Fossil Friday: New Fossil Evidence Challenges Another Icon of Evolution
This Fossil Friday features the skull of Cynognathus crateronotus, a mammal-like reptile from the Middle Triassic of the southern hemisphere landmasses that had formed the ancient supercontinent Gondwana. It belongs to a group called cynodontians. The recent analysis of the jaw anatomy of fossil cynodonts from South America challenged some longstanding evolutionary ideas.
When evolutionists are asked what in their view represents the best evidence for the Darwinian story of common descent with modification, they will generally refer to the fossil record and especially to supposed transitional series like those of horses, elephants, whales, hominins, fishapods to tetrapods, dinos to birds, and most of all the transition from reptiles to mammals. The latter allegedly shows an unambiguous transformation of the jaw articulation from a primitive reptilian state to the derived mammalian condition, correlated with a reduction of bones and an incorporation of the original jaw articulation into the mammalian ear as auditory ossicles (Reichert-Gaupp theory).
A More Complicated Picture
However, a closer look at the actual fossil evidence shows a much more complicated picture that involves multiple independent origins of anatomical similarities. In a seminal study on the evolution of the mammalian middle ear, the authors admitted that “current hypotheses on the convergent evolution of middle ear bones are complex and controversial, partly because of a lack of phylogenetic resolution and partly because the interpretation of the fossil evidence is difficult” (Ramírez-Chaves et al. 2016). They concluded that “the departure of postdentary bones from the dentary to form a partial mammalian middle ear (PMME); … occurred convergently in the northern hemisphere ancestors of therians and the southern hemisphere ancestors of monotremes … the transition from a PMME to a definite mammalian middle ear (DMME) ocurred [sic] multiple times, including at least three cases of independent evolution within extant mammals (in monotremes, metatherians and eutherians).”
Now, a new study complicated this scenario even more: The scientists studied the well-preserved fossil remains of three key species of probainognathian cynodonts, viz. Brasilodon quadrangularis and Riograndia guaibensis from the Late Triassic of Brazil, as well as Oligokyphus major from the Early Jurassic of Great Britain. They used CT scanning to digitally reconstruct the jaw joint of these animals and found something very unexpected and surprising (Luo 2024). The jaw joint anatomy of the two Brazilian species was very different, with the joint of Riograndia being more mammal-like than that of Brasilodon, even though the later genus is considered as closer related to modern mammals. Furthermore, Riograndia was dated to be about 17 million years older than any other previously known mammal-like reptile with such an advanced jaw articulation. The authors concluded that “the dentary-squamosal contact, which is traditionally considered to be a typical mammalian feature, therefore evolved more than once and is more evolutionary labile than previously considered.”
Interesting News for a Departed Colleague
The press release unashamedly speaks about “rewriting our understanding of mammal evolution” (News Staff 2024), and elaborates that:
This indicates that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts, earlier than expected. The findings suggest that mammalian ancestors experimented with different jaw functions, leading to the evolution of mammalian traits independently in various lineages. The early evolution of mammals, it turns out, was far more complex and varied than previously understood.
The lead author of the new study, Dr. James Rawson from the University of Bristol, said (quoted in News Staff 2024):
This indicates that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts, earlier than expected. The findings suggest that mammalian ancestors experimented with different jaw functions, leading to the evolution of mammalian traits independently in various lineages. The early evolution of mammals, it turns out, was far more complex and varied than previously understood.
The lead author of the new study, Dr. James Rawson from the University of Bristol, said (quoted in News Staff 2024):
What these new Brazilian fossils have shown is that different cynodont groups were experimenting with various jaw joint types, and that some features once considered uniquely mammalian evolved numerous times in other lineages as well.
Dr. Zhe-Xi Luo, one of the world’s leading experts on mammalian origins and not involved in the new study, commented that this is “a jaw-dropping discovery about early mammals” (Luo 2024). It certainly is, and it definitely looks like we are witnessing the beginning of the dismantling of yet another icon of evolution, which would have been very interesting news to my recently deceased friend and colleague Jonathan Wells, who had described many such cases in his ground-breaking books.
References
News Staff 2024. New Cynodont Fossil Discoveries are Rewriting Our Understanding of Mammal Evolution. SciNews September 25, 2024. https://www.sci.news/paleontology/brazil-cynodonts-13286.html
Luo Z-X 2024. A jaw-dropping discovery about early mammals. Nature 634, 305–306. DOI: https://doi.org/10.1038/d41586-024-03038-5
Ramírez-Chaves HE, Weisbecker V, Wroe S et al. 2016. Resolving the evolution of the mammalian middle ear using Bayesian inference. Frontiers in Zoology 13: 39, 1–10. DOI: https://doi.org/10.1186/s12983-016-0171-z
Rawson JRG, Martinelli AG, Gill PG, Soares MB, Schultz CL & Rayfield EJ 2024. Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint. Nature 634, 381–388. DOI: https://doi.org/10.1038/s41586-024-07971-3
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