Development has often been viewed as a constraining force on morphological adaptation, but its precise influence, especially on evolutionary rates, is poorly understood. Placental mammals provide a classic example of adaptive radiation, but the debate around rate and drivers of early placental evolution remains contentious. A hallmark of early dental evolution in many placental lineages was a transition from a triangular upper molar to a more complex upper molar with a rectangular cusp pattern better specialized for crushing. To examine how development influenced this transition, we simulated dental evolution on "landscapes" built from different parameters of a computational model of tooth morphogenesis. Among the parameters examined, we find that increases in the number of enamel knots, the developmental precursors of the tooth cusps, were primarily influenced by increased self-regulation of the molecular activator (activation), whereas the pattern of knots resulted from changes in both activation and biases in tooth bud growth. In simulations, increased activation facilitated accelerated evolutionary increases in knot number, creating a lateral knot arrangement that evolved at least ten times on placental upper molars. Relatively small increases in activation, superimposed on an ancestral tritubercular molar growth pattern, could recreate key changes leading to a rectangular upper molar cusp pattern. Tinkering with tooth bud geometry varied the way cusps initiated along the posterolingual molar margin, suggesting that small spatial variations in ancestral molar growth may have influenced how placental lineages acquired a hypocone cusp. We suggest that development could have enabled relatively fast higher-level divergence of the placental molar dentition.
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Temnospondyl specimens collected recently in the Middle-?Late Triassic of the Ruhuhu (Tanzania) and Luangwa (Zambia) rift basins are described and figured. They are attributed to cf. Cherninia megarhina (Chernin & Cosgriff, 1975), Mastodonsauroidea indet., Stereospondyli indet., and cf. StereoÂspondyli, as well as intercentra of small adult individual(s) which may belong to a new taxon. Although fragmentary, this new material allows taxonomic updates to the Triassic temnospondyl assemblages of Tanzania and Zambia and documents an interesting phylogenetic and ecological diversity. For example, among the Triassic mastodonsauroids of Zambia, Cherninia megarhina coexisted with Stanocephalosaurus pronus (Howie, 1970) in nonmarine environments. Similar to that of the South African Karoo Basin, these temnospondyl assemblages also illustrate the rapid recovery of the group after the Permian-Triassic mass extinction and contribute to a better understanding of the impact of this extinction on the tetrapod faunas of southern Pangea.
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Lionel Cavin, Andrà Piuz, Christophe Ferrante & Guillaume Guinot (2021)
Giant Mesozoic coelacanths (Osteichthyes, Actinistia) reveal high body size disparity decoupled from taxic diversity.
Scientific Reports 11, Article number: 11812
doi:
https://doi.org/10.1038/s41598-021-90962-5 https://www.nature.com/articles/s41598-021-90962-5Free pdf:
https://www.nature.com/articles/s41598-021-90962-5.pdfThe positive correlation between speciation rates and morphological evolution expressed by body size is a macroevolutionary trait of vertebrates. Although taxic diversification and morphological evolution are slow in coelacanths, their fossil record indicates that large and small species coexisted, which calls into question the link between morphological and body size disparities. Here, we describe and reassess fossils of giant coelacanths. Two genera reached up to 5 m long, placing them among the ten largest bony fish that ever lived. The disparity in body size adjusted to taxic diversity is much greater in coelacanths than in ray-finned fishes. Previous studies have shown that rates of speciation and rates of morphological evolution are overall low in this group, and our results indicate that these parameters are decoupled from the disparity in body size in coelacanths. Genomic and physiological characteristics of the extant Latimeria may reflect how the extinct relatives grew to such a large size. These characteristics highlight new evolutionary traits specific to these "living fossils".