It's not easy determining who had the idea first, but Hanna-Barbera's Valley of the Dinosaurs came along the same year that Sid & Marty Krofft's Land of the Lost did, in 1974. Valley aired on CBS, which put Shazam! directly opposite Land over on NBC, if my memory serves me correctly.
Here's the intro:
It's worth noting that Fred Freiberger, whose sci-fi cred includes writing and/or producing for Star Trek and Space: 1999, and who also created, produced and served as principal writer for another H-B frosh in '74, Korg: 70,000 B. C., which aired on ABC, was a writer on Valley. Freiberger would leave H-B to work on Space: 1999, ending a short but productive run with the studio. Alert viewers will recognize the voices of Mike Road (Jonny Quest, Herculoids) and Shannon Farnon (Super Friends) as John Butler and his wife. Don Markstein, in writing an entry on Valley for his Toonopedia site a week ago, got the mistaken notion that Jackie Earle Haley ("Watchmen", "Nightmare on Elm Street", Human Target) voiced John Butler. Impossible, considering Haley was just a kid back then, 2 years away from the original "Bad News Bears". Maybe if they remade the show today, or if they'd done a parody on Robot Chicken, but definitely not in 1974.
As for which was the most successful dinosaur show of the period? That's easy. It was Land of the Lost (previously reviewed), which lasted 3 seasons in its initial run, and 2 more when ABC revived it in the 90's. Korg and Valley were both cancelled after 1 season.