In incomplete skeletal fossils, how is it determined which bone goes where?

Question

A good example:

http://palaeo.gly.bris.ac.uk/Palaeofiles/whales/pictures/pakicetus.jpg

How is it determined what each bone is and where it goes when there is so little of the full skeleton remaining? Is it mostly based on similar species with more complete remains?

Answer 1

The shin bones connected to the ankle bone...

Answer 2

Comparisons with more complete fossil material, and with living critters.

I can't see any problems with the suggested placement of the bones in those sketches.  What they show (I determined by reading the file name!) is /Pakicetus/.  That's a whale relative.  There's obviously a lot of the spine available.  If it were all jumbled up, it'd still be possible to identify which bones belong to the spine, and there are distinctions between the ones from different areas of the spine.  You may not be able to recognize them, but others can.  You'll surely be able to recognize some details, for example, which of the bones can only be ribs, and which can only be limbs.  The hips and head are even more obviously unmistakable.

As far as I'm aware, these fossils were found largely in articulation.  That means the bones were still joined up and in more or less the correct positions when excavated.  That sometimes happens.  In such cases you don't need to work out what goes where.

<<Is it mostly based on similar species with more complete remains?>>

That's the general idea.  In the case of those specimens, however, a paleontologist would term them as being near complete.

There's not much missing that can't be safely assumed.  There are gaps in the tail.  However, the former bones would have been much like those in front and behind excepting for some difference in size.  Tail bones grow progressively shorter towards the end.  As it's a placental mammal, the ribs would have continued down for about two-thirds of the chest.  They also grow shorter as you go down.  For the limb bones, it's usually easy enough to distinguish the rear leg bones from front ones.  The rear ones are thicker.  Rear legs are always stonger than front ones.  That's because, when moving, the rear legs provide most the power while the front ones are more about maintaining balance.  Distinguishing upper limb bones from lower ones is also a piece of cake.  For example, you've only got a single upper arm bone.  There are two in your forearm.  The upper arm bone must be thicker.

Answer 3

Compared anatomy, yes.