If paleontologists have said it once, they have said it a hundred times:
When Dinosaurs Ruled the Earth doesn't begin with a shot of a steaming, primeval forest, but of a gaggle of tanned and oiled cavepeople who have crawled out of their cliffside dwellings to engage in their regular "let's pick which blond woman we want to sacrifice" ritual. Naturally, the prospective victims are not very happy about this—one throws herself off a cliff—but when they try to escape they are hindered by the fact that they are wearing prehistoric underwear so skimpy that it actually makes it more difficult for them to run away. It would have made more sense for them to lose the push-up bras and just bolt for it, though I imagine going streaking during prehistory would have presented its own unique risks.
In any event, one of the Cenozoic supermodels—named Sanna—does manage to escape by jumping into the sea and is promptly rescued by a conveniently placed group of fishermen whose unfortunate garments remind us why it's never wise to wear thongs in a windstorm (I wish I were talking about sandals here—yikes). It is among this group of unfortunately attired men that we meet Tara, our film's scruffy male lead. Meanwhile, back at the ranch, the musclebound leader of the cavepeople is clearly upset that the sacrifice did not go as planned; he shouts incomprehensible phrases and gestures widely to get people to go do whatever it is they do. Maybe this was intended as a bit of fun for the audience—make up your own dialogue as you go along—especially since words like "akita" appear to mean: "Over there"; "Stop"; "Give me that"; "Come over here" and "Let's have pancakes for dinner tonight."
For me, though, the film's real stars are the prehistoric creatures that help to thin out the cast, and the audience's first look at one of the film's exquisite stop-motion monsters comes when the fishermen return with the woman to their camp. While the dudes were out fishing, someone brought a plesiosaur (which is, of course, not a dinosaur) to the big clam bake, but damned if they knew what to do with the thing. It was too angry to just stick an apple in its mouth and start slow-roasting it, and when half the village runs over to examine their new visitor their dinner tries to make a break for it. Unfortunately, though, the plesiosaur wanders right into a mess of fluid the tribe uses for lighting fires, and soon the only question on anyone's mind is: "White meat or dark?"
Things don't look so rosy the next day. The cliffdwellers are still miffed that their sacrifice just up and left, and Tara's wife isn't too happy that he came back with a new, blond girlfriend. When Sanna's captors show up, she makes a break for it, and thanks to an assist from an angry Chasmosaurus she gets a little extra time to get away. That does little to help the fisherman and his friends, though—when they set out after her the same dinosaur causes them a spot of trouble before throwing itself into what sounds like a bottomless pit (lots of roaring, but no crash). Sanna also encounters some of the dangerous local fauna when she finds herself being enveloped by a carnivorous plant, although I would not recommend her escape technique of reaching outside to stab inwards at the plant's tough outer hide (pointsy towardsies = bad).
The remainder of the film is little more than an excuse to watch Victoria Vetri run around in an embarrassingly small bikini. Thankfully, there are a few more prehistoric critters to help break the movie's naked tedium. A newly-hatched baby something-o-saurus and its mother (which look like cousins of the Beast from 20,000 Fathoms) provide a brief bit of comic relief as they try to figure out whether Sanna is friend or food; an attack by an oversized Rhamphorhynchus livens things up a bit, and when Tara returns home to find that his tribe doesn't think it's cool that he ran off with someone else's sacrifice, they try to serve him up on a raft to the local Tylosaurus. (The marine reptile responds by tossing him off the raft. "Yecch! Human? No thanks - I'm trying to cut back on junk food.") Given how good these stop-motion creatures look, though, it is sad that the film also resorts to gluing plates and spikes on alligators and monitor lizards and making them fight, a practice that is despicable as it is lazy.
In the end, a giant tidal wave wipes away the coastal village but delivers our heroes to a mountaintop to observe a lunar eclipse. Dumb, but attractive, they would go on to found a settlement along the southern coast of California which would eventually be named Los Angeles. What happened to all the prehistoric monsters is unclear, though. Perhaps they got so tired of the cavepeople's shenanigans that they eventually died of boredom—a risk I certainly felt while watching this vintage 1970s schlock.