Medium 9780253356758

Gaining Ground, Second Edition: The Origin and Evolution of Tetrapods

Views: 658
Ratings: (0)

Around 370 million years ago, a distant relative of a modern lungfish began a most extraordinary adventure—emerging from the water and laying claim to the land. Over the next 70 million years, this tentative beachhead had developed into a worldwide colonization by ever-increasing varieties of four-limbed creatures known as tetrapods, the ancestors of all vertebrate life on land. This new edition of Jennifer A. Clack's groundbreaking book tells the complex story of their emergence and evolution. Beginning with their closest relatives, the lobe-fin fishes such as lungfishes and coelacanths, Clack defines what a tetrapod is, describes their anatomy, and explains how they are related to other vertebrates. She looks at the Devonian environment in which they evolved, describes the known and newly discovered species, and explores the order and timing of anatomical changes that occurred during the fish-to-tetrapod transition.

List price: $49.99

Your Price: $39.99

You Save: 20%

Remix
Remove
 

10 Chapters

Format Buy Remix

1 Introduction: The Origin and Evolution of Tetrapods

ePub

1.1. Family tree of the living tetrapod groups.

The Origin and Evolution of Tetrapods

Approximately 380 million years ago, something strange and significant happened on Earth. That time is part of an interval of Earth’s history called the Devonian period by scientists such as geologists and paleontologists, but in more popular imagination, it is known as the Age of Fishes. The reason for this is that after about 200 million years of earlier evolution, the vertebrates—animals with backbones—had produced an explosion of fishlike animals that lived in the nearshore lagoons, river estuaries, and lakes of the time. The strange thing that happened from the middle to the later parts of the Devonian period is that some of these fishlike animals evolved limbs with digits—fingers and toes. Over the ensuing 350 million years, these tetrapods gradually evolved from their aquatic ancestry into walking terrestrial vertebrates. These have dominated the land ever since this initial explosive radiation allowed them to colonize and exploit the land and its opportunities. The tetrapods, with their limbs and fingers and toes, include ourselves as humans, so that this distant Devonian event is profoundly significant for humans as well as for the planet.

 

2 Skulls and Skeletons in Transition

ePub

2.1. Skull of Eusthenopteron to show structures. (A) Left lateral view of skull roof and lower jaw. (B) Dorsal view. (C) Lateral view with skull roof bones shown transparent, and palate, braincase, and gill arches visible beneath. (D) Ventral (undersurface) view with braincase in position. (E) Medial (internal) view of lower jaw. (F) Lateral view of braincase. (G) Ventral view of braincase. Based on Jarvik (1980).

This chapter is an introduction to the skeletal anatomy of animals that exemplify the fish–tetrapod transition. The first part examines how the skulls and skeletons of lobe-finned fishes and tetrapods were built, and will introduce the terminology used for the bones. Unfortunately, many of the terms will be unfamiliar to the nonspecialist reader, but at least some of them need to be assimilated because in most cases, there simply are no other words available to describe them. It is probably a good idea to refer constantly to the diagram of skull structure (Fig. 2.1).

 

3 Relationships and Relatives: The Lobe-Fin Family

ePub

3.1. Cladogram of lobe-fins from Cloutier and Ahlberg (1997).

Introducing the Lobe-Fins

This chapter introduces the tetrapods’ closest relatives, explains how tetrapods fit into the scheme of relationships with other lobe-fins, and explores how ideas about the ancestry of tetrapods have evolved with changing perspectives.

There are a few characteristics of lobe-fins that distinguish them as a group, but the most conspicuous is the eponymous lobed fin, described more fully in Chapters 1 and 2, in which the paired fins are anchored to the respective limb girdle by a single bone. The structure appears to be a true shared, derived character of the group. Another shared feature is the possession of two dorsal fins, instead of the single one found in early ray-fins, and a third is the occurrence in the tail of a second series of fin rays growing above the body portion. This has enabled lobe-fins to evolve symmetrical tails without too much modification of the existing pattern, and they have done so in a number of separate lineages. The latter two features may after all be primitive for gnathostomes, but this is not clear. In addition, most early members of the lobe-finned group show an intracranial joint or hinge line. A hinge linked the front and back parts of the skull roof just behind the eyes, reflected in a matching hinge across the underlying braincase (see Chapter 2). The hinge line occurs at the point where the otoccipital portion of the braincase meets the ethmosphenoid dorsally, and the ventral otic fissure separates them ventrally (see Figs. 2.1, 2.3, 2.11, 2.13), an important landmark in building the vertebrate skull. The hinge itself is not seen in ray-finned fishes, and it is also lost from several of the lobe-finned groups independently, including tetrapods.

