October, 1928 



EVOLUTION 



Page Five 



The Earliest Backboned Animals 



By Frederic A. Lucas, 



Honorary Director, American Museum oj Nalural Histoiy 



w 



E all like to know how ihings began and it is natural that 

 we should ask, "What is the oldest known backboned 

 animal and what did it look like?" Nature has been 

 questioned in many ways to yield a satisfactory answer, the most 

 direct being to trace back the history of animal life from 

 fossil remains. But this method cannot go beyond 

 a certain point, for the soft bodies of the very earli- 

 est animals were not preserved. To help out, the 

 embryologist studies the early stages of developing 

 individual animals, for these throw sidelights on 

 the race history. Finally, invertebrates are studied, 

 the structural pans of some being like those of 

 vertebrates, while others actually are vertebrates in 

 disguise. But these methods, have yielded variant 

 answers, or the replies have been variously interpret- 

 ed, so that some naturalists consider vertebrates 

 descended from worms, others find their beginnings 

 in crustaceans kindred to tlie King Crab. 



Everyone knows how hard it is to trace a jamily 

 pedigree back a few centuries, how the family name 

 becomes changed and the line of descent obscure 

 and how wide gaps soon appear. It is, of course, 

 much more difficult to trace the pedigree of a race 

 that extends, not over a few centuries, but over 

 millions of years. The word "old" as we apply it 

 to fossils has only vague meaning, for we call a 

 family old whose pedigree runs back four or five 

 hundred years, when it is actually but an affair of 

 yesterday compared with even recent fossils. We may 

 appreciate this better by recalling that, since the dawn of ver- 

 tebrate life, enough of the earth's surface has been worn away 

 and washed into the sea to form strata fifteen to twenty miles 

 thick. 



At the bottom, then, of twenty miles of rocks the naturalist 

 finds the first scant remains of fish-like prevertebrates, the 

 forerunners of the back-boned animals. The earliest consist 

 of small bony plates, traces of cartilaginous back-bones, and 

 some small conical teeth thought to be those of an animal like 

 the lamprey. A little higher in the rocks, but not in the scale 

 of life, are found a better preserved fish-like creature not over 

 two inches long which is also related (probably) to the lamprey 

 and the hag-fishes. These early vertebrates were so small and 

 soft as to be preserved only when buried in mud immediately 

 after death. Under the later pressure of miles of overlying 

 rocks, their remains are often pressed out thinner than a sheet 

 of paper, so that their shadowy outlines are very hard to trace. 

 With such drawbacks to contend with, it is no wonder that 

 naturalists should differ as to their relationships. 



Still higher up we find abundant remains of small fish-like 

 animals, clad in bony armor, indicating that they lived in 

 troublous times when only the well protected could survive. Their 

 plate armor enabled them to defy their enemies or rendered 

 them such poor eating as not to be worth the taking. 



Pterichthys, the wing fish, was another small, quaint, armor 

 clad animal whose remains were once taken for those of a crab. 

 Its buckler, the part usually preserved, with its jointed, bony 



Diniclitli.vs. a giant 



Devonian Fisli. as ri>- 



stored by Dr. Hussaliot. 



Courtesy 



Am. Mus. Nat. Hist. 



arms, certainly looks more like a crustacean than a fish. From 

 occasional well preserved specimens we have a very exact idea 

 of its protective cuirass. It had a mouth, of course, but no 

 jaws, for the two do not necessarily go together. Also it had no 

 hard backbone and the hard parts of its fins or arms were not 

 inside but outside, like the legs of a crab. 



These fishes and their allies were once the domin- 

 ant type of life. In favored localities we find great 

 deposits of their protective shields jumbled together. 

 It is thought that portions of the Old Red Sandstone 

 owe their peculiar tenacity to these dead fish. Just 

 as boiling a plaster cast in oil makes it more 

 durable, so the oil from these rotting fish would 

 toughen the surrounding sandstone. Great numbers 

 were probably caught in tidal pools along shallow 

 shores and died when the pools were drained by 

 the tide going out. 



These fishes were little fellows and may be termed 

 orphans of the past as they have no living relatives. 

 Their immediate successors, preserved in the Devoni- 

 an strata, were the giants of those days, termed, 

 from their size and presumably fierce appearance, 

 Titanichthys and Dinichthys (terrible fish) and are 

 related to the living Ceratodus of Australia. We 

 know almost nothing of their external appearance, 

 however, for though they had powerful jaws and 

 armored heads, they had no bony skeletons, as if 

 they devoted their energies to preying on iheir 

 neighbors rather than on internal improvement. They 

 attained lengths of ten to eighteen feet; Titanichthys, with a gape 

 of four feet, being able to devour anything known to live then. 

 Succeeding them, in Carboniferous times, came a host of shark- 

 like fishes, known mainly from their teeth and spines, their skel- 

 etons being only cartilage. Almost their only living relative 

 is the little Port Jackson Shark. He also has a spine in front 

 of his back fin and fortunately a mouthful of diversely shaped 

 teeth; fortunately, because through their aid we can form some 

 idea of the arrangements of the teeth found scattered through 

 the rocks. For the teeth were not planted in sockets, as in higher 

 animals, but simply rested on the jaws and were easily detached 

 after death. To complicate matters, the teeth in different parts 

 of the jaws were so unlike that when found separated they would 

 hardly be suspected of belonging together. As the soft parts 

 perished, the teeth and spines were naturally scattered, but from 

 rare specimens that show, not only these hard parts, but the 

 faint imprint of body and fins, we have learned just what teeth 

 and spines go together and often find that one fish has received 

 names enough for an entire school. 



These ancient sharks were not the large and powerful species 

 of today— these came upon the scene later— but were small and 

 fitted (as their spines show) quite as much for defence as 

 offence. But they quickly became the masters of the world, then 

 as quickly declined in numbers, eventually almost to extinction. 

 iWhile sharks again increased, they never reached their former 

 abundance and the species that arose were swift, predatory forms, 

 better fitted to survive in the struggle for existence. 



Pterichthys, the Wing Fish. 



Cephalaspis, An Ancient Armored I're-Fish. 



Loricaria, a Modern Armored True Fish 



