Issue link: https://wardsworld.wardsci.com/i/1491083
5 + ward ' s science them) are distinctive and quite possibly represent phyla or at least major branches of phyla that soon became extinct, geo- logically speaking. The soft-bodied groups were also markedly diversified, although their record is so spotty that their history cannot be traced in detail. A single, exceptionally preserved soft-bodied fauna from the Burgess Shale of British Columbia, which is about 530 million years old, contains not only living soft-bodied worm phyla, but extinct groups (perhaps a dozen) that cannot be placed in living phyla and do not seem to be ancestral to them. Among these is Wiwaxia, a novel form that bears spinelike elements similar to some of the small shelly fossils of Early Cambrian age, thus providing evidence that the small shelly forms did indeed include novel types. Clearly, an early radiation of animals produced a vast array of novel body plans in an evolutionary episode that has never been surpassed. Following the early phase of rampant diversification and of some concurrent extinction of phyla and their major branches, the subsequent history of the durably skeletonized groups can be followed in a general way in the marine fossil record (Fig. 3). The composition of the fauna changed continually, but three major associations can be seen: one dominated by the arthro- podlike trilobites during the early Paleozoic [panel (a) of Fig. 3], one dominated by articulate brachiopods and crinoids (Echi- nodermata) in the remaining Paleozoic [panel (b) of Fig. 3], and one dominated by gastropod (snail) and bivalve (clam) mollusks during the Mesozoic and Cenozoic [panel (c) of Fig. 3]. The mass extinction at the close of the Paleozoic that caused the contrac- tions in so many groups (Fig. 3) may have extirpated more than 90% of marine species and led to a reorganization of marine community structure and composition into a modern mode. Resistance to this and other extinctions seems to have been a major factor in the rise of successive groups to dominance. On land, the fossil record is too spotty to trace in detail the inva- sions of the terrestrial environment, which must have been under way by the mid-Paleozoic. Annelids, arthropods, and mol- lusks are the more important invertebrate groups that made the transition to land. The outstanding feature of terrestrial fauna is the importance of the insects, which appeared in the late Paleo- zoic and later radiated to produce several million living species, surpassing all other life forms combined in this respect. Chordate origins The phylum Chordata consists largely of animals with a backbone, that is, the Vertebrata, including humans (Fig. 4). The group, however, includes some primitive nonvertebrates, termed the protochordates: lancelets, tunicates, acorn worms, pterobranchs, and possibly the extinct graptolites and con- odonts. The interrelationships of these forms are not well understood. With the exception of the colonial graptolites, they are soft-bodied and have only a very limited fossil record. They suggest possible links to the Echinodermata in developmental, biochemical, and morphological features. In addition, some early Paleozoic fossils, the carpoids, have been classified alter- natively as chordates and as echinoderms by various investiga- tors, again suggesting a link. In spite of these various leads, the origin of the chordates remains basically unclear. Chordates are characterized by a hollow, dorsal, axial nerve chord, a ventral heart, a system of slits in the pharynx that serves variously the functions of feeding and respiration, a postanal swimming tail, and a notochord that is an elongate supporting structure lying immediately below the nerve chord. The protochordates were segmented, although sessile forms (such as the tunicates) show this only in the swimming, larval phase. Even the free-swimming forms were not truly active, and they used the gill apparatus primarily for feeding on small particles trapped on mucous streams generated in the pharynx. There are two hypotheses concerning the nature of the proto- chordates. One holds that all protochordates were originally sessile, although having active larvae, and that the vertebrates arose from the larval phase through increased development of the segmented muscular trunk and associated nervous system. The alternative hypothesis proposes that the sessile condition seen in many protochordates is secondary. The first vertebrates were fishlike animals in which the pharyn- geal slits formed a series of pouches that functioned as respira- tory gills. An anterior specialized mouth permitted ingestion of food items large in comparison with those of the filter-feeding protochordates. Vertebrates are first known from bone frag- ments found in rocks of Cambrian age, but more complete remains have come from the Middle Ordovician. Innovations, related to greater musculoskeletal activity, included the origin of a supporting skeleton of cartilage and bone, a larger brain, and three pairs of cranial sense organs (nose, eyes, and ears). At first, the osseous skeleton served as protective scales in the skin, as a supplement to the notochord, and as a casing around the brain. In later vertebrates, the adult notochord is largely or wholly replaced by bone, which encloses the nerve chord to form a true backbone. All vertebrates have a heart that pumps blood through capillaries, where exchanges of gases with the external media take place. The blood contains hemoglobin in special cells that carry oxygen and carbon dioxide. During the exchanges, the oxygen content of the cells is increased and the content of carbon dioxide is decreased. In most fishes, the blood passes from the heart to the gills and thence to the brain and other parts of the body. In most tetrapods and in some fishes, blood passes to the lungs, is returned to the heart after oxygen- ation, and is then pumped to the various parts of the body. Animal Evolution (continued)