Coelacanths - a primer
1938 postcard commemorating Marjorie Courtenay-Latimer (South African Institute for Aquatic Biology, Public Domain)
One of the most familiar and widely-celebrated "living fossils", the deep-ocean fishes known as coelacanths are actually far-removed from their remote kin.
Though their morphology betrays similarities with a few Mesozoic species, the sequencing of the coelacanth genome in 2013 revealed evidence of substantial genetic evolution across tens of millions of years (Amemiya, et al. 2013). And a quick glance at the fossil record shows that long extinct coelacanths were far more diverse in form than our surviving species. These are just some of the many observations that have led scientists to argue that the term "living fossil" itself should be retired (Yong, 2013).
Discovery
It has been roughly 87 years since humanity was reintroduced to the coelacanth. Though claimed to have long been familiar to West African fishermen, it turns out that accounts of the discovery have confused the local word gombessa/ngombessa with coelacanths, when in fact the term refers to the oilfish Ruvettus pretiosus. When Hendrik Goosen and his crew on the Nerine hauled up a live coelacanth from 240 feet deep on December 22, 1938, it was completely unfamiliar and believed by the deckhands to be a "great sea lizard". Fortunately for museum curator Marjorie Courtenay-Latimer of the neighboring East London Natural History Museum, the fish had been saved for her (Courtenay-Latimer had established connections with a number of South African fishermen to aid in research, and Goosen was particularly fond of her). She boarded the Nerine, observed the still-blue, 54-inch fish with much delight, and had it taxied to the museum.
Photos were taken, but by that point the fish had been already rotting and decaying; there was no choice but to remove everything and save only the skin for taxidermy. Fortunately, Courtenay-Latimer had made a small sketch of the animal and sent it to zoologist James Leonard Brierly Smith, who was then South Africa's only ichthyologist. Reaching him on January 3rd, Smith reached the same conclusion as his colleague: it had features resembling a lungfish, but was clearly distinct. Subsequent research lead Smith to the fossils of two Mesozoic coelacanths, Macropoma & Undina, and it was then that everything clicked. Thought to be extinct since the Late Cretaceous Epoch, Goosen and Courtenay-Latimer had revealed that coelacanths were alive and well in the 20th Century. Smith formally described the West Indian coelacanth as Latimeria chalumnae in honor of Courtenay-Latimer.
More work had to be done to be sure, however, as all that was left of the first catch was the skin. So following World War II (1946), Smith put up a reward poster in English, Portuguese, and French for £100 to any South African who could dredge a new specimen. It wouldn't be until the night of December 27, 1952 when another was finally caught by Ahmed Hussein Bourou and his friend Soha off Anjouan Island in the Comores. They had been in a dugout canoe over 656 feet offshore and had hooked one, killing it with a hit to the skull. It was brought to Captain Eric Hunt, an avid fish enthusiast, who had it salted, bisected, and injected with formalin to preserve its internal organs. An anxious and desperate Smith traveled to the Comores and finally met his fabled fish in a fit of sobs. The journey of discovery of these coelacanths is vividly detailed in the 1991 book Living Fossil by Keith S. Thomson.
Since that time quite a number of coelacanth specimens have been unearthed, including the recognition of a second species from Indonesian waters Latimeria menadoensis in 1999. However, throughout this period, the increased frenzy caused by coelacanths saw a sizable reduction in numbers (Plante, et al. 1998), and today the two species are classified as threatened and endangered species.
Miguashaia of the Middle Devonian (DiBgd, CC BY-SA 4.0)
Natural History & Diversity
Coelacanths are considered lobe-finned fishes of the clade Sarcopterygii (Greek for "fleshy-fins"), so called because the base of their fins are fleshy lobes supported by bone and muscle. Specifically, the skeletal structure of the fins are essentially blueprints of the arms of tetrapods: a femur connecting to a radius & ulna, which connects to a wrist and tiny digit bones, all of which is supported on shoulder or hip joints. Coelacanths (clade Actinistia) are distinguished among the lobe-fins by a particular arrangement of skull and tail morphology. Specifically, there is the triangular operculum which covers the gills, while the tail sports its own special lobe (the epicaudal) that aids in underwater locomotion (Prothero, 2022; Maisey, 1996).
