[Paleontology • 2025] Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous

Nanotyrannus   

in Zanno et Napoli, 2025.

 DOI: doi.org/10.1038/s41586-025-09801-6 

Artwork: Anthony Hutchings

 x.com/JGN_Paleo

Tyrannosaurus rex ranks among the most comprehensively studied extinct vertebrates1 and a model system for dinosaur paleobiology. As one of the last surviving non-avian dinosaurs, Tyrannosaurus is a crucial datum for assessing terrestrial biodiversity, ecosystem structure, and biogeographic exchange immediately preceding the end-Cretaceous mass extinction —one of Earth’s greatest biological catastrophes. Paleobiological studies of Tyrannosaurus, including ontogenetic niche partitioning, feeding, locomotor biomechanics, and life history have drawn upon an expanding skeletal sample comprising multiple hypothesized growth stages—and yet the Tyrannosaurus hypodigm remains controversial. A key outstanding question relates to specimens considered to exemplify immature Tyrannosaurus, which have been argued to represent the distinct taxon Nanotyrannus. Here, we describe an exceptionally well-preserved, near somatically mature tyrannosaur skeleton (NCSM 40000) from the Hell Creek Formation that shares autapomorphies with the holotype specimen of N. lancensis. We couple comparative anatomy, longitudinal growth models, observations on ontogenetic character invariance, and a novel phylogenetic dataset to test the validity of Nanotyrannus, demonstrating conclusively that this taxon is distinguishable from Tyrannosaurus, sits outside Tyrannosauridae, and unexpectedly contains two species—N. lancensis and Nanotyrannus lethaeus, sp. nov. Our results prompt a re-evaluation of dozens of existing hypotheses based on currently indefensible ontogenetic trajectories. Finally, we document at least two co-occurring, ecomorphologically distinct genera in the Maastrichtian of North America, demonstrating that tyrannosauroid alpha diversity was thriving within one million years of the end-Cretaceous extinction.

A pack of Nanotyrannus attacks a juvenile Tyrannosaurus rex.

Artwork: Anthony Hutchings

  Lindsay E. Zanno and James G. Napoli. 2025. Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous. Nature.  DOI: doi.org/10.1038/s41586-025-09801-6  [30 October 2025]

http://x.com/JGN_Paleo/status/1983926921898660347

[Paleontology • 2024] Labocania aguillonae • A New Tyrant Dinosaur (Theropoda: Tyrannosauridae) from the Late Campanian of Mexico Reveals a Tribe of Southern Tyrannosaurs

  

 Labocania aguillonae

    Rivera-Sylva & Longrich, 2024

 www.mdpi.com/2813-6284/2/4/12  

facebook.com/NickLongrich

Artwork by Andrey Atuchin.

Abstract

The end of the Cretaceous saw the Western Interior Seaway divide North America into two land masses, Laramidia in the west and Appalachia in the east. Laramidian dinosaurs inhabited a narrow strip of land extending from Mexico to Alaska. Within this geographically restricted area, dinosaurs evolved high diversity and endemism, with distinct species in the north and south. Here, we report a new tyrannosaurid from the Late Campanian-aged Cerro del Pueblo Formation of Coahuila, Mexico, which is part of a tribe of tyrannosaurs originating in southern Laramidia. Phylogenetic analysis recovers the new tyrannosaur as part of a clade including Labocania anomala from the La Bocana Roja Formation of Baja California Norte, Bistahieversor sealeyi from the Kirtland Formation of New Mexico, Teratophoneus curriei from the Kaiparowits Formation in Utah, and Dynamoterror dynastes from the Menefee Formation of New Mexico. Distinct frontal morphology and the younger age (~72.5–73 Ma versus >75.8 Ma for L. anomala) support recognition of the new tyrannosaur as a distinct species of Labocania, Labocania aguillonae. The Labocania clade dominated southern Laramidia at a time when the north was dominated by daspletosaurins and albertosaurines. The high endemism seen in tyrannosaurids is remarkable, given that modern apex predators have large geographic ranges and hints that the diversity of carnivorous dinosaurs has been underestimated.

