a new hypothesis of dinosaur relationships and early dinosaur evolution

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A new hypothesis of dinosaur relationships and early dinosaur evolution

Affiliations.

• 1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
• 2 Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
• PMID: 28332513
• DOI: 10.1038/nature21700

For 130 years, dinosaurs have been divided into two distinct clades-Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.

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• Palaeontology: Dividing the dinosaurs. Padian K. Padian K. Nature. 2017 Mar 22;543(7646):494-495. doi: 10.1038/543494a. Nature. 2017. PMID: 28332523 No abstract available.
• Nomenclature: Share names for dinosaur divisions. Holtz TR. Holtz TR. Nature. 2017 May 3;545(7652):30. doi: 10.1038/545030d. Nature. 2017. PMID: 28470208 No abstract available.
• Evolution: Uprooting the Dinosaur Family Tree. Brusatte SL. Brusatte SL. Curr Biol. 2017 May 22;27(10):R390-R392. doi: 10.1016/j.cub.2017.04.001. Curr Biol. 2017. PMID: 28535391

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A new hypothesis of dinosaur relationships and early dinosaur evolution

During the Middle to Late Triassic period, the ornithodiran archosaur lineage split into a number of ecologically and phylogenetically distinct groups, including pterosaurs, silesaurids and dinosaurs, each characterized by numerous derived features 1. By the Carnian stage of the Late Triassic (around 230 million years ago (Ma)), dinosaurs had diversified into three major lineages, Ornithischia, Sauropodomorpha and Theropoda, and, by the Norian stage (around 208 Ma), some dinosaur groups had become species-rich and numerically abundant 2. Since 1887 (ref. 3) theropods and sauropodomorphs, which retain a classically reptile-like pelvic anatomy, have been regarded as forming a natural group (Saurischia), distinct from Ornithischia, which was characterized by 'bird-hipped' pelvic anatomy 3,4. For nearly a century, ornithischians and saurischians were regarded as unrelated, each descended from a different set of 'thecodont' (a primitive archosaur) ancestors 5. A formal hypothesis proposing dinosaur monophyly was proposed in 1974 (ref. 6), and consolidated in the 1980s 7. As a direct result of these and other analyses, Ornithischia and Saurischia came to be regarded as monophyletic sister-taxa: this hypothesis of relationships has been universally accepted ever since 2,8–13. Recent phylogenetic analyses of early dinosaurs have also supported the traditional scheme (Saurischia and Ornithischia), but those studies that concentrated on the earliest divergences within the clade have been limited to include only a handful of the relevant taxa and incorporate numerous a priori assumptions regarding the relationships within and between the higher taxonomic groups 8,9,14. Most recent studies of basal dinosaur relationships have tended to focus on a handful of taxa contained within one or two dinosaur clades (usually Saurischia), with Ornithischia represented only as either a single supraspecific taxon or by a small number of basal forms, such as Heterodontosaurus and Pisanosaurus 2,10–12. No studies on early dinosaur relationships have included an adequate sample of early ornithischians and the majority of studies have also excluded pivotal taxa from other major dinosaur and dinosauromorph (near dinosaur) lineages 2,10. Furthermore, and possibly in part owing to the unique anatomy of ornithischians, many studies of early dinosaur evolution have tended to score ornithis-chian taxa only for either anatomical characters that are thought to be dinosaur symplesiomorphies (ancestral traits or characters shared by two or more taxa) or characters that are related to discussions of ornithischian monophyly 9,11,14. As a result, these studies have incorporated numerous, frequently untested, prior assumptions with regard to dinosaur (and particularly ornithischian) character evolution, and have overlooked the possibility that some of the characters found in orni-thischian taxa are homologues of those in saurischian dinosaurs, even though several authors have commented on the anatomical similarities shared by ornithischians and theropods 13–16. In order to examine the possible effects of these biases on our understanding of dinosaur evolution, we carried out a phylogenetic analysis of basal Dinosauria and Dinosauromorpha and compiled, to our knowledge, the largest and most comprehensive dataset of these taxa to date. Although this study has drawn upon numerous previous studies, no prior assumptions were made about correlated patterns of character evolution or dinosaur interrelationships. The results of this study challenge more than a century of dogma and recover an unexpected tree topology that necessitates fundamental reassessment of current hypotheses concerning early dinosaur evolution, palaeoecology and palaeobiology. We analysed a wide range of dinosaurs and dinosauromorphs, including representatives of all known dinosauromorph clades. Our dataset included taxa that allowed wide spatiotemporal sampling worldwide, from the Middle Triassic to Cretaceous, with particular emphasis on taxa from the Middle Triassic to Early Jurassic, with varied body sizes, morphologies and levels of skeletal completeness. We attempted, as objectively as possible, to score all taxa for all characters (where applicable), a level of inclusivity that is unmatched by previous studies. For example, we are, to our knowledge, the first to score basal ornithischian taxa, such as Lesothosaurus diagnosticus and heterodon-tosaurids, for characters obtained from studies that focused on early theropod or saurischian relationships 10,11. In this way, we rigorously tested for anatomical similarities and differences between all of the included basal dinosaur taxa. However, some characters were inap-plicable in some taxa and these were treated as uncertainties using the notation (−) in this analysis. Taxa were scored from a combination of personal observations, information from the literature and a small number of unpublished photographs. In total 74 taxa were scored for 457 characters. Phylogenetic trees were produced and analysed in TNT 1.5-beta 17. Bremer support decay indices were also calculated using TNT 1.5-beta 17. Constraint trees For 130 years, dinosaurs have been divided into two distinct clades—Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.

