Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem

First appearing in the latest Cretaceous, Crocodylia is a clade of semi-aquatic, predatory reptiles, defined by the last common ancestor of extant alligators, caimans, crocodiles, and gharials. Despite large strides in resolving crocodylian interrelationships over the last three decades, several outstanding problems persist in crocodylian systematics. Most notably, there has been persistent discordance between morphological and molecular datasets surrounding the affinities of the extant gharials, Gavialis gangeticus and Tomistoma schlegelii. Whereas molecular data consistently support a sister taxon relationship, in which they are more closely related to crocodylids than to alligatorids, morphological data indicate that Gavialis is the sister taxon to all other extant crocodylians. Here we present a new morphological dataset for Crocodylia based on a critical reappraisal of published crocodylian character data matrices and extensive firsthand observations of a global sample of crocodylians. This comprises the most taxonomically comprehensive crocodylian dataset to date (144 OTUs scored for 330 characters) and includes a new, illustrated character list with modifications to the construction and scoring of characters, and 46 novel characters. Under a maximum parsimony framework, our analyses robustly recover Gavialis as more closely related to Tomistoma than to other extant crocodylians for the first time based on morphology alone. This result is recovered regardless of the weighting strategy and treatment of quantitative characters. However, analyses using continuous characters and extended implied weighting (with high k-values) produced the most resolved, well-supported, and stratigraphically congruent topologies overall. Resolution of the gharial problem reveals that: (1) several gavialoids lack plesiomorphic features that formerly drew them towards the stem of Crocodylia; and (2) more widespread similarities occur between species traditionally divided into tomistomines and gavialoids, with these interpreted here as homology rather than homoplasy. There remains significant temporal incongruence regarding the inferred divergence timing of the extant gharials, indicating that several putative gavialids (‘thoracosaurs’) are incorrectly placed and require future re-appraisal. New alligatoroid interrelationships include: (1) support for a North American origin of Caimaninae in the latest Cretaceous; (2) the recovery of the early Paleogene South American taxon Eocaiman as a ‘basal’ alligatoroid; and (3) the paraphyly of the Cenozoic European taxon Diplocynodon. Among crocodyloids, notable results include modifications to the taxonomic content of Mekosuchinae, including biogeographic affinities of this clade with latest Cretaceous–early Paleogene Asian crocodyloids. In light of our new results, we provide a comprehensive review of the evolutionary and biogeographic history of Crocodylia, which included multiple instances of transoceanic and continental dispersal.

Phylogenetic position of Neotropical Bursera-specialist mistletoes: the evolution of deciduousness and succulent leaves in Psittacanthus (Loranthaceae)

<p><strong>Background:</strong> The phylogenetic relationships of the <em>Bursera</em>-host specialist <em>Psittacanthus nudus</em>, <em>P. palmeri </em>and <em>P. sonorae</em> (Loranthaceae) remain uncertain. These mistletoe species exhibit morphological and phenological innovations probably related to their dry habitats, so that determining their phylogenetic position is key to the understanding of factors associated with the morphological evolution within <em>Psittacanthus</em>.</p><p><strong>Questions:</strong> (1) Is the evolution of some morphological innovations in the <em>Bursera</em>-host specialists associated with the ecological conditions linked to host diversification? (2) Does time of diversification in both lineages coincide?<strong></strong></p><p><strong>Study species:</strong> Fourteen species of <em>Psittacanthus</em>.</p><p><strong>Methods: </strong>Sequences of nuclear (ITS) and plastid (<em>trnL-trnF</em>) markers are analyzed with Bayesian inference, maximum likelihood and maximum parsimony methods, and molecular dating under a Bayesian approach estimated to elucidate the phylogenetic position and divergence timing of the<em> Bursera</em>-host specialists.</p><p><strong>Results:</strong> The <em>Bursera</em>-host specialists form a strongly supported clade, named here the ‘<em>Bursera</em> group’. The divergence time for the <em>Bursera</em>-host specialists was estimated at 7.89 Ma. Interestingly, phylogenetic relationships between <em>P. nudus</em> and <em>P. palmeri</em>, as currently circumscribed, were not fully resolved, making <em>P. palmeri</em> paraphyletic.</p><p><strong>Conclusions</strong>: Based on these results, the plants collected by type locality of <em>P. nudus</em> in Honduras should be named <em>P. palmeri</em>. The seasonal deciduousness of <em>P. palmeri </em>(including <em>P. nudus</em>) and morphology of <em>P. sonorae</em> (small size, fleshy leaves) are clearly adaptations to dry ecosystems where these species have lived for a long time. In parallel, the evolutionary history of these mistletoes seems to be correlated with the evolutionary history and diversification patterns of <em>Bursera</em>.</p>

Progress and future directions in archosaur phylogenetics

The basic structure of archosaurian phylogeny is understood to include two primary crown-group lineages—one leading to living crocodiles and including a broad diversity of Triassic animals (e.g., phytosaurs, rauisuchians, aetosaurs), and another leading to dinosaurs (living and extinct). These lineages were established by the middle Triassic. A few extinct groups remain controversial, such as the pterosaurs, and debate persists over the phylogenetic relationships among extant bird lineages, which have proved difficult to resolve, and divergence timing estimates within Aves and Crocodylia remain the source of contention. A few analyses support a close relationship between archosaurs and turtles, or even a nesting of turtles within Archosauria. All sources of information used to resolve these issues have weaknesses, and these problems all involve highly derived lineages when they first appear in the fossil record.