A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7–75.2 Ma) precedes the younger new species (~75.1–74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.

The repeated discovery of adult dinosaurs in close association with egg clutches leads to speculation over the type and extent of care exhibited by these extinct animals for their eggs and young. To assess parental care in Cretaceous troodontid and oviraptorid dinosaurs, we examined clutch volume and the bone histology of brooding adults. In comparison to four archosaur care regressions, the relatively large clutch volumes of Troodon, Oviraptor, and Citipati scale most closely with a bird-paternal care model. Clutch-associated adults lack the maternal and reproductively associated histologic features common to extant archosaurs. Large clutch volumes and a suite of reproductive features shared only with birds favor paternal care, possibly within a polygamous mating system. Paternal care in both troodontids and oviraptorids indicates that this care system evolved before the emergence of birds and represents birds' ancestral condition. In extant birds and over most adult sizes, paternal and biparental care correspond to the largest and smallest relative clutch volumes, respectively.A vian reproduction differs from that of other vertebrates in the extensive contribution of males to the care of eggs and young. Males participate in parental care in more than 90% of extant bird species (1). By comparison, males contribute to parental care in fewer than 5% of mammalian species and even more rarely among extant non-avian reptiles (1). The origin of this paternal contribution as well as the overall parental care system in ancestral birds remains controversial (2, 3). Maternal care predominates in crocodilians (the closest living sister taxon to birds), and the two major clades of extant birds use differing parental care systems. Neognathes (i.e., galliforms through passerines) typically exhibit biparental care, with females and males variably sharing incubation and care of the young (1-3). In contrast, males of nearly all Paleognathes (ratites, tinamous) incubate and care for the young alone (4). Cretaceous troodontid and oviraptorid dinosaurs share a close ancestry with birds and display some of their reproductive attributes, including multilayered eggshells, asymmetric eggs, and monoautochronic ovulation (5). Additionally, adult Troodon formosus, Oviraptor philoceratops, and Citipati osmolskae have been discovered on top of egg clutches, with some specimens retaining avian-like brooding postures (5, 6). We assessed the parental care system of these dinosaurs with the use of clutch volume-adult body mass models and bone histology data from brooding adults (7).Complete egg clutches for Troodon and the oviraptorids contain 22 to 30 large eggs (7). Total clutch volumes far exceed those of extant crocodilians, more closely matching those of polygamous ratites with similar adult size (Fig. 1). We generated regression models describing the adult (table S2). For birds, paternal care is associated with relatively larger clutch volumes than either biparental or maternal care over most body sizes; biparental care is associated wi...