Increasing size

[Graeme Worth <endocrin@opera.iinet.net.au>]

A somewhat belated contribution to the recent discussion regarding
increasing size over time. I've simply taken all the dinosaurs for which I
have a size estimate and divided them into groups; late Triassic, early-,
mid- and late- Jurassic, early- and late- Cretaceous, then calculated the
mean size for each time period for theropods, sauropods and ornithischians.
Where a genus existed in more than one period I included the size in both
period groups; when a size range was given I used the midpoint. Note that
these are all based on length - I suspect that the number of instances of
well documented weights is probably far fewer than length estimates. Given
all these sampling caveats, the results are as follows (Time period; mean
length; standard deviation; number of points; range).

Theropods

LT      4.15    3.46    10      1 - 12m
EJ      4.49    2.62    8       0.9 - 9m
MJ      6.26    2.91    9       1.8 - 11m
LJ      6.38    3.67    19      0.5 - 12m
EK      6.08    4.28    28      0.5 - 14m
LK      5.18    3.66    59      1 - 14m

Sauropods

EJ      10.63   4.38    4       6.5 - 18m
MJ      13.8    4.05    12      4.8 - 18m
LJ      20.5    6.3     19      12 - 40m
EK      20.8    9.35    18      9.5 - 45.5m
LK      18.97   9.78    21      5.3 - 42.5m

Ornithischians

LT      1.3     0.3     3       0.9 - 1.5m
EJ      1.8     1.1     10      0.9 - 4m
MJ      3.45    1.04    6       1.7 - 5m
LJ      4.14    2.1     17      0.6 - 9m
EK      4.42    2.53    34      0.9 - 9.5m
LK      5.95    3.42    73      0.5 - 15m
[LK     4.64    2.45    50      0.5 - 11m - without hadrosaurs]

I worked these out on a hand calculator, and it was too much effort to
calculate t-test significance, but some trends are obvious. There is a
definite size increase from the late Triassic/early Jurassic to the mid
Jurassic and beyond (there was never much argument about that, I guess), but
from the mid Jurassic onwards it is very difficult to make a case for any
size increase, either from mean value or range - if anything there seems to
be a trend to a slight decrease from the early to late Cretaceous in
theropods and sauropods. The only situation where size continues to increase
throughout is for ornithischians, and the late Cretaceous figure for these
are heavily biased by the inclusion of the hadrosaurs - if they are removed,
the mean length is not different from preceeding periods.
Obviously these results need to be treated with some caution - there are a
lot of dinosaurs for which I don't have a length estimate - but the numbers
in most groups are reasonable and fairly representative and would probably
not change greatly with more inclusions. Of course, these results are based
on lengths for individual genera, and say very little about the size of the
entire dinosaur population for any time period - you would need reliable
estimates of total numbers of each species to do that.
Basically, once things were well established ie early-mid Jurassic, nothing
seems to have changed greatly. There was a good spread of small, medium and
large critters at any one time.
Graeme Worth
The Dinosaur Encyclopaedia