Elongated body, later arrival and a bit of a mystery. Endeiolepis is an extinct genus of jawless fish (agnathan) that lived during the Upper Devonian period, approximately 383 to 359 million years ago.
It’s an anaspid, a unique group of early vertebrates that diverged early in the evolutionary history of vertebrates, specifically on the cyclostome lineage, which doesn’t lead to normal fish but instead jawless modern hagfishes and lampreys.
Endeiolepis showcases fascinating evolutionary adaptations and is particularly notable for its elongated body, seemingly oversized Hypocercal tail and scale patterns, which have intrigued palaeontologists since its discovery.
Endeiolepis Taxonomy and Classification
- Domain: Eukaryota
- Kingdom: Animalia (Metazoa)
- Subkingdom: Eumetazoa
- Infrakingdom: Bilateria
- Superphylum: Deuterostomia
- Phylum: Chordata
- Subphylum: Vertebrata
- Evolutionary Sig: stem-cyclostome (cyclostomiformes)
- Class: anaspida
- Order: endeiolepidiformes
- Family: endeiolepididae
- Genus: Endeiolepis
Geological Time and Habitat
- Period: Devonian
- Stratigraphic Range: Upper Devonian (approximately 383-359 million years ago).
- Habitat: Endeiolepis seems to have inhabited shallow marine environments, likely along coastal areas where nutrient-rich waters supported a variety of early vertebrate life. The unique adaptations seen in Endeiolepis suggest it was well-suited to these dynamic environments.
Evolutionary Significance
Endeiolepis is a member of the anaspida, a group of jawless fishes (agnatha) that are significant for being stem-cyclostomes, basal members of the cyclostome lineage, a sister clade to the gnathostome (jawed vertebrate) line.
That’s an important distinction because, until the early 2020s, scientists thought that all cyclostomes (jawless fishes or agnathans) were the precursors to gnathostomes (jawed fishes). But recent findings show that this is not the case as we found agnathans and gnathostomes living side by side in the same strata.
Jaws showed up much earlier than we thought and that allows us to make distinctions between those jawless fishes that were on the way to developing jaws (stem-gnathostome), and those that were never going to develop jaws (cyclostomes) because their evolution just didn’t go along those lines.
Super Fishy, But Not a Gnathostome Fish?
Although anaspids like Endeiolepis looked a lot like fish, they didn’t have jaws, were not on their way to developing jaws and even their fins were a completely different evolutionary development than the fins of the true fish we know today.
They also didn’t have the distinctive bony headshields of most gnathostome-lineage agnathans that eventually led to ostracoderms and placoderms (where jaws did finally develop).
Anaspids like Endeiolepis are more closely related to modern-day true jawless fish (cyclostomes) like lampreys and hagfishes, and actually quite far removed from the fishes we know today. In fact, they are not in the cyclostome crown group, but rather basal- or stem-cyclostome, meaning they are an evolutionary offshoot along the lineage that would eventually become the cyclostome we know today.
Their absolute closest relatives are conodonts – extremely small eel-like creatures with no jaws, who must have developed just after anaspids (even though we find their fossils millions of years before anaspids).
Physical Characteristics
Endeiolepis was a relatively small jawless fish, with an estimated length of 10-20 cm.
Its body was elongated and fusiform, a shape that is typical of many early vertebrates, and likely contributed to its swimming efficiency. One of the most distinctive features of Endeiolepis was its unique scale patterns, which differed significantly from the bony scales found in gnathostome-line jawless fishes of the time. These scales provided some level of protection while still allowing flexibility for movement.
In addition to this, Endeiolepis possessed a series of small, median fins running along its length, which may have played a role in stabilisation during swimming. These fins were not homologous to the true fins of other fish (they were their own, unique evolutionary experiment), highlighting the unique evolutionary adaptations of anaspids – and that it is truly evolutionarily “far away” from the fish we know today.
Endeiolepis Synapomorphies
Endeiolepis inherited all the normal vertebrate features, including a vertebral column: bony or cartilaginous vertebrae around the ancestral notochord and nerve chord to make up the backbone or spine; a neurocranium: cartilaginous or bone brain casing/skull; tri-partite brain: a brain with 3 distinct lobes: Olfactory (smell), Optic (sights) and Otic (hearing).
