Rhexoxylon piatnitzkyi and its ancient adversaries: deciphering plant-beetle interactions and its palaeoecological significance in the Ischigualasto Formation (Upper Triassic, Argentina)

We present the anatomical study of axes of the corystosperm Rhexoxylon piatnitzkyi and the interactions of this plant with wood-boring beetles. We used a CT scanner and thin sections to reconstruct the boring pattern inside the plant axes, as well as the tissues affected by the beetle. The galleries were abundant in the branch of R. piatnitzkyi, including both longitudinal and transverse borings, and less frequent in the trunks. The galleries are bored preferentially in the secondary phloem and in the pith. The position of the galleries as well as the cell and tissue composition of the frass indicates that the borer was mainly a corticophagous insect. The sample also presents evidence of white rot by lignivorous fungi, and fragments of tracheids can be recognised in the frass that fills the galleries, indicating that the borer had some capacity to feed on decayed wood. The anatomical features of the secondary xylem and phloem suggest that the plant deployed defence mechanisms against the fungal decay and/or the boring beetle. This is the first evidence of insect galleries in axes of corystosperms, a dominant group of plants during the Triassic in Gondwana, and the first possible record of cerambycid borings.