Saichania

Mounted specimen on display at Dinosaur Kingdom, in Nakasato, Japan 

Reconstruction by Andrey Atuchin

When: Late Cretaceous (~83 to 70 million years ago)

Where: Mongolia 

What: Saichania was an armored plant eating dinosaur that roamed the deserts of Mongolia in the late Cretaceous. It was about 22 ft (~6 meters) long and heavily built. It was more fearsome looking than most armored dinosaurs as it did not just have flat armor plates on its body, but rather was covered with spikes. These dinosaurs were armored all over, there is even evidence of armored eyelids! This suit of armor would have protected Saichania from predators in the late Mesozoic mongolian desert. Fossils are typically found in deserts and badlands worldwide, but typically these areas were very different environments when the species represented by the fossils were alive. The ancient Gobi Desert was much closer to the harsh modern environment than most. Saichania was well adapted for desert life, with its stocky body and teeth designed for grinding the toughest of the desert plants. 

Saichania falls within Ankylosauridae, a group of armored dinosaurs found almost worldwide. It is one of the last and most derived of the ankylosaurids. One good way to differentiate the deserved ankylosaurids from their armored close relatives is the presence of a tail club. Saichania did not have the most massive club known, but it was still a significant feature. Ankylosaurids were one of the dinosaur groups that made it right up to the end of the Cretaceous period, vanishing with the rest of the non-avian dinosaurs. 

Nemegtbaatar 

When: Late Cretaceous (~80 - 71 million years ago [the beds are poorly dated])

Where: Gobi Desert, Mongolia

What: Nemegtbaatar is a multituberculate. Multituberculata is an extinct group of mammals, but even though they did not make it to the modern day, they are one of the longest lived clades of mammals. Ranging from the early Jurassic (~160 million years ago) to the Oligocene (just about 35 million years ago). The evolutionary history  of multituberculates is fairly well known, as there are a multitude of almost complete skeletons known from these 125 million years. One of them is Nemegtbaatar, a fairly typical example of this clade. 

Nemegtbaatar was about 6 inches long (~15 cm), not counting its tail. It is reminiscent of a rodent in its appearance, primarily due to its enlarged incisors and the gap between these teeth and its cheek teeth. But a closer look at its denition shows it is very different from rodents, and all other living mammals. Its lower 4th premolar is a blade, and most of the rest of its premolars and all of its molars are elongated anterior to posterior with multiple parallel rows of many cusps. These molars are what gives the clade its name; Multituberculata means ‘lots of cusps’ to use a loose translation. Nemegtbaatar, like most of the other 80 genera of multituberculates, lived in a niche analogous to living rodents such as rats. At the bottom of the food chain, eating whatever it could get its teeth on, with the ability to quickly scurry away if need be. Despite their 125 million years of exsistance, multis always remained fairly evolutionary conservative, they found a niche that worked and stuck with it. Not to say there isn’t diversity here, but Nemegtbaatar  is a good ‘default’ multituberculate. 

The phylogenetic position of multis within Mammalia is unclear. There are two proposed placements, with good arguments for both possibilities. Multituberculates either fall between the monotremes and the therians (placentals and marsupial mammals), or they are outside all living mammals.