Re: Geological periods

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

>I had thought that there was consensus about the dates used for
>Triassic/Jurassic/Cretaceous, but apparently the reference I've been using
>for my latest project is contradicted by some later material.
>
>Sattler's DINOSAURS OF NORTH AMERICA (1981) gives:
>        Triassic 190-225 mya
>        Jurassic 140-190 mya
>        Early Cretaceous 100-140 mya
>        Late Cretaceous 65-100 mya
>
>But, in trying to track down why one reference thinks Dilophosaurus might
>have been Late Triassic and another calls it Early Jurassic, I ran into
>this schedule; it's in Lessem's KINGS OF CREATION, and matches the one in
>the Dino Society Encyclopedia -

Dilophosaurus is definitely Early Jurassic: the Kayenta Formation contains
Early J taxa of all sorts.  This dating is done by biochronology,
independant of the numerical data obtained by radioactive dating.

(For those who don't know, the stratigraphic column was developed using
fossil appearance and extinction for boundaries.  The numerical dates came
much later, after radioactive decay was understood).

>
>        Triassic 208-245 mya
>        Jurassic 145-208 mya
>        Cretaceous 65-145 mya
>
>So we agree on the end of the Cretaceous, but we're 5 million years
>different on its beginning; we differ by 18 million years on the beginning
>of the Jurassic; we differ by 20 million years on the beginning of the
>Triassic.

Going by my handy _A Geologic Time Scale 1989_ card (carry one wherever you
go!), the boundaries (as accepted by Harland et al.) are as given by Lessem.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084