In this article we will discuss about the historical resume and classification of jawless vertebrates.
L. Agassiz (1857) placed the agnaths under the class Myzontes and proposed the following scheme for the lampreys and hagfishes:
Class I. Myzontes. Order 1. Myxinides and Order 2. Cyclostomi.
E. S. Goodrich (1909) gave the following classification in his book “Cyclostomes and Fishes”. Branch I and Class Cyclostomata.
Subclass 1: Myxinoidea and Subclass 2. Petromyzontia:
In 1930, Goodrich amended his scheme. Branch Monorhina (single nose) which includes Class 1. Cyclostomata (Marsipobranchii), Subclass 1. Myxinoidea, Subclass 2. Petromyzontia and Class 2. Ostracodermi that includes Order 1. Anaspida (no shield), Order 2. Cephalaspidomorphi (head shield form), Order 3. Pteraspidomorphi (fin-shield) and Order 4. Pterichthyomorphi.
A. S. Romer (1933) included the jawless vertebrates under the class Agnatha in his book “Vertebrate Palaeontology”. Romer and T. S. Parsons (1977, 1986) propose the scheme — Class 1. Agnatha, Order 1. Osteotraci, Order 2. Anaspida, Order 3. Heterostraci, Order 4. Petromyzontia (lampreys) and order 5. Myxinoidea (hagfishes). They did not mention any name of “Superclass” under subphylum vertebrata.
L. S. Berg (1940) grouped the extinct and extant jawless fishes under the following classes:
(iii) Pteraspides and
But Young (1981) placed the surviving agnathans under the Superclass Agnatha (= Cyclostomata) and Class Cephalaspidomorphi (= Monorhina) which includes the order Cyclostomata (round mouth) and suborders (i) Petromyzontidae (lampreys) and (ii) Myxinoidea (hagfishes).
McFarland et al, (1985) placed the extant jawless fishes under Superclass and Class Agnatha but lampreys are placed under Subclass Monorhina (Cephalaspidomorphi) and Order Petromyzonteformes, and hagfishes are placed under Subclass Diplorhina (Pteraspidomorphi) and Order Myxiniformes.
Scheme of Classification:
The classification of agnathans adopted in the 7th edition of this book, is based on Nelson’s scheme (1994), taken from the book “Fishes of the World”. Nelson (1994) has represented the following scheme of classification for a primitive grade of vertebrates.
Superclass: Agnatha (Cyclostomata, Marsipobranchii)
Class 1 : Myxini (Hagfishes)
Order 1 : Myxiniformes
Class 2: Pteraspidomorphi (Diplorhina)
Order 1: Arandaspidiformes
Order 2: Pteraspidiformes (Heterostraci)
Order 3: Thelodontiformes (Coelolepida)
Class 3: Cephalaspidomorphi (Monorhina)
Order 1: Petromyzontiformes (Lampreys)
Order 2: Anaspidiformes (Birkeniae)
Order 3: Galeasidiformes
Order 4: Cephalaspidiformes
Classification with Characters:
Superclass Agnatha (Cyclostomata, Marsipobranchii):
[Agnatha, jawless; Cyclostomata, round mouth; Marsipobranchii, pouch-like gills]
1. Jaws that are derived from branchial arches are absent.
2. Notochord is only persistent throughout life but vertebral centra are never present.
3. Pelvic fins are absent.
4. Gill arch skeleton is fused with neurocranium, external to gill-lamellae.
Class 1. Myxini [About 43 species]
1. Eel-like, cylindrical animals, commonly called “hag fishes”.
2. Body is naked and highly slimy. Slime is produced from thread cells which lie single in the epidermis.
3. Buccal funnel is absent, and three pairs sensory tentacles around the surface of the mouth.
4. Eyes are vestigial.
5. The undivided, ill-developed dorsal fin (Nelson’s report supports the absence of dorsal fin) is continuous with the caudal fin. Hardisty (1979) reports about the dorsal fin as a little skin fold without fin rays.
6. Paired fins are absent.
7. Adult hagfishes lack a pineal organ and a lateral line system.
8. The teeth-like processes on their tongues use to rasp flesh from the prey.
9. Gill pouches vary from 5-14.
10. Nasohypophysial duct opens into the pharynx.
11. Single circular canal which is formed by the fusion of two semicircular canal is present.
12. Testes and ovaries are found in the same individual but they are not hermaphrodite, because the single gonad (either testes or ovaries) becomes functional.