Keywords: Tyrannosauridae; dinosauria; Campanian; Cretaceous; Laramidia

Skeletal reconstruction of Labocania aguillonae (CPC 2974) showing preserved material. Scale = 1 m.

Systematic paleontology

Dinosauria—Owen 1842  

Theropoda—Marsh, 1881  

Coelurosauria—von Huene 1914  

Tyrannosauridae—Osborn, 1906 

Tyrannosaurinae—Osborn, 1906  

Teratophoneini—Scherer & Voiculescu-Holvad, 2024  

Labocania—Molnar 1974  

Labocania aguillonae sp. nov.

Diagnosis: Tyrannosaurine diagnosed by the following character combination (*—autapomorphies): orbit large and circular, with a very strongly convex anteroventral margin of the lacrimal* with a prominent, strongly dorsally extended* subocular process on the lacrimal along the anteroventral margin of the orbit; strong posteroventral extension of the antorbital fossa onto the lacrimal to end below the anterior margin of the orbit; lateral ends of frontals with a very prominent, acutely triangular shape, with strong transversely expanded overlap of frontal onto lacrimal; broad anteromedial extension of supratemporal fossa extending far forward onto frontal*; strong medial projection of postorbitals behind frontals onto dorsolateral surface of postorbital process, such that frontal underlaps the postorbital anteriorly*, lateral teeth with a figure-eight basal cross-section, and deep, prominent grooves or fullers extending from the root up the labial and lingual surfaces of the crown.

 Etymology: The species name honors Martha C. Aguillón, discoverer of the specimen and a long-time career paleontologist of Coahuila.

Conclusions: 

CPC 2974 represents a new species of tyrannosaurid, Labocania aguillonae, closely related to Labocania anomala, Bistahieversor sealeyi, and Teratophoneus curriei. It adds to the diversity of the Cerro del Pueblo Formation and shows that endemicity existed within tyrannosaurs in the Late Campanian, with distinct species and clades inhabiting the northern Great Plains in the north and the American Southwest and Mexico in the south. Given the limited geographic sampling, many tyrannosaur species likely remain undiscovered. Competition between species likely helped to enforce endemism among dinosaurs. Why large dinosaurs seem to have such unusually high levels of endemism compared to modern mammals remains unclear.

 Héctor E. Rivera-Sylva and Nicholas R. Longrich. 2024. A New Tyrant Dinosaur from the Late Campanian of Mexico Reveals a Tribe of Southern Tyrannosaurs. Foss. Stud. 2024, 2(4), 245-272. DOI: doi.org/10.3390/fossils2040012  www.mdpi.com/2813-6284/2/4/12

  facebook.com/NickLongrich/posts/10160809409234125

[Paleontology • 2024] Tyrannosaurus mcraeensis • A Giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the Evolution of tyrannosaurid gigantism

Tyrannosaurus mcraeensis 

Dalman, Loewen, Pyron, Jasinski, Malinzak, Lucas, Fiorillo, Currie & Longrich, 2024

 

DOI: 10.1038/s41598-023-47011-0

Artwork by Sergei Krasinski.

Abstract

Tyrannosaurid dinosaurs dominated as predators in the Late Cretaceous of Laurasia, culminating in the evolution of the giant Tyrannosaurus rex, both the last and largest tyrannosaurid. Where and when Tyrannosaurini (T. rex and kin) originated remains unclear. Competing hypotheses place tyrannosaurin origins in Asia, or western North America (Laramidia). We report a new tyrannosaurin, Tyrannosaurus mcraeensis, from the Campanian–Maastrichtian Hall Lake Formation of New Mexico, based on a fossil previously referred to T. rex. T. mcraeensis predates T. rex by ~ 6–7 million years, yet rivaled it in size. Phylogenetic analysis recovers T. mcraeensis as sister to T. rex and suggests Tyrannosaurini originated in southern Laramidia. Evolution of giant tyrannosaurs in southern North America, alongside giant ceratopsians, hadrosaurs, and titanosaurs suggests large-bodied dinosaurs evolved at low latitudes in North America.

