Article: Brachiopod ecology and Lower Greensand palaeogeography
F. A. Middlemiss
Living brachiopod colonies show a marked preponderance of young stages hi the population but this is not a necessary criterion of a life-assemblage of fossil brachiopods. The occurrence of well-preserved whole shells of all growth stages is one of the criteria of a life-assemblage. The percentage of disarticulated and broken valves present in a fossil assemblage is roughly proportional to the distance drifted from the original site of life, but this refers to total distance, not necessarily net distance. In the Lower Greensand (Aptian) of Faringdon and the Bargate Beds (Lower Greensand) of Surrey the brachiopod faunas do not occur as life assemblages but as redistributed communities in the immediate neighbourhood of their sites of life. Study of the lithology, fauna, and thickness variations of the Bargate Beds, and of bore-hole evidence, suggests that the beds were deposited very close to shore. This conclusion is extended by analogy to the Faringdon Greensand. Similar evidence is reviewed for the Lower Greensand of Upware and leads to a similar conclusion. The Ferruginous Sands (Aptian) of the Isle of Wight were deposited under neritic conditions but many miles from a shore. Here, in at least two horizons, life assemblages of articulate brachiopods are preserved; their sedimentary environment suggests that shallow, well-oxygenated conditions with currents were necessary for brachiopod life.