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Tyrannosaur growth and babies

Several different answers to recent tyrant dino posts:

Dwight Stewart writes:
>       if Tyrannosaurs (& perhaps other dinosaurs) grew throughout their
>life, do we know what the
>       growth rate was after reaching "maturity".  The analogy I'm thinking
>of here is crocodiles.  If memory serves, the largest salt water crocodile
>ever recorded was well over 20 feet & almost a ton.  
>       Of course, with crocodiles, the growth rate slows considerably after
>a certain age, but it never
>       stops.  So, how much larger could a 30 year old Tyrannosaurus rex be
>than, say a 10 year
>       old T. rex?

Yep, damn good question.

Oh, you were looking for an *answer*!  Unfortunately, we don't know at
present.  Most evidence for most groups of dinosaurs suggest that growth
rates drop off dramatically at some point (by analogy with modern animals,
at or near sexual maturity), but that growth did continue.

The problem (well, one of the problems) is getting life age dates for
dinosaur specimens.  The idea that growth rings (LAGs, lines of arrested
growth) can be counted like tree rings doesn't pan out, as recent studies by
DeRicqles, Padian, Horner, and others show that different bones in the body
of the same individual have different numbers of growth rings!

Kristy Currie presented a paper at SVP concerning growth rates of
_Apatosaurus_, showing (from three different lines of evidence) that it
probably only required 10 years to reach sexual maturity, but did not
investigate (or at least present) work on data post this age.

So, like a lot of things about dinosaurs, we still don't know at present.

Kari Lynn Baker writes
>I've got a question for any of you.  
>I've been doing research on the tyrannosaur for almost a yaer now, and
>haven't come across or heard about any finds on baby Tyrannosaurs.
>This has bothered me for some time now, and I know I'm not the only bothered
>by this, but why haven't there been any baby Tyrannosaurs found???  There
>have been juevinilles found in the Tyrannosaur family yet no babies.  Why?? 
>I would like to hear your hypothesis as to why that is.  Or am I mistaken
>and there have been babies found??  I've kept up with new finds as much as
>possible and don't think I've missed that particular find.
>I'd appreicate it if I could get at least theory or hypothesis as to why
>this is.

Some of the main reasons for this may be what Brandon Haist points out: we
haven't found them yet because a) tyrannosaurs are rarer than herbivores and
b) they may have lived in different places as juveniles than as adults.
Also, he correctly points out, there may be preservational biases: some
formations which preserved big bones well are lousy at preserving small
fossils, and vice versa.

On top of this, there is the luck factor.  People have been working the Hell
Creek Formation for over a century now, and even by Barnum Brown's report of
19xx (anyone recall the date? c. 1915?) literally hundreds of _Triceratops_
skulls had been found.  And when was the first baby _Triceratops_ reported?
1997.  (Although it had been found earlier).  People have been working the
Morrison even longer, with greater total field-hours, and yet it was only in
the 1990s that the ankylosaurs _Mymoorapelta_ and _Gargoyleosaurus_ were
discovered and named.

I would dearly love to see baby tyrannosaur skeletons, but unfortunately
they haven't been found yet.  There are now juveniles known for most of the
important taxa, but these are still individuals with skull lengths of c. 30
cm or more (i.e., if they were alive today, they would be one of the biggest
predators on land).

Allan Edels wrote
>    I've also heard some people's opinion that _Nannotyrannus_ was a
>juvenile Tyrannosaur.  (Bob Bakker wouldn't like that - I'm sure).  I wonder
>what the current thinking on that is?
No, Bob Bakker doesn't like that...  Thom Carr has been working on the
subject from an ontogenetic and bone tissue standpoint, and his evidence
certainly suggests that the _Nanotyrannus_ individual is a juvenile (I've
seen the bone texture: yep, this guy was still just a kid!!).

My phylogenetic work places _Nanotyrannus_ and _Maleevosaurus_ as sister
taxa outside a clade comprised of _Tyrannosaurus rex_ and _Tyrannosaurus
bataar_.  As I said at the 1997 SVP meeting, this can be interpreted in two
ways: a) _Nano._ and _Maleevo._ are the primitive relatives to
_Tyrannosaurus_ species, and the characters that unite _T. rex_ and _T.
bataar_ are evolutionary; b) _Nano._ and _Maleevo._ are juveniles of _T.
rex_ and _T. bataar_, respectively, and the characters that unite _T. rex_
and _T. bataar_ in my analysis are actually growth characters.  Phylogenetic
analysis does not, at the moment, exclude either possibility (just because
_Nano._ is a juvenile doesn't mean that it is necessarily a juvenile _T. rex_).

However, my suspicions are that "_Nanotyrannus_" is indeed a juvenile _T.
rex_.  This can be disproved by the discovery of a) an adult _Nanotyrannus_,
distinct from _T. rex_ or b) a juvenile _T. rex_, distinct from _Nanotyrannus_.

Hope this helps.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661