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Craniofacial Growth, Evolutionary Questions Development 103 Supplement, 3-15 (1988) Printed in Great Britain © The Company of Biologists Limited 1988 Craniofacial growth, evolutionary questions CARL GANS Department of Biology, The University of Michigan, Ann Arbor, Michigan 48109-1048, USA Contents Transition to gnathostomes Diversity offish heads Introduction Fish jaws Theory Metamorphosis Principles Transition to terrestrial tetrapody Tasks of the craniofacial system Avian adaptations The process of change The mammalian condition Predictions from the record Concluding discussion Functional stages in the vertebrate head The system in the prevertebrates Key words: functional morphology, cephalization, cranial Transition to vertebrates kinesis, craniofacial evolution, adaptation. Introduction involved. In contrast, the concept of history docu- ments that adaptation does not act de novo to Understanding the growth of craniofacial systems in generate phenotypes out of infinitely plastic raw mammals, particularly in man, has always posed material. As the phenotypes of extant organisms problems. Such craniofacial systems are formed onto- derive from those of ancestral ones, the phenotypic genetically of multiple tissue types, and the contri- match to new environments reflects the genetic and butions of these tissues do not obviously match the developmental plasticity of possible precursor divisions of adult skeletal elements (see Thorogood, organisms. this volume). Even the kind and number of segments This evolutionary viewpoint is here utilized in a in the head region continue to attract attention brief review. It begins with the primary roles of the (Maderson, 1987). Furthermore, craniofacial systems structures in fishes that are homologous to the cranio- appear to show trends toward an unusual number of facial system of mammals. (Role is here considered to developmental abnormalities or teratologies. Many be that part of the function of a phenotypic aspect of these teratologies suggest that we are not looking that contributes evolutionary benefit, i.e. for those at a simple coordinated whole (Salinas, 1982; Shprint- organisms that have it, the role of a structure is its zen, 1982); rather, it seems as if multiple cranial and adaptive function in a particular environment.) In the facial components incur differential growth either process, I attempt to clarify some aspects about the symmetrically or asymmetrically. way the fossil and surviving members of early ver- It seems instructive to treat the basis of this curious tebrate groups should be considered. array of complications from an evolutionary view- This evolutionary analysis is based upon past point, considering two aspects, adaptation and his- scenario generation and is obviously reconstructive. tory. Adaptation implies that the structures of organ- There being but a limited fossil record, the gaps are isms are not randomly assembled, but have been and filled by extrapolations based upon the structure, remain at any moment under the influence of selec- physiology and ecology of animals living today (Gans, tion (Gans, 1988). Implicit within this concept of 1985). After developing the roles of the system in adaptation is that each phenotypic array must incur fishes, I shall review the several changes of role from both a cost and a benefit, the cost in terms of fishes to mammals. Throughout, I emphasize the generating and of maintaining the phenotype, the components of the head, the separate changes each benefit in terms of the roles in which the phenotype is has undergone and the aspects that keep them associ- C. Gans ated. It is hoped that such a data set will provide a framework setting the stage for interpretation of the more developmental aspects. Theory Principles The craniofacial system of mammals is supported by a skeletal array consisting of a braincase, sensory capsules and an 'upper jaw' generally fused into a single unit, a single lower jaw composed of bilateral mandibular units more or less tightly attached to that of the opposite side and a varied set of skeletal elements in the throat (Fig. 1). Just posterior to the jaw joint lies the auditory meatus leading to the middle ear, incorporating several reduced skeletal elements once involved with the jaw suspension (Van de Water et al. 1980). The historical view will document that the mam- malian craniofacial system derives from several initially independent components which probably account for the extent and nature of the observed teratologies. Each of them continues to incur and respond to independent demands of the environ- ment. It is unlikely that the demands occurred in parallel or were equivalent. Hence, the assembly will reflect different initial (and subsequent) roles, and shifts incurred in their component materials and proportions that are coordinated neither phylogeneti- cally nor ontogenetically. Tasks of the craniofacial system The craniofacial system of mammals is homologous to parts of the anterior end of the earliest vertebrates. As such, it provided the stiffening and reinforcement of this end, precluding deformation during pen- etration of liquid or solid aspects of the environment. Next, it encapsulated the central nervous system, protecting it both from anteriorly and posteriorly directed forces. The system also permitted the an- terior end to bear paired, external sensory organs, protecting them and permitting them accurately to scan the anterior environment by maintaining their position relative to the axis of the trunk. The com- ponents performing most of the roles thus far listed presumably were homologous to the mammalian cranial component. The anterior end of vertebrates retains the front of the alimentary canal and the external openings of the gas exchange system. The craniofacial system conse- quently must maintain its patency during the acqui- sition and ingestion of nutrients and respiratory fluids. Whereas these tasks are the major ones in Fig. 1. Three sketches of a cat skull each with one major mammals, earlier vertebrates incorporated extensive functional system emphasized by shading. (A) Sensory portions of the anterior pharyngeal tube in the system; (B) braincase; (C) mandibular apparatus. Craniofacial growth, evolutionary questions 5 cephalic structures, and all of these, as well, were tions of evolutionary change requires information associated with the tasks of food handling and gas about the genetics of their developmental mechan- extraction. This is a second set of roles for the isms. There are too many unknowns, making the structures contributing to what later became known system too complex for facile analysis. as the facial component. However, it is possible to predict the kinds of genetic and hence phenotypic changes that would be The process of change likely in such a framework. It is probable that there The mechanism of evolutionary change, the in- will be a relatively large number of what might be vasions of adaptive niches and radiation into these, considered to be variations on a theme; thus, many are fundamental to understanding evolutionary pro- different phenotypes will be produced by relatively cesses. As in all our attempts to trace history, the small changes in their various components. This Recent animals we see are but remnants of past would be impossible if the shape and growth rate of historical processes. The meaning of this idea of individual structural elements, for instance of the remnant is important because it establishes the kinds skeleton, were to be determined on what might be of biological conclusions permitted on the basis of called a one character/one gene basis. However, we surviving members of a group. know that changes are interactive and coevolved so We must begin with a population or populations of that modification of individual components is still organisms characterized by a set of phenotypes allow- likely to generate viable phenotypes after a number ing them to make a living in the niches they then of minor genetic changes. occupy. For various reasons, a substantial fraction of There remains the potential for more substantial the population will have phenotypes that exceed the phenotypic changes, resulting from mutations which minimal level characterized as necessary (Gans, are minor with respect to gene alteration, but major 1979). This lets them be opportunistic, to 'test' the in their effect on the phenotype. Such early minor edges of the environmental niche they occupy and changes in the sequence forming a particular charac- sometimes to discover new sites located in ranges teristic (Gans, 1987), will affect more of the develop- peripheral to their ancestral ones. mental pathway and, by implication, more pathways. The earliest invaders of a 'new' niche may be less However, in stochastic terms, viable changes of this effective at exploiting its resources than will sub- magnitude are likely to be less frequent; the greater sequent forms. Adequacy rather than perfection is the change the less the potential that the resulting demanded. The new habitat imposes new selective phenotype will be viable and reproductively success- factors and may lead to 'improved' genotypes. ful. If the resources of the new habitat are living ones, Now, what has all this to do with fish heads, they will in turn incur advantages for defence mech- whether pertaining to fishes surviving now or to those anisms or other aspects that make the task of the seen in the fossil record? First of all, it explains the predator more difficult. From this viewpoint, the Red seemingly random experimentation
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