Thorny devils are well-known for the many, large spines on their bodies. Their large spines are entirely boneless, only two parietal spines are supported by modest bony bosses on the skull. Their bodies are covered with thick shields and spiny cones and they are marked with golden and brown patches that serve to camouflage them in their arid habitats. They are extremely difficult to find because of their camouflage and escape detection even at close quarters and with experts who are actively looking for them. The head is ornamented with a large, spiny boss on the back and the eyes are small. Even the ventral surface is covered in conical, non-overlapping scales. It was long assumed that the spiny armor of thorny devils was exclusively a defensive trait, protecting them from predators. However, the complicated surface texture of thorny devils also helps them to retain and absorb water through body contact, especially condensation of water on body surfaces. Thorny devil skin color varies with temperature; individuals are brown or olive in the mornings and become light yellow as temperatures rise in the afternoon. Observations suggest thorny devils use postural changes to regulate body temperature, either maximizing contact with a warm surface (the ground) or minimizing it via standing with two feet against a shrub.
Females are larger than males, averaging 45.5 g (range 33 to 88.7 g) and 91 mm snout-vent length (range 80 to 110 mm). Males average 31.2 g and 78.7 mm snout-vent length and never weigh more than 49 g or measure more than 96 mm. Body weight changes substantially throughout the year and can vary quickly, losing or gaining up to 30% of their body mass in a few months. Oviposition in females results in a substantial loss of body mass, which can be regained quickly - in one case a female gained 40% of her body mass in a little over a month.
Thorny devils have been widely compared with Phrynosoma species, North American horned lizards. Thorny devils and Phrynosoma species are an example of convergent evolution towards a highly armored, slow-moving, thermally labile, ant-specialist lizard niche.
Unlike most Australian agamids, thorny devils have no femoral pores on the ventral surface of their thighs. They also lack a supratemporal bone. There are 21 presacral vertebrae and 22 postsacral vertebrae. Thorny devils have short digits and vary in the number of phalangeal bones, with a reduced formula throughout most of their range. The primitive agamid phalangeal formula is found in portions of the range.
Thorny devil teeth are modified for their ant specialist diet. Ants are relatively hard-bodied insects, with high levels of chitin. The mandibular teeth are modified to fit neatly between two maxillary teeth, creating a shearing apparatus. Thorny devils have also lost the anterior pleurodont teeth, reflecting their use of the tongue to capture prey, rather than the teeth.
Range mass: 28.5 to 57 g.
Range length: 76 to 110 mm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: female larger