Have there ever been as many mammals as there are now?
There can be perhaps 10 trillion rodents and bats on the planet, so the humans and livestock probably are small compared to a rainforest rodents and bats. The biggest bat colony is 40 million, they decline without unmanaged forests, can reach 4-10 bats per hectare, and the world population might normally be in 10s of billions, save for the use of pesticides and deforestation. 

Here's a bit of maths to say that we have erased 1 trillion rodent habitats through monocultures and ploughing. Every cow and sheep field would have previously supported 10-20-50 times more rodents, and they aren't adapted for pesticides and ploughs.

37% of the world's land is farmed 
Extent of total cultivated land in 2006: 1.5 billion hectares.

If there were 14 mammals per km2 of farmland, then we have displaced 20 billion mammals.

If the pristine habitats previously supported 1 rodent/mammal every 5m2, we would have destroyed the homes of 2000 mice,voles,shrews,squirrels and other mammals by hectare, that's 3000 Billion mammals (worst case scenario) due to monocultures and farming, and 1000 Billion in a best case scenario. Bangladesh rice farms and UK fields both are hostile to mammals. 

A 20g mouse for 5m2 comes to the same as one human for 15 square kilometers. A mouse only needs a 3g of seeds/roots/beetles/snails per day to survive, that's about a kilo per year. "blue oak, interior live oak, and black oak respectively produce 2955, 1074, and 616 kg/ha of tree canopy" invertebrates provide 100-300kg/ha.

Perhaps we have erased 1 trillion mammals in the amazon alone. 

https://www.worldatlas.com/articles/list-of-primates-by-population.html

Also we have depleted the fish stocks. It is reported that in medieval times, the seas heaved with incredible amounts of dolphins, whales and fish stocks, pristine seal and dolphin numbers could have been about 1 billion. Perhaps today it's about 1/4 of that. 

If i had to bet all I own on your theory, I'd say it happened at another time in the previous 60mn years, probably not today.