Wednesday, April 11, 2012

A Beautiful Feathered Tyrant

photo credit: Brian Choo
The tyrannosaurids belong to a group of carnivorous dinosaurs called theropods within the Saurischia ("reptile-hipped") dinosaurs. The Tyrannosauroidea was one of the longest-lived theropod subgroups, extending from the Middle Jurassic to the Upper Cretaceous. They are characterized by massive skulls with short but deep jaws containing large sharp teeth, elongate hindlimbs, small eyes, and highly reduced forelimbs. The Tyrannosauridae taxon includes such creatures as Albertosaurus, Gorgosaurus, Daspletosaurus, Tarbosaurus, and of course Tyrannosaurus rex. These were the dominant large carnivores during the Late Cretaceous in North America and Asia, with T. rex being the last and largest of the terrestrial carnivores. Their fossils are actually relatively common in North America, particularly their teeth. Smaller members of this dinosaur subgroup (adults with a body mass less than 1,000kg) have recently been reported from the Lower Cretaceous of China. These relatively small tyrannosaurs range from about 1.4m to about 10m in body length, and they vary in their morphology with some resembling highly specialized Tyrannosauridae and others resembling coelurosaurs (theropods more closely related to birds than carnosaurs, such as Velociraptor). This variation in earlier species suggests that there may have been some significant radiation in this group.

A new paper in the journal Nature reports the discovery of a new feathered tyrannosauroid from the Lower Cretaceous. Three nearly complete skeletons representing two distinct ontogenetic stages were found in the Yixian Formation of Liaoning Province of China. The new species has been named Yutyrannus huali. Examination of bone characteristics and phylogenetic analyses place this new dinosaur among basal (not derived) tyrannosauroids yet close to Tyrannosauridae. In particular, the pneumatic midline crest of the cranium has homologous structures to Late Cretaceous tyrannosauroids while also having features that occur consistently in basal tyrannosaurids. Other cranial features (such as a large, deep skull) and a posteriorly tapering main body are similar to derived species, and the shoulder girdle, forelimbs, and hindlimbs are similar to basal species.

Figure 3: A simplified cladogram showing the systematic position of Y. huali among the Tyrannosauroidea
Finding three specimens at two distinct ontogenetic (or developmental) stages also allowed the researchers to look for morphological changes with increasing maturity. They found that as Y. huali matured it's skull and premaxilla changed significantly. These changes could be due to individual genetic variation or sexual dimorphism. The overall growth pattern of Y. huali differed from that of derived tyrannosaurids, with Y. huali showing negative allometry (relationship of size to shape) rather than the positive allometry and near isometry seen in tyrannosaurids.

Interestingly, filamentous integumentary structures (a.k.a. feathers) were found in all three of the Y. huali specimens. At this point you may be thinking: Feathers? In a fossil? And you'd be correct. The preservation of feathers isn't found all that often in dinosaur species, either because they had no feathers or because the feathers did not preserve. In these new specimens, the feather preservation is patchy (not uncommon even in species known to have a lot of plummage) but undoubtedly there. The feathers ran parallel to each other down the spine at a 30° angle with the long axis of the tail. These dorsal feathers were at least 15cm long and densely packed. Additional feathers were found extending from the dorsal side of the neck and near the humerus. These neck feathers were longer, measuring more than 20cm. The distribution of these feathers suggests that Y. huali may have been extensively feathered in life. However, the patchy nature of the preservation of the feathers may also indicate that the distribution of the feathers on the body may have been restricted and function as display structures as they do in some other theropod groups.

In terms of evolution, feathers and hair are often developed as insulators. It has been observed that some large mammals have become almost hairless because they have a low surface-to-volume ratio that allows them to retain body heat even though they have no hair. It has been suggested that the very large Late Cretaceous tyrannosaurs lack extensive feathering for similar reasons. There is fossil evidence for patches of scaly skin and no evidence for the presence of feathers in these gigantic tyrannosaurs. However, Y. huali is an earlier specimen. The authors speculate that the discovery of Y. huali "indicates that at least one gigantic dinosaur had an extensive insulative coat of feathers, showing in turn that drastic reduction of the plumage was not an inevitable consequence of very large body size. If Late Cretaceous tyrannosaurids such as Tyrannosaurus rex were similar to Y. huali in this respect, both basal and derived tyrannosauroid dinosuars would differ from mammals in lacking a tendency to lose their integumentary covering as result of gigantism." Alternatively, if scales were the dominant characteristics of the Late Cretaceous tyrannosaurs (who lived in a warm climate) then the presence of feathers in Y. huali could have been an adaptation to a cold environment. Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., Hu, D., Cheng, S., & Wang, S. (2012). A gigantic feathered dinosaur from the Lower Cretaceous of China Nature, 484 (7392), 92-95 DOI: 10.1038/nature10906

Nature article "Researchers Unearth Largest Feathered Dinosaur"

There is also a Nature Podcast of this story that aired on April 5, 2012 and it includes an interview with the study's author

(first image from the Nature story linked above)
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