THE LONG-PROBOSCID POLLINATION SYNDROME IN MID MESOZOIC EURASIA: IMPLICTIONS FOR YIXIAN FORMATION AGE

The discovery of a new of preangiospermous pollination mode provides context for verifying the age of the Yixian Formation, highlighting how philosophical predispositions bear on whether certain pollinating insects indicate the early presence of angiosperms. An earlier study of the Yixian Formation in Liaoning Province in northeastern China, indicated that the presence of brachyceran flies with characteristic long, siphonate proboscides suggest co-occurrence of correspondingly deep throated angiosperms, even though evidence for coeval flowering plants was sparse, and the few known taxa lacked nectaries and floral structures consistent with fluid-feeding by long-proboscid insects. This study inferred that the Yixian Formation was of Late Jurassic (Tithonian) age and the presence of long-proboscid flies suggested the simultaneous occurrence of angiosperms about 15 m.y. earlier than the fossil record indicated.

Recently, an alternative hypothesis proposes that mid Mesozoic long-proboscid flies instead were pollinating gymnospermous fructifications that variously possessed long micropyles, extended integumental channels, catchment funnels and other tubular features requiring a long proboscis to reach pollination drops compositionally comparable to angiosperm nectar. This hypothesis is inconsistent with earlier angiosperm presence in the fossil record, and is consilient with a mid Early Cretaceous (Barremian) age for the Yixian Formation, as varied, modern evidence now indicates. Currently, additional long-proboscid insect taxa, including three distinctive but related lineages of scorpionflies (Mecoptera), three families of brachyceran flies (Diptera), and now a separate lineage of distinctive planipennians (Neuroptera), have been discovered, suggesting that the siphonate proboscis originated at least five times when evaluated at the family level, within approximately a 15 m.y. interval during the Middle Jurassic (Bathonian–Callovian). This correspondence of insects with stereotyped mouthparts and plants of appropriate reproductive structure indicates a Eurasian gymnospermous pollination syndrome extending to 35 m.y. before the earliest fossil flowering plants (Valanginian). At the end of the Early Cretaceous, these gymnospermous associations were extinguished.