Author: Carolyn Gramling / Source: Science News for Students
“There’s a very faint dimple here,” Sterling Nesbitt says. He holds a palm-sized fossil up to the light.
The fossil is a pelvic bone. It belonged to a creature called Teleocrater rhadinus. The slender, 2 meter (6 foot) long reptile ran on all fours and lived 245 million years ago. That’s about 10 million to 15 million years before scientists think dinosaurs first appeared.Nesbitt is a paleontologist at Virginia Tech in Blacksburg. He tilts the bone toward the overhead light. That illuminates a small depression in the fossil. The dent is about the size of a thumbprint. It marks the place where the leg bone fit into the pelvis. In a true dino, there would be a complete hole in this 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 beneath their bodies. Without it, their legs would have splayed to the sides like a crocodile. Until recently, that hole was among a handful of telltale features — physical traits — used to identify a dino.
Another no-fail trait was a particular depression at the top of the skull. Until, that is, Teleocrater mucked things up. The creature lived before dinos. But it had the “dinosaur” skull depression.
The list of “definitely a dinosaur” features was once quite long. Over the past few decades it has been shrinking. That’s thanks to new discoveries of close dino relatives such as Teleocrater. An April 2017 report of Teleocrater’s skull depression knocked yet another tell-tale trait off the list.
Today, only one feature is still unique to Dinosauria. That’s the great and diverse group of animals we know as dinosaurs. They inhabited Earth for about 165 million years. Then, some mix of asteroid impact and volcanic eruptions wiped out all dinos except birds.
“I often get asked ‘what defines a dinosaur,’” notes Randall Irmis. He’s 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.”
Recently, scientists have dug up lots of new info about dinosauromorphs (Dy-no-SOR-oh-morfs). That’s a group that includes dinosaurs and dino-like creatures that lived right before and along with early dinosaurs. New discoveries are now calling some of those dino-diagnostic features into question. And that has been shaking up long-standing ideas about what the dino family tree should look like.
Today, only one fossil feature can be attributed solely to members of Dinosauria. That is a complete hole in the hip socket of a fossil.
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. This was 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, the dinosauromorphs S. opolensis and Asilisaurus kongwe may have had epipophyses. These are 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 the dinosauromorph Marasuchus lilloensis.
In 1841, British paleontologist Sir Richard Owen coined the term “dinosaur.” Owen was thinking about fossil remains from three giants. There was a carnivore named Megalosaurus. There was the plant-eating Iguanodon. And then there was the heavily armored Hylaeosaurus. These animals shared several important features with one another but not other animals, he showed. (In particular, he noted, the creatures’ giant legs were upright and tucked beneath their bodies. Plus, each of the animals had five vertebrae fused together and welded to the pelvis.)
Owen decided the animals should be classed together as their own biological group, or taxon. He named it “Dinosauria.” That roughly translates to “fearfully great lizards.”
Stephen Brusatte is a paleontologist at the University of Edinburgh in Scotland. Back in Owen’s day, he says, it was a bit easier to spot similarities between fossils. Why? “There were so few dinosaurs,” he notes. The more fossils you find, he points out, the harder it can be to know which are most closely related. “With every new discovery, you get a different view of what features define a dinosaur,” he notes. “It’s nowhere near as clear-cut as it used to be.”
The largest extinction of species on Earth is known as the “Great Dying.” It happened about 252 million years ago. That was at the end of the Permian Period. About 96 percent of marine species and 70 percent of land species died out.
The period that followed was the Triassic. It spanned from 252 million to 201 million years ago. During that time, new reptilian species emerged and flourished. This was the age of early dinosauromorphs and crocodylians. (Those last are the ancestors of crocodiles.) And, of course, there were the true dinosaurs themselves. No one knows exactly when dinos arose. Scientists suspect it was likely some 230 million years ago.
For tens of millions of years, dinos lived beside numerous other reptile types. But at the end of the Triassic, dramatic climate change helped trigger another mass extinction. Dinosaurs somehow survived. They went on to dominate the planet during the Jurassic Period.
Paleontologists once assumed the dinos were somehow superior. Some physical traits, the thinking went, helped them outcompete other reptiles. “But that’s not borne out by new dinosaur relatives,” Nesbitt says. Dinosaurs were quite similar to dinosauromorphs, researchers found. The new trove of dinosauromorph fossils unveils a repeating pattern of parallel evolution. (That’s when similar features evolve independently in two different lines of organisms.) Dinos and other dinosauromorphs developed features such as lengthening legs or having legs directly under the body. In short, Nesbitt says, dinos “are not doing anything different than their closest relatives.”
As a result, many paleontologists now suspect there’s another reason…
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