Re: dinodimorphism

["Adam Britton" <abritton@wmi.com.au>]

From: "Rob Gay" <rob@dinodomain.com>
Sent: Friday, December 21, 2001 12:09 PM


> I'd like to think so too, but I think that the patterns seen in birds may
> not be the same reflected in dinosaurs, or may not be informative (i.e.
> males taking on both robust and gracile morphs, depending on taxa), and
I'm
> not sure how much of a degree of dimorphism is present in crocodylians.

Sexual dimorphism in crocs is dependent upon size, and upon species.
Juveniles of all species are virtually impossible to differentiate based on
skeletal characteristics, but as they grow dimorphism often becomes more
apparent. Adult males can eventually be differentiated from females based
purely on total length (typically 1.5 to 1.8 times max female total length)
and even their skull can be differentiated by the naked eye as it becomes
significantly broader and more rugose. Quite a bit of morphometric work has
been done on Crocodylus porosus, and discriminant analysis of a large wild
dataset (using primarily skull dimensions) began to successfully separate
sex from a total length of only 0.6 metres, with near 100% confidence after
1.2 metres (only 20 to 25% of typical adult size for males). Pretty
impressive, although this level of discrimination may not be possible for
all species. C. porosus, for example, show dimorphism in tail length, but C.
niloticus do not. Other differences in diet and ecology are clearly going to
be important.

Of course, this is based upon discriminant analyses of large, known datasets
from wild animals. When you're dealing with a few scant fossil remains, the
problem is trying to determine which characteristics - if any - are
dimorphic, and which range belongs to which sex. At the very least, you
could perhaps suggest that the extremely large crocodyliform fossils of
Deinosuchus, Sarcosuchus etc were probably male (based on comparisons with
extant species), but it's not impossible that they were females - nothing is
known of the range of size of each sex. As for dinosaurs, while it seems
that determining sex is a possibility, I imagine it would also require a
relatively large dataset, or some very clear characters associated with
reproductive evidence (to assign sex).

Adam Britton