The realm of the Tyrant

Close up of “Sue” at the Field Museum of Natural History in Chicago, IL, 2009 (From Wikimedia Commons)

After the extinction of many carnivorous crurotarsan lineages (phytosaurs, ornithosuchids, and rauisuchians) at the Triassic–Jurassic boundary, theropod dinosaurs became the primary large-bodied flesh-eaters in terrestrial ecosystems. The group reached a great taxonomic and morphological diversity during the Jurassic and Early Cretaceous. Some major groups include Ceratosauria, Megalosauroidea, Spinosauridae; Carnosauria, and Coelurosauria. In the last decades, the study of Gondwanan non-avian theropods has been highly prolific, showing that the group reached a great taxonomic and morphological diversity comparable to that of Laurasia. Notwithstanding, there is a qualitative difference between Jurassic and Early Cretaceous assemblages relative to the latest Cretaceous (Campano-Maastrichtian) assemblages with abelisaurids dominating Gondwanan continents, and tyrannosaurids ruling Asiamerican ecosystems. 

Tyrannosaurus rex, the most iconic dinosaur of all time, and its closest relatives known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids. The clade originated in the Middle Jurassic, approximately 165 million years ago, and was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway (about 99.5 Mya). Over the past 20 years, new discoveries from Russia, Mongolia and China helped to build the Tyranosaurs family tree.

Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. (Adapted from Brusatte et. al., 2010)

All large-bodied carnivorous theropod dinosaurs passed through a wide range of body sizes. Therefore, the ecological niche of any given individual shifted throughout its lifetime. From the Jurassic through the early Late Cretaceous, this transformation occurred in the context of ecosystems in which the juveniles and subadults potentially competed with other theropod species with medium adult body sizes. But sometime after the Turonian something changed.

A new study by Thomas Holtz, a principal lecturer in the University of Maryland’s Department of Geology, surveyed the record of 60 dinosaur communities from the Jurassic and Cretaceous periods, revealing a drop-off in diversity of medium-sized predator species (50–1000 kg) in communities dominated by tyrannosaurs. On the other hand, the study also showed that the diversity of prey species did not decline. The proposed explanation for this phenomenon is the “tyrannosaurid niche assimilation hypothesis”. In this scheme, juvenile and subadult members of Tyrannosauridae were the functional equivalent of earlier middle-sized theropod carnivores. This absence of other potential mid-sized competitors in Campano-Maastrichtian Asiamerica could be a factor in some evolutionary transformations in Tyrannosauridae such as bite force and agility.

 

References:

Thomas R. Holtz, Theropod guild structure and the tyrannosaurid niche assimilation hypothesis: implications for predatory dinosaur macroecology and ontogeny in later Late Cretaceous Asiamerica, Canadian Journal of Earth Sciences (2021). DOI: 10.1139/cjes-2020-0174

Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, et al. (2010) Tyrannosaur paleobiology: new research on ancient exemplar organisms. Science 329: 1481–1485. doi: 10.1126/science.1193304

Zanno, L., Makovicky, P. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nat Commun 4, 2827 (2013). https://doi.org/10.1038/ncomms3827

 

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