Origin Of The Vertebrates

The skeleton

The skeleton of vertebrates is made from bone and cartilage. Bone consists of a network of collagen fibers on which needle-like crystals of hydroxyapatite (a form of apatite, calcium phosphate, CaPO4) accumulate. Hence bone has a flexible component and a hard component, which explains why bones may undergo a great deal of strain before they break, and also why bones do not break along simple brittle faces. Cartilage is a flexible, gristly tissue, usually unmineralized, and containing collagen and elastic tissues. In humans, most of the bones are laid down in the early embryo in the form of cartilage, and this progressively mineralizes by deposition of apatite. In adult humans, cartilage can be found in flexible parts like the ears and the nose, as well as at the ends of the ribs and some limb bones.

The first vertebrates probably had a cartilaginous skeleton. Some of the oldest fish fossils, such as Sacabambaspis from the Ordo-vician of Brazil (Fig. 16.1a), had the beginnings of a bony skeleton, but only on the outside of the body, and there is no trace preserved of an internal mineralized skeleton. The rigid armor is made up of lots of little tooth-like structures, each equivalent to an individual shark scale, but united by continuous sheets of bone arranged like plywood.

This shows how adaptable the vertebrate skeleton can be, and this is perhaps why vertebrates became such a diverse and abundant group. The internal skeleton of vertebrates has a unique property - it allows them to

Haikouichthys Images

20 mm

20 mm

pectoral fin

pectoral fin dorsal plate rostral plate dorsal spine orbital plate dorsal plate rostral plate dorsal spine orbital plate

10 mm ventral plate branchial plate

Figure 16.1 Early jawless fishes: (a) Sacabambaspis from the Mid Ordovician of Brazil, the oldest well-preserved fish; (b) the osteostracan Hemicyclaspis from the Devonian; and (c) the heterostracan Pteraspis, also from the Devonian. (a, b, based on Gagnier 1993; c, based on Moy-Thomas & Miles 1971.)

10 mm ventral plate branchial plate

Figure 16.1 Early jawless fishes: (a) Sacabambaspis from the Mid Ordovician of Brazil, the oldest well-preserved fish; (b) the osteostracan Hemicyclaspis from the Devonian; and (c) the heterostracan Pteraspis, also from the Devonian. (a, b, based on Gagnier 1993; c, based on Moy-Thomas & Miles 1971.)

grow very large because the skeleton can grow with the animal. An external skeleton cannot grow so fast, and is less adaptable in supporting a large volume of soft tissues. Further, the external skeleton is vulnerable to damage and either has to be repaired by extending fleshy parts outside the shell (mollusks, graptolites) or by molting the skeleton (arthropods), a wasteful process that uses up energy and leaves the animal vulnerable until the new exoskeleton hardens. By contrast, the vertebrate skeleton is maintained and remodeled constantly within the body, and can act as a support for small, medium, large and massive organisms.

Jawless fishes: slurping rather than biting_

Two key defining characters of vertebrates are the head and neural crest tissues. Our head is so essential that we rarely stop to think that actually only vertebrates have heads - indeed vertebrates are sometimes called craniates, meaning "with a skull". Mollusks, worms, brachiopods and echinoderms do not have heads - we might call the front end of a worm its "head", but it really is not any more than its front end. The vertebrate head is unique in providing an organized structure that contains the brain, the major sense organs and the mouth.

The vertebrate head is formed from cells derived from the neural crest, a second key apomorphy of vertebrates. The neural crest appears in the early embryo as a strip of cells lying just below the outer skin, the ectoderm, of the embryo, above the line of where the backbone will develop. As tissues begin to differentiate in the early embryo, cells derived from the neural crest spread through the embryo and stimulate the development of muscles, nerves and blood vessels along the trunk and around the heart and gut, but a major target is the head region. The cranial neural crest cells give rise to bones, cartilage, nerves and connective tissue in the head and neck region, forming the face, teeth, eyes, inner ear, the thymus, thyroid and parathyroid glands, and the gills and gill arches of fishes.

The first vertebrates had no jaws (Fig. 16.1). Until recently, these first fishes were said to be Ordovician in age, but controversial new specimens from the remarkable fossil sites at Chengjiang in China (Box 16.1) have pushed the range back to the Early Cambrian. In the Late Cambrian and Ordovician, the commonest vertebrates were the conodont animals. Fishes became common and diverse during the Late Silurian and Devonian.

