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A toolkit of teeth

Despite often being included with dinosaurs in playsets and films, Dimetrodon was more closely related to mammals than to reptiles. Dmitry Bogdanov via Wikipedia under CC By-SA 3.0

Most mammals have a dental tool kit of differently shaped teeth. In our own mouths, for example, we have incisors to nip with, canine teeth to puncture, and premolars and molars to crush and mash grub. The diversity lets mammals handle a great variety of food and make the most of our meals, whether it’s a wolf nipping the last muscle of an elk leg or an elephant chewing grass.

Paleontologists can see the beginnings of this differentiation in synapsids more than 295 million years old. Despite its lizard-like appearance, the sail-backed Dimetrodon was a synapsid and more closely related to us than any dinosaur or other ancient reptile. Such pre-mammal synapsids are often called “proto-mammals” as their anatomy set the stage for what mammals would eventually become. Dimetrodon, in particular, illustrates an early dental shift that mammals would later take to extremes. The ancient carnivore’s name means “two measures of teeth,” referring to the stark difference between the large, piercing teeth in the canine tooth position and smaller teeth behind it along the jaw. The difference is the beginning of what anatomists call heterodonty, or having differently shaped teeth in different jaw positions. The condition differs from most reptiles, which are homodont and have teeth about the same size and shape along their jaws. As early synapsids went about feeding on the plants and animals of their world, what started as basic, conical teeth were modified into different feeding specialties. Mammal teeth eventually became so diverse in shape and so distinctive that paleontologists often tell the difference between one species and another based on their dental details.

Long lost ribs

Ophiacodon is one of several synapsids found with gastralia, or belly ribs. ДиБгд at Russian Wikipedia / Public Domain

The mammalian backstory isn’t just one of gaining new features. Some ancient traits were lost and had a major influence on mammal evolution. One of the significant losses among mammal ancestors was gastralia, or belly ribs.

Early synapsids like Ophiacodon had thin ribs running along their bellies between their shoulders and hips. Synapsids of the time sprawled with their legs out to the side, like lizards, and so the belly ribs offered some extra protection from the rough ground. As synapsids continued to evolve during the Permian Period, however, they lost their belly ribs. Creatures like our cynodont ancestors, as well as the saber-toothed gorgonopsians, didn’t have belly ribs. Instead, organs like the heart and lungs would be enclosed in the rib cage, and lower organs, such as the stomach and intestines, would be held in by the body