Locality and stratigraphy of Tyrannosaurus mcraeensis sp. nov., NMMNH P-3698.
(A), type locality in Sierra County, New Mexico; (B), stratigraphy of fossil and the Hall Lake Formation (C), recovered skull elements. Scale = 10 cm. Map by Ron Blakey.

Mandibular elements of Tyrannosaurus mcraeensis sp. nov. (NMMNH P-3698).
 Left dentary in (A), medial view; (B), lateral view; (C), dorsal view; (D), right splenial, medial view; (E), right angular, medial view; (F), right prearticular, medial view. ang angular contact, de shelf for dentary, mec Meckelian canal, pre prearticular facet, a1, a5, a15 alveoli 1, 5, and 15, sp splenial, sym symphysis. Scale = 20 cm.

Dinosauria Owen, 1842.

Theropoda Marsh, 1881.

Tetanurae Gauthier, 1986.

Coelurosauria von Huene, 1914.

Tyrannosauridae Osborn, 1905.

Tyrannosaurinae Currie, 2003.

Tyrannosaurini Olshevsky, 1995.

(Tyrannosaurini is here defined as the last common ancestor of Tarbosaurus baatar and Tyrannosaurus rex and all its descendants).

Tyrannosaurus mcraeensis sp. nov.

Diagnosis: Large tyrannosaurin distinguished from Tyrannosaurus rex (Fig. 4; SI) by the following characters (*autapomorphies): postorbital with low, posteriorly positioned cornual process; postorbital with anteriorly projecting prefrontal/frontal articular surfaces; squamosal with ventrally projecting quadratojugal process; squamosal with a concave medial margin; strong ridge bounding the anterior margin of the squamosal ventral pneumatic fossa; dentary very shallow posteriorly and with a convex posteroventral margin*; splenial with anteriorly positioned apex*; splenial with shelf-like dentary overlap*, splenial with deep, posteriorly directed angular process; prearticular weakly bowed*; small ventral prearticular-angular contact; articular T-shaped in dorsal/ventral view; retroarticular process deep and quadrangular in posterior view.

Holotype: NMMNH P-3698, partial skull including right postorbital and squamosal (Fig. 2), left palatine, fragment of maxilla, and lower jaws (Fig. 3) including left dentary, right splenial, prearticular, angular and articular, isolated teeth and associated chevrons.

 

Horizon and locality: Uppermost Campanian or lower Maastrichtian of the Hall Lake Formation, McRae Group, NMMNH locality 343, near Kettle Top Butte, Sierra County, New Mexico21 (Fig. 1A). The site lies 43 m above the base of the Hall Lake Formation. A tuff 33 m below the tyrannosaur site has a U/Pb age of 73.2 ± 0.7 Ma22 (Fig. 1B).

Etymology: The species name, mcraeensis, refers to the McRae Group of western New Mexico.

 

Reconstruction of Tyrannosaurus mcraeensis.

Artwork by Sergei Krasinski.

 

Sebastian G. Dalman, Mark A. Loewen, R. Alexander Pyron, Steven E. Jasinski, D. Edward Malinzak, Spencer G. Lucas, Anthony R. Fiorillo, Philip J. Currie and Nicholas R. Longrich. 2024. A Giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the Evolution of tyrannosaurid gigantism. Scientific Reports. 13: 22124. DOI: 10.1038/s41598-023-47011-0

[Paleontology • 2023] Exceptionally preserved Stomach Contents of A Young tyrannosaurid reveal An Ontogenetic Dietary Shift in an iconic Extinct Predator

Gorgosaurus libratus feeding on Citipes elegans.

in Therrien, Zelenitsky, Tanaka, Voris, ... et Kobayashi, 2023.

