Lizards were not harmed
Elsewhere, Woody Welch has written:
"Safe Lizard sfx Journey to the Center of the Earth
Here's the scoop of lizard poop. Back in the day, movie lava was made out of oatmeal, red poster paint, and some blackened cork to throw in for good measure.... gasoline could be added for flames, but not in this case.
Other movie lava was made out of glycerine and fullers earth(this was sometimes used as "quicksand" in movies) and red dyes.
The lava covering the Tegu lizard is of the oatmeal variety and it seems that the "steam" is actually chunks of dry ice imbedded into the oatmeal and poster paint. The room temperature would cause the chunks of dry ice to vaporize, and, ....LOOK like steam.
The Tegu lizard was gently cooled to 40 degrees which made him groggy.
The dimetrodons were actually a little herd of rhinocerus iguanas and the same cooling effect was used on them so that their special make-up (designed and crafted by Herb Cheek, who was in charge of the special effects miniature shop) could be applied without stress (to the lizards or the appliers)
The miniature spear going into the iguana was actually shot in reverse...a cut off spear was attached with spirit gum to the iguana and pulled away...this was printed in reverse for the film, giving the effect that the spear was "entering" the little beast. The spear miniature evidently blurred to the point that you couldn't tell it was really sharp at the end.
The same technique was used for the devouring scene.
The rhino iguanas were sleepy with the cooling and were gently attached to the "corpse" and they were jiggled back and forth. This "jiggling" was doctored in the printing by removing frames and it was done to give the look as if the flesh eating beasts were gnawing and tearing at the meat of the downed dimetrodon.
Reverse photography was also used in the shots when the beasts bit down, clamping their jaws onto their fallen comerade. They were actually pulling away, but when the film was printed forward, it looked like they were chomping down.
L.B. Abbott himself had said in print that the lizards had 2 speeds....rock still, and 90 miles an hour! When they were cool they would sit there and do nothing. When the movie lights heated them up, they TOOK OFF LIKE A ROCKET. They were placed on tilt sets with the camera (high speed ) attached to the sets...when the sets were tilted, the lizards would "walk" forwards. This was used sparingly. When the dimetrodons "growled" it was simply the iguanas "panting"....they do this, just like a dog, to control their inner body temperature. With the proper sound effects....instant prehistoric beast."
http://www.woodywelch.com