When I was in high school, I learned that the definition of a species is two animals that can interbreed and give birth to fertile offspring. Like, dogs are all one species because they technically can interbreed (although, functionally, watching a Great Dane and a Chihuahua work it out might be… difficult), but donkeys and horses are different because – although they can mate and give birth – their offspring (mules) are sterile.
At the time, I thought – well, that’s pretty straight forward. Thanks, scientists, for solving yet another mystery of life.
Fast forward to a few months ago when I asked one of my taxonomist colleagues to define a ‘species’ for me. The result of that (many hour-long) conversation inspired this video. Turns out, the answer isn’t, at all, straight-forward.
The Earth plays host to a remarkable variety of creatures all of which diverged from a single, unknown common ancestor. In the 540 million year history of complex life there have been repeated turnovers in dominant life forms, from The Age of Fishes during the Devonian to The Age of Reptiles during the Mesozoic which gave way to our current era in time, the Cenozoic, or otherwise known as The Age of Mammals. But where do mammals come from…?
Pelycosaurs… The Pelycosaurs first appeared 306Ma and are the most basal synapsids. Synapsids are mammal-like reptiles; a distinguishable feature is their skull with only one post orbital fenestra, reptiles have two. The mammal-like reptiles show the earliest signs of evolution towards more mammal characteristics, Dimetrodonis a classic example. Dimetrodon had only a single temporal fenestra, this feature allowed for jaw muscle attachment to be further back providing a stronger bite force over a wider range of jaw movements, subsequently making Dimetrodon one of the most successful predators of its age. Dimetrodon quite literally means “two sets of teeth” and this refers to its elongated incisors, this is the beginnings of the mammal feature known as heterodonty (differentiated teeth). Dimetrodon also exhibits deep ridges of the inner nasal cavity providing a larger surface area for the attachment of olfactory epithelium indicating a larger reliance of the sense of smell (something that will become more important in future mammal evolution).
Therapsids… Therapsids appear around 275Ma and likely evolved from pelycosaurs or a similar sort of ancestor. They are still classed as synapsids but they even more mammal-like features than their ancestors. Therapsids have less of a sprawling gait than the pelycosaurs and other reptiles (although the herbivorous kind still retain a rather sprawling gait, this is a feature that is slow to disappear). Therapsids have an even larger temporal fenestra and more elongated incisors as the teeth continue to differentiate. The skull of therapsids is also beginning to change; mammals have a single jaw bone called the dentary, this is present in synapsids along with many other jaw bones such as the quadrate and articular, but as mammal evolution gets underway the dentary expands and the quadrate and articular reduce in size and become part of the middle ear bones. Some Therapsids were apex predators throughout the permian such as Gorgonops.
Cynodonts… Cynodont fossils are some of the most important fossils in the mammals evolutionary record and we are classed as part of Cynodontia. They are dog-like creatures and appear around 260Ma, they were hugely successful and diversified rapidly. One of the best examples of cynodonts in mammal evolution is Thrinaxodon. The cynodonts exhibit even greater tooth differentiation and tooth occlusion (precise tooth contact of the upper and lower teeth, a feature unique to mammals). Tooth occlusion also suggests the cynodonts had controlled tooth replacement like mammals (mammals exhibit diphyodonty meaning two sets of teeth, the milk teeth and the adult teeth). More importantly, cynodonts also show partial or complete secondary palates meaning they are able to swallow food and breathe at the same time, something reptiles cannot do. Many taxa also show absence of abdominal ribs allowing for the presence of a diaphragm which increased lung capacity. A diaphragm paired with the secondary palate and tooth occlusion suggests that cynodonts had a higher metabolism than other extant animals of the time. The cynodonts dentary continues to expand further back of the skull. Some cynodont skulls have small indentations around the nasal region which may be indicative of nerve passages towards sensitive hairs, possibly whiskers, if so, cynodonts had hair. Thrinaxodon and others also have a larger brain size in comparison to the rest of the body than other animals as well as enlarged auditory and olfactory regions suggesting they were nocturnal.
The Permian extinction struck 250Ma due to extensive volcanism of the Siberian Traps leading to a runaway greenhouse effect. Between 80% and 96% of living species died out, including most of the cynodonts and the therapsids. Reasons why the reptiles battled through this extinction and rose to dominate is still debated but it may be due to the fact that reptiles can secrete nitrogenous waste as a uric paste whereas mammals must secrete is as a liquid. This allowed the reptiles to conserve water and see the extinction through. Whilst most cynodont species perished, a few individuals made it through, they were mostly small, nocturnal burrowers capable of getting water from underground root nodules and tubers. However, the cynodonts would not rise to dominance again but their descendants would, although not for another 200 million years. They would spend the Mesozoic Era in the dinosaurs shadows, their evolution driven by a nocturnal lifestyle and the need for endothermy. The evolution of our ancestors was shaped by the dominance of the dinosaurs, when their reign ended 65 million years ago our true mammalian ancestors would quickly take over the niches left behind and become some of the most spectacular creatures the world has ever seen.