Whatever you do, don’t step on any butterflies.
Just stay on the path and you’ll be fine
The apatasauruses are the least of your problems. Clearly some super-villainous type is respsonsible for bringing them back–either through cloning or time travel–and this can be but the first salvo in his assault upon mankind. Your first priority should be discovering that villain’s identity, location and plan.
Hell, Skald, we already know it’s gonna be you. 
Yes, of course it’s him, no need to look anywhere else, not at all . . .
< does Finger Pyramid of Evil Contemplation >
They’re not much different in size than a whale & we don’t need no steekin A-10 or Hellfire missiles to kill those.
Just harpoon it and drive away while it thrashes about & eventually dies of blood loss. We’re not in a hurry here. It won’t be hard to track the dying animal nor to find the carcass. And the only damage will be a couple-inch hole where the harpoon went in.
The hard part will be finding a car-hop strong enough to carry the bronto ribs out to the flintmobile.
Don’t be silly. I’m of the non-subtle-villainy camp. If I’d brought the brontos back, they’d have materialized in the Capitol during the State of the Union address.
Quite right! Although in the old days, you didn’t kill the whale with a harpoon - that was just to stop him escaping. You killed him with a lance. And eventually, with an exploding bomb lance, launched from a shoulder gun. Like these ones: http://www.whalecraft.net/Shoulder_Guns.html
Okay, so the shoulder guns weren’t very successful, but that was in the old days when both the propellant and the explosive were black powder. A lighter bomb lance filled with RDX, and a muzzle brake to kill some of the recoil, and I think Mr. Brontosaurus is going to have a bad day…
Use A Gun For Dinosaur.
Nuke 'em from space. It’s the only way to be sure.
Ask Batman.
He’s prepared.
It’s been done: Invasion of the Dinosaurs - Wikipedia
Few people realize that the batplane is actually an A-10 Thunderbolt with a really cool paint job.
Wow, 50 posts before that one came out.
I was sure it’d be within the first 10
GE’s XM-214 minigun was a man-portable variant of the M-134, capable of firing linked 45 mm NATO rounds at up to 4000 rpm. Using a tripod or armored vehicle mount, this weapon could be used effectively to mow down the hordes of people who object to the name “brontosaurus.” Then you could tell the meat companies to go screw, get the brontosaurus habitat set aside as a national preserve instead, open up a “Brontosaurus Park” and rake in the sudden lavish influx of ecotourist dollars.
And could this method also be used to settle the dispute over whether the United States is a Republic or a Democracy?
Hmm…could you consume the meat from something that’d been nerve gassed?
No.
Go with Napalm, air-deployed.
One Hell of a cookout! 
Broto-roasts, Bronto-ribs, Bronto-burgers…Fred Flintstone would be pleased.
Assuming that you want to be sporting about the whole thing, I’m thinking a 2-bore calibre Express Rifle and a Land Rover.
Failing that, a 25pdr Field Gun, firing Soft Point shells. 
You mean 5.56x45mm NATO rounds, which aren’t “45mm rounds” - that would be a gun (“cannon”), not small arms. Huuuuuuuuuuuge difference.
Oh, and a hellfire missile wouldn’t be effective. There’s no way the targeting system would have any idea of how to track “Brontosaurus head” as a target. Actually - the old style laser guided ones could probably do it with manual gunner control, but the newer radar guided ones probably aren’t programmed to respond to the radar signature of a Brontosaurus head 