[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: theP-T boundary

Ron asked:

>         This set me to thinking about how I made that error. The K-T is 
> only the second greatest mass extinction in earth's history, dwarfed by
> the Permo-Triassic (P-T). And yet on land at least, the scenery didn't seem 
> to change that much. 
>         On the other hand, most estimates I've seen say about 95% of all
> species checked out at the end of the Permian. But the synapsids made it
> thru. In fact the land vertebrates of the lower Triassic don't look all
> that different from the Permian, do they? Granted no one group dominated the
> Permian the way dinos did the Mesozoic, but did any major land vertebrates
> cash in their chips at the P-T? How about fish? Higher plants? Was the scenery
> very different but just not as well known, or well publicized, given that
> neither the P or T is as charismatic as the Jurrasic?

>         All of this raises lots of questions about what was different about
> the two events. The causes? No one has suggested ameteor strike at the P-T. 
> The two theories I am familiar with are massive volcanism and the 
> oceans-turned-
> to-soda-pop theory (CO2 bubbling up from the ocean floor) This last would
> explain a greater effect on marine life. 

There is large consensus about the mass extinction that occurred in 
marine faunas at the PT boundary (or better said: the PT boundary has 
been  drawn at the major extinction event that is documented in 
marine sediments), the latest thought and theories being summarized 
by Erwin.
However, concerning terrestrial faunas, the case is indeed not that 
clear. Most studies have focused on the excellent, continuous 
accumulation of Permian through Jurassic sediments in the Karoo 
Basin, and  some authors (especially Gillian King and Roger Smith) 
state that faunal diversity was already declining during the last 
stages of the Permian, which they linked with increasing aridity and 
a change from the typical Permian Glossopteris flora via an 
intermediate flora to the typical Triassic Dicroidium flora. 
Moreover, at different localities in South Africa, a 20 to 60m thick layer has 
been identified where Lystrosaurus specimens occur together with 
typical the Permian Dicynodon. And several 'transitional' faunal 
assemblages are known: the upper Guodikeng (or something) fauna in 
China where Lystrosaurus occurs with the Dicynodon like Jimusaria and 
Striodon, the Viazniki fauna of Russia, where Archosaurus is found 
together with large therocephalians and dicynodonts, and recently, 
Lystrosaurus is identified in the Madumobina formation of Zambia, 
which is dominated by typical Permian forms as Diictodon, Oudenodon, 
Dicynodon and Procynosuchus.
Although the Lowest Triassic Lystrosaurus fauna is indeed an 
impoverished one and lacks large species, several lineages have crossed 
the PT boundary: therocephalians were still a significant part of the 
Lystrosaurus fauna (scaloposaurs, moschorhinids and bauriamorphs 
survived), galesaurid cynodonts such as the well known Thrinaxodon 
are also known from Upper Permian strata (Cynosaurus), Lystrosaurus 
is clearly related to the Permian Dicynodon trigonocephalus, 
Kombuisia is an (albeit poorly know) dicynodont which is clearly 
"Permian like" (probably related to Emydops) but is found in the 
Cynognathus zone, and the Triassic kannemeyerid dicynodonts may have
descended from a Pristerodon-like  ancestor which survived the PT.  The 
anapsid Owenetta is both found in Permian and Triassic strata. The 
capitosaurid temnospondyl Uranocentrodon, formerly thought to be a 
typical Lower Triassic genus, is probably Upper Permian.
Gorgonopsians did vanish however, but they were already  less diverse in the 
Uppermost Permian, being represented mainly by large forms such as 
Dinogorgon and Rubidgea from South Africa and Sauroctonus and 
Inostrancevia from Russia. Dinocephalians were already gone from the 
end of the Kazanian/Tapinocephalus assemblage zone, quite long before 
the end of the Permian...
According to this 'gradualist' scenario, decreasing faunal diversity 
and extinction of many typical Permian taxa (at least in the Karoo)
was linked to a north movement of Gondwana, a fall in base-level and 
ground water, climatic deterioration with increasing aridity and less 
periodical rainfal, and a change from a Glossopteris and horsetail 
(Schizoneura/ Phyllotheca) dominated flora to a "tougher" Dicroidium flora. 

Pieter Depuydt