VERTEBRATES, OVERVIEW Carl Gans* and Christopher J. Bell† *Department of Integrative Biology, University of Texas at Austin and †Department of Geological Sciences, University of Texas at Austin I. Introduction neurectoderm An embryonic tissue that gives rise to II. General Vertebrate Characteristics the central tube of the nervous system. III. Early Chordate and Vertebrate History notochord A stiff, flexible, longitudinal rod running IV. Vertebrate Classification along the middorsal portion of the chordate body. V. Definitions and Diagnoses of Major Chordate It is situated dorsal to the coelom and ventral to the Groups central tube of the nervous system. pharynx The anterior portion of the alimentary canal, characterized by lateral buds that provide skeletal GLOSSARY support for the gill region. tuberculum interglenoideum An anterior projection of chordate A member of the group Chordata. The the first (cervical) vertebra in salamanders. The tu- Chordata includes the most recent common ancestor berculum interglenoideum bears articular facets that of tunicates and cephalochordates and all of that insert into the foramen magnum of the skull and ancestor’s descendants. Tunicates, lancelets, hag- provide additional articulation points between the fishes, and vertebrates are all chordates. skull and the vertebral column. ectoderm An embryonic tissue that provides the future outside layer of the animal. ectothermy A method of body temperature control in which the animal utilizes external sources for gaining VERTEBRATES INCLUDE ALL the fishes, amphibians, and giving up heat, thus achieving temperature con- reptiles, birds, and mammals. These animals are united trol without affecting metabolic rate. in a more inclusive group, the Chordata, that includes endothermy A method of body temperature control in the closest living relatives of vertebrates, the hagfishes, which the animal modifies its metabolic rate to lancelets, and tunicates. There are approximately achieve the desired body temperature. 54,450 known species of chordates, over 51,000 of neural crest An embryonic tissue intermediate be- which are classified as members of Vertebrata. Nearly tween neurectoderm and ectoderm, with cells mi- half (approximately 24,000) of the known species of grating widely to their final destination. This tissue vertebrates are members of a single group of ray-finned gives rise to anterior skeletal elements, many por- fishes, the actinopterygians. The vertebrates are found tions of the future head and pharynx, and all pigment on all major land masses and in all major oceans and cells. Sometimes also referred to as mesectoderm. seas on Earth. Encyclopedia of Biodiversity, Volume 5 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 755 756 VERTEBRATES, OVERVIEW I. INTRODUCTION basal chordates or echinoderms), or historical stages in the development of groups of vertebrates (i.e., fishes, The vertebrates include most of the major groups of reptiles, and mammals). animals that humans encounter and interact with on Although a few arthropods and mollusks achieve Earth, and in common parlance, the term ‘‘vertebrate’’ large, indeed giant, sizes, most of the largest species, is often equivalent to ‘‘animal.’’ Vertebrates include all whether terrestrial, aquatic, or aerial, are vertebrates, the fishes, amphibians, reptiles, birds, and mammals; and this latter group includes the largest animals they are found on every major landmass and in all of known. However, some vertebrates are small, almost the world’s oceans and major seas. The fossil record of tiny, and this poses interesting questions of how they the group is excellent and extends back to the late perform the basic functions of reproduction, locomo- Cambrian period of the early Paleozoic, more than 500 tion, respiration, food processing, and waste discharge. million years ago. The vertebrates are members of a Vertebrate size reduction involves two strategies: (1) nested series of more inclusive groups, extending all maintenance of the adult pattern at a reduced body size the way down the tree of life to a common ancestor and (2) the acquisition of metamorphosis, meaning that they share with bacteria, the most primitive life forms the life processes, and the structures that facilitate them, known. In order to properly understand the vertebrates, change markedly at one or more stages and sizes of de- it is necessary not only to briefly discuss the characteris- velopment. tic features unique to the group but also to evaluate Vertebrates may be visualized as elongate, tubular the features they share with their closest living relatives, creatures possessed of a central tubule, this being a gut the hagfishes, lancelets, and tunicates. These organisms or alimentary canal. The anterior