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RE: they keep on growing despite early fusion

It isn't all that surprising that this happens in the limbs, considering
that vertebral arches also fuse to each other and the centrum in early
ontogeny of (at least) all squamates. Growth occurs through remodeling at
the bone surface (external and internal), and snakes lack separate epiphyses
at all stages of ontogeny.  Of course, David is the only person in the world
who thinks that squamates are especially relevant to pterosaurs, but they do
show that caution is indeed required more generally.

Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa

-----Original Message-----
From: David Peters [mailto:davidrpeters@earthlink.net] 
Sent: 24 January, 2008 4:44 AM
To: Dinosaur Mailing List
Subject: they keep on growing despite early fusion

Was wondering if anyone has any thoughts on this?
In a nutshell, Jessica Maisano found early fusion of many skeletal  
elements, yet major growth continued in certain squamates.
It has the potential to put a cautionary note on putative size- 
independent criteria for determining maturity in fossil squamates  
(including pterosaurs & their kin).

David Peters
St. Louis


Department of Geology and Geophysics, Yale University, New Haven,  
Connecticut 06520
ABSTRACT-Terminal fusions-fusions of skeletal elements thought to  
mark the cessation of significant growth-
can be used to estimate the relative maturity of fossilized  
individuals. Little is known of how the absolute timing of
terminal fusions relates to sexual and skeletal maturity in  
squamates. Examination of postnatal ontogenetic series of
extant representatives of 14 crown squamate clades reveals that no  
terminal fusion universally coincides with the
achievement of either sexually or skeletally mature size; however,  
certain fusions may serve as benchmarks of maturity
within particular crown clades. Complete fusion of the braincase is a  
reliable benchmark for skeletal maturity in
scleroglossans, but not iguanians. Complete fusion of long bone  
epiphyses will indicate that a squamate individual is
within roughly 20% of maximum size, whereas complete fusion of the  
scapula and coracoid, the pelvis, and the
astragalus and calcaneum can be taken as evidence only that an  
individual has achieved at least half of the maximum
size of its species.

and from near the end of the text:

Because terminal fusions are thought to mark the cessation
of significant growth, it is difficult to understand how individuals
could continue to grow after such fusions are complete.
This is especially true of the fusion of the long bone epiphyses,
which may reach completion in individuals that are only 63%
of the maximum reported size for their species (Callisaurus).
It may very well be that these individuals have achieved their
personal maximum size, and this could be tested via longitudinal
allometric studies employing X-radiography. However,
even a considerable range in individual maximum size cannot
account for the extreme case presented by Bipes. In neonates
of this fossorial species, the braincase, scapulocoracoid, and
vertebral elements are already fused, yet Bipes almost triples in
length-from 90 to 240 mm SVL-during postnatal ontogeny.
How Bipes accomplishes this is unknown, but clearly, ''terminal''
fusions cannot be taken as de facto evidence that an individual
squamate has stopped growing. Only by continuing to
examine the timing of terminal fusions in extant taxa-especially
via more longitudinal studies of individual ontogenies-
will we gain a better understanding of their relationship to
growth, and thus be able to more rigorously utilize them in the
interpretation of fossil individuals.