 DOI: 10.1126/sciadv.adi0505

Illustration by Julius Csotonyi 

RoyalTyrrellMuseum.wpcomstaging.com

 

Abstract

Tyrannosaurids were large carnivorous dinosaurs that underwent major changes in skull robusticity and body proportions as they grew, suggesting that they occupied different ecological niches during their life span. Although adults commonly fed on dinosaurian megaherbivores, the diet of juvenile tyrannosaurids is largely unknown. Here, we describe a remarkable specimen of a juvenile Gorgosaurus libratus that preserves the articulated hindlimbs of two yearling caenagnathid dinosaurs inside its abdominal cavity. The prey were selectively dismembered and consumed in two separate feeding events. This predator-prey association provides direct evidence of an ontogenetic dietary shift in tyrannosaurids. Juvenile individuals may have hunted small and young dinosaurs until they reached a size when, to satisfy energy requirements, they transitioned to feeding on dinosaurian megaherbivores. Tyrannosaurids occupied both mesopredator and apex predator roles during their life span, a factor that may have been key to their evolutionary success.

Juvenile Gorgosaurus TMP 2009.12.14 preserving stomach contents.
Photographs of specimen in (A) right lateral view and (B) left anterolateral view. (C) Interpretive illustration of specimen in right lateral view. Skeleton consists of a nearly complete skull, the left side of the body and limbs, and a nearly complete pelvis. Red rectangle delineates location of stomach contents. (D) Histological photomicrograph of tibia showing the presence of five lines of arrested growths and two annuli (marked by asterisks), indicating that the individual was between 5 and 7 years old.

Scale bars, 50 cm (A) to (C) and 1 mm (D).

  

Juvenile Citipes remains preserved as stomach contents.
(A) Diagram illustrating relative body sizes of predator and prey and skeletal elements preserved in TMP 2009.12.14. Scale bar, 50 cm.
Histological photomicrographs of (B) posterior Citipes individual (metatarsal II) and (C) anterior Citipes individual (tibia), showing highly vascularized woven bone with reticular and longitudinally oriented vascular canals and lacking growth lines, indicative of young individuals that are less than 1 year old. Scale bars, 500 μm.

Gorgosaurus libratus feeding on Citipes elegans.

Illustration by Julius Csotonyi 

RoyalTyrrellMuseum.wpcomstaging.com 

 Francois Therrien, Darla K. Zelenitsky, Kohei Tanaka, Jared T. Voris, Gregory M. Erickson, Philip J. Currie, Christopher L. Debuhr, and Yoshitsugu Kobayashi. 2023. Exceptionally preserved Stomach Contents of A Young tyrannosaurid reveal An Ontogenetic Dietary Shift in an iconic Extinct Predator. SCIENCE ADVANCES. 9, 49. DOI: 10.1126/sciadv.adi0505

 ucalgary.ca/news/whats-dinner-ucalgary-paleontologist-finds-out-through-remarkable-specimen

 RoyalTyrrellMuseum.wpcomstaging.com/2023/12/08/a-young-tyrannosaurs-last-meal

 

[Paleontology • 2022] Daspletosaurus wilsoni • A Transitional Species of Daspletosaurus Russell, 1970 (Theropoda: Tyrannosauridae) from the Judith River Formation of eastern Montana, USA

Daspletosaurus wilsoni

 Warshaw​ & Fowler, 2022 

DOI:  10.7717/peerj.14461 

 twitter.com/DF9465 

Illustration: Andrey Atuchin & Badlands Dinosaur Museum

Abstract 

Here we describe a new derived tyrannosaurine, Daspletosaurus wilsoni sp. nov., from Judithian strata (~76.5 Ma) intermediate in age between either of the previously described species of this genus. D. wilsoni displays a unique combination of ancestral and derived characteristics, including a cornual process of the lacrimal reduced in height relative to D. torosus and more basal tyrannosaurines, and a prefrontal with a long axis oriented more rostrally than in D. horneri and more derived tyrannosaurines. The description of this taxon provides insight into evolutionary mode in Tyrannosaurinae, lending strength to previous hypotheses of anagenesis within Daspletosaurus and increasing the resolution with which the evolution of this lineage can be reconstructed. Cladistic phylogenetic methods, stratigraphy, and qualitative analysis of the morphology of relevant taxa supports an anagenetic model for the origin of morphological novelty in this genus, highlighting the predominance of anagenetic evolution among contemporary dinosaur lineages.

