Several very different genera of prehistoric animals have been named Palaeosaurus or Paleosaurus since the 1830s. Further confusing the matter, all of the species are poorly known or poorly preserved and both spellings have been used interchangeably, even by the same authors.
The repeated recurrence of this name is most likely due to poor communication between 19th century scientists. Another factor is the name means 'ancient lizard' (Greek palaios/παλαιος meaning 'ancient' and sauros/σαυρος meaning 'lizard'), which is an appropriate name for any fossil reptile.
- Palaeosaurus (no species named) – a teleosaurid crocodile. May be a junior synonym of Aeolodon.
- "Palaeosaurus" cylindrodon – an indeterminate archosaur (later renamed Palaeosauriscus cylindrodon)
- "Palaeosaurus" diagnosticus a prosauropod dinosaur; synonym of Efraasia
- "Palaeosaurus" fraserianus – an indeterminate phytosaur
- "Palaeosaurus" platyodon – an indeterminate phytosaur
- "Palaeosaurus" stricklandi – an indeterminate phytosaur
- Palaeosaurus is first named as a genus by French scientist Isidore Geoffroy Saint-Hilaire. No species is named, but this genus, found in Germany, is now considered a junior synonym of the Jurassic sea-going teleosaurid crocodile Aeolodon.
- Two British scientists, Henry Riley and Samuel Stutchbury, briefly and informally publish on two new fossil teeth found in or near the city of Bristol, England, which they call Palaeosaurus cylindrodon and P. platyodon. Riley and Stutchbury did not mean to assign these species to Geoffrey Saint-Hilaire's genus; they simply did not know the name had been used. Thecodontosaurus is also named in this publication.
- Riley and Stutchbury more fully describe their two species of Palaeosaurus, each based on a single sharp tooth from the Late Triassic Period. The spellings are corrected to read Paleosaurus cylindrodon and Paleosaurus polyodon.
- Sir Richard Owen creates the name Dinosauria. In the same publication, he attempts to redescribe Riley and Stutchbury's Paleosaurus and Thecodontosaurus, which he does not consider to be dinosaurs. Inadvertently, he changes the spelling back to Palaeosaurus, and this spelling is predominantly used from here on. Owen assigns other bones to Palaeosaurus, causing much confusion later, as these bones actually belong to the prosauropod dinosaur Thecodontosaurus.
- Thomas Henry Huxley describes both Thecodontosaurus and Palaeosaurus as dinosaurs for the first time. He considers Palaeosaurus platyodon to be equivalent to Thecodontosaurus antiquus, most likely due to the Thecodontosaurus bones that had been assigned to it by Owen. However, Huxley regards P. cylindrodon as a carnivorous theropod.
- American paleontologist Edward Drinker Cope names the species Palaeosaurus fraserianus for an isolated tooth found in Triassic rocks in Pennsylvania, which belongs to a phytosaur.
- Yet another species is created, Palaeosaurus stricklandi, for some teeth which probably belong to a phytosaur.
- Von Huene recognizes the tooth of Palaeosaurus platyodon as a phytosaur and places it into the genus Rileya, forming the new combination Rileya platyodon.
- Von Huene describes numerous prosauropod bones found in Germany. By this time Palaeosaurus cylindrodon has been recognized as a prosauropod because of the prosauropod bones assigned to it by Owen. Von Huene therefore refers his new species to Palaeosaurus, creating the name P. diagnosticus.
- Another German paleontologist, Oskar Kuhn, finally recognizes that the genus Palaeosaurus created by Riley and Stutchbury in 1836 is preoccupied and creates the new generic name Palaeosauriscus to contain Palaeosaurus cylindrodon and several of the other species that had been incorrectly assigned to Palaeosaurus.
- Because of the mistaken association of prosauropod remains with carnivorous teeth, American Edwin Harris Colbert classifies prosauropods into two groups. Palaeosauria included Palaeosaurus and Teratosaurus, which were thought to be carnivorous. Thecodontosaurus and Plateosaurus, which had been found with the correct skulls, were included in Plateosauria, which was described as a herbivorous group.
- Peter Galton, a British paleontologist, moves the species Palaeosaurus diagnosticus into its own genus, creating the new combination Efraasia diagnostica. For several decades, most scientists consider Efraasia a junior synonym of Sellosaurus, however.
- Thecodontosaurus is redescribed by a team of paleontologists led by Michael Benton. Most of the skeletal bones ever assigned to Palaeosaurus cylindrodon and P. platyodon are reassigned to Thecodontosaurus. The genera Rileyasuchus and Palaeosauriscus, as well as the species Palaeosaurus cylindrodon and Palaeosaurus platyodon are all declared nomina dubia.
- Adam Yates, another British palaeontologist, redescribes the bones assigned to Sellosaurus. He resurrects the genus Efraasia for some of these bones, to which he assigns the bones that had been described as Teratosaurus minor as well, although not the teeth, which are non-dinosaurian. Like Galton in 1973, Yates' Efraasia also includes the remains previously known as Palaeosaurus diagnosticus, although unlike Galton, Yates calls the species Efraasia minor because von Huene described Teratosaurus minor several pages before Palaeosaurus diagnosticus in his 1908 publication. The name minor therefore takes precedence over diagnostica for this species.
- Peter Galton, reviewing the archosaurian fossils of the 1834 Bristol finds, reaffirms the identification of the two teeth and humeri of Palaeosaurus platyodon (Rileyasuchus) as belonging to a phytosaur, and regards P. cylindrodon (Palaeosauriscus) as an indeterminate archosaur. He concurs that Rileyasuchus is dubious, but suggests that Palaeosauriscus could be valid, based on its now-lost tooth with a "subcircular cross-section and fine, obliquely inclined denticles".
Effect on paleontology
The confusion surrounding Palaeosaurus extends well beyond nomenclature. Richard Owen's mistake of associating prosauropod skeletal remains with the carnivorous teeth which Riley and Stutchbury called Palaeosaurus, combined with von Huene's Teratosaurus minor, which was also a combination of carnivore and prosauropod remains, led paleontologists to view prosauropods as carnivorous animals for quite a long time. This error made it into several textbooks and other dinosaur reference works.
- Galton, Peter (2007). "Notes on the remains of archosaurian reptiles, mostly basal sauropodomorph dinosaurs, from the 1834 fissure fill (Rhaetian, Upper Triassic) at Clifton in Bristol, southwest England". Revue de Paléobiologie 26 (2): 505–591.
Riley, H., and S. Stutchbury, 1836, "A description of various fossil remains of three distinct saurian animals discovered in the autumn of 1834, in the Magnesian Conglomerate on Durdham Down, near Bristol", Proceedings of the Geological Society of London 2:397–399