From: jespah@carson.u.washington.edu (Kathleen Hunt)
Newsgroups: rec.equestrian
Subject: Evolution of horse teeth
Date: 14 Oct 92 02:22:55 GMT
Article-I.D.: u.1992Oct14.022255.10008
Organization: University of Washington, Seattle

Jill Helms says:
>May I just say something here?  The idea that Tracy has presented a 
>morphological argument for human beings eating nuts and berries because of the 
>shape of their teeth is a new one on me.  I'm a dentist (periodontist) who does
>research and I have never, ever heard of this argument for why teeth have the 
>morphology that they have!  Where exactly did you get this information?  I've 
>been at this for over 10 years, so I'm not exactly a neophyte... just confused
>by your arguments!

Teeth are a paleontologists' be-all and end-all for figuring
out what an extinct animal ate.  Mammals are unusual among the vertebrates
for having distinctively different types of teeth for different diets.  I
imagine a human dentist might not have heard of this because human teeth
are fairly unspecialized, and we have a very diverse diet, so the tooth-diet
relationship isn't obvious in us.  When you look at other species, though,
the tooth shape--diet relationship is very obvious.  This is usually covered
in vertebrate bio, evolution, and paleontology courses.

In another post Tracy mentioned evolution of horse teeth as an 
example.  The horse family started out as browsers (eating soft
vegetation such as fruits and leaves) and switched to grazing, and
their teeth changed substantially.  The details are listed below for
anyone interested.

Kathleen

**********
  The first equid, little doggish Hyracotherium, had "primitive mammalian
teeth" with 3 incisors, 1 canine, 4 simple premolars and 3 low-
crowned, grinding molars in each side of each jaw.  Good teeth for an
omnivorous browser who mostly ate fruits and soft leaves.
  The first change was the addition of more teeth for grinding vegetation.
This was accomplished by each of the premolars, in turn, becoming larger
and squarer and exactly like a molar.  This occurred through the Eocene
from about 50 to 35 million years ago, in the equids Orohippus, Epihippus,
and Mesohippus.  By Mesohippus, equids had ended up with a battery of six
similar grinding "cheek teeth" which are actually 3 molars and 3 premolars,
with the last little lonely simple premolar still in front.  These teeth were
well-suited for browsing on leaves.  They would have been no good for
chewing grasses, but that didn't matter because grasses hadn't evolved yet.
  The second change happened when grasses evolved and spread across the 
world to create grassy plains and savannahs -- a brand new habitat.  Horses
took advantage of this habitat right away.  Their teeth changed to be 
better suited for chewing harsh, abrasive grass.  Small crests on the teeth
enlarged and connected together in a series of ridges for grinding. There 
was a gradual increase in the height of the tooth crowns, so that the teeth 
could grow out of the gum continuously as the tops were worn down 
("hypsodont" teeth).  And, in addition, the tooth crowns became harder due 
to the development of a cement layer on the teeth.  These were *major*
changes.  It happened in the early Miocene, from about 23 to 17 million years
ago.  There was a big burst of equid evolution just then, changing from
small forest browsers to large, speedy plains grazers.  The changes 
started in Parahippus and culminated in Merychippus, the first truly
horsey looking equid.  (The big teeth gave it a long, horsey-looking head.)
  Horse teeth now have the same general blueprint as Merychippus' teeth,
though the cusp and ridge pattern is a little different, and the teeth
are straighter.

(This information came from Simpson's classic book on horse evolution,
"Horses", updated with Prothero & Schoch's "The Evolution of Perissodactyls".)


-- 
Halfway round the course, up spoke the noble rider:  
 "I fear we must fall back, for she's going like a tiger."
Up spoke the noble horse:  "Ride on, my noble master,
  For we're halfway round the course, and now we'll see who's faster."