MIGHTY MARINE REPTILES: ICHTHYOSAURUS

Blue Lias Ichthyosaur / Photo: Lewis Winchester-Ellis

Ichthyosaurs were particularly abundant in the later Triassic and early Jurassic periods, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Cretaceous periods.
In the Late Cretaceous, ichthyosaurs were hard hit by the Cenomanian-Turonian anoxic event. As the deepest (benthos) layers of the seas became anoxic, poisoned by hydrogen sulphide, deep water marine life died off. This caused a cascade that wreaked havoc all the way up the food chain. At the end of that chain were our mighty predaceous marine reptiles. Bounty turned to scarcity and a race for survival began. The ichthyosaurs lost that race as the last lineage became extinct. It may have been their conservative evolution as a genus when faced with a need for adaptation to the world in which they found themselves and/or being outcompeted by early mosasaurs.

NORTH CASCADES

North Cascades, Cascade Range, western North America

OREGON: FOSSILS ALONG THE FARALLON PLATE

Had you been swimming with the marine fossils that were laid down in the Eocene Epoch in Oregon, some 55 to 38 million years ago, you'd be treading water right up to where the Cascade Mountains are today.

The Farallon Plate took a turn north some 57 million years ago, sweeping much of western coastal Oregon along with it. The Cascades were beginning to uplift and acting as the breakwater for a retreating Pacific Ocean. By the middle Oligocene, the Cascadia Subduction Zone was in full force. The growing pressure fracturing our magma shield and causing volcanic eruptions along the Western Cascades. Lassen Peak erupted twice in fairly recent history, 1914 and 1921. Mount St. Helens has had a long history of minor eruptions but there was a massive eruption as recently as 1980.

We see a fair bit of volcanic action in Oregon right through to the Miocene. We also see lovely marine fossils from this same era. The soft ocean sediments of Oregon contain beautifully preserved gastropods, bivalves, wood, bone and cephalopods that range in age from 15 to 30 million years old.

VERTIPECTEN FUCANUS

Vertipecten fucanus / Miocene / Clallam Formation

Some water worn samples of the fossil bivalve Vertipecten fucanus from lower Miocene deposits in the Clallam Formation.

These were collected on the foreshore near Clallam Bay, Olympic Peninsula, northwestern Washington.

Until Addicott's study from 1976, the area was considered middle Miocene. The new lower Miocene designation can be credited in large part to the restricted stratigraphic range of Vertipecten fucanus (Dall) and the restricted and overlapping ranges of several other mollusks.