Highly controversial, Rhyncholepis is an extinct genus of jawless fish (agnathan) that lived during the Sheinwoodian to Gorstian stages of the Silurian Period, approximately 433.4 to 421.3 million years ago.
Significantly, Rhyncholepis replaced the former genus Anaspis, the namesake of the entire clade of Anaspida. Initially, Rhyncholepis was discovered first, but Anaspis was later found, and the clade was named after it.
However, upon closer inspection, Anaspis turned out to be a junior synonym of Saarolepis, and then Saarolepis was found to be Rhyncholepis. So, the genus Anaspis was absorbed, and all previous findings were consolidated under Rhyncholepis.
Unlike most early vertebrates, Rhyncholepis is notable for its elongated body, prominent dorsal and anal fin spines, and scale-covered body.
Although Rhyncholepis is classified under Birkeniiformes, its placement in a specific family group was debated until it was placed in Rhyncholepididae.
Rhyncholepis Taxonomy and Classification
- Domain: Eukaryota
- Kingdom: Animalia (Metazoa)
- Subkingdom: Eumetazoa
- Infrakingdom: Bilateria
- Phylum: Chordata
- Subphylum: Vertebrata
- Evolutionary Lineage: Stem-cyclostome (Cyclostomiformes)
- Class: Anaspida
- Order: Birkeniiformes
- Family: Rhyncholepididae
- Genus: Rhyncholepis
- Species: Rhyncholepis parvula, Rhyncholepis butriangula, Rhyncholepis oeselensis
Geological Time and Habitat
- Period: Silurian
- Stratigraphic Range: Sheinwoodian Stage (Wenlockian Series) to the Gorstian Stage (Ludlow Series) of the Silurian Period (approximately 433.4–421.3 million years ago)
- Habitat: Rhyncholepis inhabited shallow, nutrient-rich marine environments, most likely near the coastal regions of what is now Norway. These environments were home to a diverse array of early vertebrates. Rhyncholepis used its elongated body and fin structures to navigate these ancient waters efficiently.
Evolutionary Significance
As a member of the Anaspida, Rhyncholepis belongs to a unique group of jawless fishes that diverged early in vertebrate evolution, specifically along the Cyclostome lineage.
This evolutionary branch makes Rhyncholepis more closely related to modern hagfish and lampreys than to other jawless vertebrates on the gnathostome lineage.
Unlike many early vertebrates, Rhyncholepis lacked the bony armour that many agnathans developed, instead evolving prominent dorsal and anal fin spines and a scaleless, elongated body.
Physical Characteristics
Rhyncholepis was a small vertebrate, measuring about 10-15 cm in length. Its body was elongated, covered in scales, and featured prominent dorsal and anal fin spines. Like other anaspids, Rhyncholepis lacked paired fins, instead exhibiting body fins that ran along its length. These fins evolved independently from the paired fins of gnathostomes and served as a stabilising feature in their swimming.
Rhyncholepis Synapomorphies
As a vertebrate, Rhyncholepis shared several fundamental features with other early vertebrates:
- Vertebral Column: Bony or cartilaginous vertebrae around the ancestral notochord and nerve cord, forming the backbone or spine.
- Neurocranium: A cartilaginous or bony brain casing protected its brain.
- Tri-partite Brain: Like other vertebrates, it had a brain divided into three distinct lobes: olfactory (smell), optic (sight), and otic (hearing).
Specific to Anaspida, Rhyncholepis also exhibited:
- Hypocercal Tail: The bones of the vertebrae extended into the bottom of the tail, while the fin structure made up the top of the caudal fin.
- Body Fins: Unbroken single fins running along its body, distinct from the paired fins found in gnathostome vertebrates.
Controversy & Mysteries Around Rhyncholepis
One of the most interesting aspects of Rhyncholepis is how it absorbed the former genus Anaspis, the very genus after which the Anaspida clade was named.
Initially, palaeontologists thought Anaspis was a distinct genus, but further investigations revealed that Anaspis and Saarolepis were actually junior synonyms of Rhyncholepis.
Additionally, the gap in the fossil record for anaspids remains perplexing.