end of the gut forms plus vertebrates constitute the group Chordata. In the a mouth and buccal structures, whereas the posterior pages that follow, we summarize some generalized char- end is modified for the storage and ultimate elimination acteristics of chordates and vertebrates, examine the of waste. The central tubule is placed in a lined cavity, early history of these groups as it is revealed from the the coelom, that also houses various diverticulae of the study of the fossil record, and provide explicit defini- gut that support aspects of such visceral functions as tions and diagnoses for all of the major chordate groups. gas exchange (i.e., gills and lungs, heart, and vessels), digestion (pancreas and liver), fluid balance (kidneys), and reproduction (gonads). The entire animal is coated by a multilayered epidermis that protects the internal II. GENERAL VERTEBRATE tissues from mechanical stresses and can serve as a CHARACTERISTICS physiological barrier. The space between coelom and epidermal layer is occupied by the somatic musculo- The diversity among vertebrate groups makes it possible skeletal system that facilitates locomotion and supports to utilize functional analyses and paleontological data and maintains the shape of the coelom and hence of to reconstruct a phylogenetic sequence of vertebrates the entire animal. and to place the currently surviving (extant) vertebrate The musculoskeletal system involves the cartilages groups into a historical sequence. This sequence per- and bones of the internal skeleton and the striated so- mits the development of explanatory schemes for some matic musculature. Externally, the system underlies the conditions observed in humans. Consequently, the epidermis, this portion forming a dermis having skele- study of the comparative morphology and physiology togenic properties. Two additional median and tubular of vertebrates has proved important for the understand- structures complete this architectural scheme. The first ing of the human condition. These studies in turn led tube, the notochord, lies just dorsal to the gut and is to the development of curricular variants for students filled with vacuolated cells, their turgor pressure turn- training themselves for careers in human medicine. Ex- ing the notochord into a stiffened skeletal rod. The amples are approaches to applied physiology and mor- second, still more dorsal, tube consists of neuronal cells phology of the precursors’ conditions. Such instruc- that involve sensory function and transmission of sig- tional variants may be presented as the sequential nals to the musculature. development of the history of a topic such as gas ex- The entire animal may have various appendages such change, digestion, or embryology. Alternatively, the as fins or limbs, each having characteristic internal skel- study may involve a series of steps, each explaining the etal supports and motile capacity developed by the con- condition in one of the subgroups of animals represent- traction of the attaching striated musculature, which is ing putative precursors for the vertebrate condition (i.e., subdivided into myotomes, paired blocks that parallel VERTEBRATES, OVERVIEW 757 the notochord. The notochord provides a capacity for and major physiological systems, for instance the multi- resisting longitudinal stresses, while the appendages ple senses, the head skeleton, the vertebral column, and involve support of the animal above the substrate and the digestive system. This clear diversity of vertebrate the transmission of laterally directed force patterns. specializations has long raised questions about the Whereas this general pattern seems to characterize adaptive reasons to explain why these comparable char- vertebrate structure, the group shows major curiosities. acteristics are modified in vertebrates and why and how The various groups of nonvertebrate creatures can gen- a multiplicity of such characteristics might be associated erally be diagnosed by a relatively few characteristics. In and consequently subject to possibly simultaneous se- contrast, there are no fewer than 25 new characteristics lection. Actually, the answer to this conundrum has shared by vertebrates (Table I). These involve multiple been available for some 30 years. Rather than looking at the functions of the many ‘‘vertebrate characteristics,’’ namely, aspects seen in the adult condition, it is more informative to base comparisons on the stages of verte- TABLE I brate embryology or development. The traditional rules Derived Characters of Vertebrata and Their Embryonic Origina that posited firm conditions, such as that the ectoderm formed all the epidermis and the mesoderm all of the Vertebrate characters Embryonic origin striated musculature, soon broke down.