Flipper size in Mesozoic Enaliosaurs.

Earlier we looked at three tylosaurs distinguished by their widely varying flipper size. Today we’ll do the same with a few closely related pliosaurs (Fig. 1) and ichthyosaurs (Fig. 2).

Figure 1. Three pliosaurs, Trinacromerum, Brachauchenius and Kronosaurus, to scale, all Late Cretaceous.

Figure 1. Three pliosaurs, Trinacromerum, Brachauchenius and Kronosaurus, to scale, all Late Cretaceous. Variations in flipper size, rib count and girdle size mark the major differences here. The girdles anchored large ventral swimming muscles. Scale bar = 1 meter. How flipper size affected speed and agility is a topic that has not been brought up yet, as far as I know.

Pliosaurs were some of the largest marine reptiles of all time.
They reached an acme in the Late Cretaceous with a variety of morphologies derived from smaller ancestors. Shown here are Trinacromerum, Brachauchenius and Kronosaurus, three taxa rarely shown together to scale. The big difference is in flipper size. It’s also worth noting the size of the girdles anchoring each flipper set. In whales, flipper size also varies greatly and not always for swimming advantage.

Ichthyosaurs,
even closely related ones (Fig. 2), also had widely varying flipper sizes. Here we see ‘Cymbospondylus’ buchseri and the related Guizhouichthyosaurus. Both were probably sinuoous swimmers, rather than tail or flipper swimmers.

Figure 2. Two closely related ichthyosaurs, Guizhouichthyosaurus tangae and "Cymbospondylus" buchseri, one with large flippers, one with small.

Figure 2. Two closely related ichthyosaurs, Guizhouichthyosaurus tangae and “Cymbospondylus” buchseri, one with large flippers, one with small.

References
Carpenter K 1996. A review of short-necked plesiosaurs of the Western Interior, North America. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 201(2):259-287.
Hampe O 2005. Considerations on a Brachauchenius skeleton (Pliosauroidea) from the lower Paja Formation (late Barremian) of Villa de Leyva area (Colombia). Fossil Record – Mitteilungen aus dem Museum für Naturkunde in Berlin 8 (1): 37-51.
Longman HA 1924. A new gigantic marine reptile from the Queensland Cretaceous, Kronosaurus queenslandicus new genus and species. Memoirs of the Queensland Museum 8: 26–28.
O’Keefe FR 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1-63.
Owen R 1840. British Fossil Reptiles. Chapter V Order—Sauropterygia, Owen. Genus—Pliosaurus, Owen. 152-165.
Romer AS and Lewis AD 1959. A mounted skeleton of the giant plesiosaur Kronosaurus. Breviora 112: 1-15.
Williston SW 1903. North American plesiosaurs. Field Columbian Museum, Pub. 73, Geological Series 2 (1):1-79.
Williston SW 1907. The skull of Brachauchenius, with special observations on the relationships of the plesiosaurs. United States National Museum Proceedings 32: 477-489.
Williston SW 1908. North American Plesiosaurs: Trinacromerum. Journal of Geology 16(8): http://www.jstor.org/stable/30068152

wiki/Brachauchenius
wiki/Pliosaurus

wiki/Kronosaurus
wiki/Trinacromerum

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