Vertebrate

• 1. The Five Percent

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  The smallest frog known to science, Brachycephalus pulex, measures just 6.45 millimetres from snout to vent. The largest animal alive, the blue whale, stretches up to 33 metres and weighs around 150 tonnes. Both belong to the same subphylum: Vertebrata. They share a vertebral column wrapped around the spinal cord, and a cranium guarding the brain. Around 65,000 species carry this body plan, the largest ranked grouping inside the phylum Chordata. Mammals, birds, amphibians, reptiles, and the many classes of fish all fall under it. Yet for all their reach, vertebrates account for less than five percent of every described animal species. How did a single architecture, a spine and a skull, spread across water, land, and air? Why does a hagfish belong in the same group as a human, and why did zoologists argue about it for over a century? And what is now pushing many of these animals into steep decline? The answers begin more than 500 million years ago, in a shallow sea.

• 2. Spine And Skull

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  A notochord, a hollow nerve cord, a post-anal tail, an endostyle, and pharyngeal gills in pairs: these five synapomorphies define the phylum Chordata. Vertebrates carry all five, sharing them with their chordate relatives. What sets vertebrates apart is a second, longer list of unique traits. They have a vertebral column, a skull of bone or cartilage, and a large brain divided into three or more sections. They have a muscular heart with multiple chambers, an inner ear with semicircular canals, and sense organs including eyes, ears, and nose. Their digestive equipment includes intestines, liver, pancreas, and stomach. The word itself comes from the Latin vertebratus, meaning jointed, traced back through vertebra, a joint, to vertere, to turn. As embryos, vertebrates still grow a notochord. In all but the jawless fishes, a vertebral column of bone or cartilage replaces it during development. Vertebrate embryos also form pharyngeal arches, which support the gills in adult fish and become other structures in adult tetrapods. Beneath all this anatomy lies a quieter signature, written not in bone but in proteins.

• 3. The Molecular Signature

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  Five molecular markers are shared exclusively by all vertebrates and reliably tell them apart from every other animal. These conserved signature indels appear in protein sequences such as protein synthesis elongation factor-2, eukaryotic translation initiation factor 3, and adenosine kinase, along with a protein related to ubiquitin carboxyl-terminal hydrolase. Two further markers point to a specific kinship between vertebrates and tunicates. The proteins Rrp44, linked to the exosome complex, and serine C-palmitoyltransferase are shared by species from these two subphyla alone. Cephalochordates do not carry them, a detail that would later reshape the family tree of the chordates. That shared chemistry hints at an origin the fossil record places in a single dramatic chapter of early animal life.

• 4. Age Of Fishes

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  The earliest known vertebrates lived about 518 million years ago, members of the Chengjiang biota that emerged during the Cambrian explosion. Among them were Haikouichthys, Myllokunmingia, Zhongjianichthys, and probably Yunnanozoon. They already had the basic vertebrate body plan, a notochord, rudimentary vertebrae, and a defined head and tail, but no jaws. Some researchers argue that Haikouichthys and other Myllokunmingiidae are basal stem group craniates rather than true vertebrates. Small eel-like conodonts, known from microfossils of their paired tooth segments, span from the late Cambrian to the end of the Triassic. Their hard teeth set off a long debate over whether teeth or bones mineralized first, with the evidence favouring the mineralized skeleton. Jawed vertebrates may have arisen in the late Ordovician, around 445 million years ago, or in the Silurian, and grew common during the Devonian, the period often called the Age of Fishes. The bony fishes appeared in the Silurian and spread widely in the Devonian. By the middle of that period, a lineage of bony fishes called the sarcopterygii carried both gills and air-breathing lungs, suited to swampy pools. They pushed themselves across land on muscular paired fins. Those fins, already built with bones and joints, became the two pairs of walking legs of the first tetrapods in the Famennian stage of the Devonian.

