December 3rd

Box #3 in our count down is coroniceras…

Coroniceras: the “Curved Horn”

Coroniceras was an ammonite from Early Jurassic England, and commonly found along what is known as the Jurassic Coast. The picture above shows the strong ribs along the segments of growth, but if you where to turn it so that it could look at you with both eyes it would look quite thin. A raised keel marks the center line of this thin coil, and a groove on either side makes this ammonite easy to identify from others on the beach.

These ammonites where the largest from the Blue Lias Formation, and grew up to 70 cm in diameter. They are a small part of the great diversity of life in this marine environment.

A few other critters that lived alongside it? Lots and lots of fish, such as the coelocanth Holophagus, the pterosaur Dimorphodon, and marine reptiles like Ichthyosaurus, Plesiosaurus, and Attenborosaurus. Dinosaurs like Scelidosaurus roamed the forested islands in the area.

Creatures like ammonites are helpful for figuring out the age of certain rock layers, since ammonite shells fossilize relatively easily, and they are often very distinct from each other. They also change and diversify into different species rather quickly compared to other animals (clams and other mollusks with shells are useful for the same reason), and so make it much easier to tell different layers of rock apart from each other. Together with the difference between mudstone, limestone, shale, or sandstone rock layers, ammonites help us discover a lot about prehistoric environments.

3 thoughts on “December 3rd

  1. Firstly, I want to mention that you misspelled Attenborosaurus. Secondly, I am awed by the selection of an ammonite. Most genera don’t get enough attention and most interpretations don’t have a specific name assigned.
    For my own specific advent calendar, I would have Thalassocnus selected. Thalassocnus was a sloth that lived during the Neogene division of the Tertiary period (Miocene and Pliocene epochs) of South America’s western coastline, including my mother’s own country, Peru. This mammal is noteworthy as


    1. Thank you! I’m glad you like it. For such a common fossil, there is depressingly little information on Coroniceras. I had to look up ancient Greek to find out what it’s name meant.

      Typo fixed 🙂

      Thalassocnus sounds really cool. A semiaquatic ground sloth? Definitely worthy of more attention. 🙂

      Liked by 1 person

    2. (Sorry, my comment hasn’t finished yet)
      … it was a sloth like no other. It lived a semi-aquatic lifestyle and likely dug up food using its claws. They coexisted with whales and sharks, and were most likely preyed upon by them. The genus has been found found in five areas as of now: the Pisco Formation of Peru, the Tafna Formation of Argentina (which implies that it may also have lived a terrestrial lifestyle), and the Bahía Inglesa, Coquimbo, and Horcón formations of Chile. As the Pliocene concluded, Thalassocnus went extinct due to the majority of seagrass from the coastline it lived around and on declining. The negative buoyancy that the sloths adapted to for eating seagrass became more difficult in cooler water temperatures and as a result, the sloths would have been poorly adapted to these changing conditions, leading to their extinction.


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