|Bones: Dermal Bones|
|The Vertebrates||Opercular Series: Opercular|
The opercular, sometimes called the "opercle" is, obviously, the first and principle component of the opercular series. As we have mentioned, it is likely that the opercular is serially homologous with the branchiostegal rays and that both began as a series of dermal opercular scales as is observed in some acanthodians. However, the embryology of the operculum in the zebrafish suggests that the operculum may also have some deep connection with the splanchnocranial (gill-arch derived) bones. It is probably too early to say much. This is another of those interesting regions in which two very different developmental domains converge. See the discussion in Meckel's Cartilage and the basisphenoid for other, much more complex, examples.
The desirability of having some kind of hinged cover for the gills is plain. Even the chondrichthyans, who have multiple gill slits and no dermal bone to speak of, have independently evolved the operculum twice, and probably three times (Holocephali, Iniopterygii & Hypnosqualea). However, the importance of this particular bone and its homologues varies considerably, even between taxa which are fairly closely related. Thus, for example, Porolepiformes had rather small opercula, while Actinistia have unusually large opercula. Among actinopterygians, the Amiidae have a large operculum, Pycnodontiformes had small opercula, and the Chondrostei may have none at all. In general, these differences seem to correlate with lifestyle factors, such as how hard the gills have to work to get oxygen and whether dirt and parasites are likely to gain entrance to the sensitive gill membranes.
This correlation lends itself to some interesting lines of speculation, if we may extrapolate the same observations back to earlier forms. First, were the early jawless fishes ecologically restricted because of the absence of opercula? If so, the operculum may have been a key ingredient in the radiation of fishes onto the broad, shallow continental seas of the Late Silurian and Devonian. Second, all but the most feeble gill covering must be mechanically coordinated with respiration or it will interfere with gill function. But once a muscular gill cover is coordinated with respiration, it gains the potential to assist in actively pumping water over the gills and so (a) increasing ecological range to more poorly aerated waters and (b) speeding oxidative metabolism in well-oxygenated waters. In fact, under the right circumstances, one might even imagine an early analogue of the opercular apparatus driving the development of jaws, rather than vice-versa.
The (at least tacit) assumption seems to be that the primitive operculum was only marginally coordinated with the jaw. However, as far back as we can study jaw mechanics with any confidence, there seems to have been some level of coordination. Thus, in Mimia and Cheirolepis the opercular is braced against the hyomandibular, which is braced against the palatoquadrate. Accordingly, when the jaw is opened, the opercular must move, although the specifics of its movement are not so easily determined. At least some acanthodians (Climatiiformes, in particular) seem to have had a similar arrangement, although reconstructing the details of cranial anatomy in this group still involves a fair amount of guesswork, and the mechanics are complicated by auxiliary gill covers which were not mechanically coupled to the hyomandibular in any obvious way. Interestingly, a preopercular is already present in many of these early fishes. The function of this latter bone is normally to couple the opercular series with the dermal bones of the jaw, i.e. the maxilla. So, its early appearance argues for an early mechanical relationship between operculum and jaw movements. This is also consistent with embryological evidence which shows that the operculum and preoperculum substantially precede the other elements of the opercular series in development.
Whenever muscular control of the operculum was established (presumably very early), it was probably based on the cleithrum. The cleithrum is the origin of the hypaxial musculature which generally serves to open the mouth and expand the gill chamber. In any case, the braincase and cleithrum are virtually the only stable platforms from which cranial muscles can act in these early fishes. The first evidence of a completely independent set of muscular controls comes from the Halecostomi. The halecostomes, of which Amia is an example, have an opercular dilator muscle originating on the mandible which opens the operculum. It is unclear just how novel this muscle really is at this level. Although it is apparently unknown in chondrosteans, it is believed to be homologous to lamprey velar muscles. Schilling & Kimmel (1997). Since chondrosteans have minimized the opercular as a whole, they are not a particularly good basis on which to draw any conclusions. These dilators are opposed by opercular adductors originating on the hyoid arch. Id.
What is truly new at the halecostome level is the m. levator operculi which runs the other way, so to speak, raising the opercular as a method for depressing the jaw. The levator muscle is what creates the "new jaw" of the neopterygians. As this has been discussed in the overview section, and in connection with Neopterygii, we will refrain from repeating it all again. ATW030207.