The new species (Daspletosaurus wilsoni, 76.5 Ma) is transitional in form and age between D. torosus (77 Ma) and D. horneri (75.6 Ma). This suggests that Daspletosaurus underwent linear evolution - where one form evolves into the next without splitting or branching.

by Dickinson Museum Center

The new specimen, "Sisyphus", is one of four tyrannosaur skeletons recently collected by Badlands Dinosaur Museum. Here the four tyrannosaurs dispute ownership of the fresh carcass of a Centrosaurus.

by Rudolf Hima & Badlands Dinosaur Museum.

Tyrannosaurinae Matthew & Brown, 1922 (sensu Sereno, McAllister & Brusatte, 2005)

Daspletosaurus Russell, 1970

Daspletosaurus wilsoni sp. nov.

Diagnosis: D. wilsoni can be assigned to Daspletosaurus based on the following characteristics: extremely coarse subcutaneous surface of the maxilla with no elevated ridges or corresponding fossae (Carr et al., 2017; Voris et al., 2020); cornual process of the postorbital approaching the laterotemporal fenestra (Carr et al., 2017); dorsal postorbital process of the squamosal terminating caudal to the rostral margin of the laterotemporal fenestra (Carr et al., 2017; Voris et al., 2019); and extremely coarse symphyseal surface of the dentary (Voris et al., 2020).

...

Etymology: wilsoni, Latinization of “Wilson,” after John Wilson, the discoverer of the holotype specimen.

Conclusions: 

D. wilsoni sp. nov., a stratigraphic and morphological intermediate between D. torosus and D. horneri, is hypothesized to represent a transitional form along an anagenetic lineage linking both previously named species of Daspletosaurus. This finding, in concert with previous identifications of anagenesis in contemporary dinosaur lineages, emphasizes the explanatory power of anagenesis in the production of evolutionary trends among dinosaurs of the Late Cretaceous Western Interior (Scannella et al., 2014; Freedman Fowler & Horner, 2015; Fowler & Fowler, 2020; Wilson, Ryan & Evans, 2020). Indeed, as anagenesis continues to be identified among fossil lineages, the predominant relative frequency of strictly cladogenetic evolutionary models (e.g., punctuated equilibria; Eldredge & Gould, 1972) must eventually come under scrutiny. Future explorations of evolutionary mode in fossil taxa, including further tests of the hypotheses presented here, will be important in this regard, and have the potential to refine understanding of the pattern and process of dinosaur evolution.

 

 

Elías A. Warshaw​ and Denver W. Fowler. 2022. A Transitional Species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana.  PeerJ. 10:e14461. DOI:  10.7717/peerj.14461

  twitter.com/DF9465/status/1596149970779897862

 phys.org/news/2022-11-species-tyrannosaur-daspletosaurus-wilsoni-hints.html

[Paleontology • 2021] Intraspecific Facial Bite Marks in Tyrannosaurids provide insight into Sexual Maturity and Evolution of Bird-like Intersexual Display

Artistic reconstructions of hypothesized face-biting behaviour in Tyrannosauridae.

Left, two Gorgosaurus individuals aggressively square off.

Right, the aftermath of face-biting behaviour, 

showing recent wounds and older scars. 

in Brown, Currie & Therrien, 2021.

DOI: 10.1017/pab.2021.29

  RoyalTyrrellMuseum.wpcomstaging.com

Image © Royal Tyrrell Museum, illustration by Julius Csotonyi.

