Leptocleidus

Leptocleidus was a genus of plesiosaur which lived in the oceans during the early Cretaceous Period about 135 to 125 million years ago. It measured just 10 feet long, which is quite small for a plesiosaur.

Three species of Leptocleidus are currently known:

  1. Leptocleidus capensis, found in the Swartkops River Valley, Cape Province, South Africa (collection ID code: SAM-K5822).
  2. Leptocleidus clemai, found near Kalbarri, Australia (collection ID codes: WAM 92.8.1-1 to 68).
  3. Leptocleidus superstes, found in Sussex, England (collection ID code: NHMUK R4828).

The fact that this genus has been found at opposite latitudes indicates that Leptocleidus may have had a global distribution during the early Cretaceous Period. As such, its fossils are likely to be found elsewhere within rocks of that same age bracket.

Leptocleidus is the eponymous member of the plesiosaur family Leptocleididae. At first glance, the leptocleidid plesiosaurs of the early Cretaceous bear a strong resemblance to the “cryptoclidid” plesiosaurs which existed earlier during the Jurassic Period such as Cryptoclidus and Pantosaurus. Both the cryptoclidids and the leptocleidids possessed large flippers for their body size and had moderate-length necks, as opposed to the short necks of the “rhomaleosaurids” which existed during the early and middle Jurassic, and the long necks of the “elasmosaurid” plesiosaurs of the late Cretaceous. However, when you look closer, there are some discernable differences.

The skeleton of Brancasaurus brancai, a leptocleidid plesiosaur from Westphalia, Germany. Scale bar = one-half of a meter. Sachs, Sven; Hornung, Jahn J.; Kear, Benjamin P. (2016). “Reappraisal of Europe’s most complete Early Cretaceous plesiosaurian: Brancasaurus brancai Wegner, 1914 from the ‘Wealden facies’ of Germany”. PeerJ, volume 4: e2813 (December 22, 2016). Page 63.

The skulls of the leptocleidid plesiosaurs are noteworthy for having a prominent ridge running up the middle of the skull extending from the snout to the eye sockets. This can be seen in the skull of Leptocleidus capensis from South Africa, as well as in the skull of a related genus named Nichollssaura borealis from Canada. This ridge was probably not visible on the animal’s head in life, as it supported the fleshy covering over the nares openings, which formed the animal’s nostrils. The elasmosaurid plesiosaurs which would come later also had a small ridge running down the middle of the skull, as seen in Libonectes, although it wasn’t as prominent as seen in the leptocleidids. Another difference between the cryptocleidids and leptocleidids is the size of the teeth. Unlike the small thin needle-like teeth of the cryptoclidids which were ideal for catching fish and belemnites, the leptocleidids had much larger cone-shaped crocodile-like teeth, indicating that they had a more varied diet.

Multiple views of the upper jaw of Leptocleidus capensis, from South Africa: top view, underside view, and left side view. Images from Cruickshank, Arthur R. I. (1997). “A lower Cretaceous pliosauroid from South Africa”. Annals of the South African Museum, volume 105 (February 1997). Pages 207-226.
https://www.biodiversitylibrary.org/page/40648901#page/221/mode/1up.

Fossil skeleton of Nichollssaura borealis, a leptocleidid plesiosaur from Canada. Note the ridge in the mid-line of the skull. On display in the Royal Tyrell Museum of Natural History, Drumheller, Alberta, Canada. Photos by Etemenanki3 (August 22, 2017). Wikimedia Commons, Creative Commons Attribution-Share Alike 4.0 International license.
https://commons.wikimedia.org/wiki/Category:Nichollssaura_fossils.

Fossils of Leptocleidus have been found in rock layers which were deposited in estuarine and shallow marine environments, indicating that this animal lived and hunted close to the shore and often could be found at river mouths. It’s possible that this animal occupied a similar niche as large seals do today. It has also been hypothesized that this animal occupied shallow water environments as a way to be safe from larger predators which inhabited deeper water.

Below is a drawing that I made of the early Cretaceous plesiosaur Leptocleidus. The rows of pronounced oval-shaped dermal scutes running along its neck and back, which give it something of a crocodilian appearance, are based upon a reference that I found in an article by the British paleontologist Dr. Darren Naish. He had been informed by another paleontologist that skin impressions had been found associated with a plesiosaur found in New Zealand, consisting of “a smooth surface, studded regularly with low, rounded scutes or scales” (Tetrapod Zoology. “Plesiosaur Peril — the lifestyles and behaviours of ancient marine reptiles”, by Darren Naish (March 3, 2014)). However, Dr. Naish is careful to state that just because this one species looked like this, that doesn’t necessarily mean that all plesiosaur species looked like this too. The drawing was made with No.2 and No.3 pencil on printer paper.

Leptocleidus. © Jason R. Abdale (May 13, 2022)

Keep your pencils sharp, everybody.

Sources

Articles
Cruickshank, Arthur R. I. (1997). “A lower Cretaceous pliosauroid from South Africa”. Annals of the South African Museum, volume 105 (February 1997). Pages 207-226. https://www.biodiversitylibrary.org/page/40648901#page/221/mode/1up. Accessed on February 22, 2022.

Sachs, Sven; Hornung, Jahn J.; Kear, Benjamin P. (2016). “Reappraisal of Europe’s most complete Early Cretaceous plesiosaurian: Brancasaurus brancai Wegner, 1914 from the ‘Wealden facies’ of Germany”. PeerJ, volume 4: e2813 (December 22, 2016). Pages 1-79.
https://pdfs.semanticscholar.org/7713/44b7925bc68cff779e496540bbdcf629f262.pdf. Accessed April 1, 2022.

Websites
Tetrapod Zoology. “Plesiosaur Peril — the lifestyles and behaviours of ancient marine reptiles”, by Darren Naish (March 3, 2014).
https://blogs.scientificamerican.com/tetrapod-zoology/plesiosaur-peril-the-lifestyles-and-behaviours-of-ancient-marine-reptiles/. Accessed on April 6, 2022.

The Plesiosaur Directory. “Leptocleidus”. https://plesiosauria.com/directory/genera/leptocleidus/. Accessed on February 22, 2022.



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