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Re: Hadrosaurs outgrew tyrannosaurs

I think the emphasis is on individual reproductive fitness. From a selective point of view reducing the time your offspring has to spend in a more vulnerable size range reduces the odds of it not surviving to reproduce itself. Of course as HP Holtz has already mentioned hadrosaurs inherited the condition from their ancestors, so the real question is why did tyrannosaurs prolong their growth trajectory (or indeed, where did the trend first begin?).

Just speculating, but here's a totally non predator-prey explanation for the growth rate of tyrannosaurs:

We know that basally dinosaurs grew fast, and that gigantism seems more often than not to have come by increasing growth rates (rather than the traditional diapsid solution of extending the growth period while retaining the ancestral growth rate). We also know that the size reduction seen in stem-birds seems to have resulted in reducing growth rates rather than dramatically shortening the growth period. Finallly, we know that this happened in the Early Cretaceous.

Tyrannosaur relatives at the time are small, and may be reasonably assumed to have utilized lower growth rates relative to the basal tetanuran condition to assume that size range. So...what if in the 50+ million years between that stage and the eventual gigantism of end-cretaceous tyrannosaurs the linneage canalized the growth rate, making gigantism only (or at least primarily) achievable via lengthening the growth period.

This speculation generates several testable hypotheses:

1) Indentified basal tyannosaurs with small body (e.g. Dilong) size should have a growth rate similar to Late K tyrannosaurs (and lower than the basal tentanuran rate).

2) Initial size increases (e.g. Alioramus, aublysodonts) should show an increase in growth period rather than an incease in size.

If either proves to be false then we can rule out the idea of a canalized growth rate. Also, if initial size increases in the tyrannosaur linneage do show a growth rate increase (followed by a subsequent decrease in end K tyrannosaurs) then we probably need to invoke some environmental selective factor (including P/P interactions) and/or a population dynamics constraint (can only fit so many adult tyrannosaurs into the western US and Asia).

Just food for thought...

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333


-----Original Message----- From: john hunt <john.bass@ntlworld.com> To: xrciseguy@q.com; 'Dinosaur Mailing List' <dinosaur@usc.edu> Sent: Thu, 7 Aug 2008 11:31 am Subject: RE: Hadrosaurs outgrew tyrannosaurs

While this is all good stuff on growth rates I don't really understand why
such significance is placed on hadrosaurs outgrowing tyrannosaurs. There
were always going to be younger hadrosaurs smaller then the tyrannosaurs -
its not like they were only allowed to eats dinosaurs the same age ;-)

-----Original Message-----
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of
Sent: 07 August 2008 01:36
To: Dinosaur Mailing List
Subject: Hadrosaurs outgrew tyrannosaurs

Interesting work! Here is the press release:


And the full paper:


Guy Leahy