4.2. Vertebrates

While lancelets and sea squirts had a notochord, they lacked a skull and vertebral column. These features are only found in the subphylum Vertebrata (also called Craniata) containing about 64,000 species. Vertebrates include the jawless fish and the jawed vertebrates, which include the cartilaginous fish (sharks and rays) and the bony fish. All tetrapods (amphibians, reptiles, birds and mammals) originated from bony fish.

The oldest vertebrates are the jawless fish (Agnatha). Their cranium is normally represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, comprising about 120 species in total.

Lamprey

Adults superficially resemble eels in that they have scaleless, elongated bodies. They have a cartilaginous skeleton (including skull) and can range from 13 to 100 cm (5 to 40 inches) in length. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord. Lacking paired fins, adult lampreys have large eyes, one nostril on the top of the head, and seven gill pores on each side of the head.

The adult lamprey may be characterized by a jawless, toothed, funnel-like sucking mouth. The teeth are actually keratinous structures. They lack dentin an enameloid (a broader term that comprehends enamel and several variants), and they are not homologous to vertebrate teeth. The pharynx is subdivided: the ventral part forms a respiratory tube that is isolated from the mouth by a valve called the velum. This is an adaptation to how the adults feed. It prevents the prey's body fluids from escaping through the gills or interfering with gas exchange, which takes place by pumping water in and out of the gill pouches instead of taking it in through the mouth. Near the gills are the eyes, which are poorly developed and buried under skin in the larvae. Parasitic lampreys feed on prey as adults by attaching their mouths to the target animal's body, then using their teeth to cut through surface tissues until they reach blood and body fluid.

Hagfish

These are eel-shaped, slime-producing marine fish (occasionally called slime eels) about 0.5 m (19.7 in) in length. They are the only known living animals that have a skull but no vertebral column. Along with lampreys, hagfish are jawless; they are the sister group to vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago. The skin of hagfish has copious slime glands, the slime constituting their defense mechanism. The slime can sometimes clog up enemy fishes' gills, causing them to die.

Other jawless fishes (Ostracoderms) were prominent in the oceans of the early Paleozoic. Cyclostomes (lamprey and hagfish) apparently split from the other agnathans before the evolution of dentine and bone, which are present in many fossil Ostracoderms. They were frequently armored with heavy bony plates. These Ostracoderms reached the high point of their evolution in the Late Silurian. Most of them, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the cyclostomes (lamprey and hagfish). Agnathans declined in the Devonian and never recovered.

Jawed vertebrates

The infra-phylum Gnathostomata is comprised of vertebrates with jaws. It is believed that the jaws evolved from anterior gill support arches that had acquired a new role. Instead of being used for biting, their main role was to pump water over the gills by opening and closing the mouth more effectively – the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would, with time, become stronger and tougher, being transformed into biting jaws.

Jawed fishes had two large initial radiations (Placodermi and Acanthodii) but both became extinct during the Paleozoic. Placoderms were the oldest and they had the head and thoracis highly armored. Some of them reached very large sizes, like Dunkleosteus terrelli measuring up to 6 m (20 ft) long and 1 ton in weight. It was an apex predator.

The Acanthodii shared features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They represent several independent phylogenetic branches of fishes leading to the still-extant Chondrichthyes.

The remaining main radiations of fishes are the cartilaginous fishes (Chondrichthyes) and the bony fishes (Osteichthyes).

Cartilaginous fishes

This group includes sharks, rays and a few other fishes. They are distinguished from all other jawed fishes by having a cartilaginous skeleton. This is believed to be a secondary loss because the common ancestor of jawed fishes is thought to have been a placoderm fish with bony skeleton. A notochord, is present in young cartilaginous fishes, but it is gradually replaced by cartilage.

Cartilaginous fishes, such as sharks and rays have simple skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. There are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.

Bony fishes

Bony fishes (Osteichthyes) are characterized by a relatively stable pattern of cranial bones with medial insertion of a mandibular muscle in the lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, but this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by a fissure. They also have an operculum, which helps them breathe without having to swim. Mucus glands coat the body and most species have smooth and overlapping scales.

The Osteichthyes are divided into two groups: ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The former group accounts for 99% of the nearly 30,000 known species of fish. Although currently small, the later group gave origin to all tetrapods.

Ray-finned fishes

There has been considerable modification from the primitive skull pattern in ray-finned fishes. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.

Lobe-finned fishes

Early lobe-finned fishes were bony fish with fleshy, lobed, paired fins, which are joined to the body by a single bone. All sarcopterygians possess teeth covered with true enamel. They present a well developed internal skeleton and the ability of breathing air. Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish was Rhizodus hibberti from the Carboniferous period of Scotland which may have exceeded 7 meters in length. Among the two groups of extant (living) species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching 2 m (6.5 ft) in length and weighing up 110 kg (240 lb). The largest lungfish is the African lungfish which can reach 2 m (6.6 ft) in length and weigh up to 50 kg (110 lb). These fishes share many morphological characteristics with the common ancestors of tetrapods due to common ancestry.

Summary

While a notocord is a common feature of chordates, a cartilaginous or bony skull is a more recent feature and it is only found in vertebrates. Bony jaws make another feature that evolved later in groups of fishes that became highly diverse.

Key terms

Jawless fish, lamprey, hagfish, ostracoderms, cyclostomes, jawed fish, gnathostomes, placoderm, acanthodian, cartilaginous fish, bony fish, Chondrichthyes, Osteichthyes, actinopterigian, sarcopterigian

Figure credits

Figure 1 by Petter Bøckman - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12990830

Figure 2 by Tracyanne - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18114948

Figure 3 by M.Buschmann - Germany, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=269108

Figure 4 byblic Domain, https://commons.wikimedia.org/w/index.php?curid=238724

Figure 5 byblic Domain, https://commons.wikimedia.org/w/index.php?curid=3656653

Figure 6 by Terry Goss, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1561215

Figure 7 byblic Domain, Wikipedia, https://en.wikipedia.org/wiki/Actinopterygii

Figure 8 by FASTILY (TALK) - I created this work entirely by myself., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6829574

Figure 9 by Tannin at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=179605