DESCRIPTION OF THE PHYLUM VERTEBRATA (CUVIER 1812)

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Vertebrata (vur-te-BRAH-tuh) from the Latin, vertebra, which means a joint. The reference is to the articulation of the vertebrae that form the axis of vertebrates.

INTRODUCTION TO THE VERTEBRATA The vertebrates occupy almost every environment on the surface of the earth. They are among the most obvious animals in marine and freshwater habitats; also, they occur in a bewildering array of terrestrial habitats where they may be found on the ground, in trees, underground, in the air, etc. They range in size from animals of just a few grams to giants like elephants and whales. Indeed, the mass of the female blue whale is truly astounding and marks it as the largest vertebrate that has ever lived. Furthermore, we find the vertebrates particularly intriguing because our species is found among them. All vertebrate animals have an anterior brain that is encased, in part, by a cranium of cartilage or bone. The hagfishes (Figure 1, clade 3) have the typical chordate features (notochord, pharyngeal gill slits, and hollow dorsal nerve cord) plus the cranium. All other members of the phylum have articulating skeletal elements (the vertebral column) that develop with or supplant the notochord. The internal skeleton of the vertebrates has been highly successful and allowed for the evolution of frames that swim, walk, run, fly, etc. (Figure 1). They likely appeared in the early Paleozoic and became dominant aquatic organisms by the Devonian Period (sometimes called the Age of Fishes). Tetrapods (vertebrates with four legs) appeared also in the upper Devonian.

MAJOR CLADES OF THE VERTEBRATA
	1. Animals with a hollow dorsal nerve cord, notochord, and pharyngeal gill slits. 2. Brain encapsulated, or at least partially so, by a cartilaginous or bony cranium. Vertebrae, associated with or replace notochord. First duplication of genome (1R). 3. Loss of vertebrae. 4. Mineralized bone. Second duplication of the genome (2R). 5. Head shield of dermal bone; bony scales. 6. Paired spines or fins. 7. Neurocranium encloses brain dorsally. 8. Mouth formed by articulated jaws. 9. Teeth erupt from dental lamina. 10. Paired fin radials barely extend beyond body. 11. Gills covered by an operculum. 12. Pectoral and pelvic girdles anchored to vertebral column. 13. Digits reduced to 5 or fewer; radius as long as the ulna. Operculum lost. 14. Premaxilla less than 2/3 skull width. 15. Egg with an amnion membrane outside of the embryo. 16. One temporal fenestra formed by the squamosal and jugal bones. 17. Large post-temporal fenestra; suborbital foramen in palate. 18. Two temporal fenestrae; upper one formed by the squamosal and postorbital bones. 19. Trunk ribs single-headed, end of humerus robust.
Figure 1. This is a simplification of Benton (2005) and Janvier (2013) with 21 vertebrate groups that we interpret at the superclass and class-level. We have illustrated the extant or living groups with photographs. The 12 extinct taxa are given in red letters. The two extant agnathan classes are clearly members of a different clade (called the cyclostomes). The other agnathans (Conodonta – Cephaloaspidomorphi) are called the stem gnathostomes (Donoghue and Purnell 2005) or ostracoderms and grade gradually to the gnathostomes (Placodermi – Archosauromorpha). The gnathostome fishes (Placodermi – Osteichthyes) grade to the tetrapods (Stegocephali – Archosauromorpha). The tetrapods have three major extant clades: the Batrachomorphs (tetrapods with aquatic larvae), Mammals (tetrapods with a lower jaw made of a single bone and three bones associated with the middle ear), and the Sauromorphs (Anapsida – Archosauromorpha).

