700th Synapsida

Well, here we are again with another biennial look-back at the last 100 posts, highlighting what may have been missed and taking a brief peek at where I might go next. I can't say it's entirely gone without a hitch over the last couple of years, but I've mostly kept to the weekly schedule... although you might get more regular gaps in the summer going forward.

Evolution remained the single most common category for the past 100 posts, which is hardly surprising given how many are about extinct species, and how important it can be to understanding how the living ones got to be the way they are. However, diet and habitat, which are often linked, have overtaken animal behaviour to nab the second and third spots... although the latter has remained common. Since I pick most topics on the day, without any grand plan to even things out, that's probably just the vagaries of what I happen to have come across. 

600th Synapsida

The time has come around once again for my approximately biennial piece of navel-gazing, as I celebrate this blog reaching its 600th post, almost 12 years after it first started. Since the 500th post, the world has (mostly) emerged from the perils of COVID and now tends to have other issues on its mind that biological science is less likely to be helpful with - not that I deal with epidemiology or virology here anyway. But let's take the traditional look back at what did appear on the blog during that timeframe and, allowing for the fact that I usually pick the topic on the day the post goes up, what might be coming over the next two years.

Animal behaviour and evolution remain the two most popular topics of my posts, and it's unlikely that that's going to change too much. However, there were also several posts on sociability specifically, as well as things such as diet and habitat and a more than usual number of posts on anatomy. In terms of the types of animals covered, where posts were on a particular species and not part of an ongoing series, rodents top the list, followed by bats. These are, of course, the top two mammalian groups in terms of number of species, but, while rodents are relatively easy to study, bats are rather less so... but perhaps that just means that studies on them tend to stand out more and pique my interest.

500th Synapsida

Yes, it's been almost ten years since I started this blog and, while I don't do anniversary posts, since there are typically about 50 posts a year, this does happen to be my 500th post. Which is a fair few when you think about it. And, in general celebration of round numbers, that means it's time for another review of what's appeared in the last couple of years or so, and what might be coming next.

Even allowing for the series on Miocene animals, evolution has been one of the most common topics that I've written about in the last 100 posts, second only to animal behaviour. Which is hardly surprising, I suppose, given the number (and general popularity) of the posts I write about fossil species in general. Also common has been the perennial topic of reproduction, and there have been quite a few posts on animal communication and conservation as well.

400th Synapsida

With this being the 400th post at Synapsida, it is once again time for my biennial piece of navel-gazing, in which I see what has happened over the last 100 posts, and where I might be going next.

There's no change to the most frequently used tags over the last 100 posts, with evolution and behaviour topping the list, as might be expected, given the general theme of the blog. (I'm ignoring tags directly related to particular 'series' here, of course). Bubbling under are topics related to diet, sociability, and reproduction. Given some of the animals I have been looking at, it is unsurprising - if rather depressing - that endangered species are also mentioned quite frequently.

300th Synapsida

Here, have a marsupial. Or four.

This is, by my count, the 300th post to appear at Synapsida. Which means that, once again, it's time to look back over the last 100 posts to see what has, and has not, been covered, and where I might be going next. Which, considering that I pick the topic for over half my posts on the same day on which I write them, is likely to be a bit vague, but there you go.

The blog has been running for almost six years now, and has settled into a typical audience of between 200-300 hits per day (whatever the heck that means in terms of actual readers). The most frequently used tags over that period have been behaviour and evolution, although this may just reflect how I tag things. Biogeography, for instance, may well have cropped up in all sorts of posts that I didn't specifically note as such.

Looking back over the last 100 posts, carnivorans have probably been the most common animals covered, although there are also plenty of rodents, not to mention cetaceans, bats, primates, and all the rest. As I mentioned last time, my coverage of the main mammal families has been, if not entirely comprehensive, at least pretty broad. Since then, and ignoring posts on fossil beasts, I think I've added four more families to the list of those with a headline mention, all of them either small or obscure (or both): hyenas, degus, cheirogaleid lemurs, and echidnas.

But still no pigs.

200th Synapsida

Yes, this is the 200th post on Synapsida, and with it, time for my biennial piece of navel-gazing. If the number of hits is any measure, the blog has certainly increased in popularity over the last two years, typically getting over 200 per day now. I originally picked the title to be distinctive, and, assuming you know to look for it, that's not doing too badly - I'm on the first page of Google and Yahoo searches for the term. Although, obviously, one does need to know it's there, and not be looking for the taxonomic definition of things like pelycosaurs!

