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Re: Sympatry, Allopatry, and sternum of mammals



>   David Marjanovic also made comments on how allopatry and sympatry would
> be synonymous to some degree,

In the definition you gave, where you related the terms to genealogy rather 
than geography.

> but they are in fact mutually exclusive:

Of course.

>   David also made comments on my use of sympatric speciation as being the
> reason why my definition was different from his for "sympatry" but I was
> under the impression we were discussing _evolution_.

I was under the impression that I was just nitpicking, discussing what the 
_word_ means. :-) Looks like we agree on what it means.

Original Message by Jaime A. Headden
Sunday, 29 December 2002 22:37
Subject: Re: Dinosaur Genera List update #196

> <Wrong. The mammalian sternum is, as detailed in the description of
> *Zhangheotherium*, a composite of interclavicle + manubrium sterni +
> sternebrae (+ xiphisternal stuff);>
>
>   Okay, maybe I should clarify my statement.

I understood it.

> No mammal has bilaterally paired sternal elements. Period.

See fig. 4a of
Hu Yaoming, Wang Yuanqing, Luo Zhexi & Li Chuankui: A new symmetrodont mammal 
from China and its implications for mammalian evolution, Nature 390, 137 -- 
142 (13 November 1997)
I can send the pdf, and I believe it's available for free at 
www.nature.com/nature. It's interesting to see that we have retained all 
pectoral bones, even though often as tiny fused elements, after the cleithrum 
disappeared in the Permian.

> The espisternum is formed from the
> interclavicle, and this is similarly not paired, comprising a singular,
> originally Y-shaped cartilage that bound the clavicles, coracoids, and
> non-mammalian paired sternals.

The interclavicle is indeed unpaired, but a dermal bone like the clavicles, 
not an endochondral one like the sterna, the scapula and the two coracoids. 
In crown Theria it is fused to the rest of the sternum, in *Zhangheotherium* 
it isn't, in the platypus it's gigantic.

> Before the development of crown mammals,
> the manubriim and all other sternal cartilages (xiphisterni)

-a, because it isn't -sternus. -- The xiphisternal element(s) lie caudal to 
the last sternebra and the last false sternal ribs. If I run down my sternum 
with a finger, there's a step at the lower end which is floored by a hard 
plate which appears to continue into a cartilaginous spike just above my 
stomach. That's something xiphisternal, isn't it?

> were singular elements and median, not bilateral.

In adults.

> and wether you are referring to
> division of the episternum or manubrium either sagittally, or
> transversely.

The interclavicle, AFAIK sometimes called episternum, is 
cranial-cranioventral to the manubrium sterni.

> Cynodonts show a trend towards singularlization

= fusion along the symphysis

> of sternal elements [...]

which, you are right, became so small that only the first pair of ribs 
articulates with them in crown mammals, while the 2nd pair articulates with 
the junction between the manubrium and the 1st sternebra. Maybe the 
sternebrae are neomorphs, I don't have an idea.

> the interclavical, which became the only "true" sternal bone that existed
> earlier to crown mammals.

The interclavicle (os interclaviculare) has never articulated with ribs, and 
its fused remnant in Theria is just big enough to let the clavicles, or 
apparently rather the procoracoids (fig. 4b and the legend of fig. 1), 
articulate.