Agnatha

• 1. Introduction

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  Agnatha means jawless fish, and it names a group defined by something missing. These animals have no jaws. Around 500 million years ago, two kinds of recombinatorial adaptive immune systems arose in vertebrates, and the jawless line took a path nobody else followed. Today only about 120 species of jawless fish survive, a minor element of modern marine life. Yet in the early Paleozoic they were everywhere, and one of their armored kinds gave rise to bony fish, and through them to tetrapods, including humans. How does a fish without a jaw eat, defend itself, and reproduce? Why do the two surviving kinds, hagfishes and lampreys, refuse to fit neatly into the vertebrate family tree? And why can the word Agnatha not even be made into a coherent group without either gutting it or swallowing all of Vertebrata?

• 2. Life Without A Jaw

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  Agnathan teeth cannot move up and down, so the kinds of food these animals can take are limited. Hagfish use a row of sharp teeth to break down dead animals, feeding mostly as scavengers, though they have also been observed hunting. Lampreys feed on carrion as well as on other fish and marine mammals, while some species are not carnivorous at all. When a lamprey latches onto a host, it injects anticoagulant fluids that prevent the blood from clotting, making the host yield more blood. Inside, the body is just as spare. Agnathans have no distinct stomach, only a long gut that is more or less the same along its whole length. Being ectothermic, they do not regulate their own body temperature, and their metabolism runs slow in cold water. A slow metabolism means they do not have to eat very much, an economy of effort that suits an animal built without a biting jaw.

• 3. Skin, Slime, And Armor

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  Hagfish skin carries copious slime glands, and that slime is their defense. The slime can clog the gills of an enemy fish and kill it. Modern agnathans are covered in plain skin, with neither dermal nor epidermal scales over the body. The skeleton is cartilaginous, the heart holds two chambers, and lampreys carry a light-sensitive pineal eye that is homologous to the pineal gland in mammals. Almost all agnathans, living and extinct, lack paired appendages, though most do have a dorsal or a caudal fin. The extinct relatives tell a sharply different story. Many sported extensive exoskeletons of massive, heavy dermal armor or small mineralized scales. Some fossil agnathans, such as osteostracans and pituriaspids, even had paired fins, a trait their jawed descendants would inherit.

• 4. Thirty Eggs A Lifetime

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  It is believed that a hagfish produces only 30 eggs over an entire lifetime. Much about hagfish reproduction stays unknown; even the mode of fertilization has not been established, and there is little of the larval stage that defines the lamprey. Lampreys reproduce in freshwater riverbeds, where pairs work together to build a nest and bury their eggs about an inch beneath the sediment. Fertilization in lampreys is external, development is probably external in both groups, and there is no known parental care. A lamprey can only reproduce once. The hatchlings that emerge spend four years in larval development before they become adults.

• 5. Two Living Survivors

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  Myxini, the hagfishes, are eel-shaped slime-producing marine animals sometimes called slime eels. They are the only known living animals that possess a skull but no vertebral column, and they have undergone the most extensive gene loss of any vertebrate, missing 1,386 gene families. Living hagfish remain similar to hagfish of 300 million years ago, true living fossils. Lampreys belong to Petromyzontida, also called Hyperoartia, a disputed group covering modern lampreys and their fossil relatives. Early examples in their record, Endeiolepis and Euphanerops, were fish-like animals with hypocercal tails that lived during the Late Devonian. Even the existence of Hyperoartia is contested, with some analyses treating the basal forms as a lineage called Jamoytiiformes that may sit very close to the ancestors of jawed vertebrates. Both surviving kinds share an immune system unlike anything in jawed animals. Where jawed vertebrates build their defenses by rearranging V(D)J gene segments, jawless vertebrates rely on variable lymphocyte receptors assembled from highly diverse leucine-rich-repeat sequences. Three such genes, VLRA, VLRB, and VLRC, appear in lampreys and hagfish. VLRA and VLRC cells are T-cell-like and develop in a thymus-like structure called a thymoid, while VLRB cells are B-cell-like and mature into plasma cells that secrete VLRB antibody.

• 6. Teeth As Timestamps

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  Conodonts were eel-like jawless animals that lived from the Cambrian until the beginning of the Jurassic, and for over a century they were known only from microscopic phosphatic tooth structures called conodont elements. Not until the mid-1980s did body fossils turn up, in Scotland and Wisconsin, revealing what these animals actually looked like. Their teeth make excellent index fossils, since many species lived and died out over short spans, useful from the late Cambrian to the Triassic. Conodonts were strikingly varied in lifestyle, ranging from filter feeders to macropredators. They peaked in diversity during the middle Ordovician, were struck hard by the Ordovician-Silurian extinction, surged again in the mid-late Devonian, then declined in the Carboniferous and grew rare in the Permian. In the early Triassic their numbers rose dramatically. They finally went extinct in the lower Jurassic, with some of the last populations holding on in Japan, perhaps protected by the remoteness of the area. They were once thought wiped out by the end-Triassic extinction, but it now appears newer Mesozoic taxa out-competed them.

• 7. The Age Of Armor

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  Two Early Cambrian animals with fins, vertebrate musculature, and gills are known from the Maotianshan shales of China: Haikouichthys and Myllokunmingia, tentatively assigned to Agnatha by Janvier. A third possible agnathan from the same region is Haikouella, and Simonetti reported another, undescribed, from the Middle Cambrian Burgess Shale of British Columbia. The myllokunmingiids, endemic to the Maotianshan Shales, count as the earliest known craniates, chordates with a skull of hard bone or cartilage. The first armored agnathans, the ostracoderms, appear in the middle Ordovician, and by the Late Silurian the agnathans reached the high point of their evolution. Many Ordovician, Silurian, and Devonian forms wore heavy bony-spiky plates. The ostracoderms themselves were a varied lot. Pteraspidomorphi bore extensive head shielding and dermal bone, with sucking mouth parts and hypocercal tails, some perhaps living in fresh water, holding the subgroups Heterostraci, Astraspida, and Arandaspida. Thelodonti, the nipple teeth, carried small spiny scales rather than large plates, and those scales scattered so easily after death that they became the most common vertebrate fossil of their time. Anaspida, meaning without shield, were small marine forms with a striking hypocercal tail, classically regarded as the ancestors of lampreys. Cephalaspidomorphi, named for the osteostracan genus Cephalaspis, were heavily armored with well-developed head shields, lateral line grooves across the surface, small eyes set on top of the head, and a mouth ringed by small plates that allowed flexible lips but no ability to bite.

• 8. A Group That Cannot Hold

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  Most ostracoderms, including thelodonts, osteostracans, and galeaspids, were more closely related to the jawed gnathostomes than to the surviving cyclostomes. Cyclostomes apparently split from other agnathans before dentine and bone evolved, traits present in many fossil agnathans, including conodonts. This is the knot at the heart of the group. Conodonta was jawless, yet had it kept going, its descendants would be more closely related to humans than to lampreys. Because of that, Agnatha cannot be made into a coherent grouping without either removing every non-cyclostome or swallowing all of Vertebrata and becoming a junior synonym for it. The agnathans declined in the Devonian and never recovered. Molecular data from rRNA and mtDNA, together with embryological data, strongly supports the view that hagfishes and lampreys are more closely related to each other than to jawed fish, forming the superclass Cyclostomi. Hagfish count as members of Vertebrata because they secondarily lost their vertebrae, a conclusion drawn from molecular and developmental data, and the newer phylogeny from Miyashita and colleagues in 2019 is considered compatible with both the morphological and the molecular evidence.