A couple of weeks ago Brian Switek's blog Laelaps included a post on transitional fossils (those things that some creationists will tell you simply don't exist... ) Brian's post was sparked by this story (OK, maybe the writer of was aiming for 'balance', but really!) & he suggested that others might like to emulate him & write something about their own favourite transitional fossil. The idea's been simmering away in my mind since then.
Brian's challenge required some pause for thought - what is my favourite transitional fossil? I do like the whales, but he beat me to it on that one. And then I thought, what about Diarthrognathus???
Diarthrognathus lived back in the Triassic (ie while the dinosaurs were the dominant land mammals) & was a 'mammal-like reptile' - as the name suggests, a reptile with some mammal-like features of its anatomy. These include things like the presence of different types of teeth in the mouth; development of a secondary bony palate that separates the nasal passage from the mouth - thus allowing you to breath with your mouth full!; legs that are more under the body rather than jutting out to the side like those of a lizard or crocodile; and changes in the structure of lower jaw & jaw joint. I love the name Diarthrognathus, because it tells you what's special about this beast: Diarthrognathus literally means 'double-jointed jaw', & when you look at the skull & jaw, that's just what you see.
But why is this so special? The answer has a lot to do with the evolution of jaws & ears - specifically, the bones of the middle ear. The 3 bones of the mammalian middle ear (the 'anvil', 'stirrup', & 'hammer') have a long & interesting evolutionary history that begins with the bones of reptilian jaws (which in turn have a history of their own...). Anyway, when you compare the lower jaws of a mammal & a reptile, something stands out straight away: a mammal's lower jaw is made up of just one bone, called the dentary (so named because the teeth are rooted in it). Reptiles have a dentary too, but there are several other bones in their jaws or associated with the jaw joint: the articular, angular, surangular, and coronoid, plus the quadrate where the jaw articulates with the skull. Please bear with the names, as the story's easier to tell with them in it! Turn to the ears, and while we have those 3 familiar little bones, in a reptile you'll see only one, the stapes (stirrup).
Once developmental biologists started looking closely at embryonic development, they found something rather surprising. When a reptile embryo's developing, two of its early head bones end up as the quadrate & articular bones of the jaw joint. But trace those same bones in a mammal embryo & you'll find they become the malleus (hammer) & incus (anvil) of the middle ear, while the jaw articulation now involves the dentary in the jaw & squamosal bone of the skull. This leads to the conclusion that these 2 mammal ear bones are homologous to the quadrate & articular in a reptile's jaw. And indeed, if you look at a series of increasingly mammal-like reptiles, you'll find those 2 bones getting smaller & smaller & moving to the back of the jaw - in primitive mammals like Yanoconodon (as in platypus embryos today) they appear to have become detached from the jaw joint but remain attached to the jaw itself by a cartilagenous rod. (So you could - rightly! - say that there are a whole lot of transitional fossils in this story.)
What's so neat about Diarthrognathus is that it retains both jaw joints, so that the lower jaw articulates with the skull at two points, rather than one: it has the reptilian articular & quadrate joing, & the mammalian dentary/squamosal version. A lovely example of a transition if ever I saw one :-)