This
unique stromatolite comes from an inland freshwater lake in the
State of Chihuahua, Mexico, probably a hypersaline aquatic environment.
In terms of geologic time, this is very young stromatolite (Cretaceous).
While Phanerzoic stromatolite such as this is far less common
than in the Proterozoic, it is distributed worldwide; recently
a 12 foot tall, living stromatolite dome was discovered in a freshwater,
hypersaline lake in Africa.
In
this specimen, the microbial colony began on a still intact host
rock of volcanic origin and then grew in ever-expanding bands.
It is apparently a distinct stromatolite rather than a part of
a large bacrobial mat. Such forms are often called Oncolite, and
are typically found in ancient lakes. The form is typical of younger
stromatolite owing to the vast number of life forms that would
feed on the bacteria as compared to the Precambrian when stromatolite
was at its zenith on Earth, and had little competition.
This
specimen has been cut along one horizontal plane and highly polished
one side to show the intricate detail.
Of
interest from a scientific perspective is the young age. This
dinosaur-age, Cretaceous stromatolite was formed long, long after
stromatolite-building organisms had to compete with other life
forms, and hence had become uncommon on earth, similar to modern
times. The microorganism(s) that form this stromatolite is unknown.
Perhaps it was photosynthetic cyanobacteria, other prokaryotic
bacteria, Eukaryotic algae (often considered a simple plant),
or perhaps some combination of these types of organisms competing
to survive in a not-so-friendly aquatic environment.
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