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| Jeffersoniana #8 |
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ABSTRACT
"Double-headed" ribs, which are known to occur in modem baleen whales, are reported from a middle Miocene cetothere. These" double-headed" ribs are actually the result of the fusion of an extra rib with the normal first rib. A comparison of the posterior cervical and anterior thoracic vertebrae indicates that this whale has only six cervical vertebrae. The extra pair of ribs is a result of the development of the seventh vertebra as a thoracic rather than a cervical. This type of development is consistent with genetic controls on segmented development observed in arthropods, and with the lowered constraints on skeletal development in mysticetes.
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