In some localities, multiple species or genera coexisted in different ecological niches. It contains the ear canal (external ear) with the eardrum, and small bones (ossicles) in the middle ear. Cetacean - Cetacean - Paleontology and classification: Cetaceans are distant descendants of a group of poorly defined mammals known as condylarths. The mesonychids had distinct specialized teeth that are similar to the triangular teeth of the Basilosaurus. However, it had a skull bone (bulba) that was modified to pick up vibrations from the ground. This suggests that it is a transitional species between extinct land mammals and modern whales. 12.19) pictures the skull of the upper Paleocene mesonychid Sinonyx beside that of Pakicetus (lower Eocene), a supposedly transitional form between mesonychids and early whales. In terms of their skeletal structure, however, whales are distinguished from the similar Mesonychids using five basic anatomical characteristics: (1) all of the incisors are parallel with the tooth row, (2) the medial lamboidal crest is semicircular, (3) the nasals are retracted, (4) the protocones are small, and (5) the accessory cusps are large. When transitioning to live in the sea, the mesonychids developed weak vestigial hind legs which the Basilosaurus had as well. The mouth had triangular arch and bulla that supported the ear drum that was poorly connected to the skull. Scientists believe that because this animal had inner-ear structures very similar to modern-day cetaceans and also had teeth that were very whale-like. Mesonychids had triple-tipped mandibles which suggests that some of them were adapted for hunting fish, like their relatives the toothed whales. Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, In A Whale of Trouble Evolutionists would like you to believe that whales are an excellent example of evolution. †Mesonychidae What Have We Learned from Prehistoric Cave Paintings. The cranium (skull) is the skeletal structure of the head that supports the face and protects the brain.It is subdivided into the facial bones and the brain case, or cranial vault (Figure 6.16).The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. †Hapalodectidae Although the snout is elongate, the skull shape resembles that of the mesonychids, especially Hapalodectes. . Therefore, it is seen as a kind of “missing link” between land animals and whales. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyles before the group became extinct.[2]. Gingrich and others (1983) published this reconstruction of the skull of Pakicetus … The head consisted of a very wolf-like structure, with sharp teeth … Archaeoceti is an extinct parvorder of Cetacea containing ancient whales. Although no post-cranial bones of Pakicetus were found, it seemed logical to assume, from the teeth and ear structure, that the animal spent a … Pakicetus (early-mid Eocene, 52 mya), the oldest fossil whale known, had the same skull features as Hapalodectes . The head consisted of a very wolf-like structure, with sharp teeth used for diving for fish in coastal regions. If you look at Rodhocetus pictures, then you’ll see an animal that looks like a cross between a dolphin and an alligator. Mesonychids in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene. Mesonychids were typically larger than there other two groups of carnivores that they shared the planet with at the time: the miacids (which evolved into modern carnivorans) and creodonts, another carnivorous group which mesonychids were once classified as. The body mass of Pakicetus was estimated at 45 kg (100 pounds), roughly the size of a wolf or large dog.The dentition of the animal indicates that it had a diet primarily of fish; however, its skeleton and skull suggest that it spent a considerable amount of time on land.. Mesonychids originated in Asia (which was an island continent) and quickly spread across much of the northern hemisphere, including Europe (which was an archipelago at the time), and North America (which was separated from South America by the ocean). . Mesonychids had longer, flatter skulls, and an exaggerated sagittal crest, a skull crest which would have been used as an anchor for jaw muscles. whales have a specific ear design that isn't found in any other animal, thus a similar ear design can point towards a relation. Since other predators, such as creodonts and Carnivora, were either rare or absent in these animal communities, mesonychids most likely dominated the large predator niche in the Paleocene of eastern Asia. These changes affect the shape of the skull, the shape of the teeth, the position of the nostrils, the size and structure of both the forelimbs and the hindlimbs, the size and shape of the tail, and the structure of the middle ear as it relates to directional hearing underwater and diving. For this reason, scientists had long believed that mesonychids were the direct ancestor of Cetacea, but the discovery of well-preserved hind limbs of archaic cetaceans, as well as more recent phylogenetic analyses[8][9][10] now indicate cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychids, and this result is consistent with many molecular studies. But the most distinctive feature was the inner ear. One genus, Dissacus, had successfully spread to Europe and North America by the early Paleocene. Thewissen recognized it immediately: the tympanicum that surrounds the eardrum shared the same characteristicly thick nature as all ancient and modern whales found to date. Reptile - Reptile - Skull and dentition: The skulls of the several subclasses and orders vary in the ways mentioned below. physics, biology, astronomy, chemistry, and futurism. The ear structure is also more similar to land animals because the bone structure isn't suited to water. He thought he had found the skull of a wolf, but it had a structure that looked like the middle ear of a whale. It had nostrils in the front of the head and no blowhole. . 2) Later traits that we find are nostrils shifting back toward the top of the skull; the diminishing of hind legs,forelimbs become paddle like and then we also start observing strong swimming tail with flukes form. Mesonychians were long considered to be creodonts, but have now been removed from that order and placed in three families (Mesonychidae, Hapalodectidae, and Triisodontidae), either within their own order, Mesonychia, or within the order Condylarthra as part of the cohort or superorder Laurasiatheria. Male mesonychids had thick carnassals which would have been useful for crushing bone, pointing to a scavenger lifestyle. It contains one species, A. natans.The genus name comes from Latin ambulare "to walk" and cetus "whale", and the species name natans "swimming". Recent finds have shown Pakicetus itself was a terrestrial animal with fully developed legs. Its ear bones also show that it did not have external ears but instead used the same method of hearing as modern whales - picking up vibrations through the jawbone. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes, became extinct in the early Oligocene. The teeth also are quite distinct among among mammals in that they are especially primitive, mostly-unspecialized, triangular, shrew-like, almost reptilian teeth, just like those of some modern whales. The skull of Ambulocetus was that of cetacean, the muzzle was long with uniform teeth like archaeocete. . Some mesonychids are reconstructed as predatory (comparable to canids), others as scavengers or carnivore-scavengers with bone-crushing adaptations to their teeth (comparable to the large hyenas), and some as omnivorous (comparable to pigs, humans, or black bears). Pakicetus The first fossil, a lone skull, was thought to be a mesonychid, but Gingerich and Russell recognized it as an early cetacean from characteristic features of the inner ear, found only in cetaceans. Early mesonychids probably walked on the flats of their feet (plantigrade), while later ones walked on their toes (digitigrade). Its very terrestrial ear drum could not work well in water. [5], Most paleontologists now doubt that whales are descended from mesonychids, and instead suggest mesonychians are descended from basal ungulates and cetaceans are descended from advanced ungulates (Artiodactyla), either deriving from, or sharing a common ancestor with, anthracotheres (the semiaquatic ancestors of hippos). The first species to be discovered (Rhodocetus kasrani) exhibited such features as a large pelvis fused to the vertebrae, hind legs, and differentiated teeth.Of a recently discovered species (Rodhocetus balochistanensis), the ankle bones were recovered, further strengthening the already well-founded link to artiodactyls, and weakening the link to mesonychids. Ambulocetus is a genus of early amphibious cetacean from the Early Eocene Kuldana Formation in Pakistan roughly 48 or 47 million years ago. [12] However, the close grouping of whales with hippopotami in cladistic analyses only surfaces following the deletion of Andrewsarchus, which has often been included within the mesonychids.

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