I know my thumb is shorter than my fingers, but I did not know until a few minutes ago that the thumb is missing the Intermediate phalange bone that all the other digits have. Does a mutation ever occur where the thumb has an intermediate phalange? How do we know that the tip of the thumb is a distal phalange, and not an intermediate phalange missing a distal part (the shape of the tip of the thumb, I’ll bet)? Any mutations where other digits are missing an intermediate as the thumb?
Because distal means “(furthest) away from the main trunk of the body” and intermediate means “in the middle.” If there’s no middle, all you have is distal (and proximal, of course).
As for the rest of your questions, I’ve seen finger deformities where there was no intermediate phalange, but I don’t know what it’s called.
AFAIK, terrestrial vertebrates in general have had no more than 2 phalanges on the innermost digit for hundreds of millions of years. The mutation may occur, I suppose, but it would be pretty bizarre.
The phalangeal formula for early land vertrbrates, from inner to outer, is 2,3,4,5,4 (still observable in lizards, etc). This was reduced in mammals to the formula we have, that is 2,3,3,3,3. This is a very very old arrangement.
Tell me more, tell me more!
All mammals? What about birds-- what’s their fromula?
Depending on who’s doing the counting, it’s either 0,2,2,1,0 or 2,2,1,0,0 (the controversy being whether a bird’s manus consists of digits I, II and III or digits II, III and IV; the latter is the more genrally accepted version).
And, obviously, the phalangeal formula of 2,3,3,3,3 does not apply to all mammals. Consider, for example, the ungulates, which have taken the extreme evolutionary pathway of doing away with all but one digit.
I should clarify this: the phalangeal formula 0,2,2,1,0 corresponds to the II, III, IV digit formula.
Is that true of all ungulates? I thought some were two-toed. And did the other toes completely disapear or just atrophy?
I’m getting sloppy in my old age.
Horses have taken the extreme route, retaining only the third digit (other digits may occasionally appear as splints, but these seldom, if ever, develop beyond metacarpals/metatarsals). Other ungulates may retain 2, 3 or even 4 digits (Artiodactyls retain only digits III and IV, for example), with the others much reduced or absent altogether. The first digit of the forelimb is lost in all ungulates, however (tubulidentates, e.g., the armadillo, have 4 digits on their forelimbs, and 5 on their hindlimbs).
Among those various digit arrangements, though, the phalangeal formula for a given digit is generally maintained; horse retain but the third digit, and it retains three phalanges, for example.
I use to work for a guy who had four phalanges on the ring finger of his right hand. Right at the last knuckle, his finger split into two fingertips. Very creepy. He would always stick his hand out to shake hands, and note how the other person reacted.
You meant aardvark, right?
Yes, that. Like I said…sloppy. I give up before I put my foot further into my mouth…
All five toes, or just one? 
(consults family cockatiels. Also photos of family lovebirds and jenday conure)
Um… what about birds like parrots, which have four digits per foot?
More sloppiness on my part: that formula is for bird-hands. Most birds have four toes (some chicken breeds have five, while some other groups, like the ostrich, only have two) with the pharyngeal formula 2,3,4,5 (that’s also the pattern seen in Archaeopteryx, indicating it’s likely the ancestral condition).
Of course, I was referring to the basal formula. Also it might be mentioned that snakes have a much reduced formula compared to other “reptiles”: 0,0,0,0,0. 
The latter is possibly the strongest line of evidence advanced by the dwindling ranks of the birds-are-not-dinosaurs camp, since theropod dinosaurs clearly are I,II,III. The evidence for birds being II,III,IV is from embryology; the other is based on paleontology. However, a reconciliation of the two views has been proposed: 1,2,3 = 2,3,4: A solution to the problem of the homology of the digits in the avian hand
Sloppy, indeed. DF, have you been drinking? 
As **Darwin’s Finch ** suggests, birds have lost one of the toes of the hind foot, that is, the fifth toe. The remaining toes, I,II,III, and IV, have the basic reptilian formula of 2,3,4,5.
Note that in birds that retain four toes, digit 1, the hallux (thumb/big toe) usually faces backwards. It has only 2 phalanges. The common arrangement of 3 toes forward and one back is called anisodactyl.
Parrots, like woodpeckers, cuckoos, and a few other birds, have two toes facing forward and two back. The outermost toe has become reversed, so that the two forward toes are II and III, and the two back are I and IV, with their corresponding phalangeal formulas.
The trogons, a group of tropical birds, are the only ones with the reverse arrangement, called heterodactyl, in which the innermost toe has become reversed, so they have III and IV in front, and I and II back.
HInd foot? Is there some kind of bizarre four-legged bird of which I was previous unaware, but will now haunt my nightmares?
Hooleehootoo writes:
> Does a mutation ever occur where the thumb has an intermediate phalange?
This isn’t quite the same thing, but C. S. Lewis discusses at one point the fact he, his brother, and his father were all missing a joint in their thumbs.
As a matter of fact, the Terror Bird Titanis walleri was enough to give anyone nightmares, even without the clawed forefeet. It stood somewhere between 7 and 9 feet tall, and roamed Florida in the Pleistocene.
A few more jolly restorations:
http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/BioMech/ImagesAtlas/Titanis_Horse.jpg
http://www.geocities.co.jp/NatureLand/5218/thitanisu.JPG
And a skeleton:
Back to my earlier question about mammals… I wasn’t so much interested in ungulants who might have lost some digits, but whether all mammals had the pattern outlined by Colibri– if we look at primitive mammals like a platypus, does it have the typical mammlian pattern?