| Platecarpus | |
|---|---|
 | |
| Cast ofP. tympaniticus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Clade: | †Mosasauria |
| Family: | †Mosasauridae |
| Clade: | †Russellosaurina |
| Subfamily: | †Plioplatecarpinae |
| Genus: | †Platecarpus Cope,1869 |
| Species: | †P. tympaniticus |
| Binomial name | |
| †Platecarpus tympaniticus Cope, 1869 | |
| Synonyms | |
Platecarpus coryphaeus | |
Platecarpus ("oar wrist") is anextinctgenus of aquaticlizards belonging to themosasaur family, living around 84–81million years ago during the middleSantonian to earlyCampanian, of theLate Cretaceousperiod.Fossils have been found in theUnited States and possible specimens inBelgium andAfrica.[2] A well-preserved specimen ofPlatecarpus shows that it fed on moderate-sizedfish,[3] and it has been hypothesized to have fed onsquid, andammonites as well.[2] Like other mosasaurs, it was initially thought to have swum in aneel-like fashion, although another study suggests that it swam more like modern sharks. An exceptionally well-preserved specimen ofP. 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 theNatural History Museum of Los Angeles County.[3] Isotopic analysis on teeth specimens has suggested that this genus andClidastes may have entered freshwater occasionally, just like modern sea snakes.[4]
Platecarpus had a long, down-turned tail with a large dorsal lobe on it, steering flippers, and jaws lined with conical teeth. A complete specimen LACM 128319 shows that it grew up to 5.67 m (18.6 ft) long.[3] The platecarpine mosasaurs had evolved into the very specialized plioplatecarpine group by the end of theCretaceous.
The skull structure ofPlatecarpus is unique among mosasaurs. This genus is characterized by a short skull, and has fewer teeth than any other mosasaur (around 10 teeth in eachdentary).[note 1] LACM 128319 preserves matter within thesclerotic ring that may possibly be theretina of the eye. Small structures in the retina, each around 2 μm long and observed byscanning electron microspectroscopy, may represent retinalmelanosomes preserved in their original positions.[3]
Therespiratory tube is also known in LACM 128319, preserved ascartilaginous tracheal rings. Only the posterior-most end of the tracheal tube – at the end of the neck near thepectoral girdle – is known. The section where the twobronchi split was also preserved in the specimen, but was destroyed during excavation. This is an indication thatPlatecarpus 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.[3]
Skin impressions are known fromPlatecarpus, preserved in LACM 128319 as soft impressions andphosphate 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 mostsquamates andarchosaurs. 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.[3]
Internal organs, orviscera, may also be preserved in the specimen as reddish areas. One is located in thethoracic cavity low in the ribcage, while the other is located in the upper portion of theabdominal cavity just behind the ribcage. The reddish areas were analysed withmass spectrometry and were shown to contain high levels of compounds made ofiron andporphyrin. These substances are evidence ofhemoglobin 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 akidney, although it is in a more anterior position than the kidneys ofmonitor 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 ofcetaceans.[3]
Part of the digestive tract is also preserved and is filled with mid-sized fish remains. The shape of these remains may outline the true shape of the corresponding part of the digestive tract, most likely thecolon. The presence of scales and undigested bones in the colon suggests thatPlatecarpus and other mosasaurs processed food quickly and did not thoroughly digest and absorb all food in the gastrointestinal tract. Coprolites from the mosasaurGlobidens are also suggestive of low digestion and absorption rates as they contain masses of crushedbivalve shells.[3]
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 inichthyosaurs andmetriorhynchidcrocodyliforms.[3]
Various skeletons of this mosasaur have been found in Cretaceous deposits in Kansas, but only one complete skull has ever been recovered.[5]Platecarpus fossils have been found in rocks that date back to the lateSantonian through the earlyCampanian in the Smoky Hill Chalk.
Platecarpus was often regarded as the most common genus of mosasaur in theWestern Interior Sea during the deposition of theSmoky Hill Chalk in Kansas, andPlatecarpus ictericus was regarded as the most commonly occurring species.[5] However, scientists now consider it to be of theparaphyletic genus. Therefore, some species were reassigned to their own genera. The type specimen ofPlatecarpus planifrons was discovered by Professor B. F. Mudge and was classified byEdward Drinker Cope asClidastes planiforns.[5] In 1898, on further analysis of the remains, it was determined that the mosasaur be placed in the genusPlatecarpus.[6] The type specimen underwent another taxonomic review in 1967, whenpaleontologistDale Russell determined that the remains were too fragmentary to place within any genus, and deemed it a specimen of "uncertain taxonomic position".[7] A 2006 discovery in theSmoky Hill Chalk of Kansas re-affirmed this position with the discovery a complete fossilized skull being unearthed.[8] In2011, a new generic name,Plesioplatecarpus, was erected by Takuya Konishi and Michael W. Caldwell to incorporateP. planifrons, which they found was distinct fromPlatecarpus in aphylogenetic analysis.[9] In 1994,Angolasaurus was synonymized with this genus.[10] However, many recent studies re-validate this genus.[9] Furthermore,Platecarpus is consideredmonotypic, asP. coryphaeus andP. ictericus were synonymized with its type species,P. tympaniticus.[9][11]
Thecladogram below follows the most resolved topology from a 2011 analysis by paleontologists Takuya Konishi and Michael W. Caldwell.[9]
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Compared to the tylosaurs, plioplatecarpine mosasaurs had much less robust teeth, suggesting that they fed on smaller (or softer) prey such as small fish and squid.[5]
While mosasaurs are traditionally thought to have propelled themselves through the water by lateral undulation in a similar way to eels, the deep caudal fin ofPlatecarpus suggests that it swam more like a shark. The downturned caudal vertebrae ofPlatecarpus suggest it had a crescent-shaped tail fluke. At the point of the tail where the fluke begins the vertebral centra are shortened and disk-like. Their reduced size likely allowed for greater flexibility at an area that would have experienced high stresses during swimming. The neural spines of these vertebrae also have grooves for the insertion ofinterspinal ligaments and dorsal connective tissues which would have aided in lateral movement of the fluke. The ligaments were probably made of collagenous fibers that acted as springs to move the tail back into a resting position after energy was stored in them. These types of ligaments work in some living fish to conserve energy during repetitive bending of the tail. While the fluke and back of the tail undulated inPlatecarpus, the base of the tail remained stable. This form of movement is known ascarangiform locomotion.[3]
The structure of the scales ofPlatecarpus may have been another adaptation toward a marine lifestyle. The small size and similar shape of these scales throughout the body would have stiffened the trunk, making it more resistant to lateral movement. This stiffness would have improvedhydrodynamic efficiency by improving the flow of water across the body. The earlymosasauroidVallecillosaurus also preserves body scales, but they are larger and more varied in shape, suggesting that the animal relied on undulatory movement in its trunk rather than just its tail.Plotosaurus, a more derived mosasaur thanPlatecarpus, has even smaller scales covering its body, indicating that it had even more efficient locomotion in the water.[3]
Williston 1898 – includes drawings of the skull ofPlatecarpus ictericus
