Re: Nanotyrannus

["Matthew Bonnan" <mbonnan@hotmail.com>]

Hi Chris:

I am no expert on theropod dinosaurs and so I cannot help you with specific 
questions related to Nanotyrannus itself or any recent phylogenetic work 
that has been done.  Dr. Thomas Holtz is someone on this list you may wish 
to contact in that regard.

To answer some of your other inquiries, you said:
I thought that
> many dinosaurs did not grow proportionally but varied in rate at different
> times during their life, like humans.  Can such a comparison be made
> between adult s and juveniles unless the two are found in close proximity?

As far as I know, all animals vary in growth rates during different 
ontogentic (growth) stages of their lives.  It is important to remember that 
reptiles are generally animals with indeterminate growth.  Unlike mammals, 
reptiles (and dinosaurs) have no ossified epiphyses, at least in the sense 
that mammals do.

To clarify this (and this is a very simple summary: bone growth is 
complicated!), mammalian bone forms first by laying down a cartilaginous 
"blueprint" that is then ossified over time.  The shaft, or diaphysis, 
begins ossifying at its center, and this ossification continues outward 
towards its ends, or epiphyses.  The epiphyses, in turn, also begin 
ossifying from the inside out.  Growth of the mammal stops when the 
epiphyses and diaphysis contact one another and fuse.  Although bone is 
constantly remodeled during the life of a mammal, growth has essentially 
ceased.

So, we can tell if a mammal skeleton is juvenile or adult by looking to see 
whether or not the diaphyses and epiphyses have fused.  Where and when this 
fusion takes place is used as a way to identify the age of human remains, 
along with the eruption of certain teeth (in us, wisdom teeth appear at 
about age 21).

Reptiles (and dinos), on the other hand, have epiphyses that do not 
completely ossify and whose articular surfaces are almost completely made of 
cartilage.  The diaphysis still grows similar to that of a mammal, but it is 
in a sort of race to catch the epiphyses, which it never does.  While 
reptiles will reach an optimum size where growth slacks off, they continue 
grow, even if slowly, their entire lives.  This makes determination of 
juvenile dinosaurs from adults a bit more difficult.

Fusion in the skull works a bit differently than what I described, but in 
general, the more fusion of bones present, the older the individual.  From 
my memory on this issue, I think the general consensus was that Nanotyrannus 
was an adult because its skull bones are all pretty much fused, although 
memory might serve me poorly.

As far as juvenile series go, the hadrosaurs Maiasaura described by Horner 
et al. seem to be some of our best in terms of how some dinosaurs grew and 
changed ontogenetically.  Without this information, one can still 
differentiate between adults and juveniles, but of course it is much more 
difficult.

In general, juvenile animals are characterized by proportionately larger 
eyes than adults, poorly fused skull and girdle bones, a head which is 
proprotionately larger than the body than that found in an adult, etc.  In 
theropods, I believe the hindlimb proportions tend to be different than 
adults, but I am in no way an expert on this either.

Size, of course, isn't a bad criterion when incorporated with other data.  
Finding an animal that is fairly small but that looks like a T. rex or some 
other such theropod might be a juvenile.  It is of course difficult without 
an adult and juvenile side by side to make a determination in some 
instances, but there are clues to help.

Olivier Rieppel of the Field Museum has written a number of papers on 
reptilian ontogeny.  While I don't have these or the references, you may 
wish to speak with him on obtaining this information.

Good luck with your question,

Matt Bonnan
Dept Biological Sciences
Northern Illinois University


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