Russian Trilobites

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The 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 trilobites fromPseudoasaphinus  tecticaudatuss Russian Trilobite what is today Russia developed unusual shapes, some with Boedaspis ensifereyes 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.

While the 20 million year span, give or take a million, that begins the Cambrian period known as the Cambrian Explosion 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 the ecosystems, 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 Russian Asaphus Trilobites Evolutionary Sequence: Asaphus applananatusperiodically cut off from the ocean to the west, Asaphus kowalewskii Russian Trilobite with Enormous Eyestalksresulting 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 to another 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 that is more than 50 feet think. As with the examples above, lines of descent can be traced 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.

The Ordovician 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.



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