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| Some of the eggs are infilled by silt (which could not have happened unless they were cracked), some are infilled by calcite crystals (calcium carbonate) and barites (barium sulphate) deposited by percolating ground waters, and others by a mixture of silt and calcite. Bones are composed partly of calcium phosphate. Very dilute acetic acid will break down the calcareous matrix of the silt and dissolve the calcite, but it does not attack the bones so readily. | At intervals the specimen is washed thoroughly to remove salts and acid, dried in an oven, and any exposed bone is impregnated with a plastic to prevent damage by further immersion in acid. This process is repeated again and again, over many weeks. |
| If an embryo died very young then its bones are less well ossified than in an embryo which died at full-term. The position of the bones within the egg also varies according to the stage of development at which the embryo died. It seems that earlier embryos are usually preserved around the center of the egg, perhaps suspended on the dried surface of the yolk. | Mid-stage embryos are often stuck to the roof of the egg, perhaps having been buoyed up by gas produced during decomposition, and subsequently sealed in that position when the egg dried up. By contrast, the more dense bones of late-stage embryos are usually spread over the floor of the egg. Drying of the egg also seems to be an important component in the preservation of unossified tissue such as cartilage. |
| The only sure way of identifying a dinosaur egg is to identify an embryo within it. On this basis, one of the four kinds of eggs available to us has been identified as a therizinosaur: these are an enigmatic group of dinosaurs of uncertain affinity, perhaps sauropodomorphs (big four-footed forms), perhaps herbivorous theropods (normally meat-eaters), or perhaps they are a group in their own right? |
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