A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system

Abstract

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.

Complex neuroanatomy in the rostrum of the Isle of Wight theropod Neovenator salerii

“…In addition, this ability would help explain the rarity of evidence for tooth-bone contact in the non-tyrannosaurid Mesozoic fossil record 9, 44 . We also note that enhanced facial sensitivity may potentially have played a role in intraspecific communication, consistent with data suggesting that ritualised face-biting – and thus facial contact in general – was part of theropod behavioural repertoire 11, 45 . Coronal CT slices are also known for a ventrally compressed Tyrannosaurus rex 46 specimen and reveal a (presumably deformed) maxillary channel and nerve (V 2 ).…”

A Problematic Tyrannosaurid (Dinosauria: Theropoda) Skeleton and Its Implications for Tyrannosaurid Diversity in the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta

“…The iliofibularis tubercle of the laterally adjoining fibula ( Fig. 13B-D,J) is bounded by a single crest laterally, rather than by two crests as in adult tyrannosaurids (Mader and Bradley, 1989;Carr et al, 2017). The astragalus (Figs.…”

“…This ridge is present in most tyrannosaurids except “ R. kriegsteini ” and Qianzhousaurus sinensis (Carr et al, ). The antitrochanter of the acetabular crest is very weakly developed, in contrast with those of early tyrannosauroids (Holtz Jr, ; Carr et al, ). The base of the pubic peduncle is much longer craniocaudally than the ischial peduncle, similar to the condition seen in the juvenile “ R. kriegsteini ” and early tyrannosauroids (Brusatte et al, ; Li et al, ).…”

Peer Review #3 of "A tyrannosauroid metatarsus from the Merchantville Formation of Delaware increases the diversity of non-tyrannosaurid tyrannosauroids on Appalachia (v0.1)"

“…More recently, Denton et al (2011) assigned the teeth and limb bone fragment apparently referenced by Gallagher (1993) and Gallagher (1997) Campanian deposits in North Carolina (Baird & Horner, 1979;Weishampel & Young, 1996). Lambe, 1917;Holtz, 2004;Weishampel et al, 2004;Loewen et al, 2013;Thomson, Irmis & Loewen, 2013;Fiorillo & Tykoski, 2014;Peecook et al, 2014;Carr et al, 2017). This range is obviously larger latitudinally, as the main Late Cretaceous outcrop from Appalachia ranges only from New Jersey to the southern states of Alabama,…”

“…Mississippi, and Georgia and westward to southeastern Missouri (e.g., Schwimmer, 1997;Schwimmer, 2002;Fix & Darrough, 2004). Thus, the number of tyrannosauroid taxa from Appalachia is not directly comparable to that from Laramidia, especially when the significant taphonomic biases against the preservation of dinosaurs from Appalachia are considered (e.g., Schwimmer, 1997;Schwimmer, 2002 Lambe, 1917;Russell, 1970;Weishampel et al, 2004;Carr & Williamson, 2010;Carr et al, 2011;Loewen et al, 2013;Carr et al, 2017) Appalachiosaurus, and the Merchantville taxon in a distinct clade ( Supplementary Fig.1-2). America.…”