Over the last 100 posts, I have added a further 16 living families (plus some fossil ones) to the list of those I've covered, from the reasonably well known, such as armadillos, shrews, and porpoises, to the possibly slightly more obscure, such as tuco-tucos and beaked whales. That, even after four years of the blog, leaves an awful lot of families that I still haven't touched. Many of them are, unsurprisingly, small or obscure families, some of them with just a single living species (the aardvark, say, or the Asiatic linsang). But there's still some obvious gaps. I said in my 100th post that I hadn't covered pigs, for instance, and I still haven't, apart from some fossil species.

Q&A 2012

This is a synapsid, and not a reptile. Yes, really.

This will be the last post at Synapsida before the New Year, so I thought I'd do something a little different this time. A lot of the hits on this blog are from Google searches, with a few from similar services like Bing. Often, the searches that bring people here are questions, and the blogger.com interface lets me see what those questions were. I don't check it religiously, so I'm sure there must be many questions that brought people here that I've never seen. But, just for fun, this week I'm going to answer some of the ones I have seen.

It's not going to help the people who asked the questions in the first place, of course. And, if anyone asks the question again, there's no guarantee they'll be directed to this page, rather than wherever they were directed before. But what the heck - why not, right?

So without more ado:

Is [X] a Synapsid?

This seems to be among the top two or three questions that bring people to the site. Is a cow a synapsid? Is a tiger a synapsid? What about a monkey? Or a kangaroo? And so on.

Life of an Almost-Mammal

Scaloposaurus, a therocephalian.
This artist has chosen to show it hairy, which is plausible,
but uncertain.

When I was at school, I was taught that mammals evolved from reptiles. However, at least under the modern definition of 'reptile', that isn't really true. In fact, the mammals belong to their own, distinct, evolutionary lineage, one that has existed alongside the reptiles ever since the latter first came into being. This is a staggeringly long period of time - by definition, it's from long before there were any dinosaurs, for example.

However, this evolutionary group didn't just include the mammals themselves. Mammals may not have evolved from reptiles, but they haven't been around anything like as long in the fossil record. So, if we look back to before the first mammals evolved, we find that there were many other kinds of creature belonging to this group that are no longer around today. Because evolution isn't really a ladder, some of these animals survived alongside the early mammals. In fact, the last ones didn't die out until well after the split between placentals and marsupials.

Studying fossils of these creatures can tell us a lot about where mammals came from, and how they became so different from the reptiles.

100th Synapsida

This is the 100th post on Synapsida. Granted, that's counting the introductory post, which didn't really say much, but it's far easier to keep track of that way. Synapsida started in October 2010, initially with around 2 or 3 hits a day, and currently tends to get around 80. Not the most successful blog in the history of the internet, then, or even of science blogging, but hardly a disaster, either.

In that time, I've covered a wide range of mammalian topics, although there's still a number I believe haven't yet had a fair shake. The subject I've written most about turns out to be reproduction, followed, more generally, by animal behaviour. These are topics there's a lot to say about, and there's plenty of research ongoing, as well as being easier to make interesting than, say, the diversity of nitrogen isotopes in reindeer. Still, I'd like to do a bit more on morphology - the general shapes and anatomy of animals, and how that differs between groups.

I've covered members of 33 different families of mammals in the main articles, and so far, I've generally not gone into detail on any given species more than once. Still, some types of mammals have definitely featured more frequently than others. Aside from the obvious, the most commonly mentioned family of mammals turns out to be the dogs, followed by the bears. In general, carnivorous mammals have got the most airtime, but there are also a number of posts on marsupials, bats, primates, and cetaceans, among others.

What is a Mammal?

I'm going to briefly pause from looking at recent mammalogical news to examine a more general question about mammals. And they don't come much more general than "what is a mammal?"

Doesn't sound a very difficult question, does it? A mammal is a warm-blooded, air-breathing vertebrate that, crucially, feeds its young with milk from its mammary glands. Right? Well, kind of...

For the last twenty years or so, animals have been placed into groups on the basis of something called cladistics. Essentially, the idea is that a proper, meaningful, group of animals will be one that contains a single common ancestor and all of its descendants. Or, to put it another way, that everything in the group has to be more closely related to other animals in that group than it is to anything else. Which seems pretty straightforward and common-sense, and, indeed, is a very useful way of doing things. But applying it strictly does sometimes lead to some fairly surprising results.