Abstract

Intraspecific aggression, or agonism, is a widespread intrasexual selective behavior important to understanding animal behavioral ecology and reproductive systems. Such behavior can be studied either by direct observation or inferred from wound/scar frequency in extant species but is difficult to document in extinct taxa, limiting understanding of its evolution. Among extant archosaurs, crocodylians display extensive intrasexual aggression, whereas birds show extreme visual/vocal intersexual display. The evolutionary origin of this behavioral divergence, and pattern in non-avian dinosaurs, is unknown. Here we document the morphology, frequency, and ontogeny of intraspecific facial bite lesions (324 lesions) in a large sample of tyrannosaurids (202 specimens, 528 elements) to infer patterns of intraspecific aggression in non-avian theropods. Facial scars are consistent in position and orientation across tyrannosaurid species, suggesting bites were inflicted due to repeated/postured behavior. Facial scars are absent in young tyrannosaurids, first appear in immature animals (~50% adult skull length), are present in ~60% of the adult-sized specimens, and show aggressor:victim size isometry. The ontogenetic distribution of bite scars suggests agonistic behavior is associated with the onset of sexual maturity, and scar presence in approximately half the specimens may relate to a sexual pattern. Considered in a phylogenetic context, intraspecific bite marks are consistent and widely distributed in fossil and extant crocodyliforms and non-maniraptoriform theropods, suggesting a potential plesiomorphic behavior in archosaurs. Their absence in maniraptoriform theropods, including birds, may reflect a transition from boney cranial ornamentation and crocodylian-like intrasexual aggression to avian-like intersexual display with the evolution of pennaceous feathers.

Skull bones with tooth-strike scars across tyrannosaur specimens of increasing sizes.

    

Presence of facial tooth marks, ornamentation of the skull, and presence of feathered forelimbs, across various members of Theropoda.

Caleb M. Brown, Philip J. Currie and François Therrien. 2021. Intraspecific Facial Bite Marks in Tyrannosaurids provide insight into Sexual Maturity and Evolution of Bird-like Intersexual Display. Paleobiology. FirstView. pp. 1 - 32. DOI: 10.1017/pab.2021.29

Clash of the Titans: Facial Scars as Indicators of Aggressive Behaviour and Sexual Maturity in Tyrannosaurs

 RoyalTyrrellMuseum.wpcomstaging.com/2021/09/07/tyrannosaur-facial-scars/

[Paleontology • 2021] Absolute Abundance and Preservation Rate of Tyrannosaurus rex

 Tyrannosaurus rex

in Marshall, Latorre, Wilson, ... et Poust, 2021. 

DOI: 10.1126/science.abc8300 

Illustration: JuliusCsotonyi  facebook.com/JuliusCsotonyi 

Abstract

Although much can be deduced from fossils alone, estimating abundance and preservation rates of extinct species requires data from living species. Here, we use the relationship between population density and body mass among living species combined with our substantial knowledge of Tyrannosaurus rex to calculate population variables and preservation rates for postjuvenile T. rex. We estimate that its abundance at any one time was ~20,000 individuals, that it persisted for ~127,000 generations, and that the total number of T. rex that ever lived was ~2.5 billion individuals, with a fossil recovery rate of 1 per ~80 million individuals or 1 per 16,000 individuals where its fossils are most abundant. The uncertainties in these values span more than two orders of magnitude, largely because of the variance in the density–body mass relationship rather than variance in the paleobiological input variables.

 Charles R. Marshall, Daniel V. Latorre, Connor J. Wilson, Tanner M. Frank, Katherine M. Magoulick, Joshua B. Zimmt and Ashley W. Poust. 2021. Absolute Abundance and Preservation Rate of Tyrannosaurus rex. Science. 372, 6539; 284-287. DOI: 10.1126/science.abc8300 

  facebook.com/JuliusCsotonyi/posts/10157612917441396

COVER: Tyrannosaurus rex prowl a Cretaceous coast. 

By using the rich fossil record of T. rex, including its body mass, growth and survivorship curves, geographic range, and geologic longevity, together with the relationship between population density and body mass among living animals, it is possible to ascertain how many T. rex ever roamed the planet and to estimate the fossil recovery rate for this species.

 Illustration: Julius Csotonyi

Estimating dinosaur abundance

Estimating the abundance of a species is a common practice for extant species and can reveal many aspects of its ecology, evolution, and threat level. Estimating abundance for species that are extinct, especially those long extinct, is a much trickier endeavor. Marshall et al. used a relationship established between body size and population density in extant species to estimate traits such as density, distribution, total biomass, and species persistence for one of the best-known dinosaurs, Tyrannosaurus rex, revealing previously hidden aspects of its population ecology.

[Paleontology • 2021] Baby Tyrannosaurid Bones and Teeth from the Late Cretaceous of western North America

 a juvenile tyrannosaur. 

in Brusatte, Scannella, Horner & Currie, 2021.

