Tyrannosauroids were the top predators in latest Cretaceous ecosystems in Asia and North America, reaching sizes of up to nine metres and having adaptive features such as rapid growth, specialised bone-crushing jaws and apparently well-developed senses of smell and vision. However, their time as top predators appears to have been limited to the last 15 million years of the Cretaceous, with the group having a much longer history as smaller predators, first appearing in the Middle Jurassic but being overshadowed by larger groups such as the Allosaurs. How the Tyrannosauroids made this transition from small to large predators is difficult to understand, particularly as the transition appears to have happened in North America, where there is a long gap in the Tyrannosauroid fossil record, between the small species of the Jurassic and Early Cretaceous, and the large species that appeared at the End Cretaceous.
In a paper published in the journal Communications Biology on 21 February 2019, Lindsay Zanno of Paleontology at the North Carolina Museum of Natural Sciences, the Department of Biological Sciences at North Carolina State University, and the Section of Earth Sciences at the Field Museum of Natural History, Ryan Tucker of the Department of Earth Sciences at Stellenbosch University, Aurore Canoville, Haviv Avrahami, and Terry Gates, also of Paleontology at the North Carolina Museum of Natural Sciences and the Department of Biological Sciences at North Carolina State University, and Peter Makovicky, also of the Section of Earth Sciences at the Field Museum of Natural History, describe a new species of small Tyrannosauroid from the earliest Late Cretaceous Cedar Mountains Formation of Utah.
The new species is named Moros intrepidus, where ‘Moros’ means ‘impending doom’ and ‘intrepidus’ means ‘intrepid’. The species is described from a partial right hindlimb, comprising portions of the femur, tibia, fourth, and second metatarsals and phalanges of the fourth digit, excavated from the ‘Stormy Theropod’ exposure of the Cedar Mountains Formation in Emery County, Utah, the precise location of which is restricted by Utah state statute. Analysis of zircons from the same horizon suggests that the specimen is no more than 96.4 million years old.
Right femur of Moros intrepidus. (a) Lateral, (b) cranial, (c) medial, (d) caudal, (e) proximal, and (f) distal views. Partial mid-diaphyseal cross-section of the femur shown in (g) polarized light with lambda filter, (h) natural light with numbered arrows and tracings indicating seven growth cycles, and (i) polarized light. Abbreviations: ar adductor ridge, at accessory trochanter, Ca caudal aspect, Cr cranial aspect, ft fourth trochanter, if intercondylar fossa, inf intertrochanteric nutrient foramen, L lateral aspect, L2 lobe on lesser trochanter, lic linea intermuscularis caudalis. lt lesser trochanter, M medial aspect, mdc mesiodistal crest, pf popliteal fossa, pld lateral depression, proximal. pnf principle nutrient foramen, sat semicircular accessory tuberosity, ts trochanteric shelf. Scale bars (a)–(e) 5 cm; (g)–(i) 1mm. Zanno et al. (2019).
As well as the hindlimb, Zanno et al. describe two isolated premaxillary teeth from separate exposures of the Cedar Mountains Formation at Suicide Hill and the Cliffs of Insanity. These are flattened in aspect with one concave edge, interpreted as the inner surface, and distinct carinae (grooves) on their front and back surfaces, all features typical of Tyrannosauroids.
(c) Silhouette of Moros intrepidus showing recovered elements. Isolated indet. tyrannosauroid premaxillary tooth recovered from nearby strata in (d) occlusal, (e) mesiodistal, and (f) lingual views. Holotype specimen of Moros intrepidus composed of (g) femur, (h) tibia, (i) fourth metatarsal, (j) second metatarsal, and (k) pedal phalanges of the fourth digit. Scale bar (c) 1 m, (g)–(k) 5 mm. (d)–(f) Enlarged to show detail, not to scale. Zanno et al. (2019).
Based upon the size of the known bones, Moros intrepidus is estimated to have had a mass of about 78 kg. Examination of the microstructure of the bones of the specimen suggests that it was at least six-to-seven years old when it died (Tyrannosauroids are known to have had seasonal growth, resulting in bands of denser material within their bones that form growth rings similar to those seen in trees; because bone is sometimes re-absorbed by the body during growth, this cannot provide as an absolute age, but counting the rings does give a rough minimum age for a specimen). This implies a growth rate similar to that seen in Jurassic Asian Tyrannosauroids such as Guanlong wucaii, a specimen of which with a similar size to Moros intrepidus has been estimated to have been about seven years old when it died, and is in marked contrast to later North American Tyrannosaurids such as Gorgosaurus ibratus, which would have been around three times as large at a similar age. This suggests that the emergence of large Tyrannosauroids really was restricted to the last few million years of the Cretaceous, and may have been closely linked to the disappearance of the earlier large Allosaurs.
Phylogenetic relationships, chronostratigraphic, and palaeoecological implications of Moros intrepidus. (a) Graphic illustrating temporal range of North American Tyrannosauroids including species-level range prior to the discovery of Moros intrepidus, extension of current range, and hypothesized range based on isolated teeth. The current gap in the North American Tyrannosauroid record spans from the Tithonian to the Aptian. Faunal composition of Late Cretaceous ecosystems was established between the Albian and Turonian, as recognized by the stratigraphic appearance of major clades. (b) generalized phylogenetic relationships of Tyrannosauroidea, showing the appearance of select traits related to cursoriality in Tyrannosaurs that are newly optimized as a result of the discovery of Moros intrepidus. (c) Stratigraphic distribution of Allosauria in North America (including Megaraptora) documents overlap with Moros intrepidus in early Late Cretaceous ecosystems leading to (d) refined calibration on the origin of late diverging Tyrannosauroids and clade-level faunal turnover within apex predator roles throughout the Late Jurassic–Late Cretaceous of North America. Coloured polygons are stylized call-outs and are not intended to reflect two dimensional data. Zanno et al. (2019).
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