Cambrian is one of the most sensational periods of paleobiology on earth.
An explosion of diversity and form of eukaryotic
life occurred resulting in the appearance of most major phyla of life extant
to day, as well as other phyla that appeared and disappeared. By the Ordovician
(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
from what is today Russia developed unusual shapes, some with eyes on long
stalks, and others with jagged spines. Ultimately these magnificent animals,
in whole or part, met their demise in earth's
biggest ice age.
the 20 million year span, give or take a million, that begins the Cambrian
period known as the Cambrian
gets all the glory, the Lower Ordovician was pretty spectacular itself
in terms of marine animal diversity expansion. In fact, some call it “The
Great Ordovician Radiation”, or even “The Ordovician Explosion”.
The Cambrian – Ordovician extinction even was pretty severe for
some marine life, trilobites among them. This, of course, cleared out
and left a lot of niches to be filled by newer evolved forms. Trilobites
responded with a long run of new adaptations, fostered by shallow coastal
waters and increasingly complex reef systems.
Evolutionary sequences among Russian asaphid trilobites
Nowhere is this
great Ordovician radiation better illustrated than the region around
Saint Petersburg, Russia that during the period was part of a shallow
inland sea. Saint Petersburg sits atop an enormous limestone mass, teeming
with preserved trilobites. Evidence has led scientists to conjecture that
the inland sea basin sea was periodically
cut off from the ocean to the west, resulting in long oscillations in sea
salt and particulate concentrations.
These changes, in turn, were (see Asaphus trilobite evolutionary sequence
image to the left). Asaphus
expansus branched into two Asaphidae lines of
decent, one leading to Asaphus
kowalewski, and another branching to Asaphus
plautini with its large, sharp genal spines genal, and the other leading
sequence of ever higher eye stalks in Asaphus kotlukovi, to Asaphus punctatus,
to Asaphus intermedius and finally Asaphus convincens. These observed evolutionary
trends are supported by a century of cladistics research that document
about two million years of adaptive radiation within the Asery horizon
more than 50 feet think. As with the examples above, lines of descent can
from the ancestors on lower horizons (Wolchow and Kundra), to descendants
in the upper horizons (Lasnamjagi and Llandeilo), as shown in the figure.
The matrix of
the region is soft for limestone, and preserves the trilobites in apparently
near full three dimensional high relief. Especially the Asaphids
(like many asaphids worldwide in the Ordovician) can be quite large. The
result, with professional preparation, are exquisite chocolate trilobites
on light limestone. Many trilobite orders and families come from the strata.
Besides prodigious asaphids of family Asaphidae, Illaenidae and Remopleuridae
are also represented. Pliomeridae, Cheiruridae, Encrinuridae and Phacopidae
are among Phacopids found. Super spiny members of the Lichadidae come from
the area, as well as Harpedidea, the least prevalent of all trilobite orders.
There are some 20 genera known, and more than a hundred species.
trilobites from Saint Petersburg dwindle and then disappear in the upper
horizons. This roughly corresponds to a time frame when trilobite
domination of reef systems worldwide was being diminished by an ever increasing
diversity of competing animals and predators. The later descendants also
lose their impressive eyestalks, as the opening from the inland sea reopened,
reducing turbidity and the survival need for them. Such descent with modification
is an exemplar of evolution, just as Darwin observed in finch populations
with different beak sizes in the Galápagos Islands.