From The Ledger in Lakeland, Florida. How did we ever have time to leave the house?
Land of the Lost (NBC) is up against Valley of the Dinosaurs (CBS), an animated series with a very similar premise. This is the day both shows premiered. It was the debut of the new Saturday season, actually: The New Adventures of Gilligan, Partridge Family 2200 A.D., Devlin, and Korg: 70,000 B.C., another prehistoric-themed adventure (listed as `Kong – 70,000 B.C.’ in the listing), were also new.
2 Warps to Neptune 
I was (and still am) a huge fan of “Land of the Lost”. Probably why I didn’t even know about “Valley of the Dinosaurs” until it was in syndicated repeats years later. I had always thought it was a knockoff of LOTL (c’mon, a raft? A family? Dinosaurs? Cavemen?) It wasn’t until recently I learned, as you stated, they premiered on the same day!
My guess is that CBS got wind of LOTL and created Valley to compete on the cheap, but I have no proof. As you say, they’re just too much alike, and LOTL was too sophisticated to be the knockoff.
I just started watching LOTL for the fifth or sixth time through since it came out on DVD. It really does hit a nerve.