Related Papers

Matthew Baron

For 130 years dinosaurs have been divided into two distinct clades – Ornithischia and Saurischia. This dissertation looks at the earliest evolution of the clade Dinosauria by focusing upon the interrelationships of the major subsidiary clades within it. It does this following examination, comparison and description of early dinosaur material, and by utilising modern phylogenetic analysis techniques, to rigorously and objectively test the fundamental groupings within the clade Dinosauria using a newly compiled dataset of early dinosaurs and other dinosauromorphs (= close dinosaur relatives). The current consensus on how the principal clades within Dinosauria (Theropoda, Sauropodomorpha and Ornithischia) are related to one another is challenged by the results of these analyses. This study finds, for the first time, a sister-group relationship between Ornithischia and Theropoda, here termed Ornithoscelida. Consequently, a new definition for Dinosauria is presented, as the historic definition would exclude all members of Sauropodomorpha from the clade. As well as this, I propose revisions to the definitions of each of the principal dinosaurian sub-divisions and propose a new timeframe and geographic setting for the origin of Dinosauria. These new hypotheses force re-evaluations of early dinosaur cladogenesis and character evolution, suggest the independent acquisition of hypercarnivory in multiple dinosaur groups and offers an explanation for many of the anatomical features previously regarded as striking convergences between theropods and early ornithischians. As well as presenting new anatomical data on many early dinosaurs and dinosauromorphs, including a comprehensive re-description of the postcranial anatomy of Lesothosaurus diagnosticus (Chapter 2), and a new anatomical dataset of early dinosaurs (the largest ever compiled), this thesis goes on to implement the new dataset to investigate a number of important outstanding questions about early dinosaur evolution and provides new lines of enquiry for future workers to pursue. The results of this thesis reveal the oldest known members of the dinosaurian clades Theropoda and Sauropodomorpha (Chapter 6), as well as a new clade within Ornithischia; a taxon previously thought to represent a derived theropod has been recovered as a potential ‘missing link’ between theropods and ornithischians using the new dataset. This work now provides a unique tool for the assessment of the phylogenetic affinities of early dinosaurs and dinosauromorphs and, once published, will hopefully become the benchmark dataset for palaeontologists working in this area.

