[cheryl j (OP)]

Global Warming Led to Dwarfism in Mammals -- Twice
http://www.sciencedaily.com/releases/2013/11/131102095546.htm

"In 2006, Gingerich proposed that mammalian dwarfing could be a response to the lower nutritional value of plants grown under elevated carbon dioxide levels. Under such conditions, plants grow quickly but are less nutritious than they would normally be."

Their explanation doesn't really make sense to me. Any ideas?

[CliffordK]

Odd, I had read theories that snakes and reptiles grew larger in warmer climates.  It can be observed in the world today with little tiny lizzards in northern climates and massive crocodiles in the tropical regions.  Likewise, little garter snakes here, and massive pythons and anacondas down south.  And the far more massive titanoboa lived during some of the warmest extended period in the last hundred million years.

The nutrition idea is interesting.  Perhaps during periods with higher CO₂, there is increased plant growth, in particular more sugars and carbohydrates.  But, if the phosphates and ammonias are fixed, perhaps the larger plants are depleted in other vital nutrients.

Dwarfism is often found on small islands with infrequent contact with the mainland.  But, the demands just seem different than on a continent.

Hmmm,
The article you found focuses on the PETM.  It is an extremely brief period of rapid positive & negative temperature change.  It sounds like it was a huge stress on the animal kingdom, and many mammals became extinct during this period.  But, there would have been large changes throughout the ecosystems. 

I believe the Hyracotherium was a UK animal.  The UK is extremely northerly.  Very dependent on the gulf stream.  I presume it had a mix of plains and forests.  Horses are generally a plains type of animal.

A lot of things could have gone wrong which could have damaged the plains animal's abilities to compete including different vegetation and expansion of forests.  Could the UK have experienced harsher winters if the gulf stream was altered, even if global temperatures generally increased?

[Lmnre]

Two terrestrial ecogeographical rules may come into play.

According to Bergmann's Rule on body size first proposed in 1847, within the same species of warm-blooded animals, individuals from populations living in warmer environments tend to have less body mass than do those from populations in colder areas because of the selective advantage that less body mass provides. A smaller body produces less internal heat and radiates relatively more of it into the environment because the skin’s surface area is relatively larger.

According to Allen's Rule on appendage size first proposed in the 1870s, within the same species of warm-blooded animals, individuals from populations living in warmer environments tend to have longer/larger appendages (limbs, ears, tail, etc)  than do those from populations in colder areas because of the selective advantage longer/larger appendages provide. Longer or larger appendages provide greater surface area and can radiate more heat into the environment because the skin’s surface area of the appendages is greater.

Similarly, the fennec fox, the smallest species of canid in the world, lives in the desert and has large ears.