Placental and Marsupial Mammals

My brother is a nature expert. Other than people who study in the field, if you go up against my brother when it comes to nature in a trivia challenge, you’ll lose. However, he’s an expert on extant species, when it comes to extinct species, I’m kind of a total nerd. I love dinosaurs, and anything over 100,000 years old. I do tend to dabble inbetween though.

Had I not been bitten by the tech bug, I’d be a paleontologist or anthropologist. Because damn it, old bones are awesome. So maybe for this new year I’ll talk a bit more about “old fossils for nerds”.

Today’s lesson comes from the Nat Geo program called Life after Dinosaurs. It’s a quick 45 minute documentary on what happens after the majority of the dinosaurs got wiped out. Anyway, there are plenty of shows out there that talk about paleo-history and all that jazz, what I like about this show is that it talks about the history between placental mammals (like us), vs. marsupial mammals (like kangaroos). Take, for example, the concept of the apex predator in an environment … say … the big cat. In one corner we have Smilodon.

People like to call these things “Sabre Tooth Tigers”. There really was no such animal, there were plenty of “sabre tooth cats” of different kinds though. In popular science though, when people think of “Sabre Tooth Tigers”, they probably mean Smilodon, even though there were plenty of big cats with large canines like that back in the day, and heck, even plenty of different subspecies of Smilodons. but whatever, what’s interesting is that in South America they had a different kind of sabre-tooth:

Wow, you’ve got to be kidding me. Another sabertooth. This guy has really wicked protector-things under his jaw too. Not only does he have the fangs, he’s got the killer scabbard to go along with it, I mean, take a look at this skull:

So, in North America, you have Smilodon, and in South America you have Thylacosmilus. So what happened? How come everyone knows Smilodon, and no one has every heard of a marsupial sabretooth, which could probably eat you in at least as many ways as the normal “sabre tooth tiger”?

Well, according to the show, it was a simple matter of evolution. When Smilodon showed up, Thylacosmilus died out relatively quickly, but why?

Thylacosmilus was a marsupial, and since marsupials grow up in pouches, they need to get out early and quickly, and crawl their way to suck on their mother’s nipple. And to suck you need to have a skull that is relatively formed, as a result, marsupials need to have their skulls shaped WAY before placental mammals, who have longer gestation periods in their mother’s womb. So … as a result, marsupial mammals tend to have smaller brains that placental mammals. For those of you not following along, we’re plancental mammals, there’s this weird organ that develops between a mother and child that acts as a filter, and as it ends up, is one of the reasons why placental mammals are so successful. So is that really it? The setting of the skull is difference between success and extinction?

And, marsupial mammals grew on smaller continents, Australia and South America vs. North America and Asia; according to the show, the larger the land mass, the higher the evolutionary competition. And since Smilodon had a biggger brain (longer gestation period), even thought they were both large cats, and a textbook example of convergent evolution, the fossil record shows Smilodon moving into South America, and the Thylacosmilus just totally going away in a relative short period of time, even though his kind had ruled the landscape for over 60 million years.

Two top predators, so similar that they look similar, even though they’re totally different. If you search around the web, the pictures of Smilodon and Thylacosmilus look really close. So similar, and yet, not so much, something to think about the next time you look at a different primates and think they’re so different, or the next time you look at a shark and think they all look the same. When you think about it, the difference between a successful species and a footnote in a history book is not that far apart.

Watch it for yourself here:

And while Smilodon won this battle, remember that at the end of the day when it came to success as the top predator in North America, between Smilodon, Terror birds, and wolves … teamwork wins! We don’t got to Yellowstone to see terror birds or Smilodons!