G.S. Bever, Tyler R. Lyson, Daniel J. Field & Bhart-Anjan S. Bhullar (2016)
The amniote temporal roof and the diapsid origin of the turtle skull.
Zoology (advance online publication)
The origin of turtles has long remained cryptic due to a relative lack of clearly informative transitional fossils.
Euntosaurus africanus is a Middle Permian amniote from South Africa that helps fill this transitional role.
μCT analysis of Eunotosaurus skull reveals an upper temporal fenestra, potentially homologous with that of other living reptiles.
Preliminary analysis of other early amniotes reveals considerable variation in cranial roof architecture.
Advanced imaging holds great promise in clarifying the deep-time origin of modern amniote clades.
Fossils provide a glimpse into the architecturally complex origins of modern vertebrate body plans. One such origin that has been long debated is that of turtles. Although much attention has been directed toward the origin of the shell, the enigmatic evolution of the turtle skull and its anapsid temporal region has long clouded our understanding of reptile phylogeny. Two taxa, Eunotosaurus africanus and Pappochelys rosinae, were recently and independently described as long-anticipated stem turtles whose diapsid skulls would cement the evolutionary link between turtles and other modern reptile lineages. Detailed μCT analysis of the stratigraphically older and phylogenetically stemward of the two, Eunotosaurus, provides empirical insight into changing developmental trajectories that may have produced the anapsid cranial form of modern turtles and sets the stage for more comprehensive studies of early amniote cranial evolution.
Barry G. Lovegrove (2016)
A phenology of the evolution of endothermy in birds and mammals.
Biological Reviews (advance online publication)
Recent palaeontological data and novel physiological hypotheses now allow a timescaled reconstruction of the evolution of endothermy in birds and mammals. A three-phase iterative model describing how endothermy evolved from Permian ectothermic ancestors is presented. In Phase One I propose that the elevation of endothermy – increased metabolism and body temperature (Tb) – complemented large-body-size homeothermy during the Permian and Triassic in response to the fitness benefits of enhanced embryo development (parental care) and the activity demands of conquering dry land. I propose that Phase Two commenced in the Late Triassic and Jurassic and was marked by extreme body-size miniaturization, the evolution of enhanced body insulation (fur and feathers), increased brain size, thermoregulatory control, and increased ecomorphological diversity. I suggest that Phase Three occurred during the Cretaceous and Cenozoic and involved endothermic pulses associated with the evolution of muscle-powered flapping flight in birds, terrestrial cursoriality in mammals, and climate adaptation in response to Late Cenozoic cooling in both birds and mammals. Although the triphasic model argues for an iterative evolution of endothermy in pulses throughout the Mesozoic and Cenozoic, it is also argued that endothermy was potentially abandoned at any time that a bird or mammal did not rely upon its thermal benefits for parental care or breeding success. The abandonment would have taken the form of either hibernation or daily torpor as observed in extant endotherms. Thus torpor and hibernation are argued to be as ancient as the origins of endothermy itself, a plesiomorphic characteristic observed today in many small birds and mammals.
Matthew J. Phillips (2016)
Geomolecular Dating and the Origin of Placental Mammals.
Systematic Biology 65 (3): 546-557
In modern evolutionary divergence analysis the role of geological information extends beyond providing a timescale, to informing molecular rate variation across the tree. Here I consider the implications of this development. I use fossil calibrations to test the accuracy of models of molecular rate evolution for placental mammals, and reveal substantial misspecification associated with life history rate correlates. Adding further calibrations to reduce dating errors at specific nodes unfortunately tends to transfer underlying rate errors to adjacent branches. Thus, tight calibration across the tree is vital to buffer against rate model errors. I argue that this must include allowing maximum bounds to be tight when good fossil records permit, otherwise divergences deep in the tree will tend to be inflated by the interaction of rate errors and asymmetric confidence in minimum and maximum bounds. In the case of placental mammals I sought to reduce the potential for transferring calibration and rate model errors across the tree by focusing on well-supported calibrations with appropriately conservative maximum bounds. The resulting divergence estimates are younger than others published recently, and provide the long-anticipated molecular signature for the placental mammal radiation observed in the fossil record near the 66 Ma Cretaceous–Paleogene extinction event.
Leonidas Brikiatis (2016)
Late Mesozoic North Atlantic land bridges.
