From the Collections Room (Planocephalosaurus robinsonae)

It has been almost two years since I last talked about VMNH’s collection of Triassic fossils from the fissure-fill deposits of Britain. These sediments were deposited in caves and sinkholes that had been dissolved out of Carboniferous limestones. Apparently Triassic critters were just as prone to falling into caves as modern ones, and these sediments are filled with bones from small Triassic animals.

My lack of posts on the fissure fills does not reflect the scope of our collection from these deposits. My predecessor at VMNH, Nick Fraser (now at the National Museum of Scotland), brought a large amount of fissure-fill rock to Virginia in the 1980’s, and VMNH has more than a case of this material, including many thousands of bones and additional unprocessed material. Most of VMNH’s fissure-fill material comes from the Cromhall Quarry, the type locality of the small dinosaur Agnosphitys cromhallensis. Agnosphitys is one of the rarest components of this assemblage, but much more common is a small reptilian called Planocephalosaurus robinsonae.

Planocephalosaurus was a member of the Order Sphenodontia (or Rhynchocephalia). Today sphenodontians are only represented by the genus Sphenodon (the tuatara), which includes two endangered species found on a few islands of the coast of New Zealand. Even though they’re almost extinct today, sphenodontians used to be much more diverse, and several genera are found in the fissure fills.

Like most of the fissure fill fossils, Planocephalosaurus is known from fragmented remains, and they’re tiny; all the scales in these images are in millimeters (Planocephalosaurus was smaller than the living tuatara). At the top of the page is the tip of the left dentary (lower jaw), seen in lateral view. Below is same element in lingual view, showing the small mandibular symphysis at the tip of the jaw:

One of the outstanding features of Planocephalosaurus is the last tooth in the lower jaw, which is hugely enlarged:

Here’s a partial right maxilla, seen in lingual view:

If you look closely you can see wear facets on the posterior surfaces of several teeth.

You don’t see this in many mammals, but lots of vertebrates have teeth on the palatal bones in addition to the premaxillae, maxillae, and dentaries. Here’s the left palatine in ventral view, with two rows of tiny teeth:

Here’s a reconstruction of Planocephalosaurus drawn by Nick Fraser for an exhibit we had some years ago on the fissure fills:

While dinosaurs and other big impressive animals get all the attention (even whales, to a certain extent), most of history’s vertebrate biodiversity is made up of small animals; things like fish, rodents, and lizards may not get the glory but they’re important components of their ecosystems. The fissure fills give us an interesting glimpse into a time when the sphenodontians were major, if very small, players.

Reference:

Fraser, N. C., 1982. A new rhynchocephalian from the British Upper Trias. Palaeontology 25:709-725.

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