Popular Science Monthly/Volume 77/August 1910/Contributions to Morphology from Paleontology
| CONTRIBUTIONS TO MORPHOLOGY FROM PALEONTOLOGY |
By Professor WILLIAM BULLOCK CLARK
THE JOHNS HOPKINS UNIVERSITY
OUR knowledge of the morphology both of the animal and plant kingdoms has been largely extended by the work of the paleontologist. Mention needs only to be made of the many species, genera and families, even orders and classes, established solely for fossil forms to show how much we owe to paleontology. There is not a single sub-kingdom but has been immensely enriched from this source.
Some of the fossil species possess morphological characters so closely allied, on the one hand to earlier, and on the other to later, forms as to indicate that they occupy a position in the line of descent, and phylogenetic series have been established frequently on this basis. As examples we have the well-known developmental series of the horse and the camel. Other illustrations may be found in the Paludinas of the Slavonian Pliocene and in the Planorbis types of Steinheim.
Still other fossil forms combine in the same species several morphological features which later become segregated and characterize different types. Such "synthetic types" serve to show the common origin of the forms in question if not their actual ancestors and have greatly enlarged our knowledge of the morphology of the several groups involved. These early forms are, for the most part, highly generalized, while their descendents are variously specialized. Take, for example, the mammalian Condylartha, small, generalized Ungulata with an astragalus shaped almost as in the Carnivora; or the reptilian Anomodontia with intermediate skeletal characters between the highest labyrinthodonts and the lowest mammals; or again, the early Paleozoic cystoids with generalized characters in their calyx plates which appear in more specialized forms in later crinoids and blastoids. An almost indefinite number of such illustrations might be cited.
Still other fossil forms present morphological characters so different from other fossil or living species that the genetic relationships may not be determined accurately. Some of these are possible of reference to already defined orders, while others present so many diverse morphological characters as to require the establishment of new divisions for their reception.
A survey of the known fossil and living forms shows that not only have old species constantly become extinct during the progress of geological time, but new species have been as frequently appearing. This is equally true of genera, families, orders or even classes. Some forms have appeared and disappeared, as the case may be, suddenly; others slowly. The great group of the Ammonites, for example, disappeared suddenly at the close of the Cretaceous after showing many degenerate characters, while the Trilobites gradually declined during late Paleozoic time before their final extinction. One of the most striking features in the developmental history of plants and animals is found in the great number of fossil types which have left no descendants.
Both the animal and plant kingdoms furnish a wealth of material with which to demonstrate the aid which paleontology has rendered to morphology.
The contributions of invertebrate paleontology are numerous and striking:
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The Protozoa afford in the Carboniferous Fusulinidæ and in the Tertiary Nummulinidæ forms with very different morphological characters from those living today, while the numerous extinct species of the Lituolidæ and Textularidæ in the Cretaceous and of the Miliolidæ and Globigerinidæ in the Tertiary have greatly widened our knowledge of the entire subkingdom.
The Cœlenterata in the Paleozoic Tabulata and Graptoloidea show types so different from living forms that the systematist has never been able to satisfactorily assign them to a position within the limits of the phylum. Many external and internal characters appear that are quite unknown in later forms. On the other hand, the paleontological subclass of the Tetracoralla long imperfectly understood is now regarded with a fuller knowledge of the morphology as affording the probable ancestors of the later Hexacoralla.
The Echinodermata have furnished two classes, the Cystoidea and the Blastoidea, unknown after the Paleozoic, whose morphology aids very materially in an interpretation of later and more highly differentiated forms among the Pelmatozoa. Thus the cystoids, which have been regarded as the ancestral type from which the crinoids have sprung, afford forms like the Camarocystites, in which the arms are similar to those of the crinoids although the calyx plates are irregularly arranged and thus cystoidean in character. Both the Asterozoa and Echinozoa are represented in the fossil state by many species that greatly widen our knowledge of the morphology of this group. Take for example, the Echinocystites, regarded as belonging to the Palechinodea which has a valvular pyramid of calcareous anal plates so highly characteristic of the cystoids.
The Molluscoidea, to which phylum belong the Bryozoa and Brachiopoda, would be but imperfectly understood from a morphological standpoint but for the vast number of fossil forms. The Brachiopoda have been estimated to have less than 150 living species, while probably more than 6,000 fossil species have been described. Of the 31 families only 7 have living representatives. We are dependent, therefore, largely on the fossil forms for our knowledge of the morphology of this class.
The Mollusca with their varied forms, although so well represented to-day, have furnished in the fossil state one of the most interesting and important orders in the animal kingdom, the Ammonoidea with its 5,000 and more species ranging from the Devonian to the Cretaceous. Even the allied Nautiloidea, although containing living forms, attained its chief development in the Paleozoic, and it is from these ancient forms that we obtain our chief knowledge of the morphology of this group with their early straight and irregularly coiled types.
The Arthropoda afford in the Paleozoic the important groups of the trilobites and euripterids, forms that have aided greatly in the interpretation of the entire phylum. The trilobites from their morphological features have been generally regarded as entomostracous crustaceans with relationships on the one hand to the Phyllopoda and on the other to the Merostomata, while the coalescing of the caudal segments suggests also a relationship to the Isopoda.
