Theropod guild structure and the tyrannosaurid niche assimilation hypothesis: implications for predatory dinosaur macroecology and ontogeny in later Late Cretaceous Asiamerica¹

Abstract

Well-sampled dinosaur communities from the Jurassic through the early Late Cretaceous show greater taxonomic diversity among larger (>50 kg) theropod taxa than communities of the Campano-Maastrichtian, particularly to those of eastern/central Asia and Laramidia. The large carnivore guilds in Asiamerican assemblages are monopolized by tyrannosaurids, with adult medium-sized (50–500 kg) predators rare or absent. In contrast, various clades of theropods are found to occupy these body sizes in earlier faunas, including early tyrannosauroids. Assemblages with “missing middle-sized” predators are not found to have correspondingly sparser diversity of potential prey species recorded in these same faunas. The “missing middle-sized” niches in the theropod guilds of Late Cretaceous Laramidia and Asia may have been assimilated by juvenile and subadults of tyrannosaurid species, functionally distinct from their adult ecomorphologies. It is speculated that if tyrannosaurids assimilated the niches previously occupied by mid-sized theropod predators, we would expect the evolution of distinct transitions in morphology and possibly the delay of the achievement of somatic maturity in species of this taxon.

Ecomorphospace occupation of large herbivorous dinosaurs from Late Jurassic through to Late Cretaceous time in North America

“…Whether this signal is more a reflection of differences in community structure resulting from differences in life history ( e.g. , Codron et al, 2012 ; Codron, Carbone & Clauss, 2013 ; Schroeder, Lyons & Smith, 2021 ; Holtz, 2021 ) or more of an artefact of taphonomic biases ( e.g. , Brown et al, 2013a ; Brown et al, 2013b ), is still debated and beyond the scope of the present study.…”

“…Although some research has been conducted on these alternative explanations, as we continue to sample sediments from the Early Cretaceous and the early Late Cretaceous, follow-up assessments of these hypotheses will be necessary. Insight into potential driver(s) behind the ‘sauropod decline’ may also provide information into the factor(s) behind turnover observed in other dinosaur groups, such as large carnivorous theropods ( Nesbitt et al, 2019 ; Holtz, 2021 ). Similarities in the positioning of clades for analyses with and without non-North American representatives despite differences in community composition between North American ecosystems and those on other landmasses further suggests that non-competitive factors were more important in the North American ‘sauropod decline’.…”

“…Large (>1,000 kg) herbivorous dinosaurs (=megaherbivores) dominated terrestrial North American communities for the last ∼98 Myr of the Mesozoic Era both in diversity and abundance ( O’Gorman & Hone, 2012 ; Codron et al, 2012 ; Codron, Carbone & Clauss, 2013 ; Brown et al, 2013a ). Between the Late Jurassic and Late Cretaceous, a major change in megaherbivore faunal composition is observed: previously dominant sauropods are replaced by ‘duck-billed’ hadrosaurids and ‘horned’ ceratopsids, stegosaurs become extinct and armoured ankylosaurs experience an increase in diversity ( Zanno & Makovicky, 2011 ; Barrett, 2014 ; Nesbitt et al, 2019 ; Holtz, 2021 ). The decline and decrease in sauropod diversity (herein the ‘sauropod decline’) occurred between the Cenomanian and Campanian/Maastrichtian—much later than the complete loss of stegosaurs that appears to have occurred between the Tithonian and Albian ( Williamson & Weil, 2008 ; D’Emic, Wilson & Thompson, 2010 ; Mannion & Upchurch, 2011 ; D’Emic & Foreman, 2012 ).…”

Complex neurovascular system in the dentary of Tyrannosaurus

“…The size of the element is 890 mm in anteroposterior length, 104 mm in mediolateral width, and 279 mm in dorsoventral height. The lingual bar anteriorly reaches its dorsal margin alongside the first two alveoli, a diagnostic character for genus Tyrannosaurus (Carr 2005) comprising Tyrannosaurus rex and T. bataar (Carr 2005) although the latter species is often regarded as belonging to a separate genus Tarbosaurus (e.g., Hanai and Tsuihiji 2019;Voris et al 2020;Holtz 2021). The identical condition of the lingual bar is also seen in the dentary of Zhuchengtyrannus Hone et al 2011, the latter of which erroneously lacks a numeral for the anteriormost alveoli that is described in the text), which makes the character synapomorphic for a clade formed by Tyrannosaurus, Tarbosaurus and Zhuchengtyrannus.…”

Response to Comment on “The influence of juvenile dinosaurs on community structure and diversity”

“…When examining formations that fit our criteria, the communities remaining to analyze (and indeed those we used) are simply the most taxonomically diverse communities available (Fig. 1E); it is telling that many of these same communities were selected independently in a similar analysis by a separate research group (5).…”