Wednesday, September 8, 2021

[Paleontology • 2021] Ulughbegsaurus uzbekistanensis • A New Carcharodontosaurian Theropod Dinosaur Occupies Apex Predator Niche in the early Late Cretaceous of Uzbekistan

Ulughbegsaurus uzbekistanensis 
Tanaka, Anvarov, Zelenitsky, Ahmedshaev & Kobayashi, 2021 

 life reconstruction by Julius Csotonyi 

Carcharodontosauria is a group of medium to large-sized predatory theropods, distributed worldwide during the Cretaceous. These theropods were probably the apex predators of Asiamerica in the early Late Cretaceous prior to the ascent of tyrannosaurids, although few Laurasian species are known from this time due to a poor rock record. Here, we describe Ulughbegsaurus uzbekistanensis gen. et sp. nov. from the early Late Cretaceous (Turonian) of Central Asia, which represents the first record of a Late Cretaceous carcharodontosaurian from the region. This new taxon is represented by a large, isolated maxilla from the Bissekty Formation of the Kyzylkum Desert, the Republic of Uzbekistan, a formation yielding a rich and diverse assemblage of dinosaurs and other vertebrates from fragmentary remains. Comparison of the maxilla with that of other allosauroids indicates Ulughbegsaurus was 7.5–8 m in body length and greater than 1000 kg in body mass, suggesting it was the previously unrecognized apex predator of the Bissekty ecosystem while smaller known tryannosauroids and dromaeosaurids were probable mesopredators. The discovery of Ulughbegsaurus records the geologically latest stratigraphic co-occurrence of carcharodontosaurid and tyrannosauroid dinosaurs from Laurasia, and evidence indicates carcharodontosaurians remained the dominant predators relative to tyrannosauroids, at least in Asia, as late as the Turonian.

Keywords: Uzbekistan, Bissekty Formation, Carcharodontosauria, dinosaur, Late Cretaceous, Tyrannosauroidea  

Figure 1. Left maxilla of Ulughbegsaurus uzbekistanensis (UzSGM 11-01-02).
(a) Lateral, (b) medial, (c) ventral, (d) anterior and (e) posterior views.
 (f) Reconstruction of skull (grey missing bones are based on Neovenator, modified from Naish et al. [2001]).
Black arrows indicate tubercles on the rim of the antorbital fossa and dashed lines indicate depressions.
Abbreviations: af, accessory fossa; amp, anteromedial process; aofe, antorbital fenestra; aofo, antorbital fossa; idw, interdental wall; mes, medial shelf; mfe, maxillary fenestra; mx1-8, first to eighth maxillary tooth; nuf, nutrient foramen; pmfo, promaxillary fossa; pr, pneumatic recess.

Systematic palaeontology
Dinosauria Owen, 1842 
Saurischia Seeley, 1887 

Theropoda Marsh, 1881 
Tetanurae Gauthier, 1986
Allosauroidea Marsh, 1878

Carcharodontosauria Benson et al., 2010 

Ulughbegsaurus uzbekistanensis gen. et sp. nov.
Holotype: UzSGM 11-01-02, a left maxilla.

Diagnosis: Carcharodontosaurian theropod with a series of shallow, oval depressions on the lateral surface along the ventral edge of maxilla; tubercles along with the ridged rim of the antorbital fossa; vertically oriented ridges on the lateral surface of maxilla and large foramina at a dorsal edge of dental plates in maxilla.

Referred specimens: CCMGE 600/12457 is a jugal ramus of a left maxilla and ZIN PH 357/16 is a posterior end of the right maxilla, previously referred to the dromaeosaurid Itemirus medullaris ([18]: see ‘5. Comments on isolated teeth and bone fragments of theropods from the Bissekty Formation’).

Locality and horizon: The holotype and referred specimens were recovered at Dzharakuduk, central Kyzylkum Desert, Navoi Viloyat, Uzbekistan, where the Upper Cretaceous (middle to upper Turonian, ca 90–92 million years ago) Bissekty Formation is exposed; the precise location is unknown as geographic coordinates were not noted in the field. The holotype was brought to UzSGM by a field team member of Lev Alexandrovich Nessov.

