Riograndia and Brasilodon
Jorge Blanco
As I confidently predicted in the conclusion to my last blog post, that might have been that day's casual refutation of creationism, but another would be along shortly.
Of course, I can't claim any credit for such an easy prediction because refutations of creationism are ten-a-penny in scientific literature, occurring often multiple times a day, simply by revealing facts that run counter to creationist claims.
This particular refutation deals with the discovery that the precursor to the mammalian jaw joint and middle ear bones occurred several times with this particular one appearing in the fossil record about 17 million years ago. The mammalian middle ear bones or ossicles are a modification of the reptilian jaw and the single reptilian middle ear bone.
What information do you have on the evolution of the mammalian jaw joint and the middle ear bones? The evolution of the mammalian jaw joint and the middle ear bones is a fascinating example of how structures can be repurposed over evolutionary time. This transition is one of the key features distinguishing mammals from their synapsid ancestors and other vertebrates.The discovery was made by palaeontologists from the University of Bristol, UK, working with colleagues in Brazil and Argentina. Their findings are the subject of an open access paper in Nature and a Bristol University news release:
Key Evolutionary Changes
- Reptilian Ancestors:
- Early synapsids (mammal-like reptiles) had a jaw structure similar to reptiles. The jaw joint was formed by the articular bone of the lower jaw and the quadrate bone of the skull.
- The middle ear contained a single bone, the stapes, as in most other tetrapods.
- Shift in Jaw Joint:
- Over time, some of the bones from the reptilian jaw joint were reduced in size and repurposed for hearing.
- In early mammalian ancestors, a new jaw joint formed between the dentary (the main lower jaw bone) and the squamosal (part of the skull). This transition allowed the quadrate and articular bones to become less critical for jaw movement.
- Formation of the Mammalian Jaw and Ear:
- As the dentary-squamosal joint became the primary jaw joint in mammals, the quadrate and articular bones were no longer needed in the jaw joint.
- These bones gradually became smaller and were incorporated into the middle ear, where they evolved into the incus (from the quadrate) and malleus (from the articular). These bones, along with the stapes, form the three-bone middle ear characteristic of modern mammals.
- Importance of the Three-Bone Middle Ear:
- The shift to a three-bone middle ear allowed for more sensitive hearing, particularly of higher-frequency sounds. This is thought to have been an important adaptation for early mammals, which were likely nocturnal and benefited from enhanced auditory sensitivity.
- The evolution of the middle ear also involved changes in the angular bone, which became part of the mammalian ear's tympanic bone (forming the tympanic ring that supports the eardrum).
Fossil Evidence
- The transition from the reptilian jaw joint to the mammalian configuration is well documented in the fossil record, especially in therapsids, a group of synapsids closely related to mammals.
- Notable fossils like Morganucodon and Hadrocodium exhibit intermediate forms, with both jaw joints in place (the old articular-quadrate joint and the new dentary-squamosal joint), showing how this evolutionary transition occurred gradually.
Evolutionary Significance
- The repurposing of the jaw joint bones for hearing represents an example of exaptation (a shift in function of existing structures). It allowed early mammals to develop enhanced auditory capabilities without losing essential jaw functionality.
- This adaptation is part of what allowed mammals to thrive in new ecological niches, including more nocturnal activity, where acute hearing would provide a survival advantage.
In summary, the evolution of the mammalian jaw joint and middle ear bones illustrates a profound anatomical transformation that played a key role in the evolutionary success of mammals.
Brazilian fossils reveal jaw-dropping discovery in mammal evolution
The discovery of new cynodont fossils from southern Brazil by a team of palaeontologists from the University of Bristol, alongside colleagues from Argentina and Brazil, has led to a significant breakthrough in understanding the evolution of mammals.
These fossils, belonging to the mammal-precursor species Brasilodon quadrangularis and Riograndia guaibensis, offer critical insights into the development of the mammalian jaw and middle ear, revealing evolutionary experiments that occurred millions of years earlier than previously thought.
Mammals stand out among vertebrates for their distinct jaw structure and the presence of three middle ear bones. This transition from earlier vertebrates, which had a single middle ear bone, has long fascinated scientists. The new study explores how mammal ancestors, known as cynodonts, evolved these features over time.
Using CT scanning, researchers were able to digitally reconstruct the jaw joint of these cynodonts for the first time. The researchers uncovered a ‘mammalian-style’ contact between the skull and the lower jaw in Riograndia guaibensis, a cynodont species that lived 17 million years before the previously oldest known example of this structure, but did not find one in Brasilodon quadrangularis, a species more closely related to mammals. This indicates that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts, earlier than expected.
These 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 acquisition of the mammalian jaw contact was a key moment in mammal evolution. 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.
James Rawson, Lead author
Palaeobiology Research Group
School of Earth Sciences
University of Bristol, Bristol, UK.