Despite molecular clock data suggesting that anaspids should have been present since the Cambrian, their fossils don’t appear until the Silurian, raising questions about the early evolution of anaspids and their habitat.
Discovery & Classification
- Discovery Date: 1911
- Discovery Location: Kristiania (now Oslo) area, Norway
- Discovering Paleontologist(s): Johan Kiær
- Naming: The genus name Rhyncholepis is derived from the Greek words “rhynchos,” meaning “snout” or “beak,” and “lepis,” meaning “scale.”
Known Species of Rhyncholepis
The genus Rhyncholepis consists of three known species:
- Rhyncholepis parvula: The species name parvula is derived from the Latin word “parvulus,” meaning “small.”
- Rhyncholepis butriangula: Likely refers to an angular feature of the fossil.
- Rhyncholepis oeselensis: Named after the island of Ösel (now Saaremaa, Estonia), where the fossil was discovered.
Behaviour and Ecology
Rhyncholepis likely played a role as a filter feeder or scavenger, relying on its gill structures to extract food particles from the nutrient-rich waters it inhabited. Its small, elongated body, coupled with its unique fin structures, made it an agile swimmer, well-suited to navigating the shallow marine environments of Silurian Norway.
Fossil Discoveries
The fossils of Rhyncholepis were discovered in the Kristiania (Oslo) area of Norway, dating back to the Sheinwoodian and Gorstian stages of the Silurian Period. The well-preserved fossils display the genus’s body structure, fin configuration, and scale pattern, providing critical insights into the anatomy of early anaspids.
There are 46 specimens of Rhyncholepis recorded in the GBIF (Global Biodiversity Information Facility), most of which are from Norway, with additional specimens found in Estonia, Sweden, and one specimen from the United States, hinting at near-global distribution.
Scientific Research and Theories
Johan Kiær’s 1911 study, published in Videnskapsselskabet i Christiania, described the Rhyncholepis genus, focusing on its unique features within the Birkeniiformes order. The research also explained the evolutionary significance of Rhyncholepis, particularly how it absorbed other genera and its placement within the Cyclostome lineage.
Current Specimen Locations
Specimens of Rhyncholepis can be found in several prestigious institutions, including:
- Gothenburg Natural History Museum (GNM)
- Museum of Comparative Zoology, Harvard University
- Natural History Museum (London)
- Vertebrate Paleontology Division, Yale Peabody Museum
- Swedish Museum of Natural History (NRM)
Other institutions likely hold additional specimens, referenced in databases such as the Paleobiology Database and Arctos.
Another Curious Aspect of Rhyncholepis
Despite its fish-like appearance, Rhyncholepis and other anaspids exhibit structural features that are not homologous to true fish fins or scales. These features evolved independently and serve as an example of convergent evolution.
Interestingly, Rhyncholepis and other early anaspids looked much more fish-like than their later Devonian relatives, such as Endeiolepis and Euphanerops, which developed more eel-like forms typical of modern Cyclostomes.
Extinction Theories
The extinction of Rhyncholepis and other anaspids likely resulted from environmental changes during the Silurian. Rising sea levels, shifts in marine ecosystems, and increased competition from more advanced vertebrates may have contributed to their decline.
Interesting Facts & Trivia
- Rhyncholepis parvula was named for its small size, a distinctive feature among anaspids.
- Rhyncholepis replaced the former genus Anaspis, which had originally given the Anaspida clade its name.
- The discovery of Rhyncholepis helped clarify a previously confusing classification of early jawless fishes.
Additional Resources & References of Rhyncholepis
Kiær, J. (1911). A new Downtonian fauna in the sandstone series of the Kristiania area: A preliminary report. Videnskapsselskabet i Christiania.
Discover More Cool Anaspids
Rhyncholepis is part of the Birkeniiformes order of anaspids alongside the likes of Rytidolepis, Birkenia, Pharyngolepis, Pterygolepis and Lasanius, etc.
Specifically, it’s part of the Rhyncholepididae family, along with more fragmentary genera like Rytidolepis, Schidiosteus, Silmalepis, Vesikulepis and Maurylepis that are basically only known from their scales and small fragments.
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 Cowielepis
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 more focused on the evolution of vertebrates, it’s still an awesome documentary to watch. It’s available to purchase from Amazon.
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