already occurred. When first encountered, at a somewhat 

 later time, the gnathostomes also are highly diversified as 

 to form. 



The fossil evidence can be interpreted as indicating that 

 the development of a bony armor occurred first among the 

 agnaths and that this group was more widespread in dis- 

 tribution and its individuals were more numerous than the 

 gnathostomes. The large gaps in the fossil record can only 

 be interpreted as failures to record vertebrates rather than 

 as evidence of their nonexistence. The nature of the fossil 

 record does not support the view that the vertebrates arose 

 only shortly before the first fossils, of Lower Ordovician age, 

 were entrapped. It is quite possible that vertebrates, or pro- 

 tovertebrates, were present in the Cambrian along with 

 representatives of other phyla. They may have been very 

 few in numbers, local in occurrence, and limited to habitats 

 in which fossilization was difficult or impossible. 



The fossil record suggests that the earliest vertebrates 

 were naked and that one of the first evolutionary trends 

 among vertebrates was an increase in size and the devel- 

 opment of some kind of support mechanism (skeleton), 

 either internal or external. The fossil record does not reveal 

 much as to the marine or fresh-water origin of these organ- 

 isms. The fact that the earliest fossils occur in marine or 

 brackish water deposits says little since these were already 

 widespread, highly developed, and diversified creatures. 



THE SHARK AS A PRIMITIVE VERTEBRATE 



As a second question, let us consider the generally held 

 view that the shark is a primitive vertebrate and is repre- 

 sentative of a type from which the higher forms have 

 stemmed. The general statements of this view vary some- 

 what. Some believe that Squalus is like the type, while others 

 are less positive and suggest that it is the most direct deriva- 

 tive of the common ancestor, many of whose features it re- 

 tains. The idea of the shark being like the ancestral vertebrate 

 is reflected by the palaeontologists continual references to 

 sharks, or shark-like vertebrates, in the early fossil record. 

 As we have come to know more about the structure of 

 vertebrates, and sharks, it is evident that the shark, as it is 

 now defined, was not present until the late Devonian and 

 was then quite distinct from the modern forms. The De- 

 vonian sharks are of several diverse kinds and these may not 

 be closely interrelated. 



The earliest gnathostomes (Upper Silurian, or if the 

 Palaeodus and Archodus denticles are teeth, the Lower Ordo- 

 vician) are described as acanthodians, and these are some- 

 times identified as sharks; however, the anatomy of an 

 acanthodian, as least as far as it is known, is quite differ- 

 ent from that of a shark. The general idea one can draw 

 from the fossil record is that the shark is only one of several 

 kinds of vertebrates produced by the first radiation; it is 

 not the ancestral type nor necessarily more similar to the 

 ancestral type than the others. 



Yet the value of the shark to comparative anatomy is 



still rather high, since it does retain many features of the 

 early vertebrate. What are some of these primitive features? 

 One cannot say that the amphistylic or hyostylic jaw sus- 

 pension of the shark is any more primitive than the kinds 

 of jaw suspensions observed in palaeoniscoid or primitive 

 choanates. In fact, it is doubtful that the palatoquadrate of 

 the shark includes the autopalatine portion observed in the 

 osteichthian. The orbital process of the shark palatoquadrate 

 is perhaps a parallel development comparable to the epi- 

 pterygoid. The solidly roofed chondrocranium may be some- 

 what more primitive than the more open roof observed in 

 other groups, but this solid roof might also be a modification 

 reflecting the simple observation that the shark did not 

 develop a dermal roof of bones. The variations in chon- 

 drocrania observed in the shark, acanthodians, and arthro- 

 dires suggest an early period of experimentation. The type 

 observed in the palaeoniscoid fishes and choanates may 

 represent an independent development. 



The circulatory system, the digestive system, the excretory 

 system of the shark seem to retain many, if not most, of the 

 ancestral features, and it is in this area of soft anatomy that 

 the shark is most revealing. However, the development of a 

 pronephric Mullerian duct in the shark and the peculiar 

 holonephric kidney may be specializations peculiar to this 

 group. The rectal gland of the shark, although it may be 

 homologous to the bladder or allantois of higher forms, is 

 unique in terms of its detailed structure. In terms of the fin- 

 fold theory, it has already been pointed out that the fin of 

 the shark may be primitive only for a part of the vertebrate 

 array; it is not primitive for all kinds of gnathostomes and 

 probably not for the Osteichthyes. 



THE ORIGIN OF THE TETRAPODS 



A third area of interest concerns the origin of the tetrapods. 

 It is generally believed (Barrell, 1916) that the Devonian 

 was a dry period, during which many ponds and streams 

 tended to dry up seasonally, and that certain crossopteryg- 

 ian fishes were better able to move from drying pools to 

 those of a less transitory nature. This thesis accounts for the 

 development of the limb of the amphibian through the 

 necessity of periodic escapes from drying pools. .Although 

 this view is generally accepted, it is now under attack. The 

 attack is based on the observation that escape from drying 

 pools would occur at a time when the moist skin of the fish 

 would suffer most from drying. Movement over land by 

 fishes is best done under moist conditions, not dry condi- 

 tions. Further, movement over land can be accomplished 

 by the use of fins, not necessarily by the use of limbs. 

 It would seem more probable that the fin would be of more 

 use to an aquatic form and would be as usable as a limb 

 for short periodic escape jaunts over dry land. The idea 

 that this periodic escape led directly to terrestrial habitation 

 by these vertebrates was rejected by Romer (1958), for he 

 finds it is fairly evident from the fossil record that am- 

 phibians and early reptiles were water inhabitants until 



444 • A SUMMATION OF CHORDATE MORPHOLOGY 



