RECORD OF GALL ABUNDANCE AS A POSSIBLE EPISODE OF RADIATION AND SPECIATION OF GALLING INSECTS, TRIASSIC, SOUTHERN BRAZIL

A possible episode of coevolution between galling insects and plants is reported. It was identified by a high infestation (density) of fossil gall found on the compressed Dicroidium Flora from the Triassic (Ladinian-Camian) Passo das Tropas Member, Santa Maria Formation, Parana Basin. The galls were found on phytofossils in 2.2 m thick mudstones with horizontal lamination, intercalated between coarse and medium-grained sandstones with clasts and cross-lamination at the base and top of the section. The associations were identified with a stereomicroscope, after which they were quantified within each botanical group, along with their type, in order to calculate the average number of galls per leaf (density). The overall density found was 4.018. Corystospermales was the predominant botanical group (87.81%), with a gall density of 4.281, followed by Ginkgoales, Equisetales, and Incertae sedis. Current ecological studies indicate that a xerophytic paleoenvironment correlates with high infestations of galling insects. However, many taphonomic, biological, and ecological factors may be involved, resulting in different hypotheses from the same data. On the other hand, they support the interpretations of this record in other aspects discussed. Furthermore, the identified gall density can be interpreted from a paleobiogeographical perspective, showing dispersion at low altitudes and intermediate subtropical latitudes (25 degrees to 38 degrees S or N).