weights etc of dinosaurs

[Helen Anderson <redledger@cableinet.co.uk>]

Dear Derek

I've been reading your pleas for a response on the list - so here goes.
I think the main problem with any reply is that it has to be based on
the only evidence we have - ie fossil evidence.  This does not just
include the bones found but eggs, nests and footprints as far as I am
concerned.  However, it is rather difficult to estimate such matters as
pack/herd size from taphonomy alone.  If a palaeontologist encounters a
vast area of articulated dinosaur skeletons, she may decide that some
disaster wiped them all out at the same time (as is possible with the
coelophysis (?is this still the correct name) skeletons at Ghost Ranch
and that this is evidence that many dinosaurs of the same species were
together in one place at the same time, ie a herd or flock.  On the
other hand, if the skeletons are disarticulated and badly rolled and
contain more than one species or genera, she might imagine that a mixed
herd had been crossing a river in flood, quite a lot had not made it
(wildebeest do this, following zebras across) and their bodies had been
washed down river to end up on a sand bar, in a bit of a mess.  Again,
this could be evidence of herding behaviour.  Lastly, many bones may be
found together, badly broken but approximately articulated, with
different genera and species all mixed up.  Many palaeontologists might
still assume a herd but it is also quite likely that the animals died in
a severe drought, at a water hole.  They would gather here as food ran
out and become weaker until they eventually died.  This may not be
evidence of a herd.  The dry seasons may have been annual or every 100
years.  One animal in a species every 10 years for 100 years makes quite
a nice little social group - that never existed together!  It is the
fact that the bones are broken that gives some of the game away.  Other,
later animals will walk over the skeletons to get at the water.  Sorry
not to be able to be more positive about herds, although I'm convinced
that some dinosaurs did live in herds, by the way.  Trackway evidence
can also confuse as dinosaurs may have walked about together, or may
have followed the same paths (because of some geographical feature that
no longer exists) every day, alone.  I'm obviously exaggerating here to
prove a point.  We cannot know any of this, its all supposition based on
the best evidence available.  Quite often, the simplest explanation
turns out to be true.

As for types of prey and vegetation eaten - some dinosaur fossils have
stomach contents but many scientists have argued that (for the
vegetation at least) this is merely in-fill from the surrounding
sediments.  We have a particular problem in England, where most
dinosaurs are fragmentary at best, and the skeleton does not end up
where it died.  Even if we find fossil plants in the same locality as
the bones, we cannot positively say the dinosaur ate them.  However,
looking at the animal's teeth gives you a very good idea of what they
ate.  Short stubby teeth with a lot of wear on them would indicate
coarse vegetation whereas more slender teeth may indicate softer stuff.
Plants around at the time varied enormously throughout the Mesozoic.  It
is said that iguanodonts ate horsetail ferns, which are still around in
some forms today.  Sauropods may have eaten conifers, they were
certainly tall enough.  Cycads were pretty dominant too.  For animal
prey, I think it is necessary to look at extant animals for analogues of
behaviour.  Most mammalian carnivores today eat quite small prey.  Lions
will not take on anything too large unless they are in a pride hunting.
For this reason, elephants, unless very sick, juvenile or old, are free
from predation.  I guess fully grown sauropods would also have been.  If
you look at foxes, for example, in the UK their main diet consists of
earthworms, rather than chickens!  Anything the dinosaur wanted to eat
would have to fit inside its head.  Of course, it could tear off chunks
of flesh and swallow them and I'm sure this happened with the larger
ones.  I think a lot of the smaller predatory dinosaurs ate things very
much smaller than them - such as our ancestors!

Skin colouring is going to be pure speculation.  I believe that
dinosaurs could see in colour, like reptiles and birds.  For that
reason, I would like to see them represented as highly coloured where
appropriate.  Iguana iguana lizards change colour from very bright as
babies to duller as they age and this is observed by adults who do not
try to chase them off until their colour changes.

There are two books that you might be able to get hold of which are not
about dinosaurs but would certainly give you food for thought:

"Why big fierce animals are rare: How the natural world works" Paul
Colinvaux, 1980, George Allen & Unwin, London, 224pp. ISBN 0-04-574015-1

Behrensmeyer, AK; Hill, AP (eds) 1980, "Fossils in the Making:
Vertebrate Taphonomy and Paleoecology" The University of Chicago Press,
Chicago. 338pp.

Quite frankly, about most of these things - your (educated) guess is as
good as anyone's.  If you learn about modern taphonomy and animal
behaviour, you can put these ideas into practice when thinking about
dinosaurs.

I'll get back to you with weights and dates,

Helen