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Re: Tyrannosaur numbers

It seems they were either selectively preserved (perhaps their risky behavior put them at greater probability of getting buried rapidly), or there were lots of them, or they had some characteristic that made their bones more likely to survive long enough to be fossilized (tasted bad, being just big, or something). Phil is correct in observing there are lots of fossils from young T-rex (so they were successful breeders) about the upper Cretaceous and quite a few random fossils from more adult versions too at least in my little corner of Hell Creek. I suspect that there are a good number of them in the ground yet to be found. The number alive at anyone time and predator/prey ratios can be deceptive though. For instance, a 10 foot thick layer of lake mud could have been deposited over a really large amount of time and could account for many, many generations of animals and literally thousands of possible preservations in any particular location. If you have an organism that has a higher likely hood of being preserved (for any reason) it will definitely skew the ratio of preserved individual compared to other less likely to be preserved organisms. You have to remember the enormity of geologic time.

Certain types of preservational situations also will selectively attract predators. One vegetarian prey animal gets stuck in the mud, ten T-rex move in and all get stuck and there you are with a 10 to 1 observable preservational ratio. (Phil Currie is sure looking for that situation again.) We may never know though because they are always obscured in the ground but we only find the one on the edge of the deposit and stop before the next specimen (one inch under the last shovel scoop) . The obfuscations are many. There may be 10 more animals buried along with Sue but if someone doesn't dig deeper there. We will never know.

A building theory is (gaining followers) that T-rex were pack hunters. Strength multiplied geometrically in numbers. There really might have been a lot of them and there also may have been a lot more prey than is indicated in the record.

I also don't think the general public understand the enormous amount of work that it takes to get a huge animal like an adult T-rex out of hard rock. I understand it took 30,000 hours to get Sue out of the ground and prepped. (a few hours of legal work too!) The number of prepared and displayed animals will always fall under the laws of supply and demand. There are only so many institutions that will pay to get more of the same out of the ground. A lot of researchers aren't interested at all in more "common" dinos like Triceratops, T-Rex and certain hadrosaurs. They wouldn't cross the street to dig one up let alone spend funds to study them.

I think they are pretty common relatively. With that in mind, try to go out and find one! Good luck.

Frank Bliss
MS Biostratigraphy
Weston, Wyoming.

On May 23, 2005, at 8:32 PM, Jordan Mallon wrote:

On 5/23/05, Phil Bigelow <bigelowp@juno.com> wrote:
John Horner has an informed opinion on the number of _T. rexs_ that
existed at any one time, and from my limited experience as an interested
amateur, I think I'll agree with him. _T. rex_ appears to be quite
common in the Hell Creek Formation. 

Despite my limited field experience, I can say with some confidence
that the same seems to be true in the Scollard of Alberta.  I spent a
couple of days prospecting the Scollard Fm outcrops at Dry Island
Provincial Park last year, and it seems many (if not, most) of the
scattered remains my team and I came across were attributable to
Tyrannosaurus (surangular, longbone diaphysis, metatarsal, etc.).  No
idea as to accurate species ratios, mind you, despite a microsite we
collected and analysed nearby.

--
Jordan Mallon

B.Sc. (Honours), Carleton University
Vertebrate Palaeontology & Palaeoecology

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