until
then had been small, timid animals, began their rise to true
dominating of the planet, were be a matter of the Mammals, ready to
occupy every ecological niche left free, assuming therefore the shapes
and dimensions more disparate .
|
|
Ten million years after, Mammals had already created many species, but in particular only one interests us, draft of the Mesonychide, a large land predator of
aspect and dimensions variable (along in average 1.5 m) supplied of
one skull a lot developed and large teeth, carnivore who did eat
probably fishes and carrions.
|
The aspect of the emerged lands on
which the Mammals were developed was much
various from the one of our days, the so-called drift of the continents was, therefore
like now, modifying the disposition of seas and lands. In
the area of the current Mediterranean Sea, between Europe and Africa, existed, 55 million years ago (the Eocene), the so-called Sea of
Tethys, one immense marshy area and brackish from with few deep sounding-depth and mild climate
|
|
It will be just the Mesonychide to giving origin to the first
cetacean said Archeoceti (from Greek archùs "the ancient"and from the Latin cetus"cetaceo") and to the Condylarthra an ungulate mammalian
progenitor of the current ungulates (ex. horse, camel, ox etc).
|
The study of the Archeoceti has allowed to specify several species
grown in the following 30 million years, the Ambuloceti,
the Protoceti, the Pakiceti, the Dorudonti, the Remingtonoceti, the
Basilosauri. It was dealt of animals not still only aquatic
but going to specialize. They lived in the so-called Sea of
Thethys the body lengthened (from a pair of meters until 21 m)
equipped of one long tail to whose sides had grown two lobes for increase the propulsion in water, posterior legs reduced or passings
and the front ones transformed in fins, the nostrills moved towards the
top of the head; the teeth still were distinguished in incisors,
canines and molars; the auditory apparatus still very developed. They did eat mainly of crustaceans and fish that
roused in low waters.
Turtles, crocodiles, fish lived in their same coastal ambient fish-cat
and molluscs.
 But they
slowly began to specialize themself in the
hunting to the wide sea, they were the Basilosauri,
that had a snaked body. The Basilosauri can subdivide in two familys: Basilosaurinae and Dorudontinae. Just from
these last ones it is thought (but the contrary opinions do not lack)
have had origin the current Cetaceans. Approximately 25 million years
ago in fact all the Archeoceti disappeared after to have given life to
two groups of Cetaceans, current the Odontoceti (from the Greek odontòs
"tooth", therefore cetaceans with the teeth) and Misticeti (from the
Greek mystax "moustaches", therefore cetaceans with the whalebones). In
the first ones teeth became uniform (omodontia) and development the
use of the biosonar in order to characterize the prey; not all the
Odontoceti born in those years survived until to our days the
Squalodontidi and the Eurinodelfinidi as an example has been
extinguished. Second ones (misticeti) born 34 million years ago
constituted the result of the evolution of the dental apparatus that
had been modified for the capture of small prey; the teeth will yield
the place to whalebones (corneous sheets adapted to leak) and the
alimentary habits forced
to periodic migrations to the search of the
food. In both cases however the evolution had carried to animals able to colonize all the seas of the
world with a multitude of species
variety. The molecular biologist have recently begun to
study the evolution of marine mammalian confirming, through genetic
tests, how much already supported from the evolutionary theses (have
been found affinity as an example between the blood of the cetaceans and the one of cow, of the camel and the pig).
|
|
As said initially, the
study of the evolution of the cataceans is a puzzle whose pieces found are little and difficult to place in the correct place; it
testifies the fact that many key points of what we said, also being
supported from authoritative scientists, are from others refused (one
for all the controversial positioning of the Sperm whale between the
Odontoceti or the Misticeti).
|
|
|
|
|
|
