The Downer Ending isn't as out-of-nowhere as you think. In the episodes about future archaeologists speculating about the artifacts from the show era, the items are always found just where they're being used in the show's present day. Almost as if the world had so little time to change that very little in Earl's house or office had moved at all when the end came...
The fact that Mr. Richfield plans on selling arms to both sides of the war in "Nuts to War" makes more sense when you remember that his species, Triceratops, are four-legged dinosaurs.
In "Driving Miss Ethyl", Ethyl should have realized that she'd obviously be the only one in attendance at her high school reunion: she was the first dinosaur in centuries to put-off the ritual suicide in "Hurling Day" (which came before this episode), so she could continue to torment Earl. None of her classmates made this choice, it would seem.
Another possibility of her surprise that she's the last surviving graduate would be if she had actually finished school early. Though the venue for the reunion should have realized everybody should have been hurled by now.
Of course dinosaurs would celebrate Refrigerator Day instead of Christmas- They aren't making the mistake The Flintstones did of celebrating Christmas 60 million years before Jesus's birth. Literal Fridge Brilliance!
Although Ethyl says "Merry Christmas" to a police officer in "Driving Miss Ethyl". Then again, she was spouting nonsense while feigning insanity at the time.
Baby's abusive behavior towards Earl and others could just be a symptom of his being a spoiled brat on account of being the baby of the family and disgustingly cute, but considering Baby's obvious delight in both giving AND receiving physical punishment, it is also reasonable to conclude that he might have been a budding sadomasochist.
Since Baby seems to enjoy pain, bashing his father could be his way of expressing his love for him.
Ethyl also enjoys clubbing Earl with objects, so he might have picked it up from her.
In "Hurling Day", Ethyl finds an old photo of Robbie as a baby, hitting his grandfather on the head with a pot. So that abuse is possibly endemic to dinosaur children. Or at least those on Earl's family tree.
The technology seen throughout the series closely matches modern technology, meaning that the events of "Changing Nature" pretty much set technology back millions of years, since the cavepeople weren't intelligent enough at the time to know how all the various gadgets and gizmos worked. Thus, they were forced to re-invent everything the dinosaurs had perfected. In essence, this means that had humanity been more intelligent at the time, it'd be much more advanced in the 21st Century.
With all the vegetation wiped out, the cavepeople likely relied on other creatures as their major if not only source of food. This would have included the dinosaurs.
The fact that the dinosaurs have modern-level technology makes the ending even worse. Yes, the dinosaurs have things like heaters and electric blankets to help stave off the cold...but that just means that instead of freezing to death, they get to starve to death, a much worse way to go.
It's much, much worse. All the plants are dead, so all the creatures that eat plants will die of starvation, including insects, reptiles, and mammals. All the carnivores will then run out of food and die. The dinosaurs' actions have killed every living thing on the planet, including the humans and mammals. Maybe whatever's in the oceans will live, assuming runoff from the herbicide doesn't poison the water, but that won't save the land creatures.
It's much, much, much worse than that. The Sinclairs are carnivores. They are staying all together in one place as the snow piles up around their home. We all know what humans have done in such extreme survival situations, and our world has deeply-ingrained moral, ethical, and legal issues with cannibalism. The Sinclairs' world has no such issues. The last survivor will finally die of starvation and cold, surrounded by the bones of their devoured family...and even worse, Baby is the smallest and weakest. He'll be the first to go. When you remember Earl's comment early in the series about Baby's egg being "baby or breakfast"...brrrrr.
