Harding Sandstone Microfossils

[ThePhysicist]

Back in May or so I got my hands on some micromatrix from the Harding Sandstone, CO, USA. This formation dates back to the Ordovician: ~450-475 mya. It's chock full of some really cool and important fossils. It has some of the earliest vertebrate material, and some of the earliest steps in the evolution of teeth! I hope this is an informative and fun look into an important period in life's history. If you feel I have mischaracterized something or have left out pertinent information, please do speak up! I do also plan to post more pictures as I sort through material. If there's something specific you would like a better view of, let me know. So without further ado, let's dive in!

All the matrix I have came in this small vial (not all of it is in the vial - this is just what I still have to go through).

It's been heavily concentrated. What you're seeing is a mix of shells, some sandstone bits, and vertebrate remains. The majority of the vertebrate material is from ostracoderms - armored fish whose skin was made of bone. They had no jaws, teeth, or fins. They look to me like a cross between an armadillo and a potato. Most of the fossils are of their skin-armor which was studded with "tubercles:" little bumps and ridges. These are important and we'll talk about them later.  Also in the mix are scales from potentially the earliest sharks. It seems there is still debate on this, as they could also belong to another class of fish named the thelodonts. There are also the well-preserved "teeth" of conodonts. Conodonts were jawless, bug-eyed, hagfish-like animals.
 

[ThePhysicist]

We call them teeth, but it's possible they were used as more of a filtering structure. The ones from this matrix have terrific preservation. It's amazing to me to think these tiny, fragile cusps could make it to us today. They have a nice lemonade-like color. I didn't care to identify them for this post.

[ThePhysicist]

For a sense of scale:

Most of the pictures here are at 40x magnification. It's hard for me to make a scale bar for these micro-shots.

[ThePhysicist]

Next up are the "shark" scales. As you may know, sharks have little "skin teeth" called denticles which help with their maneuverability in the water. These certainly look to me (a non-expert) like shark denticles, if not precursors. I'd be very pleased if they were from sharks (because I think they're wonderful animals), but I'm not sure. Cool fossils notwithstanding!

 

[ThePhysicist]

Now for the ostracoderms. The two species in here are Astraspis desideratus and Eryptichius americnaum. The latter being much more prevalent in this sample than the former.
Eryptichius americnaum:

Astraspis desiderata:

[ThePhysicist]

Astraspis got their name from the star-like shape of the tubercles:

There were also indeterminate tubercle "buttons:"

[ThePhysicist]

Again, this was a time before things had jaws. But there were proto-teeth. The tubercles on ostracoderms look remarkably like teeth. They have the same/similar structure, and made of the same stuff! It is likely that these bumps on the skin-armor of ancient fish evolved into teeth. The smiles of every dimetrodon, apatosaurus, crocodile, shark, T. rex, and even yours got their start on the backs of ancient fish.
I was lucky to find a plate that broke in a way to show the interior of a couple of tubercles/odontodes:

I'll link here some material to read:

Shark denticles:

https://www.researchgate.net/publication/232791057_Scales_of_thelodont_and_shark-like_fishes_from_the_Ordovician_of_Colorado

Ostracoderm odontodes:

https://www.researchgate.net/figure/Late-Cambrian-Ordovician-and-early-Silurian-vertebrate-exoskeletons-a-Ideal-vertical_fig1_275360759

Why conodont "teeth" aren't evolutionarily responsible for ours:

https://www.nationalgeographic.com/science/phenomena/2013/10/25/outside-in-origin-for-your-teeth/

Evolution of teeth:

https://pubmed.ncbi.nlm.nih.gov/25219878/

 

A book by Neil Shubin also discusses the evolution of teeth from fish armor somewhat ("Your Inner Fish"). There's a 3-part documentary by the same name but doesn't really talk about the connection between fish and teeth - it focused on reptiles, rather. It's interesting in its own right so I'll link it if you'd rather not buy/read the book:

About evolutions of limbs:

https://www.biointeractive.org/classroom-resources/your-inner-fish

About evolution of teeth (most relevant to this topic):

https://www.biointeractive.org/classroom-resources/your-inner-reptile

Evolution of sight:

https://www.biointeractive.org/classroom-resources/your-inner-monkey

 

[GordonC]

Wonderful, I'd be interested in ID and dating of the conodonts. G

[Miocene_Mason]

Beautiful pictures and awesome fossils, thanks for sharing!

[fossilcrazy]

Hello ThePhysicist,

Can I ask how you free your microfossils from the Harding SS matrix. I love examining the fossils on broken surfaces but have not been able to free them. My efforts have been crush, the Sandstone rock to less than 1/4 inch, freeze thaw several times then look for free microfossils. My method and acid do not work well. 

[Tomasz]

Hello,

Thank you for sharing. This is an important and rare topic. I am deeply interested in early vertebrates evolution. This site, the same as Bighorn Mountains, Wyoming, reveals the dawn of main vertebrate lines. Mysterious shark-like scales are a good example. The same applies to Eleochora glossa sp., probably the earliest known stem gnathostome. And Skiichthys halsteadi with unique bone structure. 

best regards,

Tomasz

[ThePhysicist]

@fossilcrazy, that's a great question. I bought these from a UK seller, so I'll PM you with details so you can ask them.

 

@WhodamanHD, @Tomasz, @GordonC, glad y'all enjoyed the topic. I will do my best to id conodont elements once I find good examples for each of the species present.