This discovery has broad implications for the understanding of the early stages of mammal evolution, illustrating that features such as the mammalian jaw joint and middle ear bones evolved in a patchwork, or mosaic, fashion across different cynodont groups.
This study was made possible because of specimens from three key cynodont taxa, Brasilodon, Riograndia and Oligokyphus, which were scanned to reconstruct the jaw joint. The exciting recent specimens of Brasilodon and Riograndia from Brazil provided new information, but also important was to scan specimens of Oligokyphus from the collections of the Natural History Museum. Although Oligokyphus was initially described in the 1950s, these detailed recent scans confirmed previous interpretations of the jaw joint structure, making it possible to confidently include them in the analysis.
Dr. Pamela G. Gill, co-author
Palaeobiology Research Group
School of Earth Sciences
University of Bristol, Bristol, UK.
Over the last years, these tiny fossil species from Brazil have brought marvellous information that enrich our knowledge about the origin and evolution of mammalian features. We are just in the beginning and our multi-national collaborations will bring more news soon.
Dr Agustín G. Martinelli, co-lead author
Museo Argentino Ciencias Naturales “Bernardino Rivadavia”—CONICET
Buenos Aires, Argentina.
The research team is eager to further investigate the South American fossil record, which has proven to be a rich source of new information on mammalian evolution.
Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals.
Professor Marina B. Soares, co-author
Museu Nacional
Universidade Federal do Rio de Janeiro
Rio de Janeiro, Brazil.
By integrating these findings with existing data, the scientists hope to deepen their understanding of how early jaw joints functioned and contributed to the development of the mammalian form.
The study opens new doors for paleontological research, as these fossils provide invaluable evidence of the complex and varied evolutionary experiments that ultimately gave rise to modern mammals.
James Rawson, Lead author.
Paper:'Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint' by James Rawson et al in Nature.
AbstractSuch a lot for creationists to ignore or lie about here:
The acquisition of the load-bearing dentary–squamosal jaw joint was a key step in mammalian evolution1,2,3,4,5. Although this innovation has received decades of study, questions remain over when and how frequently a mammalian-like skull–jaw contact evolved, hindered by a paucity of three-dimensional data spanning the non-mammaliaform cynodont–mammaliaform transition. New discoveries of derived non-mammaliaform probainognathian cynodonts from South America have much to offer to this discussion. Here, to address this issue, we used micro-computed-tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes6,7,8, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major. We find homoplastic evolution in the jaw joint in the approach to mammaliaforms, with ictidosaurs (Riograndia plus tritheledontids) independently evolving a dentary–squamosal contact approximately 17 million years before this character first appears in mammaliaforms of the Late Triassic period10,11,12. Brasilodon, contrary to previous descriptions6,7,8, lacks an incipient dentary condyle and squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate–articular joint. We postulate that the jaw joint underwent marked evolutionary changes in probainognathian cynodonts. Some probainognathian clades independently acquired ‘double’ craniomandibular contacts, with mammaliaforms attaining a fully independent dentary–squamosal articulation with a conspicuous dentary condyle and squamosal glenoid in the Late Triassic. 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.
Fig. 1: Time-calibrated relationships of Cynodontia.
The topology is based on the phylogenetic analysis within this study (methodology and detailed results are shown in the Supplementary Information). The black bars at the tree tips show the stratigraphic range of each taxon, with the geological timescale below. Outlines of lower jaws and postdentaries for key cynodont taxa adapted from the literature1,2,3,4,5,26,28,46,49,62,66,83,84 are shown in medial view (not to scale), with an example Thrinaxodon jaw in the bottom left corner denoting the colour of each bone. Taxa studied here (Brasilodon, Riograndia, Oligokyphus) are highlighted in red. Ma, million years ago. Outlines of lower jaws and postdentaries adapted with permission from refs. 4,26,84 (Springer Nature Limited); ref. 28 (Cynognathus, Morganucodon; Oxford Univ. Press); ref. 5 (Thrinaxodon; Annual Review of Ecology, Evolution and Systematics); refs. 3,46 (Probainognathus, Diarthrognathus; John Wiley and Sons); and ref. 83 (Diademodon; Palaeontologia Africana).
Rawson, J.R.G., Martinelli, A.G., Gill, P.G. et al.
Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint.
Nature (2024). https://doi.org/10.1038/s41586-024-07971-3
Copyright: © 2024 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Firstly, transitional fossils showing intermediates between the reptilian and mammalian jaw joint and middle ear bones
Secondly, the age of the fossils so long before creationists believe there was an Earth for these transitional forms to live on.
Thirdly, the complete lack of any rejection of the Theory of Evolution by the scientists as the best explanation for the data.
As I said, another rebuttal of creationism simple by revealing evidence that runs counter to creationists claims. And I'll make another prediction that there will be even more shortly …
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