The jawless fishes are sometimes referred to as ostracoderms (Box 16.2). Ostracoderms were jawless, they were generally armored, although some were not, and they had their heyday in the Devonian. Osteostracans like Hemicyclaspis (Fig. 16.1b) have a semicircular head shield bearing openings on top for the eyes and nostrils, as well as porous regions round the sides that may have served for the passage of electrical sense organs, perhaps used in detecting other animals by their movements in the water. Heterostracans like Pter-aspis (Fig. 16.1c), are more streamlined in shape, and were perhaps more active swimmers. Both forms have their mouths underneath the head shield, and they probably fed by sieving organic matter from the sediment. These armored jawless fishes died out at the end of the Devonian, and their place was taken over by fishes with jaws.

Jawless fishes still exist today, the 50 or so species of lampreys and hagfishes, eel-shaped animals. Hagfishes scavenge on dead flesh, while lampreys are often parasitic. Although they have no jaws, their mouths are filled with tooth-bearing bones, and these are used to grip prey animals and to rasp off lumps of flesh. Salmon and trout are commonly caught in the American Great Lakes with huge circular craters in the sides of their bodies, where flesh has been torn out by a sea lamprey.

Conodonts: animals of mystery_

The commonest early vertebrates were the conodont animals (Sweet & Donoghue 2001). For over 150 years conodonts had been a mystery, known only from their jaw elements - no one knew which animal had produced them.

Conodonts were first identified by the Latvian embryologist and paleontologist Christian Pander in 1856. They occur as phosphatic tooth-like microfossils, termed elements. Three main conodont groups have been established (Fig. 16.3): (i) protocon-odonts such as Hertzina are simple cones with deep basal cavities; (ii) paraconodonts like

Box 16.1 The world's oldest vertebrates

There was a sensation in 1999 when Shu Degan and colleagues (Shu et al. 1999) announced a new fossil vertebrate, Myllokunmingia, from the Early Cambrian locality Chengjiang in China. This site has become celebrated for the exceptional preservation of all kinds of animal fossils, and it rivals the Burgess Shale (see p. 249) as a window into Cambrian life. Until 1999, the oldest vertebrates were much debated, with some tentative Middle and Late Cambrian candidates, but nothing really certain until the Ordovician.

Myllokunmingia (Fig. 16.2) is tiny, less than 30 mm long - you could hold a hundred or so of them, like a handful of wriggling whitebait. The head is poorly defined, but there seems to be a mouth at one end. Relatives seem to show detail in the head, possible eyes and a brain. If it has such differentiated head features, it is a vertebrate. Behind the "head" are six gill pouches, a possible heart cavity and a gut. Above these are the notochord, a key chordate character (see p. 410), and myotomes or V-shaped muscle blocks. There is a narrow dorsal fin along the back, and possibly a ventral fin below. Myllokunmingia presumably swam by flicking its body and fins from side to side and wriggling forward through the water. None of the Chinese specimens have mineralized bone - but this does not rule them out as vertebrates. Evidently, the vertebrate skeleton began as a cartilaginous structure in early forms, and became mineralized with apatite later in the Cambrian.

In the same paper, Shu et al. (1999) also named Haikouichthys, a similar early vertebrate from Chengjiang. A rival team, Hou et al. (2002), suggest that Haikouichthys was the same as Myllokunmingia, although Shu and colleagues disagree. The two groups, led by Shu and Hou, also disagree over the identification of different organs within these fossils, and this affects where they are placed in the vertebrate phylogeny. The Chengjiang fossils are preserved in grey or yellow sediment, and the fossils may be grey or reddish, with the internal organs picked out in grey, brown and black colors. Interpreting these multicolored blobs and squiggles would test the patience of a saint, and yet it is remarkable that such details have been preserved for 500 Myr. There are now more than 500 specimens of these early vertebrates, so further intensive study may clarify their anatomy further.

See http://www.blackwellpublishing.com/paleobiology/ for relevant web links.

Figure 16.2 The basal vertebrate Myllokunmingia from the Early Cambrian of Chengjiang, China: (a) photograph of specimen, and (b) interpretive drawing showing possible identities of the internal organs. (Courtesy of Shu Degan.)
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