Vertebrate Fauna of the Anisian Triassic Otter Sandstone River

The ‘Otter Sandstone River’ of the mid-Triassic and its vertebrate fauna

Author:

Hart

Abstract:

The Triassic succession of the East Devon coastline is a key component of the Jurassic Coast World Heritage Site. Within these strata are the sediments deposited by two major river systems: the Budleigh Salterton Pebble Beds and the Otter Sandstone Formation. The latter are significant as both a reservoir for the Dorset oilfields near Wareham and as a source of significant numbers of vertebrate fossils. The Otter Sandstone Formation, of Anisian (Middle Triassic) age, has a lower part formed of aeolian sands but these pass upwards into a series of river channel deposits. These river channels, which are best seen between the mouth of the River Otter and Ladram Bay, contain a series of classic features such as erosive channel bases, channel lag deposits, de-watering structures, calcified root systems (rhizoliths), reworked calcretes and (rare) overbank mudstones. The rivers that formed the various braided channel deposits flowed northwards or north-eastwards and there are at least 4 or 5 separate channel units within the Ladram Bay area. The Otter Sandstone Formation has yielded a great many rhynchosaur fragments and at least one partial skeleton. This juvenile individual appears to have fallen into one of the channels, resulting in the loss of its head (skull), as this was not recovered when the specimen was found by the author in 1990. This important specimen of Fodonyx spenceri (Benton, 1990) is housed in the Royal Albert Memorial Museum in Exeter.