最早的哺乳动物仍然发现在英格兰南部

A mystery was solved this week. There was a big announcement from the University of Portsmouth that a student doing an undergraduate project was looking at some rock samples from the south coast of England and found these tiny teeth which it was announced are the earliest evidence of mammals existing on the Earth and they date to about 147 million years ago. Chris Smith asked Cambridge University palaeontologist Jason Head to explain the findings...

Jason - They’re not the earliest mammals; there’s a little bit of confusion. What’s significant about these teeth is that they help us put down the divergence between placental mammals. So eutherian mammals which are the people in this room - your cat, your dog, most of the diversity of living mammals, and marsupials - the metatherians - so your pouch mammal, your possums, your kangaroos, your wallabies.

These are the oldest we think - unambiguous at the moment - record of placental mammals, so there are all sorts of mammals that are much older. What this is is a nice example of a pretty clear anatomical indicator that the divergence between marsupials from placental mammals that occurred basically by the beginning of the cretaceous period, around 147 million.

Chris - The mammals that were around at that time were obviously in competition with the dinosaurs. Is that what kept them small because these were small animals, we think, weren’t they - at least when these animals first evolved - probably because they were continually turning into lunch?

Jason - That’s a generally accepted idea that basically the diversification of dinosaurs would have inhibited the ability for mammals to radiate into new body sizes and new ecologies. And sure enough, after the end of the cretaceous, fairly soon after the demise of the non-bird dinosaurs you do see this big diversification - a flowering of modern mammals groups.

It takes mammals a little while to get much bigger. They don’t really achieve giant sizes until fairly late in their history. But yeah, for most of the mesozoic mammals, they’re very small, they’re probably nocturnal, they’re fairly ecologically limited. Although in the mammals defence there is dog sized mammal that we know from the early cretaceous of China called repenomamus which has dinosaur remains in it’s gut.

Chris - Oh, right. So they did get their own back to a certain extent. You mentioned something interesting there when you said they were probably nocturnal. Is that supposition of the fact that you’ve got something hungry with big teeth running around during the day that had very good eyesight - a dinosaur. So if you came out during the day you were more likely to turn into lunch, therefore, these early mammals probably came out at night and they could tolerate the cold better because they were warm blooded - is that where you’re coming from?

Jason - Actually, it has more to do with looking at behaviours in living mammals today. And one of the things that we can do is we can look at activity patterns across all of living mammalia, and we can actually reconstruct the ancestral activity pattern based on what we can see for all the living ones today. So if you look at four different groups of different mammals, and you look at their relationships and they’re all nocturnal, you can assume maybe their ancestors were nocturnal as well.

So when that’s been done for a huge number of species and mammals, what we actually find is that the most parsimonious solution is that mammals would have been primitively nocturnal. We also can see from the anatomy of their heads, their skulls, that these are small insectivorous animals with pretty decent sized eyes. And based on both of those and comparisons with modern insectivores today, the most likely interpretation is that they were nocturnal.

Chris - So we’re the unusual ones, coming out during the daytime - we’ve evolved to become day active when, actually, we’re bucking the trend? Most of our mammalian ancestors would have been night active.

Jason - Most of mammal history is being small, nocturnal, and afraid of reptiles. It is after the cretaceous again, it’s during this big flowering and big diversification of mammals in the early cenozoic that we definitely started to see really diurnal mammal ecology really develop.