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The systematic relationships of taxa traditionally referred to as ‘basal ornithopods’ or ‘hypsilophodontids’ remain poorly resolved since it was discovered that these taxa are not a monophyletic group, but rather a paraphyletic set of neornithischian taxa. Thus, even as the known diversity of these taxa has dramatically increased over the past two decades, our knowledge of their placement relative to each other and the major ornithischian subclades remained incomplete. This study employs the largest phylogenetic dataset yet compiled to assess basal ornithischian relationships (255 characters for 65 species level terminal taxa). The resulting strict consensus tree is the most well-resolved, stratigraphically consistent hypothesis of basal ornithischian relationships yet hypothesized. The only non-iguanodontian ornithopod (= basal ornithopod) recovered in this analysis is Hypsilophodon foxii. The majority of former ‘hypsilophodontid’ taxa are recovered within a single clade (Parksosauridae) that is situated as the sister-taxon to Cerapoda. The Parksosauridae is divided between two subclades, the Orodrominae and the Thescelosaurinae. This study does not recover a clade consisting of the Asian taxa Changchunsaurus, Haya, and Jeholosaurus (=Jeholosauridae). Rather, the former two taxa are recovered as basal members of Thescelosaurinae, while the latter taxon is recovered in a clade with Yueosaurus near the base of Neornithischia. The endemic South American clade Elasmaria is recovered within the Thescelosaurinae as the sister taxon to Thescelosaurus. This study supports the origination of Dinosauria and the early diversification of Ornithischia within Gondwana. Neornithischia first arose in Africa by the Early Jurassic before dispersing to Asia before the late Middle Jurassic, where much of the diversification among non-cerapodan neornithischians occurred. Under the simplest scenario the Parksosauridae originated in North America, with at least two later dispersals to Asia and one to South America. However, when ghost lineages are considered an alternate dispersal hypothesis has thescelosaurines dispersing from Asia into South America (via North America) during the Early Cretaceous, then back into North America in the latest Cretaceous. The latter hypothesis may explain the dominance of orodromine taxa prior to the Maastrichtian in North America and the sudden appearance and wide distribution of thescelosaurines in North America beginning in the early Maastrichtian. While the diversity of parksosaurids has greatly increased over the last fifteen years, a ghost lineage of over 40 myr is present between the base of Parksosauridae and Cerapoda, indicating that much of the early history and diversity of this clade is yet to be discovered. This new phylogenetic hypothesis provides a comprehensive framework for testing further hypotheses regarding evolutionary patterns and processes within Ornithischia.

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A novel hypothesis of dinosaur relationships and early dinosaur evolution

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For nearly 130 years dinosaurs have been divided into two distinct clades Ornithischia and Saurischia. Here, we present a radical new hypothesis for the phylogenetic relationships of the major dinosaurian groups, one that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study recovers, for the first time, a sister-group relationship between Ornithischia and Theropoda (Ornithoscelida), with Sauropodomorpha + Herrerasauridae forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests the independent acquisition of hypercarnivory in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as striking convergences between theropods and early ornithischians.

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A new hypothesis of dinosaur relationships and early dinosaur evolution

NATURE (2017)

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For 130 years, dinosaurs have been divided into two distinct clades-Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.

During the Middle to Late Triassic period, the ornithodiran archosaur lineage split into a number of ecologically and phylogenetically distinct groups, including pterosaurs, silesaurids and dinosaurs, each characterized by numerous derived features1. By the Carnian stage of the Late Triassic (around 230 million years ago (Ma)), dinosaurs had diversified into three major lineages, Ornithischia, Sauropodomorpha and Theropoda, and, by the Norian stage (around 208 Ma), some dinosaur groups had become species-rich and numerically abundant2. Since 1887 (ref. 3) theropods and sauropodomorphs, which retain a classically reptile-like pelvic anatomy, have been regarded as forming a natural group (Saurischia), distinct from Ornithischia, which was characterized by 'bird-hipped' pelvic anatomy3,4. For nearly a century, ornithischians and saurischians were regarded as unrelated, each descended from a different set of 'thecodont' (a primitive archosaur) ancestors5. A formal hypothesis proposing dinosaur monophyly was proposed in 1974 (ref. 6), and consolidated in the 1980s7. As a direct result of these and other analyses, Ornithischia and Saurischia came to be regarded as monophyletic sister-taxa: this hypothesis of relationships has been universally accepted ever since2,8-13.

Recent phylogenetic analyses of early dinosaurs have also supported the traditional scheme (Saurischia and Ornithischia), but those studies that concentrated on the earliest divergences within the clade have been limited to include only a handful of the relevant taxa and incorporate numerous a priori assumptions regarding the relationships within and between the higher taxonomic groups8,9,14. Most recent studies of basal dinosaur relationships have tended to focus on a...

