Day 279#: Nanosaurus agilis
Today's animal of the day is Nanosaurus agilis!
Image credit: Alex Payne
This small species of ornithischian dinosaur lived during the Late Jurassic period, around 155 to 148 million years ago, in what is now the southwestern United States. Its genus name means "small lizard", which makes sense since it was relatively small for a dinosaur, around 6 to 7 ft long. This bipedal dinosaur was first described in 1877 by Othniel Charles Marsh, who, along with Edward Drinker Cope, was one of the two paleontologists responsible for the Bone Wars.
Image credit: cisiopurple on DeviantArt
This was basically a paleontological dick-measuring competition. I'll probably go into all the details whenever I cover Elasmosaurus, since that's the guy who started it all. I'll be honest, I don't have the energy to go into all the details of this 134-year-old beef right now. All you need to know for right now is that both Marsh and Cope were trying to be the one to name as many new species and genera of dinosaurs as possible. However, because they were both in such a hurry to constantly describe new species, it often led to them accidentally claiming a newly dug-up fossil to be a whole new species, when in reality, they were just new specimens of already known species. This happened a lot with Nanosaurus.
Image credit: BBC Walking With Dinosaurs (incorrectly called Othnielia)
Marsh would describe Othnielia rex (which he named after himself, because of course he did) that same year, but in 2007, a study comparing the teeth of Nanosaurus and Othnielia would discover that these two were actually just two different individuals of the same species, and since Nanosaurus was described first, it got to remain valid. Funnily enough, in the 1990's another " new species" would be described and named Drinker nisti, after Marsh's nemesis: Edward Drinker Cope. However, Drinker would eventually be revealed to be Nanosaurus AGAIN when a new specimen was found in 2018 that filled in some of the gaps and showed that the two genera were too similar to be considered separate. There are a couple of other dubious species of dinosaurs that turned out to be Nanosaurus, but we need to move on.
Image credit: Jack Wood
A study done in 2025 showed that Nanosaurus agilis had very similar teeth and jugal bones to pachycephalosaurs, which may suggest that it was actually a very early member of that clade. There are currently no known species of pachycephalosaurs from the Jurassic, but they are theorized to have existed since it seems that this group and the ceratopsian clade both split from the Marginocephalian dinosaur family tree around the same time, and there are some known ceratopsian dinosaurs, like Yinglong downsi, that lived during the Jurassic. However, this is currently just a theory, and more evidence needs to be found before a definitive conclusion can be made. At the moment, Nanosaurus is considered to be a fairly basal member of the ornithischian or "bird-hipped" dinosaurs, which also includes both pachycephalosaurs and ceratopsians, but not birds!
Image credit: prehistoric-wildlife.com
Most of the fossils we have of Nanosaurs come from the Morrison Formation, which spans across the western United States, and would have been a semiarid savannah during the dry season and a series of flat floodplains during the wet season. It would have lived alongside giant sauropods like Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus, as well as some medium-sized herbivores like Camptosaurus, Dryosaurus, and the plate-backed Stegosaurus. It would have also had to deal with large predators, like Ceratosaurus, Torvosaurus, and several species of Allosaurus, along with the much smaller Ornitholestes. While Nanosaurus is mostly interpreted as a small herbivore, if the theory about it being a pachycephalosaur is correct, it's possible that Nanosaurus may have been an opportunistic omnivore, and would have fed mostly on plants but supplemented its diet with meat to get some extra protein and calcium.
Image credit: Timothy J. Bradley (Jurassic Park Institute)
Fun fact: Nanosaurus actually appears in the original Jurassic Park novel. The book calls it Othneilia, but we know now that Othneilia is actually just Nanosaurus. Interestingly, they are actually depicted scavenging on the carcass of a juvenile hadrosaur that had been killed earlier by the Tyrannosaurus. They're also arboreal in the books, which is pretty outdated.
