Hi Paul,

Your answer is in your question. It takes indeed quite a lot of detailed knowledge before we can tell. 

It can only be through the study of fossil assemblages of many sections and cores of the same time interval that we can then decide which species make good biostratigraphic markers and which species tend to be restricted to specific paleoenvironments.

Paul:

Spatiotemporal distribution of a species based on detailed studies would decide on its suitability as a reliable biostratigraphic and/or paleoenvironmental marker.

Best

Syed

Dear Nicolas and Syed,

I am voting up both of you on your answers.

Best wishes, Paul

I completely agree with Nicolas and Syed; you have to study a large number of floras to be really sure!

Sir ..as i can understand for biostratigraphic use the species should be index which have wide geographical range or short vertical distribution (rapidaly evolving)  most of the nectons  forms are used for this purpose. but for palaeoenvironmental purpose benthic forms are the best.

hi Paul: As the late Dr. Ch. Arnold once explained to me (eons ago), the use of biostratgraphic species depends on how few exceptions you find.  Never mind evolution!

Cheers: Erwin Zodrow, Cape Breton University, Canada

As I know, all planktic forams are more reliable for biostratigraphy (as they posses the requirement of index fossils) but also tell about paleoenvironment, However, all benthic forams are more valuable in paleoenvironment but also used in biostratigraphic but locally in particular the larger representatives. 

Dear Paul and others. I think this is not a question of either, or. Species are always both: both are intrinsic to species; it strongly depends on the spatial and temporal scales you are looking at how useful the organism/species is for your stratigraphic or environmental purposes.

We live in a deterministic system where every moment is unique and never exactly repeats any previous moment. Furthermore, organisms interact with, and are part of this environment. Both evolution (by natural selection) and ecology are thus interwoven and two sides of a coin - one does not work without the other. Thus, at every given moment in time you have a unique configuration of life forms. Furthermore, each species has a unique set of parameters with respect to its niche (role in the ecosystem) and period of occurrence. I agree with everybody that both can only be elucidated through detailed, study of the organisms in space and time. Mostly, this is a lot of work.

Interestingly, for short time intervals, it is unlikely to see species getting extinct or new species appearing. All you see is changing (relative) abundances and they reflect the changing environment (Think e.g. of reconstruction changes in the order of millennia and shorter periods; like environmental change over the last 200 yrs). Organisms that appear or disappear from your (local) record in such situations are likely to do so because of a changing local environment and haven't become globally extinct or newly evolved.

With an increasing time window, species turnover (extinction and emergence of species) plays an increasingly important role. For environmental reconstruction this is a complicating issue since it interrupts the continuity of the system. You have to find out the ecology of the newcomers and to find a way to deal the loss of information due to extinction.

Despite these complications, each species has a first appearance, and as such is a marker for a given time interval. This interval may be entirely in the past or continue into the future. As such each species is also a biostratigraphic indicator.