15 



A Summation of Chordate 



Morphology 



One of the desired goals of this text is to stimulate the 

 student to make independent formulations about chordate 

 morphology from his own summation of the facts and con- 

 cepts presented. There are, however, still many aspects of 

 such a summation which might be commented on before 

 any arrangement of vertebrate types is attempted. These last 

 comments are offered with the hope that they will inspire 

 further adventures into chordate morphology. 



It should be stated first of all that one can devise a 

 phylogenetic classification of the chordates, based on their 

 apparent morphological evolution, from the materials of this 

 survey, for similar surveys have been the source of our cur- 

 rent classifications. Although the materials now available to 

 the student are certainly not complete, they are as adequate 

 as those available to many students in the past. The student 

 should approach the formulation of a phylogeny (or classi- 

 fication) as a reasearch effort at the border of our knowledge, 

 not as a sterile exercise. This experience in systematics in- 

 volves not only information on structure but also ways in 

 which structure can be interpreted and utilized in a classi- 

 fication. An understanding of vertebrate systematics also 

 includes a realization of the limitations of any classification. 

 At this point the student should be aware that although the 

 evolutionary process is strongly indicated, the pattern of 

 phylogeny is quite hazy, but worthy of consideration. 



TERMINOLOGY AND DEFINITION 



Throughout this survey the problem of the definition of 

 terms has been encountered again and again: terms applied 

 to anatomical details or complexes, terms used at every level 

 of description or discussion. The vagaries of terminology 

 may leave the impression that comparative morphology is 

 plagued more than other areas of science. This of course is 

 not the case; semantics and definition are constant problems 

 for every scientific field. One has only to penetrate the 

 superficial aspects of any subject to encounter them. 



Terms of special interest in the formulation of a phy- 

 logeny are primitive and advanced (or specialized), as they 



are applied in an evolutionary sense. A primitive feature is 

 one presumed to be present in the common ancestor of any 

 taxonomic grade or level. From this ancestral condition by 

 specialization (radiation) modifications are produced and 

 these adaptive changes are considered advanced features. A 

 primitive feature of a particular group (genus, family, 

 order, etc.) may be so identified because all, or most, species 

 show it, because it is shared with other similar groups, or 

 because it is the simplest condition known. These criteria do 

 not always succeed in identifying the starting point of an 

 anatomical sequence. For example, most fishes, fossil and 

 living, have some sort of an operculum. Was this the prim- 

 itive stage or did the pharyngeal slits open separately to the 

 outside as in the sharks and some agnaths? The operculate 

 condition may have stemmed from one in which the slits 

 opened into bilateral atrial pouches or it may represent a 

 specialization achieved separately in each of the several 

 lines of fishes. In the case of lungs there is a very real ques- 

 tion whether lungs were present in the primitive vertebrate 

 and lost in a few of its derivative lines. There is also the 

 problem whether all lungs (primitively present or achieved 

 in some fishes) opened at first ventrally, laterally, or dorsally. 

 Usually the first view is maintained. 



In approaching a morphological definition of groups, one 

 can be impressed more by differences than by similarities. 

 These terms involve opposite approaches to comparisons 

 and each can produce a different end product. On the one 

 hand, the mammals might be viewed as constituting a well- 

 defined and homogeneous group, while, on the other hand, 

 their blending with the therapsids through several lines of 

 descent could be considered as evidence against recognition 

 of such a group. In systematics, one works with terms which 

 come to be defined in a manner that fits the materials in a 

 way that is thoroughly understood only by the person who 

 knows the materials. Even observations tend to differ in 

 terms of the background of the observer and the emphasis 

 he places on differences or similarities. An understanding of 

 the characteristics or features on which a phylogeny is 

 based is difficult to transmit from one person to another; 

 the shades of meaning and significance involved are open 

 to many interpretations. 



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