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

Re: body size and competition cont.

At 10:44 AM 5/3/98, Dave Adams wrote:

>       With regard to the size of Tertiary mammals, I will save most of my
>remarks for "stay tuned."  But notice that dinosaurs did decrease in size
>in the Cretaceous.

As demonstrated by...?  Okay, we get more and better sauropod fossils out of
the Morrison, Tendaguru, and some of the Chinese sites than elsewhere, but
that might have more to do with regional geology than with the inhabitants
of the ancient Earth.  Sauropods remain present in global faunas throughout
the Early Cretaceous, and throughout the non-North American world until the
K-T boundary.

To put the question of size into perspective:
When do the largest sauropods with bones in collections occur?  The Cretaceous.
When do the largest theropods occur?  The Cretaceous.
When do the largest thyreophorans occur?  The Cretaceous.
When do the largest ornithopods occur?  The Cretaceous, by a long shot!
When do the largest marginocephalians occur?  The Cretaceous  (not
surprising, given the lack of a Jurassic margino record...).

Did overall dinosaur size decrease in the Cretaceous?  Perhaps, especially
if there is a burst in avian diversity in this Period, as most would agree.
However, did the maximum size of dinosaurs decrease from the J to the K?  No.

When looking at dinosaur history, you have to look beyond the wesetern
interior of North America.  Which gets to the next point...

>Since the end of the Jurassic there has been a global
>advancement of open herb-dominated habitats at the expense of forests.

Based on...?  As shown by Wing, S.L., L.J. Hickey & C.J. Swisher (1993)
[Implications of an exceptional fossil flora for Late Cretaceous vegetation.
Nature 363: 342-344], at least some late Late Cretaceous floras may be
dominated by angiosperm herbs in terms of species numbers, but still
dominated by gymnosperms in terms of biomass (i.e., the angios were
taxonomically diverse but rare, the gymnosperm trees had few species but
these few were the majority of the plant mass available), with ferns a major
additional contributor.

Also, changes in western North America are different than for eastern North
America and northern Africa and Patagonia and the European archipelago and
so on.

>This has major implications for the evolution of large herbivores and their
>predators.  My remark about grassland-type habitats was in reference to
>prairies, savannahs, desert-grasslands, some deserts, and even some
>"forests."  All fire-frequented plant communities are sometimes lumped
>together in the plant ecology literature as "grasslands."  Admittedly there
>were no grasses in the Cretaceous. 

Part of the problem of doing Mesozoic terrestrial ecology is that there may
be no living analogues to some of the main biomes.  The open lands (that is,
the non-forests) seem to fall in this realm.  We don't have large scale
"fernlands" or "herblands" in the world today (due to the success of grasses
in the mid-Cenozoic).  There may not have even been fire-frequented biomes
during the wet Cretaceous, at least not comparable to the grasslands of the

There doesn't seem to be a strong evidence for a rise in open land even for
western North America.  Taggart, R.E. & A.T. Cross (1997) [The relationship
between land plant diversity and productivity and patterns of dinosaur
herbivory. DinoFest International. pp. 403-416] argue, in fact, that the J-K
shift was actually a shift from *open* to closed biomes, not the other way

See also G.J. Retallack's paper on soils (pp. 345-359) in the same volume,
which also supports a more open condition for the Morrison compared to a
more forested Hell Creek.

In other words, it is the sauropods and stegosaurs and allosaurs of the
Morrison which are in the more open environment, and the hadrosaurids and
ceratopsids and tyrannosaurids of the Hell Creek which are in a more
forested environment, not the other way around.  But again, be careful about
taking this pattern too far: conditions in central Asia, for example, DO
seem to go from wetter to drier during the Jurassic to the Late Cretaceous.

Hope this helps, or at least adds some new information on which to modify
your hypothesis.

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