Vertebrates in Freshwater

["Cliff Lovelock" <cliff@xxxxxxxxxxxx>]

In response to the suggestion, "if you don't like the current discussion, statr 
a new one", here's something I'd like help on.

I have been working on the palaeoenvironments of the Silurian rocks of 
Lesmahagow (Midland Valley Of Scotland) for the past three years and am just 
finishing writing my thesis.  One of the central questions is to what extent 
fluctuations in salinity controlled the fauna found there (for those of you not 
familiar with the area, a fine selection of articulated thelodonts and 
anaspids, along with pod-shrimps, eurypterids and some more bizarre forms have 
been collected from there).

There are two very distinct (sharing, at the most, just two species) 
arthropod-vertebrate assemblages.  The lower one is associated with pterygotids 
and carcinosomatids, typical of Kjellesvig-Waering's (1961) eco-phase 1 
(marine).  The upper assemblage is associated with mixopterids and 
stylonuroids, suggesting a less than open marine environment, again on the 
basis of Kjellesvig-Waering's phases.

This sounds all well and good, and, on the basis of the almost complete turnover in 
species between  the two assemblages, and the eurypterids, I am quite prepared 
to interpret the lower assemblage as having lived in waters tending towards 
full marine salinity, and the upper in waters tending away from fully marine.  
The one sang is that the upper assemblage has been dated as early Wenlock 
(Wellman & Richardson, 1993, Palaeontology), and I have seen several papers 
which suggests that vertebrates did not gain a tolerance to non-marine 
environments until the end of the Silurian.

Does anyone have any comments? Especially, can anyone direct me to papers which 
discuss the timing of the vertebrate invasion opf non-marine environments in 
detail (take it as read that I've already seen most of the textbooks on the 
subject, and I am aware of work at the University of Manchester regarding 
nature of K-Ws phases).

Any help will be much appreciated,

Regards,

Cliff.


 

=========================================================
Cliff Lovelock,
Dept Geology & Geophysics, University of Edinburgh
{Cliff@glg.ed.ac.uk} {+44 131 650 5918}

"As long as we don't kill the hand of the golden egg."
- Carlos Westendorp (Spanish Sec of State for Europe).