Vertebrates first freed themselves entirely from the water when they evolved a way of making their own little pools of water, surrounded by a protective shell, and leaving their tadpoles inside that pool with a supply of yolk to feed off until they became developed enough to hatch. The animals that evolved this feature are called amniotes. Later on, some mammals evolved a way of doing away with the shell, and keeping the pond inside a membranous sac in the mother's body, but the principle is the same, and they still count as amniotes.

Not long after the amniotes first appeared, they split into two great evolutionary lines. One led to the reptiles and birds, and the other, called the Synapsida, led to the mammals.

Wait a minute, I can hear you saying, but didn't mammals evolve from reptiles? Well, it depends what you mean by "reptile". Certainly, if we could look at the early synapsids today, most people would probably call them reptiles. They were cold-blooded, hairless, laid eggs, wouldn't have produced milk, and anyway, they just kind of looked reptilian. Take a look at this one, for example. By most people's standards, that's a reptile.

But here's how the early amniotes evolved into the animals we have today:

Crocodiles    Birds      Other      Turtles      Mammals

                        Reptiles 

    ^           ^          ^           ^            ^

    |           |          |           |            |

    |           |          |           |            |

    -------------          |           |            |

          |                |           |            |

          |                |           |            |

          ------------------           |            |

                   |                   |            |

                DIAPSIDA            ANAPSIDA    SYNAPSIDA

                   |                   |            |

                   ---------------------            |

                             |                      |

                            (A)                     |

                             |                      |

                             ------------------------

                                        |

                                        |

Bearing in mind out definition for what constitutes a "group" of animals (a monophyletic clade if we want to get technical - but let's not) there are two problems with this chart. The first is that reptiles aren't actually a proper group at all - they can't be, because crocodiles are more closely related to birds than they are to, say, turtles. Which isn't to say that crocodiles are particularly close to birds, admittedly, just that they're even further from turtles. (This is because birds evolved from dinosaurs, which were fairly close to crocodiles).

When we say that a proper biological group has to consist of a common ancestor, and all of its descendants, the key word is "all". The common ancestor of all living reptiles is at the point I've marked (A), and if we want to include all of its descendants, then we have to count the birds. Either birds are reptiles, or reptiles don't really exist as a meaningful group. Bummer!

Be that as it may, the other problem is that even if the reptiles are a group, their last common ancestor, and therefore the first reptile, is the creature at (A), and mammals didn't evolve from that. So mammals evolved from creatures that certainly looked very reptilian, but which weren't, in a strict scientific sense, reptiles as we understand them today. They are entirely their own line.

But, at any rate, it's fairly clear that while the early synapsids may not have been reptiles, they weren't mammals, either. At some point, they evolved into mammals, and all of the earlier forms of reptilian-looking synapsids died out. So at what point did that happen? When did not-mammals become mammals?

The obvious answer is "when they developed mammary glands and began producing milk". Which is all very well, but a bit of a bugger when all you've got to go on is fossil bones. How do you tell from the bones whether the animal produced milk or not? You can't, pretty much. So palaeontologists have to use a different definition.

Skull of a turtle - note that the lower jaw consists of at least four different bones. (d = dentary, ar = articular)

The lower jaws of the early amniotes consisted of multiple bones. For example, there was the dentary bone, which had most of the teeth, and the articular bone, which formed the hinge joint with the skull. In the line that led to reptiles and birds, this didn't change much (at least until birds evolved beaks), but in the synapsids, something strange began to happen. The dentary bone began to get larger, slowly pushing out and shrinking the other bones, until most of them disappeared altogether.

Eventually, the dentary formed its own joint with the skull, and the lower jaw actually had two joints on each side for a while. That's not much of a problem, so long as the joints are lined up properly, but there's really no need for it, so eventually, the only remaining other bone in the lower jaw, the articular, began to shrink as well. In the end, the dentary was the only bone left in the lower jaw at all; and in mammals, we call it the mandible.

Lower jaw of a mammal - note the absence of separate bones

But the articular bone, and its old joint with the skull didn't disappear altogether. Both it, and the skull bone that it used to attach to shrank and became entirely separated from the jaw, moving up the side of the head. They are still there today, still with the old joint between them, but now we call them the malleus and incus, and they form two out of the three bones in the middle ear.

And that, at least when you're looking at fossils, is the defining characteristic of a mammal: that it has one bone on  each side of the lower jaw, and three bones in each middle ear.


[Pictures from Wikimedia Commons]