They appear to be less closely related to tetrapods than lungfishes - contrary to a longstanding tradition of thought in paleontology - a fact that was initally argued from fossil data but finally confirmed through the first genomic analysis (Amemiya, et al. 2013).
Genomic phylogeny of jawed vertebrates (Amemiya, et al. 2013)
Coelacanths first evolved sometime during the Early Devonian, represented by two genera: Euporosteus & Eoactinistia of Yunnan, China and eastern Victoria, Australia, respectively. Already in Euporosteus (at least) we find anatomical characters of the skull and jaws mirrored in the extant species, including similar sensory canals (Zhu, et al. 2012).
Phylogenetic analyses do not group these as the earliest diverging coelacanths, however. That honor goes to the Miguashaiidae, which are currently only known from Late Middle to early Late Devonian fossils of two genera from Canada and Australia. A representative genus Miguashaia was shown above and showcases the plesiomorphic or ancestral state of the caudal or tail fin; it is asymmetrical like that of other lobe-fins like lungfishes.
The next to branch off were the Diplocercidae, which include the aforementioned Euporosteus and seven other genera from the Devonian to the Mississippian (Early Carboniferous). They are the first to exhibit modern coelacanth anatomy, with a double-pair of parietal bones on the roof of the skull and the triple-lobed tail. One genus, Holopterygius, has an elongated body which been described as "eel-like" (Friedman & Coates, 2005). Already by this time, coelacanth morphology was diversifying, and it is clear that they survived the various Late Devonian extinction events between 372.2 and 358.9 million years ago and flourished in both marine and freshwater contexts.
Next to emerge were the Hadronectoridae and Rhabdodermatidae, known primarily from the Carboniferous and Early Permian of interior North America, northern Europe, and Madagascar. A Greenland Triassic genus Sassenia forms a related branch. These coelacanths show a gradual decrease in bone layering (or ossification) within the skull cavity. One genus of Rhabdodermatidae (Rhabdoderma proper) has been found in riverine, wetland, brackish, and saltwater environments, perhaps suggesting a euryhaline lifestyle akin to many fish alive today.
From this point there is a major change with the emergence of the Coelacanthiformes during the Middle Permian (Ferrante & Calvin, 2025). Subsequent lineages have replaced their bony skeletons with cartilaginous ones, and their backbone is "incomplete" and even retains the ancestral notochord of far older relatives of vertebrates. Two important genera are Coelacanthus and Rebellatrix, the latter known from superb fossils that were described fairly recently (Wendruff & Wilson, 2012). These coelacanths were fast-moving predators of pelagic seas, with streamlined bodies and large, stiff fins.
Rebellatrix divaricerca (mamatlisham, CC BY-ND 3.0)
Obviously, coelacanths survived the End Permian Mass Extinction Event 251 million years ago and adapted to the reorganization of marine ecosystems into the "modern fauna". Surveying the fossil record, we find that the peak of coelacanth evolutionary diversity occurred in the subsequent Triassic Period (Ferrante & Cavin, 2025). All remaining lineages derived from the Coelacanthiformes.
Many new clades emerged in rapid succession: Laugiidae from Greenland, Europe, and Madagascar; Whiteiidae from global deposits; and the recently-named Axeliidae from as far as Chile & Svalbard. The laugiids in particular tend to have highly-derived anatomies compared to other coelacanths. For example, Laugia proper, from the Early Triassic of Greenland, sports pectoral fins located towards the top (dorsal) part of the back and pelvic fins which are connected to the shoulder girdle. This arrangement is more in-line with ray-finned fishes than with other lobe-fins (Thomson, 1991).
Lastly we find the clade Latimerioidei, which have mostly lost their peorbital bone (located above the eye-socket or orbit) and sclerotic ossicles (a ring of bone around the orbit). The part of the jaw which holds the teeth (the dentary) has become "hook-shaped" (Ferrante & Cavin, 2025). Little research has been done on the diet and feeding behaviors of latimerioid coelacanths past and present, with the most recent papers and observations showing a diverse diet of cephalopods, crustaceans, and fishes. Coelacanths do not seem to bite their prey so much as suck it up whole: one fossil of the Late Cretaceous Axelrodichthys contains an entire sizable fish of juvenile or adult age (Meunier, et al. 2018).
