No abstract

We provide an integrative view of the evolution of dispersal strategies in modern conifers, by characterizing and examining the phylogenetic distribution of diaspore functional morphotypes, diaspore structural compositions, seed coat modifications, and dispersal syndromes using the phylogeny of Leslie et al. (2012). We first classified diaspores into nine functional morphotypes, which represent overall dispersal strategies that encompass the multiple phases of dispersal. We mapped these morphotypes, the eight different structural compositions of diaspores, two types of seed coat modifications, and the four recognized dispersal vectors onto the phylogeny and used maximum parsimony and maximum likelihood to infer ancestral states and assess shifts in dispersal characteristics. We found that structural traits (diaspore composition and seed coat modifications) are more conserved than ecological traits (functional morphotype and dispersal vector/syndrome). Almost all diaspore functional morphotypes have multiple independent origins, with several instances of parallelism (using the same structures to generate a morphotype) within families, but generally functional convergence (using different structures to generate a morphotype) between families. Within extant conifer families, shifts in the dispersal syndrome occur most frequently with simultaneous shifts in both diaspore morphotype and composition. Shifts from winged wind-dispersed to fleshy animal-dispersed diaspores are infrequent and occur only in the direction from wind to animal dispersal. Shifts to gravity or water dispersal occur from both wind and animal dispersed diaspores, concurrent with the loss of dispersal structures from the diaspore. Within both wind and animal-dispersed syndromes, further shifts between functional morphotypes represent differentiation of overall dispersal strategies, and occur most frequently without corresponding changes in the structural composition of the diaspore. The recurrent evolution of distinct morphologies suggests that there Contreras et al., 2 are local adaptive maxima that balance tradeoffs in traits related to both transport and establishment, within developmental limitations. Overall, our results suggest that the ancestral diaspore type for all modern conifers consisted only of a seed. Conifers diversified in their dispersal strategies through seed coat modifications or by the incorporation of various parts of the seed cone into the diaspore, with the modern conifer families independently evolving their characteristic diaspore compositions. Almost all functional morphotypes were present prior to the Cenozoic in at least one lineage, with more recent shifts in morphotypes representing functional convergence or parallel evolution rather than ecological novelties.

Phylogenetic divergence-time estimation has been revolutionized by two recent developments: 1) total-evidence dating (or ”tip-dating”) approaches that allow for the incorporation of fossils as tips in the analysis, with their phylogenetic and temporal relationships to the extant taxa inferred from the data, and 2) the fossilized birth-death (FBD) class of tree models that capture the processes that produce the tree (speciation, extinction, and fossilization), and thus provide a coherent and biologically interpretable tree prior. To explore the behaviour of these methods, we apply them to marattialean ferns, a group that was dominant in Carboniferous landscapes prior to declining to its modest extant diversity of slightly over 100 species. We show that tree models have a dramatic influence on estimates of both divergence times and topological relationships. This influence is driven by the strong, counter-intuitive informativeness of the uniform tree prior and the inherent nonidentifiability of divergence-time models. In contrast to the strong influence of the tree models, we find minor effects of differing the morphological transition model or the morphological clock model. We compare the performance of a large pool of candidate models using a combination of posterior-predictive simulation and Bayes factors. Notably, an FBD model with epoch-specific speciation and extinction rates was strongly favored by Bayes factors. Our best-fitting model infers stem and crown divergences for the Marattiales in the mid-Devonian and Late Cretaceous, respectively, with elevated speciation rates in the Mississippian and elevated extinction rates in the Cisuralian leading to a peak diversity of ∼2800 species at the end of the Carboniferous, representing the heyday of the Psaroniaceae. This peak is followed by the rapid decline and ultimate extinction of the Psaroniaceae, with their descendants, the Marattiaceae, persisting at approximately stable levels of diversity until the present. This general diversification pattern appears to be insensitive to potential biases in the fossil record; despite the preponderance of available fossils being from Pennsylvanian coal balls, incorporating fossilization-rate variation does not improve model fit. In addition, by incorporating temporal data directly within the model and allowing for the inference of the phylogenetic position of the fossils, our study makes the surprising inference that the clade of extant Marattiales is relatively young, younger than any of the fossils historically thought to be congeneric with extant species. This result is a dramatic demonstration of the dangers of node-based approaches to divergence-time estimation, where the assignment of fossils to particular clades are made a priori (earlier node-based studies that constrained the minimum ages of extant genera based on these fossils resulted in much older age estimates than in our study) and of the utility of explicit models of morphological evolution and lineage diversification.

Compared to the osteological record of herbivorous dinosaurs from the Late Cretaceous Prince Creek Formation of northern Alaska, there are relatively fewer remains of theropods. The theropod record from this unit is mostly comprised of isolated teeth, and the only nondental remains known can be attributed to the troodontid cf. Troodon and the tyrannosaurid Nanuqsaurus. Thus far, the presence of members of Dromaeosauridae has been limited to isolated teeth. Here we describe a symphyseal portion of a small dentary with two ziphodont teeth. Based on tooth shape, denticle morphology, and the position of the Meckelian groove, we attribute this partial dentary to a saurornitholestine dromaeosaurid. The fibrous bone surface, small size, and higher number of mesial denticles compared to distal ones point to a juvenile growth stage for this individual. Multivariate comparison of theropod teeth morphospace by means of principal component analysis reveals an overlap between this dentary and Saurornitholestinae dromaeosaurid morphospace, a result supported by phylogenetic analyses. This is the first confirmed non-dental fossil specimen from a member of Dromaeosauridae in the Arctic, expanding on the role of Beringia as a dispersal route for this clade between Asia and North America. Furthermore, the juvenile nature of this individual adds to a growing body of data that suggests Cretaceous Arctic dinosaurs of Alaska did not undergo long-distance migration, but rather they were year-round residents of these paleopolar latitudes.