RE: Tyrannosaur age-population distributions

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]On Behalf Of
> Phillip Bigelow
>
> It's hard to wrap my mind around why we don't have *any* (to my
> knowledge) isolated
> Tyrannosaurus/Daspletosaurus/Gorgosaurus/Albertosaurus bones from animals
> of, say, 1.0 meter size, already in museum collections.

Well, don't get too obsessed with tyrannosaurids.

(Did **I** just type that!?!? :-S)

It isn't like collections are dripping with juvenile Triceratops or 
Edmontosaurus or Ornithomimus or (shifting provenance)
Diplodocus or Apatosaurus or Camarasaurus or pretty much ANY dinosaur.

> To be sure, most
> deceased babies and toddlers probably passed through the digestive tracts
> of predators, but some isolated elements should have been washed into
> channel deposits by floods and were preserved.
>
> Either the infant tyrannosaur bones are mislabled in the collections
> ("dromaeosaur sp. indet."),

Almost definitely. Plus, there are a fair number of small teeth of 
tyrannosaurids.

> or the baby tyrannosaurs migrated from higher
> elevation nesting grounds (which are less commonly preserved in the
> fossil record)  to the lower flood plains (Hell Creek/Judith River
> environments) at around age 2.

Could be.

By the way, here is the paper citation:
Erickson, G.M., P.J. Currie, B.D. Inouye & A.A. Winn. 2006. Tyrannosaur Life 
Tables: An Example of Nonavian Dinosaur Population
Biology. Science 313:213-217.

Abstract:
The size and age structures for four assemblages of North American 
tyrannosaurs-Albertosaurus, Tyrannosaurus, Gorgosaurus, and
Daspletosaurus-reveal a pronounced, bootstrap-supported pattern of age-specific 
mortality characterized by relatively high juvenile
survivorship and increased mortality at midlife and near the maximum life span. 
Such patterns are common today in wild populations
of long-lived birds and mammals. Factors such as predation and entrance into 
the breeding population may have influenced tyrannosaur
survivorship. This survivorship pattern can explain the rarity of juvenile 
specimens in museum collections.

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
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