Wings, Oliver: Identification, Distribution, and Function of Gastroliths in Dinosaurs and Extant Birds with Emphasis on Ostriches (Struthio camelus). - Bonn, 2004. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Oliver Wings}},
title = {Identification, Distribution, and Function of Gastroliths in Dinosaurs and Extant Birds with Emphasis on Ostriches (Struthio camelus)},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2004,
note = {Gastroliths (stomach stones) are known from many extinct and extant vertebrate clades and are potentially useful for paleobiological interpretations. This dissertation investigates the identification, function, distribution, and taphonomy of gastroliths with a focus on those of dinosaurs, including extant birds.
The terms bio-gastroliths, patho-gastroliths, and geo-gastroliths are suggested as more precise descriptions involving the origin of the stones. A review of the literature reveals that trituration and mixing of ingesta is an accepted function of gastroliths in many vertebrates (particularly in birds), whereas the influence of gastroliths on buoyancy in aquatic animals appears to be negligible. Accidental ingestion of sediment is considered to be common, as is the overlap of numerous functions.
The results of a study on German (n=135) and South African (n=212) free-ranging farm ostriches (Struthio camelus) indicate that ostriches ingest stones of greatly varying size. Adult animals typically hold one kilogram of stones in their stomach. On average, each stomach contained several thousand gastroliths with a grain size >1 mm. The mean gastrolith mass is very similar in both populations and constitutes about 1% of the mean body mass. Gastrolith mass is not significantly correlated with gender, age, season, and food contents by weight. Quartz is the predominating mineral type among ostrich gastroliths.
The taphonomic processes that may lead to the loss of gastroliths are discussed and the fossil record of the most important groups of lithophagic vertebrates (tangasaurids, crocodilians, sauropodomorph and theropod dinosaurs including birds) is reviewed. Experimental results show that gastroliths can be set free from ostrich chick carcasses with a body weight <12 kg after relatively short periods (3-6 days) in a hot and arid climate and that a separation in an aquatic environment is likely caused by prolonged floating of the carcass. A new classification of gastroliths is introduced that scales their presumed authenticity. The scale ranges from 1 (cluster of stones in the abdominal area of the skeleton) to 6 (surface finds of isolated, rounded and occasionally polished stones without associated bones).
Sedimentological and taphonomic evidence from several classic Upper Jurassic sauropod dinosaur localities shows that there are very few sauropod finds with unambiguous gastroliths. The scarcity of stones in the fine-grained sediments of most of these localities suggests that only a small number of sauropods possessed gastroliths. The occurrence of a gastric mill as seen in birds is not supported by the taphonomic evidence for sauropods. Without an association with fossil bone, there is no convincing evidence that exotic stones (exoliths) represent former gastroliths. It is more plausible that most of the surface-collected exoliths are weathering relicts of stratigraphically younger conglomerate layers.
An experiment was conducted that simulated a bird gastric mill in a rock tumbler using stones, water, plant material (grass), hydrochloric acid, and pepsin. The forces and the abrasion rate in the artificial gizzard were lower than in a real ostrich gizzard. After the end of the experiment six months later, the stones exhibited a total weight loss of 22.4%. Stomach juices and phytoliths contained in the grass had no visible effect on stone surface development. Given that no polish formed due to continuous abrasion, other causes, such as wind abrasion and diagenesis, should be considered for the polish sometimes observed on fossil gastroliths.
Finally, future research on gastroliths in crocodilians, pinnipeds, Permian tangasaurids, insectivorous species (e.g., anteaters) and other taxa is suggested. The distribution of gastroliths in extant and fossil members of Dinosauria is listed in appendices.},

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