Discovery of New Predatory Dinosaur Species Gives New Insight on Their Evolution

Jayc Sedlmayr, PhD, Assistant Professor of Cell Biology & Anatomy at LSU Health Sciences Center New Orleans School of Medicine, was part of an international team of scientists who discovered a new tyrannosaur with an unusual mode of evolution. Their findings include that Daspletosaurus horneri, or “Horner’s Frightful Lizard,” evolved directly from its geologically older relative, D. torosus, a rare form of evolution called anagenesis where one species gradually morphs into a new one. The research also changes the face of tyrannosaurs, which the team concluded was covered by a lipless mask of large, flat scales, with smaller patches of armor-like skin and horn, as well as a highly touch-sensitive snout. The work is published online in Nature Research‘s Scientific Reports, available at http://nature.com/articles/doi:10.1038/srep44942.

The team, led by Thomas Carr, PhD, of Carthage College’s Department of Biology, also included Dr. David Varricchio of Montana State University, Dr. Eric Roberts of James Cook University and Dr. Jason Moore of the University of New Mexico. Dr. Carr is an expert on the evolution and growth of Tyrannosaurus rex and its closest relatives, collectively called tyrannosaurs.

LSU Health New Orleans’ Dr. Jayc Sedlmayr, an evolutionary biologist whose research is in evolutionary anatomy, provided the biological non-boney anatomical perspective and interpretation. The team worked with excellently preserved fossils – a skull and skeleton of a subadult, a skull and skeleton of an adult, a partial lower jaw of a subadult, and isolated bones of subadults and juveniles. The scientists compared tyrannosaur skulls with those of crocodylians, birds and mammals as well as earlier research that matched bone texture with different types of skin covering.

“Much of our research went beyond field paleontology – it was generated from lab-based comparative anatomy, the dissection of birds as living dinosaurs and crocodilians as their closest living relatives, and based on the similarities of the facial nerves and arteries we found in those same groups which left a trace on the bone, we were able to then reconstruct in the new tyrannosaur species,” notes LSU Health New Orleans anatomist Jayc Sedlmayr, PhD.

His own research was based on dissecting birds, alligators, crocodiles, lizards and turtles, including injecting their blood vessels and doing angiographic studies from some of their specimens. He then made hypotheses of homology (anatomical features that are shared because of a common ancestry) based on these dissections. If they had any boney traces (osteological correlates), he looked for those in fossil specimens. If a specific nerve or artery went through a particular opening or groove in a bone (foramen) in both birds and crocodiles, and this same foramen or groove is found in dinosaurs, he could then reconstruct that nerve or artery in the extinct animal – and this is what the team did with the new species of tyrannosaur they described.

“Our findings of a complex sensory web is especially interesting because it is derived from the trigeminal nerve which has an extraordinary evolutionary history of developing into wildly different ‘sixth senses’ in different vertebrates – sensing magnetic fields for bird migration, electroreception for predation in the platypus bill or the former whisker pits of dolphins, sensing infrared in pit vipers to identify prey, guiding mammals movements through the use of whiskers, sensing vibrations through the water by alligators, and turning the elephant trunk into a sensitive hand similar to what has been done to the entire face of tyrannosaurs,” concludes Sedlmayr.

“It turns out that tyrannosaurs are identical to crocodylians in that the bones of their snouts and jaws are rough, except for a narrow band of smooth bone along the tooth row,” says Carr. “In crocodylians, the rough texture occurs deep to large flat scales; given the identical texture, tyrannosaurs had the same covering. We did not find any evidence for lips in tyrannosaurs, the rough texture covered by scales extends nearly to the tooth row, providing no space for lips.”

Daspletosaurus horneri lived in Montana 75.2-74.4 million years ago. With a body length of approximately 9 meters, this tyrannosaur had a wide snout, small orbital horns and slit-like pneumatic opening on the inside of the lacrimal bone. The large horn behind the eye is elevated beyond the side of the head, indicating a covering of keratin, the hard and shiny material that makes up human fingernails. Its prey were horned dinosaurs, crested duckbill dinosaurs, dome-headed dinosaurs and smaller theropod dinosaurs. It was the last species of Daspletosaurus (“frightful lizards”) to have evolved in the American west.

“In some ways, the facial components of the trigeminal nerve of these dinosaurs mirrors that of humans,” says Sedlmayr. “The human trigeminal nerve provides significant touch sensitivity to the face. It brings back sensation from our facial muscles allowing us to fine tune and coordinate the emotional and social displays so important to human communication. This nerve is so sensitive that in pathological conditions, trigeminal neuraligia, it can be responsible for some of the most severe pain our species can endure; in extreme cases, the pain is so great that many people suffering from it end up committing suicide."