Discover Schidiosteus: The Split-Bone Enigma of the Silurian Seas

Fragmentary, jawless, and intriguingly named – discover the story and scientific details of Schidiosteus, a split-bone anaspid from the Silurian.

Schidiosteus is an extinct genus of jawless fish (agnathan) that lived during the Homerian to Ludfordian stages of the Silurian Period, approximately 430.5 to 423.0 million years ago.

The genus name Schidiosteus is derived from the Greek words “schidion,” meaning “split” or “divided,” and “osteus,” meaning “bone,” referencing the fragmented nature of its fossil remains. 

While only fragmentary fossils of Schidiosteus have been discovered, it is a part of the family Rhyncholepididae within the Birkeniiformes order of the Anaspida.

Schidiosteus Taxonomy and Classification

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A time-calibrated phylogeny of early vertebrates, plotted against their actual fossil stratigraphic range. Featuring images of: Haikouichthys via Nobu Tamura, CC BY-SA 3.0, via Wikimedia Commons; Metaspriggina via USGS; Diarama by Masato Hattori. Map courtesy of Colorado Plateau Geosystems Inc., Public domain, via Wikimedia Commons; Arandaspis via SpinoJP, CC BY-SA 4.0, via Wikimedia Commons; Larnovaspis via Nobu Tamura (http://spinops.blogspot.com), CC BY-SA 3.0, via Wikimedia Commons; thelodont Farcaucuda via Petr Menshikov (https://twitter.com/Petr75113553; https://vk.com/prehistoricproduction), CC BY 4.0, via Wikimedia Commons; Sinogaleaspis via Xiaocong Guo, CC BY-SA 4.0, via Wikimedia Commons; and Cephalaspis via Petr Menshikov (https://twitter.com/Petr75113553; https://vk.com/prehistoricproduction), CC BY-SA 4.0, via Wikimedia Commons, and Estonian Museum of Natural History, CC BY-SA 4.0, via Wikimedia Commons.

Geological Time and Habitat

  • Period: Silurian
  • Stratigraphic Range: Homerian Stage (Wenlockian Series) to the Ludfordian Stage (Ludlow Series) of the Silurian Period (approximately 430.5–423.0 million years ago)
  • Habitat: Schidiosteus lived in shallow marine environments rich in nutrients, likely near what is now Saaremaa Island, Estonia. These ancient seas supported a diverse array of early vertebrates, and Schidiosteus would have been part of this marine community, navigating the waters using its elongated body and fin structures.

Evolutionary Significance

As part of the Anaspida, Schidiosteus belongs to a unique group of jawless fishes that diverged early along the Cyclostome lineage.

This places Schidiosteus and other anaspids closer to modern hagfish and lampreys, unlike other jawless fishes on the gnathostome lineage. Anaspids like Schidiosteus evolved without the bony armour common in other early vertebrates, highlighting their distinct evolutionary pathway.

Schidiosteus, size, anaspid, birkeniiformer, extinct, fossil, fish, cyclostome, jawless, aganathian, silurian, devonian
Schidiosteus size comparison with a human diver.

Physical Characteristics

Due to the fragmentary nature of the Schidiosteus fossil record, precise details of its anatomy remain elusive. However, based on similar anaspids, it is estimated to have been 10-15 cm in length, with an elongated body and likely a scale pattern consistent with other members of the family Rhyncholepididae.

Schidiosteus Synapomorphies

As a vertebrate, Schidiosteus shared several core features common to early vertebrates:

  • Vertebral Column: Bony or cartilaginous vertebrae surrounding 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, Schidiosteus had a brain divided into three distinct lobes: olfactory (smell), optic (sight), and otic (hearing).

Specific to Anaspida, Schidiosteus also exhibited:

  • Hypocercal Tail: The vertebrae extended into the lower lobe of the tail, while the fin structure formed the upper part of the caudal fin.
  • Body Fins: While many anaspids developed fins along their entire body, it is unknown if Schidiosteus had these fins due to the fragmentary fossils available.

Intriguing Mysteries Around Schidiosteus

As with many other anaspids, Schidiosteus presents several unresolved mysteries. Its fossils are fragmentary, leaving much of its anatomy and ecology speculative. The genus name “split-bone” could hint at the fragmented state of its fossils, reflecting how little we know about this early vertebrate.

The gap in the fossil record of anaspids is another mystery, with no evidence of their existence before the Silurian despite molecular data suggesting they must have first appeared in the Cambrian – even before their close cousins, the conodonts.

We can only assume that either Anaspids were around long before the Silurian and just didn’t fossilise, we just haven’t found the specimens yet, or we don’t know how to recognize them, or there’s something wrong with our understanding of the origin of vertebrates.

Schidiosteus, anaspid, birkeniiformer, extinct, fossil, fish, cyclostome, jawless, aganathian, silurian, devonian
A fossil image of Schidiosteus adapted from Geoloogia.info, CC BY-SA 4.0.

