My coin shop sent this to me to sell for them, an I just got it in hand last night. It felt really light, like having been struck out of aluminum. I checked the weight, and it weighs 2.34g versus the 4g it is supposed to weigh. The style looks right to me. It is Ex. Roma Numismatics E-Sale 28 lot 655, so they thought it looked ancient as well. Any thoughts about this coin? No evidence of casting on the edge:
I think it's simply struck on a thin flan. Those issues run light to begin with because of the poor-quality metal used. Moreover, because of its poor quality, metal can leach out of the coin during its time in the earth or as a result of harsh cleaning, resulting in further loss of weight. This one is less than 3 grams, too: Otacilia Severa, AD 244-249. Roman AR Antoninianus, 2.90 g, 21.4 mm, 1 h. Rome, AD 248. Obv: OTACIL SEVERA AVG, bare-headed and draped bust, r., on crescent. Rev: SAECVLARES AVGG, hippopotamus standing r.; IIII in exergue. Refs: RIC 116b; Cohen 43; RCV 9160; CRE 526; Hunter 8.
Note that if it has suffered weght loss due to leeching that it can be even more brittle than it might otherwise be so don't drop it onto hard surfaces. A lesson learned through bitter experience.
Authentic but it probably had a tough time in the soil. It looks a bit crystallized to my eye as well, which may have resulted in a chemical process that cost the coin some weight.
I'd go along with what everybody else is saying - poor metal/preservation conditions leading to crystallization and/or leaching. That being said, I have a Gordian III antoninianus that weight under 2 grams and looks just fine - good metal, style, etc. Below are photos of it, and one that weighs over 4 grams. Weights seem to have been all over the place in those days: The darker one is the heavier one:
