Palynological analysis of the highly fossiliferous marine clays of the St. Erth Beds of Cornwall has revealed the presence of dinoflagellates, acritarchs and prasinophytes, scolecodonts, microforaminiferal linings, freshwater? invertebrates, freshwater algal spores, embryophyte spores and pollen, fungal spores, and plant cuticles. These groups are documented here and several, including the dinoflagellates, are reported from the St. Erth Beds for the first time.

The marine clays were deposited in warm, shallow waters of an inlet or embayment during a probable high stand in sea level. They are generally held to be of late Pliocene age, planktonic foraminiferal evidence placing them at between 1.9 and 2.1 Ma (late late Pliocene). Dinoflagellates are consistent with a late Pliocene age. Spores and pollen, if not reworked, suggest placement no higher than the lower part of the Tiglian Stage (upper Pliocene) of The Netherlands.

Analysis of all palynological groups permits terrestrial and marine climates to be evaluated independently. The spore-pollen assemblage has some thermophilic elements but mainly reflects a cool-temperate terrestrial climate. In contrast, dinoflagellates—dominated by gymnodinialean? and protoperidinioid cysts—indicate warm inner neritic waters and present some evidence for subtropical to tropical conditions with winter sea-surface temperatures above 15°. Increased influence of the Gulf Stream during the late Pliocene, perhaps superimposed upon a phase of global warming, is hypothesized to explain these differentially elevated marine temperatures.

The St. Erth inlet was not markedly brackish or hypersaline judging from the dinoflagellates, but rare freshwater algal spores attest either to some freshwater input or redeposition from sandy soils during marine incursion. High abundance of scolecodonts, about 1,500 to 2,000 per gram dry weight of sediment, allows speculation that the inlet hosted a thriving association of latest Pliocene marine annelid worms.

A species of algae incertae sedis, Halodinium scopaeum, and three species of dinoflagellate, Algidasphaeridium? euaxum, Sumatradinium pliocenicum, and Trinovantedinium sterthense, are proposed as new. The dinoflagellate genus Selenopemphix Benedek, 1972, is emended herein. Quinquecuspis Harland, 1977, is now validated by the transfer of Trinovantedinium concretum Reid, 1978, to Quinquecuspis Harland, 1977, as Q. concreta n. comb.