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Silhouette credits from Phylopic.com: Gallimimus, Silesaurus- Scott Hartman; Desmatosuchus- Steven Traver; Effigia- Sarah Werning; Smilosuchus- Robert Gay; Gavialis- Evan Boucher; Lesothosaurus- Jaime Headden, modified by T. Michael Keesey; Ankylosaurus- Andrew Farke
We describe an exciting new species of extinct reptile (closely related to dinosaurs and crocodylians), Triopticus primus, as part of a diverse reptilian fauna from ~230 million years ago in Texas. This single group of reptiles preserves an extensive range of skeletal shapes and structures that each are re-evolved by dinosaurs and crocodylians millions of years later, presenting a striking example of convergent evolution among vertebrates.
A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives
Michelle R. Stocker, Sterling J. Nesbitt, Katharine E. Criswell, William G. Parker, Lawrence M. Witmer, Timothy B. Rowe, Ryan Ridgely, and Matthew A. Brown
- Triassic-aged (~230 million years ago) Triopticus evolved a bony dome-head 140 million years before the more famous Cretaceous-aged (~90 million years ago) pachycephalosaur dinosaurs
- After the end-Permian extinction, the biggest extinction of all time, the early archosauromorph cousins of dinosaurs and crocodylians evolved explosively into extreme body plans that were much later re-evolved by dinosaurs and crocs.
- Dinosaurs re-evolved the common and bizarre body plans present in their Triassic-aged relatives that were once thought to be unique to dinosaurs
- The early evolution of body plans may constrain later body plans in the same group
The skull of Triopticus primus in side view. Scale bar = 1 cm. Photo by Matthew Brown
Helpful Facts/Background for press articles:
- The Triassic Period lasted from ~250 million years ago to ~200 million years ago.
- These fossils were collected by the Works Progress Administration (WPA) in West Texas in the late 1930s and early 1940s.
- The skull of Triopticus was found by Stocker, Nesbitt, Criswell, Parker, and Rowe in a drawer in the Texas Vertebrate Paleontology Collections (essentially a fossil library) of The University of Texas at Austin completely surrounded in rock as part of a seminar course in 2010. Nesbitt initially noticed the intact braincase attached to the dome of the skull.
- The Mesozoic Era includes the Triassic, Jurassic and Cretaceous Periods
- The earliest dinosaurs that lived at the same time as Triopticus were small (~9 feet long) and looked like small bipedal carnivores. Later after the Triassic Period, dinosaurs evolved into their much more familiar and larger shapes.
- Triopticus is represented by a single partial skull.
- Not much is known about the skeleton of Triopticus, but it was no bigger than a lion.
- Crocodylians and birds are each other's closest living relatives and are part of the reptile group Archosauria. All dinosaurs are archosaurs.
- The end of the Permian Period (~250 million years ago) experienced the largest extinction of all time where 80% - 95% of all life went extinct. Similarly, a mass extinction also occurred at the end of the Triassic Period (~200 million years ago) where many vertebrates went extinct.
- The reoccurring body plans of dinosaurs mostly occurred over tens of millions of years after the extinction of the reptiles in the Triassic Period. Some body plans were repeated within a few million years after the end-Triassic extinction, whereas others evolved up to 100 million years later.
- The world was much hotter in the Triassic Period than it is now.
- The Triopticus skull and other fossils of reptiles found at the same fossil locality were buried in a large river system similar to the current Mississippi River.
- Triopticus was found in West Texas near the city of Big Spring, Texas.
Quotes avialbale for press articles:
-Michelle Stocker, Virginia Tech: [statements about Otis Chalk and convergence/diversification of Triassic fauna …] “The Otis Chalk fauna is an amazing single snapshot of geologic time where you have this extraordinary range of animal body plans all present at the same time living together. This fauna is one of the earliest records of Late Triassic life in the American West, and we are able to discover so much about it today because of work done by the WPA over 70 years ago.”
“Among the animals preserved in the Otis Chalk fauna, Triopticus exemplifies this phenomenon of body shape convergence because its skull shape was repeated by very distantly-related dome-headed dinosaurs more than 100 million years later.”
“Triopticus is an extraordinary example of evolutionary convergence between the relatives of dinosaurs and crocodylians and later dinosaurs that is much more common than anyone ever expected. What we thought were unique body shapes in many dinosaurs actually evolved millions of years before in the Triassic Period.”
-Sterling Nesbitt, Virginia Tech: “After the enormous end-Permian mass extinction, the group that includes the living crocodylians and birds and their close relatives exploded onto the scene and diversified into many different sizes and shapes. These early body shapes were later mimicked by dinosaurs.”
-Katharine Criswell, University of Chicago: [statements about disparity and convergence…] : “It is amazing to think that many of the iconic dinosaur features that we know and love, such as long snouts, toothless beaks, armor plates, and thickened dome skulls were arrived at completely independently up to 100 million years earlier in these distant reptilian cousins.”
“This project started out as part of a paleontology class that many of us took when we were graduate students at the University of Texas at Austin. Triopticus was collected by the Works Progress Administration in the 1940s, and had been sitting in a drawer in the paleontology collections for almost 70 years when we discovered it. With a combination of CT scans and fossil comparisons we were able to give this old fossil new life.”
-William Parker, Petrified Forest National Park: [statements about Triassic fauna…] “Triopticus reveals insight in the window of hidden diversity in the Triassic faunal assemblages.”
-Lawrence Witmer and Ryan Ridgely, Ohio University: [statements about CT/brain evolution and convergence…]
Witmer: “This project combines both old-school and hi-tech approaches. Careful excavation and cleaning of the fossils showed the team that we had something special in Triopticus, but being able to peer inside the skull with X-ray CT scanning was a game-changer. It showed us that the similarity of Triopticus with the much later dome-headed pachycephalosaur dinosaurs was more than skin deep, extending to the structure of the bone and even the brain.”
-Matthew Brown and Timothy Rowe, The University of Texas at Austin: [statements about importance of natural history collections…] Brown: “Triopticus was dug up 76 years ago along with thousands of other fossils from Texas. We can gain new insights into the history of life because specimens like Triopticus have been curated into museum collections like the one at The University of Texas at Austin. These collections are the foundation of natural history research, and this new animal illustrates how exciting discoveries are continually made thanks to the forethought and investment of past generations. It will be fascinating to see what the students of tomorrow find next.”
“This Triassic radiation of archosaurs and their close relatives delimited the majority of the possible shapes that non-avian dinosaurs were able to take on later, but it took hundreds of millions of years for dinosaurs to achieve that full range of body shapes.” Michelle Stocker, Virginia Tech
1. YouTube video of Triopticus braincase & brain endocast with labels: https://www.youtube.com/watch?v=TbnEZABTCjk
2. Sketchfab animation of Triopticus braincase with labels: https://skfb.ly/QFLx
3. Sketchfab animation of transparent Triopticus braincase revealing brain endocast: https://skfb.ly/QFKG
Lead author Dr. Michelle R. Stocker, Departement of Geosciences, Virginia Tech. Photo by Sterling Nesbitt.
Photograph from the Works Progress Administration, showing excavation in the general area from which Triopticus primus was collected in 1940 near Big Spring, Texas. Original photo on file in the Vertebrate Paleontology Collections of The University of Texas at Austin.
The skull of Triopticus primus in side view showing the reconstructed brain. Scale bar = 1 cm.