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An Earful of Jawbones

The middle ear of mammals contains a chain of three tiny bones (auditory ossicles), the hammer (malleus), anvil (incus), and stirrup (stapes). This chain transmits and amplifies sound vibrations from the eardrum to the inner ear. The eardrum (tympanic membrane) itself is stretched across an additional bone, the ecto­tympanic. In all other land vertebrates, a rod-like stapes is the only bone connecting the eardrum to the inner ear.

In 1837 the German anatomist C. Reichert, studying the development of the head in mammalian embryos, first established that the malleus and ectotympanic were originally part of the lower jaw. The malleus partially develops from a cartilaginous structure called Meckel’s cartilage. The joint between the malleus and incus corresponds to the joint between the articular and quadrate, the two bones that form the jaw joint in amphibians, reptiles, and birds. (Mammals evolved a new jaw joint between the dentary and squamosal bones.) The questions of when and how many times these elements became separated from the lower jaw during mammalian evolution have remained unanswered until now.

The discovery, just published by Meng Jin (American Museum of Natural History, New York) and his colleagues Yuanqing Wang and Chuankui Li (Chinese Academy of Sciences) in the scientific weekly Nature, of an exquisitely preserved skeleton of a new 120-million-year-old mammal, Liaoconodon hui, from Liaoning Province in northeastern China (Fig. 1), offers crucial new evidence bearing on these questions. Liaoconodon belonged to an extinct lineage of mammals, the Eutriconodonta, which were widely distributed and diverse during the Jurassic and Cretaceous periods.

Figure 1. Skeleton of Liaoconodon hui from the Lower Cretaceous Jiufotang Formation of Liaoning Province, China. Courtesy of Meng Jin (American Museum of Natural History). Overall length of skeleton 35.7 cm (14.76 in.).

When Meng and his colleagues examined the skull of the new fossil they realized that its minute ear bones were still preserved in their life position, connected to the lower jaw.

Liaoconodon and its kin are distinguished by the possession of an ossified Meckel’s cartilage (Fig. 2). In present-day mammals, a slender rod of cartilage, Meckel’s cartilage, forms early during embryonic devel­opment. Subsequently, the single large bone (dentary) that forms the lower jaw in mammals covers this rod. Most of the cartilage becomes resorbed, leaving only its back portion, which turns into bone and becomes part of the malleus. In eutriconodonts, however, most of Meckel’s cartilage became separated from the part of the malleus it formed and then turned into bone in the adults. The ear bones became connected to this ossified cartilage.

Figure 2. Three stages in the evolution of the ear bones in mammals.  The Early Jurassic mammal precursor Morganucodon still has its ear bones fully connected to the lower jaw. Liaoconodon shows further differentiation of the ear bones while retaining a connection with the lower jaw. Adult modern mammals have the ear bones completely separated from the lower jaw.  Courtesy of Meng Jin (American Museum of Natural History).

The superb preservation of the skeleton of Liaoconodon allowed Meng and his colleagues to trace the separation between the two portions of the malleus. The front portion of the malleus, which is not derived from Meckel’s cartilage, partially wraps around the ossified Meckel’s cartilage. The ectotympanic contacts the malleus.

In modern mammals, the middle ear bones are attached to the skull, and researchers had long sought the intermediate stage between this condition and that in the earliest mammals where the ear bones were still part of the lower jaw. One suggestion for such an intermediate condition was a persisting contact between Meckel’s cartilage and the malleus. Liao­conodon now provides the first actual evidence of such a contact between the ossified Meckel’s cartilage and the bones of the middle ear in an adult early mammal.

What functional role would the ossified Meckel’s cartilage in Liaoconodon have played? The ectotympanic of Liaoconodon is completely separated from the skull and apparently only supported part of the eardrum. By contrast, the ring-shaped ectotym­panic is attached to the skull in present-day mammals and supports nearly the entire eardrum. Meng and his colleagues argue that part of the eardrum in Liaoconodon would have been connected to the skull because the membrane has to be stretched taut. The ossified Meckel’s cartilage would have held the malleus and ectotympanic in place. Thus, it would have functioned as part of the ear, rather than of the lower jaw, even though it was still connected to the jaw.

There has been a lively debate whether full separation of the middle-ear bones from the jaw took place just once or perhaps more than once during the early evolutionary history of mammals. If this change occurred only once in the common ancestor of mammals, the presence of an ossified Meckel’s cartilage in Liaoconodon and some other early mammals would represent an evolutionary reversal. This is possible because, based on modern developmental studies, such a reversal would merely require slight changes in the timing of certain developmental events.

The alternative scenario is that the common ancestor of mammals retained Meckel’s cartilage throughout its life. Complete separation of the ear bones from the lower jaw would then have evolved independently in no fewer than three lineages of mammals — one leading to present-day monotremes (echidnas and plat­ypus), a second to a group of superficially rodent-like extinct mammals known as multituberculates, and a third to the present-day marsupial and placental mammals.

Meng and his colleagues favor the alternative scenario and argue that Liaoconodon and its kin retained a tran­sitional type of middle ear, which must have been more efficient in transmitting airborne sound than the middle ear in the precursors of mammals because its ear bones were already relatively smaller, the incus had greater freedom of motion, and the other ear bones were separated from the lower jaw. The loose connection of the ossified Meckel’s cartilage to the lower jaw and the contact between Meckel’s cartilage and the ear bones would have enhanced hearing by separating it from feeding. However, the hinge joint between the malleus and incus and the still incomplete suspension of the eardrum suggest that the middle ear of Liaoconodon was less efficient in transmitting sound than that of present-day mammals.

The discovery of Liaoconodon fills in the last major gap in our understanding of one of the best-documented evolutionary transitions in the entire fossil record — the stepwise development of the mammalian ear bones from bones at the back of the lower jaw in the precursors of mammals. As my Smithsonian colleague John Burns put it so memorably in his Biograffiti, “With malleus aforethought, mammals got an earful of their ancestor’s jaw.”

Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time. A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.

Comments

  1. [...] Earful of Jawbones This is quite nice review/summary/explanation: http://newswatch.nationalgeographic.com/2011/04/25/an-earful-of-jawbones/ This entry was written by jeronen, posted on 29.4.2011 at 15.27, filed under News and tagged [...]