When paleontologists began discovering feathery dinosaurs during the 1990s, every find was a tantalizing glimpse at possibilities that researchers had based on bone. Now, almost seventeen years since the Sinosauropteryx splash, fluffy dinosaurs seem almost mundane. Finding yet another small, bird-like, fuzzy dinosaur doesn’t spur the same excitement that earlier discoveries did. This is not to say that these new finds are not important. Quite the contrary. With every new feathered dinosaur named, paleontologists uncover a little more context for ongoing discussions about the evolution of flight, feathers, and birds. The latest fluffy dinosaur to join the ranks – Eosinopteryx brevipenna.

Found by a commercial collector in the roughly 161 million year old stone of northeastern China’s Tiaojishan Formation, the tiny dinosaur is preserved as a virtually complete skeleton. Encircled by feather fossils, the 30 cm long dinosaur lies with its arms held out and legs bent. The theropod almost looks like it’s ready to take off running, except for the fact that the dinosaur’s head is slightly detached from the vertebral column.

Described by Royal Belgian Institute of Natural Sciences paleontologist Pascal Godefroit and coauthors in Nature Communications, Eosinopteryx is the third fluffy dinosaur known from the same deposits. The skeletally-similar Anchiornis and Xiaotingia have both been found in Tiaojishan Formation and supposedly lived around the same time. (How old these dinosaurs are, and whether they were contemporaries, is a tricky question made all the more complicated by the fact that these specimens are often purchased from commercial dealers who do not rigorously record geological information about the fossils.) Could Eosinopteryx be an Anchiornis by another name? Godefroit and colleagues argue against synonymy, citing plumage as the key difference.

Compared to Anchiornis and other closely-related feathered dinosaurs within a group called deinonychosaurs, Eosinopteryx seemed to be missing feathers. Whereas Anchiornis had long pennaceous feathers along the tail, ankles, and feet, Eosinopteryx lacked these specialized features. Rather than being a quirk of preservation, Godefroit and coauthors argue, Eosinopteryx seems to lack these feathers because they weren’t actually there. The fact that delicate, plume-like feathers were preserved on the dinosaur’s tail, for example, hints that the dinosaur’s anatomy is preserved to such a high degree that pennaceous feathers would have turned up had they been present.

The researchers also doubt that the difference in feather types was the result of changes during growth. Based upon the degree of fusion between certain bones in the skeleton, Godefroit and collaborators hypothesize that the lone Eosinopteryx specimen was a subadult or adult when it perished. The paleontologists concede that this Eosinopteryx specimen might have lacked pennaceous tail and leg feathers because it was moulting when it perished, but, based on other features, they argue that the fossil faithfully preserves the dinosaur’s true plumage. The fact that Eosinopteryx had comparatively short arms and uncurved toe claws is consistent with a life spent scurrying over the ground rather than flapping through the air.

Of course, the plumage, relationships, and behavior of Eosinopteryx are all hypotheses that are open to testing. Paleontologists aren’t totally agreed on the usefulness of bone fusion alone to estimate the ages of dinosaurs, and claw shape isn’t necessarily a good indicator of preferred habitat or natural history. The most controversial aspect of the new study may not be the behavior or age of Eosinopteryx, though, but the feathered dinosaur family tree the researchers recovered.

The evolutionary tree, created by comparing subtle traits of various dinosaurs and early birds with each other to discern relationships, found that the famous Archaeopteryx wasn’t actually an early bird, but an archaic deinonychosaur – the larger group that contains troodontids like Anchiornis and dromaeosaurids such as Velociraptor. This placement echoes the results of a controversial study, published in Nature in 2011published in Nature in 2011published in Nature in 2011, which proposed that Archaeopteryx, Anchiornis, and Xiaotingia formed a distinct subgroup of feathered dinosaurs that was further removed from bird ancestry than traditionally thought.

Does this mean that we should stop calling Archaeopteryx the earliest known bird? Not necessarily. “[T]his phylogeny remains only weakly supported,” Godefroit and coauthors caution, and the paleontologists point out that convergent evolution among small, feathered dinosaurs might obscure the true pattern of relationships between the feathered forms. The identity of Archaeopteryx is being questioned, and rightly so, but paleontologists have yet to fully resolve which particular lineage of dinosaur spawned the first birds.

Birds are a special lineage of coelurosaurian dinosaurs. That is a fact. But the details of when and how that transition occurred, not to mention exactly from whom, are still areas of active debate. Eosinopteryx underscores the increasingly complex pattern of feathered dinosaur evolution and bird origins. The tiny dinosaur is another point of reference in an ongoing discussion about when dinosaurs took to the air, and which particular lineage left avian heirs to the Mesozoic legacy.

[The restoration of Eosinopteryx above was created by Emily Willoughby. Check out her artwork here.]

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