Download LECTURES FOR ZOO 1010—CHAPTER 1

LECTURES FOR ZOO 1010—CHAPTER 15
Vertebrate Beginnings: The Chordates
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—It’s a Long Way from Amphioxus: Amphioxus is a famous
chordate. During the 19th century, it was thought to be a good candidate for the
position of ancestor to the vertebrates. It does possess all of the cardinal chordate
features in the adult form, i.e., (1) a dorsal tubular nerve cord, (2) a supportive
notochord, (3) pharyngeal slits, (4) a postanal tail, and (5) endostyle or thyroid gland.
Today, however, it is not thought to be the vertebrate ancestor, but rather an early
offshoot of chordate evolution. It lacks a distinct head with special sense organs,
important in the shift to an active, predatory lifestyle. It resembles, however, more
closely the earliest prevertebrate more than any other animal we know.
Chordates all share the four characteristics at some stage of their life cycle. The
notochord is the structure that gives the phylum its name. It is a rodlike, semirigid
body of cells enclosed by a fibrous sheath, extending, in most cases, the full length of
the body. It is located between the gut and the nerve cord. It functions to support and
stiffen the body, acting as a skeletal axis.
Chordates retain many of the features of nonchordate invertebrates, including
bilateral symmetry, anteroposterior axis, eucoelomate tube-within-a-tube
arrangement, metamerism, and Cephalization. The exact position of the Chordata
within the animal kingdom, however is unclear.
One idea is that the chordates are allied with the arthropod-annelid-mollusk group of
protostomes, but this idea has fallen into disfavor. Nowadays, serious consideration is
given to only a sister-group relationship with echinoderms and hemichordates.
Chordates share with other deuterostomes the salient features of this branch, i.e.,
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radial cleavage, anus derived from the blastopore, and enterocoelous coelom
formation (except in vertebrates in which the coelom is basically schizocoelous).
Chordates are ecologically widespread and illustrate as a group the basic evolutionary
processes of the origin of novel structures, adaptive strategies, and adaptive radiation.
Traditional and cladistic classification of the chordates—The traditional classification
of chordates does not satisfy the cladistic requirement that only monophyletic groups
are valid taxonomic entities. Agnatha and Reptilia, as traditionally recognized are not
monophyletic. A cladistic classification of chordates, thus, would recognize a nested
set of taxonomic categories, including in descending order the Chordata (all
chordates), Euchordata (chordates retaining an axial skeleton throughout life),
Craniata (euchordates with a cranium), Gnathostomata, Teleostomi (bony fishes plus
tetrapods), Tetrapoda (four-legged vertebrates), and Amniota (tetrapods with four
extraembryonic membranes). The traditional names Protochordata, Agnatha,
Osteichthyes, and Reptilia are attached to paraphyletic groups and are not recognized
in cladistic treatments.
Four Chordate Hallmarks:
Notochord—The notochord is the flexible, rodlike structure extending the length of
the body. It is the initial portion of the endoskeleton appearing in embryogeny. It
is supportive in function and also acts as a site of attachment of segmentallyarranged muscles. The notochord is replaced by the vertebral column in all
gnathostomates.
Dorsal, tubular nerve cord—This nervous structure is the basis of the central nervous
system in chordates. It is located above the notochord. In vertebrates, the anterior
end becomes enlarged to form a brain. It is protected by the cranium and neural
arches of the vertebrae.
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Pharyngeal pouches and slits—Pharyngeal slits are openings leading from the
pharynx to the outside of the body. They form from an inpocketing of the outside
ectoderm and the inpocketing of the endodermal wall of the pharynx. In aquatic
vertebrates, these two pouches meet and break through to form the opening
described above. The perforated pharynx functioned initially as a filter-feeding
device and later came to be associated with gills for aquatic respiration. In
terrestrial vertebrates, the pharyngeal pouches do not break through; instead, they
give rise to a number of adult structures, including the Eustachian tube, middle
ear cavity, tonsils, and parathyroid glands.
Postanal tail—The tail evolved to function in propulsion in water. In fishes,
associated fins increase propulsive efficiency. Most tetrapods retain the tail,
which may be further modified for associated locomotory functions.
Ancestry and Evolution of the Chordates:
Deuterostomes, including echinoderms, hemichordates, and chordates are linked by
developmental similarities, as explained previously. They comprise a
monophyletic group of related phyla having their common origin in the seas of
the Precambrian period. The first distinctive chordates had their origin at the base
of the Cambrian period, some 570 million years ago.
Subphylum Urochordata (Tunicata):
The subphylum Urochordata is one of two invertebrate chordate subphyla. Most of
the urochordates or tunicates are sessile as adults, living a filter-feeding existence,
but all have a free-swimming larval stage possessing all four chordate features. Of
the three recognized classes (Ascidiacea, Larvacea, and Thaliacea), the ascidians
are best known.
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Subphylum Cephalochordata:
The subphylum Cephalochordata is the other of the two invertebrate chordate
subphyla. This group contains the marine lancelets that inhabit sandy bottoms of
coastal waters around the world. As a group, these small creatures possess all four
signal chordate features in the adult. In addition to these features,
cephalochordates also possess several features that foreshadow the vertebrate
body plan, including the midgut diverticulum, which secretes digestive enzymes,
segmented trunk musculature, and the basic vertebrate circulatory pattern.
Subphylum Vertebrata:
The subphylum Vertebrata includes the chordates with a vertebral column (with the
exception of hagfishes and lampreys) and a braincase or cranium. The group
consists of the jawless and jawed fishes, amphibians, reptiles, birds, and
mammals. Vertebrates have a well-developed head, high degree of motility, a
living endoskeleton allowing continuous growth and providing a sturdy
framework for efficient muscle attachment and action, a pharynx perforated by
slits (lost in tetrapods) with efficient respiratory function, a complex nervous
system of a brain and spinal cord, and paired appendages or limbs.