 DOI: 10.1139/cjes-2020-0169

Illustration: Julius Csotonyi

 

Abstract

Tyrannosaurids were the apex predators of Late Cretaceous Laurasia and their status as dominant carnivores has garnered considerable interest since their discovery, both in the popular and scientific realms. As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution. In contrast, little is known of the earliest stages of tyrannosaurid development. Tyrannosaurid eggs and embryos remain elusive, and juvenile specimens — although known — are rare. Perinatal tyrannosaurid bones and teeth from the Campanian–Maastrichtian of western North America provide the first window into this critical period of the life of a tyrannosaurid. An embryonic dentary (cf. Daspletosaurus) from the Two Medicine Formation of Montana, measuring just 3 cm long, already exhibits distinctive tyrannosaurine characters like a “chin” and a deep Meckelian groove, and reveals the earliest stages of tooth development. When considered together with a remarkably large embryonic ungual from the Horseshoe Canyon Formation of Alberta, minimum hatchling size of tyrannosaurids can be roughly estimated. A perinatal premaxillary tooth from the Horseshoe Canyon Formation likely pertains to Albertosaurus sarcophagus and it shows small denticles on the carinae. This tooth shows that the hallmark characters that distinguish tyrannosaurids from other theropods were present early in life and raises questions about the ontogenetic variability of serrations in premaxillary teeth. Sedimentary and taphonomic similarities in the sites that produced the embryonic bones provide clues to the nesting habits of tyrannosaurids and may help to refine the prospecting search image in the continued quest to discover baby tyrannosaurids.

Artist's impression of  a juvenile tyrannosaur. 

Illustration: Julius Csotonyi

Stephen L. Brusatte, John B. Scannella, John R. Horner and Philip J. Currie. 2021. Baby Tyrannosaurid Bones and Teeth from the Late Cretaceous of western North America. Canadian Journal of Earth Sciences. DOI: 10.1139/cjes-2020-0169

 nationalgeographic.com/science/2020/10/first-tyrannosaur-embryo-fossils-revealed/

 phys.org/news/2021-01-dinosaur-embryo-baby-tyrannosaur-mystery.html

Résumé: Les tyrannosauridés étaient les prédateurs apicaux de la Laurasie au Crétacé tardif, et leur statut de carnivores dominants a suscité un intérêt considérable depuis leur découverte, tant au sein du grand public que dans les milieux scientifiques. Ils sont par conséquent bien étudiés, et les connaissances sur leur anatomie, leur diversité, leur croissance et leur évolution sont vastes. En revanche, on en sait peu sur les premières étapes de leur développement. Les œufs et embryons de tyrannosauridés manquent toujours à l’appel et, bien que certains soient connus, les spécimens juvéniles sont rares. Des os et dents de tyrannosauridés périnataux du Campanien–Maastrichtien de l’ouest de l’Amérique du Nord constituent la première fenêtre sur cette période clé de la vie d’un tyrannosauridé. Un os dentaire embryonnaire (cf. Daspletosaurus) de la Formation de Two Medicine, au Montana, faisant seulement 3 cm de long, présente déjà des caractères distinctifs des tyrannosaurinés, comme un « menton » et un profond sillon de Meckel, et révèle les toutes premières étapes du développement des dents. Ces observations, combinées à une phalange unguéale embryonnaire remarquablement longue de la Formation de Horseshoe Canyon en Alberta, permettent d’estimer grossièrement la taille minimum de tyrannosauridés nouvellement éclos. Une dent prémaxillaire périnatale de la Formation de Horseshoe Canyon appartient probablement à Albertosaurus sarcophagus et présente de petits denticules sur les carènes. Cette dent démontre que les caractères typiques qui distinguent les tyrannosauridés d’autres théropodes étaient présents tôt durant la vie et soulève des questions concernant la variabilité ontogénétique de dentelures sur les dents prémaxillaires. Des similitudes sédimentaires et taphonomiques dans les sites qui ont produit des ossements d’embryons fournissent des indices sur les habitudes de nidification des tyrannosauridés et pourraient aider à préciser l’image recherchée pour la prospection dans la quête continue de bébés de tyrannosauridé. [Traduit par la Rédaction]

