Its eggshell, like avian eggs, is composed of three structural layers, but cladistic analysis suggests that its parent was a non-avian theropod.

[1] Fossils of Trigonoolithus are found in abundance at the La Cantera site of the Blesa Formation in Teruel, Spain.

The shell fragments vary between 330 and 1040 μm in thickness, including the prominent triangular protuberances ornamenting their outer surface.

[1][3] Trigonoolithus is classified in the oofamily Prismatoolithidae, alongside Preprismatoolithus, Prismatoolithus, Protoceratopsidovum, Sankofa, and Spheruprismatoolithus.

Based on its phylogenetic position, Moreno-Azanza et al. concluded it was most likely a non-dromaeosaurian, non-oviraptorid coelurosaur theropod dinosaur.