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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. Key words: Tyrannosauridae, embryo, Theropoda, Cretaceous, North America. Les tyrannosaurides etaient les predateurs apicaux de la Laurasie au Cretace tardif, et leur statut de carnivores dominants a suscite un interet considerable depuis leur decouverte, tant au sein du grand public que dans les milieux scientifiques. Ils sont par consequent bien etudies, et les connaissances sur leur anatomie, leur diversite, leur croissance et leur evolution sont vastes. En revanche, on en sait peu sur les premieres etapes de leur developpement. Les ceufs et embryons de tyrannosaurides manquent toujours a l'appel et, bien que certains soient connus, les specimens juveniles sont rares. Des os et dents de tyrannosaurides perinataux du Campanien-Maastrichtien de l'ouest de l'Amerique du Nord constituent la premiere fenetre sur cette periode cle de la vie d'un tyrannosauride. Un os dentaire embryonnaire (cf. Daspletosaurus)de la Formation de Two Medicine, au Montana, faisant seulement 3 cm de long, presente deja des caracteres distinctifs des tyrannosaurines, comme un [much less than] menton [much greater than] et un profond sillon de Meckel, et revele les toutes premieres etapes du developpement des dents. Ces observations, combinees a une phalange ungueale embryonnaire remarquablement longue de la Formation de Horseshoe Canyon en Alberta, permettent d'estimer grossierement la taille minimum de tyrannosaurides nouvellement eclos. Une dent premaxillaire perinatale de la Formation de Horseshoe Canyon appartient probablement a Albertosaurus sarcophagus et presente de petits denticules sur les carenes. Cette dent demontre que les caracteres typiques qui distinguent les tyrannosaurides d'autres theropodes etaient presents tot durant la vie et souleve des questions concernant la variabilite ontogenetique de dentelures sur les dents premaxillaires. Des similitudes sedimentaires et taphonomiques dans les sites qui ont produit des ossements d'embryons fournissent des indices sur les habitudes de nidification des tyrannosaurides et pourraient aider a preciser l'image recherchee pour la prospection dans la quete continue de bebes de tyrannosauride. [Traduit par la Redaction] Mots-cles: tyrannosaurides, embryon, theropodes, Cretace, Amerique du Nord., Introduction Tyrannosaurids were the apex predators of Late Cretaceous Laurasia and were among the largest terrestrial predators ever (Persons et al. 2020). They have garnered considerable interest since their discovery [...]

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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. [ABSTRACT FROM AUTHOR]