Colorado State Symbols - State Fossil Stegasaurus


I don't remember why, but as a child, the stegasaurus was always my favorite dinosaur.  In the pictures the face always seems so cute.  But, these huge herbivores were up to 30 feet long!  Still, that was small compared to many of the dinosaurs around at the time.

The first stegasaurus fossils were found in Colorado in 1876 by Othniel Charles Marsh in Morrison, Colorado.  But, Marsh didn't find a complete skeleton, so he thought he had found an aquatic turtle-like creature.  It wasn't until more specimens were found that we were able to figure out what the fossils were - and that there were several species of stegasaurians. 

Marsh called his first find a stegasaurus (roofed lizard), because he believed the plates on the back lay down along the back like roof shinges, or like armour.  Eventually, a fossil was discovered with the plates still attached, showing that the plates actually stood up on end. This photo of mother and child fossils is from the web site.

The stegasaurus lived in the Mid-Jurassic to Late Cretaceous period or about 150 million years ago. They were herbivores and appear to have lived in herds.  Each species had a specific number and arrangement of plates on their back, and all had spikes on their tales.  The tail had no ossified tendons, so it is likely that the tail was highly flexible and it is believed that the stegasaurus was able to use those spikes with great accuracy for defense.  Species had between 17 and 22 plates and spikes.  The species first found in Colorado, and the state symbol is Stegasaurus Armathus, also known as Stegasaurus stenops.  S. stenops was probably the largest dinosaur in this family.  Fossils from about 80 individual S stenops were found in the Morrison formation. 

It's obvious from the fossils that stenops had a short neck and a small head so it probably ate low bushes and shrubs.  When I was a kid, stenops was always depicted dragging his tail, however scientists now believe that the tail was likely held high in the air.  You can see from this photo of an actual fossil  find that it would take some time to figure out exactly what this dinosaur might have looked like - this specimen is in the National Museum of Natural History and the photo is labeled stegasaurus roadkill.

There have been four theories over the years regarding those plates on the back.

1) Marsh initially believed the plates laid flat along the back like shingles on a roof for armour.  That's why he named it the "roofed lizard" or stegasaurus.  This theory was later disproved.

2) Marsh later believed that there might have been a single row of plates standing down the back.  This theory was also disproved by later fossil finds.

3) A later theory was that there was a double row of plates along the back, in parallel.

4) The most current theory is that there were two rows of alternating plates.  By the late 1960s this had become the dominant theory based on fossils that were found with the plates still attached.

We still aren't sure what purpose the plates served, but they could be as large as two feet tall.  The plates were not attached to the skeleton.  They were bony cored scales, growing in tihe skin, similar to what you see on a crocodile or some lizards today.  The stenops plates had lots of blood vessels through them leading scientists to speculate that the plates may have served several purposes.  These plates may have helped regulate body temperature similar to the way elephants' ears help cool their bodies.  The plates may also have been used as a display, turning colors (or blushing) to attract a mate or as a warning to predators.

Although the stegasaurus was large, the other dinosaurs living at that time, such as allosaurus and ceratosaurus probably ate them.

The tiny head, in comparison to the large body, has let to the theory that S. stenops had a very small brain about the size of a walnut.  Recent research has uncovered a space along the spinal column that may have held a second brain that allowed the stenops to react quickly with those deadly tail spikes when attacked. 

There are only 6 skeletons of stegasaurus on display in the U.S. A.  One is in the Museum of Natural History in Denver - it was discovered by a teacher and students from Canyon City High School! This is a photo of that fossil on display as if under attack by another dinosaur.






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