Agnatha

Agnatha, meaning “without jaws”, refers to a paraphyletic infraphylum of jawless vertebrates within the subphylum Vertebrata of the phylum Chordata. The group includes the living cyclostomes—hagfishes and lampreys—as well as numerous extinct lineages such as conodonts, osteostracans, galeaspids, pituriaspids and heterostracans. Although now a minor component of marine faunas, agnathans were once among the most dominant vertebrates of the early Palaeozoic. Their evolutionary history, anatomy and physiology provide essential insight into the origins of vertebrate features and the transition toward the jawed vertebrates, or gnathostomes.

Taxonomic Position and Phylogenetic Relationships

Agnathans are united by the absence of jaws, a primitive condition for vertebrates. Traditionally classified as a single group, they are not monophyletic because several extinct jawless lineages are more closely related to gnathostomes than to modern cyclostomes. Molecular sequencing from rRNA and mitochondrial DNA, together with embryological evidence, strongly supports the grouping of hagfishes and lampreys into the clade Cyclostomi, which is the living sister group to the gnathostomes.
Many fossil agnathans—such as osteostracans, galeaspids and thelodonts—possessed bone and dentine, features absent in living jawless fish. Cyclostomes appear to have diverged before the evolution of these tissues. Conodonts, once enigmatic due to their tooth-like elements, are now understood as jawless vertebrates with affinities closer to the gnathostome lineage than to cyclostomes.
The infraphylum is therefore considered paraphyletic: excluding gnathostomes results in an incomplete representation of vertebrate evolution, while including them collapses Agnatha into Vertebrata as a whole. Recent phylogenetic interpretations, such as those proposed by Miyashita and colleagues, reconcile both molecular and morphological evidence in demonstrating the complex relationships among early vertebrates.

Morphological Characteristics

Modern agnathans show several defining features:

• Absence of jaws and absence of paired fins, although dorsal or caudal fins are present.
• Persistent notochord in both larval and adult stages.
• Seven or more paired gill pouches, supporting respiration.
• Cartilaginous skeleton and a two-chambered heart.
• Lack of dermal or epidermal scales; hagfishes possess extensive slime glands used for defence.
• Reduced sensory structures, though lampreys retain a functional pineal gland.

Extinct agnathans, by contrast, often bore heavy dermal armour or arrays of mineralised scales, demonstrating remarkable morphological diversity. Some groups, including osteostracans and pituriaspids, developed paired pectoral appendages, suggesting early stages of fin evolution later elaborated in gnathostomes.

Metabolism and Feeding Behaviour

Agnathans are ectothermic, their metabolic rates highly influenced by ambient water temperatures. Without a true stomach, their digestive tract is a relatively uniform tube. Feeding strategies vary markedly between hagfishes and lampreys:

• Lampreys, many of which are carnivorous or parasitic, attach to hosts and utilise anticoagulant secretions to extract blood and tissues. A proportion of species, however, are non-carnivorous and feed on detrital matter.
• Hagfishes are primarily scavengers, entering carcasses through natural openings and using keratinised tooth-like structures to tear flesh. They are also observed occasionally engaging in active predation.

The inability of agnathan teeth to articulate vertically limits the range of prey items they can process, contributing to their specialised ecological niches.

Reproduction and Development

Reproductive biology varies, though several features are shared across living agnathans:

• External fertilisation occurs in lampreys; the fertilisation mode in hagfishes remains uncertain.
• No parental care has been recorded in either group.
• Egg numbers are low, particularly in hagfishes, where individuals are thought to produce approximately thirty eggs over a lifetime.
• Lampreys display a pronounced larval phase, lasting up to four years, during which the ammocoetes filter-feed in sediments before metamorphosing into adults. Most lamprey species are semelparous, spawning once and dying thereafter.
• Nest building is common in lampreys, which excavate small depressions in riverbeds for egg deposition.

These reproductive strategies reflect adaptations to environments where juvenile survival is favoured through large numbers of eggs or, alternatively, extended larval development.

Evolutionary History

The earliest known agnathans appeared during the Cambrian. Fossils such as Haikouichthys, Myllokunmingia and Haikouella from the Chengjiang biota of China provide compelling evidence for the early evolution of vertebrate musculature, gill structures and fin-like appendages. A possible Middle Cambrian agnathan from the Burgess Shale further supports the antiquity of this group.
Conodonts emerged during the early Cambrian and persisted until the Triassic. Their phosphatic tooth elements, ubiquitous in marine sediments, serve as valuable index fossils for Palaeozoic and early Mesozoic biostratigraphy.
During the Ordovician, heavily armoured ostracoderms diversified widely. By the Late Silurian, agnathans reached peak diversity, including the armour-plated heterostracans, galeaspids and osteostracans—many of which leaned closer phylogenetically to gnathostomes than to cyclostomes.
Agnathan diversity declined steadily in the Devonian as jawed vertebrates radiated and occupied new ecological roles. Although modern cyclostomes represent only a small vestige of earlier diversity, they retain key ancestral traits critical to understanding vertebrate origins.

Immunological Innovations

Around 500 million years ago, two distinct adaptive immune systems arose in vertebrates. Jawed vertebrates developed immunoglobulin-domain receptors via V(D)J recombination and somatic hypermutation. Jawless vertebrates, on the other hand, evolved a unique system based on variable lymphocyte receptors (VLRs) constructed from leucine-rich repeat modules.
Three VLR genes—VLRA, VLRB and VLRC—correspond to T-cell-like and B-cell-like lineages. VLRA and VLRC cells mature in thymoid structures, whereas VLRB cells mature in haematopoietic tissues and differentiate into plasma cells producing secreted VLR antibodies. These findings highlight a deep evolutionary divergence in vertebrate immune strategies.

Diversity and Classification

Agnathans encompass both extant cyclostomes and several extinct clades. Representative groups include:

• Cyclostomi: hagfishes and lampreys.
• Conodonta (extinct).
• Heterostraci, Astraspida and Arandaspida.
• Pteraspidomorphi, Osteostraci, Thelodonti, Galeaspida and Pituriaspida.
• Other early forms such as Metaspriggina, Nuucathys, Palaeospondylus and Yunnanozoonidae are often discussed within broader agnathan contexts.