And then on top of those, it developed completely unique Anaspid features:
- Hypocercal Tail: where the bones of the vertebral go down into the bottom of the tail, with a fin structure making up the top of the caudal.
- Body fins and unbroken single fin-like structures that are not homologous and seem to be a completely independent evolution from the fins of fish we know today – in fact, most Cyclostomes do not have fins at all.
Intriguing Mysteries Around Endeiolepis
One of the most intriguing aspects of anaspids, including Endeiolepis, is their mysterious fossil record. They’re conspicuously missing in fossils for some 40 million years of Earth’s history.
Molecular clock data suggests that anaspids were among the first stem-cyclostomes to diverge just after the appearance of Vertebrata. Likely appearing in the earliest Cambrian, even before conodonts. So we would expect to see them somewhere between 541 and 485.4 million years ago.
But we don’t. In fact, there are no fossils of anaspids until 42 million years after the Cambrian, in the Silurian. Nearly 100 million years after they should have appeared.
That’s just weird, or we simply haven’t found enough fossils yet. In fact, the earliest anaspids may have existed in environments or conditions that did not favour fossilisation. Or we’re simply yet to discover the right fossil deposits.
Discovery & Classification
- Discovery Date: 1925
- Discovery Location: Escuminac Formation, Miguasha, Quebec, Canada
- Discovering Paleontologist(s): Erik Andersson Stensiö
- Naming: The genus name Endeiolepis is derived from the Greek words “endeios,” meaning “internal,” and “lepis,” meaning “scale.” This reflects the distinctive internal scale patterns observed in the fossilised remains.
Known Species of Endeiolepis
- The genus Endeiolepis currently includes the species Endeiolepis aneri, which was named in honour of Nils Hjalmar Olof Stensiö, a prominent Swedish palaeontologist who described the species in 1939. The species name “aneri” serves as a tribute to Stensiö’s contributions to the field of palaeontology.
Behaviour and Ecology
Endeiolepis, like other anaspids, was likely a filter feeder or scavenger, navigating its marine environment with its streamlined body and unique fin structures. The small size and fusiform shape of Endeiolepis suggest it was well-adapted to a life of continuous swimming, likely in search of food particles suspended in the water column.
The absence of jaws suggests that Endeiolepis fed on small, soft-bodied organisms or detritus, relying on its gill structures to filter food from the water.
Fossil Discoveries
Fossils of Endeiolepis are mainly from Canada, with specimens preserved in the Palaeozoological Collections (PZ) of the Swedish Museum of Natural History (NRM). These fossils are crucial for understanding the anatomy and evolutionary history of early vertebrates.
The exceptional preservation of these body fossils, particularly the scale patterns and fin structures, has allowed palaeontologists to gain a detailed understanding of the morphology and evolutionary relationships of anaspids.
Scientific Research and Theories
Endeiolepis has been the subject of various scientific studies, most notably by researchers Arsenault and Janvier, who examined the possibility of a spiral intestine in Endeiolepis aneri – a feature seen in some modern vertebrates.
Their research, published in 2010, explored the morphology, phylogeny, and paleobiogeography of this intriguing jawless vertebrate, providing valuable insights into its anatomy and evolutionary context.
These studies have shed light on Endeiolepis’s unique position within the Cyclostome lineage and contributed to a better understanding of vertebrates’ early development.
Current Specimen Collection Locations
- Swedish Museum of Natural History
Another Curios Aspect of Endeiolepis
One of the most curious aspects of Endeiolepis and other anaspids, is their striking resemblance to fish, despite being on the Cyclostome lineage.
While modern cyclostomes, like hagfishes and lampreys, have elongated, eel-like bodies, some Devonian anaspids, including Endeiolepis, appear remarkably fish-like in their morphology. This resemblance is purely convergent, meaning that the similarities in body shape and fin structures evolved independently and are not homologous to the true fins and scales of jawed fishes.
Interestingly, this fish-like appearance is more pronounced in the early anaspids of the Silurian period, while later genera like Endeiolepis and Euphanerops, which survived into the Devonian, began to exhibit more eel-like characteristics. This evolutionary trend aligns Endeiolepis more closely with other cyclostomes, further emphasizing the diversity of body forms that existed within early vertebrate lineages.