Mawsonia gigas (Cavin, et al. 2021, CC BY 4.0)
There is a great split within the Latimerioidei between two daughter clades which emerged during the Triassic Period, the Mawsoniidae and the Latimeriidae.
Mawsoniids developed ossified or bony ribs and lost or reduced several features of the skull (Ferrante & Cavin, 2025). Notable genera include Mawsonia, Trachymetopon, Diplurus, and the previously-mentioned Axelrodichthys. Fossils spanning the Mesozoic Era in the Americas, Europe, and Africa indicate that these were among the largest lobe-finned fishes if not bony fishes that ever lived; indeed, they are only exceeded by a handful of species from the entirety of the Devonian-Quaternary periods (Cavin, et al. 2021). They inhabited both freshwater and marine ecosystems and would have simultaneously been major predators of aquatic life and a good source of food for piscivorous dinosaurs and crocodylomorphs.
Latimeriids have an upper jaw (premaxilla) sporting at most four teeth, coronoid fangs (special teeth in the mandible), and have lost their spiracle, a hole for air-breathing (Ferrante & Cavin, 2025). This group survives to the present day in the form of the Latimera coelacanths, but there were many other genera from the Triassic to the Cretaceous, including Macropoma, Undina, Megalocoelacanthus, and Foreyia. The latter is a Swiss species from the Early Triassic, and despite being a highly-derived latimeriid, evidence suggests that it underwent heterochronic evolution (which occurs through changes in the embryonic stage of development) towards a more ancestral bodyplan (Cavin, et al. 2017).
Foreyia maxkuhni (Cavin, et al. 2017, CC BY 4.0)
Though coelacanths swimming in the Jurassic and Cretaceous seas were not as diverse as they had been during the Triassic, they ultimately fell victim to the great mass extinction that closed the Mesozoic, and only one lineage that gave rise to Latimeria survived. Why?
The Living Coelacanth
Today, coelacanths live in marine waters between 492 and 1,300 feet deep, close to the rocky extensions and caves of nearby islands (Nelson, et al. 2016; Prothero, 2022). Live observation has dispelled the long-held assumption that the lobe-fins are used for "walking" among the rocks; they use a unique fatty swimbladder-like organ to keep buoyant (Cupello, et al. 2015), and move rather stifly with the fins in opposite motion. They have glowing phosphorescent eyes, and their rich blue scales with white spots can be easily detected by others of the species through color vision (Yokoyama, et al. 1999). Coelacanths even have specially-armored scales supported by collagin fibril struts that provide protection from deepwater sharks (Sherman, et al. 2017).
Like other deep-sea fishes, they are remarkably long-living, with estimates from calcified growths on their scales pointing to lifespans of around 100 years (Mahé, et al. 2021). In general, they have very slow lives and only reach sexual maturity between the ages of 40 and 69 years, with a gestation of around 5 years (coelacanths are ovoviviparous, meaning they lay eggs internally and subsequently give birth to hatched, live young). Compare that to elephants and whales, which average about one-two years for one baby! Little is known about their daily lives or how they interact with others of their kind: observations published in 1991 show that they cluster into small groups and display fairly calm behaviors (unless two individuals touch, at which they quickly disperse), and they may range over an area of ~4.9 miles for at most two years (Fricke, et al. 1991).
As stated during the beginning of this post, there is no indication that coelacanth evolution has slowed down, with the recent genomic analysis pointing to continued evolution in recent epochs (Amemiya, et al. 2013). Fossil coelacanths inhabited a wide range of aquatic habitats and they seem to have been as vulnerable to the sudden environmental swings of the bolid impact in much the same way as other fish groups were. That modern coelacanths inhabit the deep oceans today - themselves dynamic ecosystems in their own right - is most likely a clue to their ultimate survival. That said, in the most recent phylogeny, it was found that the sister genus to Latimeria was Swenzia, which is only known from Late Jurassic remains (Ferrante & Cavin, 2015); meaning there is a significant gap in the fossil record between the two genera and there are many mysteries remaining about just when coelacanths moved to the deep ocean realm. As well, it remains unclear when the two living species diverged from each other: the genomic analysis failed to narrow down the previous estimate of between 40 and 6 million years of divergence (Amemiya, et al. 2013).
