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The fossils in this picture comprise the skull, pelvic bones, a few vertebrae, and hind limb bones of two organisms that have been presented as a part of an evolutionary sequence leading from terrestrial mammals to whales. Number 1 in the foreground is Rhodocetus, number 2 in the background is Basilosaurus. The specimens, on display at the Chicago Field Museum of Natural History, are meant to convey to the observer the concept of progressive adaptation to a fully aquatic behavior. However, inferences about the relation of past life forms should be introduced with awareness of the fragmentary nature of the fossil record.
A cast of a famous specimen (about 50 cm in size) of the lobe-finned fish Tiktaalik roseae, from the Devonian of Canada, on display at the Field Museum of Natural History, Chicago. Tiktaalik is often presented as a "missing link" in tetrapod evolution. However, as it often happens, the picture of "sequential transitions" is much more complicated when looked at in detail, with mosaic distribution of characters between various forms.