Earth-Science Reviews (advance online publication)
Current palaeogeographical reconstructions suggest that since the onset of the Late Jurassic and during the whole Cretaceous Period, the landmasses of North America and West Eurasia have been separated by seaway(s) exposed on the North Atlantic rift system. Such a palaeogeographical setting should have limited the possibility of direct overland dispersals between the biotas of the two continents. Strong faunal affinities between the two sides of the North proto-Atlantic Ocean presuppose faunal exchanges between the landmasses, however, suggesting the existence of ephemeral land bridges around the Jurassic/Cretaceous boundary and the mid-Early Cretaceous. This study reviews the current faunal evidence and discusses palaeogeographical evidence that might better elucidate the existence of the postulated land bridges. Evaluation of the evidence leads to the conclusion that two land bridges were exposed: 1) between East North America and Iberia during the Late Kimmeridgian (~ 154 Ma) and 2) between North America and Fennoscandia, via the Barents Shelf, around the Hauterivian-Barremian boundary (~ 131 Ma). The former land bridge was terminated in the earliest Tithonian (~ 151 Ma), and the latter was terminated in the late Early Barremian (~ 129 Ma). Furthermore, the latter land bridge was contemporaneous with regressive stages in both the Mid-Polish Trough and the sea strait crossing the Russian Platform, allowing broad, direct terrestrial communication among North America, Europe, and Asia in the earliest Barremian.
Anne O’Connor and Matthew A. Wills (2016)
Measuring Stratigraphic Congruence Across Trees, Higher Taxa and Time
Systematic Biology (advance online publication)
The congruence between the order of cladistic branching and the first appearance dates of fossil lineages can be quantified using a variety of indices. Good matching is a prerequisite for the accurate time calibration of trees, while the distribution of congruence across large samples of cladograms has underpinned claims about temporal and taxonomic patterns of completeness in the fossil record. The most widely used stratigraphic congruence indices are the stratigraphic consistency index, the modified Manhattan stratigraphic measure, and the gap excess ratio (plus its derivatives; the topological gap excess ratio and the modified gap excess ratio). Many factors are believed to variously bias these indices, with several empirical and simulation studies addressing some subset of the putative interactions. This study combines both approaches to quantify the effects (on all five indices) of eight variables reasoned to constrain the distribution of possible values (the number of taxa, tree balance, tree resolution, range of first occurrence dates, center of gravity of first occurrence dates, the variability of first occurrence dates, percentage of extant taxa, and percentage of taxa with no fossil record). Our empirical data set comprised 647 published vertebrate and invertebrate cladograms spanning the entire Phanerozoic, and for these data we also modelled the effects of mean age of first occurrences (as a proxy for clade age), the taxonomic rank of the clade, and the higher taxonomic group to which it belonged. The center of gravity of first occurrence dates had not been investigated hitherto, and this was found to correlate most strongly with some measures of stratigraphic congruence in our empirical study (top-heavy clades had better congruence). The modified gap excess ratio was the index least susceptible to bias. We found significant differences across higher taxa for all indices; arthropods had lower congruence and tetrapods higher congruence. Stratigraphic congruence – however measured – also varied throughout the Phanerozoic, reflecting the taxonomic composition of our sample. Notably, periods containing a high proportion of arthropods had poorer congruence overall than those with higher proportions of tetrapods.
Zhong-Jian Liu & Xin Wang (2016)
Yuhania: a unique angiosperm from the Middle Jurassic of Inner Mongolia, China.
Historical Biology (advance online publication)
Despite increasing claims of pre-Cretaceous angiosperms, whether there really are angiosperms in the Jurassic is apparently still an open question for many people before further evidence is available. This question can only be answered by studying more Jurassic plant fossils. Here we report a fossil angiosperm, Yuhania daohugouensis gen. et sp. nov, from the Middle Jurassic of Inner Mongolia, China. The plant includes connected stem, leaves, flowers, aggregate fruits, fruitlets, and seeds within fruitlets. The leaves are helically arranged along the curving stem, linear in shape, with 5–6 parallel veins. The aggregate fruit is pedicellate, composed of over 20 carpels/fruitlets helically arranged. Each fruitlet encloses a seed. The reproductive organs in various stages are found in the same plant, allowing us to understand the development of Yuhania. The occurrence of Yuhania in the Middle Jurassic re-confirms the Jurassic history for angiosperms that has been suggested by other independent research and adds to the on-going study on the early evolution of angiosperms.