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Acro's Dino Facts. We produce and share art, media, educational services and programs regarding prehistoric life.

31/05/2025
21/07/2023

219th anniversary of Richard Owen's birth, the man who created the word "dinosaur".

21/07/2023

Did you know we have fossilized four-eyed lizards at Fossil Butte?

Saniwa ensidens was the first fossil lizard to be named in North America and was first described in 1870 by paleontologist Joseph Leidy. Leidy noticed that the specimen possessed similarities to the modern Nile monitor lizard. S. ensidens is a member of the Varanidae family, and is the sister taxon to Varanus, which is the family that contains monitor lizards.

Like most other lizards, monitor lizards have a third eye on top of their head, also called a parietal eye. It doesn't have sight like a regular eye, but it is sensitive to light and helps to the individual's circadian rhythm and hormone production. It is also present in tuatara, some amphibians and bony fish, and lampreys. It is notably absent from snakes, crocodilians, and turtles.

Saniwa ensidens took it a step further and has 2 parietal eyes! The epithalamus comprises two separate parts, the pineal and the parapineal organs, which have similar functions. For most vertebrates, the third eye forms from the pineal gland. However, in tuatara and modern lizards, it is formed from the parapineal gland. It is the only jawed vertebrate known to have both pineal and parapineal eyes, which makes it pretty special!

Photo Credit: NPS

Image Description: A cast of a juvenile Saniwa ensidens. Skeleton similar in appearance to that of modern monitor lizards, with it's tail making up nearly 2/3 of the body length.

21/07/2023

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