The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It’s small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear.
Here’s the beautiful little beast; as you can see, it’s very small, and we need to look very closely at some details of its morphology to see what’s special about it.
The first significant feature to examine is the jaw. This animal is from the Mesozoic, and a time when evolution was generating some radical changes in the feeding and sensory structures of the mammalian lineage. So first, a little background.
The primitive tetrapod jaw is a compound structure built up from multiple bones. In embryonic development, a rod-like structure called Meckel’s cartilage is first to form; in modern mammals, it is resorbed later in development, and really only forms a temporary scaffold. The dentary, as you might guess from the name, is the tooth-bearing portion. Farther back are several bones, including the angular and the articular, which contribute to the body of the structure and its articulation with the skull. One of the roles of these bones is to connect to the auditory apparatus of the cranium—the jaw conducts vibrations to these bones, which then transmits them to the organs of hearing. This is not a particularly sensitive way to sense sound, since it means sound waves traveling through the air (or the ground) are going to have to be picked up by a high impedance element, the bulky jaw, before being transmitted to the ear.
In the early mammalian lineage, there is a pattern of progressive reduction of the various secondary jaw elements and an expansion of the dentary bone to take over the whole job of the jaw. We have an excellent record of the transformation of the jaw and skull elements in mammalian evolution — in short, what we see is that everything but the dentary gets smaller and smaller, and gets pushed farther and farther back towards the skull. We have transitional forms that have double articulations of the jaw with the skull—one between the old articular bone and the quadrate bone of the skull, and another between the dentary and the squamosal (the current jaw joint in modern mammals)—and then forms where the old hodge-podge of bones have been cast free of the jaw altogether.
In us, the old articular and quadrate bones have completely lost their role in supporting the jaw as a joint and instead have become imbedded in the middle ear of mammals, suspended with the stapes between two delicate membranes to specialize in conducting sound vibrations to the inner ear. What does the hearing apparatus look like in Yanoconodon?
Start by looking at a, b, c, and d in this diagram. Highlighted in blues and purples at the back of the jaw are these small bones in Morganucodon (a) and Yanoconodon (b). In d is the jaw of Repenomamus, a large Cretaceous mammal. Don’t miss c—that small object is the collection of middle ear bones from Ornithorhyncus, better known as the platypus.
What we see here is that the three Mesozoic mammals all retain Meckel’s cartilage as a slender, ossified splint clinging to the inner side of the jaw. In b and c, we can see that middle ear bones of both Yanoconodon and the platypus are remarkably similar, but there is one significant difference. In the platypus, those bones are not connected to the jaw at all—they have the standard mammalian middle ear, with the bones suspended remotely from other bones of the jaw and skull. In Yanoconodon, we are almost at that point. The middle ear bones are clearly delicate and specialized for function in hearing, but they retain one last tentative, delicate connection with the jaw through a contact with Meckel’s cartilage. In this animal, we’ve caught the mammals just before they’ve taken that last step of fully separating the middle ear bones from the jaw.
This animal is from that time just before the hearing apparatus has let loose of its last bony mooring and said bon voyage to the jaw. It’s a significant moment in history, I think.
Also look at g. This is a ventral view of the left jaw of Yanoconodon, and again you can see the middle ear bones connected by that tiny strut to the jaw. h is a drawing of the middle ear bones of the platypus in the same orientation, and i is especially neat: that’s the jaw of a platypus embryo, before Meckel’s cartilage is resorbed, and you can see that the middle ear bones are connected in the same way, transiently. It’s one module in development that flaunts a lovely example of embryonic recapitulation of evolutionary history.
I said there were two novelties in this specimen. One is the beautiful connection between the middle ear bones and the jaw; the other is a curiosity in the number of vertebrae. Compare yourself to Yanoconodon, for instance:
|Cervical||7, no ribs||7, no ribs|
|Thoracic||12, with ribs||18, with ribs|
|Lumbar||5, no ribs||8, with floating ribs|
Not only is the total number of vertebrae very much on the high end of what we see in any modern mammals, making for a rather sinuous and flexible body, but there’s that odd business of the lumbar (our lower back) vertebrae having riblike bones attached to them. The authors make the point, too, that the boundary between thoracic and lumbar in Yanoconodon is somewhat arbitrary—there’s a continuous gradation rather than a sharp delineation. Gain and loss of lumbar ribs seems to be a fairly common event in these early mammalian clades, as illustrated below.
The inset diagram illustrates an experiment in mouse embryos: knocking out the three Hox10 genes in mice produces a transformation just like that seen in Yanoconodon, with all the lumbar vertebrae also producing small ribs. That’s very cool, in that it suggests a molecular mechanism in that the evolution of the Hox10 genes was probably responsible for the morphological variation we see in Mesozoic fossils.
Luo Z-X, Chen P, Li G, Chen M (2007) A new eutriconodont mammal and evolutionary development in early mammals. Nature 446:288-293.