Some of the Ancient West’s most distinctive and downright weird dinosaur species — from the famous Tyrannosaurus rex to the odd therizinosaurs — turn out to be members of an exclusive group: According to new research, they and others like them were the only true feathered dinosaurs.

Since the discovery of feathers in dinosaur fossils nearly 20 years ago, it has become something of an article of faith among some scientists that many, if not all, dinosaurs sported some fluff — either in the form of plumage or feather-like structures called protofeathers.

But a comprehensive new survey of the known fossils and impressions of dinosaur skin suggests that feathers were only characteristic of a particular group: two-legged meat-eaters that are most closely related to modern birds. [Learn more about dinosaur evolution: “‘Unusual’ Fossil Egg Reveals Evolutionary Link Between Dinosaurs and Birds“]

“Although feathers are an important part of the dinosaur story, they seem to be peculiar to those dinosaurs ancestral to birds; there’s currently no reason to suspect that all dinosaurs from Triceratops to Brachiosaurus were covered in feathers or fuzz,” said paleontologist Paul Barrett of London’s Natural History Museum, in a statement.

Barrett and his colleague David Evans of the Royal Ontario Museum revealed their findings today at the annual meeting of the Society of Vertebrate Paleontology in Los Angeles, California.

In analyzing the catalog of dinosaur skin fossils and impressions, the scientists found that some traits were shared among almost all known dinosaurs, including scales and bony plates.

But the presence of feathers and similar microstructures that resemble them were unique to particular clade, or group of related species.

Known as coelurosaurs, this group includes tyrannosaurs like the famous T. rex; large, leggy “bird mimes” called ornithomimosaurs; and maniraptors, among whose ranks are both modern birds and therizinosaurs, the strange bipeds whose most distinctive trait was their nimble forelimbs with sharp, meter-long claws. [See a photo gallery of a T. rex fossil recently discovered in Montana.]

Of course, some of these animals seem more bird-like than others, and the duo notes that more research is needed on the less obviously feather-like structures that appear in some of these and other dinosaurs, to provide a clearer evolutionary picture.

“What’s really needed now is some detailed work on the structure and chemistry of the more equivocal skin features, like quills and some ‘protofeathers,’ to see how much they really resemble feathers or not,” Barrett said.

“The idea that some of these features are merely modified scales or other soft tissues has yet to be firmly ruled out.”

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