 

4 Setting the Scene: The Devonian World

ePub

4.1. Graph of (A) and (B) levels through time from Graham et al. (1997). C = Carboniferous; ε = Cambrian; D = Devonian; J = Jurassic; K = Cretaceous; T = Tertiary; Tr = Triassic; O = Ordovician; P = Permian; PAL = present atmospheric levels; S = Silurian.

Devonian Biogeography and Climate

The Devonian period opened onto a world far different from the present day. In the earliest stages, over 400 million years ago, even the oxygen content of the air was different from today. According to some models, at the beginning of the Devonian, the air contained about half the present levels of oxygen (O2) but about 10 times the present amount of carbon dioxide (CO2) (Fig. 4.1). Estimates of O2 and CO2 levels were produced by Berner and colleagues (Berner 1993, 1999) based on factors including increasing radiation from the sun over the last 570 million years and how that has affected weathering rates of carbonates and silicates, and how the increase in plant cover changed the rate of weathering as well as the uptake of CO2 from the air. The models were backed up by evidence from carbon isotope studies of fossil soils. The effects mean that because CO2 is less dense than O2, the total air density was less than it is now. Generally speaking, the levels of CO2 are estimated to have been higher than present levels throughout the Devonian, dropping from about 0.35% in the Early and Mid Devonian to about 0.3% by the end of the Famennian. Causes for this drop are suggested to be the increased rates of burial of organic carbon and enhanced weathering of silicates. These processes also had an effect on climate cooling (Algeo et al. 2001). Present atmospheric levels of 0.03% were reached by the mid-Carboniferous (late Mississippian) (Fig. 4.1). It is an example of the influence of the increase in land plants covering the Earth, using CO2 and giving out O2.

 

5 The First Feet: Tetrapods of the Famennian

ePub

5.1. Flying over Kejser Franz Joseph Fjord in a Twin Otter in July 1987. Ymer Ø and Celsius Bjerg in the foreground with some of the classic Ichthyostega sites on its northeastern side and Gauss Halvø in the background. Photograph by R.N.G.C.

By the latter part of the Late Devonian, the Famennian, vertebrates with indisputable limbs bearing digits—tetrapods—had appeared. Some remarkably well-preserved material from several localities provides details of their anatomy and lifestyles. This chapter examines each of these in turn, and then goes on to see what pointers they may give to the origin of the group.

Famennian Tetrapods from East Greenland

It is East Greenland that has provided the most detailed knowledge of Devonian tetrapods. East Greenland has been studied by geologists for many decades. One of the reasons is that the terrain lies within the polar semidesert, so it is relatively sparsely vegetated and the geology can be seen during the short summer season, when the ice and snow melt around the coast. At the same time, in contrast to many desert areas, availability of water is not a problem for the visiting scientists, though they are often plagued by mosquitoes, and accessibility to the area is often hampered by bad weather. Most of the sites are usually only reached by helicopter, though in the past, icebreakers carried the scientific teams to the fjords from where they reached the sites by inflatable dinghies. Some of the most recent collecting expeditions have been carried out in collaboration with the Denmark and Greenland Geological Survey, who use as their base camp the Danish air force airstrip of Mestersvig.

 

6 From Fins to Feet: Transformation and Transition

ePub

6.1. Illustrated cladogram with Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, and Dendrerpeton.

Reading the Evidence: Eusthenopteron—Panderichthys—Tiktaalik—Ventastega—Acanthostega

Chapter 2 looked at opposite ends of a spectrum. At one end was the structure of a fish such as Eusthenopteron, and at the other end was that of a tetrapod such as Dendrerpeton, an early tetrapod belonging to the group known as temnospondyls (see Chapters 8 and 9). The problem embodied in the phrase “the fish–tetrapod transition” is how evolution proceeded from one to the other. One of the ways to study this is to look at intermediate forms, but what makes a suitable intermediate form? In the past, a temnospondyl such as Eryops would have been featured in the role of primitive tetrapod, and Ichthyostega would have been seen as an intermediate between Eusthenopteron and Eryops. Recent analyses, however, have suggested that Ichthyostega has some highly specialized features that may make it unsuitable as a representative Devonian tetrapod; it is now also clear that Eryops is a highly specialized and unrepresentative temnospondyl. Although Eusthenopteron is not as close a relative of tetrapods as used to be considered, it still provides good information about basal tetrapodomorph structure.

