Clifford D. Simak 
Mastodonia 

Asa Steele was unprepared for the incredible events that began to unfold when Rila Eliot – a long-lost love – stepped out of the past and his faithful dog Bowser started loping into it through time trails he’d discovered in his own backyard. Rila’s appearance was mere coincidence, but Bowser’s retrieval of fresh dinosaur bones was as inexplicable as was the curious crater in Asa’s backyard that seemed to have been made by a spaceship from the stars. Soon Asa himself tripped in time, led into prehistoric eras by an enigmatic cat-faced alien. In short order, the time trails in the quiet town of Willow Bend became the focus of global attention and government scrutiny. Then time-travelling turned into big business and led to big trouble, when Asa and Rila uncovered an interstellar mystery from before recorded time!