Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record

Abstract

To date, eco-evolutionary dynamics in the ascent of tyrannosauroids to top predator roles have been obscured by a 70-million-year gap in the North American (NA) record. Here we report discovery of the oldest Cretaceous NA tyrannosauroid, extending the lineage by ~15 million years. The new taxon— Moros intrepidus gen. et sp. nov.—is represented by a hind limb from an individual nearing skeletal maturity at 6–7 years. With a ~1.2-m limb length and 78-kg mass, M . intrepidus ranks among the smallest Cretaceous tyrannosauroids, restricting the window for rapid mass increases preceding the appearance of colossal eutyrannosaurs. Phylogenetic affinity with Asian taxa supports transcontinental interchange as the means by which iconic biotas of the terminal Cretaceous were established in NA. The unexpectedly diminutive and highly cursorial bauplan of NA’s earliest Cretaceous tyrannosauroids reveals an evolutionary strategy reliant on speed and small size during their prolonged stint as marginal predators.

The distribution of dental features in non-avian theropod dinosaurs: Taxonomic potential, degree of homoplasy, and major evolutionary trends

“…Longitudinal grooves have also been observed on the crowns of Byronosaurus (Makovicky et al, 2003), as well as Austroraptor and Buitreraptor by Gianechini et al (2011b), yet they result from the longitudinal ridges delimiting them, and we only consider the presence of ridges in Austroraptor, Buitreraptor, and Byronosaurus. An indeterminate tyrannosauroid from the Cenomanian of Utah also presents the particularity of having a longitudinal groove centrally positioned on the lingual side of a single mesial isolated crown (Zanno et al, 2019). Like these authors, who regard this dental feature as autapomorphic, we did not observe such groove in the mesial dentition of any other theropods.…”

“…Unserrated teeth are also present in the premaxillary teeth of the "Jordan Theropod" LACM 28471 (Molnar, 1978), considered to be a specimen of Nanotyrannus (Larson, 2013) or a juvenile individual of Tyrannosaurus (Carr and Williamson, 2004). In mature individuals, unserrated teeth are restricted to the mesial dentition of some coelophysoids such as Megapnosaurus rhodesiensis (Raath, 1977) and 'Syntarsus' kayentakatae (Rowe, 1989), and the basal pantyrannosaurian Xiongguanlong (Li et al, 2010;Figure 11.4) and an indeterminate tyrannosauroid from the Cenomanian of Utah (Zanno et al, 2019). Most compsognathids such as Compsognathus, Juravenator and Sinosauropteryx also show unserrated mesial crowns, but these taxa may not have been fully grown individuals (i.e., juveniles to subadults; Currie and Chen, 2001;Peyer, 2006;Chiappe and Göhlich, 2010).…”

“…per-sonal obs.). A U-shaped cross-section of the crown base is visible in the mesial teeth of the pantyrannosaurians: Albertosaurus (Currie, 2003;Buckley et al, 2010), Appalachiosaurus (RMM 6670), Daspletosaurus (TMP 1994.153.01;Lehman and Carpenter, 1990), Dilong (IVPP V14243; , Eotyrannus (Hutt et al, 2001; Figure 13.2), Gorgosaurus (1991.36.500), Nanotyrannus (Molnar, 1978;Carr and Williamson, 2004), Raptorex (LH PV18; Sereno et al, 2009), Tarbosaurus (Hurum and Sabath, 2003), Timurlengia (Averianov and Sues, 2012), Tyrannosaurus (Smith, 2005), Xiongguanlong (Li et al, 2010) and an indeterminate tyrannosauroid from the Cenomanian of Utah (if the mid-crown cross-section is indeed salinonshaped, the base crown is here considered Ushaped; Zanno et al, 2019). A similar morphology is seen in the premaxillary teeth of the non-tyrannosauroid coelurosaur Zuolong (Choiniere et al, 2010a) and in the first two premaxillary teeth of Ornitholestes (AMNH 619).…”

“…A longitudinal ridge centrally positioned on the lingual surface of the crown and delimited by two concave surfaces is typical of tyrannosauroid mesial teeth (Carr and Williamson, 2004). This feature has been observed in the pantyrannosaurians Albertosaurus (Carr and Williamson, 2004), Appalachiosaurus (Carr et al, 2005), Daspletosaurus (Lehman and Carpenter, 1990;TMP 1994.143.01), Gorgosaurus (Cillari, 2010), Nanotyrannus (Carr and Williamson, 2004;n.b., the small-sized Lmx1 of LACM 28471 displaying a U-shaped cross-section and a medial ridge is here considered to be a premaxillary tooth; Molnar, 1978;contra Carr and Williamson, 2004), Raptorex (Sereno et al, 2009; LH PV18; Figure 22.2), Timurlengia (Averianov and Sues, 2012), Tyrannosaurus (Smith, 2005;MOR 1125), Xiongguanlong (Li et al, 2010;Figure 11.4) and an indeterminate tyrannosauroid from the Cenomanian of Utah (Zanno et al, 2019). A discrete ridge also occurs on the lingual side of the mesial premaxillary teeth in the basal tyrannosauroids Dilong (IVPP V11979, V14243) and Proceratosaurus (Lpm2; NHMUK PV R.4860).…”

Caenagnathids of the Dinosaur Park Formation (Campanian) of Alberta, Canada: anatomy, osteohistology, taxonomy, and evolution

“…Current evidence indicates that the earliest caenagnathids, including giant forms, had dispersed to North America by the Albian-Cenomanian (Makovicky and Sues 1998;Simon et al 2019). During this interval, terrestrial ecosystems in North America were undergoing drastic reorganizations, including turnovers in both the flora and fauna (Cifelli et al 1997;Sues and Averianov 2009;Makovicky 2011, 2013;Prieto-Marquez et al 2016;Jud et al 2018;Nesbitt et al 2019;Zanno et al 2019). The presence of caenagnathids in North America during this interval may have enabled them to occupy opening niches and expand in body size range.…”