13. Eggs are large and yolky, enclosed in a horny shell.
14. Meroblastic cleavage.
15. There is no larval stage.
First fossil hagfish — Myxinikela siroka has recorded from the Pennsylvanian of Illinois.
Temperate zones of the world and Gulfs of Panama and Mexico (Fig. 5.4A).
The distribution of hagfishes has been shown in Fig. 5.4A.
Habit and habitat:
Hagfishes are marine and scavengers, and remain buried in mud or sand. They consume polychaete and other invertebrates, and also feed on the dead and dying fishes.
1. Eye musculature is absent.
2. Single olfactory capsule is present.
3. Nasopharyngeal duct opens posteriorly into the pharynx.
4. The dorsal and ventral roots of the spinal nerves are united.
5. Branchial basket is ill-developed.
6. They are the only vertebrates in which the body fluid is isotonic with sea-water.
All the members are placed under a single family Myxinidae which includes two subfamilies:
(i) Myxininae and
(ii) Eptatretinge. Myxinidae includes 6 genera.
Myxine (Eastern and Western North Atlantic, Greenland, Cape of Good Hope, Gulf of Panama, South Chile to South Argentina, East Coast of Japan), Eptatretus = Bdellostoma (Fig. 5.17) (South Africa, Australia, East Coast of Japan, Formosa, Gulf of Mexico, Chile, Alaska, California and New Zealand), Notcmyxine (Argentina to Patagonia), Neomyxine (New Zealand) and Nemamyxine (New Zealand).
Distribution of all genera has been shown in Fig. 5.4A.
Stensio (1968) stated that hagfishes may be the derivatives of pteraspidomorphs (fin- shield) and lampreys are of the cephalaspidomorphs (head shield form). With the further knowledge the trend of the views has changed. Forey and Janvier (1994) state that the hagfishes are the most primitive vertebrates because they lack the rudiments of vertebrae (arcualia).
Nelson (1994) states that the living agnathans and the heavily armoured fossil group — Ostracoderms are not natural evolutionary taxa. Kardong (2002) points out the physiological similarity of the hag-fishes with the invertebrates, and distinctiveness from other vertebrates.
Class 2. Pteraspidomorphi, fin shield (= Diplorhina, double nose)
The members of this group appeared in the Ordovician and continued up to the Late Devonian.
1. Two semicircular canals are present.
2. Paired nasal openings are present.
3. Bone is present but true bone cells are absent.
4. Most head shields are formed by fusion of several large bony plates.
The fossils have recorded from marine sediments and mainly of the Ordovician of the Southern Hemisphere.
1. Numerous external branchial openings present.
2. Pineal and Para pineal openings lateral to one other.
e.g., Arandaspis, Andinaspis, Astraspis
Order Pteraspidiformes, fin shield form (Heterostraci, different shells):
The members of this order were heavily armoured ostracoderms of the Lower Silurian to the Upper Devonian.
1. Most pteraspidomorphs possessed head shields of bony plates and scales covering the trunk and tail.
2. Maximum length is 1.5m.
3. Eyes were very small and lateral in position.
4. The fins were absent.
5. The tail-fin was of heterocercal type.
6. The mouth was a slit-like aperture.
7. Snout was prolonged into a long rostrum.
e.g., Pteraspis (Fig. 5.2), Psammosteus, Eglonaspis, Anetolepis.
Order Thelodontiformes (Coelolepida):
The numbers of this group have recorded from Upper Ordovician to Upper Devonian.
The position of this group is still uncertain.
1. The fossils of this group were identified by scales, a single dorsal fin, and in some forms with paired fins.
2. In some head and anterior part of the body are depressed.
3. The tail is heterocercal.
4. An anal fin is present.
5. A lateral pair of ribbon-like fins were present.
e.g., Thelodus, Lanarkia, Apalolepis, Katopordus
Class 3. Cephalaspidomorphi, head shield form (Monorhina, single nostril):
The fossils of this group have recorded from the Late Ordovician to the Late Devonian.
1. Single median nasal aperture is far back on the head.
2. Two semicircular canals are present.
3. The eyes are closely placed on the dorsal side of the head.
4. The head was dorsoventrally flattened and was covered by a cephalic shield (carapace) which was prolonged laterally into lateral horns.