• 5. Onto Dry Land

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  In the Carboniferous, the tetrapods that had crawled from Devonian waters established themselves on land as amphibians. The same period saw the amniotes branch off from amphibious tetrapods, carrying membranes around the embryo that let it survive away from water. The Mesozoic opened in disaster. Every larger vertebrate group was devastated after the largest mass extinction in earth history. The recovery brought a wave of new groups still alive today, a moment described as the origin of modern ecosystems. On the continents appeared the ancestors of modern lissamphibians, turtles, crocodilians, lizards, and mammals, alongside the dinosaurs, which gave rise to birds later in the era. In the seas, various marine reptiles evolved, and new groups of fish appeared. The Mesozoic ended with another extinction event that wiped out the dinosaurs, except for birds, and many other vertebrate groups. The era that followed, the Cenozoic, is sometimes called the Age of Mammals. Placental mammals came to dominate the Northern Hemisphere, while marsupials held the Southern. Sorting all these creatures into a tidy system, though, proved far harder than the story of their descent.

• 6. Naming The Vertebrates

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  In 1801, Jean-Baptiste Lamarck defined the vertebrates as a phylum distinct from the invertebrates he studied. He split them into four classes, fish, reptiles, birds, and mammals, while treating cephalochordates and tunicates as molluscs. In 1866, Ernst Haeckel grouped his Craniata and his Acrania together under the name Vertebrata. Ray Lankester in 1877 gathered craniates, cephalochordates, and tunicates as Vertebrata. Francis Maitland Balfour placed the Vertebrata as a subphylum within the chordates in 1880 to 1881. In 2018, Naoki Irie and colleagues proposed promoting Vertebrata to a full phylum. The conventional system groups extant vertebrates into seven classes, three of fish and four of tetrapods. It lists Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia, with two extinct armoured fish classes, Placodermi and Acanthodii, alongside. The trouble is that many of these groups are paraphyletic. Birds derive from a group of reptiles, so Reptilia excluding Aves is not a natural grouping. For that reason, most scientists working with vertebrates now use a classification built purely on phylogeny. No creature tested that approach more than the hagfish.

• 7. The Hagfish Question

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  In 1758, Linnaeus classified hagfishes not as vertebrates but as Vermes, the worms. In 1806, Andre Marie Constant Dumeril grouped hagfishes and lampreys as Cyclostomi, marked by horny teeth on a tongue-like apparatus, a large notochord in adults, and pouch-shaped gills. Edward Drinker Cope coined the name Agnatha, meaning jawless, in 1889, splitting vertebrates into the Agnatha and the jawed Gnathostomata. In 1927, Erik Stensio suggested the two living agnathan groups arose independently from fossil ancestors. Then in 1977, Soren Lovtrup argued that lampreys are more closely related to gnathostomes, citing radial muscles in the fins, true lymphocytes, neuromasts in the inner ear, and a cerebellum. That implied Vertebrata and Craniata were distinct taxa, leaving the hagfish outside. In 2002, Delarbre and colleagues used mtDNA sequencing and found Myxini more closely related to Hyperoartia than to Gnathostomata, meaning the living jawless fishes form a clade, Cyclostomata. The debate closed in 2010 when French paleontologist Philippe Janvier accepted that both vertebrates and cyclostomes were monophyletic, adding that hagfishes are strongly degenerate and have lost many characters over time. In 2019, Tetsuto Miyashita and colleagues reconciled the molecular and morphological pictures, supporting the Cyclostomata hypothesis with morphology alone. The same year, they placed fossil agnathans like the Myllokunmingiida tentatively in the vertebrate total group, outside the crown group that led to all living vertebrates.

• 8. A Steep Decline

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  The Living Planet Index tracked 16,704 populations across 4,005 vertebrate species and recorded a 60 percent decline between 1970 and 2014. Freshwater species fell by 83 percent since 1970, and tropical populations in South and Central America dropped by 89 percent. The authors caution that an average trend in population change is not an average of total numbers of animals lost. According to WWF, the trajectory could lead to a sixth major extinction event. Five causes drive the loss: land-use change, overexploitation of natural resources, climate change, pollution, and invasive species. These pressures fall on a group split roughly evenly, in described and living species, between non-tetrapod fish and tetrapods. By the 2014 tally, described vertebrates numbered 66,178, set against more than 1.3 million described invertebrate species. The amphibians alone, at 7,302 described species, sit at the front line of freshwater decline traced by that index.