Extinction Theories
From what we can tell, Endeiolepis went extinct in the Devonian, likely due to the significant environmental changes that occurred at the end of the Devonian period – there are a number of extinction events in the period and not many are well understood, but scientists recognise that there must have been a very extreme mass extinction towards the end of the Devonian.
Fluctuations in sea levels, changes in climate, and the emergence of more competitive jawed vertebrates could have contributed to the decline and eventual extinction of these once-successful jawless fishes.
The end-Devonian extinction event, one of the five major mass extinctions in Earth’s history, had a profound impact on marine life, wiping out many species, including the anaspids.
Interesting Facts & Trivia
- There’s a nearly 100 million-year gap in the fossil record of anaspids, including Endeiolepis, and it’s one of the most intriguing mysteries in early vertebrate evolution.
- Endeiolepis aneri was named in honour of Nils Hjalmar Olof Stensiö, a key figure in palaeontology who described the species.
- The independent evolution of fin structures in anaspids like Endeiolepis highlights the diverse evolutionary strategies employed by early vertebrates.
Additional Resources & References of Endeiolepis
Arsenault, M., & Janvier, P. (2010). Is there a spiral intestine in the anaspid-like jawless vertebrate Endeiolepis aneri Stensiö, 1939, from the Upper Devonian Escuminac Formation of Miguasha, Quebec, Canada? In: Elliott, D.K., Maisey, J.G., Yu, X., & Miao, D. (eds): Morphology, Phylogeny, and Paleobiogeography of Fossil Fishes. Verlag Dr. Friedrich Pfeil, München.
Discover More Cool Anaspids
Being it’s own order endeiolepidiformes and family endeiolepididae, and tending toward the more slender shapes, Endeiolepis is probably closest related to non-birkeniid anaspids like Jamoytiiformes, including Euphanerops and Jamoytius.
Time-wise, it lived alongside other Devonian anaspids: Kerreralepis, Vilkitskilepis and Euphanerops.
But, of course, it has some more famous cousins like Birkenia, Pharyngolepis, Pterygolepis, the puzzlingly fin-less Lasanius and the ultra-famous (because the genus that gave anaspida its name, i.e. Anaspis, later turned out be wrongly named, it was actually a specimen of…) Rhyncholepis.
Museums with Anaspid Collections & Exhibits
- The on-site museum at Miguasha in Canada
- Swedish Museum of Natural History
- Hunterian Museum and Art Gallery at the University of Glasgow
- Natural History Museum Denmark
- Grant Museum of Zoology, London
- The Royal Ontario Museum, Canada
- American Museum of Natural History
- University of California Museum of Paleontology
Documentaries Featuring Early Jawless Vertebrates Similar to Anaspida like Endeiolepis
They’re not always 100% accurate because we make new scientific discoveries all the time. But documentaries at least deliver the gist of the information in a fun and engaging way. Just click play, hit full screen and enjoy hours’ worth of awesome documentaries on first animals, right here…
1. Ancient Oceans
2019
Not just about vertebrates, but featuring some of the most awesome ancient fishes. This short documentary about early oceans and extinction events has two episodes and is available through Curiosity Stream here.
2. Life on Our Planet Episode 2
2023
Though not focused on vertebrates specifically, episode 2 of Steven Spielberg’s Netflix documentary, narrated by Morgan Freeman, features a look at early life and the land transition, featuring some vertebrates like Sacabambaspis.
Watch on Netflix here.
3. Animal Armageddon Episode 2
Animal Planet, 2019
It’s hard to track down a good copy of this one, but Animal Armageddon features quite a few vertebrate evolutionary stories, pegged on the major mass extinctions they faced. This one specifically is during the Devonian, the age of fishes (vertebrates).
Also available on Amazon here.
4. Mankind Rising
2012
The 2012 TV movie production of Mankind Rising is a single animated journey from the earliest vertebrates ancestors right through to present-day humans – it’s pretty cool, maybe a bit dated, but still a lot of fun!
5. Walking with Monsters
BBC, 2005
A bit older now and flashing past early animals a bit too quickly, this one in the “Walking With” series is still pretty cool for its Cambrian bits. It’s also available via Amazon.
6. David Attenborough’s Rise of Animals: Triumph of the Vertebrates
BBC, 2013
Although it doesn’t feature specific past genera, instead using modern, living animals to show what those ancient early vertebrates must have been like, it’s still an awesome documentary to watch. It’s available to purchase from Amazon.
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