Friday 14 March 2014

A dwarf Tyrannosaurid from the Late Cretaceous of northern Alaska.

Tyrannosaurids were large, predatory Dinosaurs from the Late Cretaceous of North America and Eurasia. They are among the best known of all Dinosaurs, since they were the biggest carnivores in their ecosystems, making them (particularly the eponymous Tyrannosaurus rex at 13 m and up to 6.8 tonnes) a must have for any Hollywood movie featuring Dinosaurs. The group has been extensively studied, with a fairly large number of well preserved specimens known, making it possible to assign fragmentary remains and even individual bones to the group with some confidence.

In a paper published in the journal PLoS One on 12 March 2014, Anthony Fiorillo and Ronald Tykoski of the Department of Paleontology at the Perot Museum of Nature and Science in Dallas describe a new species of dwarf Tyrannosaurid from the Late Cretaceous of Northern Alaska.

The new species is named Nanuqsaurus hoglundi, where ‘Nanuqsaurus’ derives from ‘nanuq’ the Iñupiaq word for Polar Bear, plus the Greek ‘sauros’ for Lizard, thus ‘Polar Bear Lizard’, and ‘hoglundi’ honours Forrest Hoglund ‘for his career in earth sciences and his philanthropic efforts in furthering cultural institutions’. Nanuqsaurus hoglundi is described from three disarticulated skull fragments from the Kikak-Tegoseak Quarry in North Slope Borough, Alaska, calculated to be between 70 and 68 million years old, based upon isotopic dating of volcanic ash beds that occur in the same formation.

Nanuqsaurus hoglundi. (A) Reconstruction of a generalized tyrannosaurine skull, with preserved elements of specimen shown in white. Arrow points to autapomorphic, reduced, first two dentary teeth. (B–E) Photographs and interpretive line drawings of right maxilla piece in medial (B, C); and dorsal (D, E) views. (F–I) Photographs and interpretive line drawings of partial skull roof in dorsal (F, G); and rostrolateral (H, I) views. (J–M) partial left dentary in lateral (J); medial (K); dorsal (L) views; and close-up of mesial alveoli in dorsal (M) views. Abbreviations: a, alveolus, with number indicating position in tooth row; d, dentary tooth, with number indicating position in tooth row; f, frontal; gr, orbital groove; lac.f, lacrimal facet of frontal; l.f, left frontal; na, nasal contact surface; oif, oral intramandibular foramen; p, parietal; po.f, postorbital facet of frontal; prf.f, prefrontal facet of frontal;. r.f, right frontal; ri, ridge separating sockets in nasal articulation of maxilla; rpf, rostral process of frontal between prefrontal and lacrimal facets; rsp, rostral spur of parietal; sac, sagittal crest; so, socket for nasal articulation of maxilla; stf; supratemporal fossa. Gray fill indicates missing bone or broken bone surfaces and cracks. Scale bar in A equals 10 cm. Scale bars in B–L equal 5 cm. Scale bar in M equals 1 cm. Fiorillo & Tykoski (2014).

Nanuqsaurus hoglundi appears to have been exceptionally small for a Tyrannosaurid, Fiorillo & Tykoski estimate that it would have had a total skull length of less than 700 mm. They further theorize that this related directly to the environment in which the animal lived. In the Late Cretaceous the area would have been on the north coast of the landmass of Laramida (North America was divided by an internal sea, due to much higher sea-levels in the greenhouse world of the Late Cretaceous), and further isolated from the rest of this landmass by a chain of mountains. The area was already in the Arctic Circle by this time, and while it is unlikely that there was any extensive ice cover in the warm climate of the time, there would still have been a period of complete darkness lasting several weeks in the mid-winter, as well as a longer period of extremely short days, events which would have had a profound effect on plant productivity, and therefore the amount of food available at all levels of the food-chain.

As well as the dwarf Tyrannosaurid Nanuqsaurus hoglundi, the Alaskan Arctic has produced numerous teeth of a Trudontid Theropod of exceptional size, suggesting that this member of a normally diminutive Theropod group was exceptionally large, and indeed in a similar size range to Nanuqsaurus hoglundi. Fiorillo & Tykoski speculate that in the Alaskan Arctic of the Late Cretaceous this may have been the optimum size for a predatory Theropod Dinosaur, and that the two animals, from groups which normally showed greatly divergent sizes and therefore presumably feeding stratergies, therefore converged upon this middling size in response to environmental pressures.

Relative size of Nanuqsaurus hoglundi. Silhouettes showing approximate sizes of representative theropods. (A) Nanuqsaurus hoglundi. (B) Tyrannosaurus rex, large specimen. (C) Tyrannosaurus rex, smaller specimen. (D) Daspletosaurus torosus. (E) Albertosaurus sarcophagus. (F) Troodon formosus, lower latitude individual based on multiple sources and size estimates. (G) Troodon sp., North Slope individual based on extrapolation from measurements of multiple dental specimens. Scale bar equals 1 m. Fiorillo & Tykoski (2014).

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