Two groundbreaking studies published back-to-back in Nature offer a fascinating glimpse into the evolution of mammals. Led by Dr. Mao Fangyuan and Dr. Zhang Chi from the Chinese Academy of Sciences, the research team unearthed two remarkable Jurassic mammaliaform fossils from China. These fossils provide crucial insights into the early diversification of teeth, the development of the middle ear, and the transformation of a key jaw joint – shedding new light on the path that led to modern mammals.
Mammaliaforms encompass both extinct and extant (living) organisms closely related to mammals. Studying these creatures allows paleontologists to piece together the evolutionary puzzle and understand how various mammalian traits came to be.
The newly discovered fossils belong to a group called shuotheriids, known for their unique “pseudotribosphenic” molars – lower molars with a basin-like structure in front of the main chewing surface. This contrasts with the “tribosphenic” pattern seen in most living mammals, where a shearing structure (talonid) sits behind the chewing surface (trigonid) for efficient food processing.
Traditionally, shuotheriids were grouped with “australosphenidans,” which include the unusual egg-laying monotremes. However, this classification raised several questions about morphology, distribution, and functionality in early mammals.
The first study delves into the teeth of the fossils. Analyzing two complete sets of teeth from the Middle Jurassic Daohugou Formation in Inner Mongolia, the researchers established a new genus and species: Feredocodon chowi. Interestingly, their analysis revealed a novel interpretation. They propose that the seemingly unique pseudotribosphenic molars of shuotheriids are actually homologous (sharing a common origin) to the molar pattern observed in docodonts – another extinct mammaliaform group.
This new understanding of dental evolution led them to propose a revised evolutionary tree. Their analysis suggests that a common ancestor similar to Morganucodon (a Jurassic mammaliaform) independently gave rise to three major mammaliaform lineages: Docodontiformes (including docodonts and shuotheriids), Allotheria, and Holotheria (which encompasses therians – placental mammals and marsupials – and their kin).
Essentially, the research highlights that while all these groups evolved from a common ancestor with simple triconodont-like teeth, their molar structures diversified in different directions, each becoming broader and more complex to improve food processing.
The second study focuses on another crucial aspect of mammalian evolution – the middle ear. Here, the researchers examined the mandibular middle ears (MdME) of two species: Feredocodon chowi and a Morganucodon-like animal called Dianoconodon youngi from the Early Jurassic Lufeng Biota.
These fossils revealed new morphological features that support the theory of how jaw joint bones transformed into middle ear ossicles in early mammals. The study suggests that in Dianoconodon, one of the two jaw joints (articular-quadrate joint) lost its load-bearing function, while the middle ear structures became better adapted for improved hearing.
The fossils of the shuotheriid Feredocodon show even further advancement, with the postdentary bones (bones supporting the lower jaw) exhibiting characteristics suited purely for auditory purposes.
The research also sheds light on the role of ossified Meckel’s cartilage, a temporary bone in the embryonic jaw, which likely served as a stabilizing mechanism during this evolutionary shift. Additionally, the study highlights the critical role played by the medial displacement of the quadrate bone relative to the articular bone in the transformation from a jaw joint to the middle ear.
These groundbreaking discoveries provide compelling evidence for the gradual evolution of the mammalian middle ear, a classic example of vertebrate adaptation. By revealing the early diversification of teeth, the development of the middle ear, and the transformation of a key jaw joint, the research team offers a significant advancement in our understanding of how early mammaliaforms paved the way for the diverse mammalian world we see today.
Source: Chinese Academy of Sciences