 

7 Emerging into the Carboniferous: The First Phase

ePub

The Carboniferous World

At the end of the Devonian, a major extinction event hit most groups of vertebrates, both marine and nonmarine. Although an earlier extinction event at the Frasnian–Famennian boundary has been recognized for many years, it appears to have affected invertebrates, especially marine ones, with most vertebrate groups essentially passing through it unscathed. By contrast, a massive vertebrate faunal turnover at the end of the Devonian, associated with the geological phenomenon known as the Hangenberg event, saw the extinction of many groups of vertebrates such as placoderms, and most acanthodians and sarcopterygians (Sallan and Coates 2010). Of those acanthodians and sarcopterygians that did survive, most were represented by only a remnant of their former populations, and these too eventually became extinct. After the extinction, a few groups notably survived well. These included the ray-finned fishes, which had begun their radiation in the Late Devonian but which expanded greatly in numbers, species, and niches in the Early Carboniferous. The chondrichthyans, although they had been persistently present through the Devonian, again became more widespread and numerous, particularly later in the Early Carboniferous. Finally, the tetrapods really began their radiation at this stage. From this point on, the multidigited forms from the Devonian were rare or absent, and five-digited forms became dominant.

 

8 East Kirkton and the Roots of the Modern Family Tree

ePub

8.1. (Color Plate 15) The East Kirkton Quarry site once clearance was complete and a section had been excavated through the sequence, with the author standing at the level of unit 82, where most of the best tetrapod specimens have come from. Photograph by R.N.G.C.

Background to the East Kirkton Locality

A small former mining town called Bathgate, about 20 miles from Edinburgh, Scotland, has recently been made famous in the paleontological world for being the location of a window through which to view an extraordinary episode in evolutionary history. At the edge of a housing estate lies a quarry where in the 19th century a rock called the East Kirkton Limestone was dug out. It had some curious qualities that made it attractive as a building stone and hard wearing for making the local farm walls.

It is composed of thinly alternating bands of dark carbonaceous limestone, pale silica, and hardened gray volcanic ash called tuff, and often the bands are speckled with small white nodules or twisted and distorted into intriguing curls and waves. In the 1830s, fossil collectors also found some unusual specimens, which are now recognized as the carapaces of eurypterids or sea scorpions, as well as many plant remains. The quarry was closed in about 1844, and though geologists visited occasionally afterward, it was eventually forgotten and became grown over.

 

9 The Late Carboniferous: Expanding Horizons

ePub

9.1. Drawings of Late Carboniferous plants. (A) Sigillaria sp., a lycopsid (club moss) with a trunk up to 1 meter in diameter. (B) Psaronius sp., a tree fern related to the modern family Marattiales. (C) Callistophyton sp., a trailing pteridosperm. (D) Lepidodendron sp., a lycopsid (club moss) up to 54 meters high. (E) Calamites carinatus, a horsetail (Equisetales). (F) A member of the Cordaites family, a gymnosperm with trunk diameter up to 1 meter.

Late Carboniferous/Early Permian Biogeography and Paleoecology

During the Late Carboniferous, the continents, which had slowly moved southward through the Devonian and Early Carboniferous, changed direction and began to rotate. Gondwana and Euramerica gradually collided, initiating the formation of the supercontinent Pangaea. The world’s vegetation had differentiated into continental regions so that, for example, the Gondwana flora became quite distinct from those of Euramerica and of what are now China and Siberia. At this time, Euramerica, positioned in the tropics, was covered by a vast swamp forest, while to the north and south of it, evaporite deposits speak of arid climates (Milner 1993a).

 

10 Gaining Ground: The Evolution of Terrestriality

ePub

10.1. Occipital views of tetrapods. (A) The embolomere Pholiderpeton. (B) The seymouriamorph Seymouria. (C) The temnospondyl Eryops. (D) The early amniote Paleothyris. Opisthotic/supraoccipital, dark shading; basioccipital, medium shading; exoccipital, light shading.

Steps toward Terrestriality

This extended survey of the anatomy and lifestyles of tetrapods throughout the Paleozoic has explored the evidence and speculation bearing on the advent of tetrapods onto land. This final chapter now goes on to consider the evolution of several key aspects of their biology and how they became truly adapted to terrestriality. The solutions that these early tetrapods arrived at laid the foundations for terrestrial living in a huge group of vertebrates that have ultimately become a highly conspicuous part of the fauna of the planet. How these changes were achieved over that time has influenced the anatomy, morphology, and evolutionary pathways of all subsequent tetrapods and is still reflected in our own anatomy.

 

Details

Print Book
E-Books
Chapters

Format name
ePub
Encrypted
No
Sku
B000000031685
Isbn
9780253005373
File size
14.7 MB
Printing
Allowed
Copying
Allowed
Read aloud
Allowed
Format name
ePub
Encrypted
No
Printing
Allowed
Copying
Allowed
Read aloud
Allowed
Sku
In metadata
Isbn
In metadata
File size
In metadata