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*To*: dinosaur@usc.edu*Subject*: Increasing size*From*: Graeme Worth <endocrin@opera.iinet.net.au>*Date*: Wed, 10 Jun 1998 09:35:38 +0800*Cc*: cadams@hh.gpz.org*Reply-to*: endocrin@opera.iinet.net.au*Sender*: owner-dinosaur@usc.edu

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

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