INSECT HERBIVORES OF A BROADLEAVED CONIFER FROM PALEOGENE PATAGONIA: THE ROLE OF A DISTINCTIVE PLANT-INSECT ASSOCIATIONAL SUITE IN HOST TAXONOMIC ASSIGNMENT

The leaf, “Zamia” tertiaria, initially was described from the Paleogene of Patagonia as a cycad, but recently is considered a broadleaved conifer with affinities either to Agathis (Araucariaceae) or to Podocarpaceae. Recent attempts to resolve the affinities of this leaf morphotype have failed because of too little amber material for an interpretable signal in spectral analyses, or the absence of diagnostic leaf cuticle. Moreover, the new patagonian records of “Z.” tertiaria belong to an evolutionarily iterative and undiagnostic homeomorph series throughout the early and middle Paleogene that feature lanceolate, entire-margined, broad leaves with parallel veins, absence of a keeled or discernible midvein, attributable to unrelated gymnospermous lineages.

Fortunately, “Z.” tertiaria has a distinctive suite of specialized insect herbivore damage types (DTs). These include slot feeding (DT8) between primary veins; and a distinctive, frass-laden, elongate blotch mine (DT88). Piercing-and-sucking damage occurs as interveinal, ellipsoidal punctures with prominent reaction rims (DT48), or ellipsoidal scale-insect impressions (DT86) on leaf surfaces with thick rims surrounding an internal region of undulose texture. Two distinctive gall types are present: ellipsoidal blister galls (DT115) with thickened, carbonized margins encircling an epidermal covering; and, vascularly deep-seated and protruding columnar galls (DT116) with amber or sediment infill of the central chamber. These six and other associations constitute a component community mostly of host-specialized endophytic feeders on “Z.” tertiaria, some of which occur on the same host at multiple localities. Most revealing regarding the affinities of “Z.” tertiaria is the leaf mine, which resembles blotch structures made by the plutellid Chrysorthenclites (Lepidoptera: Plutellidae) on extant Podocarpus from New Zealand and Tasmania. Also, the columnar gall is very similar to certain pitch moths (Lepidoptera: Tortricidae) on some Oceanean Podocarpaceae. Affinities of the other DTs remain enigmatic. Although modern Agathis has similar mines produced by Parectopa (Lepidoptera: Gracillariidae), the bulk of evidence (mine and gall) suggests that “Z.” tertiaria is more likely to be a podocarp.