The dinosaurs are extremely civilized, in some ways more so than modern human civilization is (for instance, dinosaurs are legally required to periodically prove their aptitude at being spouses or parents), with one huge, rather glaring exception which is never explicitly stated but permeates the entire series: it appears that murder is not a crime. Even setting aside the fact that dinosaurs regularly devour still-living and equally-sentient lesser creatures, things aren't that much better when you're a dinosaur. When it is discovered that Mr. Richfield murdered all of his daughter's boyfriends, all he gets is a slap on the wrist from her, and those aren't the only murders Richfield is known to have committed (including his own nephew, whose head he bit off—how the hell must Richfield's sibling feel about that?). A huge amount of the in-universe televised entertainment relies on Death as Comedy—Mr. Lizard, Totally Hidden Predator, Pangaea's Funniest Home Injuries, all seem to involve real dinosaurs getting killed on-camera for the amusement of the audience. Fran regularly fears for Robbie's life, and there's a very strong implication that if someone did murder Robbie, there would be nothing Fran could do about it, which is all but confirmed when Baby kills the Foreign Exchange Student and Earl and Fran have to break it to the student's parents, who simply have to accept it. This is a world where murder is perfectly legal and life is not held in very high regard. How... very disturbing.
Well, it's been mentioned in a couple of episodes that dinosaurs, while anthro'd, do eat each other, sometimes, so....
The first season spends some time discussing that the idea of living together in families and a society is very novel idea with Fran and Earl saying that they are the first generation that settled down to raise their children rather than engage in a eat-or-be eaten fight with them, and throughout the series things like laws and societal conventions are discovered as the episodes went along. It's possible that the blase attitude toward murder is still a holdover from that time and, if dinosaur civilization had continued, it might have changed.
The early episodes often make it clear that the show is set in the year 60,000,003 BC. That's awfully specific. But wait... would that mean that by the end of the series, it's 60,000,000 BC precisely? Were they always intending to end the show with extinction? Heck, in the very first episode, Robbie wonders why the years run in reverse, what they're counting down to. Spooky, writers. Spooky...
An odd bit of Fridge Logic cum Fridge Horror this is 6 million years after the extinction of dinosaurkind in our world. From there, notice how realistic the fossils in the clip show look. The sapient dinosaurs lived for only 6 million years...
"The Terrible Twos" has the plot of Baby Sinclair becoming a psychotic little troublemaker (even moreso than already) when he is two years old. What's distressing is that Ethyl makes it clear that this process is normal for dinosaurs, and the years where Robbie and Charlene were two years old were apparently so traumatic that Earl and Fran blocked them from their memories. The fridge horror doesn't end there, either, for one also can't help but wonder what Earl, Fran, Ethyl, Roy, Spike, and Monica (to name but a few) were like when they were in the terrible twos.
Oh, it gets worse! Much worse! Just imagine what a terror Mr. Richfield must have been as a two-year-old!
During Ethyl's second Near-Death Experience in "The Last Temptation of Ethyl", her husband Louie's spirit encourages her to enjoy life while she can and assures her that she'll be with him forever "soon enough." As of the final episode, it will be sooner than anyone expected, and Earl, Fran, the kids, and the whole dinosaur species will be going with her.
Even worse is the moment where Baby loudly asks if he can go to the afterlife and Fran tells him no, not for a long, long time. Yeah...
One that's both this and Fridge Brilliance, the reason Earl isn't the smartest tool in the shed? He's suffered some degree of brain damage from Baby hitting him on the head with a frying pan so many times!
The episodes that show him before Baby was born don't depict him as being terribly smart to begin with, so if he did suffer brain damage....
It's probably for the best that Mr. Richfield didn't attend dinner with the Sinclairs in "Employee of the Month". Considering that they had a live cavewoman in the house at the time, it could have resulted in Earl being fired, or worse...Richfield making a quick meal out of "Sparky".
Ethyl is the first dinosaur in ages to not get killed after turning 72, which means that geriatric medicine doesn't exist. If the dinosaurs didn't all die in the series finale, old dinosaurs like Ethyl would've spent decades suffering from age-related physical and mental problems, but doctors would have no idea what to do.
There was one episode (I think it was a clip show episode) featuring archaeologists digging up dinosaur remains. At one point, the Archaeologist sees the dinosaur equivalent of a TV, and comments that they must have sat around it till they died. Considering what happened in the final episode, chances are the dinosaur families were huddled around the TV, watching the news about the snowstorm getting worse and praying for the best while freezing/starving to death.
