Robo Alive Dino Fossil Find Series 1

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Before the body disappears completely, it is buried by sediment - usually mud, sand or silt. Often at this point only the bones and teeth remain.

Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived. [50] Because of the specialized and rare circumstances required for a biological structure to fossilize, only a small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point. [51] Many ammonite shells have been found with round holes once interpreted as a result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, is more likely evidence of the bite of a medium-sized mosasaur preying upon ammonites. Ammonites came in a range of sizes, from just a few millimetres to times bigger, with larger sizes more common from the Late Jurassic onwards.McLoughlin S., Vajda V.; Vajda (2005). "Ancient wollemi pines resurgent". American Scientist. 93 (6): 540–547. doi: 10.1511/2005.56.981. In contrast to index fossils, living fossils are organisms that have existed for a tremendously long period of time without changing very much at all. For example, the Lingulata brachiopods have existed from the Cambrian period to the present, a time span of over 500 million years! Modern specimens of Lingulata are almost indistinguishable from their fossil counterparts (Figure 11.8).

Any one of the above three definitions, but also with a relict distribution in refuges. [ citation needed]

Bivalves

Oxycone – Strongly involute and very narrow, with sharp ventral keels and a streamlined, lenticular ( lens-shaped) cross-section. These ammonoids are estimated to be nektonic (well-adapted to rapid active swimming), as their shell form incurs very little drag and allows for efficient, stable coasting even in turbulent flow regimes. [10] Monks, Neale; Palmer, Philip (2002). Ammonites. Smithsonian Institution Press. ISBN 978-1-58834-024-5. Living fossils exhibit stasis (also called "bradytely") over geologically long time scales. Popular literature may wrongly claim that a "living fossil" has undergone no significant evolution since fossil times, with practically no molecular evolution or morphological changes. Scientific investigations have repeatedly discredited such claims. [1] [2] [3]