5. The trunk was covered by elongated vertical scales.
6. The tail was heterocercal.
7. Pectoral fins were present though to be homologous to gnathostome pectoral fins.
Order Petromyzontiformes [About 41 species]:
1. Body is eel-like and naked.
2. Single nasal opening is found on the top of the head.
3. Well-developed eyes are lateral except Mordacia which are dorsal.
4. Adult lampreys are predaceous with rounded oral suckers.
5. Rasping tongue.
6. Median fins are present but paired fins are absent. The fins are not supported by true fin-rays.
7. Seven pairs gill pouches are present which open into seven pairs lateral gill openings.
8. Tail is isocercal in adults but hypo cercal in larval (ammocoete) stage.
9. Two semicircular canals are present.
10. Brachial basket is well-developed.
11. They lack bone entirely.
12. Nasopharyngeal duct is closed behind.
13. The dorsal and ventral roots of the spinal nerves remain separate.
14. Metamorphosis present. A larval stage is present which may be regarded more primitive than adult.
15. They are commonly called lampreys/lamper eels/lampers/sand pride.
Though the two groups (petromyzontids and myxinids) have been included under a single class for a long time. But McFarland et al., 1985; Romer and Parsons, 1986 and Nelson, 1994 consider that they are phylogenetically apart.
The order recognises 3 living families but Nelson (1994) recognises a single family — Petromyzontidae with 4 subfamilies:
(iii) Mordaciinae and
Temperate seas of both Northern and Southern Hemispheres. They also found in fresh water and salt water of rivers and lakes of both hemispheres. The distribution of lampreys has been shown in Fig. 5.4B.
Bond (1996) has reported two fossil representatives — Mayomyzon and Hardistiella montanensis, both from the Carbonifersous period of U.S.A.
Habit and habitat:
Freshwater, marine, anadromous, and predators. Larva leads independent life.
Petromyzon (common in the rivers of Europe, Japan, North America and western parts of Africa), Entosphenus (found in the western coasts of North America), Ichthyomyzon (abundant in the east coast of North America), Lampetra (the Eurasian fresh and salt water lamprey), Geotria (present in the rivers of Chile, Australia and New Zealand), Mordacia (occurs in Tasmania and Chile), Lethenteron (White sea to North Pacific, Alaska, Siberia, Great Lakes of N. America, St. Lawrence Basin, Mississippi drainage, Atlantic coastal region, Japan, N. Italy), Caspiomyzon (Caspian basin), Tetrapleurodon (Mexico), Eudontomyzon (Some tributaries of Baltic and Black Sea). Distribution of all genera has shown in Fig. 5.4B).
Order Anaspidiformes (no shield form) (Birkeniae):
The fossils have recorded from the upper Silurian to Devonian from the freshwater sediments:
1. Body was stream-lined and somewhat compressed.
2. A single median nostril was on the top of the head.
3. Dorsal fin was absent and replaced by scutes.
4. The body was covered by a thinner scale like plates.
5. The tail was hypo cercal.
7. Eyes were lateral.
8. They had pectoral spikes.
e.g., Birkenia (Fig. 5.3), Lasanius, Jamoytius (Fig. 5.5), Pharyngolepis etc”.
Order Galeaspidiformes (Helmet shield form):
They recorded from Lower Silurian to Middle Devonian from China.
1. They had a large number of branchial openings.
2. No paired fins.
3. They had a cephalic shield.
4. Front part of the body was drawn out into lateral spines.
5. A large median opening was present in front of eyes.
6. The structures of the brain and other parts were seen in the fossils.
e.g., Duyunolepis, Galeospis, Eugaleaspis, Huananpis etc.
Order Cephalaspidiformes (Head shied form) (Osteostraci, bone shell):
The fossils have recorded from Upper Silurian to Upper Devonian, predominantly from freshwater habitats. The group is well represented as compared with other fossil agnathans.
1. The animals were small, attaining maximum length about 60 cm.
2. Head was flattened and triangular posteriorly.
3. Eyes were dorsal.
4. There were 10 pairs ventral gill openings with large brachial chambers.
5. The entire head was enclosed by a solid, sculptured bony shield.
6. The tail was covered by scales or plates.
7. Lobe-like pectoral projections indicate the most peculiar feature among the agnathans.
e.g., Hemicyclaspis, Cephalaspis (Fig. 5.6), Kiaeraspis (Fig. 5.7).