Montevideo 100 mya

http://www.planetaryvisions.com/Project.php?pid=2216

One hundred million years ago, Uruguay was in a more southern location much closer to Africa. Perhaps closer to 45 degrees latitude and 15 degrees east of the prime meridian.Global temperatures were much higher at the time so despite being at a lower latitude, temperatures would have probably remained closer to the same as modern day Uruguay.
Air masses during the Cretaceous would have no doubt been in different locations and would have offered differing effects on past temperatures. With the development of the new Atlantic Ocean, permanent current patterns would have just begun to form but assuming a shallower sea level, they would have not had the depth to be influenced by flow from the colder Arctic and Atlantic water as they are in today's ocean.

A proposed map showing potential locations of ancient climate air
masses that might have affected Uruguay. Red indicates maritime
tropical, gold continental tropical, light blue maritime polar and pink
as a developing maritime tropical air mass over the very warm shallow
Atlantic sea. 

It seems reasonable that the warm shallow sea to the east would have begun to moderate the climate of Uruguay and made it slightly warmer than modern Montevideo. The forming ocean most likely increased humidity in the air and caused a rise in precipitation levels but nothing that would have by that time created the more lacustrine environments seen in Uruguay in later years. Because there was slightly more land mass in the southern hemisphere at the time, there might have been a bit more variety in seasonal temperatures.

Climograph for Modern day
Uruguay

Climograph for Cretaceous Uruguay. Temperature in burgundy
and precipitation in purple.