Platecarpus

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Platecarpus
Temporal range:
Ma
Cast of P. tympaniticus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Clade: Mosasauria
Superfamily: Mosasauroidea
Family: Mosasauridae
Clade: Russellosaurina
Subfamily: Plioplatecarpinae
Genus: Platecarpus
Cope, 1869
Species:
P. tympaniticus
Binomial name
Platecarpus tympaniticus
Cope, 1869
Synonyms

Platecarpus coryphaeus
Platecarpus ictericus

Platecarpus ("flat wrist") is an

ammonites as well.[1] Like other mosasaurs, it was initially thought to have swum in an eel-like fashion, although another 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.[2] Isotopic analysis on teeth specimens has suggested that this genus and Clidastes may have entered freshwater occasionally, just like modern sea snakes.[3]

Description

Skull in Naturmuseum Senckenberg
jaws of mosasaur Platecarpus coryphaeus
P. coryphaeus jaws

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.[2] 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 (around 10 teeth in each

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.[2]

Size comparison

The

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.[2]

Soft tissues in the head and neck of specimen LACM 128319: Tracheal rings are shown in the bottom three photographs.

Skin impressions are known from Platecarpus, preserved in LACM 128319 as soft impressions and

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.[2]

LACM 128319
Life reconstruction of same specimen

Internal organs, or

cetaceans.[2]

Platecarpus skeleton

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 the

bivalve shells.[2]

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

History

Fossils

Various skeletons of this mosasaur have been found in Cretaceous deposits in Kansas, but only one complete skull has ever been recovered.[4] Platecarpus fossils have been found in rocks that date back to the late Santonian through the early Campanian in the Smoky Hill Chalk.

Taxonomic history

Skeleton in Naturhistorisches Museum Wien
Specimen in Naturalis in Leiden formerly referred to P. ictericus
P. somenensis type specimen

Platecarpus was often regarded as the most common genus of mosasaur in the

monotypic, as P. coryphaeus and P. ictericus were synonymized with its type species, P. tympaniticus.[8][10]

The cladogram below follows the most resolved topology from a 2011 analysis by paleontologists Takuya Konishi and Michael W. Caldwell.[8]

Clidastes propython

Kourisodon puntledgensis

Russellosaurina

Yaguarasaurus columbianus

Russellosaurus coheni

Tethysaurus nopcsai

Tylosaurus kansasensis

Tylosaurus proriger

Plioplatecarpinae

Ectenosaurus clidastoides

Angolasaurus bocagei

Selmasaurus johnsoni

Selmasaurus russelli

Plesioplatecarpus planifrons

Platecarpus tympaniticus

Latoplatecarpus willistoni

Latoplatecarpus nichollsae

Platecarpus somenensis

Plioplatecarpus primaevus

Plioplatecarpus houzeaui

Plioplatecarpus marshi

Paleobiology

Diet

Restoration of the skull, showing dentition

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.[4]

Locomotion

Restoration

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 of Platecarpus suggests that it swam more like a shark. The downturned caudal vertebrae of Platecarpus 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 of

interspinal 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 in Platecarpus, the base of the tail remained stable. This form of movement is known as carangiform locomotion.[2]

Reconstruction showing bi-lobed tail fluke

The structure of the scales of Platecarpus 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 improved

mosasauroid Vallecillosaurus 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 than Platecarpus, has even smaller scales covering its body, indicating that it had even more efficient locomotion in the water.[2]

Notes

  1. Demopolis Formation
    in Alabama that has a lower number of teeth in its jaws.

References

  1. ^ .
  2. ^ .
  3. .
  4. ^ a b c d Everhart, Michael J.. Oceans of Kansas: A Natural History of the Western Interior Seaway. c. 2005. pp. 165–169
  5. ^ Williston (1898a)
  6. ^ Russell (1967)
  7. ^ (Everhart and Johnson, 2001)
  8. ^
    S2CID 85972311
    .
  9. ^ Lingham-Soliar T. 1994. The mosasaur "Angolasaurus" bocagei (Reptilia: Mosasauridae) from the Turonian of Angola reinterpreted as the earliest member of the genus Platecarpus. Palaeont. Z. 68 (1/2): 267–282.
  10. ^ Kiernan CR, 2002. Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with an historical review of Alabama mosasaur discoveries. Journal of Vertebrate Paleontology 22 (1): 91–103.

Further reading

Williston 1898 – includes drawings of the skull of Platecarpus ictericus