Discovery & Classification

  • Discovery Date: 1856
  • Discovery Location: Saaremaa Island, Estonia
  • Discovering Paleontologist(s): Christian Heinrich Pander
  • Naming: The genus name Schidiosteus is derived from the Greek words “schidion,” meaning “split” or “divided,” and “osteus,” meaning “bone.”

Known Species of Schidiosteus

The only known species of Schidiosteus is Schidiosteus mustelensis, named after the region of Mustjala in Estonia, where its fossils were found.

Behaviour and Ecology

Due to the fragmentary nature of Schidiosteus fossils, its specific ecological role remains unclear. However, like other anaspids, Schidiosteus may have been a filter feeder or scavenger, using its gill structures to extract nutrients from its environment. The streamlined body structure would have aided in swimming and navigating the nutrient-rich waters of Silurian Estonia.

Schidiosteus, anaspid, birkeniiformer, extinct, fossil, fish, cyclostome, jawless, aganathian, silurian, devonian
A fossil image of Schidiosteus adapted from Geoloogia.info, CC BY-SA 4.0.

Fossil Discoveries

Schidiosteus is known from fragmentary fossils found on Saaremaa Island, Estonia, dating to the Homerian and Ludfordian stages of the Silurian Period. These fragments are part of a significant fossil record that reveals the biodiversity of early vertebrates in the region, though much remains unknown about the genus itself.

Scientific Research and Theories

Significant research into Schidiosteus includes studies on the environmental conditions of the Vesiku Bone Bed, where its remains were found.

A notable study by Blom et al. (2002) focused on the broader family Rhyncholepididae and the evolutionary implications of these early anaspids. More recent studies have focused on the geochemistry of the site, helping to contextualise the ecosystem in which Schidiosteus lived.

Current Specimen Locations

Specimens of Schidiosteus are housed in several notable collections, including:

  • TalTech fossil collections
  • Paleozoological Collections (PZ), Swedish Museum of Natural History (NRM)

Another Curious Aspect of Schidiosteus

One fascinating aspect of anaspids like Schidiosteus is their lack of bony armour. Unlike other jawless vertebrates that developed head shields or bony plates, anaspids evolved to be more streamlined, likely aiding in mobility.

The split-bone nature of its fossil remains also raises intriguing questions about how well-preserved this genus might be in the future, as more discoveries may fill in the gaps in its evolutionary story.

Extinction Theories

Like other anaspids, Schidiosteus likely faced extinction due to environmental changes during the Silurian. Shifts in marine ecosystems and competition from more advanced vertebrates may have contributed to its decline.

Interesting Facts & Trivia

  • Schidiosteus mustelensis was named after Mustjala, a region in Estonia where the fossils were found.
  • The genus name Schidiosteus reflects the fragmented nature of its fossil remains, referencing the Greek word for “split” or “divided.”
  • Despite its fragmentary fossil record, Schidiosteus is an important genus for understanding the early evolution of jawless vertebrates.

Additional Resources & References of Schidiosteus

  • Fadel, A., Žigaite, Ž., Blom, H., Perez-Huerta, A., Jeffries, T., Märss, T., Ahlberg, P. E. (2015). Palaeoenvironmental signatures revealed from rare earth element (REE) compositions of vertebrate microremains of the Vesiku Bone Bed (Homerian, Wenlock), Saaremaa Island, Estonia. Estonian Journal of Earth Sciences, 64(1), 36-41. DOI: 10.3176/earth.2015.07
  • Blom, H., Märss, T., Miller, C. G. (2002). Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Transactions of the Royal Society of Edinburgh Earth Sciences, 92(3), 263-323. DOI: 10.1017/S0263593300000250

Discover More Cool Anaspids

Rytidolepis is part of the Birkeniiformes order of anaspids alongside the likes of RhyncholepisBirkeniaPharyngolepisPterygolepis and Lasanius, etc.

Specifically, it’s part of the Rhyncholepididae family, along with more fragmentary genera like RytidolepisSilmalepisVesikulepis and Maurylepis that are basically only known from their scales and small fragments.

They might even be cousins of other fragmentary specimens, such as the Tahulalepididae, namely: Tahulalepis and Trimpleylepis.

Rhyncholepis, anaspid, anaspida, extinct, fossil, jawless, fish, vertebrate, cyclostome, reconstruction, silurian, Birkeniiformes, Pharyngolepididae, fossil discoveries, ancient marine life
A reconstruction of Rhyncholepis adapted from Jeffrey0409, via Prehistopedia Fandom.

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 Anaspida, Related to Rytidolepis

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

See the full video here.

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

See the full video via Dailymotion here.

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.

About PrehistoricLife.co

I aim to have everything as up-to-date as possible and will continually update this post. Please notify me of any necessary updates, information you think should be included, and references or citations needed.

Enjoy the journey of discovery through Earth’s ancient past, geology, big ideas and, of course, awesome prehistoric life.

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