|
''Platecarpus'' ("flat wrist") is an extinct genus of aquatic lizard belonging to the mosasaur family, living around 84–81 million years ago during the middle Santonian to early Campanian, of the Late Cretaceous period. Fossils have been found in the United States as well as a possible specimen in Belgium and Africa.〔 ''Platecarpus'' probably fed on fish, squid, and ammonites. Like other mosasaurs, it was initially thought to have swum in an eel-like fashion, although a recent study suggests that it swam more like modern sharks. An exceptionally well-preserved specimen of ''P. tympaniticus'' known as LACM 128319 shows skin impressions, pigments around the nostrils, bronchial tubes and the presence of a high profile tail fluke, showing that it and other mosasaurs did not necessarily have an eel-like swimming method but were more powerful fast swimmers. It is held in the Natural History Museum of Los Angeles County. ==Description== ''Platecarpus'' had a long, down-turned tail with a large dorsal lobe on it, steering flippers, and jaws lined with conical teeth. It grew up to long, with half of that length taken up by its tail. The platecarpine mosasaurs had evolved into the very specialized plioplatecarpine group by the end of the Cretaceous. The skull structure of ''Platecarpus'' is unique among mosasaurs. This genus is characterized by a short skull, and has fewer teeth than any other mosasaur (approximately 10 teeth in each dentary).〔Burnham (1991) recently reported an unclassified species of ''Plioplatecarpus'' from the Lower Demopolis Formation in Alabama that has a lower number of teeth in its jaws.〕 LACM 128319 preserves matter within the sclerotic ring that may possibly be the retina of the eye. Small structures in the retina, each around 2 µm long and observed by scanning electron microspectroscopy, may represent retinal melanosomes preserved in their original positions.〔 The respiratory tube is also known in LACM 128319, preserved as cartilaginous tracheal rings. Only the posterior-most end of the tracheal tube – at the end of the neck near the pectoral girdle – is known. The section where the two bronchi split was also preserved in the specimen, but was destroyed during excavation. This is an indication that ''Platecarpus'' and other mosasaurs had two functional lungs. Snakes, which are closely related to mosasaurs, have only one functional lung with the second often being vestigial or absent. Unlike terrestrial lizards, however, the bronchi separate in front of the area of the forelimbs rather than at the level of the limbs.〔 Skin impressions are known from ''Platecarpus'', preserved in LACM 128319 as soft impressions and phosphate material. Scales on the tip of the snout and the top of the skull are somewhat hexagonal in shape and do not touch one another. The scales on the jaws are longer and rhomboidal in shape, overlapping one another. The scales on the snout indicate that the nostrils were placed far in front of the skull at its tip and faced laterally as in most squamates and archosaurs. The body scales are all rhomboidal in shape and form tightly connecting diagonal rows that overlap each other at their posterior edges. They are generally the same size throughout the entire length of the body. The caudal scales on the tail are taller and larger than those of the rest of the body, although those covering the lower surface of the tail are more similar to body scales.〔 Internal organs, or viscera, may also be preserved in the specimen as reddish areas. One is located in the thoracic cavity low in the ribcage, while the other is located in the upper portion of the abdominal cavity just behind the ribcage. The reddish areas were analysed with mass spectrometry and were shown to contain high levels of compounds made of iron and porphyrin. These substances are evidence of hemoglobin decomposition products that may have formed in the organs as they decomposed. Based on its position, the organ in the thoracic cavity is probably the heart or liver, or even both of those organs. The organ in the abdominal cavity may be a kidney, although it is in a more anterior position than the kidneys of monitor lizards, mosasaurs' closest living relatives. The anterior position of the kidneys may have been an adaptation toward a more streamlined body, as their presumed position is similar to that of cetaceans.〔 Part of the digestive tract is also preserved and is filled with fish remains. The shape of these remains may outline the true shape of the corresponding part of the digestive tract, most likely the colon. The presence of scales and undigested bones in the colon suggests that ''Platecarpus'' and other mosasaurs processed food quickly and did not thoroughly digest and absorb all food in the gastrointestinal tract. Coprolites from the mosasaur ''Globidens'' are also suggestive of low digestion and absorption rates as they contain masses of crushed bivalve shells.〔 The caudal, or tail vertebrae, are sharply downturned. The vertebrae at the bend (called the caudal peduncle) are wedge shaped with neural spines that are wider at their ends than they are at their bases. This downturned area likely supported a fluke similar to modern sharks. Two lobes would have been present: a lower one supported by the downturned vertebrae and an upper, unsupported one. The tail fluke was probably hypocercal, meaning that its lower lobe was longer than its upper lobe. This condition is also seen in ichthyosaurs and metriorhynchid crocodyliforms.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Platecarpus」の詳細全文を読む スポンサード リンク
|