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Plesiosuchus

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Plesiosuchus
Holotyp von Plesiosuchus in Seitenansicht

Holotype of Plesiosuchus in side view

Temporal occurrence
Upper Jurassic (upper Kimmeridgian to lower Tithonian)
154.7 to 147.7 million years
Locations
  • England
  • Spain (?)
System
Crocodylomorpha
Mesoeucrocodylia
Thalattosuchia
Metriorhynchidae
Geosaurinae
Plesiosuchus
Scientific name
Plesiosuchus
Owen, 1884
Species
  • Plesiosuchus manselii

Plesiosuchus (Greek plesios = “near” and Suchos = “Sobek”) was a genus of large metriorhynchids from the Upper Jurassic of Western Europe.

The holotype consists of an incomplete skull with an isolated right articular bone. Other fossils include parts of the lower jaw, isolated teeth, a humerus, and a larger number of ribs and vertebrae.

Description

Plesiosuchus resembled other metriorhynchids, also called “marine crocodiles”. They had evolved paddle-shaped limbs and a caudal fin. Furthermore, they were the only Crocodylomorpha completely adapted to a life in the sea.

Plesiosuchus in size comparison with a diver

The skull of Plesiosuchus is only fragmentarily preserved, present are the cranium, the rostrum and the mandible. The snout was long and narrow, there are a total of six teeth on the praemaxillary, 13 on the maxilla and 14 on the mandible. The largest specimen has an estimated skull length of 125.5 cm and was probably close to seven meters long. It thus reached similar dimensions to the pliosaur Liopleurodon ferox, an apex predator from the Oxford Clay Formation in England.

According to Delfino and Dal Sasso (2006), it is possible to determine whether an animal was already adult in Thalattosuchia (and other Crocodylomorpha) by the fusion of the cervical vertebrae. However, because the vertebrae of Plesiosuchus are too poorly preserved to see this fusion, it is not possible to determine whether the fossils belonged to an adult or subadult animal.

System

The fossils of Plesiosuchus have been assigned to various genera throughout history, including Steneosaurus and Dakosaurus

Young et al. (2012) showed through phylogenetic analysis and a more detailed description of the fossil record that Plesiosuchus should be considered a distinct genus and belongs to the subfamily Geosaurinae along with Dakosaurus, Torvoneustes, and Geosaurus, among others. In recent analyses, Plesiosuchus is the sister genus of Suchodus. Together they form the subtribe Plesiosuchia.[1]

Abbreviated cladogram according to Foffa et al. (2017)

Metriorhynchidae

Metriorhynchinae

Geosaurinae

Neptunidraco

Geosaurini
“subclade T.”

Tyrannoneustes

Torvoneustes

Geosaurina

Ieldraan

Geosaurus

Dakosaurina

Dakosaurus

Plesiosuchia

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Plesiosuchus

Paleoecology and nutrition

The skull of Plesiosuchus seen from above.

Plesiosuchus shared its habitat with other large marine reptiles, most notably the geosaurine Dakosaurus maximus. Although closely related, the two species differ in terms of skull structure and a consequent diet. Dakosaurus maximus is characterized by a robust snout that was adapted to stress from twisting and bending, the teeth were also adapted to high pressure at the base of the tooth crown, and the tips are often broken or chipped.
Plesiosuchus manselii, on the other hand, was larger overall and had a less robust snout with mostly intact dental crowns. Also, the “optimum gape” (the angle of the open mouth at which the prey comes into contact with the most teeth) was larger in P. manselii (24°) than that of D. maximus (19°).

Young et al. conclude that D. maximus was a generalist that sucked up smaller prey, but its mouth was also designed to eat larger prey by tearing off smaller pieces of their bodies, while P. manselii specialized in prey that matched its optimum gape. Due to the high optimum gape and the overall larger body, the prey of Plesiosuchus was probably larger on average than that of Dakosaurus maximus
The authors compare these results with the feeding behaviour of North Atlantic killer whales. Here, too, two “feeding types” are known. The first is smaller and feeds by sucking up fish, occasionally seals. Type 2 is larger and specializes in eating other whales.

Other carnivorous marine reptiles from Plesiosuchus ‘ former habitat included the similarly sized ichthyosaur Brachypterygius and the giant Pliosaurus macromerus. The different adaptations in feeding behavior and prey range probably made it possible for these different large predators to exist simultaneously in the same habitat.

Find history

The holotype of Plesiosuchus comes from the bay of Kimmeridge, a village in the English county of Dorset. It was found there in the 1860s by John Clavell Mansel-Pleydell in a reef after it was exposed by low tide. Mansel-Pleydell gave the fossils to the British Museum in 1866.

Literature

  • Mark T. Young, Stephen L. Brusatte, Marco Brandalise de Andrade, Julia B. Desojo, Brian L. Beatty, Lorna Steel, Marta S. Fernández, Manabu Sakamoto, Jose Ignacio Ruiz-Omeñaca, Rainer R. Schoch: The Cranial Osteology and Feeding Ecology of the Metriorhynchid Crocodylomorph Genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. In: PLoS ONE. Vol. 7, No. 9, 2012, e44985, doi:10.1371/journal.pone.0044985.

Web links

Commons: Plesiosuchus– Collection of images, videos and audio files

Individual references

  1. Foffa, D., Young, M. T., Brusatte, S. L., Graham, M. R., & Steel, L. (2017). A new metriorhynchid crocodylomorph from the Oxford Clay Formation (Middle Jurassic) of England, with implications for the origin and diversification of Geosaurini. Journal of Systematic Palaeontology, 1-21, doi:10.1080/14772019.2017.1367730