New fossils are redefining what makes a dinosaur

January 18, 2017by admin0

Defining what’s unique about these ‘fearfully great lizards’ gets harder with new finds


“There’s a very faint dimple here,” Sterling Nesbitt says, holding up a palm-sized fossil to the light. The fossil, a pelvic bone, belonged to a creature called Teleocrater rhadinus. The slender, 2-meter-long reptile ran on all fours and lived 245 million years ago, about 10 million to 15 million years before scientists think dinosaurs first appeared.

Nesbitt, a paleontologist at Virginia Tech in Blacksburg, tilts the bone toward the overhead light, illuminating a small depression in the fossil. The dent, about the size of a thumbprint, marks the place where the leg bone fit into the pelvis. In a true dinosaur, there would be a complete hole there in the hip socket, not just a depression. The dimple is like a waving red flag: Nope, not a dinosaur.

The hole in the hip socket probably helped dinosaurs position their legs underneath their bodies, rather than splayed to the sides like a crocodile’s legs. Until recently, that hole was among a handful of telltale features paleontologists used to identify whether they had their hands on an actual dinosaur specimen.

Another no-fail sign was a particular depression at the top of the skull. Until Teleocrater mucked things up. The creature predated the dinosaurs, yet it had the dinosaur skull depression.

The depression in Teleocrater’s hip bone (bottom half) marks where the leg bone fit into the pelvis. Dinosaurs have a complete hole in that part of the hip socket.


<img src=”” alt=”” class=”caption” style=”float: right; width: 370px; height: 238px;” title=”The depression in &lt;em&gt;Teleocrater&lt;/em&gt;’s hip bone (bottom half) marks where the leg bone fit into the pelvis. Dinosaurs have a complete hole in that part of the hip socket. ~~ S.J. Nesbitt” />

The once-lengthy list of “definitely a dinosaur” features had already been dwindling over the past few decades thanks to new discoveries of close dino relatives such as Teleocrater. With an April 2017 report of Teleocrater’s skull depression (SN Online: 4/17/17), yet another feature was knocked off the list.

Today, just one feature is unique to Dinosauria, the great and diverse group of animals that inhabited Earth for about 165 million years, until some combination of cataclysmic asteroid and volcanic eruptions wiped out all dinosaurs except the birds.

“I often get asked ‘what defines a dinosaur,’ ” says Randall Irmis, a paleontologist at the Natural History Museum of Utah in Salt Lake City. Ten to 15 years ago, scientists would list perhaps half a dozen features, he says. “The only one to still talk about is having a complete hole in the hip socket.”

The abundance of recent discoveries of dinosauromorphs, a group that includes the

dinosaur-like creatures that lived right before and alongside early dinosaurs, does more than call diagnostic features into question. It is shaking up long-standing ideas about the dinosaur family tree.

To Nesbitt, all this upheaval has placed an even more sacred cow on the chopping block: the uniqueness of the dinosaur.

“What is a dinosaur?” Nesbitt says. “It’s essentially arbitrary.”

Yesterday’s diagnostics

Today, only one fossil feature can be attributed solely to members of Dinosauria: a complete hole in the hip socket.

Several others, including the four below, are no longer surefire dinosaur signs:


  1. Until Teleocrater came along, only dinosaurs were known to have a deep depression at the top of the skull, an attachment site for some jaw muscles probably related to bite strength.
  2. Dinosaurs and some other dinosauromorphs such as Silesaurus opolensis have an enlarged crest on the upper arm bone where muscles attached.
  3. Along with dinosaurs, dinosauromorphs S. opolensis and Asilisaurus kongwe may have had epipophyses, bony projections at the back of the neck vertebrae.
  4. An extra (fourth) muscle attachment site, called a trochanter, at the point on the femur that meets the hip is also found in dinosauromorph Marasuchus lilloensis.

Sources: S.J. Nesbitt et al/Nature 2017; S.L. Brusatte et al/Earth-Science Reviews 2010


Shared traits

In 1841, British paleontologist Sir Richard Owen coined the term “dinosaur.” Owen was contemplating the fossil remains of three giant creatures — a carnivore named Megalosaurus, the plant-eating Iguanodon and the heavily armored Hylaeosaurus. These animals shared several important features with one another but not other animals, he determined. (In particular, he noted, the creatures’ giant legs were upright and tucked beneath their bodies, and each of the animals had five vertebrae fused together and welded to the pelvis.)

Owen decided the animals should be biologically classified together as their own group, or taxon. He named the group “Dinosauria” for “fearfully great lizards.”

In Owen’s day, it was a bit easier to spot similarities between fossils, says paleontologist Stephen Brusatte of the University of Edinburgh. “Back then, there were so few dinosaurs. But the more fossils you find, the patterns become more complicated,” he says. “With every new discovery, you get a different view of what features define a dinosaur. It’s nowhere near as clear-cut as it used to be.”

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Dino survivors

The largest extinction of species on Earth, the “Great Dying,” happened about 252 million years ago at the end of the Permian Period (SN: 9/19/15, p. 10). About 96 percent of marine species and 70 percent of land species succumbed.

In the period that followed, the Triassic, spanning 252 million to 201 million years ago, new reptilian species arose and flourished. This was the time of the dinosauromorphs, crocodylians (the ancestors of crocodiles) and, of course, the dinosaurs themselves. No one knows exactly when dinosaurs arose, although it was probably around 230 million years ago.

For tens of millions of years, the dinosaurs lived alongside numerous other reptile lineages. But at the end of the Triassic, dramatic climate change played a role in another mass extinction. Dinosaurs somehow survived and went on to dominate the planet during the Jurassic Period.


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