Jurassic fossil juvenile reveals prolonged life history in early mammals
By: Elsa Panciroli et al.
Summarized by: Thomas Smith, Senior at Binghamton University
What was the hypothesis being tested? Mammaliaformes were the immediate predecessors of modern mammals. Scientists believed they had a life history that included rapid juvenile growth, early maturation into adulthood, and relatively short lifespans, like modern mammals of that size (156 grams or less). However, examination of two newly found hyrax-like Krusatodon kirtlingtonenesis fossils from the Jurassic Period revealed that this species, and possibly all mammaliaformes did not follow the life history of their modern equivalents. Scientists tested this hypothesis in this study by reconstructing the growth stage and life history of these new fossils.
What data were used? The two fossils studied in this experiment were of the species Krusatodon, resembling modern shrews, but were likely closer to modern hyraxes, small mammals with elongated front teeth. Despite being a 166-million-year-old Jurassic fossil, an adult Krusatodon fossil was 95% intact; a juvenile fossil (between 7 and 24 months old at time of death) at around 40% intact was also examined for this study. Both samples had their dentition (teeth) preserved well enough to study. The discovery of the juvenile fossil finally allowed researchers to examine the growth patterns and life histories of mammaliaformes, and to compare it to modern mammals.
Methodology: Researchers used an imaging technique called propagation phase-contrast synchrotron radiation x-ray micro-computed tomography to create highly detailed digital, 3D models of both the juvenile and adult skeletons (Figure 1). Researchers compared the adult and juvenile skeletons to each other, and to modern mammals. Researchers also studied the skulls of the samples to determine the stages of dental eruption (the point in which “baby teeth” are replaced by “adult teeth”) that each exhibited.
Results: The juvenile fossil had a partial eruption of its adult teeth at the time of its death. The first two molars on both the upper and lower jaw had erupted, with one of the upper molars in the process of erupting. The remaining teeth were either first generation teeth (i.e. baby teeth) or were unable to be classified. The presumed-adult fossil displayed full permanent dentition, confirming the maturity of the sample. Comparing the eruption status of the juvenile Krusatodon fossil to modern mammals of the same size and age, scientists determined that the Krusatodon developed slower, and matured into its adult form later on into its life; mammals are typically fully matured by the time their last permanent tooth erupts, so the lack of complete dental eruption in the juvenile fossil indicates that even at its relatively older age, it was still far from adulthood. Additional comparisons between the timing of the dental eruption and jaw length suggest that the Kusatodon’s growth patterns are proportional to modern mammals, just at a slower rate. All of these results together indicate that the Krusatodon had a longer life cycle than would be expected of modern mammals in their size range.
Why is this study important: The typical pattern that mammals follow in terms of life histories, is that as body mass increases, the time to reach sexual maturity and lifespan extend, with longer juvenile phases and a slower metabolic rate. The results of this study disrupt this pattern, with the relatively small Krusatodon having a relatively long youth and lifespan. A species’ growth pattern and life history strategy plays an important role in its survival, both at the organism and population level. This study indicates a massive shift in the knowledge base, suggesting large chunks of information about Jurassic mammals and their life history strategies could be incorrect.
Broader Implications beyond this study: This finding might indicate that the life histories of other early mammals need to be reexamined. More research on other Jurassic mammal species is needed to determine if this discovery was species specific or if it applies to more Jurassic mammals, but this study could alter what scientists know about the evolution of this pattern in life histories.
Citation: Panciroli, E., Benson, R. B., Fernandez, V., Fraser, N. C., Humpage, M., Luo, Z.-X., Newham, E., & Walsh, S. (2024). Jurassic fossil juvenile reveals prolonged life history in early mammals. Nature, 632(8026), 815–822. https://doi.org/10.1038/s41586-024-07733-1