PÃter L. Pap, ÂGergely OsvÃth, ÂTimea Daubner, ÂAndreas Nord Â& Orsolya Vincze (2020)
Down feather morphology reflects adaptation to habitat and thermal conditions across the avian phylogeny.
Evolution (advance online publication)
Down feathers are the first feather types that appear in both the phylogenetic and the ontogenetic history of birds. Although it is widely acknowledged that the primary function of downy elements is insulation, little is known about the interspecific variability in the structural morphology of these feathers, and the environmental factors that have influenced their evolution. Here, we collected samples of down and afterfeathers from 156 bird species and measured key morphological characters that define the insulatory properties of the downy layer. We then tested if habitat and climatic conditions could explain the observed betweenâspecies variation in down feather structure. We show that habitat has a very strong and clearly defined effect on down feather morphology. Feather size, barbule length and nodus density all decreased from terrestrial towards aquatic birds, with riparian species exhibiting intermediate characters. Wintering climate, expressed as windchill (a combined measure of the ambient temperature and wind speed) had limited effects on down morphology, colder climate only being associated with higher nodus density in dorsal down feathers. Overall, an aquatic lifestyle selects for a denser plumulaceous layer, while the effect of harsh wintering conditions on downy structures appear limited. These results provide key evidence of adaptations to habitat at the level of the downy layer, both on the scale of macroâ and microâelements of the plumage. Moreover, they reveal characters of convergent evolution in the avian plumage and mammalian fur, that match the varying needs of insulation in terrestrial and aquatic modes of life.
Genetic isolation due to geographic separation (vicariance) is the best understood cause of vertebrate speciation. Nevertheless, it has never been demonstrated in the fossil record across a wide range of taxa. Here, by reviewing in-depth the available data of the Late Palaeozoic (~â350â250 million years ago), I reconstructed an early Pangaean junction-disjunction palaeogeographic model and showed that it coincides strongly with time-calibrated cladograms of the Late Palaeozoic synapsids (the primitive ancestors of modern mammals). The temporal development of the vicariant topology seems to fit closely with the emergence rhythm of the recovered synapsid taxa, suggesting vicariance due to Pangaean separation as the cause of early amniote evolution. The inferred vicariant topology also accounts for the observed pattern in the North American marine biostratigraphic units. Accordingly, the model demonstrates the link between the evolution of life on Earth and palaeogeographic evolution and strongly supports allopatric speciation through vicariance as the prominent mode of amniote evolution. Furthermore, correlations between state-of-the-art biochronostratigraphic charts and this palaeogeographic model suggest that the arido-eustasy model can explain the mid-Permian biotic extinction event and depositional cycles, such as the pre-Zechstein of the Central European Basin.
A new pycnodont taxon, Njoerdichthys dyckerhoffi gen. et sp. nov., from the Turonian of the Lower Saxony Basin of Germany is described and its systematic positions is established based on phylogenetic analyses of three specimens using slightly altered data matrices. All analyses display some differences to previous analyses but show very similar results to each other apart from the interpretation of the position of several taxa such as, e.g., Palaeobalistum. The new pycnodontiform specimens from northwestern Germany are unambiguously identified as a derived member of Pycnodontidae with close relationships to Abdobalistum and Nursallia? goedeli because of the unique combination of characters. One of the three specimens represents a juvenile form. Its morphological characters are limited, but it shares some characters with Njoerdichthys dyckerhoffi gen. et sp. nov. and is consequently allocated to the new taxon. The systematic placement of the new taxon, Njoerdichthys gen. nov., within Pycnodontidae is surprising since it does not display the one autapomorphic character (postparietal brush-like extension for muscle attachment) previously proposed to define this clade, but rather displays a combination of derived and homoplastic characters indicating that the definition of supra-generic taxa needs to be re-evaluated in the future by including more and new taxa. The distribution of pycnodontiform fishes in the Cretaceous appears to concur with changes in global climatic conditions, where high upper-ocean temperatures and high sea levels allow these fishes to migrate into higher latitudes as evidenced by the occurrence of the new taxon and Anomoeodus subclavatus in the Campanian of Sweden.