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More about Erickson et al. 2004



Greetings,

Some additional notes.
First, I do want to add that the original researchers, peer reviewers, and
everyone (not just the press) are bound by embargoes.  Communication
*between* these parties is encouraged, for the reasons outlined in Christian
Darkin's post.

Now, some of the implications concerning the paper:
Phylogenetic implications:
The growth rate during "adolescence" (again, I use this informally here: we
don't know when fertility begins in tyrannosaurids) for Gorgosaurus and
Albertosaurus are very close to each other (114 kg/yr, 122 kg/yr
respectively). That of Daspletosaurus is elevated compared to these: 180
kg/yr. Tyrannosaurus is way up at 767 kg/yr. The authors postulate an
increased growth rate as a potential evolutionary change at the base of
Tyrannosaurinae, and another on the line leading to T. rex. I would predict
that Tarbosaurus' rate will fall intermediate between Daspletosaurus and
Tyrannosaurus.

Where are the old tyrants?:
If we were to look at modern and fossil proboscidean or other large mammal
populations, we'd find a substantial fraction of it made up of adults. In
fact, a growth curve for Loxodonta or Mammuthus would be very similar to the
T. rex curve, but would continue on for several more decades. In contrast,
fully grown tyrants didn't hang around for very long. (There would seem to
be no taphonomic reason why they would preferentially not be preserved, and
identification of the External Fundamental System allows for esimation of
how long the adult dinosaur had lived since the essential cessation of
growth). If a Tyrannosaurus lifespan were like a Loxodonta, we'd expect to
find 40, 50, maybe even 60 or 70 year old individuals.  But they haven't
been found yet.

The reason may be related to something that Carrano & Janis, Paul, and
others have noted before: that non-avian dinosaurs were more r- than
K-selectors in terms of ecology.  That is, their much higher rate of
replacement (a dozen or more eggs per clutch, as opposed to a single
offspring in 2 or more years for elephant-to-indricothere sized placentals)
would mean that tyrant populations could be maintained without having to
have long-lived adults. (And the same is true for all large bodied
dinosaurs). In contrast, large-bodied mammals would need to live for longer
durations in order to ensure a sufficient number of young in a population.
So natural selection can favor long-lived placentals where the same type of
selection would be weaker on big dinos. Large dinosaurs appeared to have
grown fast like mammals, but don't appear to have their long lifespans.

Incidentally, P. Martin Sander (2000. Paleobiology 26:466-488) found that
the ontogenetically oldest Janenschia specimen he sampled was probably about
38 years old, and had been fully grown since about 26.

Lifestyles of young tyrants vs. older tyrants:
The authors note that the young T. rex are below Hutchinson & Garcia's 1000
kg limit for feasible fast-running Tyrannosaurus. So, perhaps, Tyrannosaurus
was functionally a different animal ecologically (one might dare say a
"Nanotyrannus"... :-) as a youngster than as an adult.

Good stuff.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796