What was the biggest example of prehistoric quadruped carnivores?
As an example of the type of animal I am thinking of, I am including a link to a “postosuchus”, which was about twenty feet long:
(I know that there were giant crocodilians which attained lengths of forty feet or more, but I am thinking of purely terrestrial creatures.)
Tyrannosaurus rex was up to 13 metres long. It was a quadruped in the same way as kangaroos are quadruped: it had four limbs, but only used two for locomotion.
Then how is it (or the kangaroo) anymore of a quadruped than a bipedal human?
Well, I meant a creature that actually walked on all four limbs, like Postosuchus.
I thought that I remembered that some such “quadruped carnivores” got significantly bigger than that. I may be wrong, though…
I think you’re confusing quadruped with tetrapod. 
Excluding amphibious crocodilians, I don’t think there are any much larger than Postosuchus. This site says the related Saurosuchus was 7 meters (compared to 6 for Post) and the largest Rauisuchian (the order they all belong to). Doesn’t say if it was heavier/more massive. Could all be in the tail.
Actually, in terms of mass, the big rauisuchians are about the same size as the largest mammalian carnivores, like Andrewsarchus or Kodiak bears.
I eventually did find the info on Saurosuchus, the largest Rauisuchian, at 7 meters, which is an impressive size. I was thinking that there was another group of reptiles built on similar lines, not because of being related to the Rauisuchians, but due to convergent evolution. My mind could be playing tricks on me, though.
It just seems to me that this type of body plan would enable a predator to grow larger than even the tyrannosaurs, to the point where they could pose a threat to even the biggest sauropods!
It’s possible that Spinosaurus walked on all fours at least part of the time; other spinosauridae have extremely well-developed forelimbs that appear to be capable of bearing weight (unlike those of, say, T-Rex).
Since S. aegyptiacus looks to have been the largest terrestrial predator of all, I think it would qualify.
That’s been suggested in the past (late 80s or 90s, IIRC) but it’s very unlikely, since theropod dinosaur hands were incapable of pointing palms downward the way they’re often portrayed in movies, etc.
http://www.sltrib.com/ci_11828632
(contrary to the implication in the article, this has been known for a long time, the prints are just the first direct evidence outside anatomical studies).
If spinosaurs were quadrupeds, they would had to have been knuckle-walkers, or walked on the sides of weirdly contorted forelimbs similar to the feet of ground sloths.
There are a couple of more recently extinct reptiles from Australia that were possibly comparable in size to the one cited in the OP (7 meters or so, though possible smaller):
Giant monitor lizard: Varanus (Megalania)prisca
Terrestrial crocodile: Quinkana
The proto-mammalian gorgonopsids also got pretty big, the largest was Inostrancevia. http://en.wikipedia.org/wiki/Inostrancevia
Still not as large as the rauisuchians though.
The Crocodile gets pretty damn big- up to 25 feet long and over 3000#. They have rumours of some 30’ long and fantastic stories (confirmed only by trails) of crocs up to 50’ long.
However, the Deinosuchus (a extinct croc) got up to 40’ long (12M) and 9 tons, with high end estimates being 50’ long.
That makes them the biggest “walking on 4 legs” “land” carnivore ever, bigger even than old T. rex.
Is it anatomically possible for them to have been knuckle walkers?
At this point, I think that it was the gorgonopsid group that was on my mind. I was thinking that they had attained greater size, but it appears that I was wrong about that.
I really appreciate all the information! Thanks to all who contributed!
Not without heavy modification from the ancestral condition. Theoretically possible, but extremely unlikely. There’s no real reason to think they weren’t bipeds. I don’t think that idea has ever been floated in print, all the older sources about possible quadruped spinosaurs were based on outdated anatomy.
Thanks! It’s always cool to see an uber-lurker decamp to spread some knowledge. 
The probable biggest all-terrestrial mammal carnivore: Andrewsarchus. Note that there are bears in competition with it for the mammal szie crown.
And it looks like their closest living relatives are the whales. Very cool!
How big did Dimetrodon get?
About 9 feet or so. Think of a croc with a big fin.