Much remains to be learned about these mysterious and increasingly-rare fishes. Though it is true they are the most distantly related of lobe-fins to tetrapods like ourselves, they can still teach us so much about the evolution of land vertebrates. As recently as last year, it was confirmed that pulmonary organs (related to the lungs) were preserved in the fossil coelacanth Macropoma (Cupello, et al. 2024). Not only that, but they were also detected in living coelacanths too! It was known for sometime that coelacanths had a vestigial lung (which is more developed in embryos), but this was the first time that a pair of pulmonary arteries was actually found in a specimen. It has been argued that ancestral bony fishes (including the first ray-finned lineages as well as lobe-fins) sported unpaired lungs, and now we have strong evidence confirming that the lungs of both fish groups are homologous, inherited from a single common ancestor. This can not only tell us how paired lungs evolved in our direct ancestors, but how those of the coelacanth became vestigial in the first place.
There is no doubt just from the sample of papers used to build this blog post that we are witnessing a rapid increase in our understanding of coelacanth behavior and evolution. Who knows what surprises we'll find next?
West Indian Coelacanth off South Africa (Fraser, et al. 2020, CC BY 4.0)
Book References
John G. Maisey. Discovering Fossil Fishes (Westview Press, 1996)
Joseph S. Nelson, et al. Fishes of the World - 5th Edition (Wiley, 2016)
Donald Prothero. Vertebrate Evolution: From Origins to Dinosaurs and Beyond (CRC Press, 2022)
Keith S. Thomson. Living Fossil: The Story of the Coelacanth (W. W. Norton & Company, 1991)
Paper & Article Citations
Chris T. Amemiya, et al. 2013. Analysis of the African coelacanth genome sheds light on tetrapod evolution (Nature)
Lionel Cavin, et al. 2021. Giant Mesozoic coelacanths (Osteichthyes, Actinistia) reveal high body size disparity decoupled from taxic diversity (Nature Scientific Reports)
Lionel Cavin, et al. 2017. Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland (Nature Scientific Reports)
Camila Cupello, et al. 2024. Pulmonary arteries in coelacanths shed light on the vasculature evolution of air-breathing organs in vertebrates (Nature Scientific Reports)
Camila Cupello, et al. 2015. Allometric growth in the extant coelacanth lung during ontogenetic development (Nature Communications)
Christophe Ferrante & Lionel Cavin, 2025. A deep dive into the coelacanth phylogeny (PLOS One)
Michael D. Fraser, et al. 2020. Live coelacanth discovered off the KwaZulu-Natal South Coast, South Africa (South African Journal of Science)
Hans Fricke, et al. 1991. Coelacanth Latimeria chalumnae aggregates in caves: first observations on their resting habitat and social behavior (Environmental Biology of Fishes)
Matt Friedman & Michael I. Coates, 2005. A newly recognized fossil coelacanth highlights the early morphological diversification of the clade (Proc. R. Soc. B.)
Kélig Mahé, et al. 2021. New scale analyses reveal centenarian African coelacanths (Current Biology)
François J. Meunier, et al. 2018. The diet of the Early Cretaceous coelacanth Axelrodichthys araripensis Maisey, 1986 (Actinistia: Mawsoniidae) (Cybium)
Raphaäl Plante, et al. 1998. Coelacanth population, conservation and fishery activity at Grande Comore, West Indian Ocean (Marine Ecology Progress Series)
Vincent R. Sherman, et al. 2017. A comparative study of piscine defense: The scales of Arapaima gigas, Latimeria chalumnae and Atractosteus spatula (Journal of the Mechanical Behavior of Biomedical Materials)
Andrew J. Wendruff & Mark V. H. Wilson, 2012. A fork-tailed coelacanth, Rebellatrix divaricerca, gen. et sp. nov. (Actinistia, Rebellatricidae, fam. nov.), from the Lower Triassic of Western Canada (Journal of Vertebrate Paleontology)
Shozo Yokoyama, et al. 1999. Adaptive evolution of color vision of the Comoran coelacanth (Latimeria chalumnae) (PNAS)
Ed Yong, 2013. The Falsity of Living Fossils (The Scientist)
Min Zhu, et al. 2012. Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian (Nature Communications)
