Re: The Mesozoic timeline

[tholtz@geochange.er.usgs.gov]

I've forwarded this reply to the whole dinonet, since I'm sure there are
others out there who are interested:

>> Cretaceous           Late Cretaceous         Maastrichtian   65.0
>>                       Early Cretaceous         Albian         97.0
>> Jurassic             Late Jurassic            Tithonian       145.6
>>                      Middle Jurassic          Callovian       157.1
>>                      Early Jurassic            Toarcian        178.0
>> Triassic             Late Triassic            Rhaetian         208.0
>>                      Middle Triassic          Ladinian         235.0
>>                      Early Triassic            Spathian        241.1
>
>I have a question which has always confused me:
>
>SOmetimes the terms "Upper" and "Lower" are used instead of late or early.
>But I never get them right: is Upper/Lower the Late or the Early period?

Upper, Middle, and Lower refer to the rocks themselves: The youngest rocks
in a system or series are Upper, oldest are Lower, and sometimes there is a
Middle.  So, the Hell Creek Formation is an Upper Cretaceous rock unit.

Late, Middle, and Early refer to the time in which the rocks were
deposited: the Lower rocks in a system or series were deposited first, so
they are Early; the Upper rocks were deposited last, so they are Late.  So,
the dinosaurs whose fossils are found in the Hell Creek Formation lived
during the Late Cretaceous.

>
>I think it is Upper=Late and Lower=Early, so you look at time "accumulating"
>upward and not "coming down".

Exactly.
                                     
Thomas R. Holtz, Jr.                            tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile              Phone:          703-648-5280
U.S. Geological Survey                          FAX:            703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.