@Lin : It is hard to keep up with them all and often it isn't what we think of when we think of "extinction". The two you mention are in the top 6 or so but I can't recall myself what their circumstances were without looking them up.
<http://itsmyocean.org/?p=854> See charts.
My point about designating the partitions of deep-time was that we used extinction events as end points. These points may have coincided with ice sheet-regressions of the shores beyond continental shelves, the assemblage of land masses to eliminate shallow seas or flood basalts and asteroid impacts but it was the extinction of life forms and their absence in the geological column which we used as end points. Historical geologist of mainly Great Brittan named the periods after type locations owing to the types of life in the local assemblage, yes. But it was the end points between the ages/periods which were made on the sudden absence of the classic flora/fauna for the period. I am afraid again we are in a circular argument we can't break out of.
For a good extinction discussion From:<http://en.wikipedia.org/wiki/Extinction_event>
Which includesTriassic–Jurassic extinction event (End Triassic): 200 Ma at the Triassic-Jurassic transition. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct.[6] Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus).
The Devonian-Carboniferous "extinction" was a very protracted series with at least 3 peaks ,suggesting a aggressive struggle to keep rebounding in spite of being stomped down. Be it remembered that as this was also the time land plants, land animals-- including flying insects, etc were colonizing the margins of the shores in coal bogs so the statistics are a bit skewed as previously we were making note of only marine species alone.
( From Wikipedia: Extinction Event)
Late Devonian extinction: 375–360 Ma near the Devonian-Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
Late Devonian extinction: 375–360 Ma near the Devonian-Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
Eman
From: Lin Kerns <linkerns@gmail.com>
To: Geology2 <geology2@yahoogroups.com>
Sent: Monday, April 29, 2013 11:50 AM
Subject: Re: [Geology2] Naming "Deep Time" was The Earth Moved
I don't recall an extinction event between the Triassic and the Jurassic. Also, between the Carboniferous and the Devonian. I remember something about how man designates the end of some periods by the change in stratigraphy. Am I thinking right on this?
LinOn Sun, Apr 28, 2013 at 8:37 PM, MEM <mstreman53@yahoo.com> wrote:
It shouldn't be so counter-intuitative but we are kind of stuck in a circular argument. Extinction events were what made us take notice of the changes in lifeforms. ( Not all extinction events gave rise to an ages or period designation) They didn't fully comprehend it at the time but, "extinction followed by radiation" was what early geologist were taking mental note of when assigning names to time segments. It isn't that "bad stuff" happened at the end of these man named time periods but, we named the ends when we found the "bad stuff".
Geological-time designation is an artifact of man not nature. Extinction events are how we designate the end of all units of geological time. I can think of only one which is not extinction related: Eudicarian. This was not an extinction but a radiation where life started leaving "fossil-lizable" soft structures. Ok-- one more-- the "onset" of the Cambrian when life again left a different kind of body structure: "hard body", which could be preserved.
Eman
From: Kim Noyes <kimnoyes@gmail.com>
To: Geology2 <geology2@yahoogroups.com>
Sent: Sunday, April 28, 2013 7:07 PM
Subject: Re: [Geology2] Re: The Earth Moved
In some cases those, too. However, foremost on our minds is the End Cretaceous Event and the Cretaceous is a period. I suppose we could expand my comment to include "most eras, epochs, periods, etc. Bad shit seems to mark the boundary of one of these time frames.
No comments:
Post a Comment