Pre-release theories
This is a Planet of the Apes scenario where the astronauts traveled back in time.
On that note, the undocumented meteor that struck the ship was the one that would have wiped out the dinosaurs, meaning that history has been altered.- Jossed. Mills and the other humans in the movie are explicitly from a different planet. Furthermore, the meteor that hit the ship and caused the crash is a fragment of the KT impactor, which hangs ominously in the sky for the whole movie before hitting Earth as the last part of the climax.
The Astronauts are Human Aliens
Instead of traveling back in time, the protagonists are aliens who have crash landed on prehistoric Earth.- Confirmed.
The whole thing is a simulation
All of the other travellers are not only still alive, but separated and going through the same thing.Identity of the large theropod.
- Tyrannosaurus rex
- Daspletosaurus
- Tarbosaurus
- Nanotyrannus
Mills's ship was sent back in time intentionally
He was just kept in the dark about it by his superiors. This movie is a Spiritual Successor to After Earth
Post-release theories
The many quadrupedal theropods aren't theropods, they're undiscovered notosuchians
This would explain their quadrepedal motion, scutes, and why they're all over the place. Notosuchians are incredibly common in South America and being where the asteroid strikes, the movie seems to take place in Mexico. And, needless to say, it makes the movie more consistent with real paleontology.- The small four-legged predators have been officially identified as Lagosuchus according to the soundtrack. However, that species being from the Triassic simply raises another issue.
The quadrupedal therapods are a tongue in cheek reference to the Slurpasaur trope.
The B-movie vibe of the film has been noted by numerous reviews, it's a Sam Raimi film, it only makes sense the movie would reconstruct the trope. The predatory dinosaurs are all infected with the same parasite that Koa got.
- We're never given any indication of what would've happened to Koa if Mills hadn't removed the parasite, but given that it seemed to cause foaming at her mouth, it's not a stretch of reason to speculate that it may induce rabies-like symptoms in its host. This could easily account for the unnaturally violent, hyper-aggressive behaviour of nearly every predator in the film (as well as the ecosystem having More Predators Than Prey, since the local herbivore populations were likely among the first things the diseased predators killed off).
The advanced civilization they are members of collapsed.
The same illness that took his daughter killed them. That's why they never came back to Earth over the 65 million years. We're merely seeing a reenactment with humans.
The actual aliens were more like Starfish Aliens.- Expanding on this, it's also the explanation for the gratuitous amounts of Artistic License – Paleontology. Of course the real dinosaurs didn't look or behave remotely like the way they're presented in the film, it's just that the aliens (and the humans that eventually made contact with their distant descendants) collectively took what the survivors' accounts described about the animals they encountered and over literal eons of retellings and warped interpretations gradually mutated said animals into hideous, ever-hungry monsters. Naturally, this more than the factual history is what got carried over into the human reenactment.
This movie was planned to take place in the Triassic at some point
A number of factors suggest this, such as the presence of Lystrosaurus and a primitive-looking ornithischian in the concept art, and the various four-legged predators that resemble Triassic archosaurs (not to mention long-tailed, toothy pterosaurs more reminiscent of Triassic and Jurassic genera than anything from the end of the Cretaceous). Perhaps early on, the plan for the movie's setting was to be in the Triassic period, with time-appropriate animals, but it was decided to move it to the Cretaceous and add more familiar dinosaurs in order to make it more marketable. The Lagosuchus and the giant quadrupedal predator (which itself was going to be replaced by a Triceratops at some point) are simply relics of this original plan.- This could further be supported by the fact that the Triassic period also conveniently ended in a mass extinction, implying that the basic plot of two extraterrestrial visitors racing against the clock to dodge an impending extinction event may have been planned from the very beginning. This would've made the transition to a Day the Dinosaurs Died plot all the easier.
The alien civilization that Mills and Koa come from are the ones who built the city in the end credits.
Having learned about Earth’s habitability and the recent extinction of the dominant species (the dinosaurs) from Mills and Koa, they returned to colonize earth and either prevented Homo sapiens from evolving or were actually Advanced Ancient Humans all along and Earth is just a single colony of their civilization. 65 and Alpha (2018) take place in the same universe.
Both of them were made by Columbia/Sony Pictures, both take place during the prehistory but in different time periods (65 during Mesozoic, while Alpha during the Ice Age), both feature predators that stick around far longer than real predators reasonably would and both are guilty of badly portraying prehistoric animals (sabertoothed cave lion, long-horned bison living in Europe, quadrupedal theropods, naked Oviraptor etc.).