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Comparative Study
, 274 (1610), 635-40

No Gastric Mill in Sauropod Dinosaurs: New Evidence From Analysis of Gastrolith Mass and Function in Ostriches

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Comparative Study

No Gastric Mill in Sauropod Dinosaurs: New Evidence From Analysis of Gastrolith Mass and Function in Ostriches

Oliver Wings et al. Proc Biol Sci.

Abstract

Polished pebbles occasionally found within skeletons of giant herbivorous sauropod dinosaurs are very likely to be gastroliths (stomach stones). Here, we show that based on feeding experiments with ostriches and comparative data for relative gastrolith mass in birds, sauropod gastroliths do not represent the remains of an avian-style gastric mill. Feeding experiments with farm ostriches showed that bird gastroliths experience fast abrasion in the gizzard and do not develop a polish. Relative gastrolith mass in sauropods (gastrolith mass much less than 0.1% of body mass) is at least an order of magnitude less than that in ostriches and other herbivorous birds (gastrolith mass approximates 1% of body mass), also arguing against the presence of a gastric mill in sauropods. Sauropod dinosaurs possibly compensated for their limited oral processing and gastric trituration capabilities by greatly increasing food retention time in the digestive system. Gastrolith clusters of some derived theropod dinosaurs (oviraptorosaurs and ornithomimosaurs) compare well with those of birds, suggesting that the gastric mill evolved in the avian stem lineage.

Figures

Figure 1
Figure 1
Genuine and alleged gastroliths from sauropods and ostriches. All images have the same scale. (a) Sauropod gastroliths from Cedarosaurus (Denver Museum of Natural History 39045). Note that some pebbles are composed of relatively soft sandstone and that even the quartz pebbles exhibit a dull surface, except where covered by diagenetic hematite coating. (b) Typical alleged sauropod gastroliths, composed of quartz and exhibiting a high polish (Yale Peabody Museum 1782). (c) Natural ostrich gastrolith composed of white vein quartz (Institute of Palaeontology, University Bonn (IPB) R563). (d) Black chert pebbles before and after abrasion experiment (IPB R564). (e) Abrasion sequence of experimental granite samples (IPB R565) in the ostrich gizzards. Samples are from experiment illustrated in figure 2. Note that the general shape of the stones remained intact during the experiment and that the experimental gastroliths did not develop any polish. Numbers below specimens in (c), (d) and (e) indicate residence time in gizzard in days.
Figure 2
Figure 2
Results of the gastrolith-feeding experiments with German farm ostriches (Struthio camelus) using three different rock types: limestone; granite; and rose quartz. The mass of the fed stones is plotted against the number of days of residence in the stomach. Each data point represents one slaughtered animal that contained a sample of known residence time. Rose quartz was by far the most resistant rock type, followed by granite, limestone being destroyed almost immediately. Number of days between intake of experimentally prepared gastroliths and eventual slaughter was decided independently by the farmer. Hence, duration of the interval between samples is not of equal length.
Figure 3
Figure 3
Gastrolith masses of South African and German free-ranging farm ostriches. Both groups exhibit a normal distribution of data. The Kolmogorov–Smirnov test gave an asymptotic significance of 0.414 for German ostrich gastrolith mass and an asymptotic significance of 0.084 for South African ostrich gastrolith mass. While the South African birds were two to four months younger at slaughter, and therefore had a lower mean body mass, the proportions of gastrolith and body mass is almost identical in both groups.
Figure 4
Figure 4
Relationship of body mass to gastrolith mass in birds and dinosaurs. Data were compiled for 27 bird species of several families (ostriches, wild birds and domesticated; see electronic supplementary material) as well as for the theropod Caudipteryx and the three sauropod finds with the highest mass of associated gastroliths (type specimens of Seismosaurus, Cedarosaurus and Dinheirosaurus). Note that values of relative gastrolith mass in sauropods lie well below the regression line for birds and the Caudipteryx data.

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