Download LECTURES FOR ZOO 1010—CHAPTER 1

LECTURES FOR ZOO 1010—CHAPTER 17
The Early Tetrapods and Modern Amphibians
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—Vertebrate Landfall: Amphibians are the only living vertebrates
that have a transition from water to land in both their ontogeny and phylogeny. Few
amphibians, however, are completely land adapted. Most are only quasiterrestrial and
hover between aquatic and land environments. The name amphibian refers to this
transitional position. Most amphibians have an aquatic larval stage and a terrestrial
adult stage, although most adult amphibians cannot stray far from moist conditions.
Many amphibians, however, have evolved various types of adaptations to keep their
eggs and/or larvae out of open water where they would be exposed to predation.
Adaptation for life in a terrestrial environment is a major them of the remaining
vertebrate groups. These animals belong to a monophyletic group known as tetrapods.
Amphibians and amniote tetrapods comprise the two major extant branches of
tetrapod phylogeny.
Movement onto Land:
Life originated in water. The movement to land, therefore, constitutes a major event
in organismic evolution. Vascular plants, pulmonate snails, and tracheate
arthropods accomplished the transition earlier than did tetrapod vertebrates.
Aquatic and terrestrial vertebrates retain similarities in structure and function,
although adaptation to terrestrial life has affected almost every system in the
vertebrate body. This transition can be seen today in the ontogeny of many living
amphibians.
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Living on land presents many physiological and structural challenges. Aquatic
environments differ from those on land in terms of oxygen content, density,
temperature regulation, and habitat diversity. Despite its hazards, however, land
offers a great variety of habitats and provision of safe shelter for the vulnerable
eggs and larvae may be accomplished more readily in terrestrial habitats than in
aquatic one.
Early Evolution of Terrestrial Vertebrates:
Devonian origin of tetrapods—The Devonian period began about 400 million years
ago, at a time when mild temperatures and alternating droughts and floods
prevailed. It was during this time that primarily aquatic vertebrates evolved two
features important to the success of the invasion of land, lungs and limbs. It was
during this time that the double circulation characteristic of all tetrapods evolved,
with a systemic circulation serving the body and a pulmonary circulation
supplying the lungs.
Vertebrate limbs also evolved during this period. This adaptation to life on land began
in lobe-finned fishes. The bones in the lobes their limbs can be homologized with
those in the tetrapod limb. Lobe-finned fishes are the sister group of the tetrapods
Carbonifierous radiation of tetrapods—The Carboniferous period followed the
Devonian and was a time characterizied by a warm, wet climate and swampy
landscapes. Tetrapods radiated quickly in this environment, producing a variety of
forms.
The modern amphibians, frogs, salamanders, and caecilians, evolved for ancestral
lissamphibians, a group characterized by having 4 digits on the anterior limb
rather than the 5 typical of most tetrapods.
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The Modern Amphibians:
About 4200 species of modern amphibians have been described to date. They
comprise 3 orders sharing the following characteristics: (1) ectothermy; (2)
primarily quadrupedal; (3) glandular skin; (4) respire by use of lungs, gills, or
skin.
The three orders contain the caecilians, salamanders, and anurans. The caecilians are
a small tropical group of limbless, elongate forms. The salamanders are tailed
amphibians that have retained the generalized four-legged body plan of their
ancestors. The frogs and toads or anurans are the largest group of modern
amphibians and all are specialized for a jumping or saltational mode of
locomotion.
Most amphibians have a biphasic life cycle that begins with an aquatic larva that later
metamorphoses to produce a terrestrial adult that returns to the water to lay eggs.
Some frogs, salamanders, and caecilians have evolved direct development that
omits the aquatic larval stage and some caecilians have evolved viviparity.
Salamanders are unique among amphibians in having evolved several
permanently gilled species that retain a larval morphology throughout life,
eliminating the terrestrial phase completely. The permanently gilled condition is
obligate in some species, but others will metamorphose to a terrestrial form if the
pond habitat dries.
Despite their adaptations for terrestrial life, the adults and eggs of all amphibians
require cool, moist environments if not actual pools or streams. The eggs and
adult skin have no effective protection against very cold, hot, or dry conditions,
greatly restricting the adaptive radiation of amphibians to environments that have
moderate temperatures and abundant water.