Etymology: Ulughbeg’ refers to Timurid sultan Ulugh Beg, in recognition of his early scientific contributions as a fifteenth-century astronomer and mathematician in central Asia region (now Uzbekistan). ‘Sauros’ meaning reptiles in Latin. Specific name, ‘uzbekistan’, refers to the Republic of Uzbekistan.
Ulughbegsaurus uzbekistanensis, a giant carcharodontosaurid from Uzbekistan. This new top predator dwarfs the previously largest known theropod from the region, the tyrannosauroid Timurlengia euotica.
 life reconstruction by Julius Csotonyi 

Simplified cladograms of theropod taxa, indicating the phylogenetic positions of Ulughbegsaurus.

Figure 4. Comparisons between small tyrannosauroid and large non-tyrannosauroid predatory theropods. (a) Phylogenetic tree of Tyrannosauroidea with sympatric allosauroid taxa (1, Guanlong with sympatric Sinraptor from the Late Jurassic Shishugou Formation of China; 2, Tanycolagreus and Stokesosaurus with sympatric Allosaurus and Saurophaganax from the Late Jurassic Morrison Formation of the United States; 3, Eotyrannus and sympatric Neovenator from the Early Cretaceous Wessex Formation of the United Kingdom; 4, Moros and sympatric Siats from the early Late Cretaceous Cedar Mountain Formation of the United States; 5, Timurlengia and sympatric Ulughbegsaurus from the Turonian Bissekty Formation of Uzbekistan), indicating that sympatric large allosauroid taxa are found at least until Turonian. Phylogenetic tree is time calibrated based on Zanno et al. [2019], Carr et al. [2017] and Voris et al. [2020], except for branch length and divergence times. (b) Bivariate plot of body mass between tyrannosauroids and non-tyrannosauroid predatory theropods that stratigraphically co-occur. Only the largest taxa were represented in each formation. The ordinary least-squares regression (solid line: y = –0.689x + 4.458) with the 95% confidence intervals (dashed lines) shows a negative correlation (R2 = 0.548), indicating that tyrannosauroids were small when other large predatory theropods were present. The grey shadow is where tyrannosauroids are larger than non-tyrannosauroid theropods (y > x). Dataset for the bivariate plot is shown in the electronic supplementary material, table S2.

Although carcharodontosaurids are thought to be widespread in the early Late Cretaceous of Asia, few such fossils have been recovered due to the poor rock record from that time. Ulughbegsaurus represents a previously unknown apex predator of the Turonian Bissekty Formation and the first reported carcharodontosaurian species from Late Cretaceous Central Asia. This taxon is one of the latest surviving Laurasian carcharodontosaurians and reveals the latest known stratigraphic co-occurrence between a carcharodontosaurian and a tyrannosauroid (i.e. Timurlengia). The latter suggests that carcharodontosaurians were still dominant predators in the Turonian, at least in Central Asia, while tyrannosauroids were smaller mesopredators. Because of the geographic location between East Asia and Europe, it remains uncertain if Ulughbegsaurus had affinities with European or East Asian/Gondwanan carcharodontosaurians due to unresolved phylogenetic relationships. It is probable that fragmentary bones and isolated teeth from the Bissekty Formation of Uzbekistan may belong to carcharodontosaurians.

Kohei Tanaka, Otabek Ulugbek Ogli Anvarov, Darla K. Zelenitsky, Akhmadjon Shayakubovich Ahmedshaev and Yoshitsugu Kobayashi. 2021. A New Carcharodontosaurian Theropod Dinosaur Occupies Apex Predator Niche in the early Late Cretaceous of Uzbekistan. Royal Society Open Science. DOI: 10.1098/rsos.210923

Ulughbegsaurus uzbekistanensis, a giant carcharodontosaurid from Uzbekistan. This new top predator dwarfs the previously largest known theropod from the region, the tyrannosauroid Timurlengia euotica.  

Newly discovered dinosaur predated tyrannosaurs — and at the time was a bigger apex predator