Megalosaurus lydekkeri is a tooth first mentioned by Dawkins (in Huxley, 1869) as Megalosaurus, then called Zanclodon(?) sp. b. by Lydekker (1888), named Megalosaurus (gen. ?) lydekkeri by Huene (1926) and reassigned as Magnosaurus (?) lydekkeri by Huene (1932). It's Early Jurassic, so probably not Megalosaurus or Magnosaurus, and has since been relegated to Theropoda indet. status and never compared in detail to anything.
|Holotype tooth and maxillary or dentary fragment of Megalosaurus? woodwardi (BMNH 41352) (after Lydekker, 1888).|
However, no one has noticed that in 1909, Lydekker designated the tooth as the type of a new species, Megalosaurus woodwardi (along with the tibia that would later be named Sarcosaurus andrewsi). While he included no description or definition, he did provide an indication in the form of referencing Woodward (1908) who distinguished the tibia from M. bucklandii, Ceratosaurus and Allosaurus in its slenderness and "trochlear" shape of the astragalar facet, and from Triassic taxa in its tall astragalar ascending process.
Because Megalosaurus woodwardi has priority over Megalosaurus lydekkeri, but has not been used since it was named, ICZN Article 23.9.1 should be consulted to check if the former is a nomen oblitum. Usage of Megalosaurus lydekkeri must be maintained if (188.8.131.52) M. woodwardi has not been used as a valid name after 1899 (false, as Lydekker used it in 1909) and (184.108.40.206) M. lydekkeri has been used as a valid name "in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years" (false as far as I can tell, as an extensive search located only 13-15 works since 1962*). Thus Megalosaurus woodwardi is the valid name, unless a worker were to petition the ICZN to suppress it.
* Neaverson (1962), Steel (1970), Kuhn (1972)?, Waldman (1974), Chure and McIntosh (1989), Molnar et al. (1990), Olshevsky (1991), Glut (1997), Carrano and Sampson (2004), Holtz et al. (2004), Naish and Martill (2007), Benson and Barrett (2009), Fastovsky and Weishampel (2009)?, Benson (2010), Carrano et al. (2012).
The ironic thing here is that the tibia has also been called Megalosaurus woodwardi, though for completely different reasons. The specimen was accidentally made the type of two species simultaneously by Huene (1932) (Sarcosaurus andrewsi and Magnosaurus woodwardi). In that paper, Huene also lists Megalosaurus woodwardi in the section on Magnosaurus nethercombensis, stating both should be Megalosaurus subgenus Magnosaurus. As Magnosaurus is explicitly named as a new genus on that same page, this was probably an earlier opinion that was mistakenly retained. Note Huene uses "n. sp." and does not reference Lydekker's 1909 paper, suggesting he was not merely using Lydekker's name. Though even if he did it wouldn't matter since Lydekker made the tooth the type, not the tibia. The tibia is now called Sarcosaurus andrewsi because Huene was first revisor in 1956.
And in case you're wondering, I think Olshevsky was mistaken in thinking Walgettosuchus had been explicitly assigned to Megalosaurus as Megalosaurus woodwardi.
References- Huxley, 1869. On the upper jaw of Megalosaurus. Quarterly Journal of the Geological Society of London. 25, 311-314.
Lydekker, 1888. Catalogue of the Fossil Reptilia and Amphibia in the British Museum (Natural History), Cromwell Road, S.W., Part 1. Containing the Orders Ornithosauria, Crocodilia, Dinosauria, Squamta, Rhynchocephalia, and Proterosauria: British Museum of Natural History, London. 309 pp.
Woodward, 1908. Note on a megalosaurian tibia from the Lower Lias of Wilmcote, Warwickshire. Annals and Magazine of Natural History. 8(1), 257-259.
Lydekker, 1909. Vertebrate paleontology in 1908. Science Progress in the Twentieth Century: A Quarterly Journal of Scientific Work & Thought. 3(11), 450-471.
Huene, 1926 The carnivorous Saurischia in the Jura and Cretaceous formations, principally in Europe. Revista Museo de La Plata, 29, 35-167.
Huene, 1932. Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte [The fossil reptile order Saurischia, their development and history]. Monographien zur Geologie und Palaeontologie, serie 1. 4(1-2), 1-361.
Huene, 1956. Paleontologie und Phylogenie der niederen Tetrapoden. Fischer-Verlag, Jena.
Olshevsky, 1991. A revision of the parainfraclass Archosauria Cope, 1869, excluding the advanced Crocodylia. Mesozoic Meanderings. 2, 196 pp.