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The last 65 million years have sometimes been called ‘the Age of Mammals’ (although I’m inclined to think it should be the Age of Insects, or perhaps – as it’s always been – the Age of Bacteria; after all, in terms of sheer number of individuals, bacteria have got to be the dominant life form on the planet…). This gives the impression that mammals are a relatively recent evolutionary novelty.  But just how old is this class of organisms? Just what is the ‘age’ of the mammals?

Mammals began to diversify after the mass extinction event that marks the boundary between two geological ‘periods’ (the Cretaceous & Tertiary) and which carried off a large number of species, most notably the dinosaurs. But they’ve been around for much longer. Much, much longer – the mammal family tree has its roots way back in the Carboniferous, 350-270 million years ago.

Not that those ancestors wandering through the swampy Carboniferous forests would have looked like the mammals of today. Scientists call these ancestral beasts, ’mammal-like reptiles’ ie they were reptiles, but with some mammal-like features, including features of their skulls. Modern mammals, those early mammal-like reptiles, and everything in between, are described as synapsids.

  

Amniote skull types. a) Anapsid eg turtles b) Synapsid eg mammals c) Euryapsid eg ichthyosaurs & plesiosaurs d) Diapsid eg dinosaurs & birds. (NB the ‘amniotes’ are the reptiles, birds, & mammals – all produce an amniotic egg with a number of membranes associated with the yolk & developing embryo.)
 
The synapsid skull (b) has a single opening in the external bone layer, behind the eye – these diagrams can be a bit confusing & certainly my students sometimes find them so; it looks like the hole might go through from one side of the skull to the other, but in fact that’s not so.
 
Early synapsid reptiles roamed the Earth from the Carboniferous period until the Jurassic, and at one time were the dominant land animals. Many groups of these early reptiles were not in our direct lineage; one group that was, was the pelycosaurs - which included the fearsome predator Dimetrodon. (The name Dimetrodon means ‘two kinds of teeth’ – this is significant as most reptiles have a single type of teeth in their mouths, while modern mammals have 3-4.) One group of pelycosaurs evolved into the mammal-like  ’therapsids’, a group that included Thrinaxodon, and the therapsids in turn produced a number of descendant groups. One of them, the cynodonts, was on the line to modern mammals (but not, as it turns out, directly linked to them). 
 
By the way, it’s actually rather difficult to define a clear instant at which an animal becomes a ‘mammal’, rather than a ‘mammal-like reptile’! Therapsids have a number of mammal-like features: compared to the sprawling stance of reptiles, they held their legs more underneath the body (making locomotion more efficient); they had a distinct neck; their mouths contained several different types of teeth – something described as heterodonty; the beginnings of a secondary palate began to separate the airway from the mouth; and one of the bones in the lower jaw (the dentary) was enlarged compared to that in reptiles. But they definitely weren’t mammals.
 
Many therapsids became extinct during the mass extinction event that marked the end of the Permian, around 225 mya. (Incidentally, this was one of the greatest mass extinctions in terms of species lost – David Raup & Jack Sepkoski estimated that up to 94% of all species then alive, died in the end-Permian event. On the mammalian side the survivors included cynodonts like the herbivorous dicynodont Lystrosaurus (the distribution of Lystrosaurus fossils provided early evidence in support of the concept of continental drift) and the carnivorous ‘theriodonts’. The latter were similar to wolves in size, with large serrated canine teeth and skull modifications that provided larger jaw muscle attachment points.
 
During the Triassic cynodonts gradually became more & more like ’true’ mammals. Eventually their lower jaws comprised just a single bone (called the ‘dentary’ because it bears the teeth). Along with this came a change in the way the jaw articulated with the skull (& associated changes in the tiny bones of the inner ear). Another feature was the obvious presence of a diaphragm. OK, you say – this is so unlikely to fossilise, so how can you say this? The evidence lies in the lack of ribs attached to the lumbar vertebrae (the ones that form the ‘small’ of your back) – in modern mammals the diaphram is attached to the lower edge of the ribcage, & there are no lumbar ribs. The therapsid Thrinaxodon didn’t have lumbar ribs either, so the diaphragm must have been an early evolutionary innovation in the proto-mammal lineage.

And these beasts were probably furry :-)  Cynodont jawbones are perforated by small holes, similar to those which in modern mammals carry nerves and blood vessels to the whiskers, which implies furriness. (Cuddliness would be highly unlikely!) And a wonderful fossil from the Jurassic, Castorocauda, includes direct evidence of fur. Not only this, but a range of features tell us that Castorocauda spent a lot of time in the water. So, an immediate predecessor of modern mammals swam in the streams that T.rex would have splashed through: the earliest evidence yet of an aquatic mammal.

But the origins of ‘true’ (modern) mammals remains a bit murky. This isn’t helped by the fact that many of the early forms are known mainly by their teeth. (I’m sure it’s been said by someone before, but you could jokingly say that the evolution of mammals is traced through the matings of teeth with teeth…).So to get back to that original question: what is the age of the mammals? The answer has to be – it depends on how you define them :-)