Archive for June 26th, 2011

A new Senior Independent Study project begins in Estonia

June 26th, 2011

KURESSAARE, ESTONIA–It is always a joy to begin the fieldwork for an Independent Study project — or at least know what the fieldwork will be. This morning we visited the Soeginina Cliff locality on the Atla Peninsula of western Saaremaa and it was all we hoped it would be. Nick Fedorchuk (pictured above as a happy man with his outcrop) studied the literature about this locality during his Junior Independent Study period last semester. We confirmed today that the rocks are indeed auspicious and will work as the basis of his research.

This locality is significant because it records a time and rock boundary in the geological record. The lower portion belongs to the Wenlock Series in the Silurian System, and the upper portion is in the Ludlow Series of the Silurian. They are separated by a disconformity (an erosional horizon indicating a hiatus in the geological time record). Boundaries such as this are always interesting because they can be correlated across the globe with other rocks formed at the same time. We want to better understand what was happening in Baltica at this junction between the Wenlock and Ludlow, and then compare it to the equivalents in Sweden, Britain and North America.
The boundary rocks show a laminated unit in the uppermost Wenlock (Rootsiküla Stage) that has been interpreted as lagoonal in origin, and then a more massive limestone in the lowermost Ludlow (Paadla Stage) with oncoids (microbial accumulations) and eventually shelly beds thought to be more open shallow marine deposits. The division between them appears to be marked by a mineralized layer  (see image below). Later Nick will collect rock and fossil samples to thoroughly describe this interval and sharpen the paleoenvironmental and paleoecological hypotheses.
Rachel Matt (below) does not yet know which outcrop will be the focus of her research, but we will soon!

Our last visit of the day was to Kaarma Quarry and its exposed laminated lagoonal limestones and dolomites of the Ludlow. You can see below the team in action — and what a beautiful day it was.

Wooster’s Fossils of the Week: Tiny little oysters (Lower Paleocene of Mississippi)

June 26th, 2011

This week’s fossils are by no means rare — last year Megan Innis and I picked up dozens of them at a muddy outcrop near Starkville, Mississippi, on our Cretaceous-Paleogene boundary expedition (click “Alabama” and “Mississippi” in the tags to the right for entries from that trip). They are, though, significant indicators of a particular kind of ecological system that appeared in the oceans of southeastern North America after the cataclysm of the Cretaceous Extinction.

The specimens pictured above are Pycnodonte pulaskiensis, a local species of oyster that belongs to a very prolific genus found around the world. Pycnodonte ranges from the Lower Cretaceous (about 140 million years ago) to, it appears, today. Kase and Hayami (1992) appear to have found this oyster — or a close relative — still living in submarine caves near Japan. This makes them a kind of “living fossil”, a group with a very long history of evolutionary stability.

This longevity fits into our Pycnodonte pulaskiensis story. These fossils are very common in the lowest Paleocene sediments just above the extinction horizon that marks the fiery end of the Cretaceous. After all that devastation (and Alabama was uncomfortably close to the impact site of Chicxulub), P. pulaskiensis appeared first to reoccupy the seafloor muds. They were virtually alone in this muddy habitat, and so lived there in great numbers. We call this kind of early successional species an “opportunist” (in the good sense!) taking advantage of a recently emptied niche.

Our little oysters in the Clayton Formation near Starkville, Mississippi.

Paul Taylor, Megan Innis and George Phillips at the Cretaceous-Paleogene boundary near Starkville, Mississippi in May 2010. Our oysters were found directly below Megan's feet.

These little oysters weren’t entirely alone, though. Many of them have small beveled holes in the center of their left valves, producing the ichnofossil Oichnus. These are apparently the traces of naticid gastropod predators (see Dietl, 2003) that drilled the holes to kill and eat the oyster soft parts.  (And who can blame them?) Several shells also have encrusting foraminiferans like Bullopora and Ramulina. Small hints of a recovering ecosystem setting the stage for the modern fauna we see in the northern Gulf of Mexico today.

References:

Dietl, G.P., 2003. Traces of naticid predation on the gryphaeid oyster Pycnodonte dissimilaris: Epifaunal drilling of prey in the Paleocene. Historical Biology 16: 13-19.

Kase, T. and Hayami, I., 1992. Unique submarine cave mollusc fauna: composition, origin and adaptation. Journal of Molluscan Studies 58: 446-449.