Thursday, March 17, 2016

[Paleontology • 2016] Timurlengia euotica • New Tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies Evolution of Giant Body Sizes and Advanced Senses in Tyrant Dinosaurs

Timurlengia euotica 
 Brusatte, Averianov, Sues, Muir & Butler, 2016

Tyrannosaurs — the iconic group of dinosaurian carnivores that includes Tyrannosaurus rex —dominated latest Cretaceous ecosystems with their colossal sizes and sophisticated senses. A gap in the mid-Cretaceous fossil record between these giant apex predators and their older, smaller relatives makes it difficult to understand how the characteristic body size and ecological habits of T. rex and kin developed. A new species from Uzbekistan fills this gap. This horse-sized animal shows that tyrannosaurs had yet to achieve huge size at this time but had already evolved key brain and sensory features of the gigantic latest Cretaceous species. Tyrannosaurs apparently developed giant body size rapidly, late in the Cretaceous, and their success may have been enabled by their early-evolving keen senses.

Tyrannosaurids — the familiar group of carnivorous dinosaurs including Tyrannosaurus and Albertosaurus — were the apex predators in continental ecosystems in Asia and North America during the latest Cretaceous (ca. 80–66 million years ago). Their colossal sizes and keen senses are considered key to their evolutionary and ecological success, but little is known about how these features developed as tyrannosaurids evolved from smaller basal tyrannosauroids that first appeared in the fossil record in the Middle Jurassic (ca. 170 million years ago). This is largely because of a frustrating 20+ million-year gap in the mid-Cretaceous fossil record, when tyrannosauroids transitioned from small-bodied hunters to gigantic apex predators but from which no diagnostic specimens are known. We describe the first distinct tyrannosauroid species from this gap, based on a highly derived braincase and a variety of other skeletal elements from the Turonian (ca. 90–92 million years ago) of Uzbekistan. This taxon is phylogenetically intermediate between the oldest basal tyrannosauroids and the latest Cretaceous forms. It had yet to develop the giant size and extensive cranial pneumaticity of T. rex and kin but does possess the highly derived brain and inner ear characteristic of the latest Cretaceous species. Tyrannosauroids apparently developed huge size rapidly during the latest Cretaceous, and their success in the top predator role may have been enabled by their brain and keen senses that first evolved at smaller body size.

Keywords: dinosaur, Tyrannosauroidea, Uzbekistan, phylogenetics, evolution

Fig. 1. Skeletal reconstruction of Timurlengia euotica, with known bones colored in red. Individual bones come from different individuals, as they were surfacecollected as isolated specimens in the Bissekty Formation of Uzbekistan. The proportions of the skeleton are based on an intermediate body type between Xiongguanlong and Tyrannosaurus but should be considered provisional until associated material is found.
Bones are as follows: A, left frontal, ZIN PH 2330/ 16; B, holotypic braincase, ZIN PH 1146/16; C, cervical vertebra, ZIN PH 671/16; D, cervical vertebra, USNM (National Museum of Natural History) 538131; E, dorsal neural arch, USNM 538132; F, dorsal vertebra, CCMGE (Chernyshev’s Central Museum of Geological Exploration) 432/12457; G, anterior caudal vertebra, ZIN PH 951/16; H, middle caudal vertebra, ZIN PH 120/16; I, distal caudal vertebra, ZIN PH 507/16; J, pedal ungual, USNM 538167; K, manual ungual, ZIN PH 619/16; L, right articular and surangular (reversed), ZIN PH 1239/16; M, left quadrate, ZIN PH 2296/16; N, right dentary, ZIN PH 15/16; and O, right maxilla (reversed), ZIN PH 676/16. (Individual scale bars, 2 cm.)
Skeletal drawing courtesy of Todd Marshall.

A fossil tooth (front and back) of the newly discovered dinosaur. The blade-like teeth, says Hans Sues, were well-suited for "slicing through meat."

Systematic Paleontology
Dinosauria Owen, 1842; Theropoda Marsh, 1881;
Coelurosauria Huene, 1914; Tyrannosauroidea Osborn, 1905;

Timurlengia euotica  gen. et sp. nov.

Holotype: ZIN PH (Paleoherpetological Collection, Zoological Institute, Russian Academy of Sciences, Saint Petersburg, Russia) 1146/16, a well-preserved braincase missing only the paroccipital processes and much of the parabasisphenoid (Figs. 1 and 2 and Figs. S1−S3).

Etymology Timurlengia, in reference to the fourteenth-century Central Asian ruler Timurleng (English: Tamerlane), and euotica, meaning “well eared” in reference to the large inner ear of the holotype.

Horizon and Locality: Dzharakuduk, central Kyzylkum Desert, Navoi Viloyat, Uzbekistan. Bissekty Formation, Upper Cretaceous, Middle-Upper Turonian (ca. 90–92 million years ago)

A reconstruction of the horse-sized tyrannosaur Timurlengia euotica, named for the charismatic Central Asian ruler Tamerlane, shows the species' long, slender legs, large head and teeth built sharp like a steak knife.
NMNH, Original painting by Todd Marshall 

Fig. 3. Phylogenetic relationships of Timurlengia euotica among theropod dinosaurs. Strict consensus of 99,999 most parsimonious trees (length = 3,367, consistency index = 0.322, retention index = 0.777) recovered from cladistic analysis in which T. euotica is scored based on the holotypic braincase and series of referred specimens. Numbers next to nodes are Bremer/jackknife support values, thick lines next to each taxon depict temporal range (which in most cases is age uncertainty and not true range), colors of lines denote geographic areas, and silhouettes are in relative proportion and scaled to total body length (T. rex = 13 m).
Geographic silhouettes are from Loewen et al. (3), and taxon silhouettes are courtesy of (Kileskus, T. M. Keesey; Guanlong, S. Hartman; Juratyrant, S. Hartman, T. M. Keesey; Eotyrannus, S. Hartman; Dryptosaurus, T. M. Keesey; Albertosaurus, C. Dylke; Daspletosaurus, S. O’Connor, T. M. Keesey; Tyrannosaurus, S. Hartman).

Timurlengia is a long-awaited diagnostic tyrannosauroid from the middle part of the Cretaceous. It indicates that these predators were still far from giants during this time, but had already evolved signature brain and sensory features that may have been tied to the extraordinary success of the last-surviving, latest Cretaceous species like Tyrannosaurus. However, Timurlengia remains a single data point from a still murky interval in dinosaur history, and future discoveries from this gap will undoubtedly lead to a better understanding of how tyrannosauroids rose from marginal creatures into some of the largest terrestrial predators in Earth history.

 Stephen L. Brusatte, Alexander Averianov, Hans-Dieter Sues, Amy Muir and Ian B. Butler. 2016. New Tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies Evolution of Giant Body Sizes and Advanced Senses in Tyrant Dinosaurs. Proceedings of the National Academy of Sciences of the United States of America. in press. DOI:   10.1073/pnas.1600140113

The discovery of a tiny Tyrant set the stage for the evolution of T. rex via @SmithsonianMag 
@NMNH #PreRex
 Behold Timurlengia—the Elusive Missing Link in Tyrannosaur Evolution
Timurlengia euotica: Distant relative of T-rex shows how dinosaur became giant  via @IBTimesUK #dinosaurevolution