Siamosaurus suteethorni

Siamosaurus is one of those theropods that's huge, yet sorely overlooked. Why? well, it's a relatively new discovery.The type species, Siamosaurus suteethorni, was formally described by Buffetaut and Ingavat in 1986. There is very little information on this Cretaceous meat-eater, but it is known from teeth that closely resemble those of Spinosaurus

As most spinosaurs are, Siamosaurus is pretty massive. Stretching nine meters(30 feet) long and weighing two and a half tons. It's name means "Siamese lizard" as it's remains were found in Thailand.

A 2010 publication by Romain Amiot and colleagues found that oxygen isotope ratios of spinosaurid bones indicates semiaquatic lifestyles. Isotope ratios from teeth from the spinosaurids Baryonyx, Irritator, Siamosaurus, and Spinosaurus were compared with isotopic compositions from contemporaneous theropods, turtles, and crocodilians. The study found that, among theropods, spinosaurid isotope ratios were closer to those of turtles and crocodilians. Siamosaurus specimens tended to have the largest difference from the ratios of other theropods, and Spinosaurus tended to be have the least difference. The authors concluded that spinosaurids, like modern crocodilians and hippopotamuses, spent much of their daily lives in water.

A lot is yet to be known about these piscovorious giants, and once more fossils are unearthed, we might just get an insight into their ecology.