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Re: If No. 9 is a hatchling



----- Original Message -----
From: "david peters" <davidrpeters@earthlink.net>
Sent: Thursday, September 30, 2004 12:14 AM

> Okay. Let's play.
>
> What are the 'obviously juvenile characters' of the little tiger?
>
> I can start with first set of teeth about to be replaced with second set.
>
> And small.
> But we don't know that, because we have no other bigger ones to compare
to.

The teeth are a good idea as long as we're dealing with mammals, but not
with other tetrapods -- sure, it does occur that tooth shape and count
change with age, but this is not predictable as long as we don't know
juveniles and adults from close relatives.

One surefire criterion for mammals are the epiphyses; they fuse to the
diaphyses around maturity. Does not work for other tetrapods; but still the
joint ends should ossify more with age.

Various fusions all over the skeleton. I would not be surprised if the adult
house cat would turn out to have more sacral vertebrae than the tiger baby;
remember that the seemingly subadult *Dilophosaurus* has 4 while all other
theropods have 5 (or more). One must, however, look at the whole skeleton to
get a good idea of the age of the specimen, if no reasonably close relatives
are available. If you have the vertebral column of a crocodile, you can tell
with quite much precision how old the individual was, because the neural
arches fuse to the centra in a certain sequence (it begins IIRC in the
proximal tail and spreads in both directions, reaching the neck last). In
different clades it can be different. (Besides, it's an autapomorphy of
something like Amniota that neural arches and centra _ever_ fuse.)
        In humans, all skull bones start to fuse around age 40 and finish
around age 80. In extant birds, this happens much earlier. In taxa with
indeterminate growth, it happens never -- although the sutures become more
and more fractal. Fontanelles become sutures, and straight sutures become
complex. In birds the sterna fuse around hatching age; in mammals they fuse
much, much earlier. (Whales are probably an exception.) In birds about the
proximal half of the sterna is ossified at hatching; in mammals the
individual sternebrae -- which don't occur elsewhere -- are AFAIK all
ossified at birth.

Then there are bones that only appear at a certain age. Various sesamoids
count, as do carpals, tarsals and small phalanges in some clades. Baby
pterodactyloids indeed lack the small toe phalanges.
        Likewise, the sternal keel of birds ossifies later than the rest of
the sterna.

Proportions. Babies have bigger heads and smaller (not longer) extremities.
(I'd rather not count the big, round eye sockets -- they are more
size-related.) Complications can occur, however.
        One is phylogeny. What qualifies as a relatively big head in one
clade is a relatively small one in another. Azhdarchids and elasmosaurids...
        Snout length should not be included in head size; it increases
during ontogeny. The sooner it does, the more precocial the species can be
assumed to be. Then there are more complicated growth patterns. In humans
the arms and legs are relatively longest not during adulthood, but at the
age of 9 or 10 or so -- they grow first, the trunk grows later. Horses and
antelopes have the same pattern, just shifted: their legs are relatively
longest at _birth_. It is assumed that this is an adaptation so the newborns
can keep up with the herd. A precocial basal pterosaur that can fly and hunt
soon after hatching but takes several years to reach adulthood may well
require a different ontogeny.
        I wouldn't count on relatively bigger hands and feet. In primates
they don't occur, and for biomechanical reasons they shouldn't -- the baby
has to cling to Mom's fur, and this demands certain proportions. In
pterosaurs the size of the hands at least has such a great influence on the
aerodynamics of the animal that I'd be even more careful.

Immature bone texture. There's no way to get around this with paedomorphosis
or whatever. But it does mean that one has to take a _very_ close look at
the fossil.