Vertebrate paleontology has also furnished much to morphology.
The Fishes would be but imperfectly known in their wonderful variety but for the fossil types. The problematical group Agnatha found only in the Silurian and Devonian affords no certain evidence of a lower jaw or paired limbs, and in some of the genera of the Ostracoderma mimic in a curious way the contemporaneous euripterids, which has led some to erroneously ally them with the Merostomata. The dermal armor of most of these forms is also a striking morphological feature.
Woodward divides the fishes proper into Elasmobranchii, Holocephali, Dipnoi and Teleostomi, and considers that the common ancestors of all were Elasmobranchs. Numerous fossil forms among the Elasmobranchs and Dipnoids as well as the Crossopterygians which have been thought by many to bridge the gap between the Telelostomi and Dipnoi have added largely to our knowledge of the phylum.
The Batrachians which consist to-day largely of diminutive forms were represented in the later Paleozoic and early Mesozoic by the Stegocephalia which contain the giant labyrinthodonts with their highly complex infolding of the walls of the teeth.
The Reptilians which began their existence toward the close of the Paleozoic became so numerous and diversified during the Mesozoic that this division of geological time has been referred to as the age of reptiles. Several orders of Saurians containing many giant types flourished during this time, but became practically extinct before the close of the period. With the adaptation of some for walking on their hind legs, of others for swimming, and still others for flight we have developed a great variety of morphological features that would never have been suspected from a study of living forms.
The Birds which are recognized as possessing certain dinosaurian relationships and were doubtless derived from one of the reptilian orders are unknown prior to the Jurassic. The Mesozoic forms are generalized, the tail at first not being atrophied and the pelvis imperfectly developed as in later forms. The vertebra? also had not acquired their saddle-shaped articulation while teeth were present in the jaws of the adults. Such forms certainly add greatly to our knowledge of the morphology of this class.
The Mammals which began in the early Mesozoic were represented throughout the Cenozoic time by highly diversified forms, many of which have left no descendants. The gradual evolution of the mammalian skeleton has brought about many morphological modifications from those shown in the Batrachia and Reptilia. We find the skull loses the prefrontal and postfrontal bones, the mandible is simplified, the limb bones show a development of terminal epiphyses with ossification to the center of the vertebras and the bones of the pelvic arch are ossified. From the beginning of the Tertiary time a marvelous variety of morphological characters appears, and without the fossil types we should have but an inadequate conception of this great phylum.
The contributions of paleobotany to morphology are in some respects quite as striking as those of paleozoology.
The fossil Thallophytes have not furnished any very striking variations from their present morphological features, while the Bryophytes are scarcely represented as fossils except in very recent deposits.
The remaining phyla, the Pteridospermatophytes, the Pteridopliytes and the Spermatophytes have their oldest known beginnings as far back as the Devonian and their study has enormously widened the bounds of plant morphology.
The Pteridospermatophytes, which are confined to the Paleozoic, are in habit and vegetative morphology ferns—in methods of reproduction and in the morphology of their reproductive organs typical seed plants. They alter our whole conception of ferns and seed plants and in their significance are comparable to archetypal vertebrata, the acquisition of the seed habit in plants and the vertebral column in animals probably marking the culmination of the transfer of vital activity from aquatic to terrestrial conditions.
In the Pteridophytes the extinct Paleozoic class, the Sphenophyllales, is significant, since the morphology of the distinct lycopod and Equisetum lines seems to merge in this group. The lycopod type, itself represented in the existing flora by six or seven genera of herbaceous plants, monotonously uniform in their morphology, is found in the Paleozoic to constitute one of the chief units in the arborescent flora with numerous species of complex organization, whose stem, foliar and reproductive morphology was quite unknown to botanists (Lepidodendron, Sigillaria, etc.). The Equisetum type furnishes a like case. With few existing species of minor importance and uniform morphology we find in the Paleozoic a host of forms, many of them arborescent and of varied and complex structure {Calamites, Archceocalamites, etc.). Similar examples could be drawn from the fossil representatives of the true ferns.
In the Spermatophytes another wholly extinct class, the Cordaitales, embraces a curiously organized group of conifers extending back to the oldest horizons from which land plants are found, and continuing to the close of the Paleozoic as one of the most abundant as well as the highest type of pre-Mesozoic plant. In the older Mesozoic we find two groups of plants which have made similar great contributions to morphology. The Cycadales or cycad-like plants, which to-day are an inconspicuous group, were one of the dominant Mesozoic types, and any understanding of the modern forms rests entirely upon a study of their immensely abundant Mesozoic ancestors. The other group, the Ginkgoales, represented in the existing flora by a single species, the ginkgo, is found in the Mesozoic to have been represented by many genera and species of great diversity.
The dominant plants of to-day, the conifers on the one hand, and the angiosperms on the other, have each afforded many extinct genera, the former with more fossil than recent species, and only understandable in the light of their fossil ancestors. Vegetable morphology based only upon existing plants abundantly demonstrated its sterility before the relative recent study of fossil plants placed it upon an altogether new basis.