[Paleontology • 2020] Thanatotheristes degrootorum • A New Tyrannosaurine (Theropoda: Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides Insight Into the Evolution and Biogeography of Tyrannosaurids

Thanatotheristes degrootorum 

Voris, Therrien, Zelenitsky & Brown, 2020

DOI: 10.1016/j.cretres.2020.104388 

 RoyalTyrrellMuseum.wpcomstaging.com

 SmithsonianMag.com 

Illustration: Julius Csotonyi facebook.com/JuliusCsotonyi

Highlights

• A new genus of tyrannosaurid from the mid Campanian Foremost Formation of southern Alberta is described.

• The new genus is found to be the sister taxon to Daspletosaurus spp., and together form the new clade Daspletosaurini.

• Tyrannosauridae is revealed to be comprised of multiple, multigeneric clades rather than a series of monogeneric branches.

• Geographic segregation of clades provides evidence for provinciality of Tyrannosauridae within North America during the Campanian.

Abstract

Late Cretaceous tyrannosauroid material from North America was primarily known from upper Campanian through Maastrichtian formations until the recent discovery of derived tyrannosaurid taxa from lower-to-mid Campanian deposits in the southwestern United States. However, diagnostic material from contemporaneous deposits further north in Alberta (Canada) and Montana (USA) has yet to be documented. Here we report the discovery of a new tyrannosaurid from the mid-Campanian Foremost Formation of Alberta, Thanatotheristes degrootorum gen. et sp. nov., which helps fill this gap. Thanatotheristes is found to be the sister taxon to the late Campanian tyrannosaurine genus Daspletosaurus based on several synapomorphies, such as an extremely coarse subcutaneous surface of the maxilla, a constricted jugal ramus of the maxilla, a shallow angle of the anteroventral corner of the maxilla, a high tooth count, a wide prefrontal, and a dentary chin located ventral to either the third alveolus or third interdental plate. Together, these taxa provide evidence for the existence of a clade of long-, deep-snouted tyrannosaurines endemic to northern Laramidia during the Campanian. Our study demonstrates that Tyrannosauridae consists of several geographically segregated clades rather than a series of monogeneric successive sister taxa as recovered by previous studies. The geographic segregation of tyrannosaurid clades within North America provides renewed evidence for provinciality among large theropods during the Late Cretaceous.

Keywords: Theropoda, Tyrannosauridae, Campanian, Evolution, Provinciality, Biogeography

SYSTEMATIC PALEONTOLOGY 

Dinosauria Owen, 1842 

Theropoda Marsh, 1881 

Tetanurae Gauthier, 1986 

Coelurosauria von Huene, 1914 

Tyrannosauridae Osborn, 1906 

Tyrannosaurinae Osborn, 1906 

Daspletosaurini clade nov.  

INCLUDED TAXA: Daspletosaurus torosus Russell, 1970, Daspletosaurus horneri Carr et al., 2017, and Thanatotheristes degrootorum gen. et. sp. nov. 

Skull reconstruction of Thanatotheristes degrootorum holotype TMP 2010.5.7. Known bones appear in white. Missing bone morphologies and proportions are based on the holotype of Daspletosaurus torosus (CMN 8506). Scale bar equals 10 cm.

Thanatotheristes gen. nov. 

Thanatotheristes degrootorum gen. et sp. nov. 

ETYMOLOGY: From Thanatos, the Greek god of and embodiment of death, and theristes (Greek), one who reaps or harvests, a reaper. The specific name, degrootorum, is in honor of John and Sandra De Groot of Hays, Alberta, who discovered the holotype specimen and have been supportive of paleontological research in the area.  

 Jared T. Voris, François Therrien, Darla K. Zelenitsky and Caleb M. Brown. 2020. A New Tyrannosaurine (Theropoda: Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides Insight Into the Evolution and Biogeography of Tyrannosaurids. Cretaceous Research. In Press - 104388. DOI: 10.1016/j.cretres.2020.104388  

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