Dinosaurs are getting flashier all the time. Aside from the crests, sails, horns, and other bizarre skeletal structures, beautifully-preserved soft tissues have revealed that many dinosaurs wore colorful coats of fuzz, bristles, and feathers. Dinosaurs such as the fluffy Sinosauropteryx may have shown off their striped tails, the long arm feathers of adult Ornithomimus may have marked sexual maturity, and the iridescent gloss of Microraptor might have made the bird-like dinosaur all the more attractive to potential mates. Now an in-press paper by paleontologists Scott Persons, Philip Currie, and Mark Norell proposes that oviraptorosaurs – feathery, beaked, omnivorous dinosaurs – had flexible tails which would have allowed them to strike a variety of poses.
The new Acta Palaeontologica Polonica study focuses on the tail muscles of oviraptorosaurs, building off prior studies by Persons and Currie on Tyrannosaurus and the stubby-armed Carnotaurus. (Although I hasten to point out that there has been great deal of previous work on dinosaur tails by other researchers, much of it uncited by Persons and Currie, summarized by John Hutchinson, Karl Bates, and Vivian Allen here.) Whereas those big theropods had long tails, though, oviraptorosaurs are well-known for having abbreviated tails. More than that, some oviraptorosaurs have a series of fused terminal tail bones – called a pygostyle – and the dinosaurs Caudipteryx and originally-named Similicaudipteryx had fans of elongated feathers jutting out from the tips of their tails.
After recreating a three-dimensional model of the likely tail musculature for the oviraptorosaurs Khaan mckennai, “Ingenia” yanshini, and Nomingia gobiensis, the researchers found an odd combination of features. For one thing, a muscle related to walking – M. caudofemoralis - remained robust. This contradicts hypotheses that these dinosaurs were secondarily flightless and, owing to this shift, should have had relatively reduced walking muscles. Stranger still, major muscles related to tail motion – the M. longissimus and M. ilio-ischiocaudalis – were larger than expected. These short-tailed dinosaurs would have been capable of flexible and posing their tails in ways that were impossible for other stiff-tailed theropods.
But why did Khaan and company have such maneuverable tails? We don’t know. Figuring out the function of a structure is not the same as figuring out why such a configuration evolved in the first place. Nevertheless, based on the possible range of motion and the tail feathers found among some oviraptorosaurs, Persons and colleagues propose that “Short, muscular, and highly flexible tails would have been well-suited to support a distal pygostyle, to maneuver the tail, and to flaunt a feather fan.” If this is true, the researchers speculate, the tails of oviraptorosaurs might be sexually dimorphic – the sex doing the flaunting might have better-developed tail muscles and feathers than the other. Since sexual dimorphism has yet to be conclusively demonstrated among any non-avian dinosaurs, though, this hypothesis rests on better sampling of the oviraptorosaur record and future analyses.
This isn’t the first time paleontologists have pondered the visual potential of dinosaur feathers. Researchers and artists have punted around the idea for years. The gaudy, strutting Gigantoraptor in Dinosaur Revolution (see the video above) envisioned just the sort of exhibition Persons and colleagues propose, and oviraptorosaurs actually aren’t unique in having prominent tail feathers. Microraptor – a small dromaeosaur that may have been able to glide, if not fly – had a prominent fan of feathers at the end of its tail, as did Jeholornis (sometimes classified as an early bird, but may be more closely related to non-avian deinonychosaurs like Microraptor).
The tail of tiny, buck-toothed Epidexipteryx was even closer to that of the oviraptorosaurs. Not only did this dinosaur have a stubby tail, but long, ribbon-like feathers jutted from the short appendage. Citing the importance of tail feathers for display among modern birds, paleontologists Fucheng Zhang and collaborators pointed out that “it is highly probable that the [elongate ribbon-like tail feathers] of Epidexipteryx similarly had display as their primary function, rather than serving other purposes such as flight or insulation.”
The idea that feathered dinosaurs in general, and oviraptorosaurs in particular, shook their tail feathers to get attention isn’t new. Nonetheless, the new research indicates just how muscular and flexible the tail of dinosaurs such as Khaan and Nomingia were. Did these dinosaurs develop short, muscular tails as a result of sexual selection, or might their tails have been molded by evolutionary pressures that we haven’t envisioned? That’s an open question.
What the paper does indicate, though, is that oviraptorosaurs were good, terrestrial dinosaurs. This might seem rather obvious, but ornithologists and paleontologists have frequently debated the identity and origins of these strange theropods. To ornithologists who are dead-set in their opinion that birds cannot be dinosaurs – an erroneous belief contradicted by the weight of accumulated evidence – the oviraptorosaurs were early, flightless birds. Even among those who identified these creatures are true dinosaurs, some have argued that they seem so bird-like because they evolved from flying forms, and might be close to the ancestry of birds.
Debate about the affinities, origins, and biology of oviraptorosaurs will undoubtedly continue, but the new study offers two significant points of reference for these discussions. The oviraptorosaurs modeled in the study did not show a reduction in the muscles related to walking, contradicting that they evolved from flying ancestors. More than that, the way the tails of these dinosaurs became shorter and specialized differs strongly with the way evolution reduced the tails of birds, indicating that oviraptorosaurs converged on bird anatomy but were not particularly close to early birds. As paleontologists continue to dig into the fossil record, features we once considered to be uniquely “avian” are spreading through the greater dinosaur family tree, and convergence can confound efforts to untangle the relationships of extinct species. Whether they had tails especially adapted for flaunting tail feathers or not, oviraptorosaurs were beautiful, perplexing dinosaurs that challenge our expectations for what a dinosaur truly is.
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