SYSTEMATICS OF THE VERTEBRATA Although the vertebrates have left a rich fossil legacy, their relationships have been clouded by preconceptions of what is advanced and what is not. That has been complicated even further by our own inclusion in the phylum so that concepts of advanced become confused with what appears more human. The standard class structure of the vertebrates can be seen below: AGNATHA -Jawless Fishes CHONDRICHTHYES -Sharks, skates, rays and chimeras OSTEICHTHYES -Bony Fishes AMPHIBIA -Amphibians REPTILIA -Reptiles MAMMALIA -Mammals AVES -Birds Growing bodies of molecular (summarized by Tudge 2000) and paleontological (Benton 2005; see Figure 1) evidence suggest a much more complex relationship between the classes. The agnathans (Clades 1-7) represent a grade in form and include at least five independent lines. Gnathostomes (Clades 8-19) represent another structural grade and require different comparative criteria for their taxonomy, which causes the number of class-level groups to increase. Not only are there many other groups at the class level, but the distinctions between some of the groups disappear entirely. For example, a group like the reptiles is a grade of paraphyletic taxa if the mammals and birds are not considered to be reptiles. Also, the bony fishes merge with the amphibians in a continuum of body form from the sarcopteryigians to the earliest tetrapods and thence to all other tetrapod descendants. The mammals merge with the basal synapsids (traditionally considered an extinct group of reptiles). Birds merge with the archosaurian reptiles (a group that contains the living crocodilians). The turtles stand alone as a separate group as do most of the modern reptiles in a collective group (lizards, amphisbaenas, snakes, and tuataras). Paleontological, physiological, and anatomical evidence (summarized by Janvier 2008) suggested that the hagfishes were fundamentally different from the other members of this phylum (e.g. hagfish do not have vertebrae, nervous control of the heart, etc.). This led to the creation of the Craniate hypothesis. That is, the hagfishes were primitive and represented a pre-vertebrate line, and the name of the phylum was Craniata with two very unequal subphyla: Myxinomorphi and Vertebrata. Thus, the old taxon, Cyclostomata (Dumeril 1806), in which the hagfishes and lampreys were grouped, simply was the result of convergent evolution. The view from molecular and developmental (evo-devo) work suggested a different picture altogether (e.g. Kuraku et al. 2008 and Oisi et al. 2013). Developmental work (summarized by Oisi et al. 2013) indicated that the hagfish emerged from a line that had vertebrae and therefore are degenerate (as originally suggested by Dumeril 1806). Furthermore, a very different explanation for the evolution of vertebrates emerged from molecular work. Ohno (1970) proposed that the vertebrates evolved from an early chordate in which the whole genome was duplicated once or twice at the base of the vertebrate line. With the rise in genomics, the whole genome duplication (WGD) hypothesis seems to have been borne out. For example, cephalochordates like amphioxus (Branchiostoma) have a single linear cluster of 14 Hox genes ( Minguillon et al. 2005), but all vertebrates have multiple Hox clusters in patterns that are consistent with genome duplication followed by massive gene loss (literature summarized in Kasahara 2007). In particular, the cyclostomes seem to be the products of a single WGD, but most of the living gnathostomes appear to have experienced a second round of WGD (2R; Luminiecki and Helden 2010)). The teleost fish, which are as speciose as all other vertebrates put together, appear to have experienced a third round of WGD (3R) in their line (Jaillon et al. 2004). Thus, WGD seems to have preceded major radiations in the vertebrate lines. The void between the living cyclostomes and gnathostomes is filled by a set of extinct animals which Donoghue and Purnell (2005) call the stem gnathostomes. Although these taxa do not have jaws, and therefore are not gnathostomes (sensu stricto), Donoghue et al. (2000), Janvier (2001), and Donoghue and Purnell (2005) argue that they possess a mosaic of characters that also occur in the crown gnathostomes (see the organization below). Zhang and Cohn (2008) further argue that, although these taxa are extinct, they possess mineralized bone, which required the genomic sophistication that a second doubling of the genome would provide. Thus, 2R (the second WGD) must have occurred prior to the appearance of the conodonts (see Figure 1). Despite the cladistic structure of this system, a classification system that places mammals within the bony fish is unsatisfactory from the perspectives of organizing biodiversity and its usefulness as a retrieval system. So, in our opinion, a strict cladistic application is inappropriate. However, the classical approach is equally unacceptable in that reptile is a form and not a taxonomic unit. We have taken the organization of the following system from Benton (2005), which was written in a modified cladistic format, and adapted it to conform to a standard taxonomic system. The following system stresses both the diversity of vertebrates and retains the retrieval function of a classification system. We know full well that our system will be unacceptable in some way to almost anyone who studies vertebrates. So, we offer it as a way to enter the conversation about craniate systematics, not to present a system in which we are fully invested.

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By Jack R. Holt. Last revised: 08/18/2020