Russian Ordoviocian Trilobites
By the Ordovician
age (504 to 441 million years ago), the outcome of the solar energy- and floral
oxygen-
driven
Cambrian explosion was manifest in the still accelerating diversity of trilobites
(fewer families than in the Cambrian, but more morphological variation). With
100 million years of selective pressure behind them, the trilobites from what
is today Russia developed unusual shapes, some with eyes on long stalks, others
with jagged spines. Ultimately these magnificent animals, in whole or part, met
their demise in earth's biggest ice age.
During
the Ordovician, what is now Eastern Europe was a shallow inland sea. The fauna
was rich and diverse, and included the early vertebrates. This was the heart of
the age of the trilobite, a time when trilobites were not only diverse, but had
undergone adaptations yielding striking morphologies. The trilobites of Russia,
and specifically those coming from the Region near Saint Petersburg, are among
those most cherished by collectors. The reasons are several, their exotic forms,
superb preservation, and availablity.
The
Baltic limestone deposits that bear these trilobites was formed
in seas of about 70 - 330 feet in depth in what was probably
a large shallow basin. Scientists believe that this basin was
repeatedly blocked from contact with the sea to the west,resulting
in changes in both turbidity and salinity. Toward the end of
the early Ordovician the connection was re-established, with
substantial deterioration in visibilty in the seas due to the
influx of more turbid water. At this time some trilobites developed
raised eyes perhaps to impove their vision both of potential
prey as well as predators. The "periscope eyes" of
Asaphus Kowalewski were not yet
evident as this species evolved somewhat later. While many trilobites
were detritivores feeding on animal and plant debris in the
bottom muck, Asaphids are presumed to have been necrophagous,
meaning that they fed on dead carcasses, due to the shape of
the hypostoma (platform for mouthparts).
The
remains of this ancient Ordovician fauna is abundant in the baltic 
region, and
the city of Saint Pertersburg literally sets on a bed of Ordovician limestone.
Many Ordovician outcrops occur south of Saint Petersburg, and far and away the
most prolific yield of trilobites comes from the so-called Asery horizon that
is some 20 metters thick near the Wolchov River. Some 20 genera and 100 species
are found in exquisite, non-compressed preservation. Moreover, the limestone matrix
(in a manner similar to that of Devonian
Oklahoma) is relatively soft, enabling an expert with modern preparatory equipment
to produce finished fossils that can seem life-like. The trilobite exoskeletons
generally are extremely well preserved, and are a milk-chocolate color that contrasts
well against the lighter limestone. Horizons above and below show the progenitors
and descendents of trilobites from the Asery level, adding diversity to the trilobites
from the region, and adding to a wonderful story of decent with modification.
Some dozen trilobite families are represented, including Asaphidae, Illaenidae,
Cheiruridae, Encrinuridae, Raphiophoridea, Lichadidae, Remopleuridae, Harpedidea
and Phacopidae.
Asaphidae
Far
and away the most prevalent, and the family best exhibiting
the outcome of adaptive strategies, is the family Asaphidae.
Some 30 species are believed
to
have descended from Asaphus broggeri that is found in the Wolchovian horizon,
with possibly Paraptychopyge as its sole ancestor. The figure shows the bushy
branching of Asaphidae as suggested by cladistic research spanning more than
a
century. Asaphus lepiderus replaced Asaphus broggeri, and represents a major
branching to: 1) the very similar Asaphus
expansus and 2) a new genus represented
by Asaphus
lamanskii. Asaphus lamanskii was successively superceded by Asaphus raniceps
and Asaphus major that occur in the kunda horizon, and the much later Middle
Ordovician
termination of the branch with Asaphus
delphinus; mysteriously, no intermediate
forms are known. Asaphus
expansus apparently gave
rise to the remainder of the Asaphus line, which itself would undergo additional
and exotic diversification. Asaphus applanatus began a tendency to larger
width
and genal angle, such as the impressive Asaphus
platyurus. Also impressive was the succession of Asaphus kotlukovi, Asaphus
punctatus, Asaphus intermedius and Asaphus convincens that express progressively
higher eye stalks. The same tendency of adaptation occured in a line that includes
Asaphus cornutus and Asaphus
kowalewskii, among
several others. The Asery layer captures some 2 million years of rapid adaptive
radiation, apparently in response to environmental challenges such as a more
shallow
sea, increased seaweed and higher acidity. The last descendents of Asaphus disappear
in the layers of the Upper Ordovician, manifest evidence that adaptation has
its
limits.
