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Re: tyrannosaurids (Nanotyrannus)

Here are a few thoughts on "Nanotyrannus" that may be pertinent to the
current discussion. It is possible to assess the ontogenetic stage of
fossil vertebrates, using osteological criteria that can be observed in the
skeletal growth of extant vertebrates. Pathologists do this sort of thing
with human skeletal remains routinely. Paleontologists, on the other hand,
have rarely attempted to determine the developmental stage at which a given
specimen has died, even though the criteria are often at hand for doing so,
even in incomplete specimens. Paleontologists have tended to simply assume
their specimens are 'adults' or 'mature' (whatever that might mean), and
proceeded from that assumption to the next levels of interpretation. The
current discussion is a good expample of what consequences this can have,
and why we need to be more attentive to identifying, at lest in some
relative fashion, the developmental stage for any specimen that comes to
our attention. Is "Nanotyrannus" a distinct genus or species? The answer
will impact systematic discussions, biodiversity estimates, and estimates
of evolutionary rates for tyrannosaurids. Is "Nanotyrannus" a dwarf
tyrannosaur as some of you suggest? The answer to this will have a lot of
significance for our general understanding of tyrannosaurid history and
biology -- not many other lineages can boast the acquisition of huge size
followed by dwarfing. In the few lineages where this is reasonably well
confirmed (e.g., proboscideans), there are fascinating evolutionary stories
to be told (e.g., insular reversal). But the answers to these and many
other questions hinge on whether "Nanotyrannus" died before it had finished
growing. If it did, then explaining dwarfism in tyrannosaurids would be
like explaining why Napoleon crossed the Mississippi. Our diversity
estimate would be inflated. Our taxonomy would be screwed up, and so on and
so on. Before this discuission gets carried away, lets try to first answer
the basic question -- at what ontogenetic stage did the specimen at issue
die? I havn't seen the specimen, but here's what I would look for. Are the
exoccipital bones _fused_ to the basioccipital (i.e., the suture is totally
obliterated by an overgrowth of bone so that the boundaries of the two
elements can no longer be discerned). Here's why this suture is so
important. As in the vertebral column, the paired occipital arches (like
the neural arches) develop from ossification centers that are distinct from
the center that forms the centrum (or the basioccipital). Growth proceeds
in the connective tissues between the ossification centers. At first the
arches (in this case the exoccipital) and centrum (in this case the
basiocciptial) are separate, then the meet the centrum , then suture (i.e.,
the two bones interdigitate), and finally fuse in such a way that the
sutural line is obliterated by bone. Growth largely ceases when fusion
If there is still a suture or a gap visible between the exoccipital and
basioccipital, then "Nanotyrannus" died before it finished growing -- and
its status as a distinct taxon would be most doubtful. Many more clues as
to its relative age at death could come from the postcranium. I understand
that no postcranial elements were associated with the skull (anyone out
there know if am I wrong on this?). Incidentally, there has been a lot of
comment about how it has "closed sutures" -- but these are all vague
statements. When Bob Bakker said this on The Infinage Voyage program, he
seemed to refer to the membranous skull roofing bones, which do not have
the same type of determinate growth pattern as the endochondral vertebral
column (and its cranial component at the rear of the braincase). A suture
distinguishing the boundaries of the separate roofing bones will persist
through life in many cases. And where the roofing bones do fuse, as the
parietals do near the back of the head in many theropds, this fusion occurs
earlier in life than that between the occipital elements. Based on the
growth patterns observed in living tetrapods, the occipital elements offer
an unequivocal signal on ontogenetic stage, whereas the membranous
ossification suture patterns are prone to interpretation. If any of you has
a chance to inspect the original specimen, I would love to know what you
see in the occiput -- with knowledge of the suture there's a cool paper to
write on this subject, whether or not its an adult. I only hope that
sufficient bone (not plaster) is preserved in this critical area. Should
any of you see the specimen, please let me know.

Timothy Rowe
Department of Geological Sciences
The University of Texas
Austin Texas 78712

phone: (512) 471-1725
fax: (512) 471-9425
e-mail: rowe@maestro.geo.utexas.edu