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A new hypothesis of dinosaur relationships and early dinosaur evolution

• Baron, Matthew G.
• Norman, David B.
• Barrett, Paul M.

For 130 years, dinosaurs have been divided into two distinct clades—Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.

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New study shakes the roots of the dinosaur family tree

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More than a century of theory about the evolutionary history of dinosaurs has been turned on its head following the publication of new research from scientists at the University of Cambridge and Natural History Museum in London. Their work suggests that the family groupings need to be rearranged, re-defined and re-named and also that dinosaurs may have originated in the northern hemisphere rather than the southern, as current thinking goes.

This conclusion came as quite a shock since it ran counter to everything we'd learned Matthew Baron

For 130 years palaeontologists have been working with a classification system in which dinosaur species have been placed in to two distinct categories: Ornithischia and Saurischia. But now, after careful analysis of dozens of fossil skeletons and tens of thousands of anatomical characters, the researchers have concluded that these long-accepted familial groupings may, in fact, be wrong and that the traditional names need to be completely altered.

The classification of dinosaurs dates back to Victorian times. Dinosaurs were first recognised as a unique group of fossil reptiles in 1842 as a result of the work of the anatomist, Professor Richard Owen (who later went on to found the Natural History Museum in London). Over subsequent decades, various species were named as more and more fossils were found and identified. During the latter half of the 19th century it was realised that dinosaurs were anatomically diverse and attempts were made to classify them into groups that shared particular features.

It was Harry Govier Seeley, a palaeontologist trained in Cambridge under the renowned geologist Adam Sedgwick, who determined that dinosaurs fell quite neatly into two distinct groupings, or clades; Saurischia or Ornithischia. This classification was based on the arrangement of the creatures’ hip bones and in particular whether they displayed a lizard-like pattern (Saurischia) or a bird-like one (Ornithischia).

As more dinosaurs were described it became clear that they belonged to three distinct lineages; Ornithischia, Sauropodomorpha and Theropoda. In 1887 Seeley placed the sauropodomorphs (which included the huge ‘classic’ dinosaurs such as Diplodocus and Brontosaurus) together with the  theropods (which included T. rex), in the Saurischia. The ornithischians and saurischians were at first thought to be unrelated, each having a different set of ancestors, but later study showed that they all evolved from a single common ancestor.   

This new analysis of dinosaurs and their near relatives, published today in the journal Nature, concludes that the ornithischians need to be grouped with the theropods, to the exclusion of the sauropodomorphs. It has long been known that birds (with their obviously ‘bird-like’ hips) evolved from theropod dinosaurs (with their lizard-like hips). However, the re-grouping of dinosaurs proposed in this study shows that both ornithischians AND theropods had the potential to evolve a bird-like hip arrangement- they just did so at different times in their history.

Lead author, Matthew Baron, says:

“When we started our analysis, we puzzled as to why some ancient ornithischians appeared anatomically similar to theropods. Our fresh study suggested that these two groups were indeed part of the same clade. This conclusion came as quite a shock since it ran counter to everything we’d learned.”

“The carnivorous theropods were more closely related to the herbivorous ornithischians and, what’s more, some animals, such as Diplodocus, would fall outside the traditional grouping that we called dinosaurs. This meant we would have to change the definition of the ‘dinosaur’ to make sure that, in the future, Diplodocus and its near relatives could still be classed as dinosaurs.”

The revised grouping of Ornithischia and Theropoda has been named the Ornithoscelida which revives a name originally coined by the evolutionary biologist, Thomas Henry Huxley in 1870. 

Co-author, Dr David Norman, of the University of Cambridge, says:

“The repercussions of this research are both surprising and profound. The bird-hipped dinosaurs, so often considered paradoxically named because they appeared to have nothing to do with bird origins, are now firmly attached to the ancestry of living birds.”

For 130 years palaeontologists have considered the phylogeny of the dinosaurs in a certain way. Our research indicates they need to look again at the creatures’ evolutionary history. This is simply science in action. You draw conclusions from one body of evidence and then new data or theories present themselves and you have to suddenly reconsider and adapt your thinking. All the major textbooks covering the topic of the evolution of the vertebrates will need to be re-written if our suggestion survives academic scrutiny.”

