
03-23-2012, 02:26 PM
|
 |
Tripletail
|
|
Join Date: Jul 2011
Location: NEW IBERIA
Posts: 759
Cash: 1,189
|
|
[SIZE=-1]The fossils of Basilosaurus cetoides (Owen) and Zygorhiza kochii (Riechenbach) were the first of many fossil finds that show that modern whales, e.g. the humpback whales evolved from dog-like creatures known as Mesonychids. Both Basilosaurus and Zygorhiza, exhibit unmistakable characteristics of the terrestrial Mesonychids from which they developed. For example, their skulls retained many of the features of the mesonychids despite a pronounced elongation. Also, the primitive whales such as Basilosaurus pocessed the distinctive, teeth set of the Mesonychids with well-defined incisors, canines, premolars, and multirooted molar. In addition, these whales, e.g. Basilosaurus, had well-defined vestigial rear legs (Gingerich et al. 1990, 1993, Thewissen 1994).[/SIZE][SIZE=-1]Later, these primitive whales gave rise to toothless and toothed whales. In case of the toothed whales, the teeth evolved into the teeth of the toothed whales, e.g.. the dolphins, killer whales, and sperm whales. The Baleen (toothless) whales, the other branch of whales, developed modified mouth structures that strained plankton from the sea water enabling them to graze the oceans. It should be noted that although it belongs to the group of primitive whales ancestral to modern whales, Basilosaurus is likely a relative of the direct ancestors of modern whales. Rather, it appears that Zygorhiza was closer to the direct line of descent of modern whales than Basilosaurus (Gingerich et al. 1990, 1993, Thewissen 1994).[/SIZE]
[SIZE=-1]For a long time, Basilosaurus was among the earliest known whales being found in rocks as old as Middle Eocene. Since its discovery in 1834, no older primitive whales transitional to ancestral land-mammals had been discovered from earlier rocks leaving an obvious gap in the fossil record. However, research in Pakistan and elsewhere have found the critical fossils to fill a substantial portion of this gap.[/SIZE]
[SIZE=-1]Among the ancestors or close relatives to the ancestors of Basilosaurus and modern whales are:[/SIZE]
[SIZE=-1]1. Rodhocetus kasrani - Eocene (Gingerich et al. 1994)[/SIZE]
[SIZE=-1]2. Pakicetus - latest Early Eocene (Gingerich et al. 1983, Thewissen et al. 1993)[/SIZE]
[SIZE=-1]3. Ambulocetus natans - Early to Middle Eocene (Thewissen 1994)[/SIZE]
[SIZE=-1]4. Indocetus ramani - earliest Middle Eocene (Gingerich et al. 1993)[/SIZE]
|