Vertebrate Palaeontology

Michael J. Benton

• Publisher: Wiley-Blackwell
• Year: 2014
• ISBN:
  9781118407646
• Edition: 4
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Vertebrate palaeontology is a lively field, with new discoveries reported every week… and not only dinosaurs! This new edition reflects the international scope of vertebrate palaeontology, with a special focus on exciting new finds from China.

A key aim is to explain the science. Gone are the days of guesswork. Young researchers use impressive new numerical and imaging methods to explore the tree of life, macroevolution, global change, and functional morphology.

The fourth edition is completely revised. The cladistic framework is strengthened, and new functional and developmental spreads are added. Study aids include: key questions, research to be done, and recommendations of further reading and web sites.

The book is designed for palaeontology courses in biology and geology departments. It is also aimed at enthusiasts who want to experience the flavour of how the research is done. The book is strongly phylogenetic, and this makes it a source of current data on vertebrate evolution.

CHAPTER 1
Vertebrates Originate

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KEY QUESTIONS IN THIS CHAPTER

1. What are the closest living relatives of vertebrates?
2. When did deuterostomes and chordates originate?
3. What are the key characters of chordates?
4. How do embryology and morphology, combined with new phylogenomic studies, inform us about the evolution of animals and the origin of vertebrates?
5. How do extraordinary new fossil discoveries from China help us understand the ancestry of vertebrates?

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INTRODUCTION

Vertebrates are the animals with backbones, the fishes, amphibians, reptiles, birds, and mammals. We have always been especially interested in vertebrates because this is the animal group that includes humans. The efforts of generations of vertebrate palaeontologists have been repaid by the discovery of countless spectacular fossils: heavily armoured fishes of the Ordovician and Devonian, seven- and eight-toed land animals, sail-backed mammal-like reptiles, early birds and dinosaurs with feathers, giant rhinoceroses, rodents with horns, horse-eating flightless birds, and sabre-toothed cats. These fossils tell us where the living vertebrates have come from, and they show us glimpses of different worlds that seem so bizarre that they would defy the imagination of a science fiction writer. Despite all this information that has accumulated over the past 200 years, the origin of vertebrates is hotly debated.

One thing is clear from the biology of living animals. Vertebrates are members of a larger group, termed the Phylum Chordata, which also includes their closest living relatives, marine animals such as the sea squirts and amphioxus (see below). These creatures do not have a skeleton, but they share other features, such as a notochord, a flexible, tough rod that runs along the length of the body down the back. The notochord in living chordates is generally made from an outer sheath of collagen, a tough fibrous connective tissue that encloses turgid, fluid-filled spaces. Invertebrate chordates also have V-shaped muscle blocks (myomeres) along the length of their body. The question about the origin of vertebrates then broadens out to include the origin of chordates.

Looked at more widely, vertebrates are a minor twig in the ‘Tree of Life’ (Figure 1.1). It is common to think of the major divisions of life as being animals, plants, protists, and simple organisms classed broadly as bacteria and viruses. However, molecular studies since the 1990s (e.g. Woese, 2000; Wolf et al., 2002) have shown that the fundamental splits were between Bacteria, Archaea, and Eukaryota. The familiar plants, animals and fungi are members of Eukaryota, all diagnosed by complex cells with a membrane-bound nucleus and the primitive presence of mitochondria. Within Eukaryota are various protist groups, as well as plants, fungi, and animals, and of course vertebrates are animals. Among animals, it has always been assumed that chordates are closely related to hemichordates (acorn worms and pterobranch worms) and echinoderms (starfish, sea lilies, and sea urchins), and this is now widely confirmed, based on morphological, developmental and molecular evidence.

Figure 1.1 The ‘Tree of Life’, the commonly accepted view of the relationships of all organisms. Note the location of ‘Animals’, a minor twig in the tree, close to plants and Fungi.

Source: Adapted from various sources.

The purpose of this chapter is to explore the various lines of evidence that can be used to reconstruct the origin of the vertebrates: the study of modern animals that are vertebrate-like in some features, the study of molecular relationships, and fossils.

1.1 SEA SQUIRTS AND THE LANCELET

There are two key groups of living non-vertebrate chordates, the sea squirts and the cephalochordates (amphioxus). The amphioxus certainly looks superficially fish-like, but adult sea squirts look like rubbery bottles, and so would hardly seem to be sensible candidates for close relatives of the vertebrates!

1.1.1 Urochordata: sea squirts

A typical sea squirt, or tunicate, is Ciona (Figure 1.2(a)), which lives attached to rocks in seas around the world. It is a 100–150 mm tall bag-shaped organism with a translucent outer skin (the tunic) and two openings, or siphons, at the top. The body is firmly fixed to a hard substrate.

Figure 1.2 The sea squirts: (a) Ciona, external view; (b) internal anatomy and cross-section of an adult; (c) swimming larva; (d) metamorphosing form.

Source: Adapted from Jefferies (1986) and other sources.

The internal structure is fairly complex (Figure 1.2(b)). A large pharynx fills most of the internal space, and its walls are perforated by hundreds of gill slits, each of which bears a fringe of cilia, fine hair-like vibratile structures. Seawater is pumped through the inhalant siphon into the pharynx by beating movements of the cilia, and the water is then passed through a surrounding cavity, the atrium, and ejected through the exhalant siphon. The pharynx serves mainly to capture food particles from the stream of seawater that flows through it. The seawater is drawn into a filter bag of mucus, which is produced inside the pharynx by an organ called the endostyle. During feeding, the endostyle continuously secretes mucus into the oesophagus, together with the food particles that it has filtered from the seawater, and the food is passed to the stomach for digestion. Tunicates also have a heart that pumps the blood around the body; an intriguing aspect is that the heart stops beating every few minutes and the direction of blood flow reverses.

Why is Ciona identified as a chordate? The pharynx and other structures are in fact very like those of the cephalochordates and lamprey larvae, but further evidence is to be found in the larval stage, when the sea squirt is a tiny free-swimming tadpole-shaped animal with a head and a tail. The larval sea squirt (Figure 1.2(c)) has a notochord that runs along the tail, and this identifies it as a chordate. There are muscles on either side of the notochord that contract alternately, causing the tail to beat from side to side, and this drives the animal forward in the water. The larva has a dorsal nerve cord, running along the tail just above the notochord, and this expands at the front into a very simple brain that includes a light sensor (an ‘eye') and a tilt detector.

The larva then settles on a suitable surface. It up-ends onto the tip of its ‘snout' and attaches itself by means of adhesive suckers (Figure 1.2(d)). The notochord and tail portion wither away, and the ...