While analysing the dinosaur family trees the team arrived at another unexpected conclusion. For many years, it was thought that dinosaurs originated in the southern hemisphere on the ancient continent known as Gondwana. The oldest dinosaur fossils have been recovered from South America suggesting the earliest dinosaurs originated there. But as a result of a re-examination of key taxa it’s now thought they could just as easily have originated on the northern landmass known as Laurasia, though it must be remembered that the continents were much closer together at this time. 

Co-author, Prof Paul Barrett, of the Natural History Museum, says:

"This study radically redraws the dinosaur family tree, providing a new framework for unravelling the evolution of their key features, biology and distribution through time. If we're correct, it explains away many prior inconsistencies in our knowledge of dinosaur anatomy and relationships and it also highlights several new questions relating to the pace and geographical setting of dinosaur origins".

The research was funded through a Natural Environment Research Council (NERC) CASE studentship.

Matthew Baron et al: ' A new hypothesis of dinosaur relationships and early dinosaur evolution ' Nature , 23 March 2017 

10.1038/nature21700

A short video guide has been prepared by the Natural History Museum to accompany this paper.

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Kulindadromeus, a small bipedal ornithischian dinosaur that is now part of the new grouping Ornithoscelida and identified as more obviously sharing an ancestry with living birds

Credit: Pascal Godefroid

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• DOI: 10.1017/S1477201906001970
• Corpus ID: 55723635

Early dinosaurs: A phylogenetic study

• M. Langer , M. Benton
• Published 1 January 2006
• Biology, Geology
• Journal of Systematic Palaeontology

214 Citations

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• Brief Communications Arising
• Published: 02 November 2017

Untangling the dinosaur family tree

• Max C. Langer 1 ,
• Martín D. Ezcurra 2 ,
• Oliver W. M. Rauhut 3 ,
• Michael J. Benton 4 ,
• Fabien Knoll 5 , 6 ,
• Blair W. McPhee 1 ,
• Fernando E. Novas 7 ,
• Diego Pol 8 &
• Stephen L. Brusatte 9  

Nature volume  551 ,  pages E1–E3 ( 2017 ) Cite this article

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A Brief Communications Arising to this article was published on 02 November 2017

arising from M. G. Baron et al . Nature 543 , 501–506 (2017); doi:10.1038/nature21700

For over a century, the standard classification scheme has split dinosaurs into two fundamental groups 1 : ‘lizard-hipped’ saurischians (including meat-eating theropods and long-necked sauropodomorphs) and ‘bird-hipped’ ornithischians (including a variety of herbivorous species) 2 , 3 , 4 . In a recent paper, Baron et al . 5 challenged this paradigm with a new phylogenetic analysis that places theropods and ornithischians together in a group called Ornithoscelida, to the exclusion of sauropodomorphs, and used their phylogeny to argue that dinosaurs may have originated in northern Pangaea, not in the southern part of the supercontinent, as has more commonly been considered 6 , 7 . Here we evaluate and reanalyse the morphological dataset underpinning the proposal by Baron et al . 5 and provide quantitative biogeographic analyses, which challenge the key results of their study by recovering a classical monophyletic Saurischia and a Gondwanan origin for dinosaurs. This shows that the Ornithoscelida hypothesis is not the final word, and that there is still great uncertainty around the basic structure of the dinosaur family tree. There is a Reply to this Comment by Baron, M. G. et al . Nature 551 , 10.1038/nature24012 (2017) .

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Oliver W. M. Rauhut

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M.C.L., M.D.E., O.W.M.R., M.J.B., B.W.M., F.E.N. and S.L.B. designed the project. M.C.L., M.D.E., O.W.M.R., M.J.B., F.K., B.W.M., F.E.N. and D.P. rescored the data matrix. M.D.E. and D.P. conducted the phylogenetic analyses. M.J.B., B.W.M. and D.P. conducted the biogeographic analyses. All authors discussed the data, wrote and approved the final manuscript (including the Supplementary Information ).

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