Fossil Friday: Controversial Gradualism in Tyrannosaurids
This Fossil Friday features the tyrannosaurid Daspletosaurus torosus from the Late Cretaceous of Canada. This dinosaur genus was recently in the news (News Release 2024) and caused a flurry of tweets (see Fowler 2024 on X) because of a scientific controversy about the origin of the famous T. rex — either by gradual transformation of Daspletosaurus into Tyrannosaurus, or by more discontinuous splitting from the daspletosaurine lineage. The former process of speciation is called anagenesis and the latter cladogenesis. Anagenesis is a special case of Darwinian gradualism, which is largely contradicted by the evidence from the fossil record, as I have discussed in several previous articles (Bechly 2019, 2024a, 2024b). Dinosaur paleontologists Elías Warshaw and Denver Fowler nicknamed their preferred view of an anagenetic origin of the genus Tyrannosaurus from Daspletosaurus the “Tyrannagenesis hypothesis” (Fowler 2024 on X).
Anagenesis as a mechanism of speciation in Daspletosaurus was already proposed by Carr et al. (2017) for the transition between Daspletosaurus torosus and the younger new species Daspletosaurus horneri from the Late Cretaceous of Montana. This latter species was named after famous dinosaur paleontologist Jack Horner, who had first suggested phyletic evolution in four lineages of dinosaurs including tyrannosaurids (Horner et al. 1992). A few years later, Warshaw & Fowler (2022) described another species of Daspletosaurus, and suggested that the three known species of this genus formed an evolutionary transition towards Tyrannosaurus (and the closely related genera Tarbosaurus and Zhuchengtyrannus), which would imply that the genus Daspletosaurus is not a distinct clade but a paraphyletic grade. This was communicated by the popular science media with headlines like “Newly-Discovered Tyrannosaur Species Fills Gap in Lineage Leading to Tyrannosaurus rex” (Anderson 2022). Darwin was proven right again, right? Right?
Well, Not So Fast
This year a new study by Scherer & Voiculescu-Holvad (2024) challenged the whole story. Their reanalysis of the dataset of their colleagues Warshaw & Fowler (2022) decisively refuted those claims of anagenesis in the tyrannosaur lineage. They concluded that “the dataset reveals that anagenesis is not found to be a driver of speciation within Daspletosaurus.” Instead they found strong evidence for “cladogenetic Tyrannosauridae composed of four morphologically and biogeographically distinct clades.” In their reconstructed phylogenetic tree, the three distinct species of Daspletosaurus formed a clade with the genus Thanatotheristes as sister group to a clade that includes Zhuchengtyrannus, Tarbosaurus, and Tyrannosaurus. The scientists explicitly cautioned that “all known species of Daspletosaurus do not meet previously established prerequisites for anagenesis and advise against prematurity when making conclusions concerning significant processes surrounding the mode of evolution of extinct genera with sparse and often incomplete fossil records.”
This unsurprisingly provoked a quick response by Warshaw et al. (2024), which is still in pre-proof stage, in which they present an updated analysis, incorporating further data and corrected “erroneous” interpretations, which allegedly overturns this challenge and again supports anagenesis in the tyrannosaur lineage.
A Striking Admission
History will tell who is right in this scientific controversy, but personally I am more convinced by the evidence against anagenesis. Therefore, I can only reiterate what I had already emphasized in my earlier article (Bechly 2019):
On the occasion of the Darwin Year in 2009, Hunt (2010) had reviewed all the fossil evidence for species transitions assembled by paleontologists in 150 years of research since the time of Charles Darwin. Hunt’s conclusion was mind blowing, even if framed in somewhat obfuscating language:
The meandering and fluctuating trajectories captured in the fossil record are not inconsistent with the centrality of natural selection as an evolutionary mechanism, but they probably would not have been predicted without the benefit of an empirical fossil record.
Apart from the obligatory verbal kowtow to Darwinian orthodoxy, this striking admission simply means that the empirical data from the fossil record strongly contradict the gradualist predictions of Darwin’s theory. There hardly exists any fossil evidence for directional and gradual species-to-species transitions, and especially not for anagenesis.
References
Anderson N 2022. Newly-Discovered Tyrannosaur Species Fills Gap in Lineage Leading to Tyrannosaurus rex. SciNews November 25, 2022. https://www.sci.news/paleontology/daspletosaurus-wilsoni-11424.html
Bechly G 2019. Apeman Waves Goodbye to Darwinian Gradualism. Evolution News September 6, 2019. https://evolutionnews.org/2019/09/apeman-waves-goodbye-to-darwinian-gradualism/
Bechly G 2024a. Fossil Friday: Chronospecies, a Sinking Ship. Evolution News February 9, 2024. https://evolutionnews.org/2024/02/fossil-friday-chronospecies-a-sinking-ship/
Bechly G 2024b. Fossil Friday: Direct Fossil Ancestors of Living Species? Evolution News March 8, 2024. https://evolutionnews.org/2024/03/fossil-friday-direct-fossil-ancestors-of-living-species/
Carr TD, Varricchio DJ, Sedlmayr JC, Roberts EM & Moore JR 2017. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports 7:44942, 1–11. DOI: https://doi.org/10.1038/srep44942
Horner JR, Varricchio DJ & Goodwin MB 1992. Marine transgressions and the evolution of Cretaceous dinosaurs. Nature 358, 59–61. DOI: https://doi.org/10.1038/358059a0
Hunt G 2010. Evolution in Fossil Lineages: Paleontology and The Origin of Species. The American Naturalist 176(S1), S61–S76; DOI: https://doi.org/10.1086/657057
News Release 2024. New research by Badlands Dinosaur Museum Paleontologists sheds light on the ancestry of Tyrannosaurus rex. Dickinson Museum Center June 29, 2024. https://www.dickinsongov.com/museum-center/page/anagenesis-and-tyrant-pedigree
Scherer CR & Voiculescu-Holvad C 2024. Re-analysis of a dataset refutes claims of anagenesis within Tyrannosaurus-line tyrannosaurines (Theropoda, Tyrannosauridae). Cretaceous Research 155: 105780, 1–9. DOI: https://doi.org/10.1016/j.cretres.2023.105780
Warshaw EA & Fowler DW 2022. A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana. PeerJ 10:e14461, 1–29. DOI: https://doi.org/10.7717/peerj.14461
Warshaw EA, Barrera Guevara D & Fowler DW 2024. Anagenesis and the tyrant pedigree: a response to “Re-analysis of a dataset refutes claims of anagenesis within Tyrannosaurus-line tyrannosaurines (Theropoda, Tyrannosauridae)”. Cretaceous Research 105957 pre-proof. DOI: https://doi.org/10.1016/j.cretres.2024.105957
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