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A Mass-Accumulation of Vertebrates from the Lower Cretaceous of Nehden (Sauerland), West Germany

D. B. Norman

Abstract

A `fauna' of vertebrates recovered from a cavern-like deposit at a quarry near the village of Nehden in Sauerland is described and reviewed in some detail. Anatomical descriptions of some skeletal elements are given, where they provide new information or supplement previous descriptions. The clays in which the fossils were buried have been dated comparatively as Aptian (late Lower Cretaceous) and approximately contemporary with the Weald Clay unit of the Wealden Formation of southeast England and the Bernissartian of southwest Belgium. Both these latter formations have yielded faunal and floral assemblages that are very similar to those found at Nehden. The vertebrates collected at Nehden include adult and numerous juvenile remains of two species of the ornithischian dinosaur Iguanodon: I. atherfieldensis and I. bernissartensis. (I. atherfieldensis is the more abundantly represented.) These can be contrasted with the mass-accumulation of Iguanodon recovered from Bernissart, where these dinosaurs are represented by predominantly adult skeletons. There is circumstantial evidence from a documented association of bones (revealed by examination of excavation plans) to support a reconstruction of a juvenile Iguanodon bernissartensis with a body length ca. 2-3 m (fully grown individuals reach a body length of ca. 11 m); this is the smallest individual of this species recovered to date. The remainder of the vertebrate assemblage consists of very fragmentary remains of crocodilians, chelonians and extremely rare fish. The presence of both a hypsilophodontid ornithischian dinosaur and a theropod saurischian dinosaur must be viewed as extremely conjectural, based as they are on two very poor specimens. Remains referred to as Vectisaurus sp. in previous accounts of this site are juvenile individuals of Iguanodon. The circumstances surrounding three mass-accumulations of fossils, Trossingen, Bernissart and Nehden, are reviewed, and evidence of mass deaths among recent vertebrates is considered. Archive records at Trossingen suggest that periodic events, such as mud-slides, may have been partly responsible for the concentration of remains. Similar events may also have been responsible for the assemblages at Bernissart and Nehden; the assemblage at Nehden may represent an accumulation caused by a flash flood or by a herd crossing a river. It is suggested that awareness of such common phenomena should be emphasized when data such as these are used in estimations of numerical abundance and diversity of species in the fossil record; localized high abundances of species, caused by phenomena of this type, may exert a strong bias on the patterns of abundance or diversity of species, generated by palaeobiologists, by creating patterns that are in effect flash-flood artefacts rather than natural censuses of fossil faunas.

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