As an Australian palaeontologist, I am all too familiar with dinosaur taxa named on the basis of a single bone.  The theropods Rapator, Walgettosuchus, Ozraptor, Kakuru, Nanantius and Timimus. The ornithopod Fulgurotherium. The ulna known as Serendipaceratops, whatever that is. Let us kick off, then, in familiar territory, with an Argentinian dinosaur named on the basis of a single incomplete bone. Meet Loncosaurus argentinus (MACN 11269; often (?mis)transcribed as MACN 1269).

Florentino Ameghino (1854–1911) was a pioneer of Argentinian palaeontology. Although he was not the first to report dinosaurs from Argentina, Ameghino did make what he considered to be the first report of a theropod dinosaur from the Republic. In 1899, on the basis of an incomplete femur and a tooth (MACN 10985) — discovered by Carlos Ameghino, Florentino’s brother — Ameghino declared that a relative of the English theropod Megalosaurus bucklandii lived in Argentina during the Cretaceous Period.
 
For the next three decades, Loncosaurus received relatively little attention from palaeontologists. However, that changed in 1929, when Friedrich von Huene (1875–1969) published what was, for many decades after, the definitive work on Argentinian dinosaurs: Los Saurisquios y Ornitisquios del Cretáceo Argentina. In this monumental paper, von Huene included a brief assessment of Loncosaurus: he seemed satisfied that the femur belonged to a coelurosaurian theropod dinosaur, and even went so far as to say that it compared favourably with ostrich dinosaurs — ornithomimids. Critically, he also illustrated the femur, showing the prominent fourth trochanter and the hollow nature of the shaft, but also highlighting just how incomplete the specimen was. Von Huene also illustrated and briefly appraised the tooth found with the femur, demonstrating beyond doubt that it, too, pertained to a theropod.

Once von Huene was done, however, the dust again settled on Loncosaurus. Although often included in dinosaur lists as a theropod — sometimes as a synonym of Genyodectes — it was not until 1980 that Loncosaurus received a new lease on life. Ralph Molnar, in a paper reviewing Australian Late Mesozoic terrestrial tetrapods, also listed the terrestrial tetrapods from the other Gondwanan continents. In so doing, he listed Loncosaurus as a hypsilophodontid ornithopod, with the following footnote appended: “Loncosaurus is represented by the proximal portion of the right femur and teeth… The teeth…are clearly theropod…[but]…The prominent fourth trochanter (partially missing) and the strong posterior development of the greater trochanter of the femur contraindicate theropod affinities. Both features are found in hypsilophodont femora…and for this reason the femur is here tentatively considered to pertain to a small ornithopod.” (Molnar, 1980:140).

Molnar was the first to call into question the association of the tooth with the femur, and in so doing he took the femur — which undoubtedly was not from a theropod — out of the shadow of the tooth — which undoubtedly was — and appraised it in its own right. His interpretation appears to have received broad acceptance: in both editions of The Dinosauria, Loncosaurus is listed as an iguanodontian (albeit as a nomen dubium). Molnar’s interpretation was reaffirmed by Rodolfo Coria and Leonardo Salgado in 1996, when they redescribed and illustrated the specimen, confirming beyond any reasonable doubt that it was an ornithopod. So really, Ameghino hadn’t just announced the first Argentinian theropod back in 1899; he’d also inadvertently, and unknowingly, made the first report of an Argentinian ornithopod too.

Loncosaurus was found at Pari-Aike on the Río Sehuen, in Santa Cruz Province. It derives from the Mata Amarilla Formation, which dates to the Cenomanian–Santonian (~100.5–83.6 Ma). Further fossil hunting in this rock unit has turned up another ornithopod — the wonderfully preserved Talenkauen santacrucensis, which I intend to see later on this trip — but no more specimens referable to Loncosaurus are yet known. Hopefully, in time, that will change.

References

Ameghino, F. 1899. Nota preliminar sobre el Loncasaurus[sic] argentinus, un representante de la familia de los Megalosauridae en la República Argentina. Annales Societas Ciencias 47:61–62.

Cambiaso, A. V. 2007. Los ornitópodos e iguanodontes basales (Dinosauria, Ornithischia) del Cretácico de Argentina y Antártida: In Facultad de Ciencias Exactas y Naturales, Vol. Ph.D., pp. 410. Universidad de Buenos Aires, Buenos Aires, Argentina.

Coria, R. A., and A. V. Cambiaso. 2007. Ornithischia; pp. 167–187 in Z. Gasparini, L. Salgado, and R. A. Coria (eds.), Patagonian Mesozoic Reptiles. Indiana University Press, Bloomington & Indianapolis, IA.

Coria, R. A., and L. Salgado. 1996. "Loncosaurus argentinus" Ameghino, 1899 (Ornithischia, Ornithopoda): a revised description with comments on its phylogenetic relationships. Ameghiniana 33:373–376.

Ezcurra, M. D., and F. L. Agnolín. 2017. Gondwanan perspectives: theropod dinosaurs from western Gondwana. A brief historical overview on the research of Mesozoic theropods in Gondwana. Ameghiniana 54:483–487.

Huene, F. v. 1929. Los Saurisquios y Ornitisquios del Cretáceo Argentina. Anales Museo de La Plata 3:1–196.

Molnar, R. 1980. Australian late Mesozoic terrestrial tetrapods: some implications. Mémoires de la Société Géologique de France (Nouvelle Série) 139:131–143.

Norman, D. B. 2004. Basal Iguanodontia; pp. 413–437 in D. B. Weishampel, P. Dodson, and H. Osmólska (eds.), The Dinosauria: Second Edition. University of California Press, Berkeley.

Norman, D. B., and D. B. Weishampel. 1990. Iguanodontidae and Related Ornithopods; pp. 510–533 in D. B. Weishampel, P. Dodson, and H. Osmólska (eds.), The Dinosauria. University of California Press, Berkeley.